Terrestrial ecosystem food webs stem their energy from two main sources: photosynthetic vegetation and non-photosynthetic vegetation. Herbivores consume photosynthetic vegetation and serve as prey for predators creating green food webs. Grazing pressure from herbivores can indirectly determine the availability of non-photosynthetic vegetation for detritivores which serve as prey for predators in brown foods webs. Brown food webs are integral in stabilizing terrestrial ecosystem dynamics highlighting the importance of understanding factors influencing their functioning. The aim of our project is to understand how herbivores influence non-photosynthetic vegetation biomass, detritivore abundances and their predators’ abundances. The first part of our experiment was aimed at understanding the influence of large herbivores on detritivore activity across a rainfall gradient while the second part was aimed at understanding the influence of herbivores on brown food webs. For our first aim, we used fenced plots designed to exclude large herbivores (kangaroos) across a rainfall gradient in semi-arid and arid Australian ecosystems and compared differences in live and dead vegetation cover and detritivore (termite) activity inside the exclosures and in associated controls. For our second aim, we compared detritivore abundances and their predators’ abundances across 3 1 ha exclosures and in associated controls.
Results/Conclusions
The results from the first part of our experiment show that live and dead vegetation cover both increase with rainfall but the response of dead vegetation cover to rainfall was dampened in plots grazed by kangaroos. Similarly, detritivore activity increased with rainfall in the ungrazed plots although this response to rainfall was dampened in the grazed plots. This suggests that herbivores, through the removal of vegetation, are indirectly reducing detritivore activity. The results from the second part of our experiment show similar results to the first part with a reduction in live and dead vegetation cover in grazed plots which leads to a reduction in detritivore biomass. Interestingly, the abundance of the detritivore’s predators such as lizards and small mammal insectivores was also reduced suggesting that large herbivores can disrupt the functioning of brown food webs. Brown food webs are an integral component of drylands due to the higher availability of dead vegetation compared with live vegetation through time. Herbivores can be considered as overabundant in some arid ecosystems due to changes in land practices and releases from top predation and our results demonstrate the influence of overabundant herbivores on a core function of aridlands.
