No organism on Earth lives in complete isolation. Every living thing depends on other living things for some part of its survival, whether for food, shelter, reproduction, or protection.
Pull any single species out of an ecosystem, and the effects ripple outward, affecting organisms that may seem completely unrelated. This web of mutual dependence, called interdependency, is what holds ecosystems together and makes them function.
Interdependency is the condition in which different species in an ecosystem rely on each other directly or indirectly for their survival and functioning.
Interdependency operates at every level of an ecosystem. It includes obvious relationships like predator and prey, but also subtler ones like the relationship between a fig tree and the specific wasp species that pollinates it, or between soil bacteria and the plant roots they supply with nitrogen.
Predation is a relationship in which one organism (the predator) kills and eats another (the prey).
Predator and prey populations are tightly linked. When prey populations increase, predator populations follow. As predators become more numerous, prey populations decline. As prey decline, predators decline too. This creates cyclical population fluctuations that have been observed in many ecosystems.
Competition occurs when two or more organisms require the same limited resource.
Competition between members of the same species for the same resources, such as food, mates, or territory.
Competition between members of different species for similar resources.
Competition drives natural selection, favoring individuals with characteristics that allow them to obtain resources more effectively than competitors.
Symbiosis is a close, long-term interaction between two different species. There are three types based on the effect on each partner.
Both species benefit from the relationship.
One species benefits, and the other is neither helped nor harmed.
One species (the parasite) benefits at the expense of the other (the host). The host is harmed but usually not immediately killed.
Plants and animals have evolved intricate interdependencies around reproduction.
Pollination is the transfer of pollen from the male part of one flower to the female part of another, enabling fertilization and seed production.
Most flowering plants depend on animals for pollination.
The relationship between many plant species and their pollinators has become so specific through evolution that the extinction of one threatens the survival of the other. The dramatic global decline of bee populations is therefore an urgent ecological concern with consequences for both wild plant communities and human food production.
After fertilization, seeds must be dispersed away from the parent plant to reduce competition for resources.
Animals assist seed dispersal in several ways:
A keystone species is a species that has a disproportionately large effect on its ecosystem relative to its abundance.
Removing a keystone species causes dramatic changes throughout the ecosystem, far greater than would be expected from the loss of just one species.
Their reintroduction in 1995 reduced elk populations and changed their grazing behavior. Vegetation recovered along riverbanks. River courses stabilized. Fish populations improved. Bird species returned. The presence or absence of a single predator species cascaded through the entire ecosystem.
Sea otters eat sea urchins. Without otters, urchin populations explode and consume kelp forests, destroying entire underwater ecosystems that support hundreds of other species.
Human activity disrupts interdependencies in ecosystems in several important ways.
Understanding interdependency is therefore essential not just for ecology but for conservation biology and the management of human impacts on the natural world.