Allee effects, mating systems and the extinction risk in populations with two sexes

Abstract

The Allee effect is one of the population consequences of sexual reproduction that has received increased attention in recent years. Due to its impact on small population dynamics, it is commonly accepted that Allee effects should render populations more extinction prone. In particular, monogamous species are considered more susceptible to the Allee effect and hence, more extinction prone, than polygamous species. Although this hypothesis has received theoretical support, there is little empirical evidence. In this study, we investigate (1) how variation in tertiary sex ratio affects the presence and intensity of the Allee effect induced by mating system, as well as (2) how this effect contributes to extinction risk. In contrast with previous predictions, we show that all mating systems are likely to experience a strong Allee effect when the operational sex ratio (OSR) is balanced. This strong Allee effect does not imply being exceptionally extinction prone because it is associated with an OSR that result in a relatively small extinction risk. As a consequence, the impact of Allee effects on overall extinction risk is buffered. Moreover, the OSR of natural populations appears to be often male biased, thus making it unlikely that they will suffer from an Allee effect induced by mating system.

Sex ratio bias, male aggression, and population collapse in lizards

“…Longer-term monitoring of another set of experimental populations (18) showed similar amplification of sex ratio bias (40% initially, 61% after 4 years, n ϭ 16), parallel with a sustained population decline and high extinction rates (18). Thus, the deleterious demographic effects of male excess on females raise a major threat to population persistence that had not been previously recognized (2,13,14). To quantify this extinction risk and to analyze its sensitivity to male and female behavior, we constructed a stage-structured stochastic population model.…”

“…In fact, males and females often differ in their vital rates, density dependence, and sensitivity to the environment (9). Nonmanipulative studies have identified possible consequences of sex structure on population dynamics (9,11), including reproductive collapse after male rarity (13). By experimental manipulation, long-term monitoring, and mathematical projections of populations of common lizards, we demonstrate rigorously that male-biased ASRs exacerbate male aggression and become deleterious to females, which amplifies further the sex ratio biases toward males and leads to a positive feedback of population decline, that is, an extinction vortex.…”

“…Population and evolutionary theory has long relied on the hypothesis of ASR stability, and thereby kept the issue of ASR dynamics largely out of the spotlight (2,(12)(13)(14). Our results demonstrate that ASR dynamics can have profound consequences for individual behavior and ultimately for population dynamics.…”

“…However, although considerable efforts have been devoted to documenting the occurrence of intrasexual competition, studies of its impact on population characteristics are scant (7) and all are correlative (9,11). Thus, whereas the need to probe the long-term social and population consequences of ASR biases has been raised repeatedly (5,(12)(13)(14), how individuals respond to ASR variation and how their response feeds back into population dynamics remains poorly understood (7,9). Using the common lizard (Lacerta vivipara Jacquin 1787) as a model organism, this study provides an experimental evaluation, in vivo and in silico, of the consequences of ASR biases, scaling up from individual behavior to population dynamics.…”

Examination of Annual Variation in the Adult Sex Ratio of Pronghorn (Antilocapra americana)

“…Also, intense selective harvesting of populations can cause annual variations in ASR (Ginsberg and Milner-Gulland, 1994;Bender and Miller, 1999;Milner-Gulland et al, 2003;Milner et al, 2007). Understanding what factors cause annual variation in the ASR of a population will assist biologists in conserving populations that possess high levels of demographic stochasticity or manage important game species that are subject to an intense annual harvest (Wildt and Wemmer, 1999;Wedekind, 2002;Bessa-Gomes et al, 2004).…”