Asymmetric competition among stream fishes: Do food web pathways affect competitive outcomes?

Abstract

AbstractLimiting resources are often unevenly distributed among competing consumers, as larger, more aggressive, fast growing, early emerging, or rapidly reproducing species disproportionately exploit food or habitat. In such cases, asymmetric competition can occur (i.e., when limited resources are divided up unequally among individuals or species), resulting in stronger negative effects for some species and comparatively weaker effects for others. We hypothesize that the accessibility of food web pathways influences asymmetric competition among stream fishes. To test this hypothesis, we studied the interactions of invasive brown trout (Salmo trutta) with native Rio Grande sucker (Catostomus plebeius or Pantosteus plebeius) and Rio Grande chub (Gila pandora) in mountain streams of New Mexico, USA, from 2020 to 2021. Our study found support for the hypothesis that the accessibility of food web pathways drives asymmetric competition in mountain stream fishes, as native chub and sucker grew at slower rates than invasive brown trout across all observed body sizes. Our conclusion is evidenced by (1) strong diet overlap driven by benthic macroinvertebrates that (2) were important in fueling the growth of native fishes but were also (3) disproportionately exploited by trout. Because trout fed on small aquatic invertebrates, large crayfish, other fish, and inputs of terrestrial prey, they could incorporate energy from differing food web pathways, whereas native fishes were mostly limited to small benthic prey (and chubs fed on terrestrial arthropods). As trout, chub, and sucker competed for small benthic prey, trout could also incorporate other sources of food, whereas chub and sucker had fewer options. Although two or more species may compete for one resource, their rates of growth and production are also dependent on, and fueled by, other food web pathways. Our work demonstrates that such food web pathways help explain differences in growth and production between invasive and native fishes. Our findings show that tracking energy flows through the different food web pathways provides a useful and intuitive means of describing the important sources of food (i.e., the trophic basis of production) that fuel growth and production and sheds light on the processes that drive asymmetric competition.