Introduction to Niches

A species’ niche is the range of environmental factors that allow that species to survive and reproduce. A particular tree species, for example, may be able to live where temperatures do not drop below −40◦, and where yearly precipitation is at least 750 mm. Perhaps it also needs open sunlight and an appropriate collection of root fungi. Such are the parameters of a niche.

G. Evelyn Hutchinson, one of the great ecologists of the twentieth century, envisioned the parameters that form a niche as an “n-dimensional hypervolume.” The Hutchinsonian niche is an “n-dimensional hypervolume,” where the dimensions are environmental conditions and resources, that define the requirements of an individual or a species to persist. The “fundamental niche” is the set of conditions allowing the species to survive if there are no other species interfering. Physical conditions are chief among those. The “realized niche” is the real life niche—where species are restricted by interactions with other species.

Niche overlap and interspecific competition

Interspecific competition may occur when individuals of two separate species share a limiting resource in the same area. When two species attempt to use the same resources or occupy the same space, it is described as niche overlap. Niche overlap results in interspecific competition  (inter- means between species, as opposed to intraspecific, which is within-species competition). If the resource cannot support both populations, then lowered fecundity, growth, or survival may result in at least one species. Interspecific competition has the potential to alter populations, communities, and the evolution of interacting species.

Consider latitude on the earth’s surface, which is connected to several parameters such as sunlight and temperature. And consider two species that can thrive anywhere between 40◦ and 60◦ latitude, and whose density drops slowly with increasing latitude (Fig 14.1).

At the top of Figure 14.1 are two nearly horizontal lines representing the abundance you might observe of the two species as you travel north. If free of Species 2 (its competitor), Species 1 (blue line) declines slowly in abundance in more northerly climates. Species 2 similarly declines in abundance (red line), but compared with Species 1 fares a little better in the north and a little worse in the south.

When these two species are together they compete with each other—each suppressing the other. In the south, where Species 1 fares better, it takes over and dominates. In the north, in contrast, where Species 2 fares better, it dominates instead (Fig 14.1 , bottom).

You see that there can be a sharp change in abundance even with only very slight changes in species characteristics. A range of one species can end and that of a new species can begin, even though you may not be able to discover anything from either species alone as to why they switch their dominance. And the switch-over point need not correspond to the exact place in which their dominance switches. Here the actual switch-over point is a few degrees to the north because of the migration simulated in the model. This phenomenon is called “competitive exclusion.”

Types of Competition

Competition is an interaction between organisms or species in which both require a resource that is in limited supply (such as food, water, or territory). Competition lowers the fitness of both organisms involved, since the presence of one of the organisms always reduces the amount of the resource available to the other. There are two types of competition: interference and exploitative.

Interference competition

During interference competition, also called contest competition, organisms of the same species or of two or more different species interact directly by competing for scarce resources (Fig 14.2). Interference competition occurs directly between individuals via aggression when the individuals interfere with foraging, survival, reproduction of others, or by directly preventing their physical establishment in a portion of the habitat.

In animals, interference competition is a strategy mainly adopted by larger and stronger organisms within a habitat. As such, populations with high interference competition have adult-driven generation cycles. At first, the growth of juveniles is stunted by larger adult competitors. However, once the juveniles reach adulthood, they experience a secondary growth cycle. Plants, on the other hand, primarily engage in interference competition with their neighbors through allelopathy, or the production of biochemicals.

Examples of intraspecific interference competition: (1) Large aphids defend feeding sites on cottonwood leaves by ejecting smaller aphids from better sites. (2) Male-male competition in red deer during rut to compete for females. (3) Male bowerbirds, who create elaborate structures called bowers to attract potential mates, may reduce the fitness of their neighbors directly by stealing decorations from their structures.

Examples of interspecific interference competition: (1) The ant Novomessor cockerelli interferes with the ability of red harvester ants to forage by plugging the entrances to their colonies with small rocks.

Exploitative Competition

Exploitation competition, or scramble competition, occurs indirectly when organisms both use a common limiting resource or shared food item (Fig 14.2). Instead of fighting or exhibiting aggressive behavior in order to win resources, exploitative competition occurs when resource use by one organism depletes the total amount available for the other organism. These organisms might never interact directly, but compete by responding to changes in resource levels. Very obvious examples of this phenomenon include a diurnal species and a nocturnal species that nevertheless share the same resources, or a plant that competes with neighboring plants for light, nutrients, and space for root growth. Exploitative competition has also been shown to occur both within species (intraspecific) and between different species (interspecific).

This form of competition typically rewards those organisms who claim the resource first. As such, exploitation competition is often size-dependent and smaller organisms are favored since smaller organisms typically have higher foraging rates. Since smaller organisms have an advantage when exploitative competition is important in an ecosystem, this mechanism of competition might lead to a juvenile-driven generation cycle: individual juveniles succeed and grow fast, but once they mature they are outcompeted by smaller organisms.

In plants, exploitative competition can occur both above- and below-ground. Aboveground, plants reduce the fitness of their neighbors by vying for sunlight. Plants consume nitrogen by absorbing it into their roots, making nitrogen unavailable to nearby plants. Plants that produce many roots typically reduce soil nitrogen to very low levels, eventually killing neighboring plants.

Competitive Exclusion Principle

Resources are often limited within a habitat and multiple species may compete to obtain them. All species have an ecological niche in the ecosystem, which describes how they acquire the resources they need and how they interact with other species in the community. Life in an ecosystem is often about competition for limited resources, a characteristic of the theory of natural selection. Competition in communities (all living things within specific habitats) is observed both within species and among different species. The resources for which organisms compete include organic material, sunlight, and mineral nutrients, which provide the energy for living processes and the matter to make up organisms’ physical structures.

The competitive exclusion principle states that two species cannot occupy the same niche in a habitat. In other words, different species cannot coexist in a community if they are competing for all the same resources. An example of this principle is shown in Figure 14.3 with two protozoan species, Paramecium aurelia and Paramecium caudatum. When grown individually in the laboratory, they both thrive. But when they are placed together in the same test tube (habitat), P. aurelia outcompetes P. caudatum for food, leading to the latter’s eventual extinction.

Resource Partitioning

Competitive exclusion may be avoided if one or both of the competing species evolves to use a different resource, occupy a different area of the habitat, or feed during a different time of day. The result of this kind of evolution is that two similar species use largely non-overlapping resources and thus have different niches. This is called resource partitioning or niche differentiation and it helps the species coexist because there is less direct competition between them. When two species differentiate their niches, they tend to compete less strongly, and are thus more likely to coexist. Species can differentiate their niches in many ways, such as by consuming different foods, or using different areas of the environment.

The anole lizards found on the island of Puerto Rico are a good example of resource partitioning. These anoles share common diets—mainly insects. They avoid competition by occupying different physical locations within the same area. For example, some live on the ground while others are arboreal (tree-living). Species who live in different areas compete less for food and other resources, which minimizes competition between species. Figure 14.4 below shows resource partitioning among 11 species of anole lizards. Each species lives in its own preferred habitat, which is defined by type and height of vegetation (trees, shrubs, cactus, etc.), sunlight, and moisture, among other factors.

Character displacement

Character displacement is trait differentiation that occurs when similar species that live in the same geographic region and occupy similar niches differentiate in order to minimize niche overlap and avoid competitive exclusion. Several species of Galapagos finches exhibit character displacement. Each closely related species differs in beak size and beak depth, allowing them to coexist in the same region since each species eats a different type of seed: the seed best fit for its unique beak. The finches with the deeper, stronger beaks consume large, tough seeds, while the finches with smaller beaks consume the smaller, softer seeds (Fig 14.5).

Character displacement is the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur, but are minimized or lost where the species’ distributions do not overlap. This pattern results from evolutionary change driven by biological competition among species for a limited resource (e.g. food). The rationale for character displacement stems from the competitive exclusion principle, which contends that to coexist in a stable environment two competing species must differ in their respective ecological niche; without differentiation, one species will eliminate or exclude the other through competition.

For example, Darwin’s finches can be found alone or together on the Galapagos Islands. Both species’ populations actually have more individuals with intermediate-sized beaks when they live on islands without the other species present. However, when both species are present on the same island, competition is intense between individuals that have intermediate-sized beaks of both species because they all require intermediate sized seeds. Consequently, individuals with small and large beaks have greater survival and reproduction on these islands than individuals with intermediate-sized beaks. Different finch species can coexist if they have traits—for instance, beak size—that allow them to specialize on particular resources. When Geospiza fortis and Geospiza fuliginosa are present on the same island, G. fuliginosa tends to evolve a small beak and G. fortis a large beak. The observation that competing species’ traits are more different when they live in the same area than when competing species live in different areas is called character displacement. For the two finch species, beak size was displaced: beaks became smaller in one species and larger in the other species.

Connecting concepts

Many competitive interactions between organisms are some combination of exploitative and interference competition, meaning the two mechanisms are far from mutually exclusive. For example, a recent 2019 study found that the native thrips species Frankliniella intonsa was competitively dominant over an invasive thrips species Frankliniella occidentalis because it not only exhibited greater time feeding (exploitative competition) but also greater time guarding its resources (interference competition). Plants may also exhibit both forms of competition, not only scrambling for space for root growth but also directly inhibiting other plants’ development through allelopathy.

Interference competition can be seen as a strategy that has a clear cost (injury or death) and benefit (obtaining resources that would have gone to other organisms). In order to cope with strong interference competition, other organisms often either do the same or engage in exploitation competition. For example, depending on the season, larger ungulate red deer males are competitively dominant due to interference competition. However, does and fawns have dealt with this through temporal resource partitioning — foraging for food only when adult males are not present.

References

Adapted from

Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/1-introduction

and

Lehman, C, Loberg, S., & Clark, A (2022) Quantitative Ecology – A New Unified Approach. LibreTexts. Retrieved from https://bio.libretexts.org/Bookshelves/Ecology/Book%3A_Quantitative_Ecology_-_A_New_Unified_Approach_(Lehman_Loberg_and_Clark)

and

Gownaris, N. & Zallek, T. (2022) Ecology for All!. LibreTexts. Retrieved from https://bio.libretexts.org/Courses/Gettysburg_College/01%3A_Ecology_for_All