Effects of dispersal on individual fitness
Understanding the causes and consequences of dispersal in natural populations has long been a goal of ecologists and evolutionary biologists. Due to the logistical difficulties of quantifying dispersal and fitness in natural systems, data directly linking movement to fitness remains rare. As a consequence, ecologists do not have a solid empirical understanding of dispersal’s effects on individual fitness for most species. For this chapter of my dissertation, I used data from a long-term mark-recapture study and multigenerational pedigrees of two wild populations of Trinidadian guppies (Poecilia reticulata) to explore the individual and environmental drivers of dispersal, and its consequences on fitness. We show that dispersal propensity and distance vary with sex, body size, and the environment. We also documented a striking and consistent reproductive benefit of dispersal for male guppies, wherein dispersing males had on average 35% more offspring than philopatric males, and were 1.6x times as likely to have any offspring at all. Our results demonstrate that dispersal is a major determinant of male reproduction in Trinidadian guppies, and suggest that selection for increased reproductive output drives male-biased dispersal in this system. This work was published in Ecology Letters in 2022.
Effects of inbreeding on mutualisms
One of the major concerns for the persistence of small isolated populations is inbreeding depression. When investigated in plants, the severity of inbreeding depression often depends on the environment in which it fitness is assessed. Here, I use the legume Chamaecrista fasciculata to explore how plant inbreeding combines with mutualistic interactions to affect plant fitness. I conducted two generations of experimental crosses to obtain C. fasciculata that differ in their extent of inbreeding, then planted the resulting seeds in a common garden where they were exposed to mutualist soil bacteria (rhizobia) and ants. I assessed how plants at different inbreeding levels interacted with rhizobia by observing root nodules, and did ant and extra-floral nectary surveys to assess plants’ mutualism with bodyguard ants. Throughout the experiment, I measured different plant fitness metrics, including total seed count. I found that plant inbreeding decreased plants’ investment in the mutualism with ants and with rhizobia. Plant fitness also decreased with inbreeding, yet the slope of that decline varied by trait and was different for plants that did or did not interact with rhizobia. Overall, I find that mutualisms are affected by plant inbreeding, and that mutualist presence or absence can change the severity of fitness losses in inbred populations.
A) The effect of inbreeding and rhizobia on total seed set. Solid blue lines depict individuals that were inoculated with rhizobia, while the dashed orange line represents plants that did not receive rhizobia. Inbreeding decreased seed production and significantly increased the probability of plants producing zero seeds. B) When looking at only plants that produced non-zero seeds, we find an interaction between inbreeding and rhizobia, where the benefit of the mutualism for seed production decreases at high values of plant inbreeding.