Factors influencing the vertical distributions of two intertidal porcelain crab populations
Creators: name: Zittin, David
Publisher: name: University of British Columbia
Year: 2010

description: Overlapping populations of porcelain crabs were observed on several beaches in Barkley Sound, British Columbia. The lower limit of the higher population (Petrplisthes cinctipes) and the upper limit of the lower population (Petrolisthes eriomerus) were observed to be temporally and spatially stable for three years. What prevented the higher zoned population from inhabiting intertidal levels below the lower limit of its population and what factors set the upper limit of the lower population? When P. eriomerus were transplanted above the upper limit of their distribution, they had higher moratality rates than controls which were located within their population band. This species cannot live as little as 0.3 m above its normal population range. The cause of these mortalities were not determined. In contrast, P. cinctipes demonstrated low mortality rates at all intertidal levels including those which were well below the lower limit of its population band. Thus, physical factors do not determine the lower distributional limit of the higher population. Observations made on intraspecific spacing patterns in a laboratory arena showed that both species tended to maintain even spacing patterns by various types of agonistic behavior which were similar for both species. The lower intertidal species, however, demonstrated a greater mean distance to nearest neighbor than did the higher species. I hypothesized that the lower species was a superior competitor for space. If I failed to reject this hypothesis, it would suggest that P. cinctipes is unable to inhabit the low intertidal zone because of exclusion by agonistic interactions with P. eripmerus. Results of the arena experiments did not support this hypothesis and in fact suggested the contrary; P. cinctipes consistently acquired and maintained a disproportionate amount of space over a favorable habitat in the arena. Intra- and interspecific effects on vertical distribution were tested for in an experiment which employed long, narrow enclosures (runs). By comparing the distributions of the two species in their single species control enclosures, I found that P. eriomerus demonstrated a strong preference for lower portions of the runs. In contrast, P. cinctipes demonstrated no preference for vertical range in the enclosures. When mixed, the experimental P. cinctipes population shifted slightly upwards. However, there was no clear indication that zonation was occurring within the enclosures. Results of the run experiments also showed that P. cinctipes reduced densities of the experimental P. eriomerus population near the upper limit of its vertical range. This is unusual in that it differs from the prevailing idea that upper distributional limits are caused by physical factors (Connell 1972) and not by biotic factors. Other zonation studies have suggested that the proximate cause of upper limits of an intertidal population are lethal stresses from the physical environment (Connell 1961a, 1961b, 1972, Paine 1974). These studies were conducted on sessile or very slow moving invertebrate species. In contrast, my results show that the proximate cause of the vertical distribution limitation for the lower population is a behavioral response of the lower species to an increasingly harsh physical environment with increasing intertidal height. However, the density of this population near its upper limit is probably also lowered by competition with the higher species. The nature of the factor(s) which determine the lower limit of the P. cinctipes population remain unresolved. However, it is unlikely that physical stresses prevent this species from living below the lower limit of its population band. I was unable to obtain evidence that competition for space with the lower species determined the lower limit of the P. cinctipes population.