Libor Závorka, Barbara Koeck, Julien Cucherousset, Jeroen Brijs, Joacim Näslund, David Aldvén, Johan Höjesjö, Ian A. Fleming, Jörgen I. Johnsson

Humans have introduced a variety of species outside their native ranges. Some of these species establish in the new environment successfully and can subsequently change the ecosystem. However, some of the effects of invaders are difficult to monitor but may have serious long-term impacts, such as population declines or extinction of native species.

Natives challenged by the presence of invaders often respond by changing traits. These traits, which can be physiological, morphological or ecological, are often associated together and there is some evidence that these trait associations are adaptive. For example, individuals that are active often have a high metabolism, which allows them to process food rapidly and maintain their elevated activity. In contrast, a high metabolism can be disadvantageous for inactive individuals, as they may not be able to find enough food to sustain themselves. An invader may affect these adaptive associations between traits by changing food distribution or activity of native individuals.

Here we test the idea that invasive species can break down adaptive associations among the traits of the natives, thereby reducing their growth rate. We used a wild population of brown trout living in a small Swedish stream, which was also occupied by non-native brook trout in the upstream stretch. Combining laboratory measurements of behaviour, metabolism, and body shape with observations of the movement and diet of individual fish in the wild, we tried to evaluate if and how trait associations were affected by the presence of the invader.

We found that associations between traits generally were weaker in brown trout living together with invasive brook trout than in brown trout living alone. When together with brook trout, brown trout consumed more terrestrial prey, and had smaller home ranges and a stouter body shape, changes that were associated with a reduction in growth. On the basis of these results, we suggest that the presence of invasive species can break down adaptive associations among traits in native species. Our results can help to explain other cases where invasive species reduce survival or growth of native species, even when direct effects like competition or predation are not observed.

Image caption: Ecologists at work. Photo provided by authors.
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