Landscape and Stream Continuum Patterns

Within terrestrial biomes, gradients in climate and geographic factors interacting with the patch scale of disturbances across landscapes produce a shifting mosaic of habitat types that affects the distribution of populations. Local extinction of demes must be balanced by colonization of new habitats as they appear for species to survive. However, colonists can arrive in terrestrial patches from various directions and distances. By contrast, distribution of aquatic species is more constrained by the linear (single-dimension) pattern of water flow. Colonists are more likely to come from upstream (if movement is governed by water flow) or downstream (flying adults), with terrestrial patches between stream systems being relatively inhospitable. Population distributions often are relatively distinct among drainage basins (watersheds), depending on the ability of dispersants to colonize new headwaters or tributaries. Hence, terrestrial and aquatic ecologists have developed different approaches to studying spatial dynamics of populations, especially during the 1980s when landscape ecology became a paradigm for terrestrial ecologists (M. Turner 1989) and stream continuum became a paradigm for stream ecologists (Vannote et al. 1980).

Distribution of populations in terrestrial landscapes, stream continua, and oceanic islands is governed to a large extent by probabilities of extinction versus colonization in particular sites (Fig. 7.2; see Chapter 5). The dispersal ability of a species; the suitability of the patch, island, or stream habitat; and its size and distance from the population source determine the probability of colonization by a dispersing individual (see Fig. 5.5). Island or patch size and distance from population sources influence the likelihood that an insect able to travel a given distance in a given direction will contact that island or patch.

Patch suitability reflects the abundance of resources available to colonizing insects. Clearly, suitable resources must be present for colonizing individuals to survive and reproduce. However, preferences by colonizing individuals also may be important. Hanski and Singer (2001) examined the effect of two host plants, Plantago spp. and Veronica spp., that varied in their relative abundances among patches, on colonization by the Glanville fritillary butterfly, Melitaea cinxia. Colonization success was strongly influenced by the correspondence between relative composition of the two host plants and the relative host use by caterpillars in the source patches (i.e., colonizing butterflies strongly preferred to oviposit on the host plant they had used during larval development). The average annual colonization rate was 5% for patches dominated by the host genus less common

Probability of species presence in an ecosystem (R), as a function of probabilities of local extinction (E) and colonization (C) over time, for specified values of v = probability of colonization over time and l = probability of extinction over time. From Naeem (1998) with permission from Blackwell Science, Inc. Please see extended permission list pg 570.

Probability of species presence in an ecosystem (R), as a function of probabilities of local extinction (E) and colonization (C) over time, for specified values of v = probability of colonization over time and l = probability of extinction over time. From Naeem (1998) with permission from Blackwell Science, Inc. Please see extended permission list pg 570.

across the connecting landscape and 15-20% for patches dominated by the host genus more common across the connecting landscape.

Individual capacity for sustained travel and for detection of cues that facilitate orientation determine colonization ability. Species that fly can travel long distances and traverse obstacles in an aquatic or terrestrial matrix better than can flightless species. Many small insects, including flightless species, catch air currents and are carried long distances at essentially no energetic cost to the insect. J. Edwards and Sugg (1990) reported that a variety of insects could be collected on montane glaciers far from the nearest potential population sources. Torres (1988) reported deposition, by hurricanes, of insect species from as far away as Africa on Caribbean islands.

However, many small, flightless species have limited capacity to disperse. Any factor that increases the time to reach a suitable habitat increases the risk of mortality from predation, extreme temperatures, desiccation, or other factors. Distances of a few meters, especially across exposed soil surfaces, can effectively preclude dispersal by many litter species sensitive to heat and desiccation or vulnerable to predation (Haynes and Cronin 2003). D. Fonseca and Hart (2001) reported that larval black flies, Simulium vittatum, were least able to colonize preferred high-velocity habitats in streams because of constraints on their ability to control settlement. Some aquatic species (e.g., Ephemeroptera) have limited life spans as adults to disperse among stream systems. Courtney (1985,1986) reported that short adult life span was a major factor influencing the common selection of less-suitable larval food plants for oviposition (see Chapter 3). Clearly, the distance between an island or habitat patch and the source population is inversely related to the proportion of dispersing individuals able to reach it (see Fig. 5.5).

Island or patch size and complexity also influence the probability of successful colonization. The larger the patch (or the shorter its distance from the source population), the greater the proportion of the horizon it represents, and the more likely a dispersing insect will be able to contact it. Patch occupancy rate increases with patch size (Cronin 2003). Similarly, the distribution of microsites within landscape or watershed patches affects the ability of dispersing insects to perceive and reach suitable habitats. Basset (1996) reported that the presence of arboreal insects is influenced more strongly by local factors in complex habitats, such as tropical forests, and more strongly by regional factors in less complex habitats, such as temperate forests.

The composition of surrounding patches in a landscape matrix is as important as patch size and isolation in influencing population movement and distribution. Haynes and Cronin (2003) manipulated the composition of the matrix (mudflat, native, nonhost grasses and exotic brome, Bromus inermis) surrounding small patches of prairie cordgrass, Spartina pectinata, that were identical in size, isolation, and host plant quality. Planthoppers, Prokelisia crocea, were marked and released into each host patch. Planthopper emigration rate was 1.3 times higher for patches surrounded by the two nonhost grasses compared to patches surrounded by mudflat (Fig. 7.3). Immigration rate was 5.4 times higher into patches surrounded by brome compared to patches surrounded by mudflat and intermediate in patches surrounded by native nonhost grass. Patch occupancy and density increased with the proportion of the matrix composed of mudflat, probably reflecting the relative inhospitability of the mudflat compared to nonhost grasses.

The increasing rate of dispersal during rapid population growth increases the number of insects moving across the landscape and the probability that some will travel sufficient distance in a given direction to discover suitable patches. Therefore, population contribution to patch colonization and genetic exchange with distant populations is maximized during population growth.

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