Multiple environmental changes interact to modify species dynamics and invasion rates

Multiple aspects of the environment often change at the same time, influencing populations directly by modifying their physiology, but also indirectly by influencing other interacting species. The impacts of each environmental change upon population dynamics are usually assumed be independent of the state of other aspects of the environment, despite evidence at the individual level indicating that the combined impacts are often non‐additive. The importance of indirect effects mediated through community interactions also has high uncertainty. We used experimental microcosms to determine whether environmental factors interact to affect species dynamics and the relative importance of direct and indirect effects on species dynamics. We factorially manipulated three aspects of the environment (temperature, food availability and salinity) and examined reciprocal invasions of competing protist species under each environment. Experimental observations were used to parameterize a dynamic model of the system. Using this model and a novel variance decomposition method, we examined the mechanisms by which environmental changes altered species invasion rates. The three environmental factors interacted when modifying species growth rates, intra‐ and interspecific competition, causing the impact of each environmental change on species dynamics to depend crucially on the state of other aspects of the environment. Indirect changes in the abundance of the resident competitor and its interspecific competitive ability were the main cause of environmental driven variation in invasion rates, whilst direct effects on species intrinsic growth rates were relatively unimportant. This indicates that, to understand and ultimately predict species and community responses to multiple environmental changes, we should consider their joint impacts and the mechanisms by which they interact to modify key ecological processes such as competition.