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The Role of Predation in Regulating Sea Urchin Populations in Eastern

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The Role of Predation in Regulating Sea Urchin Populations in Eastern OCEANOLOGICA ACTA- VOL. 19 - W 3-4 ~ -----~- Sea urchin The role of predation Strongylocentrotus droebachiensis Population dynamics Predation in regulating sea urchin populations Eastern Canada Oursin in eastern Canada Strongylocentrotus droebachiensis Dynamique de population Prédation Côte est du Canada RobertE. SCHEIBLING Department of Biology, Dalhousie University, Halifax, Nova Scotia, B3H 411, Canada. Received 14/03/95, in revised form 18/10/95, accepted 24/10/95. ABSTRACT The green sea urchin Strongylocentrotus droebachiensis is the dominant grazer in the rocky subtidal zone in eastern Canada and its local abundance largely determines the structure and dynamics of the coastal ecosystem. Predation has been cited as an important factor controlling populations of this species, although the evidence for this assertion is equivocal. In this review, 1 examine the effects of predators on the distribution, abundance and behaviour of S. droebachiensis at each life history stage and evaluate the potential for predatory control. 1 conclude that our understanding of interactions between this species and its predators is insufficient to support any generalizations about the role of predation in regula­ ting populations. Carefully designed field and laboratory experiments are requir­ ed to rigorously test hypotheses about the effects of predators under realistic conditions, and to identify critical life history stages. Numerical modelling is a promising but underutilzed approach in the study of sea urchin population dyna­ mics and predator-prey interaction. RÉSUMÉ Le rôle de la prédation dans le contrôle des populations d'oursins dans l'est du Canada. L'oursin commun (Strongylocentrotus droebachiensis) est un herbivore domi­ nant de la zone rocailleuse et sous-littorale sur la côte est du Canada. Son abon­ dance locale détermine en grande partie la structure et les dynamiques de l'éco­ système côtier. La prédation a été mentionnée comme étant un facteur important contrôlant les populations d'oursins communs, mais l'évidence de cette affirma­ tion est équivoque. Dans la présente revue, j'examine 1' effet des prédateurs sur la distribution, l'abondance et le comportement de S. droebachiensis à chaque stade biologique, et évalue l'importance de la prédation comme élément contrôleur. Je conclus que notre compréhension des interactions entre l'oursin et ses prédateurs est insuffisante pour généraliser sur le rôle primaire de la prédation sur la taille des populations. Des expériences sur le terrain et en laboratoire, soigneusement planifiées, sont requises pour vérifier rigoureusement les hypothèses sur les effets de la prédation dans des conditions réalistes, et pour identifier les stades les plus vulnérables de 1'oursin. De plus, la modélisation numérique est une approche prometteuse mais peu utilisée dans l'étude des dynamiques de popula­ tion et des interactions prédateur-proie de l'oursin commun. Oceanologica Acta, 1996, 19, 3, 421-430. 421 R. E. SCHEIBLING INTRODUCTION Nova Scotia, K.H. Mann and P.A. Breen hypothesized that the lobster Homarus americanus was the keystone predator Sea urchins, through their grazing activities, play a pivotai in this system, and that overfishing of lobsters, as evidenc­ role in determining the biomass, diversity and productivity ed by declining commercial catches during the previous of subtidal macrophyte communities (reviewed by Law­ decade, enabled the sea urchin population to expand (Mann rence, 1975; Lawrence and Sammarco, 1982; Harrold and and Breen, 1972; Breen and Mann, 1976a). Breen and Pearse, 1987). Due to their ecological importance, there is Mann (1976b) tested this hypothesis with a simulation considerable interest in understanding key factors, such as model based on estimates of sea urchin and lobster popula­ predation, which regulate sea urchin populations. Increased tion density and size structure; urchin recruitment, growth commercial exploitation of sea urchins in recent years has and natural mortality; and size-selective feeding rates of provided a further impetus for research in this area. Nume­ lobsters on sea urchins. The model predicted that lobsters ro us studies indicate that interactions with predators could control sea urchin density in kelp beds below the influence the distribution and abundance of sea urchins, level at which destructive grazing occurs (but see also Mil­ and a reduction in predation pressure has been implicated ler, 1985b). When the model was expanded to include two as a causal factor in sea urchin population outbreaks in other sea urchin predators, rock crabs (Cancer irroratus) sorne areas. However, except for the case of sea otters and wolffish (Anarhichus lupus), the critical density of (Enhydra lutris) in the Northwest Pacifie, evidence for pre­ lobsters required to control sea urchin populations was datory control of sea urchin populations is inconclusive reduced, but this effect was almost entirely due to crabs. (Harrold and Pearse, 1987). Wharton and Mann ( 1981) proposed that destruction of In eastern Canada, the green sea urchin Strongylocentrotus kelp beds by sea urchins led to further declines in lobster droebachiensis is the only herbivorous sea urchin in shal­ abundance, due to loss of habitat and productivity, resul­ low coastal waters and the dominant grazer. The abundan­ ting in a positive feedback that maintains the system in the ce of this key species largely determines the structure and barren ground state. dynamics of the rocky subtidal ecosystem (Miller, 1985a; The predation hypothesis, particularly as it relates to lobs­ Scheibling, 1986). Where sea urchins occur at low densi­ ters as keystone species, has been widely criticized ties, kelp beds (Laminaria spp.) flourish, providing a high­ (Pringle et al., 1982; Pringle, 1986; Miller, 1985b; Vadas ly productive and structurally complex habitat for a variety et al., 1986; Elner and Vadas, 1990). The fundamental of fish and invertebrates. However, sea urchins periodical­ assumption that lobsters are important predators of Stron­ ly undergo population outbreaks which result in wides­ gylocentrotus droebachiensis has not been substantiated by pread destructive grazing of kelp beds and the formation of field and 1aboratory studies (reviewed by Miller, 1985b). "barren grounds" (Mann, 1977, 1982) with much lower Analyses of the stomach contents of lobsters from kelp primary productivity (Chapman, 1981). In areas off New­ beds and barrens indicate that sea urchins are not an impor­ foundland and in the Gulf of Saint Lawrence, sea urchin­ tant component of the diet (Scarratt, 1980; Carter and dominated barren grounds persist for long periods (Him­ Steele, 1982; Elner and Campbell, 1987; but see also melman et al., 1983; Himmelman, 1986 ). Along the Breen, 1987) and laboratory feeding experiments have Atlantic coast of Nova Scotia, however, transitions bet­ shown that sea urchins are not a preferred prey of lobsters ween kelp beds and barren grounds occur on decadal time (Evans and Mann, 1977; Hirtle and Mann, 1978; Elner, scales in association with large fluctuations in sea urchin 1980). Miller ( 1985b) calculated predation rates of sea abundance (Miller, 1985a; Scheibling 1984, 1986; Johnson urchins by lobsters based on feeding studies and estimates and Mann, 1988). of lobster biomass and concluded that lobsters were unim­ Population outbreaks of Strongylocentrotus droebachiensis portant predators of S. droebachiensis in Nova Scotia. in Nova Scotia in the late 1960s were attributed to a release Keats ( 1986) and Hagen and Mann ( 1992) argue that these from predation by lobsters (reviewed by Mann 1977, 1982; calculations do not take into consideration a functional res­ Wharton and Mann, 1981). This hypothesis sparked consi­ ponse of lobsters to increased sea urchin density. However, derable interest in the role of predators in regulating sea another sea urchin outbreak off Nova Scotia in the early urchin populations, but after more than two decades of 1990s was preceded by a period of record high lobster lan­ research the issue of predatory control of S. droebachiensis dings (Scheibling, 1994). This underrnines even the corre­ remains controversial (Miller, 1985b; Pringle et al., 1982; lational basis of the initial hypothesis, that outbreaks in the Pringle 1986; Chapman and Johnson, 1990; Elner and earl y 1970s were preceded by a decline in lobster abundan­ Vadas, 1990; Hagen and Mann, 1992). In this review, I ce. summarize what is known about the effects of predators on Mann and Breen' s (1972) keystone predator hypothesis the distribution, abundance and behaviour of this species at was later modified to include crabs and fish as well as each stage of its life history. In so doing, I evaluate the lobsters (Breen and Mann, 1976 b; Wharton and Mann, potential for predatory control and identify gaps in our 1981; Bernstein et al., 1981, 1983). Miller (1985b) exami­ understanding of predator-prey interactions which may ned the evidence for crab and fish predation on Strongylo­ serve to direct future research. centrotus droebachiensis and concluded that these preda­ tors, like lobsters, can not control sea urchin populations. Predation and sea urchin population outbreaks Keats et al. ( 1986) argued th at there are insufficient data to assess the potential of Atlantic wolffish in limiting sea To account for a population outbreak of Strongylocentrotus urchin abundance in Newfoundland (see also Hagen and droebachiensis in the late 1960s in St. Margaret's Bay, Mann, 1992). As pointed out by Pringle
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