A community is a group of different species of organisms that live close enough together that they influence each other. There are two different hypothesis of why there are communities of organisms in ecosystems. The first hypothesis, integrated hypothesis, was made by F.E. Clements, who’s idea was that a community depended on the interactions between the organisms that allowed organisms to survive together. The second hypothesis, independent hypothesis, was made by Henry Allan Gleason, who believed that a community consisted of organisms who have the same living essential requirements. For example, this means that animals who share the same food source will live close to each other since they both must live near the food source in order to survive.
In a community, organisms of different species interact with each other, resulting in positive or negative effects for each species. These types of interactions are divided into four groups: competition, predation, herbivory, and symbiosis.
1. Competition. Competition is an interaction between different species when they compete for the same limited resource in the environment. Competition brings negative effects for both competing parties since each species does not have full control of the resource. In fact, when competition is strong between two species for the same limited resource, one species will adapt to use the resource more efficiently and reproduce rapidly. Over time, this species will eventually out produce the competing species, leading to the death of the other species. This phenomenon is called competitive exclusion. If two species require the same limiting resource in an ecosystem, these two species cannot coexist together.
2. Predation. Predation is the interaction between different species where one species is the predator, and it kills and eats the other species, the prey. This interaction benefits the predator since the predator gets the nutrients from the prey in order to survive and reproduce. This interaction brings about the adaptations for both the predator and the prey. Predators adapt by having sharp claws, teeth, fangs, or poison to catch and kill their prey. On the other hand, prey have adapted to prevent themselves from being killed by predators by adapting their speed, color, mimicry, and shell. Some animals have skin colors that camouflage them with the surroundings to hide from predators. Many animals synthesize toxins and poisons that make them dangerous if eaten by a predator. These animals display aposematic coloration, which are bright colors that warn predators that they are poisonous to eat. Furthermore, prey species often display mimicry, which is displaying colorations or traits that resemble other species. In Batesian mimicry, a harmless species looks like a harmful species, which prevents predators from eating them. In Mullerian mimicry, two poisonous species resemble each other in order to teach the predator that they are dangerous to eat. Prey have adapted to hide from predators or warn predators that they are dangerous to eat.
3. Herbivory. Herbivory is the interaction between an organism and a plant. The organism eats parts of the plants to get their nutrients. As a result, since plants do not have the ability to flee from an herbivore, plants have adapted by growing thorns and spikes as well as producing poisonous toxins to warn and prevent herbivores from eating them. In response, some animals and insects have sensors that allow them to distinguish between poisonous and delicious plants. Some herbivores also have specialized cells in their digestive systems that help them to digest the tough cellulose and cell walls of the plants.
4. Symbiosis. Symbiosis is a relationship between two species that directly affect each other. There are three types of symbiosis. The first is parasitism, where one species leeches off another species. The leeching species is the parasite, and it steals the nutrients from the other species, the host. As a result, the host is harmed since it loses part of its nutrients to the parasite. Furthermore, the parasite oftentimes changes the behavior of its host that eventually causes the death of the host. For example, ticks live on moose and suck on the blood of the moose. It also causes hair loss on the moose, which could further cause the death of the moose in the winter due to the cold weather. Another type of symbiosis is mutualism, which is an interaction between two or more species where both species benefit. Both species help each other out to receive more nutrients than they would have received if they were alone. For example, the mutual relationship between fungi and plant roots allows the fungi to receive nutrients and sugars from the plant while it helps the plant absorb additional nutrients from the soil. Another example of mutualism is the relationship between trees and ants. The tree provides food as well as shelter for the ants, while the ants provide protection to the tree. Day and night, the ants patrol the tree and attack potential herbivores. As a result, the ants help the tree survive longer. The third type of symbiosis is commensalisms, where the interaction between two species benefits one species without affecting the second species. These interactions are rare since most interactions will always have an effect on both species. Several examples of commensalisms occur when one species latches onto another species for a “free ride.” Algae oftentimes thrive on shells of turtles. As the turtle swims from one area to another, the algae are able to get to different areas in the ocean.
A trophic structure is the structure of the feeding structures between organisms in an ecosystem. A food chain depicts the order of which predators eat prey, but more importantly, food chains show the path that food energy is transferred between organisms and returned to the system in a cycle to recycle nutrients in the ecosystem. Energy from the autotrophs are transferred to herbivores and carnivores as they are eaten. When these animals die, they are decomposed by fungi and bacteria, which returns the nutrients to the ground to be used again by plants. This cycle ensures that the supply of food energy will not be depleted. Many food chains are connected together to form a food web, which depicts the relationships between organisms and show which organisms will eat other organisms. Food webs show the important relationships between each organism, and if one organism were to be removed, it would cause disastrous effects on the rest of the organisms in the community since many other organisms depend on the organism for food.
Many species play an important role in the community. The most highly abundant species is the dominant species. These species affect their community by causing other species to adapt around them. For example, if a certain type of tree is the most abundant in a community, it controls the shade and the amount of nutrients that are available in the soil. If this tree were to be removed, many species that depended on the tree for food may decrease and even become extinct. Another type of species are the keystone species, which are species that have huge influential power on the community. For example, sea otters prey on sea urchins, and sea urchins prey on kelp. When there are lots of sea otter in a section of water, the population of kelp is high, but when the otter are present, the sea urchins will quickly prey on the kelp, and there will be a low amount of kelp in the community. Finally, the ecosystem engineers are the species in the community that change the physical environment. Beavers are a huge example of ecosystem engineers since they use trees and branches to build dams and slow down the flow of water through a community ecosystem. As a result, ponds can form, and different species of organisms can thrive in the pond.
Reference: Reece, Jane B., and Neil A. Campbell. Campbell Biology. Harlow: Pearson Education, 2011. Print.