Foraging behaviour of a native topminnow when shoaling with invaders

Abstract

Individuals join a group when the benefits of doing so outweigh the disadvantages. Typically, groups are composed by individuals of the same species, as sharing the benefits with relatives dilutes the disadvantages of being part of a group. However, mixed species shoals do occur in the wild. The Mexican Goodeidae are a clade of viviparous topminnows endemic to Central Mexico. Survival of most species is under threat, with some already extinct in nature. Causes of decline include the introduction of exotic species. These include viviparous topminnows of the Family Poeciliidae, such as the Trinidadian guppy (Poecilia reticulata Peters, 1859), as well as the twospot-(Pseudoxiphophorus bimaculatus Heckel, 1848) and porthole livebearer (Poeciliopsis gracilis Heckel, 1848), both native to the Mexican costal slopes but frequently translocated to the central Highlands. Guppies and goodeids readily associate with each other, and guppies entering such associations improve their foraging efficiency. This has been hailed as evidence of a social skill that favours the establishment of viable guppy populations across the globe. It is as yet unknown whether this improvement in guppy foraging efficiency comes at a cost for the interacting goodeids. Here, we investigated how sharing resources with poeciliid invaders affects the foraging efficiency of the goodeid twoline skiffia (Skiffia bilineata Bean, 1887). We measured the time it took them to locate food and the total time spent eating when part of a mixed species shoal that included either a species of exotic poeciliids (guppies, twospot-or porthole livebearers), or another goodeid (Goodea atripinnis). We also measured foraging efficiency of twoline Skiffia in single-species (conspecific) shoals as a control. We found that the total foraging time of twoline skiffias is reduced when shoaling with guppies and twospot livebearers compared to when associating with conspecifics. We provide evidence to support the idea that native species’ fitness is reduced when invaders with similar ecological requirements occupy their habitats.