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4/8/13 Predator–Prey Relationships and Management Chapter 13 Chapter Objectives Introduction Predators Predator Behavior Habitat Interactions Predator-Prey Relationships in Theory Assessing Predation Rates Predator Control Population Limitation, Regulation, and Trophic Cascades Summary 1 4/8/13 Predators Many animals are opportunistic predators We will focus on mammals in the Order Carnivora and for birds, the raptors Also will include all snakes and lizards Historically, predators considered to limit game species Predator control was common Predators Little work on science of predators before 1930s Leopold began working on density-dependent and density-independent factors in population dynamics Main components: Density of prey Density of predators Predator food preference Physical condition of prey Abundance of alternative prey 2 4/8/13 Predators Errington suggested that predation only limited prey when prey populations were above K Theory of compensatory and additive mortality Predation can occur at high rates with little effect on subsequent breeding population of prey Early work on predator-prey relationships found that demographic information was critical to understanding Predators Most work historically has focused on the influence of predators on game species, species of concern, economics, or depredation Recent work has taken an ecosystem approach Predation is difficult and expensive to study Studies of prey remains and diet only show what was successfully captured Success and failure rates difficult to gather 3 4/8/13 Predator Behavior Territorial behavior by predators can keep population size below what prey can support Strongly defined boundaries for territories yearround for mammals Only during breeding season in raptors Interspecific competition among predators can reduce foraging success of individuals Predator Behavior Prey Selection For birds, mostly sight and sound For mammals, mostly sight and smell Pit vipers use infrared and bats use echo location 4 4/8/13 Predator Behavior Prey capture types Ambush predator Pursuit predator Systematic search predator Most predators are solitary when hunting Social predators include wolves and Harris’ Hawks Predator Behavior Prey use is influenced by: Presence of alternate prey Size of prey populations Age-specific vulnerability Sex-specific vulnerability Specializations of the predator Environmental conditions Competitive exclusion can be a factor Coyotes killing swift foxes and northern goshawks killing Cooper’s hawks 5 4/8/13 Predator Behavior Generalist predators Most predators are generalists Capture wide variety of prey Generally take prey in relation to availability, not abundance Prey switching occurs relative to availability Specialist predators Could not persist without main prey Black-footed ferret – prairie dog; lynx – snowshoe hare; snail kite - snail Predator Behavior Prey vulnerability impacts prey selection and predator success Factors include age, sex, physical condition, environmental condition, activity, group size Management efforts can have unintended effects when attempting to increase prey populations 6 4/8/13 Habitat Interactions Habitat changes can lead to increased impacts of predation on prey Loss of refuges and escape terrain Human impacts such as fire suppression can cause changes in successional stages Drought can concentrate animals at water Management practices can increase predation Predator-Prey Relationships in Theory Lotka-Voltera predator-prey models Model assumptions Prey populations experience exponential or logistic growth Predation rate is influenced by the functional and numerical response of the predator Functional response is the changing rate at which prey are removed as prey density changes Numerical response is change in predator density due to prey availability 7 4/8/13 Predator-Prey Relationships in Theory Predator-Prey Relationships in Theory Functional response types Type I – Number of prey consumed by predators increase linearly as prey populations increase, up to the point of limitations of handing and digestion Type II – Incorporates restraints such that the number of prey consumed reaches a threshold as prey densities increase Type III – Low predation rates occur when prey population is low, but rates increase exponentially when prey populations reach threshold density 8 4/8/13 Assessing Predation Rates Very difficult to assess predation rates because of predator mobility and secretiveness Can use radio-tagged predators and prey For prey, monitor survival and identify sources of mortality Radios can impact predation vulnerability Raptor diets analysis biased to nesting time Assessing Predation Rates Limited ability to assign predator to specific kills Possible for cougars and wolves More difficult for raptor kills Video monitoring now used for raptor nests, starting to be used for ungulate collars Direct observations of raptors also used in open areas 9 4/8/13 Assessing Predation Rates Predation rates also modeled using daily food requirements, biomass of prey, and proportion of prey in diet Must consider portion of prey not consumed Portion not consumed is highly variable Daily food requirement is highly variable Predator Control Very controversial Originally undertaken to reduce livestock losses Commonplace in 19th century to allow increases in game populations Mostly used today to benefit threatened or endangered species; still used to benefit game Lethal and nonlethal means exist today 10 4/8/13 Predator Control What are objectives? What is best strategy to achieve objectives? Difficult to reliably predict outcome of predator control on game populations Dynamic systems with varying interactions Many practical factors also, such as time and money Predator Control Need to establish criteria if predator control to be used Do predators kill substantial numbers that would otherwise survive? Will reduced predation facilitate reliably higher abundances or harvests of prey? Given less predation, can habitats support more prey? Predators will exist in and out of control area If population is near K, predator control will have little effect 11 4/8/13 Predator Control Predator control can have positive impact on prey population Survival of neonates For continued impact, predator control must continue Unintended consequences include increases in undesirable small mammal populations Rodents preying upon waterfowl eggs, loss of riparian vegetation, overgrazing by ungulates Predator Control Predatory birds usually controlled by nonlethal means Sometimes conflicts between 2 species of special concern Translocation being used as nonlethal control Hard vs soft release Decreased survival of translocated animals Many areas are at K for predators 12 4/8/13 Population Limitation, Regulation, and Trophic Cascades Do predators limit or regulate prey populations? Multiple hypotheses exist Recurrent fluctuations Low-density equilibria Multiple stable states Stable-limit cycles It is difficult to sort these out as the effect of humans has greatly altered natural systems Population Limitation, Regulation, and Trophic Cascades Well-designed studies needed to evaluate the impacts of predator control The important measure is how the prey population responds demographically Must know which animals would have died anyway when looking at predation before and after predator control 13 4/8/13 Population Limitation, Regulation, and Trophic Cascades Trophic structure and energy flow Predators eat lower level consumers – do they influence conditions at the producer level? Experiments have shown that removal of predators can have serious effects on lower levels Top-down interactions known as trophic cascade Now being studied via reintroduction of wolf in Yellowstone Summary Predator control will remain controversial Historically, predator control was thought as linear increase in game species Now understand predation as an important part of a community Modeling has helped us understand predator-prey dynamics Predation can limit and regulate prey Further study is needed to identify demographic effects on prey from predators and their control 14
