Disturbances

Within biomes, characteristic abiotic and biotic factors interact to influence the pattern of disturbances, relatively discrete events that alter ecosystem conditions, and create a finer-scale landscape mosaic of patches with different disturbance and recovery histories (Harding et al. 1998, Schowalter et al. 2003, Willig and Walker 1999). Disturbances, such as fire, storms, drought, flooding, anthropogenic conversion (Fig. 2.8), alter vertical and horizontal gradients in temperature, moisture, and air or water chemistry (T. Lewis 1998, P. White and Pickett 1985), significantly altering the abiotic and biotic conditions to which organisms are exposed (Agee 1993, Schowalter 1985, Schowalter and Lowman 1999).

Disturbances can be characterized by several criteria that determine their effect on various organisms (see Walker and Willig 1999, P. White and Pickett 1985). Disturbance type, such as fire, drought, flood, or storm, determines which ecosystem components will be most affected. Above-ground versus below-ground species or terrestrial versus aquatic species are affected differently by fire versus flood. Intensity is the physical force of the event, whereas severity represents the effect on the ecosystem. A fire or storm of given intensity, based on temperature or wind speed, will affect organisms differently in a grassland versus a forest. Scale is the area affected by the disturbance and determines the rate at which organisms recolonize the interior portions of the disturbed area. Frequency is the mean number of events per time period; reliability is measured as the inverse of variability in the time between successive events (recurrence interval).

Insects show a variety of adaptations to particular disturbance type. Some species respond positively, and others respond negatively to particular disturbances, based on adaptive characteristics (E. Evans 1988, Paquin and Coderre 1997, Schowalter et al. 1999, Wikars and Schimmel 2001). Responses differ between disturbance types. For example, Paquin and Coderre (1997) compared forest floor arthropod responses to forest clearing versus fire. Decomposers were less abundant, whereas predators were more abundant in cleared plots, relative to undisturbed plots. Arthropod abundance was reduced 95.5% following experimental fire, but some organisms survived as a result of occurrence in deeper soil levels or because of the patchy effect of fire. Abundances of some species differed between cleared and burned plots.

Following disturbance-induced change, populations and communities tend to become more similar to their starting point over time through a process known as ecological succession (see Chapter 10). Insect responses to anthropogenic dis-

Ecological Disturbance Images

| Natural disturbances include A: fire, especially in grasslands and savannas (north central United States), B: storms (north central United States), and C: floods (northwestern United States). Anthropogenic disturbances include the following: D: arid land conversion to agriculture use (center-pivot irrigation; western United States), E: forest harvest fragmentation (northwestern United States), and F: overgrazing and desertification (right of fence, compared to natural grassland on left; southwestern United States). These disturbances affect ecosystem components differentially. Adapted species survive, whereas nonadapted species may disappear. Overgrazing and desertification photo (F) courtesy of D. C. Lightfoot.

turbances reflect their adaptations to natural disturbances (e.g., forest harvest often elicits responses similar to other canopy opening disturbances); vegetation conversion to crop production elicits insect responses to changes in host density and apparency (see later in this chapter); and river impoundment elicits responses similar to landslides, which also alter drainage pattern. However, some anthropogenic disturbances are unique. Aquatic organisms historically had min-

(Continued)

imal exposure to the variety of synthetic toxins recently introduced into aquatic systems. Fires and other natural disturbances do not generate large numbers of stumps with exposed surfaces and in-ground root systems. Paving previously vegetated surfaces has created the most extreme changes in habitat conditions for organisms sensitive to high temperature and desiccation.

The effects of such changes may be difficult to predict, based on adaptations to natural disturbances, and may persist for long periods because local mechanisms are lacking for reversal of extreme alteration of vegetation, substrate, or water conditions. For example, Harding et al. (1998) reported that responses of aquatic invertebrate communities to restoration treatments reflected differences in community structure among stream segments with different histories of anthropogenic disturbances. Similarly, Schowalter et al. (2003) found that litter arthropod responses to variable density thinning of conifer forests for restoration purposes reflected different initial community structures, resulting from previous thinning as much as 30 years earlier.

Disturbances vary in intensity and severity. A low-intensity ground fire affects primarily surface-dwelling organisms, many of which may be adapted to this level of disturbance, whereas a high-intensity crown fire can destroy a large proportion of the community. Plant species capable of withstanding low-to-moderate wind speeds may topple at high wind speeds. Hurricane winds damage large areas of forest and can virtually eliminate many arthropods (Koptur et al. 2002,Willig and Camilo 1991).

Disturbances range in scale from local to global. Local disturbances affect the patchwork of communities that compose an ecosystem; global disturbances such as El Niño/La Niña events have far-reaching effects on climate fluctuation. Anthropogenic disturbances range from local conversion of ecosystems, such as altered streamflow pattern (e.g., sedimentation or stream scour resulting from coffer dam construction for logging), to global pollution and effects of fossil fuel combustion on climate. The degree of ecosystem fragmentation resulting from land-use changes is unprecedented in nature and seriously affects population distribution by reducing habitat area, isolating demes, and interfering with dispersal, potentially threatening species incapable of surviving in increasingly inhospitable landscapes (Samways et al. 1996, Shure and Phillips 1991, A. Suarez et al. 1998, Summerville and Crist 2001).

Frequency and reliability of recurrence, with respect to generation times of characteristic organisms, of a particular disturbance type probably are the most important factors driving directional selection for adaptation to disturbance (e.g., traits that confer tolerance [resistance] to fire or flooding). Effects of disturbances may be most pronounced in ecosystems, such as mesic forests and lakes, which have the greatest capacity to modify abiotic conditions and, therefore, have the lowest exposure and species tolerances to sudden or extreme departures from nominal conditions.

Individual insects have specific tolerance ranges to abiotic conditions that dictate their ability to survive local conditions but may be exposed during some periods to lethal extremes of temperature, water availability, or other factors. Variable ecosystem conditions usually select for wider tolerance ranges than do more stable conditions. Although abiotic conditions can affect insects directly (e.g., burning, drowning, particle blocking of spiracles), they also affect insects indirectly through changes in resource quality and availability and exposure to predation or parasitism (e.g., Alstad et al. 1982, K. Miller and Wagner 1984, Mopper et al. 2004, Shure and Wilson 1993). The degree of genetic heterogeneity affects the number of individuals that survive altered conditions. As habitat conditions change, intolerant individuals disappear, leaving a higher frequency of genes for tolerance of the new conditions in the surviving population. Adapted colonists also may arrive from other areas.

Some species are favored by altered conditions, whereas others may disappear. Sap-sucking insects become more abundant, but Lepidoptera, detritivores, and predators become less abundant, following canopy-opening disturbances in forests (Schowalter 1995, Schowalter and Ganio 2003). However, individual species within these groups may respond quite differently. Among Homoptera, some scale insects increase in numbers and others decline in numbers following canopy disturbance. Schowalter et al. (1999) found that species within each resource functional group responded differentially to manipulated change in moisture availability in a desert ecosystem (i.e., some species increased in abundance, whereas other species decreased or showed no change). Root bark beetles (e.g., Hylastes nigrinus) are attracted to chemicals, emanating from exposed stump surfaces, that advertise suitable conditions for brood development and become more abundant following forest thinning (Fig. 2.9) (Witcosky et al. 1986). Conversely, stem-feeding bark beetles (e.g., Dendroctonus spp.) are sensitive to tree spacing and become less abundant in thinned forests (Amman et al. 1988, Sartwell and Stevens 1975, Schowalter and Turchin 1993).

Reice (1985) experimentally disturbed benthic invertebrate communities in a low-order stream in the eastern United States by tumbling cobbles in patches of stream bottom 0, 1, or 2 times in a 6-week period. Most insect and other invertebrate taxa decreased in abundance with increasing disturbance. Two invertebrate taxa increased in abundance following a single disturbance, but no taxa increased in abundance with increasing disturbance. However, all populations rebounded quickly following disturbance, suggesting that these taxa were adapted to this disturbance.

Timing of disturbances, relative to developmental stage, also affects insect responses. However, Martin-R. et al. (1999) reported that experimental fires set during different developmental stages of spittlebug, Aeneolamia albofasciata, in buffelgrass, Cenchrus ciliaris, grassland in Sonora, Mexico eliminated spittlebugs for at least 4 years after burning, regardless of developmental stage at the time of burning. Because survival and reproduction of individual insects determine population size, distribution, and effects on community and ecosystem processes, the remainder of this chapter focuses on the physiological and behavioral characteristics that affect individual responses to variable abiotic conditions.

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Responses

  • helen
    How do wildfires effect biotic factors of the grasslands?
    6 years ago

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