Biology: 17.2 How Competition Shapes Communities

Competition: Common use of Scarce Resources When two species use the same resources (food, water, territory) they participate in an interaction called competition. Resources for which species compete include food, water, nesting sites, living space, light, and mineral nutrients. Competition occurs for resources that are in short supply.

Competition: Common use of Scarce Resources In Africa, lions and hyenas both compete for prey. Fierce rivalry between both species sometimes leads to standoffs and battles. Most competition among species does not involve direct contact. Species most often interact by their effects on the abundance of a resource.

Competition: Common use of Scarce Resources To understand how competition effects a community, scientists must focus on the day-to-day events in the community and look at the roles all organisms play in it. What does each organisms eat? Where does it live? The function of a particular species in an ecosystem is called it’s niche. A niche is how an organism lives – its’ ”job” it performs within the ecosystem.

Competition: Common use of Scarce Resources A niche may be described in terms of space utilization food consumption Temperature range Requirements for moisture Requirements for mating Various other factors A niche is an animal’s pattern of living; how it lives, not where it lives.

Competition: Common use of Scarce Resources A niche is often best described in terms of how the organism affects energy flow within the ecosystem in which it lives. For example, the niche of a deer that eats shrubs in the forest is that of a herbivore. The niche of an animal that hunts the deer is that of a carnivore. The niches of several organisms can overlap. If the resources that these organisms share is in short supply, such as food, it is likely that there will be competition between them.

Niche Size of a niche: To get a better understanding of what a niche is , we can look at a particular species. Let us look at a Cape May warbler living in a forest finding it’s food in spruce trees. The niche this bird occupies is affected by several factors. Temperature it prefers, the time of year it nests, what it likes to eat, and where on the tree it finds food are all variables that determine it’s niche.

Niche Size of a niche: For example, the Cape May warbler Spends summers in the northeast US Nests in midsummer Eats small insects Searches for food high up in spruce trees at the tips of branches The entire range of resource opportunities an organism is potentially able to occupy within an ecosystem is called it’s fundamental niche. Each warbler species feeds at a different height in the forest

Niche Dividing Resources Among Species: Let us look again at what the Cape May warbler is doing. It feeds at the very top of the spruce tree even though insects it eats are located everywhere on the tree. The Cape May warbler does not occupy it all of it’s fundamental niche that it could. It restricts itself to only a section of it’s potential feeding area.

Niche Dividing Resources Among Species: Closer study shows us that this is part of a more complex pattern called niche restriction. In the late 1950s, ecologist Robert MacArthur studied the feeding habits of five warbler species . MacArthur found that all five species fed on insects in the same trees at the same times as each other.

Niche Dividing Resources Among Species: To coexist, all five warbler species fed on the tree at the same time but each concentrated it’s efforts on a different part of the tree. All five species had overlapping similar fundamental niches but they did not use the same resources. These five species divide the range of resources between them, each taking it’s own niche in the tree; some feeding at the top only, the middle only and some at the bottom only.

Niche Dividing Resources Among Species: The part of a fundamental niche that a species occupies is called it’s realized niche. The realized niche of the Cape May warbler is only a small portion of it’s fundamental niche. The fundamental niche is all the resource opportunities that could be used; the realized niche is the resource opportunities that are actually used.

Dividing Resources Among Species: How does a species like this coexist by looking for food in only one portion of a tree? MacArthur suggested that this feeding pattern reduces competition among the five species of warblers allowing them to live in harmony. Because each warbler species uses a different set of resources by occupying a different realized niche, the species are not in competition with each other.

Dividing Resources Among Species: MacArthur concluded that natural selection has favored a “range of preferences and behaviors” that divides the available resources among those that share the environment. Most ecologists agree with MacArthur’s conclusions.

Competition and limitations of resource use: In the early 1960s, Joseph Connell observed two species of barnacles that lived off the coast of Scotland. One species of barnacle lived in shallow water areas where it was often exposed to air by the receding tides of the area. The second species lived lower down the rock surface in the water where it was rarely exposed to air.

Competition and limitations of resource use: When Connell removed the barnacle species from the deeper zone, it’s counterpart was easily able to spread downward and fill the deeper areas vacated. It was not intolerance of the deeper water that kept one species from that zone but the competition from the other competitive barnacle species in that deeper zone.

Competition and limitations of resource use: When the more aggressive barnacle that favored deeper waters was reintroduced, it easily pushed the less aggressive species back to the shallower water zone. On the other hand, when the less aggressive variety of barnacle was removed from the shallow zone, the more aggressive barnacle species did not spread into the shallower zone. It stayed in the deeper zone only. It could not tolerate the exposure to the air for any length of time.

Competition and limitations of resource use: What Connell’s experiments showed was that one species of barnacle was occupying only a small portion (it’s realized niche) of it’s possible fundamental niche because of competition. Competition can limit how a species uses resources. The rest of it’s fundamental niche was unavailable to it because of competition from the more aggressive species.

Competition Without Division of Resources In nature, shortage is the rule, and species that use the same resources are almost certain to compete. Darwin noted that competition would be most visible between very similar species as they both share the same resources in the same way. When two species compete, the species that uses the resources most efficiently will invariably drive the other species to extinction. This elimination of a competing species is called competitive exclusion.

Competition Without Division of Resources When can competitors coexist? When two species overlap substantially with their niches, competition occurs. When two very similar species can evolve by natural selection to share the same resources in a more specialized way, than their niches no longer overlap and competition among them ceases to exist.

Competition Without Division of Resources When can competitors coexist? As demonstrated in the case of the Cape May warbler, each of these warbler species has evolved through natural selection to only feed within a specific zone within the spruce tree. By doing so they have divided the food resources available between them and eliminated competition. Evolution by natural selection has moved them to separate niches. Each warbler’s fundamental niche could occupy the entire tree but it’s realized niche has evolved to a single zone within the forest.

Predation and Competition Many studies of natural ecosystems have demonstrated that predation reduces the effects of competition. We can see this clearly if we look at a study of starfish in tidepools and their affects on other varieties of species within the pools. Starfish are fierce predators of clams and mussels. When starfish were kept out of the tidepools, the mussel population grew rapidly.

Predation and Competition: Biodiversity The mussels crowded out the other species and took over the tide pool. Without the starfish to eat the mussels and keep their population in check; the mussel population pushed many of the other species out of the tide pool. By keeping the mussel population in check, the starfish kept the tide pool ecosystem in balance and kept it’s biodiversity up. Biodiversity is the variety of living organisms present in a community.

Biodiversity and Productivity A key investigation carried out on the prairies in Minnesota in the early 90s demonstrated relationships between biodiversity and productivity. Students tended 207 plot squares on a prairie ecosystem in Minnesota. The plots with more types of species (greater biodiversity) the greater the amount of total plant material was produced per plot. The greater the diversity, the richer the productivity of the plot. Also the plots with greater biodiversity bounced back faster after a drought. The greater the biodiversity, the more stable the ecosystem of the plot. Minnesota prairie