Intra-guild interactions and projected impact of climate and land use changes on North American pochard ducks.

The co-occurrence of functionally similar species is very common in nature, and is often put forward as a basis for ecosystem resilience to disturbance. At the same time, competition between similar species is also considered a strong driver of community composition. However, environmental stochasticity can alter this prediction, either because competitive abilities depend on time-varying factors or because covariance in species' responses to environmental conditions masks the effect of competition. Interactions other than competition can also influence community dynamics but have received less attention. We used a simplified community of two sympatric duck species (redhead Aythya americana and canvasback A. valisineria) and a previously published analysis of 50 years of demographic data to parameterize a stochastic, density-dependent, stage-structured model. These ducks interact via nest parasitism (mostly of canvasback by redhead) in addition to competition for food resources, with consequences at the demographic level; these interactions are modulated by habitat availability (number of ponds in the study landscape). We found that if habitat availability decreased there was a high risk of quasi-extinction, and redheads, although initially able to maintain their numerical dominance, quickly became the least abundant species because they perform worse during droughts. If habitat availability increased, we found that the initially more rare canvasback would increase in relative abundance, albeit slowly. We interpret this as a shift from a community influenced by nest parasitism (which is detrimental to canvasback) to a community mostly driven by species-specific dynamics due to relaxation of resource limitation.