Loss of dominant caterpillar genera in a protected tropical forest

Abstract

Reports of biodiversity loss have increasingly focused on declines in abundance and diversity of insects, but it is still unclear if substantive insect diversity losses are occurring in intact low-latitude forests. We collected 22 years of plant-caterpillar-parasitoid data in a protected tropical forest and found reductions in the diversity and density of insects that appear to be partly driven by a changing climate and weather anomalies. Results also point to the potential influence of variables not directly measured in this study, including changes in land-use in nearby areas. We report a decline in parasitism that represents a reduction in an important ecosystem service: enemy control of primary consumers. The consequences of these changes are in many cases irreversible and are likely to be mirrored in nearby forests; overall declines in the region will have negative consequences for surrounding agriculture. The decline of important tropical taxa and associated ecosystem function illuminates the consequences of numerous threats to global insect diversity and provides additional impetus for research on tropical diversity.

Figure 1

Caterpillar, parasitoid, and interaction richness declines across 22 years of sampling at La Selva Biological Research Station. Braulio Carillo National Forest (A.l) and surrounding areas, including La Selva (A.2) and a large adjacent banana plantation indicated by dashed white lines (A.3). Declines in caterpillar (B), associated parasitoid (C) and interaction (D) richness over the past 22 years (1997–2018) are evident within the La Selva forest patch. Dotted lines on plots depicting declines are the best fit lines from Bayesian regression, with 95% credible intervals in gray. Map designed by D.M.S.

Figure 2

Genus-level patterns in caterpillar encounter frequencies across years. (A) Point estimates for beta coefficients and associated 80% credible intervals (CI) for 64 genera that comprise a subset of all genera collected that met criteria for this analysis. Genus names are listed on the left margin and probabilities of a decline are on the right margin. Units of the year coefficient are on the log-odds scale. (B) Frequency (untransformed) across years for select genera and representative larval and adult images. Photographs taken by L.A.D, D.M.S and H.G.L.

Figure 3

Patterns in plant-caterpillar-parasitoid interactions, climate, and parasitism across time. Tri-trophic networks illustrate host plants (green), caterpillars (blue), and associated parasitoids (yellow) for the first (A) and last 5 years (B) of the study. Nodes represent families within each trophic level and are grouped by caterpillar suborder (Heterocera: light blue and Rhopalocera: dark blue) and parasitoid order (Hymenoptera: light yellow and Diptera: mustard yellow) then ranked by node degree. Edge thickness represents relative link weights. Percent parasitism across the 22 years of the study period (C) for hymenopteran (blue) and dipteran (green) parasitoids. Structural equation model examining causal relationships among time, positive precipitation anomalies and their one-year time lag and parasitism (D). Path coefficients are standardized. Standard errors are reported in brackets. Arrows represent positive associations and lines with circles represent negative associations. The model is a good fit to the data: χ² = 0.88, p = 0.35, df = 1, Parasitoid illustrations by M.L.F.

Figure 4

Structural equation models (SEM) estimating the effects of climate variables on caterpillar, parasitoid and interaction richness. (A) Associations between time (year), richness, positive temperature anomalies and precipitation anomalies; model fit: χ² = 0.16, p = 0.91, df = 2. (B) Similar causal pathways as in the previous panel, but with average daily minimum temperatures in place of temperature anomalies; this model also controls for the indirect effect of precipitation anomalies on richness mediated through effects on minimum temperature (which decrease with more flooding); model fit: χ²: 0.02, p = 0.89, df = 1. Path coefficients are standardized and width of arrows are scaled based on magnitude of path coefficients. Arrows represent positive associations and lines with circle represent negative associations. Parasitoid illustrations by M.L.F. Caterpillar images by B.L.