ABSTRACT
For every species of the living world that functions as a family or group a relationship exists between size of population and territory. Populations remain relatively stable in numbers within a territory. Since population and territory are kept in balance territory will be defended vigorously against intruders. Should a given population increase by natural means, instability will set in. The most painless solution is for part of the community to hive off from the parent body, to migrate to a new territory and to form a new community. By this self-adjusting process the parent community is restored to numbers corresponding to its ideal size. Successful and expanding species ultimately demand the penetration of new territories. Then new problems arise, for the ecosystem may differ in a number of respects from the territory that has been quitted. A species with specialist habits and requirements that leaves its territory may fail to adapt to the challenges of the new environment and is likely to perish. The most successful colonists are species that have developed a range of behaviours and are able to live off the possibly unfamiliar sources of food supply that the new environment offers.
