Just because an animal survives somewhere, doesn’t mean it thrives there.
Fitness, in the evolutionary sense, refers not only to survival, but also to reproduction. That is, when we talk about fitness, in the context of selection, an animal or plant or any other kind of organism needs to reproduce in order to be fit. This, of course, requires survival as well (dead organisms don’t reproduce) – but while survival per se is necessary, it is not sufficient, for fitness.
And so, population density, alone, of a species is not a perfect indicator of habitat quality for that species – there could be a lot of that species there, but that doesn’t necessarily mean that it’s reproducing there. This would be what we call a sink habitat (i.e, a place where a species is present – and perhaps even abundantly – but not enough to replace individuals lost through death and out-migration).
If such a habitat were in isolation, then it would of course eventually run out of individuals of that species – however, if there’s a source habitat (i.e, a place where reproduction exceeds loss due to death and migration) nearby, providing new individuals to the sink habitat that replace those lost through death or out-migration, then that sink habitat can have a consistent (and perhaps even consistently high) population density, even though its population isn’t reproducing enough to replace itself.
This means that density alone is not a clear indicator of the quality of the habitat… or, put another way:
Density as a Misleading Indicator of Habitat Quality. B. Van Horne. The Journal of Wildlife Management Vol. 47, No. 4 (Oct., 1983), pp. 893-901
One implication of this is that the habitat that is being conserved for a species might not be optimal for the reproduction of that species, and this could guide errant conservation strategies – for example, if most of the habitat were sink habitat, and it were supposed that that were the optimal habitat for the species, when in reality nearby (but seemingly less important because it includes less total area of the species of interest) is the source replenishing that population, then effort might be spent on that sink habitat at the expense of effort that would have been spent more fruitfully on the nearby source habitat.
Sinks can benefit a metapopulation, as they may decrease competition in source habitats.
Another implication of this is that the realized niche of a species might actually be larger (with respect to area) than its fundamental niche:
Sources, Sinks, and Population Regulation. H. Ronald Pulliam. The American Naturalist Vol. 132, No. 5 (Nov., 1988), pp. 652-661
A species’ niche is, broadly speaking, the setting (or, more precisely – the environmental factors) required for a species to survive and reproduce. The fundamental niche is the set of environmental factors that one would idealize that the species requires to survive and reproduce – the realized niche, though, is what actually happens, in the context of real biological communities.
It is often considered that a species fundamental niche (that is, and broadly speaking, the places where it lives) will occupy less space than its realized niche, due to competition – that is, there are places a species can live, but doesn’t, because it gets outcompeted in its niche.
A population is a set of individuals of a given species, in a given geographic area. They are presumed to be interbreeding. A metapopulation is a set of populations among or between which there is gene flow.