Assessing invertebrate herbivory in human-modified tropical forest canopies

Abstract

Studies on the effects of human-driven forest disturbance usually focus on either biodiversity or carbon dynamics but much less is known about ecosystem processes that span different trophic levels. Herbivory is a fundamental ecological process for ecosystem functioning, but it remains poorly quantified in human-modified tropical rainforests.Here, we present the results of the largest study to date on the impacts of human disturbances on herbivory. We quantified the incidence (percentage of leaves affected) and severity (the percentage of leaf area lost) of canopy insect herbivory caused by chewers, miners, and gall makers in leaves from 1,076 trees distributed across 20 undisturbed and human-modified forest plots in the Amazon.We found that chewers dominated herbivory incidence, yet were not a good predictor of the other forms of herbivory at either the stem or plot level. Chewing severity was higher in both logged and logged-and-burned primary forests when compared to undisturbed forests. We found no difference in herbivory severity between undisturbed primary forests and secondary forests. Despite evidence at the stem level, neither plot-level incidence nor severity of the three forms of herbivory responded to disturbance. Synthesis. Our large-scale study of canopy herbivory confirms that chewers dominate the herbivory signal in tropical forests, but that their influence on leaf area lost cannot predict the incidence or severity of other forms. We found only limited evidence suggesting that human disturbance affects the severity of leaf herbivory, with higher values in logged and logged-and-burned forests than undisturbed and secondary forests. Additionally, we found no effect of human disturbance on the incidence of leaf herbivory.

Keywords: Amazon rainforest; biodiversity and ecosystem functioning; environmental gradient; folivory; forest degradation; herbivore interactions; herbivory; plant; tropical forest.

FIGURE 1

Comparison between herbivory incidence (a) and severity (b) caused by chewers, miners, and gall formers in 1,076 stems. Different letters on top of boxes represent significant differences at p < .05 following a Kruskal–Wallis test. Boxplots display the distribution of the data, showing the first and third quartiles, whiskers extend up to 1.5 times the interquartile range. Light gray dots represent actual data distribution, black dots are outliers, and red asterisks represent mean values. Notice that the y‐axes in panel b are on different scales

FIGURE 2

Correlation between the different forms of herbivory incidence at the stem (a–c) and plot level (d–f). Notice that y‐axes are not on the same scale

FIGURE 3

Herbivory levels across a gradient of forest disturbance. Stem‐level incidence (a–c) and severity (d–f); plot‐level incidence (g–i) and severity (j–l). Invertebrate‐mediated herbivory forms are represented per column: chewing (a, d, and g), mining (b, e, and h), and galling (c, d, and i). Notice that severity plots’ y‐axes (d–f, j, and k) are not on the same scale. Both the box and violin plots display the distribution of the raw data, except for the only signifficative result (stem‐level chewing severity, (d) where we plot model predictions instead. Boxplots (including white ones within violin plots area) represent first and third quartiles, whiskers extend up to 1.5 times the interquartile range, data points beyond that threshold are plotted individually. Plots were colored according to forest classes, scaling from lighter to darker green representing the disturbance gradient: UF, undisturbed forests; L, logged forests; LBF, logged‐and‐burned forests; SF, secondary forests