The habitat template and species traits

Habitat template theory places particular emphasis on the matching of habitat requirements of individual species to the abiotic and biotic conditions of a locale (Southwood 1988, Town-send and Hildrew 1994). Increasingly, such efforts examine the traits of species with the expectation that attributes such as size, body shape, life span, and mode of dispersal will help us understand why certain species succeed where others do not, and also may provide clues regarding the environmental factors that are responsible. A conceptual elaboration of this approach connects the regional to the local species pool through a hierarchical series of filters that determines the likelihood that a particular subset of colonists will be successful at a locale (Figure 1.5, Tonn et al. 1990, Poff 1997). The idea that biological assemblages and their responses to environmental variables should be viewed from the perspective of species traits rather than taxonomic identity is attracting wide use (Poff et al. 2006), with applications to community assembly (Poff 1997, Lamouroux et al. 2004), assessment of stream health (Barbour et al. 1999, Statzner et al. 2004), ecosystem function (Naeem and Wright 2003), and in predicting species invasions (Olden et al. 2006).

Efforts to demonstrate that species traits match with environmental variables have met with at least moderate success. To test the hypothesis that functional organization of fish communities is related to hydrological variability, Poff and Allan (1995) described habitat, trophic, morphological, and tolerance characteristics using six categories of species traits for each of the 106 species present at 34 sites in Wisconsin and Minnesota. Two ecologically-defined assemblages were identified, associated with either hydrologically variable streams (high variation in daily flows, moderate frequency of spates) or hydrologically stable streams (high predictability of daily flows, stable baseflow conditions). Fish assemblages from variable sites had generalized feeding strategies, were associated with silt and general substrate categories, were characterized by slow-velocity species with headwater affinities, and were tolerant of sedimentation. These findings indicate that hydrologic regime acts as an environmental filter, supporting theoretical predictions that variable habitats should harbor resource generalists whereas stable habitats should include a higher proportion of specialist species.

The hierarchical filter model implies that associations should exist between traits of the species assemblage and habitat variables at multiple spatial scales. Lamouroux et al. (2004) assessed correlations between traits and environmental variables across spatial scale within two river basins in France, after first summarizing the functional composition of invertebrate communities using 60 categories of 12 biological traits (Table 10.2). Roughly half of the tests were significant at the microhabitat scale and about one fourth at the reach scale; although a number of invertebrate traits differed between basins, this was not attributable to between-basin habitat differences. In this example, filters at the microhabitat scale clearly were most influential.

Any consideration of species traits must recognize that some traits tend to co-occur and others may rarely if ever be found in the same species. For example, large body size, long life span, and low reproductive potential form one common suite of attributes; small body size, short life span, and high reproductive potential another. These are frequently referred to as slow and fast, or K and r species, respectively (Begon et al. 2005). Mixtures between these two suites of attributes are rare, suggesting a trade-off exists between two alternate lifestyles, or at least that they represent two ends of a spectrum. Using 11 ecological traits and 11 biological traits, Usseglio-Polatera et al. (2000) identified such gradients in body size, reproductive rate, and feeding ecology within 472 European macroinvertebrate taxa. Because they were able to aggregate taxa into groups with similar traits, they speculate that improved resolution of habitat affinities or response to pollution might be attained using a subset of species sharing a similar suite of traits, rather than the entire assemblage.

TABLE 10.2 Trait categories and their modalities from an analysis of relationships between species traits and environmental variables for invertebrate assemblages of streams in France. (Adapted from Lamouroux et al. 2004.)

Trait

Modalities

Maximum size Body flexibility Body form Life span Voltinism Aquatic stages Reproduction Dispersal Resistance form Respiration

Locomotion/relation to substrate Feeding habits

Seven categories from <2 to >80 mm

Streamlined, flattened, cylindrical, spherical

Egg, larva, nymph, imago

Ovoviviparous, individual eggsa, egg massesb, asexual Aquatic active, aquatic passive, aerial active, aerial passive Eggs, cocoons, cells, resists desiccation, diapause/dormancy, none Tegument, gill, plastron, spiracle (aerial)

Flyer, surface swimmer, swimmer, crawler, burrower, interstitial, temporarily attached, permanently attached Absorber, deposit feeder, shredder, scraper, filter feeder, piercer, predator, parasite, parasitoid

Isolated eggs can be free or cemented to substrate

Egg masses can be free, cemented, in vegetation, or deposited terrestrially a b

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