Speciation rate is ultimately set by niche filling (that is, ecological competition for resources), rather than by the rate of acquisition of reproductive isolation. ~ American evolutionary biologist Trevor Price et al
Multiple species occupy the same niche when they extensively consume the same resources in the same area. Sharing a habitat creates competition. There can also be competition between subpopulations of a single species for the same resources. This has been extensively seen in human tribes.
Owing to the high geometrical rate of increase of all organic beings, each area is already fully stocked with inhabitants; and it follows from this, that as the favored forms increase in number, so generally will the less favored decrease and become rare. ~ Charles Darwin
The idea of competitive exclusion is that rivalry for selfsame resources progresses to dominance by one of the ‘competitors’. Situations seldom develop as simply as Darwin portrayed. Organisms don’t necessarily reproduce to carrying capacity (as humans are apt to), nor do “favored forms” increase at the expense of “less favored” (the notion of such favoritism being some ersatz ideal of predeterminism).
That said, some species may appear to overwhelm others, as is sometimes seen with ‘invasive’ plants. This dynamic relates to local population numbers and may not directly affect speciation or extinction.
The alternative to becoming rare from competitive exclusion is to adapt to a different niche by consuming a distinctive mix of resources. In creating variation that leads to speciation, competition begets a degree of isolation. To avoid decline from food competition, closely related bird species do not coexist in the same area.
As we have already seen, variation can take many routes, driving adaptation in different directions. A confluence of adaptations may take place that put a population on the path to speciation via a variety of altered traits.
Taking up residence in more inhospitable terrain may result in augmenting defenses against predators and/or greater tolerances to adversity in whatever form. It may also lead to taking advantage of new resource opportunities that create a much different lifestyle.
Niche filling has stopped species from getting big ranges. ~ Trevor Price
Specialization has its risks. Performance may improve in a new niche, but if speciation has occurred via extirpation, that species may be especially sensitive to ecological disturbance.
The short-term functional advantages of a novel trait do not always translate into a lineage’s long-term survival or diversification. Adaptation contributes to a good fit between an organism and its current environment. What it cannot do is anticipate the future. ~ Geerat Vermeij
Generalization may decrease specific exploitation performance but offer a robustness that affords survival in a changing environment. Expanding the diet to include other foodstuffs may reduce direct competition. Adaptations to optimize nutritional extraction from alternative food sources may result.
Just as the availability of unexploited resources drives nichification, radiative speciation is constrained by lack of opportunity. The rate at which niches are created and occupied limits diversification. In absence of such opportunity, new species may arise from mating preferences, but these are relatively rare.