Introduction

Colonization is the arrival of individuals to areas of suitable habitat that are currently uninhabited by individuals of the same species. Populations are established (or re-established) in uninhabited areas by the successful colonizers that survive and reproduce. Colonization, or recolonization, is a spatial process central to several fundamental concepts in ecology, including the spatial structure of populations (see Spatial Distribution), species coexistence (see Competition and Coexistence in Model Populations and Metacommunities), succession (see Succession), disturbance and recovery (see Intermediate Disturbance Hypothesis), invasive species (see Invasive Species), and speciation (see Macroevolution).

Colonization is one possible consequence of dispersal. Dispersal is the permanent movement of an individual from one location to another (commonly, a seed, larva, or juvenile stage moving from its natal area to the area it will inhabit as an adult; see Dispersal-Migration). If the individual disperses to an area uninhabited by conspecifics, then dispersal has resulted in colonization. If that colonizer survives to reproduce, then a new population has been established.

Colonization occurs at a range of spatial scales. For sessile organisms in which space is a primary resource (e.g., plants, sessile marine invertebrates), the death of an individual may open space for recolonization by another individual. At this smallest scale, the balance between extinction (individual mortality) and colonization (settlement of another individual in that location) determines the persistence of a population. Small disturbances, such as gopher mounds in grasslands or tear-outs in intertidal mussel beds, create small habitat patches that allow good colonizers to coexist with dominant competitors, enhancing local biodiversity through competition-colonization tradeoff.Larger-scale disturbances, due to fire, logging, or sedimentation-erosion dynamics, may result in a succes-sional mosaic, a landscape in which different areas were disturbed at different times in the past, resulting in a range of successional stages and, therefore, higher regional biodiversity. Colonization is also essential for range expansion and spatial spread. Anthropogenic effects have increased the rates of spread of many species by enhancing colonization. Over a longer timescale, colonization of new niche space may lead to niche expansion or speciation.

We first consider characteristics which enhance species' colonization ability. Then, we use the central ecological concept of colonization-extinction balance to consider the implications of colonization from the smallest scale of an individual to the largest scale of species ranges. In doing so, we consider the implications of colonization for population growth, species coexistence, disturbance, succession, species invasions, and speciation.

Factors Enhancing Colonization Ability

Organisms vary considerably in their ability to colonize new habitat. Figures 1a and 1b illustrate the variation between taxa of freshwater zooplankton in the time needed to colonize experimental mesocosms, and in

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