Limiting factors are any factor that limits the growth of a
community (as mentioned above) or maintenance of it’s current
size, the usual examples given as availability of
water,
food,
mating grounds.
Queequeg mentioned the environmental concerns, but limiting factors can be also from the species itself: overpopulation, competition for food, mating ground, etc.; or from other species (lack of prey, excess predators).
Some biologists now claim that there are built in genetic limiting factors, citing the fact that several mice species and bird species voluntarily cut down on their mating and/or clutch size when after they have a "census". They cite swarms of birds calling and flying around in mass heaps as examples of this census, though there is much debate as to whether or not this is the true evolutionary function. It is important to note that mainstream biology, especially Richard Dawkins, rejects the idea that individuals sacrifice for the good of the group. Even if the built in limiting factor causes the individuals to have less offspring, the individuals probably derive a benefit that will pay off in the long term, rather than simply ensure species survival. (Possible benefits include a higher likely hood of the offspring they do have surviving, a greater amount of energy the parent can devote to the few offspring to better ensure their survival, etc.)