Community Ecology

Divya Patel, Nastassja Kosinski, Amanda

Manzione, Renée Buttel

Concept 53.1- A community's interactions include competition,predation, herbivory, symbiosis, and disease.

Interspecific Interactions and Adaptation- Evidence for coevolution, involving reciprocal genetic change by interaction populations, is scarce. However, generalized adaptations of organisms to other organisms in their environment is a fundamental feature of life.Populations are linked by interspecific interactions that affect the survival and reproduction of the species engaged in the interaction.

Competition (-/-)

Competition for resources between plants, between animals, or between decomposers when resources are in short supply. An ecological niche is the sum total of a species’ use of the biotic and abiotic resources in its environment. The competitive exclusion principle states that two species cannot coexist in the same community if their niches are identical. Character Displacement is the tendency for characteristics to be more divergent in sympatric (geographically overlapping) populations of two species than in allopatric (geographically separate) populations of the same two species.

Predation (+/-) An interaction between species in which one species, the predator, eats the other, the prey. Because of predation, many species have developed special adaptations, such as various forms of mimicry and coloration.

Herbivory (+/-) An interaction in which a herbivore eats a part of a plant or an alga. Because of herbivory many plants and algae have developed many chemical toxins and physical defenses (spines and thorns), while many herbivores have developed the ability to distinguish the difference between nutritious and malnutritious plants.

Parasitism (+/-) A type of symbiotic relationship in which the symbiont (parasite) benefits at the expense of the host by living either within the host (endoparasitism) or outside the host (exoparasitism). Parasitoidism is a type of parasitism in which an insect lays eggs on or in a living host; the larvae then feed on the body of the host eventually killing it.

Disease (+/-) Pathogens (disease- causing agents) can be bacteria, viruses, or protists, but fungi and prions can be pathogenic also. Pathogens inflict lethal harm on their hosts. Pathogens are very much like parasites.

Mutualism (+/+) A symbiotic relationship in which both participants benefit. Mutualistic relationships sometimes involve the evolution of related adaptations in both species, with changes in either species likely to affect the survival and reproduction of the other.

Commensalism (+/0)

An interaction between two species in which one of the benefits, but the neither harms nor helps the other. There few if any cases of pure commensalism.

Concept 53.2- Dominant and keystone species exert strong controls on community structure

• Species Diversity- Species diversity measures the number of species in a community- its species richness- and their relative abundance.Species richness is the total number of different species in the community. Relative abundance is the proportion each species represents of the total individuals in the community. Measuring species diversity is easier said then done, there are very few methods used to accurately measure species diversity. Measuring species diversity is important not only for understanding community structure but also to conserve biodiversity.

• Trophic Structure- The structure and dynamics of a community depend to a large extent on the feeding relationships between the organisms- the trophic structure of the community. A food chain is the transfer of food energy up the trophic levels from its source in plants (primary producers) to primary consumers to secondary and tertiary consumers. A food web is the connection of many food chains; it is the elaborate, interconnected feeding relationships in an ecosystem. The energetic hypothesis suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain. The dynamic stability hypothesis proposes that long food chains are less stable than short chains.

• Species with a Large Impact – Dominant species and keystone species exert strong controls on community structure. Dominant species are the most abundant species in a community and their dominance is achieved by having high competitive ability. Keystone species are usually less abundant species that exert a disproportionate influence on community structure because of their ecological niche. Ecosystem “engineers,” also called foundation species, exert influence on community structure through their effects on the physical environment.

• Bottom-Up and Top-Down Controls- The bottom-up model proposed an unidirectional influence from lower to higher trophic level, in which nutrients and other abiotic factors are the main determinants of community structure, including the abundance of primary producers. The top-down model proposes that control of each trophic level comes from the trophic level above, with the result that predators control herbivores, which in turn, control primary producers.

Concept 53.3- Disturbance influences species diversity and composition

• Disturbance- Increasingly, evidence suggests that disturbance and nonequilibrium rather than stability and equilibrium are the norm for most communities. According to the intermediate disturbance hypothesis moderate level of disturbance can foster higher species diversity than can low or high levels of disturbance.

• Human Disturbance- Humans are the most widespread agents of disturbance, and their disturbance to communities usually reduces species diversity. Humans also prevent some naturally occuring disturbances, such as fire, which can be important to community structure.

• Ecological Succession- Ecological succession is the sequence of community and ecosystem changes after a disturbance. Primary succession occurs where no soil exists when succession begins; secondary succession begins in a are where soil remains after a disturbance. Mechanisms producing community change during succession include facilitation and inhibition.

Concept 53.4- Biogeographic factors that affect community diversity

• Research done by Charles Darwin and Alfred Wallace proved that tropical habitats support many more species than temperate and polar regions. An example of this is that there are 711 different types of tree species in Malaysia while there are only 50 tree species in the area north of the Alps in Western Europe.

• Species richness in equatorial-polar gradients are affected by evolutionary history and time.

• As time passes, more speciation events will occur, causing species diversity to increase.

• Tropical habitats are older than both polar and temperate regions, proving that species richness is more abundant in the tropics than at an equatorial-pole gradient.

• Biodiversity is affected by climate. Solar energy and water are two components of climate that effect biodiversity.

• The species area curve was recognized by Alexander von Humboldt in 1807. This curve states that the larger area, the greater the number of species.

• The amount of species living on an island is based on the rate of immigration and the rate of extinction. These rates are affected by the size of the Island, the amount of species living on the island, and the distance from the island to the mainland. Larger islands have lower extinction rates because they contain more resources and more diverse habitats.

• The island equilibrium model was creates by MacArthur and Wilson. This model shows that when the rate of immigration is equal to the rate of extinction, an equilibrium will be reached. The island’s size and distance from it’s mainland effect species richness( at equilibrium). Many scientist have questioned the accuracy of this model.

Concept 53.5- Contrasting views of community structure are the subject of continuing debate

• The integrated hypothesis was created by F.E. Clemens, of the Carnegie Institute of Washington, in the early 1900’s. This hypothesis states that a community acts as a superorganism, as a result of required biotic interactions .

• Then, H.A. Gleason, of the University of Chicago, created the individualistic hypothesis. This hypothesis states that species with the same abiotic needs live in the same area.

• These hypotheses are similar because they state that communities interact.

• These hypotheses differ because the integrated hypothesis states that species depend upon their interactions with other species and he individualistic hypothesis states that each species lives in an environment where the abiotic factors that it needs are present.

• American scientists Paul and Anne Ehrlich proposed the idea that all species in a community depend upon and affect one another.

• The rivet model proposed the idea that species in a community interact with species in the web of life. Increasing or decreasing the population of one species will greatly affect the population of another species.

• The Redundancy model was created in 1992 by Brian Walker, an Australian Ecologist. This model proposed the idea that most species are not associated with one another and that decreasing or increasing the population of one species will not greatly affect the population of another species.

What is DDT?• DDT is a pesticide that was released

to the public after WWII.• Unfortunately, during that time and

even today, many people are unaware of the side effects of DDT in the environment especially on factors such as herbivory, predation, and interspecific competition.

• Thanks to the work of many researchers and publicists, such as Rachel Carson, people today are becoming aware of the effects of DDT and now DDT is banned for use.

Herbivory• Herbivory: an interaction in which an herbivore eats

parts of plants or alga• DDT is sprayed all over plants for the purpose of

killing unwanted insects. Because DDT is sprayed on plants, herbivores that eat those plants will take in the DDT. If the herbivores take in the DDT in large consumptions, then they will be unable to carry out necessary life functions, like reproduction, and eventually become extinct. Hence there wouldn’t be anymore interaction between the involved plant species and their herbivore predators.

• Global warming also affects herbivory. Global warming is when the global temperature increases. Many organisms, especially plants that cannot disperse rapidly over long distances, would probably not be able to survive the high rates of climate change projected to result from global warming. If plants, don’t survive, then neither will the herbivores, which are dependent on them.

Predation• Predation: An interaction between species in which one species,

the predator, eats the other, the prey.• Predation is also affected severely by DDT. After DDT was

released to the public, one of the first signs that DDT was a serious threat to the environment was when the population of various bird species, such as pelicans, ospreys, eagles, all birds that feed on herbivores that feed on plants. The accumulation of DDT in the tissues of these birds interfered with the deposition of calcium in their eggshells, a trend that may have already occurred because of environmental conditions. When these birds tried to incubate their shells, the weight of the parents broke the shells of the affected eggs, resulting a decline of reproduction rates. Basically, this shows how the interaction between predators (birds) and their preys (herbivores) leads to the decline in the population of the predator (and in some cases even the prey) species.

• Predation is affected by acid precipitation. In a study in Norway, it was found that fish were dying in ponds and lakes because of acid rain from pollutants. This would cause the whole lake and pond ecosystem to suffer not only due to the acid precipitation, but the loss of one species would indicate the loss of the predator species and eventually the elimination of certain food chains.

Interspecific Competition• Interspecific Competition: competition for resources between

plants, between animals, or between decomposers when resources are in short supply.

• DDT affects interspecific competition. Competition can limit herbivore numbers because competition may maintain herbivore numbers below what the vegetation could feed. But as mentioned earlier herbivore numbers can be easily reduced through DDT’s affect on herbivory. This will immediately affect competition. The worse part is that competition includes more than one species, so if an herbivore species, which was controlled by the competing species is eliminated, then the competing species will have nothing to compete for and one of two things will happen: (1)competition will end and the competing will find different, new preys or (2) the competing species will die of starvation. Basically, DDT has an indirect relationship with competition, but it still affects the outcome of interspecific competition.

• Competition is affected by the overloaded global carrying capacity because as we approach or exceed the carrying capacity the amount of available resources will not suffice for the amount of organisms. Hence competition will be brought to a new level and many species may face competitive exclusion.