Until recently, large apex consumers were ubiquitous across the globe and had been for millions of years. The loss of these animals may be humankind's most pervasive influence on nature. Although such losses are widely viewed as an ethical and aesthetic problem, recent research reveals extensive cascading effects of their disappearance in marine, terrestrial, and freshwater ecosystems worldwide. This empirical work supports long-standing theory about the role of top-down forcing in ecosystems but also highlights the unanticipated impacts of trophic cascades on processes as diverse as the dynamics of disease, wildfire, carbon sequestration, invasive species, and biogeochemical cycles. These findings emphasize the urgent need for interdisciplinary research to forecast the effects of trophic downgrading on process, function, and resilience in global ecosystems.

There are many cases where animal populations are affected by predators and resources in terrestrial ecosystems, but the factors that determine when one or the other predominates remain poorly understood. Here we show, using 40 years of data from the highly diverse mammal community of the Serengeti ecosystem, East Africa, that the primary cause of mortality for adults of a particular species is determined by two factors--the species diversity of both the predators and prey and the body size of that prey species relative to other prey and predators. Small ungulates in Serengeti are exposed to more predators, owing to opportunistic predation, than are larger ungulates; they also suffer greater predation rates, and experience strong predation pressure. A threshold occurs at prey body sizes of approximately 150 kg, above which ungulate species have few natural predators and exhibit food limitation. Thus, biodiversity allows both predation (top-down) and resource limitation (bottom-up) to act simultaneously to affect herbivore populations. This result may apply generally in systems where there is a diversity of predators and prey.

Snowshoe hare populations in the boreal forests of North America go through 10-year cycles. Supplemental food and mammalian predator abundance were manipulated in a factorial design on 1-square-kilometer areas for 8 years in the Yukon. Two blocks of forest were fertilized to test for nutrient effects. Predator exclosure doubled and food addition tripled hare density during the cyclic peak and decline. Predator exclosure combined with food addition increased density 11-fold. Added nutrients increased plant growth but not hare density. Food and predation together had a more than additive effect, which suggests that a three-trophic-level interaction generates hare cycles.

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Habitat destruction has driven many once-contiguous animal populations into remnant patches of varying size and isolation. The underlying framework for the conservation of fragmented populations is founded on the principles of island biogeography, wherein the probability of species occurrence in habitat patches varies as a function of patch size and isolation. Despite decades of research, the general importance of patch area and isolation as predictors of species occupancy in fragmented terrestrial systems remains unknown because of a lack of quantitative synthesis. Here, we compile occupancy data from 1,015 bird, mammal, reptile, amphibian, and invertebrate population networks on 6 continents and show that patch area and isolation are surprisingly poor predictors of occupancy for most species. We examine factors such as improper scaling and biases in species representation as explanations and find that the type of land cover separating patches most strongly affects the sensitivity of species to patch area and isolation. Our results indicate that patch area and isolation are indeed important factors affecting the occupancy of many species, but properties of the intervening matrix should not be ignored. Improving matrix quality may lead to higher conservation returns than manipulating the size and configuration of remnant patches for many of the species that persist in the aftermath of habitat destruction.incidence function ͉ island biogeography ͉ logistic regression ͉ metaanalysis ͉ occupancy H abitat loss and fragmentation are major threats to terrestrial biodiversity (1). Globally, Ϸ40% of land has been converted for agricultural use (2), and regions as diverse as the eastern United States, the Philippines, and Ghana have lost Ͼ90% of their natural habitat (3, 4). Conservation theory and practice are founded on the principle that large habitat patches have more species than small ones and connected patches have more species than isolated ones (5). Although few would dispute this basic premise, we still do not know the general value of patch area and isolation as predictors of species occupancy in fragmented terrestrial systems. Despite hundreds of patch occupancy studies over Ͼ4 decades, there has been no quantitative synthesis of these findings. Several syntheses have examined species-area and diversity relationships (6, 7), but the species occupancy patterns that underlie diversity patterns in fragmented landscapes have been overlooked (8). How important is patch isolation relative to patch size in determining where species occur, and how consistent are these effects across diverse taxonomic groups? These are foundational, yet unanswered, questions for ecology and conservation biology.We synthesized patch occupancy data from 89 studies of terrestrial fauna on 6 continents (Table S1) to determine how patch area and isolation affect species' occurrence patterns. Collectively, these studies recorded the occurrence of 785 animal species (Table 1) in 1,015 population networks surveyed in 12,370 discrete habitat p...