The effects of tropical fish range extensions on temperate communities

Abstract

As anthropogenic stressors cause our planet to warm at unprecedented rates, species must either adapt or migrate, or else face local extinction. Were a species to migrate, its geographic distribution could shift towards a more favourable environment, or away from an undesirable environment, a phenomenon known as range shifting. Seventy five percent of current marine range shifts involve the expansion of ranges in a poleward direction, into cooler waters situated at higher latitudes. The equatorial tropics in particular are warming disproportionately to the rest of the planet, and as such, range-expanding species often originate from the tropics. Many range-extending tropical species, or vagrants, are now living in temperate communities, surrounded by temperate neighbours and new predator-prey interactions, potentially leading to competition for resources such as habitat and food. To understand how novel interactions between these newly-sympatric fish species affect their performance and behaviour, I assessed the partitioning of food sources, body condition, and performance of range-extending and local fish in these mixed communities. Food partitioning, or trophic niche use, was investigated using stable isotope analysis and stomach content analysis. Fish body condition and performance were evaluated using a combination of condition proxies (Fulton’s condition index, protein content, and the ratio of carbon to nitrogen), and performance proxies (stomach fullness, bite rate, activity level, and growth rate). I discovered that tropical and temperate species largely occupy distinct trophic niches. Additionally, both groups fed on a wide variety of food items, indicating that they are trophic generalists. Tropical species exhibited a decrease in three performance proxies (bite rate, activity level, and growth rate) in cool waters as compared to their native region, yet showed no change in body condition. These tropical species likely faced a trade-off between growth and maintenance of body condition, favouring condition, due to the sub-optimal environmental conditions of the cooler south. Temperate species showed no change in body condition or performance between sub-tropical and temperate communities. As waters continue to warm and tropical species are once again in optimal thermal conditions, performance trade-offs may no longer be necessary, ultimately strengthening interactions between tropical and temperate species.