Back To CourseCLEP Biology: Study Guide & Test Prep
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We're going to start this lesson a little differently than most. We're going to start with a graph that shows the population densities of two very closely related bird species at different elevations within an ecosystem.
The elevation is measured along the y-axis, starting at sea level here at the bottom of the graph and going up to 8,000 feet at the top. The population densities of the two bird species are measured along the x-axis and increase as the lines move to the right. So, what exactly does this graph show? Well, there are two species of birds; species A is only found at elevations of about 4,800 feet or higher, and species B is only found at elevations below 4,800 feet.
It is interesting though, that both populations have their highest densities right at this elevation, and then dramatically drop to zero where the two populations meet. Normally, we wouldn't expect a population to drop to zero so suddenly, especially right after it reaches its peak density. A peak density signals that conditions are optimal for a species, so it's odd that the populations drop to zero right after what would appear to be the optimal elevation for both species.
So, what happens at 4,800 feet? It's hard to say what exactly changes at that elevation, but the rapid drops in population densities and the exclusion of each species from the other species range strongly suggest that these two species are in direct competition with each other. Competition between two species for the same resources is called interspecific competition, which is when two or more species in a community are competing for resources. From this graph, we can see that above 4,800 feet, species A is outcompeting species B, and below 4,800 feet, species B is outcompeting species A.
More than that, it appears as though these two species are subject to competitive exclusion, or when one species outcompetes another in a part of its habitat so well that the second species is excluded from that part. Competitive exclusion is one possible outcome of strong interspecific competition. In this case, it appears as though species A is excluding species B from areas above 4,800 feet in elevation, and species B is excluding species A from areas below 4,800 feet in elevation.
A second possible outcome is local extinction, which is when one species is outcompeted by another so effectively throughout the entire local habitat, that it becomes extinct in that area. For instance, in our example of the bird species, what if species A did not have an advantage at higher elevations, and instead, species B had an advantage at all elevations? In this situation, species B would likely outcompete species A to extinction in that local area.
Yet another possible outcome to strong interspecific competition is niche differentiation, or when similar species with similar niches become specialists in specific areas and create more than one specific niche, which allows both species to coexist.
You see, most species do not actually use all of the resources they are potentially capable of using. So, we can define a theoretical or fundamental niche for each species, which contains all of the resources that a population is theoretically capable of using. However, if we want to look at the actual resources a population uses, then we can look at that population's realized niche, which contains only the resources that a population actually uses.
When niche differentiation occurs, each different species becomes specialized to best exploit a small subset of resources within the fundamental niche and creates a much smaller realized niche. In this way, several different species that share a common fundamental niche can coexist together.
A good example of niche differentiation occurs in several different areas of the Caribbean Islands, Central and South America, where many different species of anole lizards (sometimes as many as 15) can be found living in the same area even though they share basically the same fundamental niche. Each species of lizard has evolved physical adaptations to a specific microhabitat within the forest, which helps them become specialists in a small, realized niche.
These sometimes very different looking, but closely related species are called ecomorphs, or populations that have recently evolved physical variations to adapt to specific microenvironments. Anole lizard evolution has followed very similar courses in many different ecosystems. This has allowed ecologists to categorize most species into one of six recognizable ecomorph types based on the microhabitat that each species chooses as a perch site within its environment.
Starting with species that live on the ground, grass-bush ecomorphs (like this Upland Grass Anole from Puerto Rico) have very slender bodies and long hind limbs, feet and tails, which make them perfectly adapted to climbing on narrow, vertical surfaces.
Trunk-ground ecomorphs generally have the longest hind limbs, making this species excellent jumpers and sprinters that are able to avoid predators in the relatively open areas that they are found in.
Trunk ecomorphs (like this bark anole) tend to be a somewhat small species that have flattened bodies and fairly average-sized limbs. These species reside almost exclusively on the trunks of trees and fence posts and are usually grayish in color, which serves as camouflage.
Twig ecomorphs (like this Puerto Rican Twig Anole) hunt for food by creeping along narrow branches and twigs, searching for insects hidden in crevices and underneath leaves. They have very slender bodies and long, narrow heads, suited to picking insects out of narrow spaces. However, they have short tails and the shortest relative limb length of all ecomorph - probably due to the fact that they rarely run or jump and, instead, prefer to walk at a steady pace.
Trunk-crown ecomorphs are perhaps the least specialized ecomorph grouping. These are small-to-medium sized arboreal species that can be found at a wide range of heights on just about any type of surface. Coloration of these species is generally green, but some species have purple or blue markings (like this blue anole from the Island of Gorgona). Trunk-crown species are also capable of changing their coloration to a dark brown.
Crown-giant ecomorphs are most distinguishable by their large size and a spiky crest that runs down their back, but they do have large, powerful heads, which allow them to add fruits and small vertebrates to their diet. The additional food items in their diet allow them to substantially separate their niche from the rest of the anole ecomorphs.
Clearly, with up to 15 different species in one area, the categorization of species into six different ecomorph types isn't sufficient on its own to explain all aspects of niche differentiation, but it does begin to explain niche differentiation along morphological lines. In addition to morphological differences, niche differentiation can also occur along different aspects of resource use, such as the temporal separation of niches between nocturnal and diurnal species.
Interspecific competition is when two or more species in a community are competing for resources. Limited interspecific competition for resources often occurs between multiple species in the same ecosystem. However, when two species are so similar that interspecific competition for limiting resources is high, these two species are unlikely to be able to coexist - presumably because this type of intense competition is unlikely to be balanced and therefore it is also unlikely to be sustainable.
Instead, three potential outcomes can result from strong interspecific competition: competitive exclusion, local extinction and niche differentiation. Competitive exclusion occurs when one species outcompetes another in a part of its habitat so well that the second species is excluded from that part. Local extinction is a more severe outcome and occurs when one species is outcompeted by another so effectively throughout the entire local habitat, that it becomes extinct in that area. Yet, another possible outcome to strong interspecific competition is niche differentiation, or when similar species with similar niches become specialists in specific areas and create more than one specific niche, which allows both species to coexist.
It's pretty clear that extinction sometimes occurs as a result of interspecific competition, but the other two outcomes are difficult to scientifically prove. However, the distribution patterns of similar species within ecosystems, and the ability to group anole species into ecomorph categories that are consistently found in many separate ecosystems, do suggest that strong interspecific competition does occur and that these are two ways it can be ecologically resolved.
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Back To CourseCLEP Biology: Study Guide & Test Prep
23 chapters | 211 lessons