Antagonistic Selection or Trait Compensation? Diverse Patterns of Predation-Induced Prey Mortality due to the Interacting Effects of Prey Phenotype and the Environment

Differentiation among closely related prey species may result from differing adaptations to heterogeneous environments. Many studies have focused on competition for shared resources as a major factor promoting differentiation, with considerably less attention focused on interacting effects of abiotic factors and predator–prey relationships. To further investigate the effects of interacting selective factors on the outcomes of mortality and survival in aquatic prey, we conducted interrelated laboratory studies examining the effects of water colour and plant density on predator-induced mortality in four dytiscid species (Coleoptera: Dytiscidae) that varied in body size (total body length), and body colouration pattern. Body size was more strongly phylogenetically conserved than colouration pattern, and larger body size generally resulted in decreased predator-induced mortality rates. In contrast, the effectiveness of body colouration patterns in decreasing prey mortality risk depended on water colour and prey body size. In clear water, small and patterned dytiscids had mortality rates equal to medium-sized plain beetles, thereby compensating for differences in mortality risk due to body size differences. Under dark water conditions, small dytiscids experienced higher mortality rates compared to medium-sized dytiscids; however, the effectiveness of colouration patterns in medium-sized beetles decreased to the point that it became detrimental to survival, revealing antagonistic selection. We suggest that colouration patterns are not ubiquitous in prey species and cospecialization in larger size and presence of colouration patterns does not generally result in higher prey survival, because the effectiveness of the two antipredator defences may be restricted to certain phenotype × environment combinations. Our results illustrate how interactions between prey phenotype and variable environmental conditions among habitats dominated by the same predator can lead to adaptive trade-offs, which can increase the number of possible outcomes of predator mediated selection.     

Beetle and plant density as cues initiating dispersal in two species of adult predaceous diving beetles

Dispersal can influence population dynamics, species distributions, and community assembly, but few studies have attempted to determine the factors that affect dispersal of insects in natural populations. Consequently, little is known about how proximate factors affect the dispersal behavior of individuals or populations, or how an organism’s behavior may change in light of such factors. Adult predaceous diving beetles are active dispersers and are important predators in isolated aquatic habitats. We conducted interrelated studies to determine how several factors affected dispersal in two common pond-inhabiting species in southern Alberta, Canada: Graphoderus occidentalis and Rhantus sericans. Specifically, we (1) experimentally tested the effect of plant and beetle densities on dispersal probabilities in ponds; (2) surveyed ponds and determined the relationships among beetle densities and plant densities and water depth; and (3) conducted laboratory trials to determine how beetle behavior changed in response to variation in plant densities, conspecific densities, food, and water depth. Our field experiment determined that both species exhibited density dependence, with higher beetle densities leading to higher dispersal probabilities. Low plant density also appeared to increase beetle dispersal. Consistent with our experimental results, densities of R. sericans in ponds were significantly related to plant density and varied also with water depth; G. occidentalis densities did not vary with either factor. In the laboratory, behavior varied with plant density only for R. sericans, which swam at low density but were sedentary at high density. Both species responded to depth, with high beetle densities eliciting beetles to spend more time in deeper water. The presence of food caused opposite responses for G. occidentalis between experiments. Behavioral changes in response to patch-level heterogeneity likely influence dispersal in natural populations and are expected to be important for observed patterns of individuals in nature. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The importance of marginal population hotspots of cold‐adapted species for research on climate change and conservation

Areas hosting hotspots of low‐latitude marginal populations of cold‐adapted plant species could be key areas for understanding geographical attributes that result in refugia during climatic shifts as well as the conservation of genetic diversity in the face of climate change. Low‐latitude populations of cold‐adapted plants are important because they may harbour the combination of alleles that foster persistence in a warmer climate. Consequently, identification of areas where arctic‐alpine, circumpolar and circumboreal species reach the low‐latitude ends of their distribution will present a unique opportunity to uncover processes that shaped current biogeographical patterns, as well as prepare for future scenarios. Here, we identify 35 main marginal population hotspots (19 and 16 areas in North America and Europe, respectively) of 183 plant taxa. These hotspots represent areas where southern marginal populations of cold‐adapted species co‐occur. The identification of hotspots was based on geographic overlap of southernmost locations of the target species, in a 50 × 50 km grid. With a threshold of two species in a single grid cell or in two contiguous cells, the analysis revealed that hotspots are in most cases located in the southern portion of major mountain chains. However, hotspots also occur in lowland areas at high latitudes (Fennoscandia, Alaska, Hudson Bay) which do not necessarily correspond to known cold‐ or warm‐stage refugia (e.g. Alps). Rockies and Sierra Nevada both in California and Spain, Apennines, and the southern Scandes, maintain their hotspot status even with more stringent cut‐off thresholds (>3 and >5 species per cell group). From a conservation point of view, our analysis reveals that only a small portion of the hotspots are currently included within protected areas. We discuss the importance of marginal population hotspots to future research on climate change and, finally, outline how conservation strategies can capitalize on the knowledge gained from studying climate change effects on cold‐adapted plants.