Competition occurs when two or more organisms require the same limited resources, such as food, water, or territory, and their use of these resources negatively affects the survival, growth, or reproduction of the other organisms.

In a predator-prey relationship, the predator is the organism that hunts, kills, and consumes the prey organism for food.

Mutualism is a symbiotic relationship in which both species benefit from the interaction. In the case of a bee pollinating a flower, the bee obtains nectar and pollen from the flower, while the flower benefits from the bee's pollination services, which aid in reproduction.

Commensalism is a symbiotic relationship in which one species benefits from the interaction while the other is neither harmed nor benefited. For example, a remora fish attaches itself to a shark and feeds on scraps of food left by the shark, without harming the shark.

Intraspecific competition occurs between individuals of the same species, competing for the same limited resources. In the case of two trees competing for sunlight, each tree tries to grow taller and spread its leaves wider to capture more sunlight, which can limit the growth and survival of the other tree.

Parasitism is a symbiotic relationship in which one species (the parasite) benefits from the interaction by obtaining nutrients or other resources from the other species (the host), while harming the host.

A keystone species is a species that has a disproportionately large impact on its ecosystem relative to its abundance. Sea otters are considered a keystone species because they prey on sea urchins, which helps to maintain kelp forests. Without sea otters, sea urchins would overgraze kelp forests, leading to a decline in kelp forest biodiversity and ecosystem function.

Mutualism is a symbiotic relationship in which both species benefit from the interaction. However, in facultative mutualism, the association between the species is not obligatory, meaning that each species can survive and reproduce independently of the other.

An obligate mutualism is a symbiotic relationship in which both species depend on each other for survival or reproduction. In the case of a bee pollinating a flower, the bee relies on the flower for food (nectar and pollen), while the flower relies on the bee for pollination, which is essential for reproduction.

Parasitism is a symbiotic relationship in which one species (the parasite) benefits from the interaction by obtaining nutrients or other resources from the other species (the host), while harming the host. In obligate parasitism, the parasite cannot survive without the host, and the host is negatively affected by the parasite's presence.

Indirect interactions occur when two species interact through a third species. In the case of a mistletoe plant growing on a tree, the mistletoe benefits from the tree by obtaining nutrients and support, while the tree is negatively affected by the mistletoe's presence, which can reduce the tree's growth and reproduction. However, the interaction between the mistletoe and the tree is mediated by the presence of birds, which disperse mistletoe seeds to different trees.

Commensalism is a symbiotic relationship in which one species benefits from the interaction while the other is neither harmed nor benefited. In obligate commensalism, the interaction is obligatory for the benefiting species, meaning that it cannot survive without the other species.

A trophic cascade is a series of indirect interactions between species at different trophic levels. In the case of a lion hunting a gazelle, the lion preys on the gazelle, which reduces the gazelle population. This, in turn, allows the vegetation that the gazelles feed on to grow more abundantly, which benefits other herbivores in the ecosystem.

Parasitism is a symbiotic relationship in which one species (the parasite) benefits from the interaction by obtaining nutrients or other resources from the other species (the host), while harming the host. In facultative parasitism, the interaction is not obligatory for either species, meaning that both species can survive and reproduce independently of each other.

Allelopathy is a type of indirect interaction between species in which one species releases chemicals that inhibit the growth or survival of another species. In the case of a mistletoe plant growing on a tree, the mistletoe releases chemicals that can inhibit the growth of other plants in the vicinity, giving the mistletoe a competitive advantage.