Interactions within Communities

A community consists of all populations of different species that interact together in a given ecosystem. Some organisms within communities cannot exist independently of one another and work together for survival.

Niches Ecological Niche: –An organisms use of and interaction with abiotic and biotic resources in its environment that influences its growth, survival and reproduction. Fundamental Niche: –The physical conditions under which a species might live, in the absence of interactions with other species. Realized Niche: –The actual niche of a species whose distribution is restricted by biotic interactions such as competition, predation, disease and parasitism.

Niches Intraspecific and interspecific interactions influences population dynamics of individual species. Interactions between two species and their effects on population density calssified into 5 categories: –Competition, Predation, Symbiosis (Parasitism, Mutualism, Commensalism).

Interspecific Competition Occurs between individuals of different populations. Serves to restrict population growth. Consists of: –Interference Competition: involves aggression and fighting. –Exploitative Competition: involves consumption/use of shared resources

Interspecific Competition Strongest competition occurs between species when niches overlap. Principle of Competitive Exclusion: –If resources are limited, no two species can remain in competition for exactly the same niche indefinitely. –Outcome? Population decline in weaker species; migration; change in behaviour.

Interspecific Competition Resource partitioning reduces competition between different species occupying the same habitat.

Interspecific Competition Considered to be a driving force for evolution of adaptations that enable species to use resources for continued survival. See the example on the two barnacle species on page 680 of your textbook.

Predation Population density of predator increases at the expense of its prey. As prey ↑, there is more food for the predator. As the predator ↑, the prey ↓. As the prey ↓, the predator ↓. When the predator population is low, the prey population can grow.

Predation The predator patterns tend to lag behind the prey patterns.

Defense Mechanisms (Plants) Defense mechanisms in prey have evolved through repeated encounters with predators. Plants use morphological defenses (thorns, hooks, spines, needles) and chemical defenses.

Defense Mechanisms (Animals) Defensive mechanisms brought out by coevolution between plants and animals. Camouflage. Bright colouration as a warning signal. Mimicry. Hiding Fleeing from predators.

Bright Colours = Dangerous

Camouflage

Defense Mechanisms: Mimicry Batesian Mimicry A palatable/harmless species mimics an unpalatable/harmful organism. Mullerian Mimicry Unpalatable species that resemble one another. Several species have the same appearance to the predator, reducing the impact on each individual species.

Symbiosis (“living together”) Mutualism Commensalism Parasitism

Mutualism Both organisms benefit from relationship. Obligatory mutualism: neither species involved could survive without the other. Examples: –Bacteria in guts of herbivores digest cellulose. –Bacteria in human large intestine produce vitamins B and K. –Oxpecker birds feed on backs of grazing animals. –Lichens – relationship between fungi and cyanobacteria.

Commensalism One organism benefits and the other remains unaffected. Example: –Remora suction themselves to sharks…do not have to expend energy to travel! –Barnacles also attach themselves to whales.

Parasitism One organism (the parasite) benefits at the expense of another (the host). The host is often harmed or killed. Microparasites (microscopic) vs. macroparasites (readily visible). Endoparasites (live & feed inside host) vs. ectoparasites (live & feed on host).

Parasitism Social parasites complete life cycle by manipulating social behaviour of host. Example: –Cowbirds lay eggs in other nests…no energy expended on building a nest and caring for young. –Cowbird egg hatches earlier, so larger chick receives more food, resulting in higher death rate of host eggs.