Logging cuts the functional importance of invertebrates in tropical rainforest

Abstract

Invertebrates are dominant species in primary tropical rainforests, where their abundance and diversity contributes to the functioning and resilience of these globally important ecosystems. However, more than one-third of tropical forests have been logged, with dramatic impacts on rainforest biodiversity that may disrupt key ecosystem processes. We find that the contribution of invertebrates to three ecosystem processes operating at three trophic levels (litter decomposition, seed predation and removal, and invertebrate predation) is reduced by up to one-half following logging. These changes are associated with decreased abundance of key functional groups of termites, ants, beetles and earthworms, and an increase in the abundance of small mammals, amphibians and insectivorous birds in logged relative to primary forest. Our results suggest that ecosystem processes themselves have considerable resilience to logging, but the consistent decline of invertebrate functional importance is indicative of a human-induced shift in how these ecological processes operate in tropical rainforests.

Figure 1. Ecosystem process rates in primary (green) and logged (orange) forest in response to experimental removal of invertebrates, fungi or vertebrates.

Ecosystem processes were quantified at three trophic levels represented by (a,b) leaf litter decomposition rate, (c,d) seed disturbance, defined as the combined removal and/or predation rate; and (e,f) invertebrate predation rate. Symbols indicate the taxa that contributed to the rates displayed. Absolute values (mean±95% CI) of the ecosystem process rates are presented in the left-hand column and are measured as (a) the slope of a linear regression model relating log_e-transformed litter mass (g) as a function of log_e-transformed time (number of days), (c) the proportion of experimental seeds removed or predated per day, and (e) the proportion of experimental mealworm larvae predated per day. Letters indicate habitat × treatment categories that did not significantly differ from each other (P<0.05). There was a significant treatment × habitat interaction for all three ecosystem processes, demonstrating that the role of invertebrates was stronger in primary than logged forests. In the right-hand column (panels b,d,f), values represent the proportional change in ecosystem process rates relative to control sites (calculated from data presented in the left-hand column). Values <1 (dashed line) indicate functions whose rate is reduced following the exclusion of a taxon; smaller values indicate larger reductions in the rate and hence a stronger contribution of that taxon to delivering the ecosystem process. Posthoc significance tests were used to examine the treatment × habitat interaction effects. For example, in panel b, the asterisk indicates that the effect on decomposition of excluding invertebrates was significantly (P<0.05) larger in primary forest than in logged forest. NS indicates nonsignificant interactions.

Figure 2. Differences in the physical environment and the functional composition of invertebrate and vertebrate communities between primary and logged tropical rainforest.

Response variables fall into the categories of microclimate (Clim), morphology and structure (Morph), biomass (Mass), diversity (Div), occurrence (Occ) and abundance (Abun). Invertebrate groups tend to be more abundant in primary than in logged forest, whereas the reverse is generally true for vertebrate groups. For comparison, all values were standardized to represent standard deviations from the mean. Dark lines represents the median, boxes the first and third quartiles, and whiskers the range. Significant differences (P<0.05) from mixed effect models on untransformed data are indicated with a green minus (−) or orange plus (+) sign if the effect of logging was negative or positive.