BEHAVIORAL DIVERSIFICATION OF PALEOZOIC INSECTS: INSIGHTS FROM THE FOSSIL RECORD OF INSECT HERBIVORY

Because most insect herbivory in modern forests occurs on the leaves of angiosperms, one might assume that insects engaged in a limited suite of feeding behaviors until angiosperms diversified during the Cretaceous. However, the role of angiosperm diversity and biomass in shaping extant ecosystems is nearly impossible to quantify due to angiosperms' overwhelming dominance. If angiosperms accounted for 70% of the diversity and biomass of land plants, any disproportionate ecological role they may fulfill—for example, if they hosted 95% of insect feeding behaviors—would be easier to establish. But, because angiosperms account for approximately 96% of described vascular plant species, any disproportionate ecological role they fulfill may be statistically indistinguishable from a null expectation (in the case of this example, that angiosperms would host 96% of insect feeding behaviors).

The only way to directly test the impact of angiosperm diversification on the breadth of insect feeding behaviors is to compare insect damage on fossil leaves throughout the Phanerozoic. But until recently, comparisons of insect damage on Paleozoic, Mesozoic, and Cenozoic leaves were hampered by the unavailability of leaf surface area data, which prevented robust sampling standardization. Coverage-based rarefaction obviates the need for surface area data when standardizing estimates of the diversity of insect herbivory. Estimates generated with this method show that the upper bound of insect damage type diversity on fossil leaves has remained remarkably consistent through the Permian, Triassic, and Cenozoic.

This finding raises the question of whether the vast majority of insect damage types in modern ecosystems occur on angiosperms because angiosperms are uniquely palatable, or simply because angiosperms now provide the bulk of the plant biomass available to herbivorous insects. It appears that the highly diverse herbivorous insects that feed on angiosperms replaced a similarly diverse suite of insects that fed on earlier plants; the intricate web of trophic interactions seen in modern ecosystems may be more ancient than previously appreciated.