Community Ecology

I. Biological community - two or more populations of different species that interact with one another in the same environment

A. Community organization

1. Habitat = the physical place where an organism lives, e.g. a pine forest or fresh water lake. Some organisms, particularly migratory birds require more than one habitat

2. Niche =  the functional role of an organism in a community, its job or position. E.g. grasses on the savanna of Africa are primary producers, wildebeests are herbivores, lions are carnivores

a. Each species has a fundamental niche - what they could do with no competitors or resource limitations

b. However, due to competition and/or resource limitations, most organisms occupy a realized niche,  the part of the fundamental niche that a species actually occupies in nature

II. Species interactions

A. Since communities consist of many species they all have various interactions going on at the same time

B. Types of species interactions:

1. Neutral - two species that don't interact at all

2. Commensalism - beneficial to one species but neutral to another. E.g. birds that nest in trees, epiphytes (plants that grow on other plants) such as tropical orchids.

3. Mutualism - an interaction that is beneficial to both species. E.g. plants and their pollinators, plants and animals that disperse their seeds, certain fungi and plant roots

a. Some species of the Yucca plant have an obligate mutual relationship with a moth. The plant relies upon the moth to transfer the pollen and the developing moth larvae feed off some of the seeds that result from the pollination

b. Ants and Acacia trees 

1) In tropical America, the bullhorn acacia provides a home for ants in its hollow thorns

2) The acacia also provides ants food from nectaries, and protein nodules called Beltian bodies

3) In return, the ant protects the plant from herbivores and other plants that might shade it

4) When the ants on an experimental tree were killed with insecticide, the tree also died

4. Parasitism - an interaction that benefits one species and is detrimental to another. Note that the host is generally not killed.

5. Predation - an interaction that beneficial to one species and is detrimental to another. In this case the prey is killed.

Predators are those that kill and eat other animals. Although many organisms eat plants they usually don’t kill them because they are a constant supply of food. Prey are those that are killed and eaten

III. Competitive interactions

A. Competition has a negative effect on both organisms competing for a resource. Because resources are limited in nature there will always be competition for them. Competition is the driving force of evolution. Those that win leave more offspring

B. Two types of competition:

1. Intraspecific - competition among individuals of the same species. E.g. humans compete against other humans

2. Interspecific - competition between different species.  E.g. humans compete against a wide variety of species seeking to utilize our food resources

C. Theory of competitive exclusion maintains that species who utilize the same resources cannot coexist indefinitely.  Stated another way this could be called the "one niche, one species" concept

D. Resource partitioning - one way for two competing species to survive is to subdivide potential resources via. In this case resources are divided, permitting species with similar requirements to use the same resources in different areas, ways and/or times

E.g. in New Guinea nine species of  pigeons eat the same fruits but each specializes in only one size fruit. Likewise, different warblers in New England have divided up spruce trees into specific feeding zones

IV. Predation and parasitism

A. Predation and parasitism often leads to co-evolution among the predator and prey, or parasite and host, as each adapts to changes in the other

B. Classic case of the Snowshoe Hare and Canadian Lynx

1. Careful records of pelts of both animals for over a century demonstrated regular fluctuations

2. To test whether the lynx or hare food supply was causing the cycling, three experiments were done:

a.  A hare population was given a constant supply of food and predators were excluded; the cycling ceased

b. Hare populations were given constant food supply but predators were not excluded; cycling continued

c. Predators were excluded but no food was added; the cycling continued

3. The interpretation of these results is that both a hare-food cycle and a predator-hare cycle combine to produce the overall effect

C. Mimicry - one species (the mimic) resembles another species (the model) giving it a defense against a potential predator

E.g. monarch and viceroy butterflies

D. Both parasites and predators may be used as biological control agents. Integrated pest management is the use of natural predators to control agricultural predators

E.g. When cactus was introduced to Australia, it spread wildly without competition on the desert. When a natural predator moth was introduced, the cactus populations plummeted dramatically

V. Community stability

A. Communities are assemblages of many different species occupying the same geographical area

B. Communities are not static, they gradually change over time because the environment changes and species themselves tend to also change their habitats

C. Ecological succession = a directional, cumulative change in the species that occupy a given area, through time

D. Two major types of succession:

1. Primary succession starts from barren ground, e.g. new islands or de-glaciated areas

2. Secondary succession starts from disturbed areas, e.g. abandoned farm land or storm ravaged land

E. Succession starts with a pioneer community, the first organisms to occupy an area. Over a period of time they gradually modify the area, often making it unsuitable for themselves and providing optimal conditions for others to move in

E.g. open ground might be colonized by sun tolerant plants. However, as they grow they produce more shade, which makes the habitat less suitable for them and more suitable for shade tolerant species.

F. As members of the pioneer community gradually change the environment several transitional communities may come and go

E.g. the soil pH may undergo considerable change over a period of time, alternately excluding and including different species

G. Ultimately a climax community, a stable, self-perpetuating array of species in equilibrium with one another and their habitat, will form

H. Glacier Bay, Alaska is one of the best documented examples of primary succession. Retreating glaciers leave barren land, on which succession has been documented for the past 200 years

The bare rocky soil starts with a pH of 8.0 and no ground cover. Mosses and mat forming plants with nitrogen fixing bacteria quickly colonize the area. Willow and alder shrubs, also containing nitrogen fixing bacteria eventually form dense thickets. Over a period of time plant litter enriches the soil with nitrogen, allowing sitka spruce trees to come in. Within 200 years the soil pH changes to 4.8 and the climax community, a hemlock-spruce forest forms

VI. Community instability

A. The introduction of "foreign" species into a community can dramatically alter its structure. Common introduced species which have altered communities in the US include: water hyacinth, Dutch elm disease, chestnut blight, Japanese beetle, sea lamprey and European sparrow

VII. Biodiversity

A. Biodiversity = the number of different species within an area

B. Biodiversity  is greatest in tropical areas near the equator and it decreases towards the poles

1. Tropical areas have more sunlight and of greater intensity, more rainfall and longer growing seasons for plants. This environment is quite stable and contains many vertical "layers" which provide more microhabitats. Therefore these areas can support more species

E.g. the number of bird species is directly correlated with latitude