Trophic Links: Predation and Parasitism

We wish to learn:

how predators affect prey populations, and vice-versa
what stabilizes predator-prey interactions and prevents their collapse
how predation can result in complex interactions in natural communities

11/02/2005

Introduction

Predation is used here to include all "+/-" interactions in which one organism consumes all or part of another. This includes predator-prey, herbivore-plant, and parasite-host interactions. These linkages are the prime movers of energy through food chains. They are an important factor in the ecology of populations, determining mortality of prey and birth of new predators. Predation is an important evolutionary force: natural selection favors more effective predators and more evasive prey. "Arms races" have been recorded in some snails, which over time become more heavily armored prey, and their predators, crabs, which over time develop more massive claws with greater crushing power. Predation is widespread and easy to observe. Neither its existence nor its importance is in doubt.

The Development of Predation Theory

Mathematical models of predation are amongst the oldest in ecology. The Italian mathematician Volterra is said to have developed his ideas about predation from watching the rise and fall of Adriatic fishing fleets. When fishing was good, the number of fishermen increased, drawn by the success of others. After a time, the fish declined, perhaps due to over-harvest, and then the number of fishermen also declined. After some time, the cycle repeated.

Linx chasing Hare

The idea that a coupled system of predator and prey would cycle gained further support from analyses of fur trapping records of the Hudson's Bay Company. The number of furs purchased at the Company's forts was meticulously recorded, for well over 100 years. An analysis of the numbers of snowshoe hares, and one of their main predators, the lynx, provides a remarkable record of a predator-prey cycle. Peaks and valleys can be easily observed at roughly 8-10 year intervals.

Logic and mathematical theory suggest that when prey are numerous their predators increase in numbers, reducing the prey population, which in turn causes predator number to decline. The prey population eventually recovers, starting a new cycle.

Paramecium, which also proved useful in test-tube studies of competition, was placed in culture with a predaceous protozoan. These laboratory studies found that cycles were short-lived, and the system soon collapsed. However, if one added more paramecium every few days, the expected cycle was observed.

These results suggested that the predator-prey system was inherently self-annihilating without some outside immigration. The question then arose: why are predator-prey cycles in nature apparently stable, while laboratory cultures quickly collapse?

predator/prey table

What Stabilizes Predator-Prey Systems in Nature?

Observing that frequent additions of paramecium produced predator-prey cycles in a test-tube led to the idea that in a physically heterogeneous world, there would always be some pockets of prey that predators happened not to find and eliminate. Perhaps when the predator population declined, having largely run out of prey, these remaining few could set off a prey rebound. Spatial heterogeneity in the environment might have a stabilizing effect.

A laboratory experiment using a complex laboratory system supports this explanation. A predaceous mite feeds on an herbivorous mite, which feeds on oranges. A complex laboratory system completed four classic cycles, before collapsing.

Observations of prickly pear cactus and the cactus moth in Australia support this lab experiment. This South American cactus became a widespread nuisance in Australia, making large areas of farmland unusable. When the moth, which feeds on this cactus, was introduced, it rapidly brought the cactus under control. Some years later both moth and cactus were rare, and it is unlikely that the casual observer would ever think that the moth had accomplished this. Once the cactus became sufficiently rare, the moths were also rare, and unable to find and eliminate every last plant. Inadequate dispersal is perhaps the only factor that keeps the cactus moth from completely exterminating its principal food source, the prickly pear cactus.

Prey defenses can be a stabilizing factor in predator-prey interactions. Predation can be a strong agent of natural selection. Easily captured prey are eliminated, and prey with effective defenses (that are inherited) rapidly dominate the population. Examples include camouflage in the peppered moth, and prey that are nocturnal to escape detection. Bats capture moths in flight, using sonar to detect them; some moths are able to detect incoming sonar, and take evasive action. Perhaps seriously unbalanced system simply disappear, and those that persist are ones in which the predator is not "too effective", likely because the prey has adaptations to reduce its vulnerability.

The availability of a second prey type -- an alternate prey -- can be stabilizing or destabilizing. Often a predator eats more than one prey. If a predator switches between prey A and B on the basis of their frequency, it will eat A when B is rare and B when A is rare. The prey should exhibit mild oscillations, and the predator should fluctuate little. This would stabilize prey abundances. However, if one prey species is abundant and the predator is unable to reduce its numbers, the result might be the maintenance of a continuously high predator density. Such an abundant predator might then eliminate a second prey species. This is a destabilizing effect of an alternative prey. The hare-caribou-lynx relationship in Newfoundland is a complex example of such a destabilizing effect.

Complex Interactions in Ecological Communities

Predation can have far-reaching effects on biological communities. A starfish is the top predator upon a community of invertebrates inhabiting tidally inundated rock faces in the Pacific Northwest. The rest of the community included mollusks, barnacles and other invertebrates, for a total of 12 species (not counting microscopic taxa). The investigator removed the starfish by hand, which of course reduced the number of species to 11. Soon, an acorn barnacle and a mussel began to occupy virtually all available space, out competing other species. Species diversity dropped from more than 12 species to essentially 2. The starfish was a keystone predator, keeping the strongest competitors in check. Although it was a predator, it helped to maintain a greater number of species in the community. Its beneficial impact on species that were weak competitors is an example of an indirect effect.

When non-native species (exotics) invade an area, they often create "domino" effects, causing many other species to increase or decrease. The rainbow trout, beautiful, tasty, and beloved by anglers, has been purposefully spread to virtually all parts of the world where it can survive. In New Zealand, it has out-competed the native fishes, which now are found only above waterfalls that act as barriers to trout dispersal. Because it is a more effective predator than the native fish species, the invertebrates that are prey to the trout are reduced in abundance wherever trout occur. Algae, which are grazed by the invertebrates, increase because of reduced grazing pressure. This is an example of a trophic cascade.

Introduction of the opossum shrimp to Flathead Lake, Montana, is yet another example of complex interactions in ecological communities.

Summary

Predation, a "+/-" interaction, includes predator-prey, herbivore-plant, and parasite-host interactions. These linkages are the prime movers of energy through food chains and are an important factor in the ecology of populations, determining mortality of prey and birth of new predators. Mathematical models and logic suggests that a coupled system of predator and prey should cycle: predators increase when prey are abundant, prey are driven to low numbers by predation, the predators decline, and the prey recover, ad infinitum. Some simple systems do cycle, particularly those of the boreal forest and tundra, although this no longer seems the rule. In complex systems, alternative prey and multi-way interactions probably dampen simple predator-prey cycles.

Predator-prey systems are potentially unstable, as is seen in the lab where predators often extinguish their prey, and then starve. In nature, at least three factors are likely to promote stability and coexistence. Due to spatial heterogeneity in the environment, some prey are likely to persist in local "pockets" where they escape detection. Once predators decline, they prey can fuel a new round of population increase. Prey evolve behaviors, armor, and other defenses that reduce their vulnerability to predators. Alternative prey may provide a kind of refuge, because once a prey population becomes rare, predators may learn to search for a different prey species.

Predation, while not the only cause of complex community interactions, has often been shown to have strong indirect effects and cascading effects. Predation also can be a strong agent of natural selection, as we saw in the case of the peppered moth.