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Predation, Herbivory, and Parasitism Types of Species Interactions When two species interact, the effects for each species can be positive, negative, or neutral. Competition -/Amensalism -/0 Commensalism +/0 Mutualism +/+ Exploitation +/- Types of Species Interactions Exploitative include: Predation Herbivory Parasitism interactions Parasitism Parasites live in or on their host's body and often spend most or all their lives eating tissues or body fluids of just one host individual. Sometimes multiple generations of parasites live on the same host. Because parasites depend on their hosts for continued feeding, they do not generally kill their hosts (at least not immediately). Parasitism Most parasites associated with a single host species have a free-living life stage during which they are not attached to a host. A great many other parasites, though, have multiple hosts with different life stages adapted to each host (and possibly free-living stages as well). Endoparasites The inside of an organism is a much more stable and protected environment than the outside, and endoparasites (e ndo = inner) take advantage of this by living and feeding inside their hosts Ectoparasites Although parasites tend to be tiny and hard to see, ectoparasites (ecto=outer), which live on the outside surface of their host, are often easier to observe. Ectoparasites By living outside their host, ectoparasites avoid having to defend themselves against the host's immune system. The trade-off, however, is that they are exposed to predation and a sometimes harsh exterior environment. Some species of predators specialize on ectoparasites. Ectoparasites The cleaner wrasse is a type of fish that lives in coral reefs. Other fish will wait patiently while a cleaner fish picks off parasites from their scales, mouth, and gills. Parasitoids and Hyperparasites Most of the parasites discussed so far do not directly kill their hosts but, parasitoids do. Parasitoids develop inside their host and essentially eat it from the inside out. When the host is completely consumed, the parasitoid transforms into an adult and crawls out to find new hosts for its offspring. Parasitoids and Hyperparasites Interestingly, the parasitoid wasp has a parasitoid of its own, known as a secondary parasitoid, or hyperparasitoid. Hyperparasitoid wasps find aphids with internal parasitoid larvae and lay eggs inside the larvae. Ecological Impacts of Parasites Parasites can have broad ecological impacts. These effects begin at the individual level; because parasites rob their hosts of resources, host survival and/or reproduction can be reduced even when hosts are not killed by parasites directly. Effects of Parasites on Individual Hosts A protozoan causes rats to become attracted to cats. A nematode turns the bellies of Amazonian ants red, attracting berry-eating birds. A fluke makes ants climb grass stems so they will be eaten by sheep. A trematode causes killifish to swim closer to the surface of the water, making them easy targets for birds. Herbivores Grazers are herbivores that specialize on herbaceous plants (grasses, forbs, and herbs), while browsers eat the leaves, bark, and twigs of woody plants. Herbivores that specialize on seeds are granivores, while those specializing on fruits are frugivores. Plant Defenses Against Herbivory Herbivory is generally not a positive experience for a plant, so plants have evolved forms of self-defense. Mechanical - Developing structures like thorns that make it harder for animals to eat them. Chemical - Producing chemicals that are noxious or poisonous to herbivores. Nutritional - Growing structures that are less nutritious for grazers (have less N and P). Tolerance - Adaptions to regrow quickly after being grazed. Impacts of Herbivory on Plant Communities Herbivory can reduce the overall number of plants and can also have a profound impact on the composition of a plant community. Impacts of Herbivory on Plant Communities When plants have evolved without selective pressure from herbivory, they may not be very well-defended, and the influence of herbivores can be even more drastic. This is a problem when new herbivores are introduced. Predation Lynx are fast, but lack endurance, so they don't chase hares over long distances. Instead, they stalk hares, hiding behind trees and brush until they can get close enough to pounce on a hare. Predation Stalking is one of a variety of strategies used by predators for catching mobile prey. Others include pursuit, where the predator chases prey over a distance; ambush, where the predator hides and waits in one spot until prey comes along; and random encounter, where the predator and prey meet by chance. Animal Defenses Against Predation Just as plants have evolved defense mechanisms to combat herbivory, animals have evolved ways to defend themselves from predation. Chemical - Producing chemicals that are noxious or poisonous to predators. Physical - Developing physical barriers to predation (e.g., shells). Behavioral - Behaving in ways that minimize risk from predation. Animal Defenses Against Predation Aposematism - Warning colors, sounds, or other characteristics to alert predators that this prey will not be tasty. Crypsis - Camouflaged colors, shapes, and other ways of hiding from predators. Mimicry - Looking, sounding, or in other ways mimicking a species that predators avoid. Predator-Prey Dynamics Lynx & Snowshoe hare population size records available back to 1800s. Fur traders Populations of lynx and hare seemed to follow an interesting cyclic pattern. Both species peak and then sharply decline about every ten years, at slightly non-overlapping intervals. Prey Dynamics: Modeling Hare Scientists often use computer models to investigate how different underlying mechanisms might produce particular dynamics in systems. Cycling and Extinction As the prey population size drops from predation, the predators have less and less food to eat, causing the predator population size to drop. With fewer predators, the prey population starts growing again, and the cycle repeats. Interactions among Trophic Levels Predators do not act alone in determining whether prey populations can survive. Prey can also be strongly affected by the availability and quality of their food. Deterministic vs. Stochastic The deterministic Lotka-Volterra predatorprey model predicts regular, even cycles of predator and prey populations, with predator cycles following prey cycles. More realistic models that include chance (stochastic) events show variation in both the period and amplitude of predatorprey cycles.
