Competition for Space: Growth Rate, Reproductive Output, and Escape in Size

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Abstract
The coexistence of 2 spp. on a homogeneous substratum was modeled for the situation where one is the fugitive (faster recruit, poorer competitor) and the other is the dominant (slower recruit, better competitor; Levins and Culver 1971; Armstrong 1976). The consequences of distinct size (or age) classes in competitive encounters have received little theoretical attention, and there are few examples of size-dependent competitive success for real organisms. A system of 3 differential equations was used in this study to examine potential coexistence between one species with 2 size classes, and a 2nd species not divided into classes. The 1st species is outcompeted by the 2nd when small but cannot be outcompeted (overgrown) when large. It thus has a size refuge from competition and the 3 equation system is stable for realistic values of recruitment and growth rates. An example of a size refuge in space competition was found in the rocky subtidal marine community in northern Massachusetts [USA]. The octocoral Alcyonium siderium is overgrown by the colonial ascidian Aplidium pallidum when the former is less than 15 mm colony diameter, but not when larger. The 2 spp. persist indefinitely (4 yr of photographic documentation) on relatively homogeneous flat surfaces of vertical rock cliffs, and together take up more than 60% of the space.