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OIKOS 103: 519–527, 2003 A mechanistic examination of diversity-stability relationships in annual plant communities Thomas J. Valone and Catherine D. Hoffman Valone, T. J. and Hoffman, C. D. 2003. A mechanistic examination of diversitystability relationships in annual plant communities. – Oikos 103: 519– 527. While recent theoretical work has demonstrated several mechanisms whereby more diverse communities can exhibit greater temporal stability, empirical examinations have been few and the subject of much debate. We show that the temporal stability of natural summer and winter annual plant communities, at spatial scales of 0.25 m2 and 0.25 ha, tends to increase with community richness. Furthermore, more diverse communities exhibited greater stability because they contained a greater abundance of individuals (overyielding effect). Statistical averaging (the portfolio effect) and negative covariances between species (insurance and competition effects) did not enhance stability. Relationships between diversity and stability tended to be weak and were significant only at the smaller spatial scale. Because more diverse communities contained higher densities of individuals, the effect of diversity per se on stability was unclear and likely small. If overyielding is common in other ecological systems, the loss of individuals and biodiversity may often result in increased variation in ecological communities. T. J. Valone and C. D. Hoffman, Dept of Biology, Saint Louis Uni6., St. Louis, MO 63103, USA ([email protected]). How the number of species affects the stability of communities has been long-debated in ecology (MacArthur 1955, Elton 1958, Pimm 1984, Johnson et al. 1996, Cottingham et al. 2001) and discussion has intensified recently (Givnish 1994, Grime 1997, Huston 1997, Wardle 1999, McCann 2000) as interest in the effects of species extinctions on ecosystem function has heightened (Chapin et al. 2000, Schwartz et al. 2000, Loreau et al. 2002). One aspect of the debate has involved definitions because stability has been defined in a variety of ways (Loreau et al. 2001). Various workers have measured stability as 1) the degree of change in an ecosystem following a perturbation (resistance), 2) the rate of recovery of an ecosystem following a perturbation (resilience), and 3) the inverse of variance of a community property over time (invariability, Schlapfer and Schmid 1999). Much recent theoretical work has examined the effect of diversity on community dynamics and thus has focused on temporal stability (invariability) (Doak et al. 1998, Tilman et al. 1998, Tilman 1999, Ives et al. 1999, 2000, Yachi and Loreau 1999, Hughes and Roughgarden 2000, Lehman and Tilman 2000, Ives and Hughes 2002). These models predict that the temporal stability of communities should increase with diversity and have revealed one statistical and several biological mechanisms whereby more diverse communities can exhibit greater temporal stability (see Cottingham et al. 2001 for review). The statistical mechanism is known as statistical averaging, or the portfolio effect. Here, the variability of an aggregate community property declines with increased diversity because the temporal fluctuations of species in a community are not correlated (Doak et al. 1998, Tilman et al. 1998, Ives et al. 1999, 2000). Several biological mechanisms also may dampen community fluctuations as diversity increases. First, if community evenness increases with diversity, the importance of statistical averaging should increase (Cottingham et al. 2001). Second, if many pairs of species Accepted 23 April 2003 Copyright © OIKOS 2003 ISSN 0030-1299 OIKOS 103:3 (2003) 519 exhibit negative covariances in abundance, this will reduce the variability in an aggregate community property: in different years, some species will be rare while others are common and so the total community will exhibit small fluctuations (Frost et al. 1995, Tilman 1999). This covariance effect can result from two biological mechanisms: species may compete strongly (competition effect) or they may exhibit different responses to environmental variation (insurance effect, Doak et al. 1998, Ives et al. 1999, 2000, Tilman 1999, Yachi and Loreau 1999). And third, a reduction in the temporal variation of a community property can occur via an overyielding effect, an increase in the mean community parameter with increases in diversity (Tilman 1999, Hughes and Roughgarden 2000). When overyielding occurs, the abundance of constituent species does not decline as community diversity increases which tends to stabilize community fluctuations (Tilman 1999, Hughes and Roughgarden 2000). While some empirical studies have reported a positive relationship between community diversity and the temporal stability of various community properties (Dodd et al. 1994, Naeem et al. 1995, Tilman 1996, McGradyStead et al. 1997, Lehman and Tilman 2000, McGradyStead and Morin 2000), many of these studies have been brought into question (Cottingham et al. 2001). Empirical studies of natural communities have been criticized for containing hidden treatment effects whereby unmeasured variables covary with diversity (Givnish 1994, Huston 1997, Loreau et al. 2001) while studies of artificially created communities may contain sampling biases in the selection of species and often result in unnatural assemblages (Huston 1997, Hodgson et al. 1998, Loreau 1998, Wardle 1998, 1999, Schwartz et al. 2000, Cottingham et al. 2001). In addition, few studies have examined explicitly mechanisms that produce greater stability in more diverse communities (Schlapfer and Schmid 1999). And, while increasing attention has focused on potential scale effects in biodiveristy-ecosystem function studies (e.g. Loreau et al. 2001), few studies of diversity-stability relationships have examined the effects of scale. Using a long-term data set, we examine relationships between the temporal stability of community abundance (total number of individuals) and community diversity in two different natural annual plant assemblages at two spatial scales to test the predicted effect of diversity on community stability. Further, we evaluate the mechanisms above to determine how more diverse communities may exhibit greater temporal stability. Methods We examined the temporal stability of community abundances of annual plant communities over an 11520 year period (1989 –1999) from a long-term study site in southeastern Arizona, United States. The site contains 24 0.25 ha experimental plots subjected to different experimental treatments that manipulate the number or kinds of rodents and ants (Brown 1998). Two temporally segregated annual plant assemblages occur at the site corresponding to the bimodal pattern of precipitation (Samson et al. 1992). The winter plant assemblage germinates following winter rains and flowers and sets seed in spring. The summer plant assemblage germinates in early July after the onset of summer rains and flowers and sets seed in late summer. A total of 37 winter and 45 summer species have been recorded at the site. On average, 27.6 winter and 21.7 summer species are observed each year (Guo et al. 2002). Three bi-seasonal species can occur in both communities. However, only two of these species may be common in some years and these species, if present, are common typically in only one season (Guo et al. 2002). Each plot contains 16 0.25 m2 permanent quadrats. Twice each year, all annual plants were counted in each quadrat, in spring and late summer, corresponding to peak flowering time for the winter and summer annuals, respectively. We used species richness as our measure of diversity. Community abundance, which correlates strongly with cover and biomass in this system (Ernest et al. 2000, Guo pers. comm.), was the community property used to calculate temporal stability. We calculated stability two ways. First we calculated the coefficient of variation of community abundance (CV) over the 11-year period. The CV of aggregate community properties has been used in other empirical studies and allows comparison to other systems. Second, to examine more specifically mechanisms that may affect stability, we used the method of Lehman and Tilman (2000) to calculate the temporal stability, ST of the community, where ST = ' % Abundance (% Variances+ % Covariances) where abundance and variances are summed over all species in a community and covariances are summed over all pairs of species. This measure of temporal stability increases with diversity if, all else being equal, more diverse communities 1) contain higher abundance (more individuals; the overyielding effect), 2) have lower summed variances (portfolio effect) or 3) have lower summed covariances (competition and insurance effects) than low diversity communities (Lehman and Tilman 2000, Cottingham et al. 2001). We examined the plant communities at the 0.25 ha plot level by pooling data from the 16 quadrats on each plot. We calculated mean species richness over the OIKOS 103:3 (2003) Table 1. Summary of ANOVA results of the effect of experimental treatments on species richness, temporal stability (ST), and coefficient of variation of community abundance (CV) for the summer and winter plant assemblages for the 24 plots. Assemblage Variable F5,18 P Summer Richness ST CV Richness ST CV 0.83 1.68 0.75 1.90 0.46 0.77 0.55 0.31 0.60 0.14 0.80 0.58 Winter 11-year period in each of the 24 plots, for both summer and winter plants, as well as summed abundance, summed variances of each species and summed covariances of all pairs of species using the 35 most common species in each assemblage. This included all species in most years. To eliminate possible effects due to the experimental treatments, and to investigate potential scale effects, we also focused on the six unmanipulated plots: four control plots and two plots that remove Dipodomys spectabilis. The later serve as additional unmanipulated (control) plots because Dipodomys spectabilis has been rare at the site since the late 1980s and locally extinct since the early 1990s (Valone and Brown 1996). First, we performed all of the above analyses on these six unmanipulated plots. Second, we examined the local plant communities on each of the 96 0.25 m2 quadrats from these six plots. We calculated the mean richness over the same 11-year period in each quadrat, for winter and summer plants, as well as the summed abundance, summed variances of all species and summed covariances of all pairs of species in each quadat. Our use of the quadrats as independent communities may raise some concerns. In the strictest sense, these quadrats lack complete statistical independence but we believe these analyses are warranted for three reasons. First, they allow examination of the effects of diversity on temporal stability in the absence of concerns about possible hidden treatment effects because they come from unmanipulated plots. Second, they allow examination of potential scale effects via comparison to the plot-level analyses (Loreau et al. 2001). And third, their use greatly increases the sample size of the analysis and thus our power to detect significant relationships. Prior examinations of diversity-stability relationships have revealed weak relationships between community diversity and stability (McGrady-Stead et al. 1997). Thus, by using the quadrat communities, we trade off some degree of statistical independence for increased power (Oksanen 2001). Accordingly, we modify our significance criterion to 0.01 for these analyses. Results Mean community richness, temporal stability and the CV of community abundance all varied independently of the experimental manipulations of the 0.25 ha plots for both the summer and winter assemblages (Table 1). Mean richness, stability and CV of community abundance differed significantly between summer and winter communities. The winter communities were significantly more diverse and exhibited smaller fluctuations than the summer communities at both spatial scales examined (Table 2). Diversity-stability patterns: all plots Over all plots, the CV of community abundance declined with plot richness for both summer and winter plant assemblages but neither relationship was significant (Fig. 1). Similarly, temporal stability increased with diversity for both summer and winter assemblages but not significantly (Fig. 2). Because stability tended to increase with diversity, we examined the mechanisms that generated these relationships. Both plant assemblages exhibited the same patterns: summed abundance tended to increased with richness consistent with the overyielding effect, but the relationship was only significant for the winter plants. Table 2. Mean (SE) and range of species richness, temporal stability (ST) and coefficient of variation of community abundance (CV) of summer and winter plant assemblages at both plot and quadrat scales. P-values indicate statistical differences using a paired t-test for N = 24 plots and 96 quadrats. Summer Plot scale Richness ST CV Quadrat scale Richness ST CV OIKOS 103:3 (2003) Winter Mean (SE) Range Mean (SE) Range P 11.0 (0.3) 7.3 (0.3) 160.4 (6.1) 8.7–15.4 6.0–10.8 90–200 13.7 (0.3) 9.6 (0.4) 130.9 (3.1) 11.7–15.4 6.7–14.6 90–170 B0.001 B0.001 B0.001 3.0 (0.1) 6.1 (0.2) 190.7 (3.3) 1.1–5.5 4.4–9.6 120–290 4.7 (0.2) 9.2 (0.3) 150.2 (5.9) 1.7–6.9 4.7–14.2 80–260 B0.001 B0.001 B0.001 521 dance of plants increased significantly with mean richness, consistent with the overyielding hypothesis. In contrast, we found weak positive (destabilizing) relationships between summed variances and summed covariances and quadrat richness (Fig. 7 and 8). Thus, the overyielding effect alone conferred higher temporal stability on more diverse quadrat communities. For both plot- and quadrat-scale analyses, we found no evidence that statistical averaging enhanced the stability of more diverse communities. Recall that the statistical averaging effect will be enhanced if community evenness increase with diversity. To examine this relationship, we analyzed data from one high and one low diversity year for each assemblage. For both assemblages at both scales, evenness was either independent of, or declined significantly, with diversity (Table 4). Other years exhibited similar patterns. Discussion For both plant assemblages, and at both spatial scales examined, more diverse communities tended to exhibit greater temporal stability. However, significant relaFig. 1. Relationships between coefficient of variation of community abundance (CV) and mean community richness for the (A) summer and (B) winter plant assemblages for the 24 0.25 ha plots. Smaller CV indicates greater stability. Each letter represents one plot assigned to the following experimental treatments: A – ant removal; C – control, D – kangaroo rat (Dipodomys) removal, R – rodent removal, DA – kangaroo rat and ant removal, RA – rodent and ant removal. Both summed variances and summed covariances increased weakly with richness and thus acted to destabilize more diverse communities (Fig. 3 and 4). Diversity-stability patterns: unmanipulated plots and quadrats Restricting analyses to the six unmanipulated plots yielded similar results to those obtained from all 24 plots. For both summer and winter plants, CV of community abundance declined with richness while temporal stability, summed abundance, summed variances and summed covariances all increased with richness (Table 3, Fig. 1 –4). Only the relationship between richness and summed covariances for the summer plants was significant. At the smaller quadrat scale, both the CV of community abundance and temporal stability varied significantly with diversity. The CV of community abundance declined significantly and temporal stability increased significantly both plant assemblages (Fig. 5 and 6). And, in both plant assemblages, the summed abun522 Fig. 2. Relationships between temporal stability and mean community richness for the (A) summer and (B) winter plant assemblages for the 24 plots. Temporal stability is the calculated ST value of Lehman and Tilman (2000). Larger temporal stability indicates greater stability. See Fig. 1 for plot symbols. OIKOS 103:3 (2003) found significant variation in community properties over a small range of diversity (S& paèková and Lepš 2001). Instead, we think the most likely explanation for the lack of significance at the larger plot scale is the Fig. 3. Relationships between summed (A) abundance, (B) variances and (C) covariances of summer plant communities as a function of mean community richness for the 24 plots. The overyielding, portfolio and covariance effects would correspond to significant positive, negative and negative relationships, respectively while observed relationships are all positive. See Fig. 1 for plot symbols. tionships were restricted to the smaller spatial scale of the quadrat communities. One explanation for the difference between the plot- and quadrat-level conclusions is that scale affects diversity-stability relationships. While spatial scale can strongly affect many ecological phenomenon (With and Crist 1996, Ludwig et al. 2000, Whittaker et al. 2001), we think that explanation unlikely in this case because all patterns observed were consistent between the plot and quadrat analyses (compare Fig. 1 –4 with 5–8). A second explanation is that the variation in species richness on plots was too small to detect significant trends. While greater variation in richness would provide greater statistical power, note that the range in species richness was similar across both plots and quadrats and that other studies have OIKOS 103:3 (2003) Fig. 4. Relationships between summed (A) abundance, (B) variances and (C) covariances of winter plant communities as a function of mean community richness for the 24 plots. The overyielding, portfolio and covariance effects would correspond to significant positive, negative and negative relationships, respectively while observed relationships are all positive. See Fig. 1 for plot symbols. Table 3. Correlation coefficients and associated probabilities for relationships between richness and coefficient of variation of community abundance (CV), temporal stability (ST), SAbundance, SVariances and SCovariances for both plant assemblages for the n =6 unmanipulated plots. Variable Summer R CV ST SAbundance SVariances SCovariances −0.71 0.66 0.64 0.24 0.88 P 0.12 0.15 0.17 0.62 0.02 Winter r −0.27 0.18 0.48 0.10 0.45 P 0.60 0.82 0.33 0.98 0.50 523 Fig. 5. Relationships between coefficient of variation of community abundance (CV) and mean community richness for the (A) summer and (B) winter plant assemblages for the 96 0.25 m2 quadrats from unmanipulated plots. Smaller CV indicates greater stability. weak nature of the relationships between diversity and stability. The amount of variance explained in our diversity-stability correlations ranged from 3 to 10%, values intermediate with respect to other published relationships (Tilman 1996, McGrady-Stead et al. 1997). Detecting significance of such weak relationships thus requires the larger sample sizes provided by the quadrat-level analyses. Previous empirical studies of the effect of diversity on temporal stability have been criticized for containing hidden treatments effects and sampling biases, and for creating unnatural assemblages (Loreau et al. 2001). For example, in perennial plant communities, individual species can differ greatly in life history and biomass such that only a few high-biomass perennial species may be present in a community. As such, species-rich communities are more likely to contain species with high biomass than species-poor communities. Thus, mean plant biomass will often covary positively with community richness and therefore increased biomass rather than increased richness may confer greater temporal stability on more diverse communities (Huston 1997). Experimentally constructed communities can 524 similarly contain hidden treatment effects, because community construction requires selecting sub-sets of species to compare communities that differ in richness. If individual species differ in biomass and high biomass species are rare, species-poor communities will, on average, contain lower average biomass than species-rich communities (Givnish 1994, Huston 1997). Furthermore, artificially constructed communities often exhibit unnatural evenness in the abundances of species (Schwartz et al. 2000). While caution is required when interpreting correlations such as ours, the above criticisms are less cause for concern for three reasons. First, variation in community diversity and evenness resulted from natural variation across the site and was not influenced significantly by the experimental manipulations. Second, our communities consisted of natural assemblages of plants of similar life history and relative biomass; there were no perennial, high-biomass species in these communities. And third, the positive relationships between community diversity and temporal stability were observed at two spatial scales, including quadrats within unmanipulated control plots. We acknowledge that in our study, diversity was not manipulated and so we cannot examine the effect of diversity, per se, on community stability because unmeasured variables likely covaried with diversity. However, our approach does allow testing of the predicted relationship between diversity and Fig. 6. Relationships between temporal stability and mean community richness for the (A) summer and (B) winter plant assemblages for 96 quadrats from unmanipulated plots. Temporal stability is the calculated ST value of Lehman and Tilman (2000). Larger temporal stability indicates greater stability. OIKOS 103:3 (2003) (Lehman and Tilman 2000). Positive covariances suggest that most species responded similarly to environmental fluctuations as might be expected in this system; in arid environments, increased amounts and more even distributions of precipitation generally result in increased numbers of individuals and species of annuals (Ernest et al. 2000). As such, most species were abundant or rare in wet and dry years, respectively, resulting in positive covariances over the 11-year time period, regardless of possible competitive interactions. The importance of competitive interactions in influencing community stability also has been the subject of much theoretical work (Ives et al. 1999, 2000, Hughes and Roughgarden 2000, Lehman and Tilman 2000). While we cannot address this directly, competitive interactions among annual plants can be strong at the scale of the 0.25 m2 quadrats that we examined (Guo et al. 1998). Our quadrat-scale analysis therefore would be more likely to detect the influence of competition, if present. However, the lack of covariance effects at that scale suggests that local competitive interactions did not strongly influence the relationship between diversity and temporal stability. Theoretical treatments of the effect of diversity on community stability have paid the least attention to the overyielding effect although overyielding is considered Fig. 7. Relationships between summed (A) abundance, (B) variances and (C) covariances of summer plant communities and mean community richness for 96 quadrats from unmanipulated plots. Note the similarity to the patterns in Fig. 3. community stability in natural systems as well as examination of various theoretical mechanisms that are predicted to enhance stability in more diverse communities. Many theoretical treatments have focused on the portfolio and covariance effects for demonstrating how greater diversity can confer greater temporal stability (Doak et al. 1998, Tilman et al. 1998, Tilman 1999, Ives et al. 1999, 2000, Yachi and Loreau 1999, Hughes and Roughgarden 2000, McCann 2000, Loreau et al. 2001). We found no evidence of a decline in summed variances or covariances with increased community diversity and so neither effect contributed significantly to the greater stability observed in higher diversity communities. In retrospect, this should not be too surprising because our communities violated an important assumption of both the portfolio and covariance effects: fixed community abundance (Doak et al. 1998, Lehman and Tilman 2000); in the annual plant communities examined, more diverse communities contained a greater number of individuals. In addition, community evenness declined significantly with diversity, further weakening the strength of the portfolio effect (Cottingham et al. 2001). In fact, summed covariances in our system tended to be positive rather than negative as is typically assumed OIKOS 103:3 (2003) Fig. 8. Relationships between summed (A) abundance, (B) variances and (C) covariances of winter plant communities and mean community richness for 96 quadrats from unmanipulated plots. Note the similarity to the patterns in Fig. 4. 525 Table 4. Summary of relationships between evenness and community richness for both plant assemblages in one low and one high diversity year. Slope characterizes the nature of the relationship between evenness and diversity. In low diversity years, sample size for quadrats is less than 96 because some quadrats contained no plants. Assemblage Year Scale Mean Richness Slope P N Summer 1990 Plot Quadrat Plot Quadrat Plot Quadrat Plot Quadrat 17.7 7.0 7.4 2.2 16.0 4.2 19.9 7.9 −0.02 −0.59 −0.58 −0.70 −0.26 −0.47 0.01 −0.54 0.93 B0.001 0.003 B0.001 0.22 B0.001 0.95 B0.001 24 96 24 72 24 90 24 96 1996 Winter 1991 1993 to be an important component of relationships between diversity and productivity (Hector et al. 1999, S& paèková and Lepš 2001). In our communities, overyielding was the sole mechanism that led to greater stability in more diverse communities. In fact, overyielding was sufficiently strong to overcome the destabilizing effect of the positive summed covariances observed. Our results suggest that overyielding may be an important mechanism that affects the stability of natural communities. The generality of this mechanism, however, remains to be determined. It may be the case that overyielding occurs mostly in annual communities and is rare in communities of perennial species. In our arid system, seasonal precipitation strongly affects the number and diversity of individuals in communities and this resulted in a strong overyielding effect. However, we suspect that in nature, community diversity in many systems likely will often covary with factors such as productivity and disturbance that are known to influence the abundance of species (Sankaran and McNaughton 1999). The abundance of individuals in a community tends to increase with productivity (Wright 1983, Rosenzweig 1995, Kaspari et al. 2000, Valone and Hoffman 2003) suggesting that more productive habitats will often have greater densities of individuals and higher diversity of species, and these communities may also exhibit greater temporal stability as a result of overyielding. As such, the greater temporal stability of species-rich communities may result because of increased abundance rather than increased diversity. Future work should focus on disentangling the causal relationship between community abundance and community richness (Kaspari et al. 2000, Hubbell 2001) to understand more fully the effects of abundance, as well as diversity, on temporal stability. The accelerating extinction rate has raised concern among ecologists about the effects of species losses on ecosystem properties (Chapin et al. 1997, Grime 1997). Our work suggests that species-poor communities will often exhibit greater fluctuations in abundance than species-rich communities, owing to a lower number of individuals. In such systems, maintaining a high total abundance of individuals as well as a large number of species will stabilize fluctuations in ecological communities. 526 Acknowledgements – Thanks to J. H. Brown, S. K. M. Ernest and M. Kaspari for helpful comments. Data collection was supported by NSF grants, most recently by DEB-0211069. C.D.H. was supported by a SLU 2000 research assistantship. References Brown, J. 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