Bassar Lab

Bassar, R. D., et al. 2010. Local adaptation in Trinidadian guppies alters ecosystem processes. PNAS.

In this paper we test the hypothesis that local adaptation causes ecosystem structure and function to diverge. We demonstrate that populations of guppies (Poecilia reticulata), characterized by differences in phenotypic and population-level traits, differ in their impact on ecosystem properties. We report results from a replicated, common garden mesocosm experiment and show that differences between guppy phenotypes result in the divergence of ecosystem structure (algal, invertebrate, and detrital standing stocks) and function (gross primary productivity, leaf decomposition rates, and nutrient flux). These phenotypic effects are further modified by effects of guppy density. We evaluated the generality of these effects by replicating the experiment using guppies derived from two independent origins of the phenotype. Finally, we tested the ability of multiple guppy traits to explain observed differences in the mesocosms. Our findings demonstrate that evolution can significantly affect both ecosystem structure and function. The ecosystem differences reported here are consistent with patterns observed across natural streams and argue that guppies play a significant role in shaping these ecosystems.

Bassar, R. D. and R. Ferriére, et al. 2012. Direct and indirect ecosystem effects of evolutionary adaptation in Trinidadian guppies (Poecilia reticulata). American Naturalist.

Several studies have examined the net effects of adaptive evolution on ecosystem properties. However, we do not know if these effects are confined to direct interactions or if they propagate further through indirect ecological pathways. Even less well understood is how the combination of direct and indirect ecological effects of the phenotype promotes or inhibits evolutionary change. We coupled mesocosm experiments and ecosystem modeling to evaluate the ecological effects of local adaptation in Trinidadian guppies (Poecilia reticulata). The experiments show that guppies adapted to life with and without predators alter the ecosystem directly through differences in diet. The ecosystem model reveals that the small total indirect effect of the phenotype observed in the experiments is likely a combination of several large indirect effects which act in opposing directions. The model further suggests that these indirect effects can reverse the direction of selection that direct effects alone exert back on phenotypic variation. We conclude that phenotypic divergence can have major effects deep in the web of indirect ecological interactions and even small total indirect effects can radically change the dynamics of adaptation.

Bassar, R. D., et al. 2015. Population size-structure-dependent fitness and ecosystem consequences in Trinidadian guppies. Journal of Animal Ecology.

The vast majority of theory and empirical research on these eco-evolutionary feedbacks has focused on interactions among population size and mean traits of populations. However, numbers and mean traits represent only a fraction of the possible feedback dimensions. Populations of many organisms consist of different size classes that differ in their impact on the environment and each other. Moreover, rarely do we know the map of ecological pathways through which changes in numbers or size structure cause evolutionary change. The goal of this study was to test the role of size structure in eco-evolutionary feedbacks of Trinidadian guppies and to begin to build an eco-evolutionary map along this unexplored dimension. We used a factorial experiment in mesocosms wherein we crossed high and low predation guppy phenotypes with population size structure. Size structure had a very large effect on invertebrate biomass in the mesocosms, but there was little or no effect of the phenotype. The effect of size structure on algal biomass depended on guppy phenotype, with no difference in algal biomass in populations with more, smaller guppies, but a large decrease in algal biomass in mesocosms with phenotypes adapted to low predation risk. Overall, these results demonstrate size structure as a possible dimension through which eco-evolutionary feedbacks may occur in natural populations.

El-Sabaawi, R. W. and R. D. Bassar, et al. 2015. Intraspecific phenotypic differences in fish affect ecosystem processes as much as bottom–up factors. Oikos.

Whereas several studies have shown that evolution can affect ecological processes as much as commonly studied biotic (top-down) ecological variables, currently we do not know how the effects of evolution compare to bottom-up (abiotic) factors, or whether the effects of evolution are sensitive to abiotic conditions. Using a factorial mesocosm experiment we compared the ecosystem effects of guppy (Poecilia reticulata) life history phenotypes in two light treatments representing a three-fold difference in light levels, which is comparable to upstream-downstream differences in light availability in Trinidadian streams.  Light and phenotype had significant effects on similar aspects of ecosystem function. Whereas light had a stronger effect on ecosystem structure (algal and invertebrate stocks) than phenotype, phenotype and light had nearly equal effects on many ecosystem processes (nutrient recycling, nutrient fluxes, ecosystem metabolism and leaf litter decomposition). Interactions between light and phenotype were weak for the majority of response variables suggesting that abiotic variability did not alter the mechanisms by which phenotypes affect ecosystem function. We conclude that subtle phenotypic differences in consumers can affect ecosystem processes as much bottom-up factors which until recently were thought to be the primary drivers of ecosystem function in nature.

Zandonà, E., S. K. Auer, S. S. Kilham, J. H. Howard, A. López-Sepulcre, M. P. O'Connor, R. D. Bassar, O. Osorio, C. M. Pringle, and D. N. Reznick. 2011. Diet quality and prey selectivity correlate with life histories and predation regime in Trinidadian guppies. Functional Ecology.

Life histories evolve as a response to multiple agents of selection, such as age-specific mortality, resource availability, environmental fluctuations, etc. Predators can affect life history evolution both directly, by increasing mortality of preys, and indirectly, by modifying preys density and resources available to the survivors. Changes in prey density may affect the links between prey and lower trophic levels. For example, increasing prey densities can intensify intraspecific competition and cause evolutionary changes in the prey selectivity, also affecting nutrient acquisition. Here we show how the evolution of different life history traits in guppies (Poecilia reticulata) is correlated with differences in resources consumption and prey selectivity. We examined differences in guppy diet among stream types with high (HP) and low predation (LP) pressure and how they are related to benthic invertebrate biomass. Fish and invertebrate samples were collected from two HP and two LP reaches of two distinct study rivers in Trinidad. Our results showed that guppies from HP environments matured earlier, had higher fecundity and reproductive allotment, fed more consistently on higher quality food, and showed higher prey selectivity. Guppies from low predation sites displayed the opposite patterns in life history traits and had diets with prevalence of detritus and algae, which are a poorer quality food. LP guppies fed on invertebrates according to their availability, while HP guppies were selective toward those invertebrates with the lower C:N body ratio, and thus with higher nutritional value. Our study corroborates the important role of predators in shaping their preys’ life histories in concert with other traits, such as resource specialization and diet selectivity.

El-Sabaawi, R., M. C. Marshall, R. D. Bassar, A. López-Sepulcre, C. Dalton, and E. P. Palkovacs. 2015. Assessing the effects of guppy life history evolution on nutrient recycling: from experiments to the field. Freshwater Biology.

The effects of animal trait evolution on nutrient recycling are poorly understood. We characterize the effects of life history evolution on nutrient recycling using the Trinidadian guppy (Poecilia reticulata) as a model system. Guppy life history traits evolve when guppies are translocated from sites with high predation risk to sites with low predation risk. After translocation guppy population density and average body size of the population also increase. Therefore, the evolution of guppy life histories has consequences for individual traits and demographic population characteristics, both of which can alter nutrient recycling. The relative contributions of these variables to guppy-driven nutrient recycling are unknown. We first synthesize data from published experiments to disentangle how differences in individual traits and population characteristics contribute to differences in guppy-mediated recycling of nitrogen and phosphorus. This synthesis shows that individual guppies adapted to the absence of predators (LP guppies) have lower nitrogen and phosphorus excretion rates than individual guppies adapted to predators (HP guppies). However, LP guppy populations excrete twice as much nitrogen as HP populations because of their larger average body size and higher population densities. This result suggests that guppy life history evolution reduces population-level differences in excretion that would be expected based on demographic shifts alone.We then compare findings from these experiments to guppy excretion data collected from HP and LP sites in four rivers in Trinidad, thereby testing for the first time how guppy life history evolution might alter nutrient recycling rates in natural ecosystems. We find that phenotypic and population differences in excretion rates are consistent with those observed in the experiments. Our study shows that life history evolution can alter nutrient recycling patterns in stream ecosystems through the combined effect of individual (i.e. trait) and demographic changes that occur in response to predator removal. Guppy population density and size structure increase after predator removal and are key variables for determining the total contribution of guppies to nutrient recycling. However, trait changes associated with the reduction in predation dampen the effects of density and size structure on nutrient recycling, and demographic characteristics cannot be considered to be independent of the evolution of individual traits. We conclude that characterizing the combined effects of traits and population demographics is essential for understanding consumer-mediated nutrient recycling in freshwater ecosystems.

Marshall, M. C., A. J. Binderup, E. Zandona, S. Goutte, R. D. Bassar, S. A. Thomas, A. S. Flecker, S. Kilham, D. N. Reznick, and C. M. Pringle. 2012. Effects of consumer interaction on benthic resources and ecosystem processes in a Neotropical stream. PLoS One.

The effect of consumers on their resources has been demonstrated in many systems, but is often confounded by interactions with other consumers. Consumers may also have behavioral and life history adaptations to each other and to co-occurring predators that may additionally modulate their particular roles in ecosystems. We experimentally excluded large consumers from tile periphyton, leaves and natural benthic substrata using submerged electrified frames in three stream reaches with overlapping consumer communities in Trinidad, West Indies. Concurrently, we assessed visits to (non-electrified) control frames by the three most common large consumers - primarily insectivorous killifish (Rivulus hartii), omnivorous guppies (Poecilia reticulata) and omnivorous crabs (Eudaniela garmani). Consumers significantly decreased final chlorophyll a content and accrual rates the most in the downstream reach containing all three focal consumers in the presence of fish predators. Consumers also significantly increased leaf decay rates the greatest in the upstream reach containing only killifish and crabs. In the downstream reach where guppies co-occur with predators, we found significantly lower benthic invertebrate biomass in control relative to exclosure treatments than the midstream reach where guppies occur in the absence of predators. These data suggest that differences in guppy foraging, potentially driven by differences in their life history phenotype, may affect ecosystem structure and function as much as their presence or absence and that interactions among consumers may further mediate their effects in these stream ecosystems.

Simon, T., and R. D. Bassar, et al. 2017. Experimental evidence that local adaptation in Trinidadian guppies alters ecosystem structure in nature. Copeia.

While previous studies have shown that evolutionary divergence alters ecological processes in small-scale experiments, a major challenge is to assess whether such evolutionary effects are important in natural ecosystems at larger spatial scales. At the landscape scale, across eight streams in the Caroni drainage, we found that the presence of locally adapted populations of guppies (Poecilia reticulata) is associated with reduced algal biomass and increased invertebrate biomass, while the opposite trends were true in streams with experimentally introduced populations of non-locally adapted guppies. Exclusion experiments conducted in two separate reaches of a single stream showed that guppies with locally adapted phenotypes significantly reduced algae with no effect on invertebrates, while non-adapted guppies had no effect on algae but significantly reduced invertebrates. These divergent effects of phenotype on stream ecosystems are comparable in strength to the effects of abiotic factors (e.g. light) known to be important drivers of ecosystem condition. They also corroborate the results of previous experiments conducted in artificial streams. Our results demonstrate that local adaptation can produce phenotypes with significantly different effects in natural ecosystems at a landscape scale, within a tropical watershed, despite high variability in abiotic factors: five of the seven physical and chemical parameters measured across the eight study streams varied by more than one order of magnitude.  Our findings suggest that ecosystem structure is, in part, an evolutionary product and not simply an ecological pattern.

Bassar, R.D., B.L. Bryan, M.C. Marshall, C.M. Pringle, D.N. Reznick, and J.Travis. 2017. Evidence for effects of local adaptation on primary production resulting from changes in producer community composition and diversity. Oikos.

Top predators can impact both community structure and ecosystem processes. Ecological research has traditionally focused on understanding how these impacts are wrought by changes in predator numbers. However, there is increasing evidence that rapid evolutionary changes in community members can also influence community structure and ecosystem process and that these phenotypic effects are often as large as the effects of doubling of predator numbers. Typically, these phenotypic effects on communities and the resulting effects on ecosystem processes are measured as a total effect across numerous ecological pathways; leaving us with a lack of understanding regarding how these phenotypic effects are generated in even in simple ecological communities. Here, we combine experimental manipulations of Trinidadian guppy (Poecilia reticulata) presence, density and phenotype with analytical tools borrowed from path analysis to understand how ecological and evolutionary changes in guppies are manifested in the volume and diversity of algal species and, ultimately, their impact on gross-primary production. Overall, our results show that factors related to guppy presences and density acted through different ecological pathways than the evolutionary effects of the phenotype. Adding guppies and doubling their densities mostly decreased gross-primary production by reducing algal volume. Results from experimental exclosures showed that this decrease in algal volume represented a balance between direct effects of guppies feeding on algae and opposing indirect effects (e.g. trophic cascades). In contrast, exchanging guppy phenotypes increased gross-primary production through an increase in the diversity of algal species. Moreover, these changes were driven by the direct effects of differences in the foraging of guppies. We discuss how these results impact the way we view the relationship between ecological and evolutionary processes.