Temperature-dependant priority effects in milkweed specialist aphids
As one of the most pressing environmental concerns facing ecosystems worldwide, climate warming has the potential to dramatically alter species interactions. Competitive outcomes and coexistence can depend on both assembly history and indirect interactions through a host, and the ways in which these complex interactions will be affected by climate remain unclear. We experimentally warmed two competing aphid species (Aphis nerii and Aphis asclepiadis) that specialise on milkweed plants (Asclepias syriaca) to determine how warming alters competitive interactions, plant-insect interactions, and the influence of priority effects on competitive outcomes. Although A. nerii is known to have higher growth rates and be generally competitively superior, it arrives at northern latitudes later in the summer, which could give A. asclepiadis an advantage. In a growth chamber, we crossed two temperatures (22.5°C and 25ºC) with six levels of competition (a no aphid control, A. nerii alone, A. asclepiadis alone, both together, A. nerii added with two weeks priority, and A. asclepiadis added with two weeks priority). We counted aphids and quantified plant death twice a week for five weeks, which is typical of the length of time that these species interact each summer in our region.
As predicted, A. nerii had higher growth rates and reached over twice the maximum population size as A. asclepiaidis over the course of the experiment. Although warming had no effect on initial growth rates for either species, it resulted in lower maximum population sizes of both species as aphid feeding accelerated plant death at high temperatures. While A. nerii was unaffected by interspecific competition, A. asclepiadis populations were strongly negatively affected by the presence of A. nerii at both temperatures. However, this negative effect of interspecific competition was alleviated when A. asclepiadis was added with two weeks priority. Overall, the combined negative impacts of warming and interspecific competition on A. asclepiadis resulted in frequent local extinction of this species. In the presence of A. nerii, priority arrival allowed the persistence of A. aslepiadis in unwarmed conditions, however in warmed conditions A. asclepiadis declined to local extinction even with priority arrival. Warming therefore negated the impact of priority effects on local coexistence by overriding the priority advantage that enabled A. asclepiadis to persist. These results demonstrate that while priority effects may promote coexistence, warming even a few degrees can eliminate the advantage conferred by early arrival and result in local extinctions.