Herbivory, intraspecific trait variability and back to herbivory

Background/Question/Methods

Ungulate herbivory is a key driver of leaf trait syndromes and defence strategies at the ecological and macroevolutionary levels. Different levels of herbivory driven by land-use changes and anthropogenic landscape interventions should also cause short-term evolution within plant populations, but few studies have experimentally tested this prediction. We set out to experimentally assess the plastic and heritable effects of contemporary history of ungulate herbivory on (a) leaf trait syndromes at the intraspecific level, (b) tolerance, (c) mechanical resistance traits (i.e. the size of spines and leaves); as well as (d) the relationship among these traits (leaf syndrome, tolerance and resistance traits). We measured leaf traits (specific leaf area, leaf dry matter content, and leaf nitrogen concentration) in populations of four woody legume species that had grown under contrasting pressures of domestic ungulate herbivory during ≥50 years. We then performed a common garden experiment, measuring the same traits. Next, we performed cutting experiments on the plants grown in the common garden to measure tolerance and leaf trait plasticity. In the woody species, we also measured the length of spines and the leaf size (average area per leaf), in the field and in the common garden experiment.

Results/Conclusions

We found that field herbivory over decades produced a displacement toward more conservative leaves and longer spines and smaller leaves, with the trait differences between histories of herbivory being mostly hereditary. Woody individuals were tolerant to the cutting treatment irrespective of their history of herbivory and other traits. These results indicate that (a) vertebrate herbivory can lead to evolutionary processes at contemporary time scales even in long-lived perennial plant species; and (b) there is no trade-off between tolerance and resistance in these woody legumes species. Our findings suggest that land-use change, in addition to its well-known effects on vegetation composition and physiognomy, drives evolutionary changes which could, in turn, modify ecosystem processes and community dynamics.