Even a slight reproductive advantage will eventually lead to the elimination of the less well adapted of two competing species.

+/-

mutualism

parasitism

Invasive species are not held in check by the predators and agents of disease that have always been in place for native species.

commensalism

a nonvenomous snake that looks like a venomous snake

two species of unpalatable butterfly that have the same color pattern

tropical regions generally have more available water and higher levels of solar radiation

+/+

cattle egrets eating insects stirred up by grazing bison

-/-

Human activities are transporting pathogens into new habitats (or communities) at an unprecedented rate.

keystone species