Other Abiotic Factors

Many aquatic insects are sensitive to water level and flow rate (Ward 1992).These factors can fluctuate dramatically, especially in seasonal habitats, such as desert playas, intermittent streams, wetlands, and perched pools in treeholes and bromeliads (phytotelmata). Water level affects both temperature and water quality, temperature because smaller volumes absorb or lose heat more quickly than do larger volumes, water quality because various solutes become more concentrated as water evaporates. Insects, and other aquatic arthropods, show life history adaptations to seasonal patterns of water availability or quality, often undergoing physiological diapause as water resources disappear (Batzer and Wissinger 1996, Ward 1992). Although most mosquitoes oviposit in surface water, floodwater mosquitoes, Aedes spp. and Psorophora spp., oviposit in soil at the high water line. Their eggs are resistant to desiccation and can remain dormant for several years. Egg hatch is stimulated by flooding, and the number of generations at a site depends on the frequency of flooding (Wiggins et al. 1980). Flow rate affects temperature and oxygenation, with cooler temperature and higher oxygen content at higher flow rates, but high flow rates can physically dislodge and remove

Relationship Between Species

Relationship between sibling species composition of black flies (Simulium truncatum, solid line, S. verecundum AA, dashed line, and S. venustum CC2, dotted line) and distance from lake outlets on the Avalon Peninsula, Newfoundland, in early June. Least squares regression equations were significant at P < 0.01; adjusted R2 values were 92%, 85%, and 68% for the three species, respectively. From Adler and McCreadie (1997) with permission from the Entomological Society of America.

Relationship between sibling species composition of black flies (Simulium truncatum, solid line, S. verecundum AA, dashed line, and S. venustum CC2, dotted line) and distance from lake outlets on the Avalon Peninsula, Newfoundland, in early June. Least squares regression equations were significant at P < 0.01; adjusted R2 values were 92%, 85%, and 68% for the three species, respectively. From Adler and McCreadie (1997) with permission from the Entomological Society of America.

exposed insects. McCreadie and Colbo (1993) and Adler and McCreadie (1997) reported that sibling species of black flies, Simulium, select different stream microhabitats on the basis of their adaptations to water velocity (Fig. 2.14).

Light is an important factor affecting development, behavior, and distribution of many insects. Some aquatic insects are negatively phototactic during most of their lives, but they may move toward light under conditions of oxygen depletion (Ward 1992). Algal feeders are more likely to occur in illuminated portions of streams. Moonlight affects drift rates for species that disperse in stream currents and is a synchronizing agent for emergence of a number of aquatic species, especially nocturnal feeders, with different species emerging during different lunar phases (Ward 1992). A variety of insects are attracted to lights at night, an attribute that facilitates collection and measurement of diversity (see Chapter 9), and normal dispersal or foraging activities may be disrupted by artificial lights.

Insects are particularly sensitive to ultraviolet radiation. Kelly et al. (2003) demonstrated, using experimental filters, that aquatic insects in the Little Qualicum River on Vancouver Island showed differential sensitivities to ultraviolet A (UVA) and UVB exposure. Caddisflies (Trichoptera), especially Dicosmoecus spp. (Limnephillidae), were most sensitive to UV exposure. Final abundances under photosynthetically active radiation (PAR) alone were 15 times higher than under PAR + UVA and 40 times higher than under PAR + UVA + UVB. Stoneflies (Plecoptera) were 51% more abundant under PAR than under UV exposure. In contrast, chironomids (Diptera) were more abundant in the UV treatments.

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