Coexistence in a Kelp Forest: Size, Population Dynamics, and Resource Partitioning in a Guild of Spider Crabs (Brachyura, Majidae)

Abstract

The population dynamics and resource partitioning of five species of spider crabs (Loxorhynchus crispatus, Pugettia producta, Mimulus foliatus, Pugettia richii, and Scyra acutifrons) were measured in a kelp forest on the central California coasts. The distribution of the five species showed zones of peak abundances along a transect from the intertidal zone through the kelp forest to a deep reef. P. richii was the most abundant species with peak densities of 6 crabs/m2 at the middle and inner edge of the kelp forest. M. foliatus was second most abundant with a peak of 3.5 crabs/m2 at the middle and outer edge of the kelp forest. S. acutifrons had peak densities of about 3 crabs/m2 at the outer edge of the kelp forest. P. producta had a peak of 2 crabs/m2 in the intertidal zone. L. crispatus was present in low densities of about 0.5 crabs/m2 from the middle of the kelp forest to the deep reef. The greatest combined density of spider crabs (11 crabs/m2) occurred at the middle of the kelp forest. Densities of all five species in the main study area in the middle of the kelp forest showed a synchronous cycle with a maximum in late summer and fall and a decline in winter to a low in spring. The cycle appeared to be caused by a combination of density—independent mortality from winter swells and storm activity and by slower development and growth rates during the upwelling period of colder water temperatures. Brooding frequencies of all the species remained high year—round. Size structures of the populations did not change during bimonthly sampling, indicating continuous recruitment. Size—frequency analysis of crab populations along the transect indicated that juveniles of Pugettia producta recruit into the intertidal and shallow Phyllospadix zones and migrate out into the kelp forest as they grow, but the other species did not have zones of recruitment separate from adult distribution. The diversity of predators which take spider crabs as a major portion of their diets indicated that the general level of predation pressure is high and may limit the overall population levels of the crabs. Predation by sea otters probably limits the density of P. producta, and the other four species are probably limited by fish predation. The crabs exhibit different color morphs, color change, and decorating behavior, which are adaptive for concealment from predators. Size at maturity of the five species spanned an order of magnitude in carapace width from small Scyra acutifrons (1 cm) to large Loxorhynchus crispatus (10 cm). Mean body size of mature females and mean body size of the entire population of all pairs of species except Mimulus foliatus/Pugettia richii had ratios larger than Hutchinson's (1959) predicted value of 1.28. Body size may reflect limitations of the crabs' utilization of crevice refuges in the microhabitat. Partitioning of the microhabitat resource was measured by substrate utilization. Mimulus foliatus had the largest microhabitat niche breadth and was found on most types of substrates, with an important refuge in kelp holdfasts. Pugettia richii was found primarily on the alga Cystoseira osmundacea and in coralline algal mats. Scyra acutifrons occurred in the interstices of algal/invertebrate turf, and Loxorhynchus crispatus was found on top of the turf. Pugettia producta occurred upon the kelp plants and had the narrowest microhabitat niche breadth. Analysis of stomach contents showed partitioning of food resources. Pugettia producta was a strict specialist grazing on giant kelp, Macrocystis pyrifera. Mimulus foliatus and Pugettia richii also had narrow diets of mostly drift M. pyrifera. Scyra acutifrons had major food categories of detritus, sponge, and pieces of M. pyrifera trapped in the algal—invertebrate turf. Loxorhynchus crispatus was a dietary generalist foraging on a broad range of invertebrates and giant kelp. Niche separation in the guild is multidimensional, and similar utilization of one resource is generally complemented by dissimilar utilization of another resource. Graphical analysis of niche separation for the three dimensions of microhabitat, food, and body size showed that Pugettia producta is an overall specialist and Loxorhynchus crispatus is a generalist. Scyra acutifrons also has a distinct niche within the algal invertebrate turf, but Mimulus foliatus and Pugettia richii exhibit extensive overlap in all of the parameters measured. However, the degree to which a species is a specialist vs. a generalist for a resource did not relate to the amount of niche overlap with the rest of the guild. Microhabitat resources appear to be the most important for niche separation and may also be explained as adaptations for minimizing predation.