PhD Oral Exam - Mahsa Hakimara, Biology

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Insect herbivory plays a vital role in forest ecosystems, structuring trophic webs and maintaining biodiversity. While dramatic insect outbreaks often dominate attention, low intensity but persistent background herbivory also contributes to nutrient cycling and ecological processes. This thesis investigates the ecological drivers of background insect herbivory in sugar maple-dominated forests, focusing on within-tree variations, between-tree diversity, and trophic interactions.

Using the "green world" hypothesis as a framework, this study examines how bottom-up and top-down forces shape herbivore populations. Bottom-up pressures from leaf physical traits varied with vertical stratification and light gradients: sunlit canopy leaves exhibited higher thickness, lower specific leaf area (SLA), and lower water content with less herbivory damage, whereas shaded understory leaves showed lower thickness, higher SLA and greater water content with increased herbivory damage. Paradoxically, feeding bioassays revealed that a local lepidopteran herbivores preferred sun-exposed leaves over shaded leaves and had a better performance on sun-exposed leaves. These findings underscore the complex interplay between leaf traits and herbivore behavior, indicating that physical defenses alone cannot fully explain observed patterns of herbivory.

Top-down forces, including predation and parasitism, were further studied. Predation rates varied across vertical gradients but not between saplings under different light conditions, with arthropods dominating shaded understories and birds in sunlit canopies. While higher bird predation may contribute to reduced herbivory in the sun canopy, limited parasitoids data prevented robust conclusions about vertical variation of natural enemies. This spatially variable top-down pressures on herbivore populations deserves further attention.

Comparisons between sugar maple and black maple (Acer nigrum) revealed key differences in leaf traits, like tougher leaves and denser trichomes in black maples, but no difference in herbivore communities. This suggests that despite notable trait differences, insect herbivores did not distinguish between the two tree species, likely reflecting their close evolutionary relationship.

By integrating within-tree and between-tree variations with trophic interactions, this thesis provides a comprehensive view of the factors shaping insect herbivory in sugar maple forests. The results contribute to ecological theory and offer insights for forest management, emphasizing the need to address both plant defense mechanisms and trophic interactions to support forest resilience.