Biogeography theory predicts that herbivory is higher at low latitudes than at high latitudes, and that, the strong feeding pressure at low latitudes would select for plants with higher resistance (lower palatability) to herbivores compared at high latitudes. This theory has been tested in the terrestrial and salt marsh ecosystems, but few studies have been conducted in mangrove ecosystems. We hypothesized that insect folivory in mangrove forests would be higher at low versus high latitudes, as would plant traits associated with plant resistance. We worked with Kandelia obovata, the most widely distributed mangrove species in southeast China. We measured herbivory and leaf traits at eight geographic sites along the southeast coast of China, spanning 19.9 °N to 28.3 °N degrees latitude, at the end of three growing seasons (2013, 2014, 2015). We also conducted a reciprocal transplant experiment by transplanting propagules of K. obovata from high, mid and low latitude provenances into three common gardens at north, mid and south locations within the geographic range of K. obovata. We monitored leaf herbivory, insect density and leaf traits of transplanted plants during the growing seasons of 2014 and 2015.
Results/Conclusions
In the geographic field survey, herbivory on leaves of K. obovata decreased with increasing latitude. Similarly, in the reciprocal transplant experiment, herbivory on leaves of K. obovata varied among common gardens, with the greatest values at low latitudes. We found no effect of plant provenance within each common garden, suggesting that differences in geographic herbivory patterns were not genetic. However, plants from the high latitude sites had a higher N content, lower leaf toughness, and total phenolic content, traits usually thought to indicate higher palatability. These results support the idea that herbivory decreases at high latitudes, but call into question the importance of leaf toughness, N content and phenolic content in mediating herbivory on mangrove leaves.