The abiotic environment

At all spatial scales in fluvial ecosystems, studies of the stream biota support the expectation that greater physical complexity of the environment promotes increased biological richness. That organisms are adapted to aspects of habitat, such that the traits of organisms reflect features of the environment, is a fundamental idea in ecology referred to as the habitat template concept (Southwood 1988). The key habitat needs of a species are commonly identified from the subset of environmental variables that best correlate with its distribution and abundance. Although additional factors also influence the composition and diversity of biological assemblages, including interactions among species and the taxon richness at the regional scale, the abiotic environment provides an important starting point in investigations of species distributions and abundances. This view has two important corollaries. First, environments that are structurally either simple or extreme tend to support fewer species, whereas more moderate and heterogeneous habitats support more species. Second, a high frequency of disturbance tends to diminish biological richness, although a moderate level of disturbance potentially may enhance diversity by maintaining an ever-changing spatial mosaic of conditions. These principles forecast the consequences of human disturbance: anthropogenic degradation and homogenization of habitat will lead to biodiversity decline with unpredictable consequences for ecosystem function.

Habitat is often described as where a species lives, and so is a subset of a species' niche. The latter term is broader, describing a species' place in a biological community and incorporating all of the physical and biological conditions needed for a species to maintain its population in an area (Begon et al. 2005). The niche concept incorporates species interactions, competition in particular, in distinguishing between the space that a species would occupy in the absence of competitors and the more restricted space, the realized niche, which it actually does occupy owing to the impact of other species. In this chapter we focus on key abiotic aspects of habitat that influence the distribution and abundance of the biota of fluvial ecosystems; later, in Chapter 10, the influence of species interactions will be explored.

Habitat features vary across small to large spatial scales, sometimes referred to as micro-, meso-, and macrohabitat (Vinson and Hawkins 1998), and from very short to long time scales (Figure 1.3). Individual taxa are adapted more or less narrowly to a specific range of habitat conditions, and will be more or less successful depending on the matching of their morphological, behavioral, and physiological traits to environmental conditions. Thus the abiotic environment, acting on species traits, serves as a filter (Figure 1.5) that determines at least the candidate taxa of a locale.

In fluvial ecosystems, key abiotic features of the environment are usually those related to current, substrate, temperature, and sometimes water chemistry variables such as alkalinity and dissolved oxygen. Water chemistry and dissolved oxygen are important under natural conditions only in some unusual environments and under low flows, but both factors can be very influential when human activities result in polluted waters (Section 13.2.3). Current is the defining feature of rivers and streams. It conveys benefits, such as transport of resources to the organism and removal of wastes, and also risks, of which being swept away is the most obvious. The substrate of running waters differs greatly from place to place, and is important to algae and many insects as the surface on which they dwell and to many fishes as the structure near which they find shelter from current or enemies. Temperature affects all life processes, and because most stream-dwelling organisms are ectothermic, growth rates, life cycles, and the productivity of the entire system are strongly under its influence. Thus current, substrate, and temperature are three physical variables that we should understand in order to appreciate the functioning of a lotic ecosystem and the adaptations of its denizens.

To decipher how organisms respond to individual habitat variables is complicated because organisms are subject to the simultaneous and interactive effects of multiple abiotic factors. The relationship of macroinvertebrate abundances to velocity, substrate size, and depth measures (Figure 5.1) illustrates differences in habitat preferences, but because these environmental factors are interrelated, it can be difficult to distinguish causal from correlated variables. Often we focus on the range of average conditions, but the variance in average conditions and the frequency and magnitude of extremes may be equally important. When environmental conditions episodically become unfavorable, such as an area of substrate that receives excessive scour or a stream section that becomes too warm for days or weeks, then patches of remaining suitable habitat provide refuge until the disturbance passes and recolonization can occur.

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