Introduction

Primary production in terrestrial environments generates about 100 gigatons of biomass annually (Gessner et al. 2010). While on average 90% of terrestrial plant biomass escapes herbivory (Cebrian 2004), herbivores nonetheless exert a pervasive influence on the quality of all plant tissues in ecological and evolutionary time (Hunter 2001; Dethier 1954; Ehrlich and Raven 1964; Thompson 1994; Karban and Baldwin 1997). By inducing chemical changes in plant tissues in ecological time, or by acting as agents of natural selection favouring defended tissues in evolutionary time, herbivores have a significant impact on plant traits. Accordingly, terrestrial herbivores may engage in density-mediated indirect effects (DMIEs) with other organisms by their consumption of 10% of plant biomass, and in trait-mediated indirect effects (TMIEs) by their ecological and evolutionary effects on the 90% of the biomass that is not consumed. Finally, assuming average assimilation efficiencies of around 20% (Speight et al. 2008), herbivores may convert around 2% of terrestrial plant biomass into animal biomass. If herbivores are not consumed by their own predators, their cadavers are subsequently available for decomposition by the soil microbial community. Cadaver inputs and burrowing or trampling (Hunter 1992) are the only direct effects (DEs) of herbivores on soil processes of which we are aware. Based on these numbers alone, we might expect TMIEs of herbivores on other organisms to be relatively more important than DMIEs or DEs. Simply put, the effects of terrestrial herbivores on plant quality may often be more important ecologically than their effects on plant biomass.

TMIEs induced by herbivores may be particularly important in soil food webs. Communities of decomposers and detritivores in the soil rely largely upon energy derived from dead plant material and are fundamental players in the global carbon cycle (Cornwell et al. 2008). If 90% of land plant biomass enters the decomposer food web without being consumed (Cebrian 2004), a focus on DMIEs or DEs alone might lead us to assume that herbivores have relatively minor effects on the population and community ecology of decomposers and detritivores. However, the activities of herbivores can lead to dramatic changes in the nutritional, chemical and structural characteristics of plant tissues (Speight et al. 2008) that subsequently mediate their rates of decomposition (Choudhury 1988; Pastor and Naiman 1992; Findlay et al. 1996; Hunter 2001; Chapman et al. 2006).