Predators and their prey

1. Predators and their prey • Numerical response • The change in number of predators in response to the change in abundance of their prey • Has a stabilizing effect on prey abundance • Functional response • The change in feeding rate by a predator on a type of prey in response to a change in prey abundance • Can have a destabilizing effect on prey abundance • Consider bobcats and cottontails

2. Bobcats and Cottontails

3. Predator Selectivity • Taking only old, weak, diseased, injured, or young • Impact on prey abundance is less than one would expect based purely on number killed by predators • How might predators actually help prey populations? • Possible reduction in intraspecific competition • Predators can allow coexistence of multiple prey species that would otherwise exclude each other • Competitive exclusion principle

4. Competitive exclusion principle • No two species can occupy exactly the same niche • No two species can use the same limited resources in the same way at the same time • If they do, one will tend to exclude the other • They may coexist if the resource is not limiting or they somehow partition the resource • A predator may reduce the number of the most abundant species, allowing the weaker competitor to exist (resources are no longer limiting)

5. Gause's experiments • Competition and predation microcosms • Paramecium and didinium • We have a harder time studying wild vertebrates

6. Home-range and Territoriality • Behavior may prevent overexploitation of prey • Home range • Area visited by an individual animal on a regular basis within which it obtains all of its needs • Territory • Area protected by an individual animal for its exclusive use • Breeding, feeding, nesting, and other types • May exclude all competitors, like species, or like sex only

7. Bobcats in Eastern Kentucky

8. Wolf/moose: Isle Royale

9. Wolf/moose: Isle Royale • 570 km2 (220 mi2) • logged around 1900 • All game was eliminated • 1912 – moose crossed the ice to the island • Early-successional forest was ideal habitat • No predators! • Population exploded to about 3000 by 1940 • There was an obvious browse line

10. Wolf/moose: Isle Royale • Wolves crossed the ice in 1949 • Reached a “steady state” • about 22 wolves and 600 moose • approximately a 30:1 prey:predator ratio • Wolves killed about 150 moose/yr • 25% of the moose population • About 7 moose per wolf per year • Wolves appeared to limit moose to a level below what food resources would allow during this period

11. Wolf/moose: Isle Royale • Territoriality and pack behavior probably limited wolf numbers • intrinsic population control • Wolf numbers dropped to 12 by 1992 • diseases • distemper • possible inbreeding • Moose population exploded to 2500 then crashed

12. Do wolves limit the Isle Royale moose populations? • Perhaps at some times but not others.

13. Wolves and Moose, caribou, and deer • Based on Isle Royale data and other studies in Canada and Alaska • Wolves limit these prey, but only when prey:predator ratio drops due to additional factors

14. We need to restore ecosystem integrity • Government kills predators in some instances (wolves and coyotes) and restores them in others (wolves, grizzly bears) • Hmm • Need to consider the ecosystem approach