Biological ispersal refers to those processes by which a species maintains or expands the distribution of a population. Dispersal implies movement—movement away from an existing population (population expansion) or away from the parent organism (population maintenance). This is necessary for members of a population because organisms of the same age require all of the same resources within an ecosystem. Dispersal relieves pressure for resources in an ecosystem, and competition for these resources may be a selection factor for dispersal mechanisms.