Appendix B. Summaries of empirical tests.
This section gives a brief synopsis of each empirical tested listed in Table 2 in the main text. Level B1 tests of the ZSM SAD prediction are discussed extensively in the main text and not described here.
Caswell (1976) studied how theShannon-Weaver index of diversity (H) varied in empirical data along gradients where we would expect competitive interactions to become progressively more important (early to late successional, temperate to tropics, etc). He found that the neutral-predicted H became systematically further from the empirical H as the expected importance of competitive interactions increased.
Zhang, Lin, Yu et al. (1997, 1998) noted that neutral theory is balanced on a knife’s edge – that if the species are not exactly competitively equivalent then neutral theory predictions degenerate into an entirely different and unrealistic set of predictions. More recently, Fuentes (2004) has updated this idea for the metacommunity based neutral models and shown that the shape of the SAD and species area relationship change drastically and unrealistically when small variations in competitive equivalence are modeled.
Terborgh et al. (1996) and Pitman et al. (2001) suggested neutral theory fails for two reasons. First there exist a few “oligarchic” species that are extremely abundant and dominate community composition across thousands of kilometers. Secondly, Terborgh et al. also emphasized that community composition changed in a predictable fashion with environmental factors (successional stage and floodplain vs. terra firme).
Pandolfi (1996, 2002) studied coral assemblages. He showed that while there were large differences between communities at very small and very large scales, at intermediate scales there was a constancy in community structure across 90,000 years and despite up to nine cycles of perturbation and re assemblage due to changing sea levels.
Condit et al.(2002) developed a model of how community similarity decreases with distance (Chave and Leigh 2002). Curve fitting estimation of some parameters enabled the curve to fit the data well for the range 100m-50km but poorly for shorter and longer spatial scales. Some authors have interpreted this as an overall success for neutral theory (Chave 2004) but the original paper (Condit et al. 2002) describes it as a failure. Given that curve fitting parameters were used, the curve must fit empirical data over some spatial range.
Clark and McLachlan (2003) derived a neutral prediction that variance in abundance across space increases without limit over time, but used pollen data from trees of Quebec to show that for empirical data the variance quickly reached a constant asymptote. See also further debate on this test (Clark and McLachlan2004, Volkov et al. 2004).The debate centers on the nature of the metacommunity and the fast-slow assumption of implicit metacommunities.
Ricklefs (2003) attempted to estimate speciation rates from an estimated q and showed that the predicted speciation rates far exceed our current estimates of speciation rates (see also main text section “Level B2 test of the ZSM SAD – a priori parameters” for further discussion).
Adler (2004) used two closely related predictions of neutral theory: (1) the species area relationship (or SAR) which predicts increasing species diversity with area due to changing community composition and (2) the species time relationship (STR) which follows the increase of species diversity at one point in space with an increasing time span (Adler and Lauenroth2003, White 2004).Adler showed that while neutral theory performed well at level A2 (curvefitting) for either the SAR or the STR, neutral theory failed at level B2 because the parameters that lead to a good fit for the STR fail badly for fitting the SAR and vice versa.
Gilbert and Lechowicz (2004) carefully selected sites so that distance and environment were independent of each other, a major confounding factor in most tests of dispersal limitation. They showed that when distance and environmental differences are decoupled, the environmental variation has far greater explanatory power for species composition than dispersal limitation (distance).
Fuller et al. (2004) studied communities of invertebrate pool detrivores and compared not mean abundance in the SAD but predicted variability in abundance. They suggest that for their data, the neutral model shows less variability than empirically observed for common species and more variability in the neutral model than empirically observed for rare species.
B. J. McGill, E. A. Hadly, and B. A. Maurer (unpublished manuscript) examined fossil assemblages giving a proxy for community structure across 900,000 years and 3000 km. They show that community composition displays much more inertia (constancy) in the empirical world than neutral theory over a time scale of 5,000 years and up.
Fargione et al. (2003) tested the invasion success of four guilds of grassland plants in invading already established plots of varying composition. The invasion success of a guild was negatively affected by the % cover in the preexisting community of the same guild and of the C4 grass guild, contradicting neutrality in these organisms.
Wootton (1994) used undisturbed intertidal invertebrate communities to parameterize a modified version of the neutral theory. They then used this to predict abundances in communities where the dominant species was removed from the regional pool and compared this to empirical communities where this removal was achieved by manual removal. The neutral theory did a poor job of predicting abundances in the manipulated communities.
Harpole and Tilman (2005) studied grassland communities. They measured in monoculture the species trait R* (minimum sustainable level of N in the soil, the limiting resource in these communities). They showed that across a variety of gradients in nitrogen (experimental manipulation, successional, and large scale natural) abundance was well predicted by R* and that R* had a tradeoff with colonization parameters such as seed size, contradicting neutral theory predictions of random assemblage independent of traits.
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