INSECT HERBIVORE DAMAGE AS AN INDICATOR OF LOCAL ECOSYSTEM DISTURBANCE IN THE OLIGOCENE TROPICS OF ETHIOPIA

Insect herbivore damage on fossil leaves has the potential to distinguish between stable and disturbed environments. Plants growing in disturbed habitats generally grow quickly to colonize the landscape, have short-lived leaves, and invest little in anti-herbivore defense (Grime 1974, Coley et al. 1985, Coley and Barone 1996). Therefore, these plants can be consumed by a wide variety of insect herbivores, including generalists that eat many different plant species. In contrast, many plants in stable habitats grow more slowly and invest much energy in anti-herbivore defenses. These plants are more immune to generalist herbivores, but susceptible to specialist herbivores, which have adapted to eat one or a few plant species. Insect damage censuses were conducted on two 27-28 Ma (late Oligocene) floras from the Chilga region of northwestern Ethiopia. The floras are stratigraphically equivalent, just 1.5 km apart, and grew under very similar climate conditions. The Guang River flora consists of 433 identifiable dicot leaf fossils from 4 sublocalities, and Bull’s Bellow flora of 710 fossils from 7 sublocalities. Leaves were identified using leaf architectural and cuticular features. Insect damage was quantified using the damage morphotype system of Labandeira et al. (2008), which also provides metrics for distinguishing damage made by generalist and specialist herbivores. This is the first analysis of insect herbivory in the Cenozoic of Africa and the second in the tropics. Almost 15% of the leaves from the Guang River flora have specialized damage, and 37.5% of damage occurrences are specialized damage types. The Bull’s Bellow flora has considerably more generalized damage, including abundant hole and margin feeding. The differences in damage composition between the two sites suggests that Bull’s Bellow represents a more recently disturbed habitat, a result that is supported by the taxonomy and leaf physiognomy of the plant species at each site.