Expanding Populations

Growing populations tend to spread geographically as density-dependent dispersal leads to colonization of nearby resources. This spread occurs in two ways. First, diffusion from the origin, as density increases, produces a gradient of decreasing density toward the fringe of the expanding population. Grilli and Gorla (1997) reported that leafhopper, Delphacodes kuscheli, density was highest within the epidemic area and declined toward the fringes of the population. The difference in density between pairs of sampling points increased as the distance between the sampling points increased. Second, long-distance dispersal leads to colonization of vacant patches and "proliferation" of the population (Hanski and Simberloff 1997). Subsequent growth and expansion of these new demes can lead to population coalescence, with local "hot spots" of superabundance that eventually may disappear as resources in these sites are depleted.

The speed at which a population expands likely affects the efficiency of density-dependent regulatory factors. Populations that expand slowly may experience immediate density-dependent negative feedback in zones of high density, whereas induction of negative feedback may be delayed in rapidly expanding populations because dispersal slows increase in density. Therefore, density-dependent factors should operate with a longer time lag in populations capable of rapid dispersal during irruptive population growth.

The speed, extent, and duration of population spread are limited by the duration of favorable conditions and the homogeneity of the patch or landscape. Populations can spread more rapidly and extensively in homogeneous patches or landscapes such as agricultural and silvicultural systems than in heterogeneous systems in which unsuitable patches limit spread (Schowalter and Turchin 1993). Insect species with annual life cycles often show incremental colonization and population expansion. Disturbances can terminate the spread of sensitive populations. Frequently disturbed systems, such as crop systems or streams subject to annual scouring, limit population spread to the intervals between recolonization and subsequent disturbance. Populations of species with relatively slow dispersal may expand only to the limits of a suitable patch during the favorable period. Spread beyond the patch depends on the suitability of neighboring patches (Liebhold and Elkinton 1989).

The direction of population expansion depends on several factors. The direction of population spread often is constrained by environmental gradients, by wind or water flow, and by unsuitable patches. Gradients in temperature, moisture, or chemical concentrations often restrict the directions in which insect populations can spread, based on tolerance ranges to these factors (Chapter 2). Even relatively homogeneous environments, such as enclosed stored grain, are subject to gradients in internal temperatures that affect spatial change in granivore populations (Flinn et al. 1992). Furthermore, direction and flow rate of wind or water have considerable influence on insect movement. Insects with limited capability to move against air or water currents move primarily downwind or downstream, whereas insects capable of movement toward attractive cues move primarily upwind or upstream. Insects that are sensitive to stream temperature, flow rate, or chemistry may be restricted to spread along linear stretches of the stream. Jepson and Thacker (1990) reported that recolonization of agricultural fields by carabid beetles dispersing from population centers was delayed by extensive use of pesticides in neighboring fields.

Schowalter et al. (1981b) examined the spread of southern pine beetle, Dendroctonus frontalis, populations in east Texas (Fig. 7.5). They described the progressive colonization of individual trees or groups of trees through time by computing centroids of colonization activity on a daily basis (Fig. 7.6). A centroid is the center of beetle mass (numbers) calculated from the weighted abundance of beetles among the x,y coordinates of colonized trees at a given time.

The distances between centroids on successive days was a measure of the rate of population movement (see Fig. 7.6). Populations moved at a rate of 0.9 m/day, primarily in the direction of the nearest group of available trees. However,

Southern Pine Beetle Texas

Spatial and temporal pattern of spread of a southern pine beetle population in east Texas during 1977. In the upper figure, cylinders are proportional in size to size of colonized trees; ellipses represent uncolonized trees within 10 m of colonized trees. In the lower figure, Julian dates of initial colonization are given for trees colonized (solid circles) after sampling began. Open circles represent uncolonized trees within 10 m of colonized trees. From Schowalter et al. (1981b) with permission from the Society of American Foresters.

Spatial and temporal pattern of spread of a southern pine beetle population in east Texas during 1977. In the upper figure, cylinders are proportional in size to size of colonized trees; ellipses represent uncolonized trees within 10 m of colonized trees. In the lower figure, Julian dates of initial colonization are given for trees colonized (solid circles) after sampling began. Open circles represent uncolonized trees within 10 m of colonized trees. From Schowalter et al. (1981b) with permission from the Society of American Foresters.

because southern pine beetle populations generally were sparse during the period of this study, indicating relatively unfavorable conditions, this rate may be near the minimum necessary to sustain population growth.

The probability that a tree would be colonized depended on its distance from currently occupied trees. Trees within 6 m of sources of dispersing beetles had a 14-17% probability of being colonized, compared to a <4% probability for trees further than 6 m from sources of dispersing beetles. Population spread in most cases ended at canopy gaps where no trees were available within 6 m. However,

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Centroids of colonization (ATK), reemergence (REM), and emergence (EMER), by Julian date, for the southern pine beetle population in Figure 7.4. From Schowalter et al. (1981b) with permission from the Society of American Foresters.

Centroids of colonization (ATK), reemergence (REM), and emergence (EMER), by Julian date, for the southern pine beetle population in Figure 7.4. From Schowalter et al. (1981b) with permission from the Society of American Foresters.

one population successfully crossed a larger gap encountered at peak abundance (see Fig. 7.5), indicating that a sufficiently large number of beetles dispersed across the gap to ensure aggregation on suitable trees and sustained population spread.

Population spread in this species may be facilitated by colonization experience and cooperation between cohorts of newly emerging beetles and beetles "reemerging" from densely colonized hosts. Many beetles reemerge after laying some eggs, especially at high colonization densities under outbreak conditions, and seek less densely colonized trees in which to lay remaining eggs. The success of host colonization by southern pine beetles depends on rapid attraction of sufficiently large numbers to overwhelm host defenses (see Chapter 3). For a given day, the centroid of colonization was, on average, twice as far from the centroid of new adults dispersing from brood trees as from the centroid of reemerging beetles (see Fig. 7.6). This pattern suggested that reemerging beetles select the next available trees and provide a focus of attraction for new adults dispersing from farther away.

Related research has reinforced the importance of host tree density for population spread of southern pine beetle and other bark beetles (Amman et al. 1988, M. Brown et al. 1987, R.G. Mitchell and Preisler 1992, Sartwell and Stevens 1975). Schowalter and Turchin (1993) demonstrated that patches of relatively dense pure pine forest are essential to growth and spread of southern pine beetle populations from experimental refuge trees (see Fig. 6.6). Experimentally established founding populations spread from initially colonized trees surrounded by dense pure pine forest but not from trees surrounded by sparse pines or pine-hardwood mixtures.

A critical aspect of population spread is the degree of continuity of hospitable resources or patches on the landscape. As described in the preceding text for the southern pine beetle, unsuitable patches can interrupt population spread unless population density or growth is sufficient to maintain high dispersal rates across inhospitable patches. Heterogeneous landscapes composed of a variety of patch types force insects to expend their acquired resources detoxifying less acceptable resources or searching for more acceptable resources. Therefore, heterogeneous landscapes should tend to limit population growth and spread, whereas more homogeneous landscapes, such as large areas devoted to plantation forestry, pasture grasses, or major crops, provide conditions more conducive to sustained population growth and spread. However, the particular composition of landscape mosaics may be as important as patch size and isolation in insect movement and population distribution (Haynes and Cronin 2003). Furthermore, herbivores and predators may respond differently to landscape structure. Herbivores were more likely to be absent from small patches than large patches, whereas predators were more likely to be absent from more isolated patches than from less isolated patches in agricultural landscapes in Germany (Zabel and Tscharntke 1998).

Corridors or stepping stones (small intermediate patches) can facilitate population spread among suitable patches across otherwise unsuitable patches. For example, populations of the western harvester ant, Pogonomyrmex occidentalis, do not expand across patches subject to frequent anthropogenic disturbance (specifically, soil disruption through agricultural activities) but are able to expand along well-drained, sheltered roadside ditches (DeMers 1993). Roads often provide a disturbed habitat with conditions suitable for dispersal of weedy vegetation and associated insects. Roadside conditions also may increase plant suitability for herbivorous insects and facilitate movement across landscapes fragmented by roads (Spencer and Port 1988, Spencer et al. 1988). However, for some insects the effect of corridors and stepping stones may depend on the composition of the surrounding matrix. For example, Baum et al. (2004) reported that experimental corridors and stepping stones significantly increased colonization of prairie cordgrass, S. pectinata, patches by planthoppers, P. crocea, in a low-resistance matrix composed of exotic, nonhost brome, B. inermis, that is conducive to planthopper dispersal but not in a high-resistance matrix composed of mudflat that interferes with planthopper dispersal, relative to control matrices without corridors or stepping stones.

Population expansion for many species depends on the extent or duration of suitable climatic conditions. Kozar (1991) reported that several insect species showed sudden range expansion northward in Europe during the 1970s, likely reflecting warming temperatures during this period. Population expansion of spruce budworm (Choristoneura fumiferana), western harvester ants, and grasshoppers during outbreaks are associated with warmer, drier periods (Capinera 1987, DeMers 1993, Greenbank 1963).

An important consequence of rapid population growth and dispersal is the colonization of marginally suitable resources or patches where populations could not persist in the absence of continuous influx.Whereas small populations of herbivores, such as locusts or bark beetles, may show considerable selectivity in acceptance of potential hosts, rapidly growing populations often eat all potential hosts in their path. Dense populations of the range caterpillar, Hemileuca oliviae, disperse away from population centers as grasses are depleted and form an expanding ring, leaving denuded grassland in their wake. Landscapes that are conducive to population growth and spread, because of widespread homogeneity of resources, facilitate colonization of surrounding patches and more isolated resources because of the large numbers of dispersing insects. Epidemic populations of southern pine beetles, generated in the homogenous pine forests of the southern Coastal Plain during the drought years of the mid-1980s, produced sufficient numbers of dispersing insects to discover and kill most otherwise-resistant pitch pines, Pinus rigida, in the southern Appalachian Mountains.

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