The impact of secondary compounds and functional characteristics on lichen palatability and decomposition

There has been much recent interest in understanding how functional traits of vascular plant species drive ecological processes such as herbivory and litter decomposition. In plants, these two processes are often driven by the same or similar suites of traits and therefore correlate across species. However, few studies have considered how traits of plant-like life forms such as lichens determine species differences in their effects on ecological processes. This is despite the significant contribution of lichens to carbon and nutrient cycling in many environments. We collected 28 lichen species that differed in their growth form, substrate type and capacity to fix N, and determined key traits for each species. For each species, we performed a feeding bioassay using the generalist snail Cepaea hortensis and carried out a laboratory bioassay to assess decomposability. We did these tests both with intact lichen material containing natural concentrations of carbon-based secondary compounds (CBSCs), and material that had been acetone rinsed to reduce concentrations of CBSCs, to evaluate the effect of CBSC on palatability and decomposability. We found that reducing CBSC concentrations greatly increased palatability for 17 species, and decomposability of 10 species. However, decomposability was correlated with several lichen traits while palatability was not, regardless of whether or not CBSCs were removed, and we therefore found no relationship between decomposability and palatability across species. Decomposability and palatability both varied, but in contrasting directions, among N-fixing vs. non-fixing lichens, lichens with different growth forms and those from contrasting substrate types. As such, N-fixing lichens had higher decomposition rates but lower consumption rates than non-fixing lichens, while foliose species had higher decomposition rates but lower consumption rates than fruticose species. Synthesis: We have shown that lichen CBSCs regulate key processes such as lichenivory and decomposition, that lichen decomposability but not palatability are related to traits, and that these two processes are unrelated across species. These results highlight the potential role of lichen species differences in influencing ecosystem processes relating to decomposition and nutrient cycling and the role that grazers may play in driving this.