Kleptoparasitism and Scavenging Can Stabilize Ecosystem Dynamics

Scavenging is ubiquitous in nature, but its implications have rarely been investigated. We used camera traps on wolf kills to investigate the role of scavenging on predator and multiprey dynamics in a northern Apennine system in Italy. In contrast to North American systems, the omnivorous wild boar successfully competes with wolves for the meat of their kills. We developed a deterministic, multitrophic web model (wolf, vegetation, deer, and wild boar), tunable through a parameter that governs the impact of prey sharing between wolves and wild boar. When prey sharing is scarce, populations oscillate, but above a threshold value the trophic web is stabilized, with the regime solution becoming a fixed, stable point. Both deer and wild boar then increase as a function of prey sharing, and the impact of herbivores on the vegetation increases. When prey sharing exceeds another threshold, the system collapses due to the extinction of both wolves and wild boar. Our analysis shows that scavenging is crucial for the dynamics of this ecosystem, and thus it should not be overlooked in food web modeling. The exploitation of wolf kills by wild boar may allow juveniles and yearlings to obtain high-quality resources that are not usually available, helping the wild boar to compensate for losses caused by hunting. This is likely to make them even more invasive and difficult to control.

Expert judgment and uncertainty regarding the protection of imperiled species

Decisions concerning the appropriate listing status of species under the U.S. Endangered Species Act (ESA) can be controversial even among conservationists. These decisions may determine whether a species persists in the near term and have long-lasting social and political ramifications. Given the ESA's mandate that such decisions be based on the best available science, it is important to examine what factors contribute to experts' judgments concerning the listing of species. We examined how a variety of factors (such as risk perception, value orientations, and norms) influenced experts' judgments concerning the appropriate listing status of the grizzly bear (Ursus arctos horribilis) population in the Greater Yellowstone Ecosystem. Experts were invited to complete an online survey examining their perceptions of the threats grizzly bears face and their listing recommendation. Although experts' assessments of the threats to this species were strongly correlated with their recommendations for listing status, this relationship did not exist when other cognitive factors were included in the model. Specifically, values related to human use of wildlife and norms (i.e., a respondent's expectation of peers' assessments) were most influential in listing status recommendations. These results suggest that experts' decisions about listing, like all human decisions, are subject to the use of heuristics (i.e., decision shortcuts). An understanding of how heuristics and related biases affect decisions under uncertainty can help inform decision making about threatened and endangered species and may be useful in designing effective processes for protection of imperiled species.

Disrupted trophic interactions affect recruitment of boreal deciduous and coniferous trees in northern Europe

Loss of large carnivore populations may lead to increased population densities of large herbivores, and subsequent cascading effects on the composition, structure, and function of ecosystems. Using a macroecological approach based on studies in multiple boreal forest landscapes in the Baltic Sea region and Russia, we tested the hypothesis that disrupted trophic interactions among large carnivores and large herbivores affect the recruitment of both ecologically and economically valuable tree species. We measured damage levels on young trees and large herbivore density in 10 local landscapes representing a gradient from extinct to extant populations of both large carnivores and large herbivores. We also tested the alternative hypothesis that forest management intensity is correlated to reduced recruitment of these tree species. At the macroecological scale there was an inverse relationship between the number of large carnivores and large herbivores. This coincided with a steep gradient in browsing damage on the ecologically important aspen, rowan and sallow as hosts for specialized species, as well as the economically important Scots pine. In one landscape hunting had replaced the presence of carnivores. Mean damage levels of these four tree species were correlated with large herbivore abundance, but not with forest management intensity. We discuss the pros and cons of this macroecological approach, as well as the challenge of governing and managing trophic interactions at multiple scales.

Modelling moose-forest interactions under different predation scenarios at Isle Royale National Park, USA

Loss of top predators may contribute to high ungulate population densities and chronic over-browsing of forest ecosystems. However, spatial and temporal variability in the strength of interactions between predators and ungulates occurs over scales that are much shorter than the scales over which forest communities change, making it difficult to characterize trophic cascades in forest ecosystems. We applied the LANDIS-II forest succession model and a recently developed ungulate browsing extension to model how the moose population could interact with the forest ecosystem of Isle Royale National Park, USA, under three different wolf predation scenarios. We contrasted a 100-yr future without wolves (no predation) with two predation scenarios (weak, long-term average predation rates and strong, higher than average rates). Increasing predation rates led to lower peak moose population densities, lower biomass removal rates, and higher estimates of forage availability and landscape carrying capacity, especially during the first 40 yr of simulations. Thereafter, moose population density was similar for all predation scenarios, but available forage biomass and the carrying capacity of the landscape continued to diverge among predation scenarios. Changes in total aboveground live biomass and species composition were most pronounced in the no predation and weak predation scenarios. Consistent with smaller-scale studies, high browsing rates led to reductions in the biomass of heavily browsed Populus tremuloides, Betula papyrifera, and Abies balsamea, and increases in the biomass of unbrowsed Picea glauca and Picea mariana, especially after the simulation year 2050, when existing boreal hardwood stands at Isle Royale are projected to senesce. As a consequence, lower predation rates corresponded with a landscape that progressively shifted toward dominance by Picea glauca and Picea mariana, and lacking available forage biomass. Consistencies with previously documented small-scale successional shifts, and population estimates and trends that approximate those from this and other boreal forests that support moose provide some confidence that these dynamics represent a trophic cascade and therefore provide an important baseline against which to evaluate long-term and large-scale effects of alternative predator management strategies on ungulate populations and forest succession.

Competition between apex predators? Brown bears decrease wolf kill rate on two continents

Trophic interactions are a fundamental topic in ecology, but we know little about how competition between apex predators affects predation, the mechanism driving top-down forcing in ecosystems. We used long-term datasets from Scandinavia (Europe) and Yellowstone National Park (North America) to evaluate how grey wolf (Canis lupus) kill rate was affected by a sympatric apex predator, the brown bear (Ursus arctos). We used kill interval (i.e. the number of days between consecutive ungulate kills) as a proxy of kill rate. Although brown bears can monopolize wolf kills, we found no support in either study system for the common assumption that they cause wolves to kill more often. On the contrary, our results showed the opposite effect. In Scandinavia, wolf packs sympatric with brown bears killed less often than allopatric packs during both spring (after bear den emergence) and summer. Similarly, the presence of bears at wolf-killed ungulates was associated with wolves killing less often during summer in Yellowstone. The consistency in results between the two systems suggests that brown bear presence actually reduces wolf kill rate. Our results suggest that the influence of predation on lower trophic levels may depend on the composition of predator communities.

AIC and the challenge of complexity: A case study from ecology

Philosophers and scientists alike have suggested Akaike's Information Criterion (AIC), and other similar model selection methods, show predictive accuracy justifies a preference for simplicity in model selection. This epistemic justification of simplicity is limited by an assumption of AIC which requires that the same probability distribution must generate the data used to fit the model and the data about which predictions are made. This limitation has been previously noted but appears to often go unnoticed by philosophers and scientists and has not been analyzed in relation to complexity. If predictions are about future observations, we argue that this assumption is unlikely to hold for models of complex phenomena. That in turn creates a practical limitation for simplicity's AIC-based justification because scientists modeling such phenomena are often interested in predicting the future. We support our argument with an ecological case study concerning the reintroduction of wolves into Yellowstone National Park, U.S.A. We suggest that AIC might still lend epistemic support for simplicity by leading to better explanations of complex phenomena.

Natural and experimental tests of trophic cascades: gray wolves and white-tailed deer in a Great Lakes forest

Herbivores can be major drivers of environmental change, altering plant community structure and changing biodiversity through the amount and species of plants consumed. If natural predators can reduce herbivore numbers and/or alter herbivore foraging behavior, then predators may reduce herbivory on sensitive plants, and a trophic cascade will emerge. We have investigated whether gray wolves (Canis lupus) generate such trophic cascades by reducing white-tailed deer (Odocoileus virginianus) herbivory on saplings and rare forbs in a northern mesic forest (Land O' Lakes, WI). Our investigation used an experimental system of deer exclosures in areas of high and low wolf use that allowed us to examine the role that wolf predation may play in reducing deer herbivory through direct reduction in deer numbers or indirectly through changing deer behavior. We found that in areas of high wolf use, deer were 62 % less dense, visit duration was reduced by 82 %, and percentage of time spent foraging was reduced by 43 %; in addition, the proportion of saplings browsed was nearly sevenfold less. Average maple (Acer spp.) sapling height and forb species richness increased 137 and 117 % in areas of high versus low wolf use, respectively. The results of the exclosure experiments revealed that the negative impacts of deer on sapling growth and forb species richness became negligible in high wolf use areas. We conclude that wolves are likely generating trophic cascades which benefit maples and rare forbs through trait-mediated effects on deer herbivory, not through direct predation kills.

Trophic Cascades by Large Carnivores: A Case for Strong Inference and Mechanism

Studies on trophic cascades involving large carnivores typically are limited by a lack of replication and control, giving rise to a spirited debate over the ecological role of these iconic species. We argue that much of this debate can be resolved by decomposing the trophic cascade hypothesis into three constituent interactions, quantifying each interaction individually, and accommodating alternative hypotheses. We advocate for a novel approach that couples the rigor characterizing foundational work on trophic cascades (i.e., from studies carried out in mesocosm and whole lake systems) with the conservation relevance of large carnivore-dominated food webs. Because of their iconic status, it is crucial that inferences about the ecological role of large carnivores rise to meet the same rigorous standards to which other studies in community ecology are held.

Yellowstone wolf (Canis lupus) density predicted by elk (Cervus elaphus) biomass

The Northern Range (NR) of Yellowstone National Park (YNP) hosts a higher prey biomass density in the form of elk (Cervus elaphus L., 1758) than any other system of gray wolves (Canis lupus L., 1758) and prey reported. Therefore, it is important to determine whether that wolf–prey system fits a long-standing model relating wolf density to prey biomass. Using data from 2005 to 2012 after elk population fluctuations dampened 10 years subsequent to wolf reintroduction, we found that NR prey biomass predicted wolf density. This finding and the trajectory of the regression extend the validity of the model to prey densities 19% higher than previous data and suggest that the model would apply to wolf–prey systems of even higher prey biomass.

Effects of predation risk on elk (Cervus elaphus) landscape use in a wolf (Canis lupus) dominated system

Food acquisition and predation avoidance are key drivers of herbivore behaviour. We investigated the interaction of top-down (predator) and bottom-up (food, fire, thermal) effects by measuring the relationship between wolf (Canis lupus L., 1758) predation risk perceived by elk (Cervus elaphus L., 1758) and elk landscape use. We conducted fecal pellet and wolf scat surveys in three valleys with three wolf population levels (Saint Mary: low; Waterton: moderate; North Fork: high). In the North Fork, 90% of quaking aspen (Populus tremuloides Michx.) stands burned recently; the other valleys had no fire. We created predictive models of elk pellet density that incorporated bottom-up and top-down variables. All valleys had a high elk pellet density (≥10 per 100 m2). Wolf scat density was similar where there was no fire, but one order of magnitude greater in burned areas. Elk pellet density was lower in the North Fork, a predation-related response. In all valleys, site-specific elk density declined as impediments to detecting or escaping wolves increased, and elk avoided aspen, except for North Fork unburned areas. Models that best predicted elk density contained bottom-up and top-down effects. At local scales, high predation risk negatively influence elk occurrence, suggesting that even with minimal wolf exposure elk avoid risky sites.