Effects of an exotic prey species on a native specialist: Example of the snail kiteby Mathieu Finkler and Hyppolyte Terrones
Published by the September 11, 2018 on 12:34 PM
Exotic species have largely been studied over the years, their effects on native populations, their consequences... Most of the studies aim to see the competition between a native species and an exotic one. Here the study focus on the effect of an exotic species on a native predator.
Florida snail kite (Rostrhamus sociabilis plumbeus) are endangered, their populations are drastically declining in recent years. It is important to study them and to determine why their numbers are falling to implement an adapted conservation strategy.
The purpose of the study is to assess the effects of the recently introduced island apple snail (Pomacea insularum) on snail kite behavior and energetics comparing with the native prey (Pomacea paludosa).
Juvenile snail kite - Cláudio Dias Timm - CC BY-NC-SA 2.0
The authors determined different parameters such as the proportion of snail dropped, the searching and handling time, the consumption rate and proportion of time in flight. Caloric intake of both species has been determined by a model (Sykes 1987) and so is the daily energetic expenditure. Caloric balance seems to be perfectly suitable in this case because the difference in intake calories could affect all the life history traits and be the cause of the fast decline of the kites.
Foraging on exotic snails led to a greater proportion of snails dropped, a lower consumption rate, a longer handling time and a lower energy balance (figure below). These conclusions are particularly true and worrying for juveniles. This results indicates that feeding on exotic snails will decrease their energy and so less energy will be available for others activities (like reproduction, growth, defence against predators...). Finally, lakes where only exotic species are present (Tohopekaliga) could form an ecological trap.
From Cattau et al. 2010
Even after this study, it will be hard to conclude on an optimal foraging theory because both snail species were never found together in a lake. Therefore it could be interesting to make the same study in a lake were both species are present. Furthermore, this study has been conducted during the breeding period. During breeding period, species will need more energy to feed their offspring, to protect them, potentially leading to a greater difference in energetic balance.
Further studies may focus on the fact that kites feed on larger exotic preys (compared to native preys). Are the smallest individuals not available for kites or do kites choose to feed on larger exotic preys to compensate for their lower energetic content ?
This method could be used in others studies of trophic relationships and not only on native-exotic conflict. For example if human overfish a species, the predator of this species will have to change preys. So it will be important to calculate the energetic balance with the new prey.
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