Dispersal is the movement of individuals away from their source and includes spread, the local movement of individuals, and migration, the cyclic mass movement of individuals among areas (L. Clark et al. 1967, Nathan et al. 2003). As discussed in Chapter 2, long-distance dispersal maximizes the probability that habitat or food resources created by environmental changes or disturbances are colonized before the source population depletes its resources or is destroyed by disturbance. However, dispersal also contributes to infusion of new genetic material into populations. This contribution to genetic heterogeneity enhances population capacity to adapt to changing conditions.
Dispersal incorporates emigration, movement away from a source population, and immigration, movement of dispersing individuals into another population or vacant habitat. Immigration adds new members to the population, or founds new demes, whereas emigration reduces the number of individuals in the population.
Effective dispersal, the number of individuals that successfully immigrate or found new demes, is the product of source strength (the number of individuals dispersing) and the individual probability of success (Nathan et al. 2003, Price 1997, see Chapter 2). Source strength is a function of population size, density, and life history strategy. Individual probability of successful dispersal is determined by dispersal mechanism, individual capacity for long-distance dispersal, the distance between source and sink (destination), patch size, and habitat heterogeneity, as described later in this section (see also Chapters 2 and 7).
Species characterizing ephemeral habitats or resources have adapted a greater tendency to disperse than have species characterizing more stable habitats or resources. For example, species found in vernal pools or desert playas tend to produce large numbers of dispersing offspring before water level begins to decline. This ensures that other suitable ponds are colonized and buffers the population against local extinctions. Some dispersal-adapted species produce a specialized morph for dispersal. The dispersal form of most aphids and many scale insects is winged, whereas the feeding form usually is wingless and sedentary. Migratory locusts develop into a specialized long-winged morph for migration, distinct from the shorter-winged nondispersing morph. Some mites have dispersal stages specialized for attachment to phoretic hosts (e.g., ventral suckers in the hypopus of astigmatid mites and anal pedicel in uropodid mites) (Krantz 1978).
Some species have obligatory dispersal prior to reproduction. Cronin and Strong (1999) reported that parasitoid wasps, Anagrus sophiae, laid >84% of their eggs in host planthoppers, Prokelisia spp., on cordgrass, Spartina alterniflora, plants isolated at 10-250 m from source populations.
Dispersal increases with population size or density. Cronin (2003) found that emigration of planthoppers, Prokelisia crocea, increased linearly with density of female conspecifics. Crowding increases competition for resources and may interfere with foraging or mating activity, thereby encouraging individuals to seek less-crowded conditions. Leisnham and Jamieson (2002) reported that more mountain stone weta emigrated from large tors with larger demes, but proportionately more weta emigrated from small tors, likely reflecting the greater perimeter-to-area ratio of small tors.
The mating status of dispersing individuals determines their value as founders when they colonize new resources. Clearly, if unmated individuals must find a mate to reproduce after finding a habitable patch, their value as founders is negligible. For some species, mating occurs prior to dispersal of fertilized females (Mitchell 1970). In species capable of parthenogenetic reproduction, fertilization is not required for dispersal and successful founding of populations. Some species ensure breeding at the site of colonization, such as through long-distance attraction via pheromones (e.g., by bark beetles; Raffa et al. 1993), or through males accompanying females on phoretic hosts (e.g., some mesostigmatid mites; Springett 1968) or mating swarms (e.g., eastern spruce budworm, Choristoneura fumiferana; Greenbank 1957).
Habitat conditions affect dispersal. Individuals are more likely to move greater distances when resources are scarce than when resources are abundant. Furthermore, the presence of predators may encourage emigration (Cronin et al. 2004). However, Seymour et al. (2003) found that a lycaenid butterfly, Plebejus argus, whose larvae are tended by ants, Lasius niger, apparently are able to orient toward patches occupied by L. niger colonies. Butterfly persistence in patches was influenced more strongly by ant presence than by floral resource density.
Dispersal mechanism determines the likelihood that individuals will reach a habitable patch. Individuals that disperse randomly have a low probability of colonizing a habitable destination. Larval settlement rates for black flies, Simulium vittatum, are lowest in the high stream velocity habitats preferred by the larvae as a result of constraints on larval ability to control direction of movement at high flow rates (D. Fonseca and Hart 2001). Conversely, individuals that can control direction of movement and orient toward cues indicating suitable resources have a higher probability of reaching a habitable destination. Transportation by humans has substantially increased possibilities for long-distance dispersal across regional and continental barriers.
The capacity of individuals for long-distance dispersal is determined by flight capacity, nutritional status, and parasitism. Winged insects disperse greater distances than wingless species (Leisnham and Jamieson 2002). Individuals feeding on adequate resources can store sufficient energy and nutrients to live longer and travel farther than can individuals feeding on marginal or inadequate resources. Although dispersal should increase as population density increases, increased competition for food may limit individual energy reserves and endurance at high densities. Furthermore, parasitized individuals may lose body mass more quickly during dispersal than do unparasitized individuals and consequently exhibit shorter flight distances and slower flight speeds (Bradley and Altizer 2005). Hence, dispersal may peak before increasing density and disease reach levels that interfere with dispersal capacity (Leonard 1970, Schowalter 1985).
Dispersing individuals become vulnerable to new mortality factors. Whereas nondispersing individuals may be relatively protected from temperature extremes and predation through selection of optimal microsites, dispersing individuals are exposed to ambient temperature and humidity, high winds, and pred ators as they move across the landscape. Exposure to higher temperatures increases metabolic rate and depletes energy reserves more quickly, reducing the time and distance an insect can travel (Pope et al. 1980). Actively moving insects also are more conspicuous and more likely to attract the attention of predators (Schultz 1983). Dispersal across inhospitable patches may be inhibited or ineffective (Haynes and Cronin 2003). However, insects in patches with high abundance of predators may be induced to disperse as a result of frequent encounters with predators (Cronin et al. 2004).
The number of dispersing individuals declines with distance from the source population, with the frequency distribution of dispersal distances often described by a negative exponential or inverse power law (Fig. 5.5). However, some species show a higher proportion of long-distance dispersers than would be expected from a simple diffusion model, suggesting heterogeneity in dispersal type (Cronin et al. 2000). A general functional model of dispersal (D) can be described by the following equation:
where c and a are shape and distance parameters, respectively, and r(1/c) is the gamma function (J. Clark et al. 1998, Nathan et al. 2003). The negative exponential (c = 1) and Gaussian (c = 2) are special cases of this formula. Similarly, effec-
u rn CC
19 25 14
1-24 25-49 50-74 75-99 100-149 150-199 200-400 >400
Distance moved (m)
Range of dispersal distances from a population source for the weevil, Rhyssomatus lineaticollis, in Iowa, United States. From St. Pierre and Hendrix (2003) with permission from the Royal Entomological Society. Please see extended permission list pg 570.
Simulated population heterozygosity (H) over time in three habitat patches. Extinction is indicated by short vertical bars on the right end of horizontal lines; recolonization is indicated by arrows. From Hedrick and Gilpin (1998).
tive dispersal declines as the probability of encountering inhospitable patches increases.
The contribution of dispersing individuals to genetic heterogeneity in a population depends on a number of factors. The genetic heterogeneity of the source population determines the gene pool from which dispersants come. Dispersing individuals represent a proportion of the total gene pool for the population. More heterogeneous demes have greater contributions to the genetic heterogeneity of target or founded demes than do less heterogeneous demes (Fig. 5.6) (Hedrick and Gilpin 1997). The number or proportion of individuals that disperse affects their genetic heterogeneity. If certain genotypes are more likely to disperse, then the frequencies of these genotypes in the source population may decline, unless balanced by immigration. Distances between demes influence the degree of gene exchange through dispersal. Local demes will be influenced more by the genotypes of dispersants from neighboring demes than by more distant demes. Gene flow may be precluded for sufficiently fragmented populations. This is an increasing concern for demes restricted to isolated refugia. Populations consisting of small, isolated demes may be incapable of sufficient interaction to sustain viability.
III. LIFE HISTORY CHARACTERISTICS
Life history adaptation to environmental conditions usually involves complementary selection of natality and dispersal strategies. General life history strategies appear to be related to habitat stability.
MacArthur and Wilson (1967) distinguished two life history strategies related to habitat stability and importance of colonization and rapid population establishment. The r-strategy generally characterizes "weedy" species adapted to colonize and dominate new or ephemeral habitats quickly (Janzen 1977). These species are opportunists that quickly colonize new resources but are poor competitors and cannot persist when competition increases in stable habitats. By contrast, the K strategy is characterized by low rates of natality and dispersal but high investment of resources in storage and individual offspring to ensure their survival. These species are adapted to persist under stable conditions, where competition is intense, but reproduce and disperse too slowly to be good colonizers. Specific characteristics of the two strategies (Table 5.1) have been the subject of debate (Boyce 1984). For example, small size with smaller resource requirements might be favored by K selection (Boyce 1984), although larger organisms usually show more efficient resource use. Nevertheless, this model has been useful for understanding selection of life history attributes (Boyce 1984).
Insects generally are considered to exemplify the r-strategy because of their relatively short life spans, Type 3 survivorship, and rapid reproductive and dispersal rates. However, among insects, a wide range of r-K strategies have been identified. For example, low-order streams (characterized by narrow constrained channels and steep topographic gradients) experience wider variation in water flow and substrate movement, compared to higher-order streams (characterized by broader floodplains and shallower topographic gradients). Insects associated with lower-order streams tend to be more r-selected than are insects associated with slower water and greater accumulation of detritus (Reice 1985). Similarly, ephemeral terrestrial habitats are dominated by species with higher natality and dispersal rates (e.g., aphids and Collembola), compared to more stable habitats, dominated by Lepidoptera, Coleoptera, and oribatid mites (Schowalter 1985, Seastedt 1984). Many species associated with relatively stable habitats are poor
Life history characteristics of species exemplifying the r- and K-strategies
Attribute Ecological Strategy
Life history characteristics of species exemplifying the r- and K-strategies
Attribute Ecological Strategy
dispersers and are often flightless, even wingless, indicating weak selection for escape and colonization of new habitats (St. Pierre and Hendrix 2003). Such species may be at risk if environmental change increases the frequency of disturbance.
Grime (1977) modified the r-K model by distinguishing three primary life history strategies in plants, based on their relative tolerances of disturbance, competition, and stress. Clearly, these three factors are interrelated because disturbance can affect competition and stress can increase vulnerability to disturbance. Nevertheless, this model has proved useful for distinguishing the following strategies, characterizing harsh versus frequently disturbed and infrequently disturbed habitats.
The ruderal strategy generally corresponds to the r-selected strategy and characterizes unstable habitats; the competitive strategy generally corresponds to the K strategy and characterizes relatively stable habitats. The stress-adapted strategy characterizes species adapted to persist in harsh environments. These species usually are adapted to conserve resources and minimize exposure to extreme conditions. Insects showing the stress-adapted strategy include those adapted to tolerate freezing in arctic ecosystems or minimize water loss in desert ecosystems (see Chapter 2).
Fielding and Brusven (1995) explored correlations between plant community correspondence to Grime's (1977) strategies and the species traits (abundance, habitat breadth, phenology, and diet breadth) of the associated grasshopper assemblages. They found that the three grasshopper species associated with the ruderal plant community had significantly wider habitat and diet breadths (gen-eralists) and had higher densities than did grasshoppers associated with the competitive or stress-adapted plant communities (Fig. 5.7). Grasshopper assemblages also could be distinguished between the competitive and stress-adapted plant communities, but these differences were only marginally significant. Nevertheless, their study suggested that insects can be classified according to Grime's (1977) model, based on their life history adaptations to disturbance, competition, or stress.
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