SpruceRoots Magazine - September 2001

 

A STUDY OF THE IMPACT OF DEER PRESENCE ON INSECT COMMUNITIES

by Sylvain Allombert

Studies of the impact of deer on plant communities, (e.g. RGIS results by S. Stockton) have shown that deer can greatly affect the structure and abundance of understory vegetation.

This can, in turn strongly affect the songbird communities by reducing nest sites and habitat of birds using the understory (RGIS results by J.-L. Martin and S. Allombert.) Another impact of deer on bird communities might be a reduction of the main food resource of songbirds-insects.

To study a potential impact of the presence of deer on insect communities, we carried out an extensive collection of insects and other invertebrates on six islands of the Laskeek Bay area. Two of the islands were deer-free, and four were affected by deer at different levels. Insects were capture by different methods, including pitfall and pan traps, sweeping, shrub beating and hand collecting.

Results show that the presence of deer can both reduce the abundance and the diversity of some invertebrates. This can often be explained by a cascading effect between differed components of the ecosystem. For example, the use of pitfall traps showed the deer-impacted islands have fewer species (from five to one species) and lower abundance (by an 11-times factor) of terrestrial mollusc. This leads in turn, to a reduced abundance of mollusc-eating ground beetles (by a factor from 2.5 to 11 depending on species) on heavily- affected islands compared to deer-free islands.

The deer, by reducing the amount of flowers, also seem to greatly reduce both the abundance and diversity of pollinating insects. Results are still under analysis, but it seems that the most important pollinators such as Bumblebees, Syrphid flies or Dance flies are all much less common and less diverse on deer affected islands. This could lead to a reduction of pollinating process and hence deer could also have an indirect negative impact on the abundance of wild flowers and berries.

In addition to improving our understanding of the impact of deer on the ecosystem, this study will also increase the knowledge of the native insect fauna of the archipelago. Up to July 1, 2001, around 350 species of insects and spiders were found in the Laskeek Bay as part of this study. The complete analysis of all samples should in the near future increase this number by at least two times, yet, this number of species is already much greater than the number of species is already much greater than the number of species with published records for the whole archipelago which is fewer than 100. Moreover, this is the first time that the insect fauna of deer-free islands has been studied, i.e. giving the opportunity to find insect species never recorded before since they might have become rare or have disappeared on the main islands which have all been affected by deer presence.


SpruceRoots Magazine - September 2001