![]() It’s also very very common because resources are finite and necessary for survival and reproduction. You can create the complementary population growth equation for species 2 by swapping the 1’s for 2’s and vice versa. For more individuals of species 2 or larger values of alpha, species 1 sees lower population growth. The new term in this equation is “minus alpha.” Alpha is the competition coefficient, and you can think of it as converting units of species 2 into units of species 1. We can consider this mathematically with a slight alteration of the logistic equation to include the detrimental effect of species 2 on species 1, where the two species are represented with subscripts. That is, individuals of each species would have more success (more resource access with less energy expenditure = greater growth, survival and reproduction) in the absence of the other species. ![]() The competition for resources that drives reductions in niche overlap is called interspecific competition, and it has a negative effect on both species in the interaction. Competitive interactions have negative effects on both species If the trait in question lacks enough heritable variation for character displacement to occur, then outcome can instead be competitive exclusion, where the more successful species will out-compete the other to the point of elimination. Character displacement is an evolutionary adaptation in a heritable trait, caused by resource partitioning. The shift in the trait value is called character displacement. ![]() These ecological interactions can exert such strong selection pressure that the trait to use the resource (like the beak of the finch to eat seeds) can adapt over generations to a distinctly new character state (like a smaller or larger beak better matched to the available seed food supply). (Image source: Brews ohare CC-BY on Wikimedia Commons.) When species use the same resource, competition is strongest where the niches overlap. Frequently in nature we observe that a species only uses part of its fundamental niche the part they use is called the realized niche. The figure above shows the fundamental niche, the full resource axis of seed sizes that a finch species is theoretically able to use. ![]() The figure below shows a range of seed sizes to define the niche breadth on the x-axis, with seed consumption on the y-axis. Niches have many dimensions, one for each requirement of the species, but we usually reduce a complex niche down to the one or two dimensions deemed most important for the question we are testing. The full range of resources that a species can use combined with the range of conditions a species can tolerate (now we can add sunlight and oxygen back into the mix) defines the ecological niche for that species. The resource that is reduced the most by use, for example water in a desert, and thereby limits population growth first is specially designated as a “ limiting resource.” Likewise, sunlight is not considered a resource. By this definition, oxygen isn’t a resource for humans unless we are living in space or underwater or somewhere requiring we get our oxygen from a finite tank. ![]() A resource in ecology is a thing or factor that causes population growth and that is reduced by use. Not all of these required elements are called resources. Many species need oxygen and carbon dioxide, but there are entire groups of bacteria and archaea that live in anoxic conditions and don’t use or require oxygen. Organisms need many things to survive, including food, water, sunlight if you photosynthesize, etc.
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