Predator-Prey Relationships

Predator–Prey Relationships 933 I. Evolutionary Time Scales Predator – prey relationships have been likened to an evolutionary Predator –Prey arms race— the prey become more diffi cult to capture and eat, while Relationships the predators perfect their abilities to catch and kill their prey. Just how strong these selective forces are probably depends on the strength of the interactions between the predators and their prey ( Taylor, 1984 ). ANDREW W. T RITES As predators, marine mammals feed primarily upon fi sh, inverte- ost marine mammals are predators, but some are also brates, or zooplankton, which in turn feed primarily upon other spe- preyed upon by other species. Theoretically, the interaction cies of fi sh, invertebrates, zooplankton, and phytoplankton ( Fig. 1 ) . between marine mammals and their prey infl uences the To capture their prey, marine mammals have evolved special sensory M abilities (e.g., vision and hearing), morphologies (e.g., dentition), and structure and dynamics of marine ecosystems. Similarly, predators and prey have shaped each other’s behaviors, physiologies, morphologies, physiologies (e.g., diving and breath-holding abilities) ( Trites et al ., and life-history strategies. However, there is little empirical evidence 2006 ). They have also evolved specialized strategies to capture prey, of these infl uences due to the relative scale and complexity of marine such as cooperation to corral fi sh, or the production of curtains of ecosystems and the inherent diffi culties of observing and documenting air bubbles used by humpback whales ( Megaptera novaeangliae ) marine mammal predator – prey interactions. to capture herring. Marine mammals have also evolved specialized Sea ice 7 Ice algae 6 3 2 4 8 9 5 10 11 18 12 19 P 14 13 15 P 16 la 17 n 21 20 t de 22 tritu Animal s detritus Figure 1 A simplifi ed depiction of the Bering Sea food web; (1) ice algae; (2) phytoplankton; (3) copepods; (4) mysids and euphausiids; (5) medusae; (6) hyperid amphipods; (7) seabirds; (8, 9) pelagic fi shes; (10) walrus; (11) seals; (12) basket stars; (13) ascideans; (14) shrimps; (15) fi lter-feeding bivalves; (16) sand dollars; (17) sea stars; (18) crabs; (19) bottom feeding fi shes; (20) poly- chaetes; (21) predatory gastropods; and (22) deposit feeding bivalves. From McConnaughey and McRoy (1976) . 934 Predator–Prey Relationships feeding behaviors to capture prey that move diurnally up and down they can indirectly affect the abundance of other species. Their inter- the water column or to capture prey that move seasonally across broad action may also affect the physical complexity of the marine environ- geographic ranges. This in turn has likely infl uenced the life-history ment ( Katona and Whitehead, 1988 ; Bowen, 1997 ; Trites, 1997 ). strategies of marine mammals and their prey. For example, baleen Predation by sea otters on sea urchins is probably the best example whales feed for about 6 months when plankton are abundant and con- of how marine mammals can alter ecosystem structure and dynam- centrated in shallow water, and then fast for the remainder of the year ics (Estes, 1996 ). Sea otters were hunted to near extinction in the when the plankton are too dispersed to make them worth fi nding. late 1800s throughout their North Pacifi c range. Without predation, As prey, marine mammals have had to escape aquatic and terres- urchin populations grew unchecked and overgrazed the fl eshy algae. trial predators ( Taylor, 1984 ; Morisaka and Connor, 2007 ). Porpoise Kelp did not replace the underwater barrens until reintroduced sea (phocoenidae) for example are preyed upon by killer whales (Orcinus otters once again began preying upon sea urchins. orca ) and may have evolved an echolocation and communication Primary production has been estimated to be three times higher system through the selective pressures of predation that falls within a in areas where sea otters are present compared to those areas where range of sounds that killer whales hear poorly or not at all ( Ͻ 2 and sea otters are absent, allowing those organisms that feed upon pri- Ͼ 100 kHz). Other species such as pinnipeds can reduce their risk of mary production to grow faster and attain larger sizes (e.g., mussels being eaten by aquatic predators (sharks and killer whales) by haul- and barnacles). The increase in primary production may even alter ing out and resting onshore; while species such as Steller sea lions settlement patterns of invertebrates. The kelp also provides habitat (Eumetopias jubatus) and northern fur seals ( Callorhinus ursinus ) for fi sh and suspension feeding invertebrates to spawn, grow, and reduce their risk of being eaten by terrestrial predators [wolves ( Canis fl ourish. It can also change water motion and reduce onshore erosion lupus ) and bears] by breeding and hauling out on offshore rocks and and may even block the shoreward movement of barnacle larvae. islands where terrestrial predators are absent. Other species, such as Thus a top predator such as the sea otter can change the structure ringed seals ( Pusa hispida ), give birth in caverns formed between ice and dynamics of marine ecosystems. and snow to avoid predation by polar bears ( Ursus maritimus ). Gray whales (Eschrichtius rohustus ) and walruses ( Odobenus ros- Fish and other cold-blooded species of prey have also evolved marus ) are other species of marine mammals whose foraging behav- a number of strategies to increase their chances of survival ( Trites ior can also affect community structure. For example, gray whales et al ., 2006 ). One is cryptic countershading that enables fi sh to blend turn over an estimated 9 – 27% of the bottom substrate each year in in with the bottom when viewed from above, and avoid detection the Bering Sea. The feeding pits created by gray whales draw 2– 30 when seen from below against a bright sea surface. Many species of times more scavengers and other invertebrates compared to adjacent fi sh, invertebrates, and zooplankton take refuge from predators in sediments. The disturbed sediments may also help maintain the high the deep, dark waters during the day and move toward the surface to abundance of gray whale prey and other early colonizing species. feed under the cover of night. Another strategy evoked by the prey Similarly, walruses turn over bottom substrate in their search for of marine mammals is predator swamping, such as large aggrega- clams and other bivalves. There is some evidence that they may feed tions of spawning salmon and herring (Clupea spp.) that reduce the selectively on certain size classes and certain species and that their numerical effect of predators on their prey populations. Schooling defecation may result in the redistribution of sediment. Thus, the is another antipredator behavior that creates confusion through the interaction of benthic feeding marine mammals with their prey can sheer volume of stimuli from a fl eeing school, making it diffi cult for result in food for scavengers and habitat for other species. a marine mammal to actively select and maintain pursuit of single Interactions between predators and prey also infl uence the individuals. Scattering and fl eeing is yet another option to reduce shapes of their respective life tables (i.e., age-specifi c survival and predation and is used by some prey when attacked by bulk feeders pregnancy rates). In Quebec, Canada, for example, there are a such as baleen whales [e.g., humpback whales and capelin ( Mallotus number of freshwater lakes that are home to landlocked harbor seals villosus )]. The line between feeding and fl eeing is undoubtedly fi ne (Phoca vitulina ). Studies have found that the trout in these lakes are for species of prey and must be continually evaluated by prey to min- younger, grow faster, attain smaller sizes, and spawn at younger ages imize vulnerability to predation. compared to adjacent lakes without seals. As for marine mammals, Marine mammals may also have indirectly infl uenced the evo- they typically have elevated mortality rates during their fi rst few lution of nontargeted species in their ecosystems by consuming years of life. This is likely due to a number of factors, including their P the predators of these species (Estes, 1996 ). The best example of relative vulnerability to predators and their inexperience at capturing this is the apparent infl uence of sea otters ( Enhydra lutris ) on kelp prey and securing optimum nutrition. and other marine algae. Most species of marine algae use second- In the Gulf of Alaska and Bering Sea, killer whales have been impli- ary metabolites to defend against herbivores. However, marine algae cated as a contributing factor, but not the main one, in the decline of in the North Pacifi c Ocean have lower levels of chemical defenses Steller sea lions and harbor seals through the 1980s ( Williams et al ., where sea otters occur compared to algae species inhabiting the 2004). Field observations along the Aleutian Islands indicate that southern oceans where sea otters are not present. Sea otter predation these population declines were followed by a decline of sea otters in on sea urchins and other herbivores may have removed selective the 1990s and that this decline was caused by killer whale predation. pressure for species of marine algae to defend themselves against Some killer whales may have begun supplementing their diet with sea herbivores. Because secondary metabolites are expensive to produce, otters because they could not sustain themselves on the low numbers this may have allowed algae, like kelp, to radiate and diversify with- of remaining seals and sea lions. What ultimately caused the decline of out the added cost of evolving and producing antigrazer compounds. Steller sea lions and began this spiraling change of events is a matter of considerable scientifi c debate. However, it is apparent from math- ematical calculations of population sizes and energetic requirements II.

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