Patterns in the abundance of kangaroo populations in arid Australia are consistent with the exploitation ecosystems hypothesis

The exploitation ecosystems hypothesis (EEH) makes predictions about trophic interactions along gradients of primary productivity. The EEH has been shown to apply to a wide range of terrestrial environments but its applicability to arid environments has received little attention. One reason for this is that arid environments may not satisfy the assumptions of the EEH because dearth of water may limit biological activity in both temporal and spatial contexts. The EEH predicts that herbivore biomass should increase linearly with primary productivity in the absence of predators; but when predators are present herbivore biomass will remain relatively constant due to top down regulation. We tested this prediction in an arid environment using rainfall as a proxy of primary productivity and an index of the abundance of the dominant herbivores (kangaroos Macropus spp.). We compared an index of kangaroo abundance at 18 areas situated along a gradient of mean annual rainfall in areas where a top predator (the dingo Canis lupus dingo) was rare and common. We also explored the relationship between the density of artificial water points (AWPs) and kangaroo abundance to investigate if the resource subsidy provided by AWPs allows kangaroos to persist in high numbers. Consistent with the EEH, kangaroo abundance showed a weak relationship with mean annual rainfall in the presence of dingoes but increased with increasing annual rainfall in the absence of dingoes. The density of AWPs was a poor predictor of kangaroo abundance. Our analysis of macro‐ecological patterns suggests that kangaroo populations are primarily top down regulated in the presence of dingoes, but are bottom up regulated in the absence of dingoes. Our findings provide evidence that top down regulation can prevail over bottom up regulation of herbivore populations in arid ecosystems and highlights the usefulness of the EEH as a predictor of macro‐ecological patterns of species abundance.     

Exploitation ecosystems and trophic cascades in non‐equilibrium systems: pasture – red kangaroo – dingo interactions in arid Australia

The exploitation ecosystems hypothesis (EEH) proposes that 1) plant biomass reflects the primary productivity of an ecosystem modified by the regulating effect of herbivory, and 2) herbivore abundance reflects the productivity of plants modified by the regulating effect of predation. Primary productivity thus determines the number of trophic levels in an ecosystem and the extent to which bottom–up and top–down regulation influence the biomass ratios of adjacent and non‐adjacent trophic levels (i.e. trophic cascading). We constructed an interactive model of plant (pasture), herbivore (red kangaroo Macropus rufus) and predator (dingo Canis lupus dingo), a system in which trophic cascades have been suggested to occur, and used it to test the effects of increasing stochastic variation in primary productivity and dingo culling on predictions of the EEH. The model contained four feedback loops: the predator–herbivore and herbivore–plant feedback loops, and the predator and plant density‐dependent feedback loops. The equilibrium conditions along the primary productivity gradient reproduced the three zones of trophic dynamics predicted by the EEH, plus an additional zone at productivities above which the maximum density of a predator is achieved due to social regulation: that zone is characterized by increasing herbivore density and decreasing plant biomass. Culling dingoes produced trophic cascades that were strongly attenuated at primary productivities below which the maximum density of dingoes was attained. Results were robust to uncertainty in kangaroo off‐take by dingoes and to the efficacy of dingo culling, but prey switching by dingoes from red kangaroos to reptiles would weaken trophic cascades. We conclude that social regulation of carnivores has important implications for expression of the EEH and trophic cascades, and that attenuation of trophic cascades increases with increasing stochasticity in primary productivity. Our model also provides a framework for understanding the conditions in which dingo‐mediated trophic cascades might be expected to occur, and generates testable predictions about the effects of higher dingo densities (e.g. by stopping culling or reintroduction to former range) on kangaroo and pasture dynamics.     

The impact of thermal seasonality on terrestrial endotherm food web dynamics: a revision of the Exploitation Ecosystem Hypothesis

Many terrestrial endotherm food webs constitute three trophic level cascades. Others have two trophic level dynamics (food limited herbivores; plants adapted to tackle intense herbivory) or one trophic level dynamic (herbivorous endotherms absent, thus plants compete for the few places where they can survive and grow). According to the Exploitation Ecosystems Hypothesis (EEH), these contrasting dynamics are consequences of differences in primary productivity. The productivity thresholds for changing food web dynamics were assumed to be global constants. We challenged this assumption and found that several model parameters are sensitive to the contrast between persistently warm and seasonally cold climates. In persistently warm environments, three trophic level dynamics can be expected to prevail almost everywhere, save the most extreme deserts. We revised EEH accordingly and tested it by compiling direct evidence of three and two trophic level dynamics and by studying the global distribution of felids. In seasonally cold environments, we found evidence for three trophic level dynamics only in productive ecosystems, while evidence for two trophic level dynamics appeared in ecosystems with low primary productivity. In persistently warm environments, we found evidence for three trophic level dynamics in all types of ecosystems. The distribution of felids corroborated these results. The empirical evidence thus indicates that two trophic level dynamics, as defined by EEH, are restricted to seasonally cold biomes with low primary productivity, such as the artic–alpine tundra and the temperate steppe.     

Changes in desert avifauna associated with the functional extinction of a terrestrial top predator

We investigated how long‐term suppression of populations of a top predator, the dingo Canis dingo, affected composition of sympatric avifauna in Australian deserts, by surveying bird assemblages across ~80 000 km² of arid dune‐fields on either side of the Dingo Barrier Fence (DBF; a 5614 km‐long fence separating ecosystems in which dingoes are abundant from ecosystems in which dingoes are functionally extinct). Using fourth‐corner modelling, incorporating species’ traits, we identified apparent declines of sedentary birds that nest in low vegetation and small birds that feed primarily on grass seed, and increases in scavenging birds associated with the functional extinction of dingoes. Occupancy differed between sites inside and outside the DBF in 13 bird species. We hypothesise that these differences in bird assemblages across the DBF result, in part, from increases in kangaroos Macropus spp. and red foxes Vulpes vulpes in arid landscapes where dingoes have been removed. Our study provides evidence that the functional extinction of a large terrestrial predator has had pervasive ecosystem effects, including shifts in composition of avian assemblages.     

The Logic and Realism of the Hypothesis of Exploitation Ecosystems

Hypotheses on trophic dynamics in terrestrial ecosystems fall into two major categories: those in which plants are assumed to be invulnerable to their consumers and those in which the build-up of plant biomass is assumed to require top-down control of folivores. The hypothesis of exploitation ecosystems (EEH) belongs to the latter category and focuses particularly on the consequences of the high energetic costs of maintenance of endotherms. Carnivorous endotherms require relatively high prey densities in order to break even. Moreover, they are dependent on folivorous prey during the limiting season, at least at high latitudes. The endotherm branch of the grazing web is thus predicted to collapse from three-link trophic dynamics (carnivores --> folivores --> plants --> inorganic resources) to two-link dynamics (folivores --> plants --> inorganic resources) along gradients of decreasing primary productivity. Consequently, the vegetation of cold and unproductive areas is predicted to be under intense winter grazing pressure, which prevents the accumulation of aboveground plant biomass and excludes erect woody plants. In the most extreme habitats (e.g., polar deserts and their high alpine counterparts), even folivorous endotherms are predicted to be absent, and the scanty vegetation is predicted to be structured by preemptive competition. Within temperature-determined productivity gradients, EEH is corroborated by biomass patterns, by patterns in the structure and dynamics of carnivore, folivore, and plant communities, and by experimental results. The general idea of top-down trophic dynamics is supported for other autotroph-based systems, too, but the relevance and sufficiency of the energy constraint in explaining patterns in trophic dynamics appears to be variable. Moreover, critical empirical evidence for or against the capacity of folivorous insects to regulate plant biomass has not yet been obtained. Another open question is the ability of boreal and temperate browsers, evolved in productive environments with intense predation pressure and abundance of forage, to prevent the regeneration of the least palatable tree species. There are, thus, many open questions waiting to be answered and many exciting experiments waiting to be conducted.     

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     

Does a top-predator provide an endangered rodent with refuge from an invasive mesopredator?

In arid environments, ecological refuges are often conceptualised as places where animal species can persist through drought owing to the localised persistence of moisture and nutrients. The mesopredator release hypothesis (MRH) predicts that reduced abundance of top-order predators results in an increase in the abundance of smaller predators (mesopredators) and consequently has detrimental impacts on the prey of the smaller predators. Thus according to the MRH, the existence of larger predators may provide prey with refuge from predation. In this study, we investigated how the abundance of an endangered rodent Notomys fuscus is affected by Australia's largest predator, the dingo Canis lupus dingo , introduced mesopredators, introduced herbivores, kangaroos and rainfall. Our surveys showed that N. fuscus was more abundant where dingoes occurred. Generalised linear modelling showed that N. fuscus abundance was associated positively with dingo activity and long-term annual rainfall and negatively with red fox Vulpes vulpes activity. Our results were consistent with the hypothesis that areas with higher rainfall and dingoes provide N. fuscus with refuge from drought and predation by invasive red foxes, respectively. Top-order predators, such as dingoes, could have an important functional role in broad-scale biodiversity conservation programmes by reducing the impacts of mesopredators.     

Top-down response to spatial variation in productivity and bottom-up response to temporal variation in productivity in a long-term study of desert ants

Under the Ecosystem Exploitation Hypothesis ecosystem productivity predicts trophic complexity, but it is unclear if spatial and temporal drivers of productivity have similar impacts. Long-term studies are necessary to capture temporal impacts on trophic structure in variable ecosystems such as deserts. We sampled ants and measured plant resources in the Simpson Desert, central Australia over a 22-year period, during which rainfall varied 10-fold. We sampled dune swales (higher nutrient) and crests (lower nutrient) to account for spatial variation in productivity. We asked how temporal and spatial variation in productivity affects the abundance of ant trophic guilds. Precipitation increased vegetation cover, with the difference more pronounced on dune crests; seeding and flowering also increased with precipitation. Generalist activity increased over time, irrespective of productivity. Predators were more active in more productive (swale) habitat, i.e. spatial impacts of productivity were greatest at the highest trophic level. By contrast, herbivores (seed harvesters and sugar feeders) increased with long-term rainfall; seed harvesters also increased as seeding increased. Temporal impacts of productivity were therefore greatest for low trophic levels. Whether productivity variation leads to top-down or bottom-up structured ecosystems thus depends on the scale and dimension (spatial or temporal) of productivity.     

Prey Vulnerability Limits Top-Down Control and Alters Reciprocal Feedbacks in a Subsidized Model Food Web

Resource subsidies increase the productivity of recipient food webs and can affect ecosystem dynamics. Subsidies of prey often support elevated predator biomass which may intensify top-down control and reduce the flow of reciprocal subsidies into adjacent ecosystems. However, top-down control in subsidized food webs may be limited if primary consumers posses morphological or behavioral traits that limit vulnerability to predation. In forested streams, terrestrial prey support high predator biomass creating the potential for strong top-down control, however armored primary consumers often dominate the invertebrate assemblage. Using empirically based simulation models, we tested the response of stream food webs to variations in subsidy magnitude, prey vulnerability, and the presence of two top predators. While terrestrial prey inputs increased predator biomass (+12%), the presence of armored primary consumers inhibited top-down control, and diverted most aquatic energy (∼75%) into the riparian forest through aquatic insect emergence. Food webs without armored invertebrates experienced strong trophic cascades, resulting in higher algal (∼50%) and detrital (∼1600%) biomass, and reduced insect emergence (−90%). These results suggest prey vulnerability can mediate food web responses to subsidies, and that top-down control can be arrested even when predator-invulnerable consumers are uncommon (20%) regardless of the level of subsidy.     

Evaluating trophic cascades as drivers of regime shifts in different ocean ecosystems

In ecosystems that are strongly structured by predation, reducing top predator abundance can alter several lower trophic levels—a process known as a trophic cascade. A persistent trophic cascade also fits the definition of a regime shift. Such ‘trophic cascade regime shifts'' have been reported in a few pelagic marine systems—notably the Black Sea, Baltic Sea and eastern Scotian Shelf—raising the question of how common this phenomenon is in the marine environment. We provide a general methodology for distinguishing top-down and bottom-up effects and apply this methodology to time series from these three ecosystems. We found evidence for top-down forcing in the Black Sea due primarily to gelatinous zooplankton. Changes in the Baltic Sea are primarily bottom-up, strongly structured by salinity, but top-down forcing related to changes in cod abundance also shapes the ecosystem. Changes in the eastern Scotian Shelf that were originally attributed to declines in groundfish are better explained by changes in stratification. Our review suggests that trophic cascade regime shifts are rare in open ocean ecosystems and that their likelihood increases as the residence time of water in the system increases. Our work challenges the assumption that negative correlation between consecutive trophic levels implies top-down forcing.     

A test of three alternative pathways for consumer regulation of primary productivity

The pathways linking consumer effects to primary productivity (PPR) are likely to vary among taxa because of species-specific trophic and functional differences. Thus, it is necessary to understand the dynamics of consumer–PPR interactions so that effects of species loss on ecosystem function can be addressed from a mechanistic approach. In this study, I used three fish taxa (orangethroat darter, Etheostoma spectabile; western mosquitofish, Gambusia affinis; and bullhead minnow, Pimephales vigilax) as model consumers with different trophic and functional characteristics to test alternative mechanisms for consumer regulation of PPR (i.e., trophic cascade, terrestrial nutrient translocation, and sedimentary nutrient translocation). Experiments were conducted in stream mesocosms fitted with a combination of fish and terrestrial insect barriers to address relative importance of consumer-driven top-down and bottom-up control of PPR. A predatory invertivore, orangethroat darter, increased PPR through an apparent trophic cascade by localized reduction of benthic grazing invertebrate densities (i.e., top-down). A surface feeding insectivore, western mosquitofish, consumed terrestrial insects on the stream surface, increasing PPR by enhancing allochthonous nutrients in the mesocosms (i.e., bottom-up). A benthic omnivore, bullhead minnow, consumed benthic food items, resulting in increased PPR by enhancing availability of autochthonous nutrients via translocation of sedimentary nutrients (i.e., bottom-up). However, under specific environmental contexts, this species also consumed terrestrial invertebrates, potentially affecting PPR through terrestrial nutrient translocation as well. In this study, the trophic and functional characteristics of different species resulted in alternative pathways that increased PPR, suggesting that in natural ecosystems multiple consumer-driven pathways may be influencing PPR simultaneously and could potentially be important for temporal persistence of ecosystem function in changing environments.     

The distribution of deer biomass in North America supports the hypothesis of exploitation ecosystems

The hypothesis of exploitation ecosystems (EEH) predicts that, along a productivity gradient in terrestrial environments, predators will regulate herbivores at a relatively constant density whenever primary productivity exceeds 700 g m⁻² y⁻¹; under this threshold, or if predators are absent, forage production determines herbivore density. I tested EEH using the pattern of deer biomass distribution over North America, the dominant family of large herbivores. Deer biomass increased from the High Arctic to the north of the boreal forest and remained in the same range southward within the gray wolf range; for the same latitude, deer biomass increased by a factor of 5 in the absence of wolves. South of the wolf range, there existed a clear relationship between actual evapotranspiration, a proxy of primary productivity, and deer biomass. Highest deer densities occurred in the south-east of the continent where only white-tailed deer are present. The observed pattern lends support to EEH and suggests that the removal of large predators in southern North America may have imposed an unprecedented pressure on plants eaten by deer.     

Complex wasp-waist regulation of pelagic ecosystems in the Pacific Ocean

‘Wasp-waist’ control of marine ecosystems is driven by a combination of top-down and bottom-up forcing by a few abundant short-lived species occupying intermediate trophic levels that form a narrow ‘waist’ through which energy flow from low to high trophic levels is controlled. It has been assumed that wasp-waist control occurs primarily in highly productive and species-poor systems (e.g. upwelling regions). Two large, species-rich, pelagic ecosystems in the relatively oligotrophic eastern and western Pacific Ocean also show wasp-waist-like structure, in that short-lived and fast-growing cephalopods and fishes at intermediate trophic levels comprise the vast majority of the biomass. Possible forcing dynamics of these systems were examined using ecosystem models by altering the biomass of phytoplankton (bottom-up forcing), large pelagic predators (top-down forcing), and intermediate ‘wasp-waist’ functional groups independently and observing how these changes propagated throughout the ecosystem. The largest effects were seen when altering the biomass of mid trophic-level epipelagic and mesopelagic fishes, where dramatic trophic cascades occurred both upward and downward in the system. We conclude that the high productivity and standing biomass of animals at intermediate trophic levels has a strong top-down influence on the abundance of primary producers. Furthermore, their importance as prey for large predators results in bottom-up controls on populations at higher trophic levels. We show that these tropical pelagic ecosystems possess a complex structure whereby several waist groups and alternate trophic pathways from primary producers to apex predators can cause unpredictable effects when the biomasses of particular functional groups are altered. Such models highlight the possible structuring mechanisms in pelagic systems, which have implications for fisheries that exploit these wasp-waist groups, such as squid fisheries, as well as for fisheries of top predators such as tunas and billfishes that prey upon wasp-waist species.     

The Ulva connection: marine algae subsidize terrestrial predators in coastal Peru

How can terrestrial animals survive in a desert with scant primary productivity? The Peruvian coastal desert is hyper‐arid, but faces one of the world's most productive marine ecosystems, the Peru–Chile cold current. Given the stark difference in productivity between these adjacent ecosystems, we expected to find strong linkages connecting the terrestrial and marine food web. We investigated how marine resources are incorporated in the diet, and influence the distribution of terrestrial consumers (geckos, scorpions, solifuges and darkling beetles). Stomach contents from geckos, and δ¹³C and δ¹⁵N values of geckos and other terrestrial consumers suggest that marine green algae of the genus Ulva provide energy and nutrients to the terrestrial food web. Isotopic values suggest that amphipods, which feed on stranded Ulva, make marine resources available to terrestrial predators by moving between the intertidal and supratidal zones. The relative contribution of terrestrial and algal carbon sources varied among terrestrial predators, because scorpions assimilated a lower proportion of energy from Ulva than did geckos and solifuges. These δ¹³C patterns reflected differences in the spatial distribution of consumers. Our study supports the idea that in places where ecosystems with contrasting productivity levels are spatially juxtaposed, it is not possible to understand the structure and dynamics of food webs without taking into account the effects of energy and nutrients flowing from adjacent ecosystems. In contrast to other studied systems, especially those in Baja California, our site in Peru receives very little rainfall and the amount of precipitation is not affected by El Niño events. The near absence of rainfall promotes an extreme dependence of terrestrial consumers on marine resources, and causes permanent indirect food‐web effects that are affected by temporal variability in marine productivity, rather than temporal patterns of plant growth.     

Invasive plants have different effects on trophic structure of green and brown food webs in terrestrial ecosystems: a meta‐analysis

Although invasive plants are a major source of terrestrial ecosystem degradation worldwide, it remains unclear which trophic levels above the base of the food web are most vulnerable to plant invasions. We performed a meta‐analysis of 38 independent studies from 32 papers to examine how invasive plants alter major groupings of primary and secondary consumers in three globally distributed ecosystems: wetlands, woodlands and grasslands. Within each ecosystem we examined if green (grazing) food webs are more sensitive to plant invasions compared to brown (detrital) food webs. Invasive plants have strong negative effects on primary consumers (detritivores, bacterivores, fungivores, and/or herbivores) in woodlands and wetlands, which become less abundant in both green and brown food webs in woodlands and green webs in wetlands. Plant invasions increased abundances of secondary consumers (predators and/or parasitoids) only in woodland brown food webs and green webs in wetlands. Effects of invasive plants on grazing and detrital food webs clearly differed between ecosystems. Overall, invasive plants had the most pronounced effects on the trophic structure of wetlands and woodlands, but caused no detectable changes to grassland trophic structure.     

Effects of Multi-chain Omnivory on the Strength of Trophic Control in Lakes

Omnivory has been implicated in both diffusing and intensifying the effects of consumer control in food chains. Some have postulated that the strong, community level, top-down control apparent in lakes is not expressed in terrestrial systems because terrestrial food webs are reticulate, with high degrees of omnivory and diverse plant communities. In contrast, lake food webs are depicted as simple linear chains based on phytoplankton-derived energy. Here, we explore the dynamic implications of recent evidence showing that attached algal (periphyton) carbon contributes substantially to lake primary and secondary productivity, including fish production. Periphyton production represents a cryptic energy source in oligotrophic and mesotrophic lakes that is overlooked by previous theoretical treatment of trophic control in lakes. Literature data demonstrate that many fish are multi-chain omnivores, exploiting food chains based on both littoral and pelagic primary producers. Using consumer-resource models, we examine how multiple food chains affect fourth-level trophic control across nutrient gradients in lakes. The models predict that the stabilizing effects of linked food chains are strongest in lakes where both phytoplankton and periphyton contribute substantially to production of higher trophic levels. This stabilization enables a strong and persistent top down control on the pelagic food chain in mesotrophic lakes. The extension of classical trophic cascade theory to incorporate more complex food web structures driven by multi-chain predators provides a conceptual framework for analysis of reticulate food webs in ecosystems.     

Multi-level trophic cascades in a heavily exploited open marine ecosystem

Anthropogenic disturbances intertwined with climatic changes can have a large impact on the upper trophic levels of marine ecosystems, which may cascade down the food web. So far it has been difficult to demonstrate multi-level trophic cascades in pelagic marine environments. Using field data collected during a 33-year period, we show for the first time a four-level community-wide trophic cascade in the open Baltic Sea. The dramatic reduction of the cod (Gadus morhua) population directly affected its main prey, the zooplanktivorous sprat (Sprattus sprattus), and indirectly the summer biomass of zooplankton and phytoplankton (top-down processes). Bottom-up processes and climate-hydrological forces had a weaker influence on sprat and zooplankton, whereas phytoplankton variation was explained solely by top-down mechanisms. Our results suggest that in order to dampen the occasionally harmful algal blooms of the Baltic, effort should be addressed not only to control anthropogenic nutrient inputs but also to preserve structure and functioning of higher trophic levels.     

The effect of rainfall on population dynamics in Sahara-Sahel rodents

In arid and semi-arid regions, rainfall is scarce, limiting primary productivity and animal reproduction. As long-term population monitoring is limited in remote arid areas, indirect and remote technicals are needed. We investigated if and how populations of rodents in Sahara-Sahel responded to past events of rainfall. Using short field surveys and remotely sensed rainfall data, spanning between years 2010 to 2015, we tested if rainfall prior to field surveys affected populations of Gerbillus rodents. Generalized additive analysis showed that amount of moonlight (the effect of number of days away from full moon) negatively correlated with number of trapped animals. When controlling for moonlight and geographic and temporal variation, rainfall up to 1 year prior to surveys positively and rainfall 2 years prior to surveys negatively correlated with number of trapped gerbils. We suggest that the effect of increased number of gerbils resulted from reproduction and population density increase after bursts of primary productivity. Negative correlation with rainfall could be related to increased predation or other ecological effects (e.g., resource collapse, pathogens spread) on prey population densities and activity levels. Our results suggest multiphase delayed effect of gerbils population response to rainfall implying interactive model of population regulation in rodent communities on Sahara-Sahel. Presented indirect method and results are readily applicable to population monitoring and management of remote and understudied areas.     

Apex predator suppression is linked to restructuring of ecosystems via multiple ecological pathways

Removal of apex predators can drive ecological regime shifts owing to compensatory positive and negative population level responses by organisms at lower trophic levels. Despite evidence that apex predators can influence ecosystems though multiple ecological pathways, most studies investigating apex predators’ effects on ecosystems have considered just one pathway in isolation. Here, we provide evidence that lethal control of an apex predator, the dingo Canis dingo, drives shifts in the structure of Australia's tropical‐savannah ecosystems. We compared mammal assemblages and understorey structure at seven paired‐sites. Each site comprised an area where people poisoned dingoes and an area without dingo control. The effects of dingo control on mammals scaled with body size. Where dingoes were poisoned, we found greater activity of herbivorous macropods and feral cats, a mesopredator, but sparser understorey vegetation and lower abundances of native rodents. Our study suggests that ecological cascades arising from apex predators’ suppressive effects on herbivores and mesopredators occur simultaneously. Concordant effects of dingo removal across tropical‐savannah, forest and desert biomes suggest that dingoes once exerted ubiquitous top–down effects across Australia and provides support for calls that top–down forcing should be considered a fundamental process governing ecosystem structure.     

Spatial patterns and dynamic responses of arctic food webs corroborate the exploitation ecosystems hypothesis (EEH)

According to the exploitation ecosystems hypothesis (EEH), productive terrestrial ecosystems are characterized by community-level trophic cascades, whereas unproductive ecosystems harbor food-limited grazers, which regulate community-level plant biomass. We tested this hypothesis along arctic-alpine productivity gradients at the Joatka field base, Finnmark, Norway. In unproductive habitats, mammalian predators were absent and plant biomass was constant, whereas herbivore biomass varied, reflecting the productivity of the habitat. In productive habitats, predatory mammals were persistently present and plant biomass varied in space, but herbivore biomass did not. Plant biomass of productive tundra scrublands declined by 40% when vegetation blocks were transferred to predation-free islands. Corresponding transfer to herbivore-free islands triggered an increase in plant biomass. Fertilization of an unproductive tundra heath resulted in a fourfold increase in rodent density and a corresponding increase in winter grazing activity, whereas the total aboveground plant biomass remained unchanged. These results corroborate the predictions of the EEH, implying that the endotherm community and the vegetation of the North European tundra behaves dynamically as if each trophic level consisted of a single population, in spite of local co-occurrence of >20 plant species representing different major taxonomic groups, growth forms, and defensive strategies.