Moreover, some kinds of information can only be determined using paleobiological data. For example, although net diversification rates can be inferred from species richness in clades and communities without a long historic record, the more useful raw speciation and extinction rates can only be determined from the fossil record. Only data on raw rates can reveal whether the striking variation in richness observed among ecological communities reflects differences in the production of species, in rates of species loss, or some combination of the two.
These are qualitatively different dynamics, and call for completely different management strategies. These issues can be addressed by studies in deep time e. However, other studies—including a number of paleoecological studies—suggest that communities can retain fundamental structure and function despite continual turnover in species composition driven by routine geographic range shifts and extinctions Valentine and Jablonski, ; Holland and Patzkowsky, ; S.
Jackson and Overpeck, ; Webb et al. This suggests that there can be considerable interchangeability among species within ecological and functional categories. Are these apparently conflicting perspectives an artifact of scale? Paleoecological studies can take advantage of natural experiments in which community. Paleobiological data can be used to provide tests of fundamental ecological theory.
Permian rocks in west Texas, deposited Ma ago, contain well-preserved and well-studied assemblages of brachiopods. Olszewski and Erwin tallied these specimens in order to determine the species abundance distributions and how these distributions changed through the million year interval.
The shapes of the distributions of the lower two intervals differ significantly from those of the upper two intervals, and these differences are attributed to restrictions in population size, decreased isolation, and decreased chances of immigration resulting from lower sea levels in the upper two intervals. Accordingly, the geologic record of Permian fossils in this area preserves the results of a natural experiment on how environmental change affects ecological community structure over long time periods.
Similarly, little is known about the timescales and patterns of survival in remnant populations and the consequences for community structure. Do new species substitute for lost ones? Are functional groups thinned proportionally?
Understanding these dynamics in terms of general principles, and at the timescales at which. Species abundance distributions of brachiopods from Permian rocks of west Texas. The red line is maximum likelihood fit to log-normal distribution; blue line is maximum likelihood fit to zero-sum multinomial distribution. Olszewski and Erwin ; used with permission. One reason why migration has emerged as such an important species response to environmental change is that species often exhibit evolutionary stasis, and are insufficiently malleable in an evolutionary sense to adapt to radically altered conditions.
The fossil record clearly indicates pervasive evolutionary stasis in diverse groups in the face of substantial environmental change Huntley et al. Jackson and Cheetham, ; J. Jackson and Johnson, However, evolutionary responses have been documented in several cases e. What permits or drives some species to depart from stasis and thus adjust to changing physical and biological environments?
Do the evolutionary responses observed represent expansion into new niche space or redistribution of existing variation Huntley, ; S. Jackson and Overpeck, ? These questions are particularly pressing in the tropics, where many species are linked in close ecological partnerships. These alternative explanations suggest radically different potentials for the regeneration capacity of coevolutionary partnerships that have been disrupted. This can only be tested—using the fossil record—by determining the antiquity of such partnerships.
Although molecular data can provide information on the age of taxa, their mutual readjustments can only be documented using morphological and paleobiogeographic data from geohistorical records e. When the rate or magnitude of the environmental change exceeds the ability of a species to adapt or migrate, the result is local, regional, or global extinction.
Because global extinction is irreversible and because even local extinction can remove key genetic resources and severely perturb communities, one fundamental aim of conservation is to minimize extinction or at least to minimize its effects. Consequently, a high priority for managing the present day biota is a set. Examples are plants and pollinators plants feed insects in exchange for pollen transport , and mycorrhizae fungi provide hard-to-get dissolved minerals in exchange for sugars from plant root.
In a given situation, these rules probably vary according to the magnitude and kind of disturbance, the intrinsic characteristics of the species involved e. The fossil record affords an opportunity to test these factors across a range of extinction intensities and drivers. Some commonalities are beginning to emerge among taxa and across time intervals e. This is an area where modeling efforts and simulations, through collaborations between paleontologists and ecologists could be particularly productive.
Such models could act as a spur and guide for paleontological field work to iteratively refine the models. Determining the characteristics of demonstrably resilient systems or groups, particularly in relation to biotic or environmental crises, will be critical for effective management of diversity as a whole. Resilient groups may not have characteristics considered desirable from a human perspective.
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Unity, Anarchy, or Both? Geologic records of different ecosystems and time periods often yield contrasting views of community unity and integrity. Paleobiological records indicate that terrestrial and temperate marine communities did not migrate as cohesive, integrated units in response to Quaternary environmental changes.
Instead, species shifted their geographic ranges individualistically, producing species associations that do not occur today Box 3. This fluid pattern of community assembly and disassembly was unexpected by ecologists, and speaks to fundamental questions of the inertia of community structure, the strength and particularity of biotic interactions among species, and the likely consequences of species extinctions and invasions for community resilience. Some systems, however, such as tropical coral reefs, show greater stability of community composition during the Quaternary J.
Jackson, ; Pandolfi, ; Pandolfi and Jackson, , and many pre-Quaternary paleobiological studies suggest long intervals of community stability Brett et al. Assessment of the origin of this variation—whether it stems from the nature of the physical environment S. Jackson and Overpeck, , differential resilience of communities to. One of the most important insights to emerge from the late Quaternary fossil record is the existence in the recent past of biotic assemblages that have no modern counterparts. Today, populations of these species do not occur within 1, km of each other. Vertebrate faunal records from Cheek Bend Cave and numerous other sites contain diverse examples of communities with no modern analogs, and similarly singular assemblages have been documented for terrestrial insects Coope, and marine mollusks Roy et al.
Similar results come from studies of past terrestrial vegetation; pollen and plant macrofossil assemblages with no modern analogs are well documented in a variety of continental settings S. Jackson and Williams, For example, vegetation of the upper Midwest was occupied during the last deglaciation by forests dominated by boreal conifers spruce and cool-temperate hardwoods elm, oak, ash, hornbeam Williams et al.
Such forests do not occur anywhere in North America today. A critical question for both ecology and conservation biology is how these singular communities arise. Their existence in the past indicates that animal and plant species have responded individualistically to environmental changes of the late Quaternary, and that communities may be ephemeral at timescales beyond a few thousand years.
Did they develop because species range adjustments through dispersal and colonization could not keep pace with the rate of environmental change? Or did they arise as a result of unique combinations of environmental variables? Paleoecologists have pursued these questions vigorously during the past two decades. A recent study by Williams et al. Regions and time periods characterized by pollen assemblages lacking modern analogs closely match those with simulated climate regimes lacking modern analogs.
Simulated climates of the late glacial period 18,, years ago were drier and had greater temperature seasonality than modern climates of the region. From this perspective, the modern suite of species associations is not fixed, and novel associations may be expected to arise if future climatic. Recent climatic trends and climate simulations all suggest that 21st century climates may have smaller diurnal and annual seasonal ranges, and the synoptic pattern of precipitation intensity and variability is likely to change Easterling et al.
An important question is whether we have sufficient ecological knowledge to predict which communities will be most sensitive to these changes, and which sorts of new communities will arise to replace them. Paleoecological studies can play two roles here. First, examination of the history of particular communities can indicate which communities have been most sensitive to climate changes of the recent past. Second, the no-analog communities of the past can serve as a laboratory for testing models relating modern biogeographic distributions and community composition to climate.
Numerous opportunities exist for collaboration among ecologists, paleoecologists, paleoclimatologists, and modelers to determine whether past community composition can be predicted accurately from modern ecological observations and paleoclimate information. Such collaborations will help assess whether we have adequate knowledge to predict biotic responses to ongoing and future climate change.
Transient effects are likely to dominate in the near term, and another pressing question is how rapidly communities will respond to climate change in the coming decades. Again, the past may serve as a guide. Analyses of high-resolution lacustrine records suggest that plant communities responded to past abrupt climate changes with lag times less than a century Birks and Ammann, ; Tinner and Lotter, ; Williams et al.
Response times of a few decades are short from a geological perspective, but highly significant on human and ecological timescales. Based on these findings, we can expect disruption of existing communities and emergence of new combinations of species associations in the near future in response to climate change, although other anthropogenic effects land use, invasive species will also contribute. More broadly, these studies underscore the value of Quaternary research for studying linkages between biological and physical systems at timescales intermediate between the human life span and the deep-time perspectives afforded by longer-term geohistorical records S.
Each plant taxon is represented by a single pollen percentage contour, and each possible combination of plant taxa is represented by a different color. Color combinations present in the ancient time periods but absent in the modern maps mark the distribution of no-analog plant associations. Rows 3 and 4 are dissimilarity maps for fossil pollen assemblages Row 3 and CCM-1 Community Climate Model-1 climates Row 4 , interpolated to a common 50 km grid. Each dissimilarity value represents the minimum distance between a pollen or climate gridpoint and its corresponding modern dataset.
High dissimilarities indicate the absence of any close modern analog. Modified from Williams et al. Jackson, , the taxonomic level of the investigation species vs. Recovery after Biotic Disturbance. Disturbance occurs on a variety of temporal and spatial scales. Many such disturbances are not amenable to contemporary analysis but require paleobiological data. An important unresolved issue is whether biotic response to perturbation scales from ecological to evolutionary events. Do the processes involved in response to ecological disturbances also apply to response to mass extinctions, or is there a threshold beyond which the processes differ?
The variety and intensity of disturbances that species and communities can tolerate or withstand is best evaluated through the paleobiological record of past disturbances. This allows definition of the limits to disturbances beyond which the system may collapse. Repeated disturbances might weaken communities by, for example, reducing the number of connections within food webs, or may harden communities through progressive subtraction of more narrowly adapted species.
Evidence for such behavior and its net consequences for community diversity and structure, and predictions for present day communities, can be acquired only from historical analysis. In the historical analysis of recoveries, it is clear that some species that appear to be adapted for particular disturbance regimes may in reality simply have fortuitous exaptations or pre-adaptations that happen to be useful.
For example, long-leaf pines in southeastern forests occur in a fire regime that paleobiological data suggest has only existed for the last Ka, and to which they could not have evolved. Fire adaptations may have arisen in a very different environment. Similarly, high carbon-use efficiency in conifers—an adaptation to cold environments—may also confer advantages in the low-CO 2 environments that arose during Quaternary glaciations S.
The preferential survival of dino-flagellates relative to other phytoplankton during the end-Cretaceous extinction events may be attributable to their characteristic resting cysts, which appear to have evolved to permit dormancy under much less extreme stresses Kitchell et al.
These examples underscore that only paleobiological analysis can reveal the behavior of species over the timescales—and through the repeated natural experiments—that permit assessment of the relative importance of the different environmental and biological factors affecting biotic recovery. Biogeochemical cycles of carbon, oxygen, nitrogen, phosphorus, sulfur, and other elements play critical roles in the earth system, ranging from effects on local productivity to global biosphere functioning. Understanding these cycles and how they are influenced by heterogeneity in space and variability in time is a critical goal for ecology.
It is also vitally important as human activities disrupt these cycles, altering their rates, pathways, and chemical transformations.
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Many of the processes involved in these cycles occur at timescales of hundreds to millions of years, and so geological perspectives are necessary. Biogeochemical cycles are global in nature, but the underlying processes, particularly biological ones, are inherently local. Geohistorical records can address fundamental biogeochemical questions at spatial scales ranging from local to global, and temporal scales ranging from the recent past to the entirety of geobiological history. Biogeochemical Fluxes and Controls: Retrospective Experiments and Chronosequences. Ecologists are concerned with understanding the processes governing biogeochemical cycles, and with assessing the sensitivity of biogeochemical reservoirs and fluxes to a wide array of perturbations—from local disturbances to global climate change.
Their efforts are hampered by the wide range of temporal scales at which relevant processes occur and the limited array and duration of observational and experimental studies available. Geohistorical records can be exploited to solve some of these problems; in particular, they can be used to design retrospective experiments and for chronosequence analysis. Retrospective experiments take advantage of natural or human-caused situations in which treatment effects and controls can be assessed using geohistorical records. Sedimentary basins—lakes, estuaries, marine basins, peatlands—provide a record of a variety of biogeochemical processes and fluxes in the form of fossil organisms, biomarkers, and sediment geochemistry.
They simultaneously record information about adjacent terrestrial ecosystems e. Quasi-experimental pairs or arrays of basins can be used to examine treatment versus control effects of perturbations e. This approach can even be used to provide estimates for variables not measured in long-term monitoring programs e. The retrospective-experimental approach greatly extends our ability to understand the controls on biogeochemical processes.
The chronosequence approach—in which variation in space is substituted for change in time—is one of the oldest and most powerful tools employed by ecologists for studying temporal change. Historically, its primary applications have involved geological or disturbance-based contexts respectively referring to the time since geologic origination of the land surfaces, and time since the last severe disturbance. The wide range in ages of the volcanic islands of the Hawaiian archipelago, for instance, have been used to identify rates of nutrient leaching, sources and rates of nutrient supply, and the climatic controls on these processes Chadwick et al.
Information from paleoecological studies comprises a vastly underutilized context for chronosequence studies of biogeochemical flux. Such studies provide site-specific information on the timing of immigration of dominant species, conversion of one vegetation type to another e.
Ecologists can utilize this information to devise sampling arrays to assess the effects of a range of factors e. Abundant opportunities exist for collaboration between ecologists, paleoecologists, and geologists to identify appropriate chronosequences at a broad range of scales. The Evolution of Global Biogeochemical Cycles. The fundamental understanding of the dynamics and controls of biogeochemical cycles must rest on a foundation composed of the entire history of the biosphere Schlesinger, Biogeochemical cycles have evolved together with the biota of the planet, and the evolution of these cycles has involved reciprocal effects of biogeochemical and environmental changes on the biota.
Furthermore, because of the diversity of rates and response times among the various components of the earth system, these effects have played out over a vast array of timescales. Determining how these cycles have changed—and why—constitutes a major scientific challenge that requires. Fish introductions and invasions are widespread in North America, and they will continue as management agencies continue to stock lakes and streams, and as fish invade new waters as a result of human activities or by natural means Rahel, Fish introductions can alter food-web structure and extirpate native amphibians, zooplankton, and benthic macroinvertebrates.
A recent collaborative study involving surveys, experiments, modeling, and paleolimnology has revealed that trout introductions to oligotrophic lakes i. Trout feed on amphibians and benthic macroinvertebrates along the shallow margins of lakes, and they excrete phosphorus-rich wastes into the water column throughout the lake. In doing so, the fish transfer phosphorus from terrestrial ecosystems and the benthos—which would normally be recycled within the benthos or trapped in sediments—to the pelagic system. Experimental and observational studies in lakes indicate that this transfer can lead to a significant increase in phosphorus availability to phytoplankton Schindler et al.
A retrospective experiment was performed, using algal biomarkers in sediments to test whether fish stocking in the s influenced primary productivity. Paleolimnological studies of three lakes, each with a different fish-stocking history, show clear responses consistent with the nutrient-transfer hypothesis: Fish were stocked in a third lake Snowflake Lake , but disappeared within two decades.
The return of algal productivity to pre-stocking levels indicated that lakes can recover when fish populations decline or are removed. Trout were introduced for the first time to Bighorn Lake in , and brook trout populations have been self-sustaining in the lake ever since. Trout were also introduced repeatedly to Snowflake Lake in the early to mids. However, these populations were not self-sustaining and were extinct by the early s. Harrison Lake has never been stocked, but it sustains a native population of bull trout.
The unstocked lake—Harrison Lake—shows little change in algal productivity during recent decades. Bighorn Lake, with fish populations since , shows a steady increase in productivity following the original stocking. Snowflake Lake shows an. The stratigraphic profiles of algal-pigment biomarkers in sediments of three lakes in the Canadian Rockies with differing fish introduction histories demonstrate how net primary productivity is influenced by fish populations as a result of their effects on nutrient cycling. Great progress in understanding the biogeochemical evolution of the biosphere has been made in the past two decades.
Carbon cycling has been a particular focus of attention e. However, many critical questions remain, and the biogeochemical history of other elements is more poorly known. Is it possible to predict past dynamics of oxygen, phosphorus, iron, and nitrogen cycles based on knowledge of past carbon cycling? Are interactions among these cycles contingent on particular configurations of climate, sea level, and continental position?
Are couplings among cycles contingent on the existence or absence of particular types of organisms or metabolic pathways? Furthermore, development of improved paleobiogeochemical proxies is needed to extend our understanding in deep time. These can include periods for which diverse, detailed information is readily available, periods with important system transitions or excursions, periods with unique configurations of the earth system, and periods for which processes can be linked with understanding and modeling of modern systems or biota.
The latter can range from anaerobic, prokaryotic systems of the Proterozoic, to the modern biota of the late Quaternary. For the Phanerozoic, periods in which correlations of events or time series between marine and terrestrial domains should be emphasized. For example, the Eocene-Oligocene interval might be studied at a coarse scale, with intensified efforts focused on specific events such as the early Eocene thermal maximum and the rapid cooling event at the Eocene-Oligocene boundary Zachos et al.
These efforts will require collaborations spanning much of the earth and environmental sciences, and integrating data collection, database development, and modeling. Some initial efforts at such collaborations have been made for some geologic intervals, notably the Proterozoic, Cenozoic, and Quaternary e.
Species introductions, both deliberate and accidental, are having dramatic ecological effects across the globe Mooney and Hobbs, ; Mack et al. The long-term ecological and evolutionary consequences of these exchanges are as yet poorly understood. Geohistorical records of past invasions induced by climate change, dispersal, and continental drift provide a series of natural experiments that can help assess future community and ecosystem consequences of biotic invasions.
For example, the past biotic interchanges from North Pacific to North Atlantic, between North America and South America, and between eastern Asia and western North America, recognized on the basis of phylogenetic and biogeographic analyses of pre- and post-interchange biotas, are rich in insights into the asymmetry seen in most biotic interchanges, the differential invasive properties of species, phylads, 2 and functional groups, and the contrasting regional biotic histories of donor and recipient areas Vermeij, ; Jablonski and Sepkoski, Paleobiological records of species expansions into new territory following climatic changes of the late Quaternary provide lessons relevant to modern invasions, including the relative rapidity and underlying mechanisms Davis, a; Webb, ; Clark et al.
Paleobiological approaches can also be used to investigate species introductions that occurred before adequate historical documentation Parkes et al. Jackson, ; Egan and Howell, There is increasing awareness of the risks of outbreaks of infectious diseases in humans and other species that are important for human use or conservation NRC, c. Although epidemiological studies will likely be central to this topic, the role of environmental change and human activities in triggering disease outbreaks is being increasingly appreciated.
Our ability to identify past disease outbreaks and their consequences using the geologic record is certainly limited. Nevertheless, we should be alert to the opportunities that may exist for useful investigations on this topic. For example, sedimentary records of dinoflagellate tests or biomarkers in selected areas may help determine whether recent outbreaks of Pfiesteria red tides—harmful algal blooms—are unprecedented and presumably related to human activities, or have occurred in the past with.
Phylad—an evolutionary lineage; a group of species, genera, or families that share a common ancestor. The ecological effects of the Cretaceous-Tertiary extinction in the oceans persisted for millions of years. Although the productivity of surface waters was restored quickly, full recovery to pre-extinction levels of organic carbon flux to the deep sea was delayed by at least three million years. They argue that the delayed recovery was either a consequence of the small size of the surviving phytoplankton or the small size of phytoplankton grazers.
The organic matter of very small phytoplankton is more likely to be recycled within the surface waters than exported to deeper water, and larger grazers are needed to aggregate biomass as carcasses or fecal material for rapid delivery to the deep sea. Similarly, molecular or serological analysis of debris from fossil rodent middens and archeological sites may provide clues on the antiquity, extent, and evolution of hantavirus.
Prehistoric outbreaks of forest pathogens in North America Davis, b; Allison et al. Application of molecular techniques to the sedimentary record may help resolve the causal agents, and high-resolution paleoclimatological studies may help identify whether climate variations interacted with pathogens or vectors e. Late Quaternary megafaunal extinctions have been attributed to microbial pathogens spread by migrating humans MacPhee and Marx, , and attempts are being made to test this hypothesis using molecular analysis of fossil material Greenwood et al.
Geographic distributions and population sizes of organisms are constrained to varying degrees by climate.
The tempo and pattern of biological invasions and disease outbreaks are frequently determined by climate, and effects of human land use and habitat alteration are contingent on climate. Atmospheric accretion of greenhouse gases, together with extensive land-cover changes by human activities, are raising concerns about human-induced climate change IPCC, , ; Pielke et al. Historical data have been crucial to disentangling potential anthropogenic and non-anthropogenic impacts.
Management of modern ecosystems e. Understanding how ecological systems have responded to past climatic and other environmental changes is necessary because the geologic record of ecological dynamics includes a vastly greater range of climatic conditions than are present today, or that are recorded in the instrumental and archival records of the past two centuries. We would be utterly ignorant of the rates of abrupt climate change in the absence of paleoclimate data NRC, a.
The rapid, ongoing development of paleoclimatic and other paleoenvironmental records, at local to global scales, and from the Proterozoic to the late Holocene, provides important opportunities for linking records of biotic changes to the existing paleoclimatic and paleobiological datasets and records. Such opportunities exist throughout the entire geologic column. Most depositional settings where high-quality, well-dated paleoclimatic records can be extracted—whether these are high-resolution time series or more scattered samples across time—can also yield paleobiological records.
Recent improvements in dating and chronostratigraphy have opened additional opportunities for correlating these records across different localities, regions, continents, and oceans, allowing climatic, environmental, and biotic dynamics to be studied together within a spatial framework. Finally, the recent and continuing development of a multitude of paleoclimate proxies Parrish, ; Bradley, is driving an important transition in paleoclimatology and paleoecology.
Paleoclimate inference is no longer heavily dependent on biological proxies, but can now draw on a greater variety of both biological and non-biological data. Thus, paleontological data on past distribution and abundance of organisms can now be used as response variables in studies in which climate change has been independently inferred see Box 3. Particular emphasis should be given to research efforts that focus on particularly well-documented and revealing time intervals in Earth history, and to work that contributes to our ability to forecast ecological responses to ongoing and future climate change.
Integrated studies of paleoclimate, paleoenvironment, and paleobiology are not practical for every interval of Earth history. With limited resources and in light of variation in the quality of the geologic record at different time periods, efforts should be targeted at selected intervals in Earth history that:.
Efforts to use geohistorical records to understand biotic responses to environmental change have often been hampered because the same data—pollen percentages, occurrences of plant or animal fossils—were used to infer both paleoenvironments and composition of past biotic communities. The proliferation of multiple paleoenvironmental proxies in the past two decades has now made it possible to link biotic responses to past environmental changes with greater confidence and precision.
For example, pollen percentages from lake and peatland sediments—long a dominant source of paleoclimatic inferences for continental regions—are now being complemented by a variety of paleoclimatological and paleohydrological proxies, including stable isotopes, geochemical, mineralogical, and molecular markers, and microfossils of aquatic organisms e.
In the figure below, postglacial pollen percentages of several important forest trees are plotted for a pond in the Berkshires of western Massachusetts, together with independently derived estimates of paleoclimate from lake levels paleohydrology and hydrogen isotopes paleotemperature from a lake km to the east in southeastern Massachusetts. Major vegetation transitions in the Berkshires coincide with climatic transitions recorded in southeastern Massachusetts, demonstrating the climatic control of vegetational composition and spatial coherence of climatic change at millennial scales Shuman et al.
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Please review your cart. You can remove the unavailable item s now or we'll automatically remove it at Checkout. Continue shopping Checkout Continue shopping. Herbivory mixes the two Barth and Jander, , thereby triggering the production and release of various reactive hydrolysis products, mainly isothiocyanates and nitriles Bones and Rossiter, , which can be directly toxic and repellent to herbivores Lazzeri et al. Consistently, glucosinolate biosynthesis can be induced by herbivory Hopkins et al. There are several cases showing that distinct defence-associated changes can be observed in tissues neighbouring the damaged area within 1 min after wounding or herbivory Schittko et al.
The extent to which the JA from this JA burst is synthesized de novo or released from storage is not always clear; it probably differs across different plant species and with the number of elicitations. The JA burst is preceded by a fast signal Schittko et al. However, molecular signals also travel from leaf to leaf, via either the phloem or the xylem Malone and Alarcon, ; Rhodes et al. However, the identity of these signals is still elusive.
Grafting experiments with tomato plants showed that JA biosynthesis, but not perception, was required for initiating the systemic signal while the downstream defence responses required perception Schilmiller and Howe, The spatiotemporal patterns of systemic responses differ across plant species, depending on differences not only in their size or age but also in plant-specific vascular architecture. Because not all leaves of a plant are connected to the same degree, the induction of defensive compounds is higher in undamaged leaves with the most direct vascular connection to the attacked leaf, the so-called orthostichous leaves Orians et al.
Heterogeneity in the levels of defensive compounds in damaged tissues can stimulate herbivores to move to other areas in an attempt to avoid the induced defences Shroff et al. For example, caterpillars of M. Also, the foraging behaviour of the generalist insect Helicoverpa armigera depends on JA: Interestingly, Plutella xylostella , which is known to be resistant to defensive chemicals in some Brassicaceae plants, did not display this behaviour Perkins et al.
Finally, heterogeneity of leaf systemic response Stork et al. Rodriguez-Saona and Thaler used normal tomato plants and JA-deficient def-1 tomato plants to analyse local and systemic induced JA responses in relation to patterns of caterpillar feeding damage. They observed that the systemic response in leaves with a stronger vascular connection was stronger than in leaves with a weaker connection, but at a similar physical distance from the damaged area. Hence the extent to which a herbivore can avoid induced defences is determined by the strength of the vascular connection of the induced leaf and the newly selected distal leaf.
It was shown that electric and vascular signals can act in synergy with airborne signals to optimize the systemically expressed resistance within a plant Heil and Ton, Moreover, exposure of plants to relatively high amounts of synthetic volatiles was shown to induce defences. For example, exposure of tomato plants to methyl jasmonate results in the accumulation of PIs in systemic leaves of the plant, in neighbouring tomato plants and even in plants of different species, such as tobacco and alfalfa Farmer and Ryan, Similarly, methyl salicylate may act as an airborne signal to activate resistance in uninfected tissues of an infected plant and in neighbouring plants Shulaev et al.
In contrast, corn plants previously exposed to the natural volatiles emitted by neighbouring plants accumulated more JA when damaged mechanically or induced with caterpillar regurgitant compared with corn plants not pre-exposed to volatiles Engelberth et al. Airborne signalling has at least two advantages over vascular signalling. This is especially relevant for bushy plants in which signals transported via the vascular system have to travel long distances.
For example, systemic induced resistance in sagebrush depends on air contact, possibly due to restrictions in vascular connections Karban et al. Thus, there is ample evidence that plant volatiles can facilitate signalling between leaves with weak vascular connections and facilitate priming in synergy with signals transmitted directly via plant tissues. Plants and herbivores have coevolved for over million years.
While plants have evolved signalling networks to regulate induced defences and diversity in their palette of secondary metabolites, herbivores have been under pressure to evade defences reviewed in Alba et al. Hence, behavioural adaptions have evolved that allow herbivores to avoid defended plant tissues Paschold et al. However, herbivores have also evolved a variety of mechanisms to cope with deterrent substances produced by their host plants. Given the enormous economic impact of herbivore resistance to agrochemicals, a large part of our knowledge of adaptations to xenobiotics comes from the field of pesticide resistance Despres et al.
However, the mechanisms at play are similar to those that enable them to resist defensive phytotoxins, and a functional overlap between adaptation to agrochemicals and to phytotoxins has been suggested Dermauw et al. Two general mechanisms allow herbivores to cope with the xenobiotics from their environment: Pharmacokinetic responses comprise a variety of adaptations that reduce uptake, increase catabolism and allow sequestration, whereas the pharmacodynamic response types comprise adaptations at the level of interactions between allelochemicals and their target-site s Taylor and Feyereisen, ; Kennedy and Tierney, Together, these mechanisms determine the level of tolerance of herbivores to xenobiotics Fig.
The herbivore resistance response. Plant secondary metabolites can have diverse target sites. Two general mechanisms allow arthropod herbivores to cope with plant secondary compounds: The midgut, fat body and Malpighian tubes are the insect tissues where these detoxification phases occur Harrop et al.
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Metabolites are excreted via faeces and urine. Some metabolites, or modified metabolites, are stored in the cuticle or in other organs and are used by the arthropod for its own protection. Plant substances are indicated in green and arthropod responses in red. The detoxification of xenobiotics usually occurs in three phases. In phase III, the phase II conjugated xenobiotic is transported out of the cell by cellular transporters. In several cases, these transporters can also act as a first line of defence, preventing allelochemicals entering the cell by rapid efflux without the need for modifications sometimes this is referred to as phase 0.
Adaptations that allow herbivores to enhance their detoxification of particular target xenobiotics often occur via mutations that increase the production of specific detoxification enzymes and transporters as well as by mutations that improve their catalytic or transport properties Brattsten, ; Despres et al. A wide range of allelochemicals, including furanocoumarins, terpenoids, glucosinolates, flavonoids and alkaloids can be metabolized by Ps of herbivorous arthropods Despres et al.
The involvement of Ps within the lepidopteran genus Papilio in furanocoumarin resistance is one of the first documented examples relating to plant—arthropod interactions Berenbaum, The Ps are by far the best studied enzymes of phase I, and have been associated with resistance to plant allelochemicals and pesticides in a wide range of species Feyereisen, ; Schuler, Although esterases have been linked to pesticide resistance in a number of cases, their role in plant allelochemical defence remains elusive Despres et al.
Notably, esterases were found not always to operate via hydrolysis but also to confer resistance to pesticides by sequestration, i. It is well imaginable that such esterases are involved in resistance or tolerance to phytotoxins as well. Conjugation of xenobiotics by GSTs has been linked to allochemical tolerance in a number of cases, although most evidence has been obtained from in vitro assays. The expression of GSTs in arthropod herbivores can be induced by a number of allelochemicals, and these enzymes are generally believed to make up an important component of the overall pharmacokinetic response.
Some GSTs of the polyphagous insect Spodoptera frugiperda are able to metabolize a variety of thiocyanate conjugates. Moreover, the diversity of glucosinolate-derived thiocyanates that insects such as the larvae of S. Recently, a role for GSTs in tolerance to glucosinolates was also suggested for the whitefly Bemisia tabaci and in the mustard-feeding specialist Scaptomyza nigrita Elbaz et al. The second important class of conjugation enzymes is that of the UGTs, which convert lipophilic aglycones into more hydrophilic glycosides by conjugation with UDP-glucose.
UGTs have been shown to be involved in resistance of lepidopterans to the alkaloid capsaicin and the detoxification of benzoxazinoids Ahn et al. Some insects have evolved traits that in principle allow them to prevent the activation of protoxins, such as glucosinolates, by plant enzymes. For example, aphids prevent glucosinolates from mixing with myrosinases by avoiding rupturing myrosinase-containing plant cells.
Moreover, the insect degrades these glucosinolates in the gut and conjugates the breakdown product to ascorbate, glutathione and cysteine. However, artificial feeding assays suggested that these conjugates also have an antifeeding effect in M. Hence, glucosinolates may also have defensive functions independent of myrosinase, via post-ingestive breakdown processes occurring in the aphid Kim et al.
This finding was based on the observation that verapamil, a known inhibitor of the ABC transporter, blocks nicotine transport in the Malpighian tubules of M. In addition, Petschenka et al. Other transporter families are also implicated in mediating the efflux of plant allelochemicals. Interestingly, other members of the same superfamily were upregulated in arthropod herbivores that were transferred from their preferred host to a more challenging, less suitable host de la Paz Celorio-Mancera et al.
Regardless of the identity of the transporters or transport mechanisms, a number of cases have been described in which the evolution of transport systems has determined herbivore success. For example, the ability to selectively transport plant glycosides has been suggested to stand at the basis of the evolution of life styles and host ranges of leaf beetles Kuhn et al. Some insects, especially host-plant specialists, have developed resistance mechanisms that differ from those described above.
For example, a flavin-dependent mono-oxygenase in arctiid moths was shown to be involved in detoxifying pyrrolizidine alkaloids Sehlmeyer et al. Moreover, specialist crucifer-feeding insects have developed the means to redirect the formation or to overcome the toxic action of glucosinolate breakdown products Pentzold et al.
Pieris rapae was shown to use a nitrile-specifier protein to divert the degradation of glucosinolates in the toxic isothiocyanates to less toxic nitriles Wittstock et al. Another specialist, the diamondback moth, Plutella xylostella , desulphates glucosinolates and thereby generates inactive metabolites Ratzka et al. This enzyme might also be crucial for countering the negative effects of glucosinolate in P. In addition, some lepidopterans have evolved the means to convert cyanide into nitrogen Engler et al.
Some herbivores make use of plant allelochemicals for their own defence against predators by storing ingested chemicals in specialized tissues or in the integument. A variety of mechanisms have been described that allow herbivores to exploit these toxic substances without suffering from their latent detrimental effects.
Compounds can be sequestered either directly or after biotransformation, such as by oxidation and conjugation. In some cases, insects have evolved the ability to synthesize these compounds de novo by convergent evolution of the biosynthetic pathways Jensen et al. Also, some insect species have evolved additional means to efficiently utilize sequestered compounds for their own defence. For example, some aphid and flea beetle species have evolved a specific myrosinase that allows them to convert glucosinolates into their toxic breakdown products in a similar way as plants do and to release these as toxic and repellent volatiles into the air Beran et al.
While many adaptations of herbivores to xenobiotics depend on mechanisms that directly or indirectly divert these substances from their target sites, some adaptations are due to reduced target-site sensitivity. Although alterations in the target site of pesticides have been associated with herbivore resistance to pesticides Ffrench-Constant, ; Van Leeuwen et al.
This lack of documented target-site resistance to phytochemicals is probably due to the fact that many of these have multiple modes of action and our knowledge of these mechanisms is limited Berenbaum, ; Despres et al. Remarkably, the same substitution was found in different insect species, all of which specialized in cardenolide-containing plant species, suggesting that this replacement must have evolved independently several times Agrawal et al.
Finally, tolerance to l -canavanine, a non-proteinogenic amino acid of leguminous plants, can also be considered as a target-site-based resistance mechanism in some insect species. The toxicity of l -canavanine is caused by its incorporation into proteins, replacing l -arginine. Insects such as the bruchid beetle Caryedes brasiliensis have specialized in feeding on the l -canavanine-rich seeds of Dioclea megacarpa and have evolved an arginyl-tRNA synthetase that can discriminate between l -canavanine and l -arginine, thereby effectively avoiding the adverse biochemical effect of l -canavanine Rosenthal et al.
In addition to target-site insensitivity, mechanisms of decreased exposure to l -canavanine have also been reported for the tobacco budworm H eliothis virescens Melangeli et al. Because many defensive actions of plant are induced or maintained by ongoing physiological processes, some herbivores have evolved the means to interfere with these processes. In this way, these herbivores may manipulate resource flows Clark and Harvell, or suppress defences Musser et al. However, these two processes will not always be easy to separate experimentally Karban and Agrawal, ; Stireman and Cipollini, ; Alba et al.
Suppression of defences is characterized by lowering the rate of production of defensive compounds. Hence, suppression can operate upstream or downstream of a defensive pathway and block it altogether or dampen it to intermediate levels, although assessing the latter can be difficult without having a suppression-free control experiment as a benchmark Alba et al. Finally, downregulation of plant defences qualifies as suppression when it is paralleled by an increase in reproductive performance of the herbivore Table 1. Suppression of plant defences is a well-known phenomenon in plant pathogens such as pathogenic bacteria Abramovitch et al.
However, nematodes and mites were also found to suppress defences. Several phytophagous nematode species interfere with host plant resistance Haegeman et al. In addition, two spider mite species were found to suppress the defences downstream of both JA and SA simultaneously in tomato Kant et al. Other examples of suppression are from insects and can be attributed to hormonal crosstalk in the majority of cases. Hemipteran phloem feeders such as the mealybug Phenacoccus solenopsis Zhang et al. In addition, the aphid Megoura viciae inhibits defensive phloem clogging Will et al.
The leafhopper Macrosteles quadrilineatus suppresses JA defences indirectly via an effector derived from a vectored phytoplasma Sugio et al. Also, chewing larvae of several lepidopteran species have been found to interfere with induced defences Bede et al. In addition, Helicoverpa zea was found to suppress nicotine accumulation in N. This is reminiscent of the downregulation of nicotine by M. Moreover, the Colorado potato beetle, Leptinotarsa decemlineata , suppresses transcription of PI genes in tomato Lawrence et al.
Finally, larvae of virulent strains of the hessian fly, Mayetiola destructor , secrete substances via their vestigial mouthparts into plant tissues, thereby suppressing the expression of PI and lectin genes Stuart et al. Thus, evidence for suppression of plant defensive processes is found across herbivorous insects, plant-eating mites and nematodes.
Whereas induction of plant defences often results from elicitor or HAMP recognition, the suppression of induced or constitutive defences is often attributed to so-called effector molecules. These molecules are especially well known from phytopathogens and had been discovered already in the s Shiraishi et al.
Although our knowledge of herbivore effectors is still limited, a staggering diversity of effectors from pathogens Deslandes and Rivas, ; Rovenich et al. Roughly, these effectors comprise the following functional groups although these are not mutually exclusive. Metabolites secreted into the host to manipulate particular physiological processes such as hormonal signalling.
Some strains of P. Enzymes secreted by pathogens to perform a metabolic conversion in the host that affects its defences. Fungi such as Septoria lycopersici produce tomatinase, which not only detoxifies the defensive alkaloid tomatine but also generates hydrolysis products that suppress the hypersensitive response Bouarab et al. Secreted proteins that interfere with transcription factors or that act as transcription factors of defence genes of the host.
The first is the case for an effector called SAP11, produced by phytoplasmas vectored by leafhoppers. The second is the case for the transcription activator-like TAL effectors produced by the Xanthomonas bacterium. Secreted proteins that interfere with host receptors involved in defences. For example, the P. Secreted proteins that interfere with defence signalling cascades downstream of receptor recognition. This is the case with AvrB of P. Secreted proteins that manipulate proteasome functioning in defensive processes.
Secreted proteins that perform proteolysis of plant defence proteins. Secreted proteins that interfere with host vesicle trafficking during immune responses. This is the case for HopM1 of P. Secreted proteins that interfere with RNAi, such as the 2b protein of the cucumber mosaic virus. Effectors may be powerful weapons whereby pathogens and nematodes interfere with plant defences, but plants have evolved a range of R genes in return, which specifically serve to recognize effectors and bypass suppression Bergelson et al. These molecular sensors are used in plant breeding to obtain pathogen-resistant crops but R gene resistance is often broken again shortly after its introduction due to counter-adaptations in pathogens Bent and Mackey, Hence, plant breeders have redirected their focus to the effector targets, also referred to as susceptibility genes or S genes van Schie and Takken, , since effector-resistant breeding targets are promising tools for obtaining more durable resistance Gawehns et al.
There are indications that plant-defence-suppressing herbivorous arthropods also secrete effectors via their saliva into their host, similar to pathogens and nematodes. The first of such salivary components that was discovered was the enzyme GOX, which is the most abundant molecule in the oral secretions of the caterpillar of H.
This enzyme catalyses the oxidation of glucose to d -gluconic acid and thereby generates hydrogen peroxide. The amount of GOX applied to T. GOX has been found in many more caterpillar species Eichenseer et al. A comprehensive study by Eichenseer et al. Moreover, a recent report showed that H. Taken together, these studies suggest that some plants, such as tomato, may have evolved a recognition mechanism for GOX, resembling R-gene-mediated recognition of effector proteins in plant—pathogen interactions. Advances in genomics and proteomics have greatly facilitated the discovery of more effector proteins in insects.
After the Acyrthosiphon pisum peach aphid salivary glands were sequenced, the first aphid effector was discovered. This protein is a kDa salivary-secreted protein of unknown function called C Mutti et al. RNAi-mediated knockdown of C expression affected A. Transient overexpression of a second aphid protein, Mp10, sufficed to suppress the flagellin-triggered oxidative burst in N. Interestingly, the performance of M. Finally, two putative effectors of Macrosiphum euphorbiae were found, Me10 and Me23, both of which increased aphid fecundity on N.
Research on gall midges has provided independent evidence for a role of effector proteins in plant—herbivore interactions. Early larval stages of the hessian fly, M. When they colonize wheat Triticum spp. More than 30 hessian fly resistance genes have been found in wheat, some of which are predicted to encode typical R proteins Liu et al. On resistant wheat, hessian fly larvae are unable to induce feeding cells, but instead induce a hypersensitive-like response that prevents them from eating Harris et al. In contrast, larvae from populations that are virulent on H13 wheat did not express vH13 , while RNAi-mediated knockdown of vH13 in avirulent larvae made some of them virulent Aggarwal et al.
These data suggest that vH13 may function as an effector on non-resistant wheat varieties. Thus, there are indications that herbivores may make use of effectors, just as pathogens do. This notion is strengthened by the existence of anti-herbivore R genes such as Mi-1 , Vat and Bph14 Rossi et al. The high diversity found among pathogen effectors discourages the use of protein homology as a strategy to identify herbivore effectors Rep, Nevertheless, most effector proteins share structural features that can be easily recognized, such as an amino-terminal signal peptide, the absence of transmembrane domains and a small protein size.
Furthermore, effectors that operate in the plant apoplastic space are usually rich in cysteine residues Rooney et al. Several studies have exploited these common properties to find novel effector-encoding genes from sequenced pathogen genomes or transcriptomes. Plants are often attacked by a diverse community of enemies, including herbivores and pathogens. Interspecific competition within phytophagous communities can be direct e. Because induction and suppression of defences manifest themselves in distal tissues Kant et al.
These interactions may lead to decreased performance, but also to facilitation: Responses induced by one particular species can result in resistance to another Long et al. Hence, inducible plant defences can be major determinants of ecological interactions; in particular, defences depending on JA and SA appear to play important roles in determining community composition. For example, JA-deficient wild tobacco plants in the field were colonized by herbivores that normally ignore these plants Kessler et al.
However, treating tomato plants with JA increased the growth of the pathogen P. Moreover, when JA and SA responses were induced simultaneously the performance of the cabbage looper caterpillar T. In addition to these field studies, experiments have also been carried out in the laboratory with model organisms to reveal the mechanisms that underlie indirect interactions. In contrast, it did reduce disease symptoms caused by P.
It also reduced infectiousness of the biotroph turnip crinkle virus due to an ethylene-primed SA response de Vos et al. In addition Thaler et al. Conversely, infection with tobacco mosaic virus induced only an SA response, causing induced susceptibility to S. Herbivores feeding on different plant organs can also affect each other through the induction of plant responses Soler et al. Similarly, Soler et al. Systemic signals that have been proposed to be involved in above—belowground interactions include the phytohormones JA, ABA, ethylene, auxin and cytokinin, but for none of these has a clear role been unequivocally established.
This suggests that the simultaneous induction of different defences may affect community members differently. Not only induction, but also suppression of defences can affect interactions between herbivores and pathogens. In principle, the benefits of suppression by a single herbivore species can be shared by other species in the community.
Within the spider mite species T. Some spider mites are very sensitive to the JA defences they induce in tomato plants. However, when these mites share their feeding site with other types that can suppress JA defences, their reproductive performance increases dramatically Alba et al. In addition, suppressor mites of the species T etranychus evansi as well as non-suppressor mites of the species T. In turn, some parasitoids have adapted to locate hosts using SA-induced volatiles, the emission of which is not suppressed by whiteflies Zhang et al.
Finally, field-grown tomato plants were frequently infested with the two-spotted spider mite T. Spider mites had much higher reproductive performance on plants infested with russet mites, an effect that was not due to the russet mite-suppressed JA response but to the antagonistic effect of the doubled SA response induced by both species.
However, this same SA response inhibited infection by P. Hence, the overall effect of selective suppression one type of defence is determined by the presence of competing plant parasites and the distinct palette of defences these induce Glas et al. During recent decades, the study of interactions between plants and other organisms has developed from the investigation of relatively simple interactions between plants and herbivores to that of more complex multitrophic interactions and their importance for the structure of communities of plants and arthropods.
In nature, plants are part of complex food webs in which the sum of direct and indirect interactions between organisms, either allies or enemies, determines the selection pressures on plants. Plant defences against herbivores play an important role in these interactions Hairston et al.
Plant defences comprise not only traits that interfere with herbivores directly Walling, , but also traits that operate indirectly by facilitating foraging predators and host-seeking parasitoids. These indirect plant defences often rely on the release of volatile compounds into the atmosphere, which can act as cues for prey-searching natural enemies Dicke et al. Plant toxins ingested by herbivores may interfere with their natural enemies. Hence plant defence may actually provide a herbivore with additional protection. While sequestration is adaptive and entails storage of ingested plant toxins in specialized tissues or eggs Duffey, ; Tooker and De Moraes, , in principle any plant substance present anywhere in a herbivore can serve as antipredator protection.
For example, tomatine incorporated in the diet of H. Nicotine in the diet of M. Alternatively, sometimes predators may themselves evolve resistance to plant toxins such as nicotine Kumar et al. In addition, glucosinolates were found to affect the performance of the second and the third trophic level Poelman et al.
Furthermore, Kauffman and Kennedy found that a high concentration of the methyl ketone 2-tridecanone in a particular accession of L ycopersicon hirsutum was toxic to the herbivore H. Moreover, the stinkbug Podisus maculiventris reared on M. This shows that not only growth, development and mortality but also the behaviour of natural enemies can be modulated by plant toxins ingested by their prey. There are also indications that JA-dependent plant metabolites can constrain herbivore predation. Thaler showed that treating tomato with JA increased parasitism of S.
In addition, Kaplan and Thaler observed lower predation of the caterpillar of M. However, manual application of JA forces the constitutive display of defences that are normally induced, and this may result in overestimation of the magnitude of the predator response. Another layer of complexity is added when communities harbour one or more herbivore species that can suppress plant defences Alba et al. Suppression of defences by one species can facilitate another competing species Sarmento et al. For instance, suppression of defences by specialized strains of the two-spotted spider mite T.
Hence, suppression of plant defence can indirectly mediate competition between herbivores, forcing the suppressor to adopt strategies to reduce competition with opportunistic herbivores. For instance, the spider mite T. Suppression of plant defences also reduces the emission of induced plant volatiles Rodriguez-Saona et al.
Moreover, suppression of plant defences by spider mites can potentially backfire in the presence of predatory mites, because these prefer the eggs of prey derived from plants with suppressed JA defences to those from plants with activated JA defences L. Ataide, University of Amsterdam, Netherlands, unpubl. Like suppression, induction also affects the performance of competitors.
When plants are simultaneously attacked by more than one herbivore species, the palette of plant defences these induce together will determine their mutual interactions and those with their natural enemies. For example, cutting and trenching prevent the plant from transporting photosynthates away from, and defence compounds towards, the tissue where the herbivore is feeding Dussourd and Denno, ; Delaney and Higley, ; Oppel et al.
This allows the herbivore to exploit a nutrient-rich part of the plant without having to deal with elevated plant defences. However, if the isolated tissue is large enough, other herbivores can profit from this resource as well, without having to invest in cutting a vein or digging a trench. Similarly, density-dependent feeding efficiency is also observed with gregarious feeding Prokopy and Roitberg, For example, gregarious aphids can create sinks in plant tissue, which are preferentially supplied with nutrients by the plant compared with parts where individual aphids feed Larson and Whitham, Also, the adult mass of froghoppers, a predictor of fecundity, peaks at intermediate juvenile group size Wise et al.
In this way, gregarious feeding provides clear benefits because multiple herbivores can feed more efficiently than single herbivores. Interactions with the third trophic level may also shift when multiple herbivore species attack a plant simultaneously. For instance, when cabbage plants are simultaneously attacked by more than one herbivore species, the blend of volatiles released from these plants is different from that released by a singly infested plant.
This new blend is less attractive to natural enemies of one of the herbivores and consequently adults of these herbivores preferred to oviposit on cabbage plants previously attacked by the other herbivore species, thus reducing the risk of parasitism of their offspring Shiojiri et al. Finally, induced plant defences can also mediate indirect intra- or interspecific interactions among plants, and this possibly arises from competition between plants for enemy-free space.
It has long been known that non-attacked plants close to attacked plants can be warned of imminent attacks through the volatile cues released by the neighbours Baldwin and Schultz, ; Bruin et al. These volatiles prime the defences of neighbouring plants such that herbivores attacking these plants will trigger the induction of defences more rapidly and usually more strongly than in non-primed plants Engelberth et al. The examples mentioned above illustrate the importance of incorporating the indirect interactions between plants, herbivores and natural enemies in the study of the evolution of plant defences.
Besides the interaction between plants and their herbivores, many other organisms can benefit or suffer, either directly or indirectly, from the defensive products induced by herbivores. The net effects of plant defences on plant fitness thus depend not only on the interaction of the plant with the inducing herbivore, but also on effects on plant fitness through the interaction web associated with the plant. The true fitness effects of plant defences can therefore only be evaluated in the natural environment in which the various interacting species have evolved.
Defence suppression is perhaps the most striking example of a herbivore strategy that affects the performance of other herbivores living on the same host plant. There are indications that defence suppression is not always restricted to the site of the suppressor, and may act systemically throughout leaflets Kant et al. For example, the performance of a non-suppressing strain of the spider mite T.
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