General Forest Predator Clearinghouse

Introduction

"Extinction rates are acknowledged to be hundreds or thousands of times higher today than they were in the prehuman past…" 39

"… widespread elimination of top predators from terrestrial ecosystems the world over has disrupted the feedback process through which predators and prey mutually regulate each other’s numbers." 40

"The role of predation has become a matter of intense interest to conservationists because mounting evidence, as we shall see, points to the pivotal role of predation in helping to preserve the biodiversity of terrestrial communities.

Theory

"Top-down [predator] effects have been shown to act on communities in two fundamentally different ways. One is through preferential feeding on a prey species that, in the absence of predation, is capable of competitively excluding other species that depend on a limiting resource… we refer to this process as the ‘Paine effect’…
The second way in which predators influence their communities is through a cascade of interactions extending through successively lower trophic levels to autotrophs at the base of the food web…" 42

Empirical Foundations

The Paine Effect

"By removing the predatory starfish Pisaster ochraceous from sections of the intertidal zone of the rocky Washington coastline, [Paine] showed that the diversity of the attached invertebrates subsequently declined as a superior competitor, the mussel Mytilus californicus, gradually occupied all available space, thereby excluding other species from the community [Mytilus is the preferred prey of Pisaster]… The primary effect of a top predator in the intertidal system is thus seen in regulating the diversity of the prey community. This is the Paine effect." 43

Anecdotal Evidence

Herbivore Release onto Predator-Free Islands

"Sailors of yore introduced herbivores to predator-free islands throughout the Seven Seas to ensure themselves of a supply of meat on subsequent voyages… in numerous instances… the introduced herbivores increased without check until they devastated the native vegetation of the island—at which point populations of the herbivores themselves often crashed." 44

Predator Elimination

"The problem of mammalian overabundance in predator-free portions of North America has become so widespread and so severe that it was recently the topic of a major symposium hosted by the Smithsonian Institution." 45

Predator Introduction

- sea otter recovery led to sharp declines of benthic grazers, that brought back the kelp forests 47
- introduction of alien top predators has devastated freshwater aquatic systems (sea lamprey in the Great Lakes, Nile perch in E. Africa, rainbow trout in the Andes, peacock bass in Panama) 47
- exotic predators have wreaked havoc on predator-free islands (mongooses on Pacific islands, brown tree snake on Guam, domestic cats in Australia, foxes in boreal to arctic regions) 47
- gray wolves have decreased elk and white-tailed deer densities in North America; wolves have influenced beaver movements in the northern Midwest 47-48

Long-term Monitoring of Predator/Prey Interactions

- Isle Royale example: gray wolves influenced moose numbers, which in turn influenced growth of balsam fir. 48

Experimental Evidence

Barro Colorado Island example, Panama (1600-hectare island created by flooding during the construction of the Panama Canal):
"The higher densities of medium and large mammals on BCI have been interpreted as evidence of a top-down effect resulting from missing top predators… This conclusion, however, has been questioned…" 49

Lago Guri islands example, Venezuela (one of the world’s largest hydroelectric impoundments made hundreds of islands out of hilltops):

"Roughly 75 to 90 percent of the species of terrestrial vertebrates that occupy the same forest type on the mainland were absent from the islands between 1 and 10 hectares in size within seven years after isolation… The absence of many species and the hyperabundance of others has created animal communities unlike any that would ever occur naturally—communities that are grotesquely imbalanced from a functional standpoint." 50

Wisconsin Lakes example
"Removal of piscivorous fish (large-mouthed bass, the top carnivore in this system) leads to order-of-magnitude increases in planktivorous fish, decreases in the size and number of zooplankton (cladocerans), and strong increases in the standing crop of phytoplankton in a textbook top-down trophic cascade. 51

Countercurrents

"… one would be wrong to conclude that predators limit the numbers of all consumers." 52

"Sheer size enables a few of the world’s largest mammals to escape predation… Some species are able to reduce (but not eliminate) predation through social mechanisms. The list of these mechanisms is long. It includes the formation of herds and flocks, sentinel behavior, and the giving of alarm calls…" 53

"In the world at large… productivity-limited (pure bottom-up) systems appear to be rare. Moderate to strong top-down regulation appears to be the norm for terrestrial ecosystems." 54

Indirect Effects and Trophic Cascades

"Having made the case for top-down regulation as a nearly ubiquitous force in terrestrial ecosystems, we now ask about the role played by top predators in maintaining ecosystem integrity." 54

"In many parts of North America, extirpation of dominant predators has resulted in a phenomenon known as ‘mesopredator release’ in areas supporting small to midsized predators (foxes, skunks, raccoons, opossums, feral and domestic housecats…)." 54-55

"Extirpation of top predators has released herbivore populations in parts of the United States with consequences that are just beginning to come to light." 55
- white-tailed deer are altering tree regeneration in eastern forests
- introduced ungulates, like Eurasion boar, are altering tree regeneration patterns
- in Lago Guri, the predator-free islands are seeing 3 generalist herbivores increase by more than an order of magnitude (howler monkeys, iguanas, leaf-cutter ants), with devastating effects on tree regeneration

"The most severe impacts of hyperabundant mesopredators and consumers appear in localities where predators are absent and hunting and trapping are prohibited." 56

"Predators prevent prey populations and mesopredators from exploding into hyperabundance while rarely, if ever, driving prey to extinction… The operation of such feedback mechanisms can be likened to "a balance of nature." Nature stays in balance so long as a fauna remains intact and the full suite of ecological processes operates unhindered." 57

Another Key to Biodiversity

"The evidence reviewed here overwhelmingly supports the strong top-down role of top carnivores in regulating prey populations — and thereby stabilizing the trophic structure of terrestrial ecosystems." 58

"In sum, then, our current knowledge about the natural processes that maintain biodiversity suggests a crucial and irreplaceable regulatory role of top predators. The absence of top predators appears to lead inexorably to ecosystem simplification accompanied by a rush of extinctions." 58