Community Ecology
Community Interactions
Community ecology focuses on the interactions species have with the other species that they share a habitat with. The major types of interactions that have been categorized are competition, predation, and the various types of symbiosis.
Symbiosis is a rather broad term which refers to any sort of close and long-term interaction between different species. Depending on the interaction and how organisms are affected, it has the subcategories of mutualism, commensalism, parasitism, and ammensalism.
Mutualism is a relationship where each species is benefiting from the interaction. This can be due to providing a needed service or resource to each other, such as how flowers provide nectar to bees, which in turn supply the flower with the service of pollination, allowing the plants to reproduce.
An interesting example of mutualism is that of the leafcutter ant. These ants do not cut the leaves in order to eat themselves; rather, they use them in order to create a fungus garden. The ants provide the plant material and conditions needed for the fungus to properly grow, and the ants will then feed on the grown fungus.
Figure 2: A leafcutter ant and its leaf cutting; this ant is a great example of mutualism.
Commensalism is a relationship where one species benefits from the interaction, while the other species remains largely unaffected. The second species will have very little, if any, benefit or harm done to it.
The cattle egret is a great example of this type of ecological relationship. As cows and other livestock feed on the grasses available to them, they stir up and disturb insects. Nearby cattle egrets will take advantage of the now displaced insects and catches itself a meal.
Figure 3: A cattle egret waiting for its next meal; an example of commensalism.
Parasitism is a relationship where one species (the parasite) relies on another species (the host) for some benefit while harming the host in the process. Many parasites live in or on the host (such as tapeworms or ticks), however, there are free-living parasites as well (such as brood parasites like the cuckoo bird).
Cymothoa exigua, more commonly known as the tongue-eating louse, is an aquatic parasite that affects marine fish. This isopod will sever the blood vessels in a fish's tongue, causing them to gradually fall off. It will then attach itself to the remaining tongue-stub and serve as the fish's tongue (see Figure 4 for an image of C. exigua at this stage). As it remains attached, it will feed off of the host fish's blood or mucus.
Figure 4: A tongue-eating louse in the mouth of a fish it is infecting; an example of parasitism.
Ammensalism is a relationship where one species causes harm to another, without receiving any harm or benefit to itself. This type of relationship can be seen as a sort of "unintended consequences" or "collateral damage" interaction.
As elephants walk and travel through their habitat, they level and crush insects and vegetation beneath their feet. The elephants do not benefit from killing ants or grasses that happened to be in their path, nor are they harmed by this.
Figure 5: An elephant and its offspring walking through vegetation; a potential instance of ammensalism.
Symbiotic relationships are not static.
Organisms are capable of changing their symbiotic relationships with each other.
A great example of this is something known as an opportunistic pathogen. These are species of microbes which are normally not disease-causing, but will infect individuals if the conditions are right (such as the presence of underlying conditions or in situations of a weakened immune system).
Staphylococcus aureus is a bacterial species that is normally found as part of the human microbiome in a commensal relationship with us. However, S. aureus is also an opportunistic pathogen and can cause meningitis, sepsis, pneumonia, and various skin infections.
Every organism needs energy in order to carry out its life processes. One type of ecological interaction that centers around this is predation. This is the interaction through which one organism (the predator) consumes part or all of another organism (the prey).
The predator will hunt, capture, and kill its prey.
Figure 6: An image of a predator (the grizzly bear) that has successfully hunted its prey (the salmon)
Organisms have particular roles they play in their ecosystem, also known as their niche. This consists of their required environmental conditions, the interactions they have with other species, and where they fit in the food web.
When different species have overlapping niches, it means they use the same limited resource (such as food, water, space, light, etc.). Competition is an interaction that occurs as organisms with overlapping niches interact to use the same limited resources.
Interspecific competition (when different species are competing) and intraspecific competition (when members of the same species are competing) both exist, although interspecific competition tends to play a larger role in ecosystems.
In interspecific competition, if one species begins to win out more often, species B will suffer a population decline if it does not have the ability to migrate to a different habitat. However, species are capable of evolving adaptations that allow them to reduce or avoid competition, such as the varying types of niche partitioning.
Resource partitioning leads to specialization of the species so that they use the same resource in different ways, allowing them to share the resource and reduce competition (see Figure 8).
Temporal partitioning will lead to species using resources at different times, such as being nocturnal vs diurnal.
Spatial partitioning can result in species using different areas of a shared habitat, such as roots growing to different depths.
Figure 7: Lions and hyenas, two predators that live in the African savannah, have overlapping prey, resulting in interspecific competition.
Figure 8: Resource partitioning in warblers. They reduce competition by using different parts of the tree.