3 Ecological Interactions
Explore how interactions among species affect survival, reproduction, population sizes, distributions, and the structure of ecological communities.
How interactions affect communities
An consists of populations of different species that live and interact in the same area. Their interactions affect survival, reproduction, and population size, and thereby influence the community’s composition and food-web structure.
A shorthand describes the effect on each of two species: , means a benefit, , means a cost, and , means no substantial effect. Typical effect pairs are ,; , , and ,; ,; and ,. These categories describe typical effects, not unchanging rules: the strength and outcome of an interaction can vary with environmental conditions and the species involved.
for limited resources
occurs when organisms use the same limited resource, including food, water, light, nesting sites, or space. It may happen among members of the same species, called , or among members of different species, called . Because competitors have reduced access to a resource, the typical effect is ,.
Strong between species can cause one population to decline or restrict where each species occurs. Competing species may coexist through : they use different foods, locations, or times. For example, bird species in one forest may feed on insects in different parts of the canopy. These differences reduce direct and help shape community composition.
and
In , a predator captures and consumes prey, usually killing it. The predator gains food and the prey loses an individual, so the typical effect is ,. If prey become abundant, predators may have more food and increase in number; increased can then reduce prey numbers. Such effects can ripple through food webs as changes in one population alter the abundance of its prey, predators, or competitors.
is the consumption of plants or algae by animals and is also usually ,. Unlike many predators, herbivores often consume only part of an organism rather than killing it. Grazing and browsing can affect plant growth, survival, and reproduction, changing vegetation and the resources or habitat available to other species. Plants may reduce damage with defenses such as thorns or distasteful chemicals, while herbivores may have adaptations that help them overcome those defenses.
Close associations between species
, , and are close associations often grouped under the term , although definitions of vary. A symbiotic relationship is not necessarily beneficial to both partners.
In , both species benefit. A pollinator obtains food from a flower while transferring pollen that can help the plant reproduce. Mutualisms can affect community structure by supporting partners’ reproduction, survival, or access to resources.
In , one species benefits and the other is not noticeably helped or harmed. A bird nesting in a tree may gain shelter while having little effect on the tree. Demonstrating a truly neutral effect can be difficult, because a relationship classified as commensal may have subtle effects that are not readily observed.
In , a parasite gains resources from a host and harms it without usually killing it immediately. Ticks feeding on mammals are an example. Parasites can reduce host condition or reproduction, influence host population sizes, and affect interactions between hosts and other community members.
Community-wide consequences
Interactions influence which species persist, how abundant they become, and where they occur. can limit access to resources or encourage species to use different niches. and transfer energy through food webs and can regulate consumer and resource populations. Mutualisms can support reproduction or resource acquisition, while parasites can weaken hosts and alter population dynamics.
Because interactions connect populations, a change in one interaction may cause indirect changes elsewhere in the community. Together, these relationships shape population sizes, species distributions, and community structure.