Diversity Versus Stability

The relationship between biodiversity and community stability remains a controversial issue (e.g., de Ruiter et al. 1995, Grime 1997, Hooper and Vitousek 1997, Schulze and Mooney 1993, Tilman et al. 1997; see Chapter 15). An early

(o Intermediate and specialized ■ Generalized ♦ All

(o Intermediate and specialized ■ Generalized ♦ All

Stability Analysis Plant

| (a) Frequency analyses (percentage) of insect damage for 14 stratigraphic horizons (with at least 200 specimens of identified dicot leaves) across the Cretaceous/Tertiary (K/T) boundary (orange bar) from the Williston Basin of southwestern North Dakota, United States. The horizontal scale is the percentage of leaves bearing insect damage (±1SD). The green line represents combined damage types; the black line is generalized damage types only; and the purple line is intermediate and specialized damage types. Because some individual leaves contain more than one damage type, the total percentage (green) is usually less than the sum of the two other data series. (b) Diversity analysis of insect damage, with raw data bootstrapped to 5,000 replicates. Vertical scale as in a.The data labels show the number of leaves in each sample. Poor preservation is probably responsible for the lack of recovered insect damage around the 30- to 40-m interval. From Labandeira et al. (2002) with permission from the National Academy of Sciences.

assumption that diversity conferred stability on communities and ecosystems was challenged, beginning in the 1970s, by modeling efforts that indicated increasing vulnerability to perturbation with system complexity (e.g., May 1973,1981, Yodzis 1980). However, new studies have addressed the importance of biodiversity for variability of ecosystem processes (e.g., de Ruiter et al. 1995, Fukami et al. 2001, Tilman and Downing 1994,Tilman et al. 1997).Among these are studies of "pest" dynamics and their effects on community structure in diverse ecosystems versus simple ecosystems (e.g., Schowalter and Turchin 1993).

Fundamental to our understanding of this relationship are definitions and measurements of diversity and stability (O'Neill 2001). As noted in Chapter 9, the variety of methods for measuring diversity has complicated comparison of communities, including assessment of community change. Should diversity be measured as species richness, functional group richness, or some diversity index using species or functional groups (de Ruiter et al. 1995, Grime 1997, Hooper and Vitousek 1997, Tilman and Downing 1994, Tilman et al. 1997)? Stability can be defined as reduced variability in system behavior. However, ecologists have disagreed over how best to measure stability. Stability has been shown to have multiple components—one representing capacity to resist change, and the other representing ability to recover following a change (i.e., succession)—which indicate different degrees of stability for a given ecosystem (see the following text). The variable(s) chosen to measure stability also can indicate different degrees of stability.

Traditionally, stability was measured by population and community ecologists as the constancy of species composition and community structure (e.g., Grime 1997, May 1973,1983). Ecosystem ecologists have emphasized the variability of ecosystem processes such as primary productivity, energy flux, and biogeochem-ical cycling, especially as variability changes during succession (e.g., de Ruiter et al. 1995, Kratz et al. 1995, E. Odum 1969, Tilman and Downing 1994). Species diversity may stabilize some variables but not others, or at one spatiotemporal scale but not another, leading to different conclusions. The extent to which diversity contributes to ecosystem integrity will be addressed in Chapter 15.

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