Herbivore-initiated interaction cascades and their modulation by productivity in an African savanna

Abstract

Despite conceptual recognition that indirect effects initiated by large herbivores are likely to have profound impacts on ecological community structure and function, the existing literature on indirect effects focuses largely on the role of predators. As a result, we know neither the frequency and extent of herbivore-initiated indirect effects nor the mechanisms that regulate their strength. We examined the effects of ungulates on taxa (plants, arthropods, and an insectivorous lizard) representing several trophic levels, using a series of large, long-term, ungulate-exclusion plots that span a landscape-scale productivity gradient in an African savanna. At each of six sites, lizards, trees, and the numerically dominant order of arthropods (Coleoptera) were more abundant in the absence of ungulates. The effect of ungulates on arthropods was mediated by herbaceous vegetation cover. The effect on lizards was simultaneously mediated by both tree density (lizard microhabitat) and arthropod abundance (lizard food). The magnitudes of the experimental effects on all response variables (trees, arthropods, and lizards) were negatively correlated with two distinct measures of primary productivity. These results demonstrate strong cascading effects of ungulates, both trophic and nontrophic, and support the hypothesis that productivity regulates the strength of these effects. Hence, the strongest indirect effects (and thus, the greatest risks to ecosystem integrity after large mammals are extirpated) are likely to occur in low-productivity habitats.

Fig. 1.

NDVI at our six experimental blocks, indicating a gradient of aboveground primary productivity. Points represent mean NDVI derived from MODIS (Moderate Resolution Imaging Spectroradiometer) images taken throughout 2003–2005. Error bars represent 95% confidence intervals. Blocks are arranged in order of decreasing productivity. Blocks 1–3 are underlain by vertisols with high clay content; blocks 4–6 are located on adjacent sandy loams. Points not connected by the same letter were significantly different (P < 0.05) in Tukey's honestly significant difference post hoc comparisons.

Fig. 2.

Effect sizes (log_e response ratios) of ungulates on four response variables regressed against aboveground primary productivity (as measured by NDVI): (A) lizard density, (B) tree density, (C) total-arthropod abundance, and (D) abundance of coleopterans only. Numbers represent individual blocks and correspond to those presented in Fig. 1. All relationships remain statistically significant after sequential Bonferroni corrections of α (0.05, 0.025, 0.017, and 0.013).