Introduction

Dispersal, or the movement and subsequent breeding of individuals from one area to another, strongly influences the population dynamics of a species. Dispersal can help regulate population size and density; many animals, such as aphids and female root voles, have increased dispersal rates under high density situations. Sometimes low density instead of high density is associated with greater dispersal rates. For example, during range expansions, peripheral populations of some grasshoppers may experience higher dispersal rates though they are of lower density than central populations, probably because of fitness costs associated with morphologies specialized for dispersal.

Such dispersal events can have large effects on neighboring populations. Marginal populations that are subject to high rates of immigration may experience a rescue effect, where despite poor genetic or ecological conditions, populations are able to persist. On the other hand, high dispersal rates can inhibit adaptation to novel environments due to constant influx ofnonadapted individuals. Small populations that experience high rates of emigration may have a higher probability of extinction under such situations.

Natural populations in highly fragmented areas, such as agricultural or urbanized settings, may not experience sufficient levels of dispersal. Lack of dispersal can lead to high rates of inbreeding, which can lead to decreased fitness in many species. Because dispersal can have such strong effects on populations, dispersal patterns and processes are important when considering the potential spread of a biocontrol agent, pathogen, or invasive species into a new range. Dispersal also has implications for species redistributions due to climate change, as the dispersal rates and distances of a species will affect its potential to shift its range in response to climate change.

Two types of dispersal are commonly distinguished: natal dispersal, which is movement and subsequent breeding away from the birth territory or area, and breeding dispersal, which is movement from one area to another after the first breeding season. Dispersal of spores, or haploid life stages (such as pollen), strongly affects patterns of gene flow in a species, but the process is not generally considered to be directly associated with population dynamics. Dispersal in plants is generally limited to natal dispersal, as little to no secondary movement is possible, while many animals disperse multiple times.

All species disperse to some extent, in part because resources become limited locally as populations grow. Seedlings of plants must grow at some distance from the parent plant in order to obtain enough water, nutrients, and light to survive. Similarly, animals must disperse to avoid competing for resources such as mates, food, and territory. Depending on intraspecific patterns of resource limitation, dispersal is often sex-biased. In mammals, females tend to disperse more often than males; the trend is reversed in birds.

In areas with high temporal environmental variation, or in areas prone to frequent disturbances, species with greater dispersal abilities are expected to have a greater likelihood of survival. When one population's habitat is rendered untenable, if the species has a high dispersal rate, many individuals in that population will be able to move to a more suitable area. In the case of nonmotile organisms such as plants, high dispersal rates increase the likelihood that another population may be established even as the original population is rendered extinct. When studying populations that specialize in habitats with high temporal environmental variation, it is sometimes appropriate to distinguish between spatial and temporal dispersal. For example, many animals and plants that live in deserts with unpredictable rainfall will produce desiccation-resistant embryos that delay maturity until favorable environmental conditions cue further development. Instead of traveling long distances to reach suitable habitat, the individuals produce offspring that are able to lie dormant until the habitat is once again suitable for survival and reproduction. Because dispersal can enable escape from low-quality environments and access to higher-quality resources, many species that specialize in colonizing disturbed areas tend to have greater dispersal abilities than species that live in relatively stable habitats.

In some cases, dispersal can have a high cost associated with it, especially if individuals that disperse experience a higher mortality rate than those that do not disperse, or that disperse only a short distance. Because individuals are moving to an area that may not be as productive, and because they may have to travel through unsuitable habitats, mortality rates during the dispersal process may be high. The number of individuals that successfully establish in a new area may be far fewer than the number of individuals engaging in the dispersal process. In plants and other organisms with no choice involved in the dispersal process (passive dispersal), many propagules may never establish simply because they land in an unsuitable habitat. In animals where some choice may be involved in the final dispersal location (active dispersal), survival of dispersing individuals may be higher than individuals of species with passive dispersal, but there are still risks associated with dispersal, such as locating an appropriate territory, finding a mate, and successfully breeding in the new area. However, the benefits of dispersal can overcome the costs if mates and/or resources are limiting in the home range.

The process of dispersal is not necessarily as simple as suggested above, as it involves both emigration (leaving the original patch) and immigration (entering a new patch). The entire process of dispersal can be divided into approximately four different stages: (1) emigration,

(2) exploring or traveling through the surrounding habitat,

(3) immigrating to a different patch, and (4) successfully breeding in the new patch. Each of these stages has a cost involved. Leaving the original patch involves leaving an area where resources are known to exist, but may have become limiting. The exploratory phase of dispersal can involve a high risk of mortality, as the individual may have to travel through territories with inadequate resources. In many plants and other passive dispersers, the exploratory phase entails a high rate of mortality, as seeds often land in areas unsuitable for growth. Even when a propagule successfully disperses to a hospitable environment, it may not be able to establish there, due to mortality rates associated with establishment. The risks involved with emigration, exploratory movement, and settling in a new patch can be outweighed by the potential benefits of dispersal if successful dispersal significantly increases the fitness of the individual.

There are varying degrees of active and passive dispersal, with many species exhibiting intermediate levels of participation in the dispersal process. In many animals, dispersal is active, involving a high level of choice during the dispersal process. In passive dispersal, there is little or no choice involved in selection of the final location. In many insects, many marine animals, and all plants, dispersal is largely passive, depending on air currents, water currents, or on the actions of vectors transporting the propagule. Larvae of many marine animals are often dispersed solely at the whims of the currents or in ship ballast. Insects are often at the mercy of the wind when entering a dispersal phase, especially if they cannot generate enough speed to overcome wind velocities. However, even dispersal of small insects need not be completely passive. Small insects, even if they are not large enough to overcome wind velocity, can have some level of choice as to where they land. They can begin exiting a wind stream when they decide to settle, then make short, self-powered trips to explore the surrounding area and find a suitable habitat.

Though considered passive dispersers, plants can regulate dispersal to some extent. Seed size, shape, and seed coat construction vary among species. Seed morphologies that aid dispersal include barbs (for attaching to animals), eliasomes (for attracting ants as dispersal vectors), or pappus scales (to assist in wind transport). However, because the seed itself is not actively involved in the decision process, it is still a passive process.

A species with little innate dispersal ability may be able to move greater distances and have higher survival than expected ifit has the ability to be spread by a vector, such as ants, birds, or other animals. Plants commonly use vector-assisted dispersal, and there are many instances of adaptations by plants to use animals as dispersal agents. For example, mistletoe seeds are eaten by birds which then fly to another tree. The seeds are adapted to survive the digestive tract, and are subsequently deposited on the tree where the bird lands, which is usually a suitable tree for growth. Such assisted dispersal can lead to dispersal distances that would be impossible to achieve otherwise.

Most vector-associated dispersal regimes have evolved over hundreds of generations. Recently, however, many species of both plants and animals have serendipitously become associated with novel and extremely efficient dispersal vectors. Species associated with humans have always been dispersed in concert with human movements. However, the last few generations ofhumans have seen an exponential increase in the rates ofmovement around the globe. Many terrestrial and marine species have been spread at unprecedented rates through ship ballast and packing materials. In addition, ornamental plants and agriculturally associated species are deliberately transported from one location to another by humans, at distances and rates that would be impossible for each species to accomplish under its own power. Hundreds of species involved in these accidental experiments in dispersal and evolution have benefited tremendously, becoming the world's invasive species. Species such as cheatgrass in North America, Caulerpa taxifolia (an alga) in the Mediterranean, and the Nile Perch in Africa have successfully outcompeted hundreds of native species, often driving them to extinction.

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