Insect herbivory and herbivores of Ficus species along a rain forest elevational gradient in Papua New Guinea.
The classic research on elevational gradients in plant-herbivore interactions holds that insect herbivore pressure is stronger under warmer climates of low elevations. However, recent work has questioned this paradigm, arguing that it oversimplifies the ecological complexity in which plant-insect herbivore interactions are embedded. Knowledge of antagonistic networks of plants and herbivores is however crucial for understanding the mechanisms that govern ecosystem functioning. We examined herbivore damage and insect herbivores of eight species of genus Ficus (105 saplings) and plant constitutive defensive traits of two of these species, along a rainforest elevational gradient of Mt. Wilhelm (200-2,700 m a.s.l.), in tropical Papua New Guinea. We report overall herbivore damage 2.4% of leaf area, ranging from 0.03% in Ficus endochaete at 1,700 m a.s.l. to 6.1% in F. hombroniana at 700 m a.s.l. Herbivore damage and herbivore abundances varied significantly with elevation, as well as among the tree species, and between the wet and dry seasons. Community-wide herbivore damage followed a hump-shaped pattern with a peak between 700 and 1,200 m a.s.l. and this pattern corresponded with the abundance of herbivores. For two tree species surveyed in detail, we observed decreasing and hump-shaped patterns in herbivory, in general matching the trends found in the set of plant defenses measured here. Our results imply that vegetation growing at mid-elevations of the elevational gradient, that is at the climatically most favorable elevations where water is abundant, and temperatures still relatively warm, suffers the maximum amount of herbivorous damage which changes seasonally, reflecting the water availability.
Sam, K., Koane, B., Sam, L., Mrazova, A., Segar, S., Volf, M., ... & Novotny, V. (2020). Insect herbivory and herbivores of Ficus species along a rain forest elevational gradient in Papua New Guinea. Biotropica, 52(2), 263-276.
Location of (a) the study area in Papua New Guinea and (b) the six study sites of the Mt. Wilhelm elevational gradient in Bismarck Range, Madang Province. An example of the original leaf photos (c) and leaf photos processed in Photoshop, ready to be analyzed in ImageJ (d)
Mean (±SE) herbivore damage (a; in percentages of individual leaves) and herbivorous arthropod abundances (b) of all species studied at each elevation combined at six study sites along the elevational gradient of Mt. Wilhelm in Papua New Guinea. Herbivore damage at each elevational site was measured at the end of the dry season (Survey 1) and 6 months later, at the end of the wet season (Survey 2)
Mean (±SE) herbivore damage (in percentages of individual leaves) of four plant species surveyed at the 200 m a.s.l. study site (Kausi). ARF = Ficus arfakensis, CON = F. conocephalifolia, HAH = F. hahliana, and TRI = F. trichocerasa. Same letters (lowercase letters for Survey A, capital letters for Survey A) mark non-significant results of the Unequal N HSD post hoc test. ** mark significant difference (p ≤ .01) within the species between the two surveys
Mean (±SE) herbivore damage (in percentages of individual leaves) on eight Ficus species surveyed along the Mt. Wilhelm elevational gradient. The values marked with different letters differ significantly from each other (Tukey test, p = .05). n.s. = non-significant differences in herbivory between elevational study sites for given species. Means are based on the average across both surveys
Scatter plots of trait values for Ficus arfakensis (a) and F. hahliana (b) fitted with second-order polynomial curves. For each plant species, the following characteristics are fitted: Upper left: trichome density per 10 mm2 (average of both sides of the leaf lamina), upper right: Shannon diversity of triterpenes, lower left: total triterpene content (total sum of peak areas/mg) and lower right: specific leaf area