Predator functional response and prey survival: direct and indirect interactions affecting a marked prey population

Abstract

Summary 1.Predation plays an integral role in many community interactions, with the number of predators and the rate at which they consume prey (i.e. their functional response) determining interaction strengths. Owing to the difficulty of directly observing predation events, attempts to determine the functional response of predators in natural systems are limited. Determining the forms that predator functional responses take in complex systems is important in advancing understanding of community interactions. 2. Prey survival has a direct relationship to the functional response of their predators. We employed this relationship to estimate the functional response for bald eagle Haliaeetus leucocepalus predation of Canada goose Branta canadensis nests. We compared models that incorporated eagle abundance, nest abundance and alternative prey presence to determine the form of the functional response that best predicted intra-annual variation in survival of goose nests. 3. Eagle abundance, nest abundance and the availability of alternative prey were all related to predation rates of goose nests by eagles. There was a sigmoidal relationship between predation rate and prey abundance and prey switching occurred when alternative prey was present. In addition, predation by individual eagles increased as eagle abundance increased. 4. A complex set of interactions among the three species examined in this study determined survival rates of goose nests. Results show that eagle predation had both prey-and predator-dependent components with no support for ratio dependence. In addition, indirect interactions resulting from the availability of alternative prey had an important role in mediating the rate at which eagles depredated nests. As a result, much of the within-season variation in nest survival was due to changing availability of alternative prey consumed by eagles. 5. Empirical relationships drawn from ecological theory can be directly integrated into the estimation process to determine the mechanisms responsible for variation in observed survival rates. The relationship between predator functional response and prey survival offers a flexible and robust method to advance our understanding of predatorprey interactions in many complex natural systems where prey populations are marked and regularly visited.

Anthropogenic resource subsidies decouple predator–prey relationships

“…We suggest that the lack of concordance between rates of nest predation and predator activity in urban landscapes arises because many synanthropic predators are heavily subsidized by anthropogenic food sources (Gehrt 2004, Marzluff and Neatherlin 2006, Withey and Marzluff 2009) and, consequently, may depredate fewer nests than less subsidized rural predators. Both theoretical (Schmidt 1999) and empirical studies (Miller et al 2006) suggest that availability of alternative foods for predators can depress rates of nest predation. In our system, most species of nest predators are omnivorous, opportunistic generalists (e.g., raccoon, American Crow) that are known to regularly consume anthropogenic foods in metropolitan areas.…”

Prey Behavior, Age‐Dependent Vulnerability, and Predation Rates

“…If coyotes live in an area with a preferred prey species that is readily available in winter (e.g., snowshoe hares [Lepus americanus]), we would expect coyotes to reduce the time spent hunting ungulates in winter, which should benefit mule deer. As with other forms of alternative prey (Patterson et al 1998;Cooper et al 1999;Ackerman 2002;Prugh 2005;Miller et al 2006), the presence of the second deer species would be likely to influence predation on the first (Robinson et al 2002) and should be considered in future models.…”

“…Seasonal and annual variation in the time predators devote to hunting a particular prey species might arise in response to a few factors, including variation in the availability of alternative prey (Lingle 2000;Miller et al 2006). We plotted three scenarios showing seasonal variation in the time one coyote pack spends hunting deer per day (t h , expressed as hunt h day Ϫ1 ): (1) coyotes spend more time hunting deer in summer (1.0 h day Ϫ1 ) than in winter (0.25 h day Ϫ1 ), (2) coyotes spend equal time hunting deer in summer and in winter (0.5 h day Ϫ1 ), and (3) coyotes spend more time hunting deer in winter (1.0 h day Ϫ1 ) than in summer (0.25 h day Ϫ1 ).…”

“…Not surprisingly, the prey species suffering the highest annual predation rates was the one that was most vulnerable at the time of year when predators devoted the most time to hunting. We introduced seasonal variation in hunt time to our model to reflect shifts in hunt activity that a generalist predator could make in response to seasonal variation in alternative prey (e.g., Miller et al 2006). Other factors, including the size and behavior of predators (Paquet 1992;Kunkel and Pletscher 1999;Pierce et al 2000;Downes 2002;Essington and Hansson 2004) and the intrinsic vulnerability of young animals themselves (Kruuk 1972;Patterson et al 1998), may also lead to seasonal variation in the attention a predator devotes to any particular prey species.…”

“…However, prey vulnerability is not the only source of temporal variation that affects predation rates. The size and hunting tactics of a predator can affect the size or age of prey it captures (Paquet 1992;Pierce et al 2000;Downes 2002;Essington and Hansson 2004), and seasonal variation in alternative prey can affect the time of year when a predator hunts any particular species (Patterson et al 1998;Cooper et al 1999;Ackerman 2002;Miller et al 2006). A prey species will presumably suffer the greatest impact from predators that target the animal at the time of year when it is most vulnerable.…”