Professor McGill University Montreal, Quebec, Canada
Abstract: Biological communities are often structured by disturbance; however, the starting composition of a community can also dictate how the community resists change and/or responds to disturbance events. The consequences of multiple disturbances interacting in time or in space are often difficult to predict and can result in unexpected, swift, and nonlinear change. The rarity of major natural disturbances and cooccurring multiple disturbances have limited the extent to which research has considered community responses. The aquatic community, including macroinvertebrates, fish, and amphibians, inhabiting a wetland sandspit in Lake Erie experienced a chance overlap of two major disturbance events: an herbicide treatment of the invasive reed, Phragmites australis australis, and a dune washover event; representing a rare opportunity to assess and compare the short-term respective impacts of a natural disturbance and an anthropogenic disturbance. While both disturbances had the capacity to destabilize and transform the landscape, and hence, alter community compositions and the distribution of animals, it is likely that the communities differed in their responses to these disturbances. In this research, we explored how the disturbance-caused habitat modifications; herbicide treatment, dune washover, and both disturbances, affected the aquatic communities using minnow traps in a 3 replicate 2x2 site design.
The dune washover created extensive overwash fans and altered the landscape drastically, creating shallow, sand-filled pools in the place of deep, vegetated ponds. Herbicide-application was effective, suppressing much of the regrowth of Phragmites reeds in treated sites. The composition of species present at our sites varied in accordance with the disturbance type. Herbicide-treated and washover sites had similar species richness to control sites, and sites affected by both disturbances had fewer species. Sites affected by both disturbances exhibited higher evenness compared to other site types. When applying Principal Response Curves to the site type communities, community compositions over time were similar among control and herbicide-treated sites, while washover communities were dissimilar from control sites. Although herbicide-treated washover site compositions changed in a similar direction to washover-only sites, the sites affected by both disturbances were most distinct in composition from control sites. Furthermore, these doubly disturbed sites represented the only site type that the federally endangered Lake Chubsucker (Erimyzon sucetta) consistently inhabited. Our research stresses the impact of compounding effects of multiple disturbances on community structure and the importance of promoting the natural disturbance regime in a landscape. Insights acquired from this research will also inform future dune and Phragmites management endeavours.
