Regulation of Net Primary Productivity by Biodiversity

The extent to which biodiversity contributes to ecosystem stability has been highly controversial (see Chapter 10). Different species have been shown to control different aspects of ecosystem function (e.g., production, decomposition, and nutrient fluxes), demonstrating that biodiversity in its broadest sense affects ecosystem function (Beare et al. 1995, Vitousek and Hooper 1993, Waide et al. 1999, Woodwell 1993). The presence or absence of individual species affects biotic, atmospheric, hydrospheric, and substrate conditions (e.g., Downing and Leibold 2002). However, relatively few species have been studied sufficiently, under different conditions, to evaluate their effects on ecosystem functions. The debate depends, to a large extent, on definitions and measures of stability (see earlier in this chapter) and diversity (see Chapter 9).

Vitousek and Hooper (1993) suggested that the relationship between biodiversity and ecosystem function could take several forms. Their Type 1 relationship implies that each species has the same effect on ecosystem function. Therefore, the effect of adding species to the ecosystem is incremental, producing a line with constant slope. The Type 2 relationship represents a decreasing and eventually disappearing effect of additional species, producing a curve that approaches an asymptote. The Type 3 relationship indicates no further effect of additional species.

Communities are not random assemblages of species; instead, they are functionally linked groups of species. Therefore, the Type 2 relationship probably represents most ecosystems, with additional species contributing incrementally to ecosystem function and stability until all functional groups are represented (Vitousek and Hooper 1993). Further additions have progressively smaller effects, as species packing within functional groups simply redistributes the overall contribution among species. Hence, ecosystem function is not linearly related to diversity (Waide et al. 1999).

Within-group diversity could affect the persistence or sustainability of a given function, more than its rate or regulation, and thereby increase the reliability of that function (Fig. 15.2) (Naeem 1998, Naeem and Li 1997). Tilman et al. (1997) reported that both plant species diversity and functional diversity significantly influenced six ecosystem response variables, including primary productivity and nitrogen pools in plants and soil, when analyzed in separate univariate regressions but that only functional diversity significantly affected these variables in a multiple regression. Hooper and Vitousek (1997) also found that variability in ecosystem parameters was significantly related to the composition of functional groups, rather than the number of functional groups, further supporting the concept of complementarity among species or functional groups. Fukami et al. (2001) investigated the mathematical relationship between such compartmentalized biodiversity and ecosystem stability. They concluded that biodiversity loss

Ecosystem reliability over time as a function of the number of functional groups (M) and number of species per functional group (S) for a probability of species colonization over time of 0.005 and a probability of species presence over time of 0.005. From Naeem (1998) with permission from Blackwell Science, Inc. Please see extended permission list pg 573.

Ecosystem reliability over time as a function of the number of functional groups (M) and number of species per functional group (S) for a probability of species colonization over time of 0.005 and a probability of species presence over time of 0.005. From Naeem (1998) with permission from Blackwell Science, Inc. Please see extended permission list pg 573.

reduces similarity in species composition among local communities and thereby reduces the reliability (stability) of continued ecosystem processes.

Dominant organisms in any ecosystem are adapted to survive environmental changes or disturbances that recur regularly with respect to generation time. Therefore, adaptation to prevailing conditions (evolution) constitutes a feedback that reduces ecosystem deviation from nominal conditions. For example, many grassland and pine forest species are adapted to survive low-intensity fires and drought (e.g., underground rhizomes and insulating bark, respectively) that characterize these ecosystems, thereby stabilizing vegetation structure and primary production. Diverse communities may be more resistant to spread of host-specific insects or pathogens (see Chapters 6 and 7). However, spread of gener-alists may increase with diversity, where diversity ensures a greater proportion of hosts (Ostfeld and Keesing 2000).

All ecosystems are subject to periodic catastrophic disturbances and subsequent community recovery through species replacement (succession). Ecosystem diversity at large spatial or temporal scales provides for reestablishment of key species from neighboring patches or seed banks. The rapid development of early successional communities limits loss of ecosystem assets, especially soil and limiting nutrients. Hence, succession represents a mechanism for reducing deviation in ecosystem parameters, but some early or mid successional stages are capable of inhibiting further succession. Herbivores may be instrumental in facilitating replacement of inhibitive successional stages under suitable conditions (see Chapter 10).

Few studies have measured the effect of biodiversity on stability of ecosystem parameters. Most are based on selection of plots that differ in plant species diversity and, therefore, potentially are confounded by other factors that could have produced differences in diversity among plots.

McNaughton (1985,1993b) studied the effects of plant species diversity on the persistence and productivity of biomass in grazed grasslands in the Serengeti Plain in East Africa. Portions of areas differing in plant diversity were fenced to exclude ungulate grazers. Stability was measured as both resistance (change in productivity resulting from grazing) and resilience (recovery to fenced control condition following cessation of grazing). Grazing reduced diversity 27% in more diverse communities but had no effect on less diverse communities. The percentage biomass eaten was 67% and 76% in the more and less diverse communities, respectively, a nonsignificant difference. By 4 weeks after cessation of grazing, the more diverse communities had recovered to 89% of control productivity, but the less diverse communities recovered to only 31% of control productivity, a significant difference.

McNaughton (1977,1993b) also compared resistance of adjacent grasslands of differing diversities to environmental fluctuation. Stability, measured as resistance to deviation in photosynthetic biomass, increased with diversity, as a result of compensation between species with rapid growth following rain but rapid drying between showers and species with slower growth after showers but slower drying between showers. Eight of 10 tests demonstrated a positive relationship between diversity and stability (McNaughton 1993b).

Frank and McNaughton (1991) similarly compared effects of drought on plant species composition among communities of differing diversities in Yellowstone National Park in the western United States. Stability of species composition to this environmental change was strongly correlated to diversity (Fig. 15.3).

Ewel (1986) and Ewel et al. (1991) evaluated effects of experimental manipulation of plant diversity on biogeochemical processes in a tropical rainforest in Costa Rica. This study included five treatments: a diverse natural succession, a modified succession with the same number and growth form of successional species but no species in common with natural succession, an enriched species diversity with species added to a natural succession, a crop monoculture (replicates of three different crop species), and bare ground (vegetation-free). After 5 years this design yielded plots with no plants (vegetation-free), single species (monoculture), >100 species (natural and modified succession), and 25% more species (enriched succession). Elemental pool sizes always were significantly larger in the more diverse plots, reflecting a greater variety of mechanisms for retention of nutrients and maintenance of soil processes favorable for plant production. The results suggested a Type 2 relationship between biodiversity and stability, with most change occurring at low species diversity. However, the absence of intermediate levels of diversity, between the monoculture and >100 species treatments, limited interpolation of results.

Tilman and Downing (1994) established replicated plots, in 1982, in which the number of plant species was altered through different rates of nitrogen addition. These plots subsequently (1987-1988) were subjected to a record drought. During the drought, plots with >9 species averaged about half of their predrought biomass, but plots with <5 species averaged only about 12% of their predrought biomass (Fig. 15.4). Hence, the more diverse plots were better buffered against this disturbance because they were more likely to include drought-tolerant species compared to less diverse plots. More diverse plots also recovered biomass more quickly following the drought. When biomass was measured in 1992, plots with >6 species had biomass equivalent to predrought levels, but plots with <5

How Primary Productivity Measured

Relationship between stability (measured as resistance [R] to change in species abundances, in degrees) and diversity (H') in grasslands subject to grazing and drought at Yellowstone National Park, Wyoming. 1, early season, ungrazed; 2, peak season, grazed; 3, peak season, ungrazed. From Frank and McNaughton (1991) with permission from Oikos. Please see extended permission list pg 573.

Relationship between stability (measured as resistance [R] to change in species abundances, in degrees) and diversity (H') in grasslands subject to grazing and drought at Yellowstone National Park, Wyoming. 1, early season, ungrazed; 2, peak season, grazed; 3, peak season, ungrazed. From Frank and McNaughton (1991) with permission from Oikos. Please see extended permission list pg 573.

species had significantly lower biomass, with deviations of 8-40% (Fig. 15.5). Tilman and Downing (1994) and Tilman et al. (1997) concluded that more diverse ecosystems represented a greater variety of ecological strategies that confer both greater resistance and greater resilience to environmental variation. However, the contribution of diversity to ecosystem stability may be related to environmental heterogeneity (i.e., diversity does not necessarily increase stability in more homogeneous environments).

A number of studies have demonstrated that ecosystem resistance to elevated herbivory is positively correlated to vegetation diversity (e.g., McNaughton 1985, Schowalter and Lowman 1999, Schowalter and Turchin 1993; see Chapters 6 and 7). As vegetation diversity increases, the ability of any particular herbivore species to find and exploit its hosts decreases, leading to increasing stability of herbivore-plant interactions.

Experimental studies relating ecosystem stability to diversity generally have been limited to manipulation of plant species diversity. However, diversity usually increases from lower to higher trophic levels. Insects represent the bulk of diversity in virtually all ecosystems (e.g., Table 9.1) and are capable of controlling a

0 5 10 15 20 25 Plant species richness before drought

| Relationship between plant species diversity prior to drought and drought resistance in experimental grassland plots planted with different species diversities. Mean, standard error, and number of plots with given species richness are shown. 1 dB/Bdt (yr-1) = 0.5ln (1988 biomass/1986 biomass), where 1988 was the peak drought year and 1986 was the year preceding drought. The biomass 1988:1986 ratio (righthand scale) indicates the proportional decrease in plant biomass associated with dB/Bdt values. From Tilman and Downing (1994) with permission from Nature, © 1994 Macmillan Magazines, Ltd.

variety of ecosystem conditions (Chapters 12-14). A few studies have addressed the significance of diversity at higher trophic levels to ecosystem processes but not to ecosystem stability (Downing and Leibold 2002, Lewinsohn and Price

Klein (1989) found that diversity of dung beetles (Scarabaeidae) and the rate of dung decomposition were positively correlated to the size of forest fragments in central Amazonia. However, abiotic conditions that also affect decomposition likely differed among fragment sizes as well.

Couteaux et al. (1991) manipulated diversity of decomposer communities in microcosms with ambient or elevated concentrations of CO2. They found that decomposition and respiration rates were significantly related to decomposer diversity, as affected by species shifts following CO2 treatment. This study demonstrated an effect of biodiversity on rates of a key ecosystem process but did not address long-term stability of this process.

Downing and Leibold (2002) evaluated the effects of manipulated species composition nested within multitrophic diversity treatments in pond mesocosms. The effect of species composition on productivity, respiration, and

Decomposers The Savanna

Relationship between plant species diversity and deviation in 1992 biomass (following drought) from mean (1982-1986) predrought biomass in experimental grassland plots planted with different species diversities. Mean, standard error, and number of plots with given species richness are shown. Negative values indicate 1992 biomass lower than predrought mean. Biomass ratio is biomass 1992/predrought. Plots with 1, 2, 4, or 5 species (but not plots with >5 species) differed significantly from predrought means. From Tilman and Downing (1994) with permission from Nature, © 1994 Macmillan Magazines, Ltd.

Relationship between plant species diversity and deviation in 1992 biomass (following drought) from mean (1982-1986) predrought biomass in experimental grassland plots planted with different species diversities. Mean, standard error, and number of plots with given species richness are shown. Negative values indicate 1992 biomass lower than predrought mean. Biomass ratio is biomass 1992/predrought. Plots with 1, 2, 4, or 5 species (but not plots with >5 species) differed significantly from predrought means. From Tilman and Downing (1994) with permission from Nature, © 1994 Macmillan Magazines, Ltd.

decomposition was equivalent to, or greater than, the effect of diversity per se. Productivity was highest in the highest diversity treatments.

Herbivore and predator diversities have not been experimentally manipulated in terrestrial ecosystems to evaluate the effect of diversity at these levels on processes at lower trophic levels, except for biological control purposes, which may not represent interactions in natural ecosystems. For example, McEvoy et al. (1993) manipulated the abundances of two insect species with complementary feeding strategies (cinnabar moth, Tyria jacobaeae, a foliage and inflorescence feeder, and ragwort flea beetle, Longitarsus jacobaeae, a root feeder) introduced to control the exotic ragwort, Senecio jacobaea, in coastal Oregon, United States. Their results indicated that increasing diversity (from no herbivores to one herbivore to both herbivores) decreased local stability of the herbivore-plant interaction, as increasing herbivory drove the host to local extinction, at the plot scale. However, this plant species persisted at low densities over the landscape, suggesting that the interaction is stable at larger spatial scales. Croft and Slone (1997) reported that European red mite, Panonychus ulmi, abundances in apple orchards were maintained at lower, equilibrial, levels by three predaceous mite species than by any single predaceous species.

Ultimately, the capacity of ecosystems to endure or modify the range of environmental conditions is the primary measure of stability (McNaughton 1993b) (see Fig. 15.2). In this regard, Boucot (1990) noted that the fossil record demonstrates that characteristic species assemblages (hence, ecosystems) often have persisted for many thousands of years over large areas.

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