Disentangling the role of environment in cross-taxon congruence of species richness along elevational gradients

Abstract

Spatial patterns of species richness have been found to be positively associated, a phenom called cross-taxon congruence. This may be explained by a common response to environment or by ecological interactions between taxa. Spatial changes in species richness are related to energy and environmental heterogeneity but their roles in cross-taxon congruence remain poorly explored. Elevational gradients provide a great opportunity to shed light on the underlying drivers of species richness patterns. We study the joint influence of environment and biotic interactions in shaping the cross-taxon congruence of plants and orthopterans species richness, along three elevational gradients in Sierras Grandes, central Argentina. Elevational patterns of species richness of orthopterans and plants were congruent, being temperature the best single predictor of both patterns supporting the energy-related hypotheses. Using a structural equation model, we found that temperature explained plant richness directly and orthopteran richness indirectly via orthopteran abundance. Cross-taxon congruence is likely due to a common response of both taxa to temperature although via different theoretical mechanisms, possibly, range limitations for plants and foraging activity for orthopterans. We disentangled the role of temperature in determining the cross-taxon congruence of plants and orthopterans by showing that a common response to the environment may mask different mechanisms driving the diversity of different taxonomic groups.

Figure 1

Elevational patterns of each environmental variable for the three gradients. The line is a local regression model for the three mountains together with a 95% confidence interval. Figure generated using R software.

Figure 2

Elevational pattern of orthoptera and plant richness for each of the elevational gradients. The line is a local regression model for the three elevational gradients together with a 95% confidence interval. Spearman correlation: r = 0.37, p = 0.03. Figure generated using R software.

Figure 3

Conditional predictions for the best fitted models for orthoptera and plant richness. Figure generated using R software.

Figure 4

Structural equation model for the alternative mechanisms of cross-taxon congruence between orthopteran and plant richness. We show standardized partial regression coefficients and their significance levels (**p > 0.01; ***p > 0.001) shown above the arrows. Black arrows represent significant paths.

Figure 5

Location of the study sites in Sierras Grandes Mountains in central Argentina. Map created using the Free and Open Source QGIS.