An Integrative Approach to the Ecological and Evolutionary Causes of Geographic Range Limits

Colorado State University, Fort Collins CO

Investigators

Abstract

The proposed research will investigate the physiological responses of fish species to altered salinity. Salinization of freshwater ecosystems has been identified as an important global threat to ecosystem goods and services. Yet very little is known about how increased salinity will impact freshwater communities and ecosystems. The proposed research will contribute a mechanistic understanding on how salinity alters suites of physiological and behavioral traits and how these changes in turn shape geographic distributions and fish community structure. The proposed research also includes an educational outreach component focused on middle school students. Live fish will be used in classrooms to teach students biological concepts through inquiry-based approaches. Assessments of a pilot program have shown improved student understanding of biological concepts and increased test scores. All species in nature show a limited geographical distribution. Indeed, one of the most fundamental goals of ecological and physiological research is to explain why some organisms occur in some environments and not others. Geographic range limits also pose a fundamental evolutionary question; why don't populations evolve over time to occupy a wider range of environments. Such simple questions remain largely unanswered for most organisms, limiting our ability to predict how organisms respond to environmental variation. The challenge persists in part because the causes of geographic range limits requires an understanding of how multiple abiotic and biotic factors covary in the environment, and how suites of different traits respond and adapt to these challenges. In the research proposed here, the distributional limits of closely related tropical fish species in the genus Poecilia are investigated. When these fish occur together they segregate along a salinity gradient, but when they occur alone they span a broad range of salinities, suggesting salinity tolerance and competitive interactions jointly shape distributional limits and community structure. The proposed research focuses on how osmoregulation, behavioral aggression, hormones, metabolic scope, and locomotor performance interact with each other. These traits are typically studied separately, however, their joint study reveals new hypotheses on how shared mechanisms can influence multiple traits in ways that lead to geographic range limits. The proposed research will use lab and field experiments to first test 1) how elevated energetic costs of salinity stress, interspecific competition, and predation pressure may make some environments too expensive to persist in, and 2) how hormonal changes associated with osmoregulation may compromise competitive ability and food acquisition. Second, the proposed research will explore the role of gene flow, additive genetic variation, and negative genetic correlations as evolutionary constraints on osmoregulation and competitive ability using a combination of lab breeding experiments, genomic approaches, and artificial selection experiments. Collectively, the proposed research not only offers an opportunity to test the eco-physiological and evolutionary causes of geographic range limits, but also provides an integrative framework that can be applied more broadly.