Sexual selection affects local extinction and turnover in bird communities

Abstract

Predicting extinction risks has become a central goal for conservation and evolutionary biologists interested in population and community dynamics. Several factors have been put forward to explain risks of extinction, including ecological and life history characteristics of individuals. For instance, factors that affect the balance between natality and mortality can have profound effects on population persistence. Sexual selection has been identified as one such factor. Populations under strong sexual selection experience a number of costs ranging from increased predation and parasitism to enhanced sensitivity to environmental and demographic stochasticity. These findings have led to the prediction that local extinction rates should be higher for species͞populations with intense sexual selection. We tested this prediction by analyzing the dynamics of natural bird communities at a continental scale over a period of 21 years , using relevant statistical tools. In agreement with the theoretical prediction, we found that sexual selection increased risks of local extinction (dichromatic birds had on average a 23% higher local extinction rate than monochromatic species). However, despite higher local extinction probabilities, the number of dichromatic species did not decrease over the period considered in this study. This pattern was caused by higher local turnover rates of dichromatic species, resulting in relatively stable communities for both groups of species. Our results suggest that these communities function as metacommunities, with frequent local extinctions followed by colonization. Anthropogenic factors impeding dispersal might therefore have a significant impact on the global persistence of sexually selected species. O ne of the key assumptions of theoretical models of sexual selection is the antagonistic effect of natural and sexual selection on the evolution of exaggerated sexual advertisements (1). On one hand, competition among males and͞or female preference can result in the evolution of costly sexual traits; on the other hand, extravagant ornaments can incur survival costs. As a consequence, the evolution of exaggerated displays is thought to displace individuals harboring such traits from their survival optimum (2). Survival costs of sexual displays have been shown both at the intraspecific and interspecific level. In a pioneering study, Endler (3,4) showed that the number and size of color spots in guppies (Poecilia reticulata) were tightly linked to the distribution of predator species in the stream; the number of color spots decreased in areas with high visual predation. A laboratory study on experimental populations of guppies experiencing various predation regimes confirmed the causative link between expression of secondary sexual traits and predation (3, 4). Comparative evidence for survival costs of sexual advertisement comes from studies on birds. Using a phylogenetic framework, Promislow and coworkers (5, 6) have shown that sexual dichromatism was associated with higher mortality rate within...

A rigorous comparison of sexual selection indexes via simulations of diverse mating systems

“…O ur understanding of sexual selection is obstructed by lack of progress on a fundamental question: How do we measure sexual selection rigorously, and how do we compare its strength between the sexes or across taxa (1-3)? Sexual selection underlies some of the most extreme traits in nature, including spectacular armaments and ornaments, and affects macroevolution via both speciation and extinction (4)(5)(6)(7)(8). It is thought to vary systematically with a species' mating system (i.e., the degree of multiple mating by each sex) and with sex-specific investment in offspring and mating partners (i.e., parental investment and direct benefits) (9,10).…”

Sexual selection forms the structure and dynamics of ecological communities

“…The basic idea is that sexual selection can easily cause populations to diverge in mate recognition systems, thus leading to rapid evolution of prezygotic barriers between populations (22). Interestingly, the results of Doherty et al (1) suggest that an increased local extinction rate is not necessarily translated into an overall loss of sexually selected species. In their study covering a 21-year time span, a higher turnover rate of these species was found to compensate for the increased local extinction rate.…”

“…However, occasionally the two fields do meet, as in the article by Doherty et al (1) in this issue of PNAS, in which the authors interpret new and exciting results obtained through statistical analysis of ecological monitoring data (2) within the framework of sexual selection (3). Through a powerful statistical analysis of data collected by volunteers, the authors are able to show, at a continent-wide scale, that sexual selection is associated with higher local extinction and turnover rates in natural bird communities, which affects biodiversity across the continent (Fig.…”

“…18 and references therein for a critical review). On the other hand, sexual selection may drive populations away from their survival optima, thereby contributing to increased local extinction rate at an ecological time scale (1,(15)(16)(17). Novel hypotheses related both to community structure and dynamics and evolutionary biology may emerge when we consider the net effect of sexual selection at both evolutionary and ecological time scales.…”

“…Doherty et al (1) provide an excellent demonstration of the potential power of using long-term ecological monitoring data to address key problems in community ecology and evolution. Conceptually, the study raises interesting questions that warrant further investigation.…”