Competitive asymmetry and local adaptation in Trinidadian guppies

Abstract

The outcome of competition between individuals often depends on body size. These competitive asymmetries can drive variation in demographic rates, influencing the ecology and evolution of life histories. The magnitude and direction of such asymmetries differ among taxa, yet little is known empirically about how adaptation to resource limitation alters competitive asymmetries. Here, we investigate the relationship between size‐dependent competitive ability and adaptation to resource limitation. We examined size‐dependent competition in two ecotypes of Trinidadian guppy, adapted to high or low levels of resource competition. Using aquaria‐based competition experiments, we describe how the size and ecotype of competitors influence somatic growth rate, whilst controlling for the confounding effect of niche differentiation. We replicated our study across two independent evolutionary origins of the “competitive” ecotype. The two “competitive” ecotypes differed markedly in size‐dependent asymmetry, indicating that adaptation to resource limitation alone is insufficient to explain changes in size‐dependent competitive asymmetry. For one origin, the ecotype adapted to resource limitation was a superior competitor over a wide range of size pairings. The equivalence of competitors varied over fivefold, dependent on size and ecotype; in three of four populations, larger individuals had a competitive advantage. Our results demonstrate that competitive asymmetry has strong effects on somatic growth. Because somatic growth contributes to demographic parameters, intraspecific trait variation is likely to play a key role in regulating demographic rates. Our findings imply that the evolution of size‐dependent asymmetries under conditions of intense competition is likely to be constrained by niche availability, although further research is needed to verify this.

Density-dependent consequences of size-selective induced life-history changes to population fitness in medaka (Oryzias latipes)

“…High-and low-density treatments consisted of eight and four fish, respectively, with a 1:1 sex ratio. Hence, densities in our experiment (0.8 and 0.4 fish•L −1 ) were lower than the densities used during the previous generations of selection (6 fish•L −1 ; Renneville et al 2020) and also biologically appropriate for small fish like medaka (e.g., Potter et al 2018). The size structure treatment was applied in both lines to assess the response of each line to size truncation induced by harvesting.…”

“…However, our results also show that the removal of large-sized individuals did not favour increased probability of reproducing for small-sized females, highlighting that direct interference competition alone would probably not influence fitness-related characteristics of smallsized fish. It is worth noting that investigating size-asymmetric competition within each tank as in Potter et al (2018) was not possible here due to the lack of individual size data throughout the experiment. Although we found that the size-structure treatment only played a minor role in structuring fitness components in the populations, we argue that further studies should investigate the complex role of size-dependent asymmetric competition (e.g., following the approach developed by Bassar et al 2016).…”

“…For instance, populations of guppies (Poecilia reticulata) that have a truncated size structure displayed stronger sizedependent competitive abilities (i.e., increased competitive ability with increasing body size) than populations with evenly distributed size structure. However, this asymmetric response was only reported at a high fish density and ultimately influenced individual somatic growth rate (Potter et al 2018). Predicting how anthropogenic selection impacts the dynamics of harvested populations requires an assessment of these size-and density-dependent processes and how they interact.…”

Substantial intraspecific variation in energy budgets: biology or artefact?

“…The distinction between guppy ecotypes is well characterised (Reznick and Endler, 1982;Reznick, 1982;Reznick and Bryga, 1987). However, there is substantial life-history variation among low-predation populations from different streams, including differences in rates of senescence (Reznick et al, 2004(Reznick et al, , 2005, juvenile growth rates (Arendt and Reznick, 2005), basal metabolic rate (Auer et al, 2018), how competitive ability scales with body size (Potter et al, 2019), and in genes associated with living in a low-predation habitat (Whiting et al, 2020). Our results here support the notion that there are multiple mechanistic routes through which the low-predation ecotype can evolve.…”

Individual differences determine the strength of ecological interactions

“…This density dependence is a fundamental principle in ecology and determines the struggle for existence that drives evolution ( 4 ). However, simple metrics, such as population density, entirely ignore differences between individuals in traits such as body size, weight, and condition, although numerous experimental studies and theory clearly indicate that the strength of interaction between individuals should depend on their traits ( 5 – 8 ).…”