I.6 Dispersal
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I.6 Dispersal Nicolas Perrin OUTLINE 1. Definition, patterns, and mechanisms 2. Evolutionary causes 3. Demographic and genetic consequences of dispersal 4. Measuring dispersal After a brief overview of the general patterns and the variety of mechanisms used for dispersal, this chapter delineates its evolutionary causes. Besides the spatial distribution and temporal dynamics of limiting resources, genetic structures resulting from mating or social systems play a role by affecting the potential for inbreeding and kin competition . Depending on conditions, however, dispersal may also have detrimental consequences at the population level, in terms of both demography and genetics. Finally, the chapter outlines recent developments in the way dispersal is measured. GLOSSARY coancestry. Probability that two alleles sampled from two different individuals are identical by descent. FST. A measure of genetic differentiation among populations , expressing the proportion of variance within a set of demes that results from the differentiation among them. genetic load. Decrease in average population fitness (relative to the fittest genotype) caused, e.g., by immigration of locally less-adapted immigrants (migration load), mating among relatives (inbreeding load), fixation of deleterious alleles (drift load), or any other population process. heterosis. Increase in fitness resulting from matings among individuals from different populations (as a result, e.g., of superdominance or drift-load effects). inbreeding depression. Drop in fitness resulting from the mating between relatives (caused, e.g., by recessive deleterious mutations). local competition. Competition among relatives for limiting resources (including mates). mass effects. Quantitative effects of dispersal on local population dynamics. Emigration from a population may have negative effects on its demography, whereas immigration may have positive (rescue) effects. outbreeding depression. Drop in fitness resulting from the mating among distantly related individuals (from, e.g., the disruption of coadapted gene complexes ). phoresis. Mechanism of dispersal by attachment of the propagule to another, actively dispersing organism. polygyny. Mating system in which a few males monopolize many females. propagule. Any part of an organism used for the purpose of dispersal and propagation. sink. Any population that consistently receives more immigrants than it sends emigrants. source. Any population that consistently sends more emigrants than it receives immigrants. 1. DEFINITION, PATTERNS, AND MECHANISMS There are many ways to define dispersal. The simplest and possibly most appealing one might be to define it as the movement of organisms away from their place of birth. The crucial feature here is that dispersers do not reproduce where they were born. This opposes dispersal to ‘‘philopatry,’’ i.e., the tendency to reproduce at the natal place. Dispersal is referred to as ‘‘effective’’ when immigrants in a population contribute to local reproduction (i.e., when the rate of dispersal translates into a rate of gene flow among populations). Dispersal is a very ubiquitous feature throughout the living world, from bacteria to animals, including organisms that spend most of their life cycle in a sessile form (such as plants and fungi, but also many filterfeeding invertebrates). Dispersal patterns can be described by a ‘‘dispersal kernel,’’ which expresses settlement probability as a function of distance from the source. The shapes of such kernels are obviously bound to depend on the dispersal mechanisms involved, which might be passive (e.g., transport by wind or water) or more active (e.g., flight). In the latter case, the kernel will also depend on behavioral strategies (e.g., random walk versus directed movement) and cognitive abilities in interaction with landscape features. In its simplest form, the kernel is an exponential negative function of distance from the source. However, even slight departures from this simple function might be of importance. Long-distance dispersers have a disproportionate impact on population processes, in particular during colonization events, by determining the rate of spread and the establishment of long-lasting genetic structures. Whether dispersal kernels have thin or fat tails (i.e., decrease faster or slower than an exponential) thus becomes an important theoretical issue. It is also one difficult to address empirically because long-distance dispersal events are rare and therefore often missed in mark-recapture experiments. Adaptations to dispersal are extremely diverse and often remarkably ingenious. Plants normally disperse passively by relying on currents (wind, water) or animals , both in the gametic (pollen) and zygotic (seed) dispersal phase. Pollen dispersal by animals involves complex interactions that are usually mutualisms (in which plants provide nectar to pollinators) but may include parasitism: more than one-third of orchid species do not provide their pollinators with either pollen or nectar rewards, relying on floral mimicry for pollination...