The ability of organisms to move among patches has broad implications for evolutionary and ecological processes such as gene flow and population persistence. Understanding these processes often rests on the assumption that movement among patches is symmetric. In reality, individual movement frequently follows environmental cues or tends to be directed toward higher quality habitat patches, and is asymmetric. This study proposes to quantify the effects of asymmetric or directional dispersal on metapopulation colonization-extinction dynamics and, therefore, persistence of patchy populations. It relies on field experiments to test how different behavioral mechanisms affect the magnitude of movement asymmetries in a common insect. Results will lead to more accurate measures of population connectivity and thus to more accurate metapopulation models
Results from the study will be made available in different formats to four distinct groups - scientists, managers and conservationists, the general public, and young students. One of the investigators is Hispanic and serves as an effective role model for undergraduates from under-served groups. Undergraduates from these groups will be recruited to the project, and diverse social media (such as YouTube, TeacherTube) will be used to disseminate research results to young scientists and teachers. Manuals for managers and policy makers will be produced in both English and Spanish; these will explain metapopulation theory and how to conduct and analyze metapopulation experiments.
Movement has implications for both basic and applied biology. In basic biology, movement is related to gene flow and the maintenance of biodiversity. In applied biology, modeling the movement of organisms is an important tool used in biological conservation and management. Many modeling approaches that describe movement assume symmetric dispersal. This assumption means that the likelihood of moving from location A to location B is the same as going in the opposite direction (from location B to location A; Figure 1). However, asymmetric dispersal (when the likelihood of moving from location A to B is not necessarily the same as going from location B to A) may be the rule more than the exception in nature where spatial variation in environmental attributes may drive asymmetric movement. We studied the patterns and mechanisms of asymmetric movement in the cactus bug (Chelinidea vittiger). The cactus bug moves between prickly pear cactus (Opuntia spp.) patches, where it breeds and feeds. Previous studies have shown that movements among these cactus patches are highly asymmetric. In this study we used a combination of an observational and experimental approach to test three hypotheses that may explain these asymmetries: (1) movements upwind, (2) movement towards larger cactus patches, and (3) movement towards or away from occupied patches. We first re-analyzed data from a mark-recapture study across a 56-patch network to test for the magnitude of asymmetric movement and if wind direction or patch area were drivers of directed movement (Figure 2). Second, we performed translocation experiments in which we manipulated patch area, wind and the presence of conspecifics to determine if and the extent to which the cactus bug biases movements in response to these factors (Figure 3). In the mark-recapture study, cactus bug movements were highly asymmetric (proportion of symmetric movements was 0.04). These asymmetric movements were weakly related to variations in patch area, but not related to wind direction (Figure 4). In the translocation experiment, we found that the immigration rate was significantly higher to larger patches (Figure 5), but no effect of wind advection or conspecifics. Together, these results suggest that asymmetric movements may be driven by local patch area, which likely reflects the amount and quality of resources (e.g., food) for this insect. Spatial variation in the environment may cause variation in movement behavior leading to asymmetric dispersal patterns. While recent metapopulation models suggest that movement asymmetries can have substantial effects on metapopulation dynamics, empirical tests of dispersal asymmetries across landscapes remain rare with even fewer tests of the mechanisms driving such patterns. We found that movements of the cactus bug were generally asymmetric and better explained by movements from small-to-large patches. These results provide empirical support for movement asymmetries and suggest that these patterns can be explained by local information on resource amount and/or quality. Much of the work on asymmetric movements requires complex modeling. As part of our broader impacts, we developed a website (www.ecosigmasquared.com) that accomplishes three goals aimed to foster greater engagement in quantitative sciences for under-represented groups. First, it provides a video for broad audiences that describe the problem of asymmetric movement and why it is relevant to conservation and ecology. Second, it provides programming code for other scientists, students, and programmers to use for our questions and other problems in quantitative ecology. Third, it provides each of these in both English and Spanish to reach a greater diversity of students and scientists.
