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

Bobcats and Cottontails

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

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)

Gause's experiments

• Competition and predation microcosms– Paramecium and didinium

• We have a harder time studying wild vertebrates

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

Bobcats in

Eastern Kentucky

Wolf/moose: Isle Royale

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

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

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

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

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

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