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

Variation in plant defences among populations of a range-expanding plant: consequences for trophic interactions

Although plant-herbivore-enemy interactions have been studied extensively in cross-continental plant invasions, little is known about intra-continental range expanders, despite their rapid spread globally. Using an ecological and metabolomics approach, we compared the insect performance of a generalist and specialist herbivore and a parasitoid, as well as plant defence traits, among native, exotic invasive and exotic non-invasive populations of the Turkish rocket, Bunias orientalis, a range-expanding species across parts of Eurasia. In the glasshouse, the generalist herbivore, Mamestra brassicae, and its parasitoid, Microplitis mediator, performed better on non-native than on native plant populations. Insect performance did not differ between the two non-native origins. By contrast, the specialist herbivore, Pieris brassicae, developed poorly on all populations. Differences in trichome densities and in the metabolome, particularly in the family-specific secondary metabolites (i.e. glucosinolates), may explain population-related variation in the performance of the generalist herbivore and its parasitoid. Total glucosinolate concentrations were significantly induced by herbivory, particularly in native populations. Native populations of B. orientalis are generally better defended than non-native populations. The role of insect herbivores and dietary specialization as a selection force on defence traits in the range-expanding B. orientalis is discussed.

Ecosystem engineering effects on species diversity across ecosystems: a meta-analysis

Ecosystem engineering is increasingly recognized as a relevant ecological driver of diversity and community composition. Although engineering impacts on the biota can vary from negative to positive, and from trivial to enormous, patterns and causes of variation in the magnitude of engineering effects across ecosystems and engineer types remain largely unknown. To elucidate the above patterns, we conducted a meta-analysis of 122 studies which explored effects of animal ecosystem engineers on species richness of other organisms in the community. The analysis revealed that the overall effect of ecosystem engineers on diversity is positive and corresponds to a 25% increase in species richness, indicating that ecosystem engineering is a facilitative process globally. Engineering effects were stronger in the tropics than at higher latitudes, likely because new or modified habitats provided by engineers in the tropics may help minimize competition and predation pressures on resident species. Within aquatic environments, engineering impacts were stronger in marine ecosystems (rocky shores) than in streams. In terrestrial ecosystems, engineers displayed stronger positive effects in arid environments (e.g. deserts). Ecosystem engineers that create new habitats or microhabitats had stronger effects than those that modify habitats or cause bioturbation. Invertebrate engineers and those with lower engineering persistence (<1 year) affected species richness more than vertebrate engineers which persisted for >1 year. Invertebrate species richness was particularly responsive to engineering impacts. This study is the first attempt to build an integrative framework of engineering effects on species diversity; it highlights the importance of considering latitude, habitat, engineering functional group, taxon and persistence of their effects in future theoretical and empirical studies.

Cascading effects of insectivorous birds and bats in tropical coffee plantations

The loss of apex predators is known to have reverberating consequences for ecosystems, but how changes in broader predator assemblages affect vital ecosystem functions and services is largely unknown. Predators and their prey form complex interaction networks, in which predators consume not only herbivores but also other predators. Resolving these interactions will be essential for predicting changes in many important ecosystem functions, such as the control of damaging crop pests. Here, we examine how birds, bats, and arthropods interact to determine herbivorous arthropod abundance and leaf damage in Costa Rican coffee plantations. In an exclosure experiment, we found that birds and bats reduced non-flying arthropod abundance by -35% and -25%, respectively. In contrast, birds and bats increased the abundance of flying arthropods, probably by consuming spiders. The frequency of this intraguild predation differed between birds and bats, with cascading consequences for coffee shrubs. Excluding birds caused a greater increase in herbivorous arthropod abundance than excluding bats, leading to increased coffee leaf damage. Excluding bats caused an increase in spiders and other predatory arthropods, increasing the ratio of predators to herbivores in the arthropod community. Bats, therefore, did not provide benefits to coffee plants. Leaf damage on coffee was low, and probably did not affect coffee yields. Bird-mediated control of herbivores, however, may aid coffee shrubs in the long-term by preventing pest outbreaks. Regardless, our results demonstrate how complex, cascading interactions between predators and herbivores may impact plants and people.

Herbivore diet breadth mediates the cascading effects of carnivores in food webs

Predicting the impact of carnivores on plants has challenged community and food web ecologists for decades. At the same time, the role of predators in the evolution of herbivore dietary specialization has been an unresolved issue in evolutionary ecology. Here, we integrate these perspectives by testing the role of herbivore diet breadth as a predictor of top-down effects of avian predators on herbivores and plants in a forest food web. Using experimental bird exclosures to study a complex community of trees, caterpillars, and birds, we found a robust positive association between caterpillar diet breadth (phylodiversity of host plants used) and the strength of bird predation across 41 caterpillar and eight tree species. Dietary specialization was associated with increased enemy-free space for both camouflaged (n = 33) and warningly signaled (n = 8) caterpillar species. Furthermore, dietary specialization was associated with increased crypsis (camouflaged species only) and more stereotyped resting poses (camouflaged and warningly signaled species), but was unrelated to caterpillar body size. These dynamics in turn cascaded down to plants: a metaanalysis (n = 15 tree species) showed the beneficial effect of birds on trees (i.e., reduced leaf damage) decreased with the proportion of dietary specialist taxa composing a tree species' herbivore fauna. We conclude that herbivore diet breadth is a key functional trait underlying the trophic effects of carnivores on both herbivores and plants.

Lethal control of an apex predator has unintended cascading effects on forest mammal assemblages

Disruption to species-interaction networks caused by irruptions of herbivores and mesopredators following extirpation of apex predators is a global driver of ecosystem reorganization and biodiversity loss. Most studies of apex predators' ecological roles focus on effects arising from their interactions with herbivores or mesopredators in isolation, but rarely consider how the effects of herbivores and mesopredators interact. Here, we provide evidence that multiple cascade pathways induced by lethal control of an apex predator, the dingo, drive unintended shifts in forest ecosystem structure. We compared mammal assemblages and understorey structure at seven sites in southern Australia. Each site comprised an area where dingoes were poisoned and an area without control. The effects of dingo control on mammals scaled with body size. Activity of herbivorous macropods, arboreal mammals and a mesopredator, the red fox, were greater, but understorey vegetation sparser and abundances of small mammals lower, where dingoes were controlled. Structural equation modelling suggested that both predation by foxes and depletion of understorey vegetation by macropods were related to small mammal decline at poisoned sites. Our study suggests that apex predators' suppressive effects on herbivores and mesopredators occur simultaneously and should be considered in tandem in order to appreciate the extent of apex predators' indirect effects.

Dietary guild composition and disaggregation of avian assemblages under climate change

Climate change is expected to cause geographic redistributions of species. To the extent that species within assemblages have different niche requirements, assemblages may no longer remain intact and dis- and reassemble at current or new geographic locations. We explored how climate change projected by 2100 may transform the world's avian assemblages (characterized at a 110 km spatial grain) by modeling environmental niche-based changes to their dietary guild structure under 0, 500, and 2000 km-dispersal distances. We examined guild structure changes at coarse (primary, high-level, and mixed consumers) and fine (frugivores, nectarivores, insectivores, herbivores, granivores, scavengers, omnivores, and carnivores) ecological resolutions to determine whether or not geographic co-occurrence patterns among guilds were associated with the magnitude to which guilds are functionally resolved. Dietary guilds vary considerably in their global geographic prevalence, and under broad-scale niche-based redistribution of species, these are projected to change very heterogeneously. A nondispersal assumption results in the smallest projected changes to guild assemblages, but with significant losses for some regions and guilds, such as South American insectivores. Longer dispersal distances are projected to cause greater degrees of disassembly, and lead to greater homogenization of guild composition, especially in northern Asia and Africa. This arises because projected range gains and losses result in geographically heterogeneous patterns of guild compensation. Projected decreases especially of primary and mixed consumers most often are compensated by increases in high-level consumers, with increasing uncertainty about these outcomes as dispersal distance and degree of guild functional resolution increase. Further exploration into the consequences of these significant broad-scale ecological functional changes at the community or ecosystem level should be increasingly on the agenda for conservation science.

Effects of red-backed salamanders on ecosystem functions

Ecosystems provide a vast array of services for human societies, but understanding how various organisms contribute to the functions that maintain these services remains an important ecological challenge. Predators can affect ecosystem functions through a combination of top-down trophic cascades and bottom-up effects on nutrient dynamics. As the most abundant vertebrate predator in many eastern US forests, woodland salamanders (Plethodon spp.) likely affect ecosystems functions. We examined the effects of red-backed salamanders (Plethodon cinereus) on a variety of forest ecosystem functions using a combined approach of large-scale salamander removals (314-m² plots) and small-scale enclosures (2 m²) where we explicitly manipulated salamander density (0, 0.5, 1, 2, 4 m⁻²). In these experiments, we measured the rates of litter and wood decomposition, potential nitrogen mineralization and nitrification rates, acorn germination, and foliar insect damage on red oak seedlings. Across both experimental venues, we found no significant effect of red-backed salamanders on any of the ecosystem functions. We also found no effect of salamanders on intraguild predator abundance (carabid beetles, centipedes, spiders). Our study adds to the already conflicting evidence on effects of red-backed salamander and other amphibians on terrestrial ecosystem functions. It appears likely that the impact of terrestrial amphibians on ecosystem functions is context dependent. Future research would benefit from explicitly examining terrestrial amphibian effects on ecosystem functions under a variety of environmental conditions and in different forest types.

Effects of predator richness on prey suppression: a meta-analysis

It is well established that species richness of primary producers and primary consumers can enhance efficiency of resource uptake and biomass production of respective trophic levels. At the level of secondary consumers (predators), however, conclusions about the functional role of biodiversity have been mixed. We take advantage of a recent surge of published experiments (totaling 46 since 2005) to both evaluate general effects of predator richness on aggregate prey suppression (top-down control) and explore sources of variability among experiments. Our results show that, across experiments, predator richness enhances prey suppression relative to the average single predator species (mean richness effect), but not the best-performing species. Mean richness effects in predator experiments were stronger than those for primary producers and detritivores, suggesting that relationships between richness and function may increase with trophic height in food webs. The strength of mean predator richness effects increased with the spatial and temporal scale of experiments, and the taxonomic distinctness (TD, used as a proxy of phylogenetic diversity) of species present. This latter result suggests that TD captures important aspects of functional differentiation among predators and that measures of biodiversity that go beyond species richness may help to better predict the effects of predator species loss.

Cascading effects of predator-detritivore interactions depend on environmental context in a Tibetan alpine meadow

Studies of grazing food webs show that species traits can interact with environmental factors to determine the strength of trophic cascades, but analogous context dependencies in detrital food webs remain poorly understood. In predator-detritivore-plant interaction chains, predators are expected to indirectly suppress plant biomass by reducing the density of plant-facilitating detritivores. However, this outcome can be reversed where above-ground predators drive burrowing detritivores to lower soil levels, strengthening their plant-facilitating effects. Here, we show that these trait-mediated indirect interactions further depend on environmental context in a Tibetan alpine meadow. In our study system, undulating topography generates higher (dry soil) patches interspersed with lower (wet soil) patches. Because the ability of detritivores to form deep burrows is likely to be limited by oxygen availability in low patches (wet soil), we hypothesized that (i) burrowing detritivores would undergo a vertical habitat shift, allowing them to more effectively avoid predation, in high - but not low - patches, and (ii) this shift would transmit positive effects of predators to plants in high patches by improving conditions in the lower soil layer. We tested these hypotheses using complementary field and glasshouse experiments examining whether the cascading effects of above-ground predatory beetles (presence/absence) on the density and behaviour of tunnel-forming detritivorous beetles, soil properties, and plant growth varied with patch type (low/high). Results revealed that predatory beetles did not reduce the density of detritivores in either patch type but had context-dependent trait-mediated effects, increasing the tunnelling depth of detritivores, improving soil conditions and ultimately increasing plant biomass in the high but not low patches. This study adds to an emerging predictive framework linking predators to plants in detritus food webs, demonstrating that these indirect interactions depend not just on the relative habitat domains of predators and prey, but also on environmental conditions that can predictably constrain the behavioural response of detritivores to predation risk.

The importance of aboveground-belowground interactions on the evolution and maintenance of variation in plant defense traits

Over the past two decades a growing body of empirical research has shown that many ecological processes are mediated by a complex array of indirect interactions occurring between rhizosphere-inhabiting organisms and those found on aboveground plant parts. Aboveground-belowground studies have thus far focused on elucidating processes and underlying mechanisms that mediate the behavior and performance of invertebrates in opposite ecosystem compartments. Less is known about genetic variation in plant traits such as defense as that may be driven by above- and belowground trophic interactions. For instance, although our understanding of genetic variation in aboveground plant traits and its effects on community-level interactions is well developed, little is known about the importance of aboveground-belowground interactions in driving this variation. Plant traits may have evolved in response to selection pressures from above- and below-ground interactions from antagonists and mutualists. Here, we discuss gaps in our understanding of genetic variation in plant-related traits as they relate to aboveground and belowground multitrophic interactions. When metabolic resources are limiting, multiple attacks by antagonists in both domains may lead to trade-offs. In nature, these trade-offs may critically depend upon their effects on plant fitness. Natural enemies of herbivores may also influence selection for different traits via top-down control. At larger scales these interactions may generate evolutionary "hotspots" where the expression of various plant traits is the result of strong reciprocal selection via direct and indirect interactions. The role of abiotic factors in driving genetic variation in plant traits is also discussed.

Human-induced trophic cascades along the fecal detritus pathway

Human presence and activity in tropical forest is thought to exert top-down regulation over the various ‘green-world’ pathways of plant-based foodwebs. However, these effects have never been explored for the ‘brown-world’ pathways of fecal-detritus webs. The strong effects of humans on tropical game mammals are likely to indirectly influence fecal detritivores (including Scarabaeine dung beetles), with subsequent indirect impacts on detrivore-mediated and plant-facilitating detrital processes. Across a 380-km gradient of human influence in the western Brazilian Amazon, we conducted the first landscape-level assessment of human-induced cascade effects on the fecal detritus pathway, by coupling data on human impact, game mammal and detritivore community structure, and rate measurements of a key detritus process (i.e. dung beetle-mediated secondary seed dispersal). We found evidence that human impact indirectly influences both the diversity and biomass of fecal detritivores, but not detritivore-mediated processes. Cascade strength varied across detritivore groups defined by species' traits. We found smaller-bodied dung beetles were at higher risk of local decline in areas of human presence, and that body size was a better predictor of cascade structure than fecal resource manipulation strategy. Cascade strength was also stronger in upland, unflooded forests, than in seasonally flooded forests. Our results suggest that the impact of human activity in tropical forest on fecal-detritus food web structure is mediated by both species' traits and habitat type. Further research will be required to determine the conditions under which these cascade effects influence fecal-detritus web function.

Modelling landscape-level numerical responses of predators to prey: the case of cats and rabbits

Predator-prey systems can extend over large geographical areas but empirical modelling of predator-prey dynamics has been largely limited to localised scales. This is due partly to difficulties in estimating predator and prey abundances over large areas. Collection of data at suitably large scales has been a major problem in previous studies of European rabbits (Oryctolagus cuniculus) and their predators. This applies in Western Europe, where conserving rabbits and predators such as Iberian lynx (Lynx pardinus) is important, and in other parts of the world where rabbits are an invasive species supporting populations of introduced, and sometimes native, predators. In pastoral regions of New Zealand, rabbits are the primary prey of feral cats (Felis catus) that threaten native fauna. We estimate the seasonal numerical response of cats to fluctuations in rabbit numbers in grassland–shrubland habitat across the Otago and Mackenzie regions of the South Island of New Zealand. We use spotlight counts over 1645 km of transects to estimate rabbit and cat abundances with a novel modelling approach that accounts simultaneously for environmental stochasticity, density dependence and varying detection probability. Our model suggests that cat abundance is related consistently to rabbit abundance in spring and summer, possibly through increased rabbit numbers improving the fecundity and juvenile survival of cats. Maintaining rabbits at low abundance should therefore suppress cat numbers, relieving predation pressure on native prey. Our approach provided estimates of the abundance of cats and rabbits over a large geographical area. This was made possible by repeated sampling within each season, which allows estimation of detection probabilities. A similar approach could be applied to predator-prey systems elsewhere, and could be adapted to any method of direct observation in which there is no double-counting of individuals. Reliable estimates of numerical responses are essential for managing both invasive and threatened predators and prey.

Lethal interactions among vertebrate top predators: a review of concepts, assumptions and terminology

Lethal interactions among large vertebrate predators have long interested researchers because of ecological and conservation issues. Research focusing on lethal interactions among vertebrate top predators has used several terms with a broad sense, and also introduced new terminology. We analysed the published literature with reference to the main underlying concepts and the use of terminology and its ecological context. The most frequently used terms in the literature were 'predation', 'intraguild predation', 'interference competition', and 'interspecific killing'. Most studies presented evidence of the killing of the victim (77%), but information regarding its consumption was not given in 48% of cases. More than half of the analysed studies (56%) had no solid information on the degree of competition between interacting species. By reviewing definitions and their underlying assumptions, we demonstrate that lethal interactions among large vertebrate predators could be designated using four terms-'predation', 'intraguild predation', 'interspecific competitive killing', and 'superpredation'-without the need to employ additional terminology that may increase confusion and misuse. For a correct framework of these lethal interactions it is critical to assess if the kill is consumed, if the victim is indeed a competitor of the killer, and if the prey is a high-order predator. However, these elements of the framework are simultaneously the most common constraints to studies of lethal interactions, since they often require a great effort to obtain. The proper use of terms and concepts is fundamental to understanding the causes behind lethal interactions and, ultimately, what is actually happening in these complex interactions.

Predator attack rate evolution in space: the role of ecology mediated by complex emergent spatial structure and self-shading

Predation interactions are an important element of ecological communities. Population spatial structure has been shown to influence predator evolution, resulting in the evolution of a reduced predator attack rate; however, the evolutionary role of traits governing predator and prey ecology is unknown. The evolutionary effect of spatial structure on a predator's attack rate has primarily been explored assuming a fixed metapopulation spatial structure, and understood in terms of group selection. But endogenously generated, emergent spatial structure is common in nature. Furthermore, the evolutionary influence of ecological traits may be mediated through the spatial self-structuring process. Drawing from theory on pathogens, the evolutionary effect of emergent spatial structure can be understood in terms of self-shading, where a voracious predator limits its long-term invasion potential by reducing local prey availability. Here we formalize the effects of self-shading for predators using spatial moment equations. Then, through simulations, we show that in a spatial context self-shading leads to relationships between predator-prey ecology and the predator's attack rate that are not expected in a non-spatial context. Some relationships are analogous to relationships already shown for host-pathogen interactions, but others represent new trait dimensions. Finally, since understanding the effects of ecology using existing self-shading theory requires simplifications of the emergent spatial structure that do not apply well here, we also develop metrics describing the complex spatial structure of the predator and prey populations to help us explain the evolutionary effect of predator and prey ecology in the context of self-shading. The identification of these metrics may provide a step towards expansion of the predictive domain of self-shading theory to more complex spatial dynamics.

Complex biotic interactions drive long-term vegetation dynamics in a subarctic ecosystem

Predicting impacts of global warming requires understanding of the extent to which plant biomass and production are controlled by bottom-up and top-down drivers. By annually monitoring community composition in grazed control plots and herbivore-free exclosures at an Arctic location for 15 years, we detected multiple biotic interactions. Regular rodent cycles acted as pulses driving synchronous fluctuations in the biomass of field-layer vegetation; reindeer influenced the biomass of taller shrubs, and the abundance of plant pathogenic fungi increased when densities of their host plants increased in exclosures. Two outbreaks of geometrid moths occurred during the study period, with contrasting effects on the field layer: one in 2004 had marginal effects, while one in 2012 severely reduced biomass in the control plots and eliminated biomass that had accumulated over 15 years in the exclosures. The latter was followed by a dramatic decline of the dominant understory dwarf-shrub Empetrum hermaphroditum, driven by an interaction between moth herbivory on top buds and leaves, and increased disease severity of a pathogenic fungus. We show that the climate has important direct and indirect effects on all these biotic interactions. We conclude that long time series are essential to identify key biotic interactions in ecosystems, since their importance will be influenced by climatic conditions, and that manipulative treatments are needed in order to obtain the mechanistic understanding needed for robust predictions of future ecosystem changes and their feedback effects.

Spider silk reduces insect herbivory

The role of predators in food webs extends beyond their ability to kill and consume prey. Such trait-mediated effects occur when signals of the predator influence the behaviour of other animals. Because all spiders are silk-producing carnivores, we hypothesized that silk alone would signal other arthropods and enhance non-lethal effects of spiders. We quantified the herbivory inflicted by two beetle species on green bean plants (Phaseolus vulgaris) in the presence of silkworm silk and spider silk along with no silk controls. Single leaflets were treated and enclosed with herbivores in the laboratory and field. Another set of leaflets were treated and left to experience natural herbivory in the field. Entire plants in the field were treated with silk and enclosed with herbivores or left exposed to herbivory. In all cases, the lowest levels of herbivory occurred with spider silk treatments and, in general, silkworm silk produced intermediate levels of leaf damage. These results suggest that silk may be a mechanism for the trait-mediated impacts of spiders and that it might contribute to integrated pest management programmes.

Scientific foundations for an IUCN Red List of ecosystems

An understanding of risks to biodiversity is needed for planning action to slow current rates of decline and secure ecosystem services for future human use. Although the IUCN Red List criteria provide an effective assessment protocol for species, a standard global assessment of risks to higher levels of biodiversity is currently limited. In 2008, IUCN initiated development of risk assessment criteria to support a global Red List of ecosystems. We present a new conceptual model for ecosystem risk assessment founded on a synthesis of relevant ecological theories. To support the model, we review key elements of ecosystem definition and introduce the concept of ecosystem collapse, an analogue of species extinction. The model identifies four distributional and functional symptoms of ecosystem risk as a basis for assessment criteria: A) rates of decline in ecosystem distribution; B) restricted distributions with continuing declines or threats; C) rates of environmental (abiotic) degradation; and D) rates of disruption to biotic processes. A fifth criterion, E) quantitative estimates of the risk of ecosystem collapse, enables integrated assessment of multiple processes and provides a conceptual anchor for the other criteria. We present the theoretical rationale for the construction and interpretation of each criterion. The assessment protocol and threat categories mirror those of the IUCN Red List of species. A trial of the protocol on terrestrial, subterranean, freshwater and marine ecosystems from around the world shows that its concepts are workable and its outcomes are robust, that required data are available, and that results are consistent with assessments carried out by local experts and authorities. The new protocol provides a consistent, practical and theoretically grounded framework for establishing a systematic Red List of the world's ecosystems. This will complement the Red List of species and strengthen global capacity to report on and monitor the status of biodiversity.

Top-down control of epifauna by fishes enhances seagrass production

Predators can influence the structure and function of ecosystems by altering the composition or behavior of herbivore communities. Overexploitation of predators, therefore, may lead to habitat loss by altering important top-down interactions that facilitate habitat-forming species. In seagrass beds, top-down control of algal growth by mesograzers appears to facilitate seagrass production. The indirect consequences of higher-order trophic interactions, however, remain unclear. Although predators may limit the beneficial effects of algal mesograzers, it is also possible that they limit the abundance of invertebrates that consume and foul seagrasses. We used experimental enclosure and exclosure cages to explore the direct and indirect effects of microcarnivorous fishes on epifaunal invertebrates, epiphytic loads, and seagrass growth in a natural eelgrass (Zostera marina) bed in San Diego Bay, California, USA. Contrary to expectations, when fishes were excluded, invertebrate abundance increased by 300-1000%, fouling on eelgrass leaves increased by 600%, and eelgrass production declined by 50%. Despite high densities of predators in enclosures, subsequent effects did not differ from ambient conditions. When predators were excluded, however, abundances of epifauna (including tube-building crustaceans and an eelgrass-grazing limpet) increased dramatically, resulting in reduced seagrass production. Our results are supported by several studies of eelgrass communities in the northeastern Pacific, characterized by coastal upwelling, inverse estuaries, and a voracious seagrass-consuming limpet. These strong, positive, indirect effects of microcarnivores on seagrass production contrast with the beneficial mesograzer paradigm, highlighting the need for hypotheses to be tested across a variety of ecosystems with varying biophysical characteristics.