A further test of a palaeoecological thermometer: Tropical rainforests have more herbivore damage diversity than temperate forests

The fossil record suggests greater diversity of insect leaf-feeding during warm climate intervals. Much published work in the palaeobotanical literature has been based on the presumed validity of this pattern. Here we investigated whether, in a tropical rainforest climate, leaves are being eaten in more ways than in a temperate forest climate. We compared forests at five sites in Malaysia (2–3°N, MAT ca. 26.5 °C) with five sites in northern Florida, USA (30°N, MAT ca. 19.5 °C) and in the north-eastern (NE) USA (40–42°N, MAT 7–9 °C). Presence and absence of damage types was determined using a standard leaf damage guide. In bulk floras, there was a consistent difference in damage diversity, per unit number of leaves (as in the fossil studies), between Malaysian and cool temperate NE USA sites. Malaysian tropical forest sites had a significantly greater number of damage types. Florida occupied an intermediate position, too similar to both Malaysia and NE USA to be statistically significant. When damage diversity on leaves of individual tree species was compared, consistently higher damage diversity was found in the five most abundant tropical species sampled, than in the five most abundant temperate species sampled in the NE USA. In Malaysia, the percentage of leaves receiving any damage was statistically significantly greater than in NE USA, a difference of around 10%. The results appear to validate that the trend of damage diversity with temperature seen in the fossil record is not a spurious effect of preservation bias. Tropical forests and tropical tree species do appear to have greater damage diversity than temperate forests and species. This may have broader, as yet unexplored, implications for community ecology.