Community Interactions

What is a Community?

Before diving into community interactions, let's define what a community is.

These populations of different species also each occupy different portions of the environment for which they are specialized, called ecological niches.

The number of different niches within a particular community determines its level of biodiversity. For example, a tropical rainforest has more niches and, thus, a higher level of biodiversity than communities with fewer niches, such as temperate deciduous forests and arctic tundra.

Community Interactions Definition

Now, let's look at the definition of community interactions.

These interactions are vital biotic components of communities that help to drive evolution by natural selection. There are three types of community interactions that are currently recognized:

1. Competition

2. Predation

3. Symbiosis

Different Types of Community Interactions

In order to shed more light on each of these types of community interactions, let's take a closer look at them one by one.

Competition

The first type of community interaction is called competition.

Competition may occur between different species (interspecific) and between individuals of the same species (intraspecific). Interspecific competition, however, can fuel both evolution and extinction.

Predation

The second type of community interaction is predation.

There are two different types of predators - true predators and grazers.

• True predators are species that kill and consume their prey. Examples of true predators include crocodilians and big cats.

• Grazers, unlike true predators, do not typically kill their prey items. Examples of grazers include herbivores, which “graze” upon plants, as well as insects like mosquitoes, which consume the blood of their prey but do not kill them.

Predators naturally exist at lower population densities than their prey and help to control the prey population through predation. In turn, the predator population is controlled by the abundance of prey.

This results in both predator and prey populations fluctuating in a stable fashion. Some of these predators are considered to be keystone species, impacting the entire ecosystem.

Predation can also fuel evolution by natural selection through adaptation. As prey species adapt to avoid predators, individuals with traits that are more beneficial for survival will breed and those that do not will be preyed upon.

Over time, even more adaptations may be selected for. Some examples of prey adaptations include camouflage (to avoid predation) and bright colors (to warn against predation).

Symbiosis

The third type of community interaction is symbiosis.

At present, there are three different types of symbiosis recognized by scientists:

1. Commensalism

2. Mutualism

3. Parasitism

Commensalism

Commensalism occurs when one species benefits from an interaction, while the other experiences no affect at all. A classic example of commensalism occurs between humans and bacteria. Many species of bacteria that reside inside our bodies are harmless commensals that provide no obvious benefits to us, nor do they harm us.

Mutualism

Mutualism occurs when both species benefit from an interaction. Going back to the previous example of humans and bacteria, many of the bacteria species present within our intestines (our microbiome) are extremely beneficial and play a crucial role in our digestive process. In return, these bacteria receive food and shelter.

Parasitism

Lastly, parasitism occurs when one species benefits from an interaction, while the other species is harmed. The benefitting organism is known as a parasite, while the harmed organism is the host. Examples of parasites include ticks and tapeworms.

Community interaction examples

Here are some more examples of how community members interact with each other.

Competition Example

In some portions of the Americas, both jaguars (Panthera onca) and pumas (Puma concolor) are sympatric. Both species are large feline predators (though the jaguar is the only true “big cat” of the two) that compete for many of the same resources.

It has been suggested that, in the rainforest communities of the neotropics, pumas have become more specialized in order to avoid competition with the jaguar. In these areas, pumas tend to be much smaller and much more arboreal than they are in the temperate regions to the north and south, while the jaguars tend to be larger and more terrestrial. This is an example of competition resulting in specialization.

Parasitism Example

Eight different species of fish are known to be hosts for parasitic isopods. These isopods, known as tongue-eating louse (Cymothoa exigua) attach to fish tongues, rip the tongues off, and replace them. While this does not kill or seriously harm the fish, it does result in the loss of the tongue and these fish often exhibit weight loss.

Mutualism Example

The capybara (Hydrochoerus hydrochaeris) is the world’s largest rodent and is native to South America. Capybaras experience a mutualist relationship with multiple bird species, including the cattle tyrant (Machetornis rixosa), which preys upon the horseflies that are attracted to the capybaras (Fig. 1).

This relationship is mutually beneficial, as the cattle tyrant gets food, and the capybaras receive fewer horsefly bites.

Figure 1: A capybara and a cattle tyrant in the Brazilian Pantanal. These two species have a mutualist relationship. Source: Brandon Sideleau, own work

Community and Ecosystem Interactions

There are six levels of ecological organization. They are, in order from largest to smallest: biosphere, biome, ecosystem, community, population, and individual.

Figure 2: The six levels of ecological organization. Source: http://ecology-project.weebly.com/

While a community consists of all populations present within a specific area, an ecosystem consists of the community (biotic component) and its surrounding environment (abiotic component). Thus, community and ecosystem interactions are interactions that involve the species present within the community and their abiotic environment. Abiotic factors, such as drought and wildfires, affect the birth and death rates of the community.