The organisms that share a community interact with one another in a variety of ways. They compete for resources, struggle to eat or escape one another, or rely on one another for food or shelter. These interactions fall into three broad categories – competition, predation, and symbiosis.

Competition
Competition is a struggle for limited or favored resources, such as food, water, space, sunlight, or mates. Competition is the most widespread type of interaction between species. It is a powerful force that drives natural selection within species and leads to niche partitioning between species.

Competition can be interspecific and occur between different species. An example would be competition between cattle and antelope on western rangelands. The antelope was historically located in the area first, but humans saw the grasslands as an opportunity to grow cattle for beef, and introduced cattle to the same area. Since then antelope and cattle have competed for grass and water. Because ranchers are interested in increasing cattle production, they may elect to further remove the competition represented by antelope through hunting or relocation.

Other competition is intraspecific, between members of the same species. Territoriality – the defense of a nesting, mating, or feeding area - is a type of intraspecific competition seen in a number of organisms from ants to robins to wolves.

Predation
Predation occurs when one organism, the predator, catches and eats another organism, the prey. Predation, like competition, is an important influence on natural selection. Although predation has obvious unfortunate consequences for prey individuals, it can have beneficial effects on prey populations. Predation can prevent over-population and the subsequent habitat degradation, and ward off disease outbreaks by removing sick individuals.

Symbiosis
Symbiotic relationships are long-term and intimate associations between different species. There are three types of symbiosis, distinguished by how the relationship impacts each of the partners: parasitism, commensalism, and mutualism.

Parasitism refers to one organism, the parasite, living in or on another organism, called the host. The parasite feeds on the host, which benefits it but harms the host. Parasites are typically microorganisms, bacteria, worms, insect larvae, fungi, protozoa, viruses and so on. Those that are overly enthusiastic in feeding or reproducing can kill the host, but many simply cause some degree of persistent debilitation.

External parasites live on their hosts. Examples include deer and dog ticks and gill flukes. Internal parasites live inside the host. These include tapeworms and liver flukes.

While we might have a negative emotional reaction to the existence of parasites, parasites have an important role in host population control, as or more important than that of predators. Humans now take advantage of parasites by using them, in lieu of chemical pesticides, to control many undesirable species, such as weed species like spotted knapweed and field bindweed, grasshoppers, and vegetable-eating caterpillars.

For the most part, parasites are very specific to the host that they feed on, so using these specific parasites to control undesirable species does not damage our food supply.

Commensalism is a type of symbiosis that benefits one partner and does the other neither good nor harm. Commensal organisms might get transportation or shelter from their partners, or feed on dropped food scraps or wastes. A woodpecker roosting in a the crook of a tree, a remora fish attached to a shark, and epiphyte orchids living on tropical trees are all examples of commensalism.

There is some disagreement among ecologists over whether commensalism is in fact possible. Some scientists argue that no relationship between living beings can be completely neutral. The barnacles that seem to ride harmlessly along on whales may cause drag or skin irritation. The epiphytic plants that appear to perch innocuously on tree limbs may intercept sunlight and water that would otherwise go to the tree.

Mutualismis a relationship that represents the best of both worlds for two species that coexist. In this interaction, both partners benefit and neither one is harmed. There are many important examples of this type of relationship.

All mammals, including humans, carry a population of bacteria in their guts which helps with food digestion. We could not obtain nourishment from our food without these necessary organisms to aid us. We benefit from the activity of the bacteria, and the bacteria obtain the nutrients required to survive from the food that we ingest.

Mutualism is widespread in plants and invertebrates as well. For example, many plants have mutualistic relationships with insects, providing nectar to the insect and getting pollinated in return.

Seventy percent of land plants, the primary producers in terrestrial ecosystem, rely on a mutually beneficial relationship with nitrogen-fixing bacteria or fungi. Over time, these relationships may be refined and strengthened further by co-evolution.