American Society of Naturalists

Andrea J. Roth-Monzón, Mark C. Belk, J. Jaime Zúñiga-Vega, and Jerald B. Johnson (June 2020)

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For a long time, scientists have studied how competition can shape species traits. Most researchers have focused on pairs of competing species, thereby using a simple system to understand the evolutionary effects of competition. However, our understanding of how traits change when more than two species are interacting is still at a beginning stage. Most natural communities consist of several species living together and potentially interacting. Previous research suggests that when multiple species interact, trait evolution can take different pathways due to the addition of complex ecological interactions. There is still much to learn from studying these more complex competitive interactions.

The authors of this paper studied a community of four species of livebearing fishes in northwest Mexico. They focused on how competition affects body shape. They asked if body shape differs among sites where no competitors are present, sites where only one competitor is present, and sites where two or more competitors are present. They discovered that body shape of all three species converged towards a similar morphology when co-occurring with competitors. They also found that when competing with two or more species, body shape is intermediate with respect to the body shape of the species with no competitors and with one competitor. Their results stand in contrast to the pattern of character divergence typically observed when only two species are competing.

Abstract

Competition has long been recognized as a central force in shaping evolution, particularly through character displacement. Yet research on character displacement is biased as it has focused almost exclusively on pairs of interacting species, while ignoring multispecies interactions. Communities are seldom so simple that only pairs of species interact, and it is not clear if inferences from pairwise interactions are sufficient to explain patterns of phenotypes in nature. Here we test for character displacement in a natural system of freshwater fishes in western Mexico that contains up to four congeneric species of the genus Poeciliopsis. We analyzed body shape differences between populations with different numbers of competitors while accounting for confounding environmental variables. Surprisingly, we found evidence for convergent character displacement in populations of P. prolifica, P. viriosa, and P. latidens. We also found that the convergence in body shape was not consistently in the same direction, meaning that when three or more competitors co-occurred, we did not find more extreme body shapes compared to when there were only two competitors. Instead, when three or more competitors co-occur, body shape was intermediate to the shape found with a pair of species or no competitor present. This intermediate shape suggests that evolution in multispecies communities likely occurs in response to several competitors, rather than to simple pairwise interactions. Overall, our results suggest that competition among multiple species is more complex than simple pairwise competitive interactions.

Más allá de las interacciones pareadas: desplazamiento de caracteres multiespecífico en una comunidad de peces dulceacuícolas Mexicanos

La competencia ha sido reconocida durante mucho tiempo como una fuerza central en la configuración de la evolución, particularmente a través del desplazamiento de caracteres. Sin embargo, la investigación en el desplazamiento de caracteres esta sesgada, puesto que se ha centrado casi de manera exclusiva en las interacciones entre pares de especies, mientras ignora las interacciones multiespecíficas. Las comunidades rara vez son tan simples para que solo pares de especies interactúen, y aún no es claro si las inferencias obtenidas de las interacciones pareadas son suficientes para explicar patrones fenotípicos en la naturaleza. Aquí nos enfocamos en examinar el desplazamiento de caracteres en un sistema natural de peces dulceacuícolas en el oeste de México que contiene hasta cuatro especies congenéricas del género Poeciliopsis. Analizamos diferencias en la forma del cuerpo entre poblaciones con diferentes número de competidores mientras consideramos variables ambientales. Sorprendentemente, encontramos evidencia de desplazamiento de caracteres convergente en las poblaciones de P. prolifica, P. viriosa, y P. latidens. También encontramos que la convergencia en la forma del cuerpo no fue consistente en la misma dirección, es decir, que cuando tres o más competidores coocurren, no se encuentran formas corporales más extremas como cuando se compara con la coocurrencia de solo dos competidores. Es más, cuando tres o más competidores coocurren, la forma de cuerpo es intermedia a la forma encontrada con un par de especies o en la ausencia de competidores. Esta forma corporal intermedia sugiere que la evolución en comunidades multiespecíficas probablemente emerja en respuesta a la presencia de varios competidores, en lugar de a simples interacciones pareadas. En general, nuestros resultados sugieren que la competencia entre múltiples especies es más compleja que las interacciones competitivas pareadas.