Macrophytic cover, predation, and survivorship
Mean numbers of prey survivors in two replicates of 24-hr exposure to predators in the experimental cover conditions, and mean numbers of survivors expected on the basis of assuming no loss to the predator are presented in Table I. The pattern of differences between observed and expected distributions is similar for both species. The number of survivors in the condition of no cover is far less than that expected; “Ambulia” and Ulva are associated with observed numbers of survivors that are much closer to the numbers expected; and in Codium there is little difference between what is observed and what is expected. This general pattern holds, even when the predator is exposed simultaneously to the two prey species. Chi-square tests comparing the observed and expected distributions (Table I) indicate that for P. pugio, whether exposed alone to the predator or simultaneously with P. vulgaris, the difference between observed and expected distributions is significant (df = 3; α = .05). For P. vulgaris, these differences are not significant.
Table I. Comparison of the observed distribution of survivors (mean of two replicates) with that expected by assuming no loss of individuals due to predation. O = observed; E = expected; “*” = significant (α = .05); “ns” = not significant (α = .05).
Predation pressure under the experimental conditions of exposure and cover, as measured by the percentage of the starting prey population missing after 24-hr exposure to the predator, is reported in Table II. Whether exposed alone or together, both prey species are effectively sheltered by Codium, where both species lose <10% of the starting population to the predator. However, when exposed alone, the protection afforded P. vulgaris by Codium is so great that even the small loss of P. pugio is 3.5 times that of P. vulgaris. Predation pressure in “Ambulia” and Ulva is similar for both prey species, but for P. pugio it is about 1.5 times as much as for P. vulgaris. In these moderately protective covers, the presence of the second prey species decreases the predation pressure on P. pugio and increases it on P. vulgaris. However, neither of the changes associated with the presence of the second prey species is very large. Exposed alone and without cover, predation pressure results in the loss of little more than one-third of the starting population of P. vulgaris but removes more than half of the starting population of P. pugio. When the two prey species are exposed together without cover to the predator, predatory loss increases for both species (6% for P. vulgaris and 18% for P. pugio). Whether exposed without cover alone or in combination with P. vulgaris, P. pugio bears about one and one-half times the predation pressure of P. vulgaris.
Results of the ANOVA performed on survivorship values (as arcsine transformed percentage of starting population) in each condition of experimental cover are reported in Table III. The lack of significant variation contributed by any interaction between and among the three independent sources of variation indicates that the prey (P. pugio and P. vulgaris) responded alike to the designed changes in cover, exposure, and species. The lack of significant variation contributed by exposure reveals that the impact of the predator upon a specific prey species was not reduced or enhanced by the presence of an equal number of individuals of the second prey species. The significant variation contributed by the independent sources, cover and species, shows that these variables influenced importantly the number of prey consumed by the predator.
Results of the multiple means analysis (T-method) of the effect of cover are given in Table IV. Predation by Fundulus in the absence of cover caused a loss >50% of the experimental population of Palaemonetes prey. This loss of population was reduced significantly by adding cover, allowing at least 70% of the prey population to survive. The different macrophytic covers, however, were not equally effective in increasing prey survivorship. While Ulva and the plastic “Ambulia” allowed about 30% loss to the predator, Codium permitted the predator to consume an average of only 2% of starting population of prey.
The results of means comparison (T-method) of the effect of species are in Table V in which it is shown that overall mean survivorship by P. vulgaris was significantly greater than that of P. pugio.