Engineering effects of herbivores surpass those of woody plants

Abstract

Herbivores and woody plants are dryland ecosystem engineers due to their effects on multiple ecosystem attributes. The abundance and diversity of such engineers may alter ecosystem functions directly, and indirectly, through interactions among themselves and with ecosystem productivity. We report on a study in 451 sites in eastern Australia, finding that herbivores influenced a larger number of ecosystem attributes than woody species and therefore played a larger role as dryland engineers. The abundance or diversity of herbivores generally reduced native species richness, plant cover, water infiltration and soil stability and enhanced the dominance of exotic plants. Woody plants, however, showed fewer effects and with more variable signs (positive for biocrusts but negative for soil nutrients). Most negative effects found for herbivores were linked to livestock, only partly linked to changes in heterogeneity, and were substantially dampened with increases in ecosystem productivity. Ecosystem productivity was a major regulator of dryland engineering effects, reducing both positive and negative effects of both herbivores and woody plants. We contribute a better understanding of the relative importance and context dependency of two major dryland ecosystem engineers, showing that prioritizing livestock management over woody removal and locating rangelands in more productive areas may improve dryland sustainability.