Role of Resource Allocation, Aggression and Spatial Heterogeneity in Coexistance of Two Competing Intertidal Starfish

Menge, Jane; Menge, Bruce

Previous field experiments indicated that the rocky intertidal starfish Pisaster ochraceus and Leptasterias hexactis compete for food. Here, the method of resource partitioning and mechanisms of competition and of coexistence are examined. Comparisons between the diets, feeding activities (high tide), and location at low tide are made for co-occurring Leptasterias (0.01-11.5 g wet weight) and small (0.01-11.5 g), medium (11.5-100 g), and large (100-550 g) Pisaster. These comparisons suggest that (1) there is no spatial or temporal subdivision of the habitat while feeding, (2) there is some microhabitat separation at low tide, with Leptasterias farther back both in crevices and under rocks, and (3) although the diets of the two species overlap greatly in both size and species of prey eaten, each obtains most of its energy from a different combination of prey size and species. Leptasterias obtains most of its energy from small- to medium-sized mobile prey; small Pisaster depend on small, sessile prey, and medium to large Pisaster depend on larger and more mobile prey. Thus, the food resource is divided along a combination of resource dimensions and not along a single resource "axis." Laboratory experiments indicate (1) that Pisaster is highly aggressive toward Leptasterias, using its pedicellariae to pinch the latter, and (2) the effect of this aggression is to reduce the feeding rate of Leptasterias. Field experiments confirm this result. This aggression is somewhat surprising in light of the complete spatial overlap of the competitors. Coexistence is possible because in nature the smaller seastar evidently acclimates relatively quickly to changes in Pisaster abundance. Leptasterias is not aggressive towards Pisaster, and its location at low tide suggests it avoids close contact with the larger asteroid. The primary competitive advantages of the larger asteroid thus are the ability to reach a larger size (and thus to capture "better" prey), and aggression. The competitive advantage of the smaller starfish is suggested by calculations of the number of calories of prey consumed per gram wet weight of starfish per tidal cycle. These comparisons indicate Leptasterias has a higher rate of energy intake than Pisaster. Since this advantage could conceivably give Leptasterias a great initial size advantage over small Pisaster, aggression may function to reduce the foraging time of the former. As Pisaster recruitment is low, small individuals of this species are rare. Leptasterias may coexist with Pisaster by (1) reproducing at a small size (2 g wet weight vs. 70-90 gin Pisaster) and (2) using a spatial portion of the intertidal (deeper parts of crevices) not normally utilized by Pisaster during low tide when the two species would be in closest association. Pisaster's aggressive superiority may render invasion of this system by large asteroid predators similar to Pisaster impossible.

Role of Resource Allocation, Aggression and Spatial Heterogeneity in Coexistance of Two Competing Intertidal Starfish

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