Intrinsic growth shapes spatial patterns of species competition

Strengths of competition between the same two species vary across space due to complex factors, including environmental conditions (e.g. physical exposure, predation) and internal processes (e.g. intrinsic growth). However, causal links among these factors are obscure. Although intrinsic growth rate reflects a species’ best possible performance under local conditions, it is rarely used to indicate environmental suitability. Using an 18-year dataset from a rocky intertidal two-species system ( Chthamalus dalli and Gloiopeltis furcata ), we explored how environmental factors and intrinsic growth rates influence strengths of competition within and between species. We tested both direct environmental effects and indirect effects that act through intrinsic growth. Results showed that intrinsic growth rates had strong influences on competition, while direct environmental effects were weak. Most environmental influence was indirect through intrinsic growth rates. These findings suggest under-appreciated role of intrinsic growth in shaping species interactions and the importance of indirect pathways for understanding competition.