Key Topics • Types of predation. • Effects of predation on prey populations and communities. • The Refuge Theory. • The keystone Predator Theory.
Types of Predation • Herbivory - eating plant tissue. • Browsers • grazers • Carnivory - eating of animal tissue. • Predation. • Parasitiods
Some Effects of Predation • Set distribution limits of prey. • Set carrying capacity. • Influence age/size frequencies within prey populations. • Influence diversity of the community.
Moose and Wolves on Isle Royale After wolves discover a moose population on an isolated island, the moose population attains a lower, more stable carrying capacity.
The Malthusian Dilemma. • Because population growth potential is geometric and world is finite, there is a struggle for existence. • Darwin reasoned that struggle should be most intense among most similar organisms - hence competition should predominate. • The competitive exclusion principal maintains that no two species could indefinitely occupy the same niche, i.e. have the same resource use. • Therefore, two related questions have driven much of the work in benthic ecology: • How do natural enemies coexist? • How can so many similar species coexist? (Why does not competition eliminate similar species?)
Two possible answers • The Refuge Theory is one hypothetical answer to the question of how predators and prey can coexist indefinitely. • The Keystone Predator Theory is one hypothetical answer to how similar species (potential competitors) can coexist.
The Refuge Hypothesis • In all stable predator - prey systems, prey attain an age/size refuge or a spatial refuge. • “Refuged” prey produce young that replenish their population and provide food for predators. • Refuges depend on adaptive limitations of the predators, e.g. intolerance of exposure at low tide, limited prey handling abilities. • In the following examples, “intertidal zonation” is attributed to spatial refuges.
The Case of Whelks and Barnacles, Connell 1970 • Location San Juan Island, Washington. • Predators, three species of whelks, Thais emarginata, T. lamellsoa, T. canaliculata. • Prey, three species of barnacles, Chthamalus dalli, Balanus glandula, Semibalanus cariosus. • Larvae of all three species occur over a broad range of shore levels, but adults show restricted vertical zonation. • Observations and experiments indicate that Balanus adults reside in a spatial refuge, Semibalanus attains and age/size refuge.
The whelk -barnacle system on two Continents. In contrast to Scotland, where competition was the limiting factor, Washington shores had intense predation. Barnacle zones are different types of prey refuges.
Connell’s Evidence • Whelks do not forage on the highest shore levels. • Balanus adults survive well in the upper zone above the reach of the whelks. • Balanus can survive in the lower, Semibalanus zone only if protected from whelks. (Cage experiment) • In the lower zone, large, adult Semibalanus survive attacks of whelks. Smaller barnacles, Semibalanus juveniles and Balanus adults, are preferentially eaten over other prey. • Therefore, the upper zone is a spatial refuge for Balanus, the lower zone is an age-size refuge for Semibalanus.
The Mytilus-Pisaster interaction, Paine 1974,1976 • Site Olympic Peninsula, Washington State • Prey, the sea mussel Mytilus californianus. • The predator Pisaster ochraceus. • Pisaster extirpates mussel on lower shores levels, promoting diversity. • Mytilus attains a spatial refuge on upper shore levels, and, infrequently, an age-size refuge on lower shore levels.
Predation by Sea Stars Pisaster ochraceous
Photograph of the “Glacier Experiment” After sea stars were removed from foreground area, mussels took over lower shore levels.
Statement of Keystone Predator Hypothesis • Keystone predators are characteristically large, or numerous consumers that prey an assemblage of competing species. • Mortality on a dominant competitor species keeps its numbers in check, freeing resources for subordinate species. • Therefore, keystones promote coexistence of competitors at lower trophic levels. • Experimental removal of the predator leads to transformation of the community. Certain competitors take over the limiting resource.
Significance of Keystone Predator Hypothesis • Arose in the context of the Malthusian dilemma. • An example of the importance of indirect effects in complex food webs. • Explains how predators can enhance the diversity of a community.
Indirect effects in food webs • Pisaster indirectly affects barnacles (Balanus spp.) and algae by preferential preying on their principal, superior competitor, mussels (Mytilus californianus), and by preying on other consumers of the barnacles.
Contradictions of the Refuge Hypothesis: Panulirus-Mytilus Interaction. The prey Mytilus sp. the mussel The predator Panulirus sp. the spiny lobster
Time lapse photography rigin the intertidal zone at low tide, Catalina Is.
Lobster Exclosure Experiments on Red Algae “Turfs” The the lobster exclosures, mussels survive and crowd out the algae, supporting the keystone predator hypothesis.
Lobster exclosure experiment in a mussel bed. Lobster predation within mussel beds affects size frequency distributions without significantly affecting percent covers of mussels. Marsh 1986 and Wootton 1993 present other examples of predator effects within mussel zones.
Some mussel beds fall completely within the foraging range of the highly mobile lobsters. See Menge and Lubchenco 1981 for discussion
Absolute age size refuges: rarely, if ever, attained.
Conclusion • The Keystone predator hypothesis is supported by the fidnings about the Panulirus-Mytilus interaction • While the refugia concept fits for some predator prey systems, it does not apply to all, e.g. the Panulirus-Mytilus interaction • Rather than invoking refuges, the most recent formulations of theory look at balances between rates of prey input (recruitment) and loss (predation) to explain co-existence.