Ecology is the study of the interaction of living organisms with their physical/chemical environment and other living organisms. When two or more species share a habitat (the physical setting in which the species normally inhabit), they form a community. By definition, a community consists of all individuals of all species that share a habitat.

The manner in which individuals within a community interact can be grouped into a few different categories:

Neutral: In this case, the coexistence of two species doesn’t significantly affect either

species.

Commensalism: In the case of commensalism, one species benefits but the other species in not affect. The book uses the excellent example of a bird nesting in a tree: the bird benefits by having a place to nest while the tree is not affected either way.

Mutualism: In a mutualistic relationship, both species benefit. Think of the fish tanks: coral and zooanthelle, clownfish and the anemone, and fish and denitrifying bacteria.

Interspecific Competition: In this case, both species suffer as they compete over shared resources. When Northern Pike were discovered in trout lakes in the Black Hills, GFP stopped stocking game fish such as trout in such lakes because of predation (pike will eat smaller trout) and interspecific competition (both going after forage fish).

Predation: A species feeds off another, usually killing the prey. Predators are usually larger than their prey items and they do not live on or within the prey.

Parasitism: In the case of parasitism, one species benefits and one species suffers. Parasites usually do not kill their prey but rather drain some nutrients and energy from the prey species. Parasites live on or within the prey species.

(Note: three of the above relationships, mutualism, commensalisms and parasitism, are cases of symbiosis, meaning the two species live in close contact with one another.)

Competitive Interaction: A basic rule of ecology states that no two species can coexist if they share the same niche. By niche, we mean what resources they utilize in a given habitat. If two species exist within a single habitat and their niches significantly overlap, one species will ultimately out compete the other species vying for the same niche. This principle is known as Competitive Exclusion.

Because of the rule of competitive exclusion, within communities we observe resource partitioning, in which each species utilizes a different portion of the available resources. Your book provides an excellent example of how different plants utilize moisture and fertilizer nutrients from different layers in the soil. This is an excellent example of resource partitioning.

One of the most extreme cases of competition between two species is scene in predator/prey relationships. Because predators are evolving to become better hunters and prey are evolving better survival mechanisms, predator-prey relationships have lead to some of the more unique adaptations we see on our planet:

Mimicry: Species often develop defenses in which they store dangerous chemicals, become bitter tasting or in some other way become discourage predators. Consider a bird eating a monarch butterfly only to find it too bitter to consume. The bird learns to associate the orange and black pattern with bitter prey items that are better left alone. Often we observe other species copying the coloring of organisms such as the monarch to fool predators into thinking that they too are dangerous and should therefore be left alone. Locally, we observe such a case with viceroy butterflies (which do not store the bitter chemical from milkweed plants) copying the coloring of monarch butterflies (which are bitter due to the accumulation of toxic chemicals from the milkweed plant). The viceroy is mimicking the monarch to gain an advantage and fool predators into avoiding them as prey items.

Warning Coloration: When you see a brightly colored organism, they are often dangerous and should be avoided. The bright coloring is a warning to predators to stay away and avoid that species. This is the opposite of camouflage, in which species try to blend in with the environment to avoid being predated upon.

Succession is the change in a community over time. In our current age, we often attempt to suppress succession by extinguishing wildfires or preventing floods. When we look at the available record (tree rings, charcoal in the soil, flood deposits) we see that succession has always been an ongoing event that is necessary for healthy ecosystems.

Consider the case of the Missouri River: On much of the Missouri, we have prevented spring floods by building some of the world’s largest dams to create huge reservoirs such as Lake Oahe. On the Missouri River by Vermillion, SD the river still floods and we see not a large lake but a wide, shallow river. When we went canoeing on this section of river, we had lunch on a large sand bar/island. On one end of the sandbar, there were few plants as they was hardly any soil, mostly sand. Sand is inorganic and contains few nutrients. When we look at the plants beginning to grow on a sandbar, we observe primary succession, which involves plants colonizing land not previously occupied by life. Primary succession is currently being observed on the mudflows and lava beds formed after the 1980 eruption of Mt. Saint Helens. Over time, primary succession will lead to the formation of soils with adequate moisture content to support other plants. The pioneer species (those that first colonize a new area) will give rise to new species and ultimately to a climax community. The climax community is a community of plants that will dominate an ecosystem for years (in the case of the Pacific coast sequoia forest, the climax community can dominate for thousands of year).

In cases of forest fires or prairie fires, we observe secondary succession. This process involves plants that colonize disturbed areas, and while there may be no visible life immediately, roots, seeds, spores, insects, bacteria and organic soils still remain. Generally, the process of secondary succession is much quicker than is primary succession (which often takes centuries).

In recent decades, one of the greatest threats to healthy natural communities has been the introduction of foreign species. Because foreign species often lack natural predators, they have an advantage when it comes to taking over available niches within an ecosystem. In many local lakes, carp (native to Asia) are the dominant species of fish. If you look out into a field of grass around Brandon, you will likely see brome grass, a Russian species that has displaced a significant portion of our native grasses. In the Southern US, a vine called kudzu was introduced in the 1970s and has taken over large areas of the South, choking out native trees and plants. Australia has a huge problem with introduced cats, rabbits and toads. In all these cases, the introduction of a species into a habitat lacking predators and parasites has lead to disastrous population explosions.

Another way communities can be upset involves the removal of keystone species, meaning organisms that play a particularly vital role in an ecosystem. Locally, the elimination of the bison was one such critical species. When bison were eliminated, we saw an increase in the number of trees, decrease in native grasses, and many predators (prairie wolf and bears) were eliminated from the prairies, largely due to the loss of one species.