In a closed system (and the biosphere as a whole is a closed system) the only way to generate additional species is through evolution.
While the evolution of new species is a necessary condition for the generation of diversity, it isn't good enough on its own. If a species splits into two daughter lineages that are unable to interbreed, you should have two species. But in order for them to coexist in a given area, some sort of ecological difference needs to have evolved. If two species occupy the same area, they are in a position to compete for resources. The more similar their needs, the more intense the competition is likely to be.
A bird species that eats berries and one that eats grass seeds are unlikely to find themselves in competition for food sources, although they might compete for nesting or breeding sites. On the other hand, two species that eat insects are more likely to find themselves in competition for food. For both these species to coexist, they may need to specialise on different types of insects, or they may segregate spatially - one species may feed in the tree canopy while the other feeds in the understorey.
Within their area of specialisation, each species can outcompete the other. As a result, coexistence is possible. For two species to coexist, there must be some minimum difference between them. This has been termed the principle of limiting similarity.
The theory that all species must have there own niche to survive feels wrong, when you have seen how many similar species that occurs in nature without any distinguishable habitat preferences. For example the only apparent difference between many species are in sexual characters like the penis in beetles and have nothing to do with niche preference. An other “thing” that comes form field biodiversity studies is that there are some common species and the more the survey continue the more rare species gets found.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
It all can possibly be explained by loss of imagination but I want to believe in the existence of some other explanations. I se two way’s of doing it one is to explore different types of competition and what it is the other is to se if some relaxations of the assumptions in the Lotka –Volterra model of interspecific competition can enable some coexistence and dispute the common theory of coexistence and competition
First we have several types of Competition
Exploitation competition
It’s where one species can survive on lower levels of the resource than the other species and reduce the resource to that level so that the other species starve.
Apparent competition
it’s where the species can tolerate a higher predation pressure than the other species and feed the predators to a population level that the other species cant tolerate.
Interference
It is direct interference between the species via some sort of aggression
But there are something important missing, the reproduction and dispersal rate were it is. The species that can reproduce and disperse better than another should out compete the other species that is similar in al other ways and it is isn’t necessary some of the other types of competition because it should be better to find empty lots in nature and we have a forth type of “competition” and one more dimension that can create niches.
But I still think there are something missing, there have come some ideas with lottery models that try to use the fact that the environment is changing and that there are lots of more or less random things going on, but I don’t think the theories really logically and mathematically work out. But in this complex mathematical landscape with dynamical and random processes there are still room for new grand theories. On more opening is that also species can change with time and evolve.
One possibility can be to loan theories from statistical thermodynamics where we can compare the individuals with the molecules in a gas because the movements of the individuals largely can be seen to be random in many species. Then we can see that with more species that are more evenly distributed we get higher entropy. The entropy is a statistical phenomena where macroscopic states (here diversity) that have more possible of microscopic configurations (here where al the individuals are) is more probable.
Another theory is to borrow the red Queen hypothesis from population genetics where the allele frequencies in a predator – prey system constantly is changing, but in the end doesn’t come anywhere. It may be possible to think that instead of different alleles in the prey population there are different species and the predator switch and adapts to the most common species so some other prey species can take over and so on. Then we can have several rare species that in that moment doesn’t have any niche but doesn’t disappear until the predator will change and they get niche.