Predators and Predation

Low success rates affect species behavior

• Carnivores are generally territorial 

  • Found in low densities

• Social groupings that hunt together may increase hunt success rate

predator management

• Predators, as well as prey species, require management in a highly human-modified habitat

• Management methods may include:

  • Protection from persecution

  • Controlled harvest

  • Habitat manipulation

  • Predator reductions by lethal or non-lethal methods

Snowshoe Hare/Lynx

• Snowshoe hare/lynx classic case of population oscillations—but is it because of predation?

  • Hares, with no predation, still oscillate

  • Other factors are not constant 

    • social status

    • population density

    • food availability

    • disease

    • other predators

    • habitat changes

    • climate, etc.

Type I functional response of predator

• Searches randomly for prey

  • Unlimited appetite—eats more, as prey density increases

  • Eats a constant proportion of prey population, no matter the density

  • Example:  Filter feeders (e.g. baleen whale)

Type II functional response of predator:

• Proportion of prey eaten is a function of the prey density

  • Number eaten increases to a point and then stabilizes

  • Proportion eaten decreases as prey density increases

  • Example:  Lynx-hare, spiders

Type III functional response of predator

• # of prey caught per predator increases slowly at low prey densities (eating a substitute prey when prey density is low)

• Fast at intermediate prey densities

• Levels off at high prey densities

• Produces an S-Shaped curve

• % of prey population eaten increases a first, then declines

• Example:  Most vertebrates

what do those two graphs mean

Predation tends to be compensatory (when prey density in graph is below A or above B prey density), rather than additive mortality

If reproduction of prey exceeds predation, then predation is not limiting

If predation exceeds prey population growth (X in figure to right), then predation limiting

Aldo Leopold proposed that there are 5 significant factors in predator-prey interactions:

• 1. Density of prey population

• 2. Density of predator population

• 3. Characteristics of prey 

• 4. Density and quality of alternative food for predator

• 5. Characteristics of predator 

prey responses to predation (3)

• Migration

• Forming groups, herds or flocks

  • Synchronizing births—predator swamping

• Becoming solitary vs. gregarious

  • Female ungulates may give birth away from group 

    • Predators may be searching where prey density is highest

wolf control in alaska

• Wolf culled in 1970s to increase moose after decline 

  • Decline due to over-hunting or severe winter or severe predation?

• Did it work?

  • Moose increased

  • Cause of decline?

    • Human hunting also a factor

    • Severe weather a factor in original decline

  • Moose would have likely recovered on own, but culling wolves expedited process

• “Balance of Nature” a myth

  • Systems are not in equilibrium

  • Major fluctuations can result in extinctions

domestic livestock predation

• If prey numbers low, predators often switch from primary prey to another prey species (called buffer species)

  • Human-wildlife conflicts may result when switch is to livestock 

    • Coyote may shift from jackrabbits to sheep when rabbit numbers are low

• coyote predation

  • Economic loss equivalent to $14 million

  • Ewe losses an additional $5 million

  • Total losses $19 million/year

• Control methods include: (4)

• Trapping

• Den hunting:

• Shooting 

• Poisoning 

effectiveness of coyote control

• 70,000-85,000 coyotes killed annually 

  • 62,000—2020 by USDA/Wildlife Services

Coyote response to removals:

• Reproduce earlier

• Reproduce more frequently

• Larger litter sizes

Animal Damage Control-1931

• 1986 its role moved from USFWS to USDA—now called USDA/APHIS “Wildlife Services”

TWS position (5)

• Must be justified biologically, socially and economically

• Encourage research

• Control efforts should be minimum required to bring damage to tolerable levels

• Use on most efficient, safe, economical, and humane methods—use lethal methods only when all other methods failed

• Regulate control methods closely