Non-additive effects of intra- and interspecific competition between two larval salamanders

1. Assessment of the relative strengths of intra- and interspecific competition has increased in recent years and is critical to understanding the importance of competition. Yet, whether intra- and interspecific competition can have non-additive effects has rarely been tested. The resulting fitness consequences of such non-additive interactions are important to provide the context necessary to advance our understanding of competition theory. 2. We compared the strength of additive and non-additive intra- and interspecific competition by manipulating densities of a pair of larval salamanders (Ambystoma talpoideum and A. maculatum) in experimental mesocosms within a response surface design. 3. Intraspecific density had the strongest effect on the strength of competition for both species, and few observed comparisons indicated interspecific competition was an important factor in predicting body size, growth or larval period length of either species. 4. Non-additive effects of intra- and interspecific competition influenced some response variables, including size and mass at metamorphosis in A. maculatum, but at a reduced strength compared to intraspecific effects alone. 5. Intraspecific competition was thus the dominant biotic interaction, but non-additive effects also impact the outcome of competition in these species, validating the importance of testing for and incorporating non-additive density effects into competition models.