All species show a limited geographical distribution. Understanding what factors cause these distribution limits is one of the main goals of ecology. In the simplest case, physical barriers limit the distribution of species. However, in most cases such barriers do not exist, and species gradually decline and eventually disappear along environmental gradients. In theory, distributional limits should emerge when the parameters responsible for an increase in population (birth and immigration) fall below those that reduce population size (death and emigration). Since Darwin, naturalists have been working to understand how these population parameters are affected by environmental factors. Historically, the impact of biotic (e.g. competition, predation) and abiotic (e.g. temperature, humidity) factors on the population parameters have been treated separately. However, theoretical developments have shown that this is an oversimplification, and that multiple factors often interact to affect population growth and distribution. Nevertheless, empirical support for how biotic and abiotic factors interact to shape range boundaries is scarce. Arguably, this limitation is due to the difficulties of isolating and testing these factors using joint field and laboratory experiments. The research proposed here takes advantage of two closely related tropical fish species distributed across a complex gradient (i.e. salinity, predation, and parasitism) to test how biotic and abiotic factors interact to limit distributions. The Trinidadian Guppy (Poecilia reticulata) and the Swamp Guppy (Poecilia picta) inhabit rivers of Trinidad and Tobago. Trinidadian Guppies are freshwater occupants whose distribution ends at the brackish/freshwater interface. In contrast, Swamp Guppies are common in brackish water and crosses into freshwater, where their abundance gradually decreases toward their upstream distributional limit. Preliminary data suggest that salinity alone has a small effect in the two species. Furthermore, Trinidadian Guppies are competitively dominant to Swamp Guppies, but may be more sensitive to the direct and indirect effects of predators and parasites. Specifically, this project will compare how these two guppy species differ in 1) their swimming performance under predator attacks; 2) their tendency to avoid unfavorable salinity conditions; and 3) determine how competition, predation, and parasitism interact with salinity to determine distributional patterns using reciprocal field translocation experiments.
Broader Impacts: Six undergraduate students to date have been supervised by the Co-PI in the context of the dissertation research, resulting in: 1) the completion of a Honor Thesis; 2) two poster presentations at the Front Range Student Ecology Symposium; and 3) independent research experience. The proposed project will involve four new undergraduates, three from Colorado State University (CSU) and one from the University of West Indies (UWI), Trinidad and Tobago. Our previous contact with faculty and students from UWI emphasized the need for projects culminating in honors theses, and we will provide this opportunity through a side project. Using supplemental funds not requested here, the Trinidadian student will be brought to CSU for additional training, and all four students will have the opportunity to be involved in lab and fieldwork. This will expose them to experimental design in the field of evolutionary ecology. Additionally, CSU students will be presenting their results at the Front Range Student Ecology Symposium, national meetings, and to co-author peer-reviewed papers. This will provide a bridge to graduate school for both national and international students. Thus we consider this an exiting opportunity for international collaboration. Results of this research will be published in scientific peer reviewed journals, presented at international meetings and published as a PhD dissertation at Colorado State University.
Understanding what limits a species distribution poses a fundamental challenge for ecology, because its often not clear what combination of environmental factors are responsible for determining the range of conditions a species can occupy. Geographic range limits also pose an evolutionary problem, because in theory all populations should evolve to expand their geographic ranges. We asked what the ecological and evolutionary causes of geographic range limits are in two closely related species of fish Poecilia reticulata (the guppy) and Poecilia picta (the swamp guppy) that are found along a freshwater-brackish water gradient on the island of Trinidad. The guppy is limited to only freshwater sections of rivers and streams, while the swamp guppy is found in both freshwater and brackish water stretches of lowland rivers. To test the causes of these distributions we conducted controlled laboratory experiments using second-generation laboratory reared genetic families. We found the swamp guppy was more tolerant of brackish water, but preferred freshwater. In contrast, the guppy was competitively dominant to the subordinate swamp guppy, and was less tolerant of brackish water. These results answer in part the ecological question of what limits each species distribution; the swamp guppy appear limited in its upstream distribution and forced into brackish water by the competitively dominant guppy. The guppy avoids brackish water, and remains in freshwater where it does best. When we compared the same genetic families in their ability to grow in brackish water and their ability to compete with the opposite species, we found a significant trade-off; genotypes good at tolerating brackish water were poor competitors, and genotypes that were good competitors did poorly in brackish water. These results imply a genetic basis between the ecological trade-off of being a good competitor versus being tolerant of salinity. Thus, the answer to the evolutionary question as to why the guppy does not evolve to expand its range into brackish water is because it would come at the expense of being a competitor. While such trade-offs have been speculated to play a role in limiting the evolution of geographic ranges, the results of this study provide empirical evidence in support of the importance of such trade-offs in natural populations. Climate change is predicted to result in rising sea levels and the transition of rivers and estuaries from freshwater to brackish water. Understanding trade-offs between salinity tolerance and other traits will be critical to making informed predictions about how freshwater systems will respond to future conditions.