Habitat construction contributions to complementarity

Along with their comparatively well-studied modes and patterns of habitat and resource utilization, species have differing impacts on their environments. This habitat-constructing dimension of functional diversity contributes to species complementarity and, consequently, to ecosystem productivity and stability. For instance, recent models indicate that such stability arises from a

At exposed montane sites such as this limestone cap

Figure 6.3 At exposed montane sites such as this limestone cap (Cataract Mountain, Glacier National Park), closely spaced neighboring plants facilitate each others' survival and growth by mitigating the cold, dry conditions. Interactions among heterospecific individuals may be predominantly positive rather than competitive in such stressful habitats, while in more favorable conditions individuals may instead compete with close neighbors for resources. photo courtesy of R. Callaway, Glacier National park, Montana, USA. For the color image, see plate 17.

specific type of community complexity: distinct, unequal inputs of energy and materials from organisms with contrasting niche-constructing roles (e.g., primary producers vs. microbial detritivores; Holt 2006). These distinct inputs are used by cooccurring organisms and integrated at higher levels of trophic and other interactions (Holt 2006). In the classic field experiment of Tilman et al. (2001, 2006), plant species diversity had progressively stronger positive effects on the total biomass of experimental plots over a decade. The species combinations in this study included taxa with differing niche-constructing impacts, such as nitrogen-fixing legumes and C4 grasses that produce very dense, shallow root systems. This aspect of biodiversity may contribute to facilitation and hence complementarity; specific beneficial habitat-constructing effects such as soil nitrogen enrichment may be one reason that both species diversity per se and the particular composition of a multispecies mixture determine combinatorial stability and productivity.

The habitat-constructing dimension of species functional diversity likewise led to complementarity in an experimental microbial ecosystem. Wohl et al. (2004) created different "microcosm" communities consisting of between one and eight randomly chosen species of cellulose-degrading bacteria isolated from leaf litter and then measured the cellulose decomposition they accomplished in a closed environment (i.e., one with finite resources). Because cellulose is an abundant but inaccessible source of carbon in many ecosystems, breaking it down into chemically available forms is a key bacterial niche-construction role that substantially benefits cohabiting organisms as well. In these microcosms, greater bacterial species richness resulted in enhanced rates of total cellulose breakdown, because the bacteria created fine-scale differences in biochemical function that allowed more individuals to be maintained and permitted species to coexist over numerous bacterial generations (Wohl et al. 2004). Species diversity in this system promoted complementary modes of habitat-constructing function, allowing for a complex community structure as well as greater ecosystem productivity.

In such cases, functional complementarity for niche-constructing effects can contribute to habitat amelioration. A recent stream bioremediation experiment (Cardinale 2011) provides a practical example. A key management objective for aquatic systems is the reduction of excess nitrogen (added by agricultural fertilizers and fossil fuel combustion), since high nitrogen levels lead to the eutrophication of aquatic systems and coastal dead zones (V. Smith and Schindler 2009). The remediation study directly tested the impact of biofilm species diversity on the ability of stream communities to take up and store excess nitrogen. Researchers manipulated the number of diatom and green algal species growing in the biofilms of 150 stream me- socosms, recreating the types of habitat variation that influence the coexistence of algal species, such as heterogeneity of flow and periodic disturbance regimes. Both nitrogen uptake rates and algal community biomass increased linearly with species richness, evidently owing to functional complementarity (Cardinale 2011).

In view of these biodiversity insights, the species losses predicted over coming decades can be expected to reduce the efficiency with which many different types of communities capture and use resources (Cardinale et al. 2007). In a global synthesis of meta-analyses, the impact of moderate-to-high rates of species extinction (40%-60% species loss) on ecosystem function is estimated to be as large as that of major drivers of environmental change, such as nutrient pollution, ozone production, and climate change (Hooper et al. 2012). Even if global levels of species loss are less drastic than these estimates, moderate-to-high levels of extinction are likely to be reached locally in many regions of intense human activity (especially with respect to top predators; Estes et al. 2011). These anticipated biodiversity losses can be expected to lower ecosystem productivity and stability through lost habitatconstructing effects as well as reduced functional diversity.

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