Switching in General Predators: Experiments on Predator Specificity and Stability of Prey Populations

Abstract

"Switching" in predators which attack several prey species potentially can stabilize the numbers in prey populations. In switching, the number of attacks upon a species is disproportionately large when the species is abundant relative to other prey, and disproportionately small when the species is relatively rare. The null case for two prey species can be written: P1/P2 = cN1/N2, where P1/P2 is the ratio of the two prey expected in the diet, N1/N2 is the ratio given and c is a proportionality constant. Predators were sea—shore snails and prey were mussels and barnacles. Experiments in the laboratory modelled aspects of various natural situations. When the predator had a strong preference (c) between prey the data and the "null case" model were in good agreement. Preference could not altered by subjecting predators to training regimens. When preference was weak the data did not fit the model replicates were variable. Predators could be trained easily to one or other prey species. From a number of experiments it was concluded that in the weak—preference case no switch would occur in nature except where there is an opportunity for predators to become trained to the abundant species. A patchy distribution of the abundant prey could provide this opportunity. Given one prey species, snails caused a decreasing percentage mortality as prey numbers increased. This occurred also with 2 prey species present when preference was strong. When preference was weak the form of the response was unclear. When switching occurred the percentage prey mortality increased with prey density, giving potentially stabilizing mortality. The consequences of these conclusions for prey population regulation and for diversity are discussed.

Not attackable or not crackable—How pre‐ and post‐attack defenses with different competition costs affect prey coexistence and population dynamics

“…Furthermore, the type of defense costs regarding competitiveness (resource uptake affinity or growth rate) plays an important role for coexistence. Theory already showed that predator‐mediated coexistence crucially depends on the environmental conditions (Chase et al., 2002), for example, the enrichment level (Genkai‐Kato & Yamamura, 1999; Leibold, 1996; Proulx & Mazumder, 1998), the prey switching behavior of the predator (Abrams & Matsuda, 1993; Fryxell & Lundberg, 1994; Murdoch, 1969), the magnitude of the trade‐off between defense and competitiveness (Abrams, 1999; Kasada, Yamamichi, & Yoshida, 2014; Tirok & Gaedke, 2010), and the difference in both the defense level and the competitiveness between the prey types (Becks, Ellner, Jones, & Hairston, 2010; Ehrlich, Becks, & Gaedke, 2017; Jones & Ellner, 2007). However, the role of different defense mechanisms and competition costs in prey communities remains unclear but holds promise to be decisive for their coexistence and the occurring population dynamics.…”

Prey switching by Acartia clausi:experimental evidence and implications of intraguild predation assessed by a model

“…This relates to the fact that the per capita prey mortality inflicted by a predator increases with prey concentration when the predator displays a type 3 response. This response is also termed a switching response, following the definition of Murdoch (1969): 'As a prey become relatively more abundant, switching occurs if the relative amount which that species forms of the predator's diet increases disproportionately in comparison with the expected amount'. Copepods have commonly been recognized as grazers which have non-increasing clearance rates with increasing p]-ey concentratlons (e.g.…”

Evolutionary consequences of a search image

“…If we make the reasonable assumption that a search image would be formed for the more abundant prey type, this leads to a disproportionally high predation rate on the more abundant prey type and, thus, to prey switching (Murdoch, 1969;Oaten and Murdoch, 1975;Van Leeuwen et al, 2007). Search images cause prey switching by a predator, which leads to coexistence between prey species even if the prey species are in direct competition (Murdoch, 1969), such behaviour could also lead to polymorphism in prey species (Allen et al, 1998;Bond and Kamil, 2006). While many predators are able to form a search image, it seems that there is a trade off.…”