Consumptive and nonconsumptive effects of cannibalism in fluctuating age‐structured populations

Abstract

Abstract. Theory and empirical studies suggest that cannibalism in age-structured populations can regulate recruitment depending on the intensity of intraspecific competition between cannibals and victims and the nature of the cannibalism window, i.e., which size classes interact as cannibals and victims. Here we report on a series of experiments that quantify that window for age-structured populations of salamander larvae and paedomorphic adults. We determined body size limits on cannibalism in microcosms and then the consumptive and nonconsumptive (injuries, foraging and activity, diet, growth) effects on victims in mesocosms with seminatural levels of habitat complexity and alternative prey. We found that cannibalism by the largest size classes (paedomorphs and !age 3þ yr larvae) occurs mainly on young-of-the-year (YOY) victims. Surviving YOY and other small larvae had increased injuries, reduced activity levels, and reduced growth rates in the presence of cannibals. Data on YOY survival in an experiment in which we manipulated the density of paedomorphs combined with historical data on the number of cannibals in natural populations indicate that dominant cohorts of paedomorphs can cause observed recruitment failures. Dietary data indicate that ontogenetic shifts in diet should preclude strong intraspecific competition between YOY and cannibals in this species. Thus our results are consistent with previous empirical and theoretical work that suggests that recruitment regulation by cannibalism is most likely when YOY are vulnerable to cannibalism but have low dietary overlap with cannibals. Understanding the role of cannibalism in regulating recruitment in salamander populations is timely, given the widespread occurrences of amphibian decline. Previous studies have focused on extrinsic (including anthropogenic) factors that affect amphibian population dynamics, whereas the data presented here combined with long-term field observations suggest the potential for intrinsically driven population cycles.

Larval growth in polyphenic salamanders: making the best of a bad lot

“…Thus, the cohorts (e.g., 1990, 1991) that trail large cohorts (e.g., 1988) develop in the context of high densities of similarly sized conspecifics and depressed resources (both conditions favoring competition). Experimental data also indicate that individuals in those trailing cohorts are vulnerable to the lethal and non-lethal effects associated with cannibalism by large larvae and paedomorphs (Wissinger et al 2010). Non-lethal effects of the threat of cannibalism such as reduced activity levels and reduced foraging contribute to reduced growth rates of small larvae, reinforcing the poor growth conditions that lead to best of a bad lot paedomorphosis.…”

“…In large cohorts that develop during years with few large larvae or paedomorphs, within-cohort competition among similarly-sized larvae is the most likely mechanism that underlies slow growth, hence paedomorphosis, whereas in cohorts that mature in the presence of large larvae or paedomorphs, between-cohort competition and canniba- Fig. 3a were immature at capture, which reduced the sample size of metamorphs in this figure lism are likely to play a larger role (Wissinger et al 2010). Thus, there is a strong positive relationship between size structure and paedomorphosis (Fig.…”

“…After becoming paedomorphic, only a few individuals (<3%) successfully metamorphose later in life (H. Whiteman, unpublished data). Growth and abundance of larval cohorts vary considerably across years and ponds, depending on the number of eggs laid in each pond and environmental effects such as pond drying and cannibalism (Wissinger and Whiteman 1992;Whiteman and Wissinger 2005;Wissinger et al 2010).…”

“…We present data for the fate (maturation as a metamorphic or paedomorphic adult) of individuals from the following pond-cohorts for which there were sufficient data for statistical analysis: the 1988 cohort in ponds 1, 3, 5, 9 and 12; the 1990 cohort in ponds 1 and 12; and the 1991 cohort in pond 9. Low recruitment and/or survival to the second year precluded analysis for other pond-cohorts (Whiteman and Wissinger 2005;Wissinger et al 2010).…”

“…Density estimates were generated within a cohort, which made up the majority of the total salamander density within the ponds, as well as among larger and/or smaller, non-cohort larvae (i.e., slightly older cohorts that were not paedomorphic as well as younger cohorts). Paedomorphic adults were the primary predators of larvae in permanent ponds (Whiteman and Wissinger 2005;Wissinger et al 2010); thus, we used paedomorphic adult densities as estimates of predation pressure. Metamorphic adult densities were not included because they were very low across permanent ponds and because metamorphs feed on different prey (Whiteman et al 1995(Whiteman et al , 1996Wissinger et al 1999;Denoël et al 2007).…”