Although the population of each group can fluctuate through death and birth rates, with their own probability of going extinct, these groups can also be supported by other groups through occasional contact. The dynamics of these groups are different not only because their living environments and landscapes are not always equally suitable, but also because of their proximity, or lack of it, to other groups. Limited contact with other groups can force one of these smaller groups to become extinct, leading to the collapse of the entire species if too many groups die out. There are several different ways in which a metapopulation might be organized though, not all fitting this classic definition. For instance, the group could have a core group, from which several subgroups branch off and / or rejoin later. There are also situations in which all members of a species exist in small patches that remain more closely entwined with each other, saving each other from extinction and freely passing members from one patch or subgroup to another. There are also situations in which subgroups are sparsely distributed over adequate environments to such an extent that they have no capacity to reach each other and some appropriate living areas remain void of this species.