Communities and Ecosystems

Negative Ecological Interactions

Sometimes only one organism or species benefits from an interaction at the expense of another organism or species. This type of negative ecological interaction can come in different forms, such as predation or competition.
Symbiosis is the ecological relationship between two or more organisms that live in direct contact with each other. These relationships can be either positive, such as mutualism or commensalism, or they can be negative, such as one species eating another. Other negative interactions result in one species living in or on the body of another organism, called the host. The species living off of the host steals the host's nutrition, but not enough to kill it.


Parasitism occurs when one organism lives in or on another, causing the first organism harm.
A parasite is an organism that lives in or on another organism and uses its host for its own nutritional needs. While a parasite consumes part of its host, it usually does not kill the host. Sometimes parasites that live inside the host can be categorized as pathogens. This means they cause definite harm to the host, such as a disease. One example of a parasitic relationship is between a tapeworm and animal such as a cow or human. Tapeworms are flatworms that attach to the inside of an animal's intestine to get food for survival. They eat the host's partially digested food. This causes the host nutrient supply to be deprived of vital nutrients for survival. Eventually, the host will die.


One organism consumes another in predation.
Predation is an interaction that has negative consequences for one species (the prey) and positive consequences for the other species (the predator). A common example of predation involves carnivores that hunt. For example, lions kill antelope for food, and owls hunt rats. Not all predators are animals. Plants, such as the Venus flytrap, can also be carnivores. The Venus flytrap digests insects that land on its leaves.

Predator-Prey Relationships

The graph describes a predator-prey model, which predicts the cyclical relationship between the population size of predator and prey. As the prey population increases, so does the predator population with a characteristic delay. These graphs are also used to evaluate how predators and prey adapt within an ecosystem for survival. An example includes the fox and rabbit population where foxes are major predators of rabbits. When more rabbits are in abundance the fox population can increase because they have more food available to them.
Predators and prey exhibit specific traits that help them survive and reproduce. Predators have traits such as sharp teeth or venom that enhance their ability to catch prey. Prey use techniques such as camouflage or mimicry to avoid detection or capture. Camouflage hides a prey animal as it blends into its background, making it harder for the predator to see it. Mimicry occurs when an animal looks different so that it is mistaken for something dangerous. For example, the Mexican milk snake has a pattern of red, black, and yellow stripes that greatly resembles the pattern of stripes that the aggressive and venomous coral snake has. This makes potential predators (including humans) avoid the Mexican milk snake.

Interspecific Competition

When two or more species compete for limited resources, it is known as interspecific competition.
Interspecific competition, another negative interaction form symbiosis, describes a type of interaction where organisms of the same species or different species use the same limited resources in a community. Examples of limited resources can be food, a space to live, or even sunlight. Interspecific competition can result in a negative interaction if the competing species fight and kill each other for the limited resource. Interspecific competition can also have dramatic results on one or both species even if the species do not directly interact. For example, consider a grassland ecosystem in which rabbits and deer both eat grass. In a year of drought, less grass grows, meaning there is a limited food resource for both the rabbits and the deer. Even if the two species never attack each other, numbers of individuals in both species may decline, and one of the species may even die out if food becomes too scarce.

Competition can also be a type of amensalism. Amensalism is an interaction between species where one species is destroyed and the other is unaffected. For example, the mold Penicillium produces a chemical that is toxic to most species of bacteria. The bacteria die, but the mold is unaffected.