Appendix C. Discussion of further examples of mid-domain effect (MDE) model predictions that are strongly collinear with environmental gradients.
The problem of collinearity confounds many other studies of latitudinal gradients. Willig and Lyons (1998) used the same New World domain gradient as Romdal et al. (2005); they therefore had the same temperature collinearity. Ellison (2002) related nearly 80% of the latitudinal variance in Pacific basin mangrove richness to a mid-domain model. However, Ellison also points out that 78% of the variation in richness is related to habitat area, and he concludes that the mid-domain effect reflects area (which, presumably, is defined in large part by temperature). Similarly, Jetz and Rahbek (2001, 2002) studied sub-Saharan birds. They argued that the Sahara formed a hard boundary analogous to a coast. By choosing sub-Saharan Africa as the study domain, there is a strong mid-domain peak of precipitation (Fig. C1). Bird richness in warm parts of the world is known to depend strongly on precipitation (Hawkins et al. 2003; H.-Acevedo and Currie 2003). Thus, the MDE predictions are collinear with the climatic gradient hypothesized to control patterns of richness.
Hawkins and Diniz-Filho (2002) observed a latitudinal mid-domain effect in North American birds that is conflated with two different factors in the northern and southern parts of its domain. They defined their domain as the Nearctic region, i.e., north of a line near the Tropic of Cancer. Species whose ranges crossed this boundary were excluded from their study. Thus, by definition, richness must decline to zero toward the southern boundary (even though total richness continues to increase southward; Hawkins et al. 2003). North of the mid-domain (roughly, north of 45°N), the MDE prediction is strongly collinear with temperature. Temperature is not an issue in the southern part of the domain because bird richness is strongly correlated with temperature (or potential evapotranspiration) only in areas north of about 45°N in North America (Currie 1991: Fig. 6; Hawkins et al. 2003). In warm areas of North America (i.e., south of the mid-domain), richness depends more strongly on precipitation than temperature. Richness is far more strongly correlated with climate in North America (e.g. R2 = 0.81; Currie 1991) than with the MDE predictions (R2 = 0.21 Hawkins and Diniz-Filho 2002). This is not surprising: MDE predictions are collinear with climate in the N–S direction axis, but not on the E–W axis. Richness tracks the climatic pattern whether the gradient is latitudinal or longitudinal, but it tracks the MDE pattern only when the MDE pattern is collinear with climate.
Richness on elevational gradients -- Grytnes (2003) found mid-elevation peaks in richness vascular plant richness in southern Norway, and monotonic gradients at sites above the Arctic Circle (where the most favorable climate is at the mountain base). Grytnes and Vetaas (2002) examined plant richness along elevation gradients in the Nepalese Himalayas. They concluded that climate, area, and mid-domain effects all could influence the distribution of species richness in the Himalayas, although they did not try to statistically disentangle these influences. Carpenter (2005) examined plant richness in the same area. He noted that land area, cultivated land, and climate all have peaked relationships with elevation, as does species richness. He concluded that mid-domain effects could not explain the mid-elevation peak in richness.
McCain (2004) examined small-mammal richness on an elevational transect in Costa Rica near the Barva transect discussed by Cardelús et al. (2006), Watkins et al. (2006), and Brehm (2007). Richness peaks in mid-gradient; however, we have shown that environmental favorability also peaks mid-gradient in the area. McCain notes that primary productivity shows a mid-elevation peak on her transect.
In all these cases, mid-domain peaks on elevation gradients occurred when they were collinear with mid-domain peaks of warm, wet conditions, and not otherwise. When mid-elevation peaks occur, they do not always occur in the predicted place (e.g., Sanders 2002: his Fig. 1; McCain 2005).
FIG. C1. The variation in precipitation among quadrats in sub-Saharan Africa. Data are from Francis and Currie (2003). Precipitation shows a mid-domain peak in this particular domain, which is collinear with the mid-domain predicted peak in richness.
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