Effects Of Biodiversity On Ecosystem Processes

Ecologists often equate ecosystem processes with the average (steady-state) flow of energy and nutrients in undisturbed ecosystems. However, there is little reason to think that the process of natural selection which accounts for patterns of biotic diversity is tightly coupled to biogeochemical cycling. To persist, individuals that comprise populations and species must (1) reproduce, and to achieve this must (2) acquire resources to maintain themselves and produce biomass. In the process, they crcate conditions that may be essential or detrimental to the existence of other species. Regardless of the impact of these interactions, the ultimate result is to select for traits that promote persistence of certain genotypes in space and time, and not maximization of production or rates of biogeochemical cycling per se. In some situations high productivity may promote persistence. For example, following disturbance, rapidly growing species quickly monopolize the available light and nutrient resources. Other species may occupy niches where slow growth and space occupancy lead to greater long-term persistence and reproductive output. In other words, high biotic diversity is not necessarily coupled to a particular rate of production or biogeochemical cycling, but may depend on the maintenance of an environmental matrix in which different strategies are favored at different times or places. For these reasons, the biological feedbacks that maintain the integrity of ecosystems arc of greater functional importance in the long term than are instantaneous fluxes of matter. The retention of soil resources, and the maintenance of structures, pools and interactions among organisms are particularly significant. The long-term persistence of ecosystem functions such as the uptake and cycling of carbon, nutrients and water will depend on the maintenance of this integrity, which thereby becomes the key component to consider when discussing feedbacks of biodiversity on ecosystem function.

There is no intrinsically unique level at which biotic diversity affects ecosystem processes. The current level of conceptual understanding of the effects of biodiversity on ecosyslcm processes is so primitive that it is easiest to recognize these linkages at the level of functional groups (i.e. groups of species which have ecologically similar effects on ecosystem processes). However, no two species or individuals are ecologically identical, so as our understanding improves we can expect to recognize situations where species diversity within functional groups or genetic diversity within species has important ecosystem consequences. Thus, the current emphasis at the level of functional groups rather than on species or genetic diversity is more a function of our ignorance than of the taxonomic level at which diversity is important.

in the next section we consider first the role of biodiversity in the steady-state turnover of energy and nutrients within ecosystems, then its impact on the size of the resource base that sustains biogeochemical cycles, and finally its impact on the long-term integrity of ecosystem processes.

2.3.1 Nutrient cycling and energy flow

Plant differences in size and relative growth rate (RGR) have large effects on steady-state acquisition and loss of energy and nutrients in intact ecosystems. Large size enhances resource capture by allowing the plant to reach the top of the canopy where light is most available and t.o exploit a large soil volume. A high RGR supported by high potentials for nutrient absorption and photosynthesis (resulting from large leaf allocation and high rates per unit leaf) enables plants to exploit successfully a high-rcsource environment. By contrast, species from low-resource environments have lower rates of growth and resource acquisition but retain these resources for longer periods of time through slow tissue turnover. In most regions there is a plant species pool with a broad range of sizes and RGR. Ecological sorting (Vrba and Gould 1986) causes species to occur at those points along resource gradients where they have the greatest competitive advantage in acquiring and conserving the resources necessary for growth, survival and reproduction (Whittaker 1953). For example, in arctic and alpine ecosystems tall shrubs dominate in riparian habitats, where nutrient and water avail ability is high and where topography provides protection from winter winds. Sites of low nutrient availability are dominated by evergreen heath species with slow growth rates and low rates of nutrient turnover. Thus, ecological sorting causes species with large size and high rates of resource acquisition to dominate nutrient-rich sites, and species with low rates of resource acquisition and turnover to occupy infertile sites. Within a given site it is the large-statured individuals that dominate resource capture and cycling, so within these ecosystems relatively few species account for most of the biogeochemical cycling. Thus, differences among individuals and species in size and RGR are extremely important in explaining (1) site differences in steady-state rates of biogeochemical cycling, and (2) the identity of spccies in a given site that are responsible for most of the productivity and nutrient cycling.

Tissue quality, which governs rates of both herbivory and decompositon, correlates closely with RGR (Chapin ¡993). Species differences in tissue quality act as positive feedback to amplify ecosystem differences in soil resources. Species from sites of low resource availability generally have low annual production and high concentrations of tannins, lignin and waxes that are toxic or indigestible to herbivores (Chapin 1980, 1987; Körner 1989). resulting in low animal feeding rates in infertile sites (Bryant et al. 1983; MacLean and Jensen 1986). By contrast, in high-resource sites, plants produce leaves with high nutrient content and low levels of secondary metabolites. These leaves can be eaten in large quantities with a high digestive efficiency. As a result of species and site effects on tissue quality, animals concentrate their activity on more fertile sites. Because animals preferentially feed on high-quality tissues within these sites and respire away much of the assimilated carbon (>98% of assimilated carbon in the case of vertebrate homeotherms), animals accelerate nutrient turnover in fertile sites (Chapin 1991). Arctic and alpine plants of a given growth rate have higher tissue N concentrations than do species from warm environments (Chapin 1987; Körner 1989).

Species differences in tissue quality arc critical controls over litter decomposition (Melillo et al. 1982). Litter from low-rcsource plants decomposes slowly because of the negative effects of lignin, tannin, wax and other recalcitrant or toxic compounds on soil microbes, reinforcing the low nutrient availability of these sites (Chapin 1991; Hobbie 1995). By contrast, species from high-resource sites produce litter with more N and P (Vitousek 1982) and fewer recalcitrant compounds. Therefore this litter decomposes rapidly. For example, arctic evergreen species have high concentrations of lignin and tannins (Chapin el al. 1986), low rates of herbivory by microtines, caribou and insects (Batzli and Jung 1980; MacLean and Jensen 1986), and slow decomposition rates (Shaver et al. 1996) compared with leaves of deciduous species (Figure 2.2). Mosses, with their low tissue quality, decompose slowly

RSBELAC RSBELAC

Species

R=Rubus; S~Sallx; B=BetuIa; E=Eriophorum;

L=Ledum; A=Aulacomnlum; C=Cetraria

Figure 2.2 Nitrogen concentration, lignin concentration, herbivore preference and decomposition rate of leaves from seven arctic tundra species: three deciduous shrubs (R, Rub us chamaemorus; S, Salix pulchra', B, Betula nanny, a graminoid E, Erio-phorum vaginatum); an evergreen shrub (L, Ledum paiustre)', a moss (A, Aulaconmhmt lurgidum); a lichen (C, Cetraria richardsonii). Nitrogen and lignin concentrations were measured on mid-summer leaves (1-year leaves of Ledum), and herbivore preferences are the average for four lepidopteran iarvac and four mammals (C'hapin et al, 1986). Plant species were ranked from 0 (never eaten) to 10 (always preferred). Decomposition rate was measured on leaf litter under optimal conditions of temperature and moisture (Shaver et al. 1996)

and accumulate much of the ecosystem nutrient capital in undecomposed peat (Clymo and Hay ward 1982). Both herbivore and decomposition feedbacks amplify initial site differences in nutrient availability and rates of biogeochemical cycling.

Plants indirectly influence rales of nutrient supply through modification of the microenvironment (Hobbic 1995). For example, mosses, with their low rales of évapotranspiration and inability to tap water at depth (leading to water-logging), and effective thermal insulation (preventing soil warming) indirectly inhibit decomposition (Tenhunen et a! 1992). Aerenchymatous tissues in sedges transport oxygen into soils, supporting soil microbial activity and decomposition in the rhizosphere. These species-specific effects could be important in determining both the pools of resources available to plants and the rates at which these pools turn over.

In summary, there arc large differences among species in traits that determine resource uptake, resource loss to herbivores or litter, and release of nutrients from this material in soil or the guts of animals. Many of these traits correlate with species differences in size and RGR. In general, there is a continuum among species in RGR and size, with different functional groups (e.g. herbs, shrubs, trees) tending to occupy different portions of this spectrum (Grime and Hunt 1975). Species that arc extreme with respect to these traits tend to occur in ecosystems of contrasting resource availability. How important is species diversity within an ecosystem in determining rates of biogeochemical cycling?

In closed communities, any reduction in the abundance of one species causes a compensatory increase in the abundance of other species due to release from competition, with little change in the total quantity of resources accumulated by vegetation at the ecosystem level. Consequently, we expect that gain or loss of a species will have relatively small effects on biogeochemical cycles within the ecosystem under "steady-state" conditions (Shaver et al. 1996). There are three sources of evidence for this hypothesis. Firstly, the pattern of biomass distribution among species in closcd arctic and alpine plant communities fits a geometric model (Pastor 1995), which theory suggests is best explained by competitive partitioning of resources among species: the dominant species preempts most resources, and the remaining resources are partitioned among specics according to a compétitive hierarchy. Secondly, experimental manipulation of resource supply causes much larger changes in the abundance of individual species than in biogeochemical pools or fluxes measured at the ecosystem level (Figure 2.3; McNaughton 1977; Lauenroth et al. 1978; Chapin and Shaver 1985). When experimental manipulations alter the abundance of the dominant species, other species change their growth and resource acquisition to utilize the remaining resources, as long as there are species present that are capable of using these resources. Biomass distribution in these experimental manipula-

Figure 2.3 Response of total plant biomass (excluding roots) to environmental manipulations simulating global change measured 9 years after initiation of treatment. Treatments are control (Cj, fertilizer added (F), temperature increased with a plastic greenhouse (Gj, combined fertilizer and greenhouse treatment (FG>, and shading to reduce light intensity by 50% (S). Response is shown for individual growth forms and for the total plant community. From Chapin a al. 1995

moss

lichen

¡3

evergreen

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