Abstract
Invasive species are one of the greatest threats to freshwater ecosystems globally. However, the causal mechanisms that drive negative impacts of many invasive species are poorly understood. In Tanzania, non-native Nile tilapia (Oreochromis niloticus) exists in sympatry with a diverse range of native species, many of which are congenerics with strong niche overlap. It has been suggested that O. niloticus can displace native species from preferred habitat through dominance during interference competition, yet interference competition between O. niloticus and a native tilapia species has never been directly tested under experimental conditions. In this study juvenile O. niloticus and Manyara tilapia (Oreochromis amphimelas), a functionally similar but endangered Tanzanian cichlid, were size matched in conspecific and heterospecific pairs. We presented pairs with limited shelter and recorded competitive interactions. We found that O. niloticus were more aggressive and faster to initiate agonistic interactions than O. amphimelas. Furthermore, O. niloticus showed a strong competitive dominance in their interactions with O. amphimelas. One-sided dominance hierarchies can drive fundamental changes in resource use by subordinate individuals, potentially resulting in habitat displacement over the long term. Based on this experimental evidence, we conclude that O. niloticus may threaten native tilapia species through dominance in interference competition.
Highlights
Invasive species pose a major threat to aquatic ecosystems and can cause biodiversity loss, species extinction and adverse socioeconomic changes (Kaufman, 1992; Pringle, 2005)
The probability of an agonistic interaction was lower in the O. amphimelas–O. amphimelas treatment than in the O. amphimelas–O. niloticus and O. niloticus–O. niloticus treatments, with these latter two treatments not differing significantly from each other (Fig. 2a)
O. amphimelas were more likely to be involved in aggression when the intruder was an O. niloticus rather than a conspecific (comparison: coef = - 2.3, exp(coef) = 0.099, lower 95% CI = 0.037, upper 95% CI = 0.267, P \ 0.001)
Summary
Invasive species pose a major threat to aquatic ecosystems and can cause biodiversity loss, species extinction and adverse socioeconomic changes (Kaufman, 1992; Pringle, 2005) Underlying these impacts are causal mechanisms that explain why an invasive species has a given effect in an ecosystem. Oreochromis niloticus (Linnaeus, 1758), is a freshwater fish in the family Cichlidae, native to Western Africa and the Nile basin (Trewavas, 1983). It has a pan-tropical non-native distribution (Canonico et al, 2005), with the spread largely due to its role in aquaculture. Successful establishment of non-native populations has taken place in the majority of countries where culture has been initiated (Ehrlich, 1989; CostaPierce, 2003)
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