Density Independent Factors

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Insect populations are highly sensitive to changes in abiotic conditions, such as temperature, water availability, etc., which affect insect growth and survival (see Chapter 2). Changes in population size of some insects have been related directly to changes in climate or to disturbances (e.g., Greenbank 1963, Kozar 1991, Porter and Redak 1996, Reice 1985). In some cases, climate fluctuation or disturbance affects resource values for insects. For example, loss of riparian habitat as a result of agricultural practices in western North America may have led to extinction of the historically important Rocky Mountain grasshopper, Melanoplus spretus (Lockwood and DeBrey 1990).

Many environmental changes occur relatively slowly and cause gradual changes in insect populations as a result of subtle shifts in genetic structure and individual fitness. Other environmental changes occur more abruptly and may trigger rapid change in population size because of sudden changes in natality, mortality, or dispersal.

Disturbances are particularly important triggers for inducing population change because of their acute disruption of population structure and of resource, substrate, and other ecosystem conditions. The disruption of population structure can alter community structure and cause changes in physical, chemical, and biological conditions of the ecosystem. Disturbances can promote or truncate population growth, depending on species tolerances to particular disturbance or postdisturbance conditions.

Some species are more tolerant of particular disturbances, based on adaptation to regular recurrence. For example, plants in fire-prone ecosystems tend to have attributes that protect meristematic tissues, whereas those in frequently flooded ecosystems can tolerate root anaerobiosis. Generally, insects do not have specific adaptations to survive disturbance, given their short generation times relative to disturbance intervals, and unprotected populations may be greatly reduced. Species that do show some disturbance-adapted traits, such as orientation to smoke plumes or avoidance of litter accumulations in fire-prone ecosystems (W. Evans 1966, K. Miller and Wagner 1984), generally have longer (2-5-year) generation times that would increase the frequency of generations experiencing a disturbance. Most species are affected by postdisturbance conditions. Disturbances affect insect populations both directly and indirectly.

Disturbances create lethal conditions for many insects. For example, fire can burn exposed insects (Porter and Redak 1996, P. Shaw et al. 1987) or raise temperatures to lethal levels in unburned microsites. Tumbling cobbles in flooding streams can crush benthic insects (Reice 1985). Flooding of terrestrial habitats can create anaerobic soil conditions. Drought can raise air and soil temperatures and cause desiccation (Mattson and Haack 1987). Populations of many species can suffer severe mortality as a result of these factors, and rare species may be eliminated (P. Shaw et al. 1987, Schowalter 1985). Willig and Camilo (1991) reported the virtual disappearance of two species of walkingsticks, Lamponius portoricensis and Agamemnon iphimedeia, from tabonuco, Dacryodes excelsa, forests in Puerto Rico following Hurricane Hugo. Drought can reduce water levels in aquatic ecosystems, reducing or eliminating habitat for some aquatic insects. In contrast, storms may redistribute insects picked up by high winds. Torres (1988) reviewed cases of large numbers of insects being transported into new areas by hurricane winds, including swarms of African desert locusts, Schis-tocerca gregaria, deposited on Caribbean islands.

Mortality depends on disturbance intensity and scale and species adaptation. K. Miller and Wagner (1984) reported that the pandora moth preferentially pupates on soil with sparse litter cover, under open canopy, where it is more likely to survive frequent understory fires. This habit would not protect pupae during more severe fires. Small-scale disturbances affect a smaller proportion of the population than do larger-scale disturbances. Large-scale disturbances, such as volcanic eruptions or hurricanes, could drastically reduce populations over much of the species range, making such populations vulnerable to extinction. The potential for disturbances to eliminate small populations or critical local demes of fragmented metapopulations has become a serious obstacle to restoration of endangered (or other) species (P. Foley 1997).

Disturbances indirectly affect insect populations by altering the postdistur-bance environment. Disturbance affects abundance or physiological condition of hosts and abundances or activity of other associated organisms (Mattson and Haack 1987, T. Paine and Baker 1993). Selective mortality to disturbance-intolerant plant species reduces the availability of a resource for associated herbivores. Similarly, long disturbance-free intervals can lead to eventual replacement of ruderal plant species and their associated insects. Changes in canopy cover or plant density alter vertical and horizontal gradients in light, temperature, and moisture that influence habitat suitability for insect species; alter plant conditions, including nitrogen concentrations; and can alter vapor diffusion patterns that influence chemoorientation by insects (Cardé 1996, Kolb et al. 1998, Mattson and Haack 1987, J. Stone et al. 1999).

Disturbances injure or stress surviving hosts or change plant species density or apparency. The grasshopper, Melanoplus differentialis, prefers wilted foliage of sunflower to turgid foliage (A. Lewis 1979). Fire or storms can wound surviving plants and increase their susceptibility to herbivorous insects. Lightning-struck (Fig. 6.4) or windthrown trees are particular targets for many bark beetles

| Lightning strike or other injury impairs tree defense systems. Injured, diseased, or stressed trees usually are targets of bark beetle colonization.

and provide refuges for these insects at low population levels (Flamm et al. 1993, T. Paine and Baker 1993). Drought stress can cause audible cell-wall cavitation that may attract insects adapted to exploit water-stressed hosts (Mattson and Haack 1987). Stressed plants may alter their production of particular amino acids or suppress production of defensive chemicals to meet more immediate metabolic needs, thereby affecting their suitability for particular herbivores (Haglund 1980, Lorio 1993, R. Waring and Pitman 1983). If drought or other disturbances stress large numbers of plants surrounding these refuges, small populations can reach epidemic sizes quickly (Mattson and Haack 1987). Plant crowding, as a result of planting or long disturbance-free intervals, causes competitive stress. High densities or apparencies of particular plant species facilitate host colonization and population growth, frequently triggering outbreaks of herbivorous species (Mattson and Haack 1987).

Changes in abundances of competitors, predators, and pathogens also affect postdisturbance insect populations. For example, phytopathogenic fungi establishing in, and spreading from, woody debris following fire, windthrow, or harvest can stress infected survivors and increase their susceptibility to bark beetles and other wood-boring insects (T. Paine and Baker 1993). Drought or solar exposure resulting from disturbance can reduce the abundance or virulence of ento-mopathogenic fungi, bacteria, or viruses (Mattson and Haack 1987, Roland and Kaupp 1995). Disturbance or fragmentation reduce the abundances and activity of some predators and parasites (Kruess and Tscharntke 1994, Roland and Taylor 1997) and may induce or support outbreaks of defoliators (Roland 1993). Alternatively, fragmentation can interrupt spread of some insect populations by creating inhospitable barriers (Schowalter et al. 1981b).

Population responses to direct or indirect effects vary, depending on scale of disturbance (see Chapter 7). Few natural experiments have addressed the effects of scale. Clearly, a larger-scale event should affect environmental conditions and populations within the disturbed area more than would a smaller-scale event. Shure and Phillips (1991) compared arthropod abundances in clearcuts of different sizes in the southeastern United States (Fig. 6.5). They suggested that the greater differences in arthropod densities in larger clearcuts reflected the steepness of environmental gradients from the clearcut into the surrounding forest. The surrounding forest has a greater effect on environmental conditions within a small canopy opening than within a larger opening.

The capacity for insect populations to respond quickly to abrupt changes in environmental conditions (disturbances) indicates their capacity to respond to more gradual environmental changes. Insect outbreaks have become particularly frequent and severe in landscapes that have been significantly altered by human activity (K. Hadley and Veblen 1993, Huettl and Mueller-Dombois 1993, Wickman 1992). Anthropogenic suppression of fire; channelization and clearing of riparian areas; and conversion of natural, diverse vegetation to rapidly growing, commercially valuable crop species on a regional scale have resulted in more severe disturbances and dense monocultures of susceptible species that support widespread outbreaks of adapted insects (e.g., Schowalter and Lowman 1999).

G. 6.5

10 C

Densities of arthropod groups during the first growing season in uncut forest (C) and clearcut patches ranging in size from 0.016 ha to 10 ha. For groups showing significant differences between patch sizes, vertical bars indicate the least significant difference (P < 0.05). HOM, Homoptera; HEM, Hemiptera; COL, Coleoptera; ORTH, Orthoptera; DIPT, Diptera; and MILL, millipedes. From Shure and Phillips (1991) with permission from Springer-Verlag. Please see extended permission list pg 570.

Insect populations also are likely to respond to changing global temperature, precipitation patterns, atmospheric and water pollution, and atmospheric concentrations of CO2 and other trace gases (e.g., Alstad et al. 1982, Franklin et al. 1992, Heliovaara 1986, Heliovaara and Vaisanen 1993, Hughes and Bazzaz 1997, Lincoln et al. 1993, Marks and Lincoln 1996, D. Williams and Liebhold 2002). Grasshopper populations are favored by warm, dry conditions (Capinera 1987), predicted by climate change models to increase in many regions. D. Williams and Liebhold (2002) projected increased outbreak area and shift northward for southern pine beetle, Dendroctonus frontalis, but reduced outbreak area and shift to higher elevations for the mountain pine beetle, D. ponderosae, in North America as a result of increasing temperature. Interaction among multiple factors changing simultaneously may affect insects differently than predicted from responses to individual factors (e.g., Franklin et al. 1992, Marks and Lincoln 1996).

The similarity in insect population responses to natural versus anthropogenic changes in the environment depends on the degree to which anthropogenic changes create conditions similar to those created by natural changes. For example, natural disturbances usually remove less biomass from a site than do harvest or livestock grazing. This difference likely affects insects that depend on postdisturbance biomass, such as large woody debris, either as a food resource or refuge from exposure to altered temperature and moisture (Seastedt and Cross-ley 1981a).Anthropogenic disturbances leave straighter and more distinct boundaries between disturbed and undisturbed patches (because of ownership or management boundaries), affecting the character of edges and the steepness of environmental gradients into undisturbed patches (J. Chen et al. 1995, Roland and Kaupp 1995). Similarly, the scale, frequency, and intensity of prescribed fires may differ from natural fire regimes. In northern Australia, natural ignition would come from lightning during storm events at the onset of monsoon rains, whereas prescribed fires often are set during drier periods to maximize fuel reduction (Braithwaite and Estbergs 1985). Consequently, prescribed fires burn hotter, are more homogeneous in their severity, and cover larger areas than do lower-intensity, more patchy fires burning during cooler, moister periods.

Few studies have evaluated the responses of insect populations to changes in multiple factors. For example, habitat fragmentation, climate change, acid precipitation, and introduction of exotic species may influence insect populations interactively in many areas. For example, stepwise multiple regression indicated that persistence of native ant species in coastal scrub habitats in southern California was best predicted by the abundance of invasive Argentine ants, Linep-ithema humile; size of habitat fragments; and time since fragment isolation (A. Suarez et al. 1998).

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