Factors affecting individual foraging specialization and temporal diet stability across the range of a large “generalist” apex predator

Abstract

Individual niche specialization (INS) is increasingly recognized as an important component of ecological and evolutionary dynamics. However, most studies that have investigated INS have focused on the effects of niche width and inter- and intraspecific competition on INS in small-bodied species for short time periods, with less attention paid to INS in large-bodied reptilian predators and the effects of available prey types on INS. We investigated the prevalence, causes, and consequences of INS in foraging behaviors across different populations of American alligators (Alligator mississippiensis), the dominant aquatic apex predator across the southeast US, using stomach contents and stable isotopes. Gut contents revealed that, over the short term, although alligator populations occupied wide ranges of the INS spectrum, general patterns were apparent. Alligator populations inhabiting lakes exhibited lower INS than coastal populations, likely driven by variation in habitat type and available prey types. Stable isotopes revealed that over longer time spans alligators exhibited remarkably consistent use of variable mixtures of carbon pools (e.g., marine and freshwater food webs). We conclude that INS in large-bodied reptilian predator populations is likely affected by variation in available prey types and habitat heterogeneity, and that INS should be incorporated into management strategies to efficiently meet intended goals. Also, ecological models, which typically do not consider behavioral variability, should include INS to increase model realism and applicability.

Short-term shifts of stable isotope (δ13C, δ15N) values in juvenile sharks within nursery areas suggest rapid shifts in energy pathways

“…Recent advances in analytical methods also enable individual-level investigations of diet changes that can provide complementary information to population-level studies, providing insight into individual differences in the speed at which foraging develops in juvenile animals (see Layman et al, 2012 for a review). For example, comparing plasma and red blood cell δ 13 C values, Rosenblatt et al (2015) found that American alligators (Alligator mississippiensis) in the southeastern United States have very stable mixtures of prey taxa in their diets, but individuals can exhibit considerable intraspecific dietary differences, creating individual variation in the topdown effects alligators exert. Further developing methods to quantify individual-level changes in trophic interactions provides a means to test hypotheses generated from population-level studies and gain greater insight at the level of individuals, which is critical for understanding the role phenotypic variability plays in population dynamics and the ecological niches species fill (Bolnick et al, 2011;Sih et al, 2012).…”

Foraging strategies of a generalist marine predator inhabiting a dynamic environment

“…This variability may arise as a result of intrinsic factors such as individual differences in physiology, morphology, or behavior (Killen et al 2011;Patrick and Weimerskirch 2014;Hoskins et al 2015), or from external factors such as prey and habitat availability (Svanbäck and Bolnick 2005;Kernaléguen et al 2015;Newsome et al 2015;Rosenblatt et al 2015). Population-level descriptions largely overlook this individual variability, which may result in incomplete descriptions of foraging behavior and obscure the responses of individuals and populations to environmental variability.…”

Urinary iodine and stable isotope analysis to examine habitat influences on thyroid hormones among coastal dwelling American alligators

“…Prey species used in end member calculations were selected based on previous diet studies of coastal alligator populations (Nifong et al, 2015; Nifong et al, 2012; Rosenblatt et al, 2015). The posterior distributions produced by SIAR provide plausible solutions for the proportional contribution of prey sources to the diet of a consumer and incorporates variation in consumer and prey isotope values as well as variation in discrimination factors to provide robust estimates of dietary patterns.…”