In an ideal ecosystem, each species has its own niche - a different "job description": what it does, what it eats, where it sleeps, and more.
But world is often not an ideal place. In many instances, two species may live in the same spot, yet overlap in some of their roles or needs. They may both compete for the same tree-holes or caves for dwelling, or they may eat the same food. It is not necessary for the two species to be aggressive toward each other, but it is likely that one of the species will be more efficient in gaining the resource than the other.
What is the "loser" to do?
One solution is to move elsewhere. This is called spatial displacement or spatial niche partitioning in ecological jargon. For example, the less competitive species can move some miles down the road, where the more efficient competitor does not live. The place may not be as good - less food and shelter, for example, but it is good enough for individuals to survive and breed, and for the population to persist for a long period of time.
Perhaps the only place to go is up - up the slope of the mountain to higher elevation. There may be stronger winds, colder winters, less vegetation to hide in from predators, and less food, but again, it may be good enough for the population to survive. If this persists for a substantial amount of time, natural selection has the opportunity to introduce new adaptations for the new environment, even to the point of evolving a new species, sufficiently different from other populations of the same species that remains in other places.
In some cases, there is nowhere to go. The two species may inhabit an island. This is often the way an invasive species drives a local island species to extinction. Or you may remember the high school experiment in which you placed colonies of two bacterial species in a petri dish, watching as one colony uses the food better, grows faster, and finally completely kills off the other species.
If there is nowhere to go in space, there is a possibility to go somewhere in time. This is called temporal displacement or temporal niche partitioning. The two species remain in the same place, but divvy up the day (or year). The more efficient (or aggressive) species keeps doing what it's always been doing. But the less efficient species embarks on a new time regime.
There have been a number of observed instances of this. The best documented one is the case of two closely related species of spiny mice inhabiting the "Evolution Canyon" in Israel. The common spiny mouse (A. cahirinus) is, like most rodents, night-active (nocturnal). The other species, the golden spiny mouse (A. russatus) is actively foraging during the day (diurnal), which is unusual for a rodent. When brought into the laboratory and isolated, monitoring of the circadian rhythms of activity showed that both species are inherently nocturnal. The golden spiny mouse forages during the day as a purely behavioral adaptation - its genetics drives it to eat at night, but its environment (including the presence of smells of the other species) dictates eating during the day.
Furthermore, as this situation has persisted for long periods of time, the golden spiny mice have evolved changes in their eyes, adapting them better for a diurnal mode of life. The genes and developmental pathways underlying the development of the eyes apparently contained more useful variation that natural selection could act upon than the underlying biological clock which is still "stuck" in its ancestral condition. This is not optimal - it would be presumably be better if all of the animal's biochemical, metabolic, physiological and behavioral functions switched to the daytime regimen, but it is also obviously "good enough" for the species to survive and thrive.
Today, a new paper in Proceeding of the National Academy of Sciences (not online yet - PNAS is legendary for being late at actually publishing their papers at the time embargo lifts, but check the link later) introduces another interesting example of temporal niche partitioning - and this time it is relevant both for conservation purposes and for human safety.
Researchers from Nepal, in collaboration with Neil Carter and colleagues at the Michigan State University, East Lansing, observed that Chitan National Park in Nepal, one of the 28 world's tiger reserves large enough to support 25 or more breeding females, has a healthy population of tigers. Yet, the Park is also full of humans, and the interactions between humans and tigers are relatively rare.
Their hypothesis was that tigers and humans may use the space of the large park differently, each species limiting its activities to particular areas of the park. Humans in the park include locals who forage, hunt and collect wood in the park, a growing number of tourists, and the military units which traverse the park in jeeps to ensure safety and prevent poaching. The two species compete for some of the same resources - mainly space, but to some extent also food. The two species are also afraid of each other and would tend to avoid meeting each other if possible.
To test this, the researchers installed motion-sensitive cameras inside the park as well as just outside of it. What they discovered was that the two species completely overlapped in space, using the same roads and trails. But, humans remained strictly diurnal animals, confounding their activities to the daylight hours and generally avoiding the darkness. On the other hand, tigers, which are normally day-active animals, switched to the night. They triggered the same cameras in the same places, but mainly at times when humans were not around - during the night.
While being interesting in its own right, as well as a potential model for future research, this study also has practical consequences. It shows that temporal niche partitioning is a strategy that can be employed by tigers, at least as a "good enough" strategy that can allow the tiger population to survive and thrive over long periods of time. This means that humans and tigers can coexist and use exactly the same spaces. The finding makes it easier to politically "sell", set up, fund and run protection areas for tigers as there may be no need to displace the resident humans as long as there is sufficient guard against poaching.
Neil H. Carter, Binoj K. Shrestha, Jhamak B. Karki, Narendra Man Babu Pradhan and Jianguo Liu. Coexistence between wildlife and humans at fine spatial scales, PNAS, September 4, 2012