Abstract
In the long-term absence of disturbance, ecosystems often enter a decline or retrogressive phase which leads to reductions in primary productivity, plant biomass, nutrient cycling and foliar quality. However, the consequences of ecosystem retrogression for higher trophic levels such as herbivores and predators, are less clear. Using a post-fire forested island-chronosequence across which retrogression occurs, we show that nutrient availability strongly controls invertebrate herbivore biomass when predators are few, but that there is a switch from bottom-up to top-down control when predators are common. This trophic flip in herbivore control probably arises because invertebrate predators respond to alternative energy channels from the adjacent aquatic matrix, which were independent of plant biomass. Our results suggest that effects of nutrient limitation, following ecosystem retrogression, on trophic cascades are modified by independent variation in predator abundance, which requires a more holistic approach to trophic ecology to better understand herbivore effects on plant communities.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Statement of authorship: AK and DW designed the study, AK collected the data, MJ calculated invertebrate biomass, AK analysed the data with substantial input from EA and wrote the manuscript with substantial input from EA, DW, MG and MJ.
Data accessibility statement: Should the manuscript be accepted, the data supporting the results will be archived in a public repository (Dryad) and the data DOI will be provided.