A growing number of species are becoming vulnerable to extinction as populations shrink and become more isolated in disappearing habitats (Boecklen 1991, M. Wilson et al. 1997) or are displaced by exotic competitors. Examples include a number of butterfly species, the American burying beetle, Necrophorus ameri-canus, and a number of aquatic and cave-dwelling species (e.g., Boecklen 1991,
cl-can open gravel paved pline
Habitat width u c o
i i i i i cl-can pline gravel open paved
Mean (+ standard error) density of fire ant, Solenopsis invicta, mounds along roads under various canopy and substrate conditions in order of increasing corridor width (a) and disturbance frequency (b) at the Savanna River Site in South Carolina. cl-can = closed canopy, pline = powerline cut, and open, gravel, and paved = open canopy roads with dirt, gravel, or paved surfaces, respectively. N = 10 for each treatment. Bars with different letters are significantly different at p < 0.05. From Stiles and Jones (1998) with permission from Kluwer Academic Publishers.
Hanski and Simberloff 1997, C. Thomas and Hanski 1997, M. Wilson et al. 1997). All of these species are vulnerable to extinction because of their rarity and the increasing fragmentation and isolation of their habitats. Maintenance or recovery of endangered species requires attention to the size and distribution of nature reserves for remnant populations.
The theory of island biogeography was a dominating paradigm in conservation biology during the 1970s and 1980s and continues to shape perspectives of nature reserves as habitat islands (e.g., Diamond and May 1981, Harris 1984). One of the important early applications of this theory was to the development of rules for refuge design. The most widely debated of these rules was the SLOSS (single large or several small) rule, based on the likelihood of colonization and persistence of large versus small islands or patches. Diamond and May (1981) noted that the value of various options for species viability depended on the habitat area required by a species and its dispersal capability. Small organisms such as insects could persist in smaller reserves than could larger organisms such as vertebrates. In fact, insects often can persist undetected on rare hosts in relatively small, isolated patches, as was the case for Fender's blue butterfly, Icaricia icarioides fenderi. This species was last seen in 1936 before being rediscovered in 1989 in small remnant patches of its host lupine, Lupinus sulphureus kincaidii, in western Oregon (M. Wilson et al. 1997). Nevertheless, species in disappearing habitats remain vulnerable to extinction, as in the case of the Rocky Mountain grasshopper (Lockwood and DeBrey 1990).
Island biogeography theory has largely been supplanted by models of metapopulation dynamics. Metapopulation models are based on the landscape pattern of demes and gene flow among demes in a nonequilibrium landscape (Hanski and Simberloff 1997, Harrison and Taylor 1997). Small demes are most vulnerable to local extinction as a result of disturbances, but their presence may be critical to recolonization of vacant patches or gene exchange with nearby demes. Dispersal among patches is critical to maintaining declining populations and preventing or delaying local extinction. Clearly, population recovery for such species depends on restoration or replacement of habitats.
Principles of metapopulation dynamics may be particularly important for conservation and restoration of populations of entomophagous predators and parasites in landscapes managed for ecosystem commodities (e.g., forestry and agricultural products). Predators and parasitoids are recognized as important natural agents of crop pest regulation but as a group appear to be particularly vulnerable to habitat fragmentation (Kruess and Tscharntke 1994, Schowalter 1995) and pesticide application (Sherratt and Jepson 1993). Hassell et al. (1991) and Sherratt and Jepson (1993) suggested that predator and parasite persistence in agroecosystems depends on the metapopulation dynamics of their prey, as well as on the frequency and distribution of pesticide use, and that connectivity between patches characterized by locally unstable predator-prey interactions could allow their mutual persistence. M. Thomas et al. (1992) found that creation of islands of grassland habitats in agricultural landscapes increased the abundances of several groups of entomophagous arthropods.
Corridors connecting otherwise-isolated habitat patches have been identified as critical needs for conservation biology. Just as roads and other disturbed corridors facilitate movement of invasive species among disturbed habitats (DeMers 1993, Spencer and Port 1988, Spencer et al. 1988), corridors of undisturbed habitat connecting undisturbed patches can facilitate movement of species characterizing these habitats.
Varkonyi et al. (2003) used mark-recapture techniques to track movement of two species of noctuid moths, Xestia speciosa, a habitat generalist that can be found in natural and managed spruce forests and also in pine-dominated forest throughout Finland, and X. fennica, a species more restricted to natural spruce forests in northern Finland. They found that both species preferred to move along spruce forest corridors and avoid entering the matrix of clearcuts and regenerating forest. Movement of X. speciosa generally covered longer distances, whereas movement of X. fennica was characterized by shorter distances confined within corridors. However, X. fennica was capable of longer-distance dispersal across the matrix.
Haddad (1999, 2000) demonstrated that corridors between patches of open-habitat, embedded in pine, Pinus spp., forest significantly increased interpatch dispersal of buckeye, Junonia coenia, and variegated fritillary, Euptoieta claudia, butterflies (Fig. 7.10). Haddad and Baum (1999) found that three butterfly species (J. coenia, E. claudia, and cloudless sulphur, Phoebis sennae) characterizing open habitat reached higher population densities in patches connected by corridors than in isolated patches; a fourth species, the spicebush swallowtail, Papilio troilus, did not show any preference for open versus pine habitat and did not differ in density between connected or isolated patches. Collinge (2000) also reported variable effects of corridors on grassland insect movement. Corridors slightly increased the probability of colonization by less vagile species but did not affect recolonization by rare species. One of three focus species significantly
Proportion of marked butterflies, Junonia coenia and Euptoieta claudia, that moved increasing distances to adjacent patches connected by a corridor or unconnected. From Haddad (1999) with permission from the Ecological Society of America. Please see extended permission list pg 570.
preferred corridors, whereas the other two moved independently of corridors. These studies indicated that corridors may facilitate movement of organisms among patches, but their effect depends on species characteristics, landscape context, patch size, corridor length, and environmental variation.
Riparian corridors have been a focus of many conservation efforts for some vertebrates, but few studies have addressed arthropod responses to riparian corridors. Riparian vegetation has been shown to protect stream or wetland conditions and maintain aquatic species (Batzer et al. 2000a, Davies and Nelson 1994, Haggerty et al. 2004, M. Stone and Wallace 1998). Davies and Nelson (1994) reported 34-62% declines in mayfly and stonefly densities in streams with riparian buffers <30-m wide, compared to streams with buffers >50-m wide (see earlier in this chapter). Cartron et al. (2003) found that carabid beetle abundance and species richness were significantly higher in riparian forests subject to periodic flooding, compared to nonflooded sites, indicating the importance of flooding for some riparian taxa. The distinct habitat characterizing riparian corridors may not be suitable for conserving upland species in areas with steep elevational gradients.
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