Myra J. Shulman

Priority Effects in the Recruitment of Juvenile Coral Reef Fishes

Competing models of community structure in assemblages of coral reef fishes have suggested that (1) these assemblages are structured by deterministic interactions between species, or between species and resources, or (2) the composition of these assemblages are determined by highly variable settlement from planktonic larvae. We examined interactions among newly recruited juvenile fishes and between juvenile fishes and transplanted resident damslfish on artificial reefs in St. Croix, United States Virgin Island. Two kinds of priority effects occurred: (1) recruitment of three species of settling juveniles significantly decreased in the presence of the territorial damselfish, and (2) prior settlement of a juvenile predator lowered successful recruitment of two juvenile prey species. The first effect increases determinism in the structure of coral reef fish assemblages, while the second decreases their predictability.

REGULATION OF INTERTIDAL FOOD WEBS BY AVIAN PREDATORS ON NEW ENGLAND ROCKY SHORES

Although there is a large body of research on food webs in rocky intertidal communities, most of the emphasis has been on the marine benthic components. Effects of avian predation on highly mobile predators such as crabs, remains practically unstudied in rocky shore ecosystems. The crab, Cancer borealis, is an important component of the diet of gulls (Larus marinus, L. argentatus) at the Isles of Shoals, Maine, USA. C. borealis prey include the predatory gastropod Nucella lapillus L., the herbivore Littorina littorea, and mussels Mytilus edulis L. We hypothesized that gulls reduce abundance of C. borealis in the low intertidal and shallow subtidal, thereby allowing C. borealis prey to persist in high numbers. A study of crab tidal migration showed that C. borealis density nearly doubled at high tide compared to low tide; thus, crabs from a large subtidal source population migrate into the intertidal zone during high tides and either emigrate or are removed by gulls during low tides. Results from a small-scale (1 m2) predator caging experiment in the low intertidal zone indicated that enclosed crabs significantly reduced L. littorea abundance when protected from gull predation. In a much larger-scale gull exclusion experiment, densities of C. borealis increased significantly during low and high tides in exclosures relative to the controls. C. borealis density was inversely correlated with changes in the abundance of two mesopredators Carcinus maenas and Nucella lapillus, and with the space-occupier M. edulis. There was a similar negative correlation between abundance of C. borealis and the change in abundance of the herbivore L. littorea, but the trend was not significant. Mortality of tethered L. littorea was associated with C. borealis density across sites. However, preferred algae did not change in response to L. littorea density during the experiment. Thus, we found suggestive, but not conclusive, evidence for a three-level cascade involving gulls, crabs, and L. littorea. Our studies strongly suggest that gulls, as apex predators, generate three-level trophic cascades in rocky intertidal food webs by preventing the highly mobile subtidal predator, C. borealis, from establishing substantial populations in the low-mid intertidal zone thereby indirectly enhancing densities of two key mesopredators (N. lapillus, Carcinus) and blue mussels (M. edulis).

PREDATION RISK, PREY ABUNDANCE, AND THE VERTICAL DISTRIBUTION OF THREE BRACHYURAN CRABS ON GULF OF MAINE SHORES

Abstract Three large brachyuran species are common in the intertidal and shallow subtidal of New England rocky shores: two native crabs Cancer borealis (Jonah crab) and Cancer irroratus (rock crab), and the introduced crab Carcinus maenas (European green crab). For these three co-occurring species in the Isles of Shoals (Gulf of Maine, USA), we compared distribution and abundance to survivorship and prey availability along a depth gradient and examined stomach contents and prey preference. The three species show differences in vertical distribution: Carcinus is more abundant in the intertidal, while both species of Cancer are more abundant in the subtidal. Survivorship of both species of Cancer increases with increasing depth, while survivorship of Carcinus decreases with increasing depth, perhaps corresponding to differential vulnerability to predation by gulls in the intertidal and by decapods and fish in the subtidal. There were notable differences in laboratory prey preference experiments: C. irroratus consumed both small mobile and non-mobile prey (amphipods, small snails, and small mussels), while Carcinus consumed primarily small mobile prey (amphipods and isopods). In contrast, C. borealis consumed larger, heavier bodied prey (larger snails and mussels) but did not eat amphipods or isopods. However, differences in prey preference among crab species were greater than the differences in realized diets. Based on stomach content analysis, the blue mussel Mytilus edulis was the majority component of stomach contents for all three species. Some differences were evident in the remaining diet components: Carcinus was the most omnivorous (> 30% green algae), C. borealis consumed more snails and arthropods, and C. irroratus consumed the most mussels. Overall, species distribution does not track the distribution of the preferred prey of each species; rather, the distribution corresponds with patterns of survivorship, indicating predominant top-down control of crab distribution.

Impact of Raccoons on Breeding Success in Large Colonies of Great Black-backed Gulls and Herring Gulls

Abstract For most ground-nesting seabirds, offshore islands usually serve as refuges from mammalian predators, however, introduced predators pose a substantial threat to many species. In New England, thousands of Herring Gulls (Larus argentatus) and Great Black-backed Gulls (L. marinus) breed on hundreds of coastal islands. The Isles of Shoals (New Hampshire/Maine) archipelago has some of the largest colonies of both gull species in New England. In 2004, we noted unusually large numbers of failed nests on Appledore Island, Maine and hypothesized that Raccoons (Procyon lotor) were the cause. Therefore, we quantified egg and chick mortality and documented the presence of Raccoons. We trapped Raccoons on Appledore and Smuttynose islands and conducted nest surveys on these and two other islands in the Isles of Shoals. Surveys of marked nests on Appledore Island revealed high levels of egg and chick mortality in early June 2004; by late June 58% of nests were empty. We also found that on Appledore and Smuttynose, where Raccoons were observed, the odds that a nest would be empty were 17 times greater than on Lunging or Duck Islands where no Raccoons were observed. During 2004, eleven Raccoons were trapped off of Appledore and Smuttynose Islands. A survey of nests in 2005 showed that gull breeding success increased significantly on Appledore and Smuttynose islands, whereas there was no significant change in the relatively high ratio of chicks to nests from 2004 to 2005 on Lunging Island where Raccoons were not observed. Thus, we concluded that the presence of a few Raccoons on Appledore and Smuttynose islands was sufficient to cause substantial breeding failure in large colonies of larids.

Intraguild Predators: Behavioral Changes and Mortality of the Green Crab (Carcinus maenas) During Interactions with the American Lobster (Homarus americanus) and Jonah Crab (Cancer borealis)

Abstract The green crab Carcinus maenas invaded the Gulf of Maine over a hundred years ago and has become a well-established member of coastal ecosystems. Carcinus is most abundant in the intertidal zone despite higher availability of invertebrate prey in the nearby shallow subtidal. A previous study revealed a higher mortality rate for Carcinus in the subtidal than in the intertidal zone. Two other decapods, the American lobster Homarus americanus and Jonah crab Cancer borealis, have their highest abundance in the shallow subtidal. These three species have similar diets and predation among and within species occurs. We performed a laboratory experiment to investigate the effects of H. americanus, C. borealis, and larger conspecifics on mortality and behavior of Carcinus. Homarus americanus was the most effective predator on Carcinus, consuming 27% within 24 h. In contrast, only 5% of Carcinus were eaten by C. borealis and none by conspecifics. The presence of American lobsters had dramatic effects on Carcinus behaviors: hiding and climbing increased while walking and standing on the open substrate decreased. In contrast, Carcinus showed no significant changes of behavior in the presence of C. borealis or larger conspecifics. The consumptive and non-consumptive (behavioral) effects of lobsters on Carcinus appear to be one major cause of the scarcity of Carcinus in subtidal communities. These effects are also likely to reduce Carcinus predation on the small invertebrates that are common prey for all the large decapods in this ecosystem.