On the elusiveness of enemy-free space: spatial, temporal, and host-plant-related variation in parasitoid attack rates on three gallmakers of goldenrods

Host shifting by phytophagous insects may play an important role in generating insect diversity by initiating host-race formation and speciation. Models of the host shifting process often invoke reduced rates of natural enemy attack on a novel host in order to balance the maladaptation expected following the shift. Such "enemy-free space" has been documented for some insects, at some times and places, but few studies have assessed the occurrence of enemy-free space across years, among sites, or among insect species. We measured parasitoid attack rates on three insect herbivores of two goldenrods (Solidago altissima L. and Solidago gigantea Ait.), with data from multiple sites and multiple years for each herbivore. For each insect herbivore, there were times and sites at which parasitoid attack rates differed strongly and significantly between host plants (that is, enemy-free space existed on one host plant or the other). However, the extent and even the direction of the attack-rate difference varied strongly among sites and even among years at the same site. There was no evidence of consistent enemy-free space for any herbivore on either host plant. Our data suggest that enemy-free space, like many ecological and evolutionary forces, is likely to operate as a geographic and temporal mosaic, and that conceptual models of host shifting that include enemy-free space as a consequence of host novelty are likely too simple.     

互利关系和植物多样性对节肢动物群落不同营养级的影响路径和强度

食物网中的上行效应和下行效应对于群落的动态和生态系统功能有十分重要的影响,旨在探讨互利关系和植物多样性对节肢动物群落中食物网不同营养级之间的影响。通过随机裂区试验方法,分别设置了3种蚂蚁-紫胶虫互利关系处理(有互利关系、无互利关系和自然对照)以及3种植物多样性处理(单一种植、2树种混植和3树种混植),于2016年8月和9月分两次用手捡法、网扫法和震落法采集试验地寄主植物上所有的节肢动物,并按照不同营养级将其分类。利用结构方程模型分析方法对不同营养级之间的相互作用的路径和强度进行了比较,结果显示:1)互利关系对捕食者和消费者均有显著的下行作用,有互利关系处理下蚂蚁对捕食者的路径强度要强于自然对照组,互利关系对捕食者的影响要强于对消费者的影响。2)植物多样性会通过影响植物的生物量而对消费者和捕食者产生显著的上行效应影响,这种影响会随着营养级的升高而显著减小。3)消费者主要受植物多样性的上行效应影响,而捕食者主要受互利关系的下行效应影响。有互利关系的食物网结构更加复杂,营养级之间的相互作用更为显著。探讨了以蚂蚁-紫胶虫互利关系为核心作用的紫胶林生态系统中互利关系和植物多样性对节肢动物食物网中各营养级的影响,揭示了上行效应和下行效应对各营养级的作用途径和强度,其结果有一定的理论参考价值。     

The tri‐trophic niche concept and adaptive radiation of phytophagous insects

A conceptual divide exists between ecological and evolutionary approaches to understanding adaptive radiation, although the phenomenon is inherently both ecological and evolutionary. This divide is evident in studies of phytophagous insects, a highly diverse group that has been frequently investigated with the implicit or explicit goal of understanding its diversity. Whereas ecological studies of phytophagous insects increasingly recognize the importance of tri‐trophic interactions as determinants of niche dimensions such as host‐plant associations, evolutionary studies typically neglect the third trophic level. Here we attempt to reconcile ecological and evolutionary approaches through the concept of the ecological niche. We specifically present a tri‐trophic niche concept as a foil to the traditional bi‐trophic niche concept for phytophagous insects. We argue that these niche concepts have different implications for understanding herbivore community structure, population divergence, and evolutionary diversification. To this end, we offer contrasting empirical predictions of bi‐ and tri‐trophic niche concepts for patterns of community structure, the process of population divergence, and patterns of evolutionary diversification of phytophagous insects.     

Diversity cascades in alfalfa fields: from plant quality to agroecosystem diversity

To examine top-down and bottom-up influences on managed terrestrial communities, we manipulated plant resources and arthropod abundance in alfalfa (Medicago sativa L.) fields. We modified arthropod communities using three nonfactorial manipulations: pitfall traps to remove selected arthropods, wooden crates to create habitat heterogeneity, and an arthropod removal treatment using a reversible leaf blower. These manipulations were crossed with fertilizer additions, which were applied to half of the plots. We found strong effects of fertilizer on plant quality and biomass, and these effects cascaded up to increase herbivore abundance and diversity. The predator community also exhibited a consistent positive effect on the maintenance of herbivore species richness and abundance. These top-down changes in arthropods did not cascade down to affect plant biomass; however, plant quality (saponin content) increased with higher herbivore densities. These results corroborate previous studies in alfalfa that show complex indirect effects, such as trophic cascades, can operate in agricultural systems, but the specifics of the interactions depend on the assemblages of arthropods involved.     

Trophic cascades in tropical rainforests: Effects of vertebrate predator exclusion on arthropods and plants in Papua New Guinea

Insect herbivores have the potential to consume large amounts of plant tissue in tropical forests, but insectivorous vertebrates effectively control their abundances, indirectly increasing plant fitness accordingly. Despite several studies already sought understanding of the top-down effects on arthropod community structure and herbivory, such studies of trophic cascades in old tropics are underrepresented, and little attention was paid to top-down forces in various habitats. Therefore, we examine how flying insectivorous vertebrates (birds and bats) impact arthropods and, consequently, affect herbivore damage of leaves in forest habitats in Papua New Guinea. In a 3-month long predator exclosure experiment conducted at four study sites across varying elevation and successional stage, we found that vertebrate predators reduced arthropod density by ∼52%. In addition, vertebrate predators decreased the mean body size of arthropods by 26% in leaf chewers and 47% in non-herbivorous arthropods but had only a small effect on mesopredators and sap suckers. Overall, the exclusion of vertebrate predators resulted in a ~ 41% increase in leaf damage. Our results, across different types of tropical forests in Papua New Guinea, demonstrate that flying vertebrate insectivores have a crucial impact on plant biomass, create a selective pressure on larger and non-predatory prey individuals and they prey partition with mesopredators.     

Behavioral adaptations increase the value of enemy-free space for Heliothis subflexa,a specialist herbivore

We investigated the importance of specialized behaviors in the use of enemy-free space by comparing the host-use behavior of two closely related moths, Heliothis subflexa Guenee and H. virescens Fabricius. Heliothis subflexa is a specialist on plants in the genus Physalis, whereas H. virescens is an extreme generalist, feeding on plants in at least 14 families. Heliothis subflexa uses the inflated calyx surrounding Physalis fruits as enemy-free space, and field rates of parasitism for H. subflexa on Physalis are much lower than for H. virescens on tobacco and cotton, common hosts found in the same habitat as Physalis. If Physalis, architecture were solely responsible for H. subflexa's low rates of parasitism on Physalis, we predicted that H. virescens larvae experimentally induced to feed on Physalis would experience parasitism rates similar to those of H. subflexa. We found, however, that specialized host-use and host-acceptance behaviors are integral to the use of enemy-free space on Physalis and strongly augment the effects of the structural refuge. In laboratory assays, we found considerable differences between the larval behavior of the specialist. H. subflexa, and the generalist, H. virescens, and these contributed to H. subflexa's superior use of enemy-free space on Physalis. We tested the importance of these behavioral differences in the field by comparing parasitism of H. virescens on Physalis, H. virescens on tobacco, and H. subflexa on Physalis by Cardiochiles nigriceps Vierick, a specialist braconid parasitoid. For H. virescens, a threefold decrease in parasitism occurred when feeding on Physalis (mean parasitism +/- SEM = 13 +/- 4%) rather than tobacco (43 +/- 4%), a difference we attribute to the structural refuge provided by Physalis. However, parasitism of H. virescens on Physalis was more than ten times as great as that of H. subflexa on Pliv.salis (1 +/- 4%), supporting the hypothesis that specialized behaviors have a substantial impact on use of Physalis as enemy-free space. Behavioral adaptations may be central to the use of enemy-free space by phytophagous insects and may act as an important selective force in the evolution of dietary specialization.     

Ecosystem alteration modifies the relative strengths of bottom-up and top-down forces in a herbivore population

1. Ecosystem alterations can affect the abundance, distribution and diversity of plants and animals, and thus potentially change the relative strength of bottom-up (the plant resource) and top-down (natural enemies) trophic forces acting on herbivore populations. 2. The hypothesis that alterations of the forest ecosystem associated with precommercial thinning have contributed to the increased severity of outbreaks of Neodiprion abietis (Harris), a sawfly defoliator, through the reduction of trophic forces acting on N. abietis larvae, was tested using exclusion techniques. 3. The relative contributions to N. abietis larval mortality of bottom-up and top-down forces both increased with increasing levels of defoliation and were both reduced by thinning. The reduction of bottom-up and top-down forces caused a 58% mean increase in N. abietis larval survival in thinned compared with untreated stands, which is less than would be expected by the sum of the effects of thinning on each source of mortality. Evidence indicates that the partly compensatory, partly additive nature of the mortality associated with trophic forces in the system under study is responsible for this discrepancy. 4. To our knowledge, this is the first study to show the impact of ecosystem alterations on the balance between bottom-up and top-down forces acting on an eruptive herbivore population along a gradient of host-plant defoliation, and how this can lead to increased outbreak severity. It is stressed that accurate estimates of the relative contributions of bottom-up and top-down forces to mortality cannot be obtained if the additive or compensatory nature of the mortality associated with these trophic forces is overlooked.     

Effects of nutrients and parasitism on the density of a salt marsh planthopper suppressed by within-trophic-level interactions

Abstract.  1. Competition, food quality, and pressure from natural enemies have all been established as important determinants of phytophagous insect densities. Examining how these forces interact, however, is important for furthering our understanding of insect population and community ecology.
2. In this paper the results of a factorial field experiment are reported in which the effects of within-trophic-level interactions on the top-down and bottom-up forces influencing the density of a salt marsh planthopper were examined.
3. Plant quality was increased by adding nitrogen fertiliser, and parasitism was decreased through the use of yellow sticky traps that removed hymenopteran parasitoids. These treatments were applied to plots with either low or high densities of a lepidopteran stem borer, which has been shown to negatively affect the planthopper.
4. Addition of nitrogen increased planthopper density significantly, but only on plots with low stem borer densities. Yellow sticky traps significantly reduced parasitism of planthopper eggs, but this resulted in increased planthopper density only on plots with low stem borer density. Thus, where stem borer densities were high, the effects of fertiliser and reduced parasitism on planthopper densities were suppressed.
5. The results of this study show that negative interactions between herbivores can affect vertical trophic linkages, and can greatly modify the strengths of top-down or bottom-up effects on herbivore density.     

Relative strengths of top-down and bottom-up forces in a tropical forest community

We tested integrative bottom-up and top-down trophic cascade hypotheses with manipulative experiments in a tropical wet forest, using the ant-plant Piper cenocladum and its associated arthropod community. We examined enhanced nutrients and light along with predator and herbivore exclusions as sources of variation in the relative biomass of plants, their herbivores (via rates of herbivory), and resident predaceous ants. The combined manipulations of secondary consumers, primary consumers, and plant resources allowed us to examine some of the direct and indirect effects on each trophic level and to determine the relative contributions of bottom-up and top-down cascades to the structure of the community. We found that enhanced plant resources (nutrients and light) had direct positive effects on plant biomass. However, we found no evidence of indirect (cascading through the herbivores) effects of plant biomass on predators or top predators. In contrast, ants had indirect effects on plant biomass by decreasing herbivory on the plants. This top-down cascade occurred whether or not plant resources were enriched, conditions which are expected to modify top-down forces. Received: 9 August 1998 / Accepted: 1 December 1998     

Nonadditive interactions between trophic levels bias the appraisal of the strength of mortality factors

The role of bottom-up and top-down factors in determining the abundance of organisms is still often evaluated in terms of the nominal (i.e., apparent) cause of death. To determine whether estimates of mortality can be influenced by nonadditive multitrophic interactions, we generated both marginal (i.e., the entirety of mortality attributable to a given factor, including its obscured fraction) and nominal estimates of the trophic forces acting on an herbivore. This was accomplished by comparing mortality rates of the willow leaf beetle Phratora vulgatissima on willows affected or relatively unaffected by local growing conditions, in the presence or absence of natural enemies. Marginal estimates indicated that bottom-up and top-down mortality factors interacted in a synergistic or compensatory way, with the nature of the interaction varying in response to the time elapsed since the last harvest of willow plantations. In contrast, nominal estimates could not discriminate synergistic and compensatory interactions from top-down influence. Combined with recent evidence of compensatory mortality between trophic forces in another system, this study suggests that nonadditive effects between bottom-up and top-down mortality factors may be common, offering an explanation through which the contrasting evidences previously presented by proponents of the bottom-up and top-down views can be understood.     

Control of arthropod abundance, richness, and composition in a heterogeneous desert city

There is a demand for mechanistic studies to explore underlying drivers behind observed patterns of biodiversity in urban areas. We describe a two-year field experiment in which we manipulated bottom-up (resource availability) and top-down (bird predation) forces on arthropod communities associated with a native plant, Encelia farinosa, across three land-use types—urban, desert remnant, and outlying natural desert—in the Phoenix metropolitan area, Arizona, USA. We monitored the trophic structure, richness, and similarity of the arthropod communities on these manipulated plants over a two-year period. We predicted that (1) increased water resources increase plant productivity, (2) increased productivity increases arthropod abundances, and (3) in the urban habitat, top-down forces are greater than in other habitats and limit arthropod abundances. We also predicted that urban remnant habitats are more similar to urban habitats in terms of arthropod richness and composition. Strong interannual differences due to an unusual cold and dry winter in the first year suppressed plant growth in all but urban habitats, and arthropod abundances in all habitats were severely reduced. In the following year, arthropod abundances in desert and remnant habitats were higher than in urban habitats. Water had positive effects on plant growth and arthropod abundance, but these water effects emerged through complex interactions with habitat type and the presence/absence of cages used to reduce bird predation. Plants grew larger in urban habitats, and phenology also differed between urban and desert habitats. The results from caging suggest that bird predation may not be as important in cities as previously thought, and that arthropods may retard plant growth. As expected, desert communities are strongly bottom-up regulated, but contrary to predictions, we did not find evidence for strong top-down control in the city. Remnant habitats were intermediate between desert and urban habitats in terms of diversity, richness, evenness, arthropod composition and phenology, with urban habitats generally lowest in terms of diversity, richness, and evenness. Our study shows that control of biodiversity is strongly altered in urban areas, influenced by subtle shifts in top-down and bottom-up controls that are often superseded by climatic variations and habitat type.     

Leafhopper-induced plant resistance enhances predation risk in a phytophagous beetle

Many herbivores elicit biochemical, physiological, or morphological changes in their host plants that render them more resistant to co-occurring herbivores. Yet, despite the large number of studies that investigate how induced resistance affects herbivore preference and performance, very few have simultaneously explored the cascading effects of induction on higher trophic levels and consequences for prey suppression. In our study system, early-season herbivory by leafhoppers elevated plant resistance to subsequent attack by chrysomelid beetles sharing the same host plant. Notably, beetles feeding on leafhopper-damaged plants incurred developmental penalties (e.g., prolonged time in early larval instars) that rendered them more susceptible to predation by natural enemies. As a result, the combined bottom-up effect of leafhopper-induced resistance and the top-down effect of enhanced predation resulted in the synergistic suppression of beetle populations. These results emphasize that higher trophic level dynamics should be considered in conjunction with induced resistance to better understand how plants mediate interspecific interactions in phytophagous insect communities.     

Relative importance of top-down and bottom-up forces in food webs of Sarracenia pitcher communities at a northern and a southern site

The relative importance of resources (bottom-up forces) and natural enemies (top-down forces) for regulating food web dynamics has been debated, and both forces have been found to be critical for determining food web structure. How the relative importance of top-down and bottom-up forces varies between sites with different abiotic conditions is not well understood. Using the pitcher plant inquiline community as a model system, I examine how the relative importance of top-down and bottom-up effects differs between two disparate sites. Resources (ant carcasses) and top predators (mosquito larvae) were manipulated in two identical 4 × 4 factorial press experiments, conducted at two geographically distant sites (Michigan and Florida) within the range of the purple pitcher plant, Sarracenia purpurea, and the aquatic community that resides in its leaves. Overall, top predators reduced the density of prey populations while additional resources bolstered them, and the relative importance of top-down and bottom-up forces varied between sites and for different trophic levels. Specifically, top-down effects on protozoa were stronger in Florida than in Michigan, while the opposite pattern was found for rotifers. These findings experimentally demonstrate that the strength of predator–prey interactions, even those involving the same species, vary across space. While only two sites are compared in this study, I hypothesize that site differences in temperature, which influences metabolic rate, may be responsible for variation in consumer–resource interactions. These findings warrant further investigation into the specific factors that modify the relative importance of top-down and bottom-up effects.     

Social predation by ants as a mortality source for an arboreal gregarious forest pest

Biocontrol of caterpillars by ants is highly variable, and we investigate how the strength of the trophic relationship between ants and an important outbreaking forest pest depends on phenological synchrony and on social foraging. We test the hypothesis that early spring foraging by ants, coupled with eusocial recruitment behavior, could undermine the caterpillar's strategies to achieve either enemy-free space or predator satiation.We use a series of field surveys and experiments in trembling aspen stands (Populus tremuloides) in the boreal forest of eastern Canada to assess the role of ants in early-instar mortality of the outbreaking, gregarious forest tent caterpillar (Malacosoma disstria). We also investigate individual-level mechanisms related to phenology and social behavior that underlie the effectiveness of ants as biocontrol on caterpillars. Our results show that ants climb trees early in the spring and harvest young forest tent caterpillars, suggesting that early phenology does not provide an entirely enemy-free space for caterpillars. Our findings further show that recruitment-based social foraging enables ants to deplete groups of gregarious prey, suggesting that these eusocial insects are particularly effective at generating predation pressure on gregarious herbivores since they do not satiate easily. Finally, a manipulative predator exclusion experiment confirms that ant predation is a significant mortality source for early-instar forest tent caterpillars. Taken together, these results suggest that phenology and sociality could modulate the role of ants as effective caterpillar predators and thus showcase the importance of considering natural history and behavioral traits when studying trophic interactions and their role in population dynamics.     

Detection of refuge from enemies through phenological mismatching in multitrophic interactions requires season-wide estimation of host abundance

The concept of “enemy-free space” (EFS) refers to ways of living that reduce or eliminate the vulnerability of a species to natural enemies. It has been invoked to explain host shifts of phytophagous insects. A demonstrated cause of EFS is escape from enemies in time, through phenological mismatching of herbivore development and enemy occurrence, leading to low percentages of predation/parasitism of herbivores occurring at a certain time. The mere measurement of percentage parasitism, however, is not sufficient to demonstrate EFS in certain cases. Here we present such a case, where parasitism was studied of a phytophagous insect (Phyllotreta nemorum), using two different host plant species in the field: an atypical, relatively rarely used, plant (Barbarea vulgaris), and a more widely used one (Sinapis arvensis). At one location we found a paradoxical result: on each separate sampling day throughout the season the percentage of parasitism of P. nemorum using a patch of B. vulgaris was not significantly different from, or even significantly higher than on a nearby patch of S. arvensis. The overall season-wide proportion parasitism of the flea beetle cohort using the B. vulgaris patch, however, was lower. We conclude that, in the year and at the location we studied, the patch of B. vulgaris provided enemy-free space to the herbivore in the form of a temporal refuge, and that the importance of enemy-free space in the use of an atypical host plant should be evaluated on the basis of season-wide sampling, including estimation of host population size.     

Phytophagous insect-microbe mutualisms and adaptive evolutionary diversification.

Adaptive diversification is a process intrinsically tied to species interactions. Yet, the influence of most types of interspecific interactions on adaptive evolutionary diversification remains poorly understood. In particular, the role of mutualistic interactions in shaping adaptive radiations has been largely unexplored, despite the ubiquity of mutualisms and increasing evidence of their ecological and evolutionary importance. Our aim here is to encourage empirical inquiry into the relationship between mutualism and evolutionary diversification, using herbivorous insects and their microbial mutualists as exemplars. Phytophagous insects have long been used to test theories of evolutionary diversification; moreover, the diversification of a number of phytophagous insect lineages has been linked to mutualisms with microbes. In this perspective, we examine microbial mutualist mediation of ecological opportunity and ecologically based divergent natural selection for their insect hosts. We also explore the conditions and mechanisms by which microbial mutualists may either facilitate or impede adaptive evolutionary diversification. These include effects on the availability of novel host plants or adaptive zones, modifying host-associated fitness trade-offs during host shifts, creating or reducing enemy-free space, and, overall, shaping the evolution of ecological (host plant) specialization. Although the conceptual framework presented here is built on phytophagous insect-microbe mutualisms, many of the processes and predictions are broadly applicable to other mutualisms in which host ecology is altered by mutualistic interactions.     

PHYTOPHAGOUS INSECT–MICROBE MUTUALISMS AND ADAPTIVE EVOLUTIONARY DIVERSIFICATION

Adaptive diversification is a process intrinsically tied to species interactions. Yet, the influence of most types of interspecific interactions on adaptive evolutionary diversification remains poorly understood. In particular, the role of mutualistic interactions in shaping adaptive radiations has been largely unexplored, despite the ubiquity of mutualisms and increasing evidence of their ecological and evolutionary importance. Our aim here is to encourage empirical inquiry into the relationship between mutualism and evolutionary diversification, using herbivorous insects and their microbial mutualists as exemplars. Phytophagous insects have long been used to test theories of evolutionary diversification; moreover, the diversification of a number of phytophagous insect lineages has been linked to mutualisms with microbes. In this perspective, we examine microbial mutualist mediation of ecological opportunity and ecologically based divergent natural selection for their insect hosts. We also explore the conditions and mechanisms by which microbial mutualists may either facilitate or impede adaptive evolutionary diversification. These include effects on the availability of novel host plants or adaptive zones, modifying host-associated fitness trade-offs during host shifts, creating or reducing enemy-free space, and, overall, shaping the evolution of ecological (host plant) specialization. Although the conceptual framework presented here is built on phytophagous insect–microbe mutualisms, many of the processes and predictions are broadly applicable to other mutualisms in which host ecology is altered by mutualistic interactions.     

Trophic Cascades in Terrestrial Systems: A Review of the Effects of Carnivore Removals on Plants

We present a quantitative synthesis of trophic cascades in terrestrial systems using data from 41 studies, reporting 60 independent tests. The studies covered a wide range of taxa in various terrestrial systems with varying degrees of species diversity. We quantified the average magnitude of direct effects of carnivores on herbivore prey and indirect effects of carnivores on plants. We examined how the effect magnitudes varied with type of carnivores in the study system, food web diversity, and experimental protocol. A metaanalysis of the data revealed that trophic cascades were common among the studies. Exceptions to this general trend did arise. In some cases, trophic cascades were expected not to occur, and they did not. In other cases, the direct effects of carnivores on herbivores were stronger than the indirect effects of carnivores on plants, indicating that top-down effects attenuated. Top-down effects usually attenuated whenever plants contained antiherbivore defenses or when herbivore species diversity was high. Conclusions about the strength of top-down effects of carnivores varied with the type of carnivore and with the plant-response variable measured. Vertebrate carnivores generally had stronger effects than invertebrate carnivores. Carnivores, in general, had stronger effects when the response was measured as plant damage rather than as plant biomass or plant reproductive output. We caution, therefore, that conclusions about the strength of top-down effects could be an artifact of the plant-response variable measured. We also found that mesocosm experiments generally had weaker effect magnitudes than open-plot field experiments or observational experiments. Trophic cascades in terrestrial systems, although not a universal phenomenon, are a consistent response throughout the published studies reviewed here. Our analysis thus suggests that they occur more frequently in terrestrial systems than currently believed. Moreover, the mechanisms and strengths of top-down effects of carnivores are equivalent to those found in other types of systems (e.g., aquatic environments).     

Habitat structure alters top-down control in litter communities

The question whether top-down or bottom-up forces dominate trophic relationships, energy flow, and abundances within food webs has fuelled much ecological research with particular focus on soil litter ecosystems. Because litter simultaneously provides habitat structure and a basal resource, disentangling direct trophic and indirect non-trophic effects on different trophic levels remains challenging. Here, we focussed on short-term per capita interaction strengths of generalist predators (centipedes) on their microbi-detritivore prey (springtails) and addressed how the habitat structuring effects of the leaf litter modifies this interaction. We performed a series of laboratory functional response experiments where four levels of habitat structure were constructed by adding different amounts of leaf litter to the experimental arenas. We found that increased leaf litter reduced the consumption rate of the predator. We interpreted this as a dilution effect of the augmented habitat size provided by the increasing leaf litter surface available to the species. Dilution of the prey population decreased encounter rates, whereas the capture success was not affected. Interestingly, our results imply that top-down control by centipedes decreased with increasing resource supply for the microbi-detritivore prey (i.e. the leaf litter that simultaneously provides habitat structure). Therefore, effective top-down control of predators on microbi-detritvore populations seems unlikely in litter-rich ecosystems due to the non-trophic, habitat-structuring effect of the basal litter resource.     

Seasonal variation in top-down and bottom-up processes in a grassland arthropod community

Both top-down and bottom-up processes are common in terrestrial ecosystems, but how these opposing forces interact and vary over time is poorly understood. We tested the variation of these processes over seasonal time in a natural temperate zone grassland, a field site characterized by strong seasonal changes in abiotic and biotic conditions. Separate factorial experiments manipulating nutrients and cursorial spiders were performed in the wet and dry seasons. We also performed a water-addition experiment during the summer (dry season) to determine the degree of water limitation during this time. In the spring, nutrient addition increased plant growth and carnivore abundance, indicating a bottom-up control process. Among herbivores, sap-feeders were significantly enhanced while grazers significantly declined resulting in no net change in herbivore abundance. In the summer, water limitation was predominant increasing plants and all herbivores while nutrient (N) effects were non-significant. Top-down processes were present only in the spring season and only impacted the guild of grazing herbivores. These results show that bottom-up limitation is present throughout the season in this grassland, although the specific limiting resource changes as the season progresses. Bottom-up processes affected all trophic levels and many different guilds, while top-down effects were limited to a select group of herbivores and did not extend to the plant trophic level. Our results show that the relative strengths of top-down and bottom-up processes can shift over relatively short periods of time in habitats with a strong seasonal component.