Functional and Numerical Responses of Predators: Where Do Vipers Fit in the Traditional Paradigms?

Abstract

Snakes typically are not considered top carnivores, yet in many ecosystems they are a major predatory influence. A literature search confirmed that terrestrial ectotherms such as snakes are largely absent in most discussions of predator-prey dynamics. Here, we review classical functional and numerical responses of predator-prey relationships and then assess whether these traditional views are consistent with what we know of one group of snakes (true vipers and pitvipers: Viperidae). Specifically, we compare behavioural and physiological characteristics of vipers with those of more commonly studied mammalian (endothermic) predators and discuss how functional and numerical responses of vipers are fundamentally different. Overall, when compared to similar-sized endotherms, our analysis showed that vipers have: (i) lower functional responses owing primarily to longer prey handling times resulting from digestive limitations of consuming large prey and, for some adults, tolerance of fasting; (ii) stronger numerical responses resulting from higher efficiency of converting food into fitness currency (progeny), although this response often takes longer to be expressed; and (iii) reduced capacity for rapid numerical responses to short-term changes in prey abundance. Given these factors, the potential for viperids to regulate prey populations would most likely occur when prey populations are low. We provide suggestions for future research on key issues in predator-prey relationships of vipers, including their position within the classical paradigms of functional and numerical responses.

Potential environmental influences on variation in body size and sexual size dimorphism among Arizona populations of the western diamond-backed rattlesnake (Crotalus atrox)

“…Thus, when only small prey are available, it is less costly to remain relatively small, but when large prey species are available, snakes are predicted to grow sufficiently large to exploit those prey sources. Although we lack information on prey type and abundance for most of our sites, the available data suggest that C. atrox may not follow this pattern (Nowak, 2009;Repp and Schuett, 2009;Spencer, 2003). In her examination of 561 museum specimens of C. atrox from across their geographic range, Spencer (2003) found no relationship between body size and type of prey (mammal, reptile, bird, or frog) consumed.…”

“…The diet-alteration hypothesis, for example, predicts that snakes will be larger when larger types of prey, usually mammals or birds, are available, and smaller when only small types of prey (e.g., squamates) are available (Boback, 2003;Forsman, 1991). Furthermore, some snake species appear to reduce (or even eliminate) small prey from their diet as they increase in body size, which is probably due to relative costs associated with pursuit, ingestion, and digestion of one large meal versus many smaller ones (Forsman, 1996;Nowak et al, 2008;Shine et al, 1998; but see Repp and Schuett, 2009). Thus, when only small prey are available, it is less costly to remain relatively small, but when large prey species are available, snakes are predicted to grow sufficiently large to exploit those prey sources.…”

Could controlling mammalian carnivores lead to mesopredator release of carnivorous reptiles?

“…Investigations into the impacts of mammalian carnivore management have generally focused on mammalian apex predators and mammalian mesopredators and have largely ignored other mesopredators such as large carnivorous reptiles [14,15]. Varanid lizards are the paramount terrestrial reptilian predator across the Old World and appear to play a role similar to small-to medium-sized carnivorous mammals [16 -18].…”

Lagomorphs As Prey of Western Diamond-Backed Rattlesnakes (Crotalus atrox) In Arizona

“…The western diamond-backed rattlesnake (Crotalus atrox) is a strict carnivore and an important meso-predator across a variety of biotic communities in the Southwest (Nowak et al, 2008). The diet of C. atrox is welldocumented in the United States and includes a variety of mammalian, reptilian, and avian prey (reviewed by Ernst and Ernst, 2011).…”

“…Because larger snakes have higher energetic needs, which may influence foraging strategies more so than do limitations of gape (Vincent and Mori, 2008), it is reasonable to suggest that they will seek out the largest prey possible, resulting in an ontogenetic shift (King, 2002). This can lead to C. atrox having an outsized predatory effect (Nowak et al, 2008).…”