Competitive exclusion, not predation, likely drove the decline in native fish diversity following carnivorous fish introductions to Lake Titicaca in the mid‐20th century

Lake Titicaca is a continental centre for ichthyological biodiversity, with over 90% of native species being endemic. However, these species face significant threats from invasive species, particularly from invasive salmonids and invasive Argentine pejerrey. While ecologists have known that these invasive species have had deleterious effects on the ichthyofauna of Lake Titicaca for decades, whether this is a consequence of competitive exclusion or direct predation remains unknown. We tested this using morphological data, testing whether the closeness of native species to invasives in morphospace was predictive of their current conservation status, and whether smaller species were likelier to be at risk than larger species. We also tested whether fisheries pressure better explained extinction risk and found species that were at higher risk of extinction were fished less than those which were at a lower risk of extinction. We found that larger species, and those closer to the invasives in morphospace, are likelier to be endangered or to have gone extinct. This strongly suggests that competitive exclusion is the primary way that invasive species cause native fish declines in Lake Titicaca. Using morphospace to assess the mechanisms of invasive species' impacts on native fish species is an alternative to longer-term monitoring which is often absent in poorly studied, though highly diverse, tropical waters. It could be used by conservationists in other regions to triage conservation efforts towards more vulnerable species and to predict which species are likelier to be harmed when a new invasive species is identified.