Population trends of insect pollinators in a species-rich tropical rainforest: stable trends but contrasting patterns across taxa

Abstract

Recent reports of insect decline have raised concerns regarding population responses of ecologically important groups, such as insect pollinators. Additionally, how population trends vary across pollinator taxonomic groups and degree of specialization is unclear. Here, we analyse 14 years of abundance data (2009-2022) for 38 species of native insect pollinators, including a range of Coleoptera, Lepidoptera and Hymenoptera specialists and generalists from the tropical rainforest of Barro Colorado Island, Panama. We estimated population trends across taxonomic groups to determine whether specialist species with a narrower range of interacting mutualistic partners are experiencing steeper population declines under environmental change. We also examined the relationship between climate variables and pollinator abundance over time to determine whether differences in sensitivity to climate predict differences in population trends among pollinator species. Our analyses indicated that most pollinator populations were stable or increasing, with few species showing evidence of decline, regardless of their degree of specialization. Differences in climate sensitivity varied among pollinator species but were not associated with population trends, suggesting other environmental factors at play for tropical insect pollinators. These results highlight the need for long-term population data from diverse tropical taxa to better assess the environmental determinants of insect pollinator trends.

Keywords: bee; beetle; butterfly; climate change; long-term monitoring; pollinator decline.

Figure 1.

Population trends of (a) 19 species of bees, (b) 13 species of butterflies and (c) 6 species of Cyclocephala beetles during 14 years in BCI. Exponentiated means of the posterior probability (filled circles) and 95% credibility intervals (horizontal lines). The red dashed line in population trend = 1 (exponentiated value of zero) is a reference for stable population trend. Examples of species with increasing abundance (a(i),b(i),c(i)), and stable or decreasing abundance (a(ii),b(ii),c(ii)) over the study period. = Nocturnal bee species.

Figure 2.

Effects of climate variables on the population trends of focal species: (a) sea surface temperature in the equatorial Pacific, (b) BCI rainfall and (c) maximum average air temperature in BCI forest. Butterfly species are indicated in orange, bees in blue and beetles in green.

Figure 3.

Relationship of the temporal trend in species abundance to (a) species specialization, (b) sensitivity to SST, (c) sensitivity to precipitation and (d) sensitivity to maximum temperature. Black lines show a smooth regression line from the linear mixed-effects model fitted values, and the grey shading shows the standard error.