How the Canadian beaver as ecosystem engineer can drive the reproduction of saproxylic beetles

Indirect effects of an ecosystem engineer: how the Canadian beaver can drive the reproduction of saproxylic beetles

A. Mourant, N. Lecomte, G. Moreau; Journal of Zoology, vol. 304, pp. 90-97, DOI: 10.1111/jzo.12506

Nutrient cycling is a fundamental ecosystem function and as such, contributing organisms are critical assets of natural systems. Conservation measures aimed at protecting and preserving decomposers and detritivores have therefore been the subject of several studies in recent years. However, the ecology of these organisms remains largely undocumented or misunderstood. For instance, beetles, the most diverse group of animals, comprise many species known to act as decomposers and detritivores. Yet, the fundamental impact of this taxon on nutrient cycling, particularly dead wood decomposition, is still poorly documented.

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Wood-boring beetles are coleopterans whose larval and/or adult forms tunnel through wood. Most species colonize weakened or freshly dead woody material. From an ecological perspective, the tunneling activity of these beetles is generally beneficial because it physically breaks down deadwood and accelerates the subsequent colonization of deadwood decomposers such as fungi and bacteria. Once their life cycle is complete, wood-boring beetles exit the debris via emergence holes. Among the factors known to influence this taxon is deadwood availability. As such, we anticipated that wood-boring beetle communities would be profoundly affected by the activity of the Canadian beaver, an ecosystem engineer that alters deadwood resource dynamics. This rearrangement is due to dam building, foraging and flooding, which create sudden large quantities of fresh deadwood in the form of vertical snags and horizontal logs. The activity of the Canadian beaver also creates large sun-exposed habitats patched throughout the landscape, modifying the floristic characteristics of these areas. These changes have been studied for their positive impacts on vertebrates but their effects on wood-boring beetles have yet to be quantified.

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To investigate the interaction between wood-boring beetles and the Canadian beaver, we tested whether beaver-induced perturbations enhance the realized fecundity and activity of saproxylic beetles by modifying their habitat and resources. We assessed this by quantifying the abundance of beetle emergence holes around beaver ponds and control unaltered watercourses within the Kouchibouguac National Park, New Brunswick, Canada. Among the variables of interest examined in our study were beaver activity and several host plant characteristics influencing beetle colonization, such as snag essence, diameter, decay stage and distance from the watercourse. We also sorted emergence holes according to their diameter and shape to determine which beetle taxon exited from a given snag.

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Our results revealed that beetle emergence was higher around beaver ponds for both bark beetles and longhorn beetles. Bark beetles are members of the subfamily Scolytinae and reproduce mainly in the inner bark of trees, making small circular holes upon emergence from tree boles. Longhorn beetles are members of the Cerambycidae family and leave behind large circular holes upon emergence from tree boles. Bark beetles emerged in masses from small trees adjacent (i.e., at ~5m) to beaver ponds, and were seldom observed on small trees located further away from beaver ponds or in control areas. This suggests that the deadwood pulse produced by the Canadian beaver in the pond caused an aggregation of bark beetles that overflowed in the periphery of the disturbed area. Longhorn beetles also emerged in greater abundance from snags located around beaver ponds than from snags located in control areas, but the effect was not limited to the surrounding of the pond and was still occurring 50m away from the disturbed area. We suspect this to be due to the generally higher dispersal capacities of longhorn beetles compared to bark beetles. Overall, our results indicate that beaver activity enhances the realized fecundity and activity of wood-boring beetles. We propose that this complementary relationship is beneficial for nutrient cycling across forest ecosystems, considering that beaver-disturbed areas could be seen as breeding hotspots for wood-decomposing organisms.

Gaétan Moreau

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