Mangroves are an overlooked hotspot of insect diversity despite low plant diversity

Abstract

Background: The world's fast disappearing mangrove forests have low plant diversity and are often assumed to also have a species-poor insect fauna. We here compare the tropical arthropod fauna across a freshwater swamp and six different forest types (rain-, swamp, dry-coastal, urban, freshwater swamp, mangroves) based on 140,000 barcoded specimens belonging to ca. 8500 species.

Results: We find that the globally imperiled habitat "mangroves" is an overlooked hotspot for insect diversity. Our study reveals a species-rich mangrove insect fauna (>3000 species in Singapore alone) that is distinct (>50% of species are mangrove-specific) and has high species turnover across Southeast and East Asia. For most habitats, plant diversity is a good predictor of insect diversity, but mangroves are an exception and compensate for a comparatively low number of phytophagous and fungivorous insect species by supporting an unusually rich community of predators whose larvae feed in the productive mudflats. For the remaining tropical habitats, the insect communities have diversity patterns that are largely congruent across guilds.

Conclusions: The discovery of such a sizeable and distinct insect fauna in a globally threatened habitat underlines how little is known about global insect biodiversity. We here show how such knowledge gaps can be closed quickly with new cost-effective NGS barcoding techniques.

Keywords: Beta-diversity; Global insect decline; Insect biodiversity; Mangroves; NGS barcoding; Southeast Asia; Species discovery.

Fig. 1

Insect alpha-diversity across tropical forest habitats. a Mangroves treated as one habitat; b Comparison of mangrove sites: Pulau Ubin (PU), Sungei Buloh (SB), Pulau Semakau old-growth (SMO), Pulau Semakau new-growth (SMN), other smaller mangrove fragments (see Additional File 1: Table S13); solid lines = rarefaction; dotted = extrapolations. The arrow on the x-axis indicates the point of rarefaction where species richness comparisons were made (see bar charts for absolute numbers with 95% confidence intervals)

Fig. 2

Insect communities across tropical forest habitats are distinct based on Bray-Curtis distances illustrated on 3D NMDS plots. Results are stable even when rare species are removed (a) or the data are split into different taxonomic groups (b)

Fig. 3

Only the diversity of phytophagous and fungivorous insects is correlated with plant diversity based on a linear regression model using rarefied insect species richness (*≤0.05, **≤0.01, ***≤0.001). Color coding of points as in Fig. 2

Fig. 4

Voronoi treemap of insect guilds across four habitats. Mangroves are represented by four sites (PU=Pulau Ubin, SB=Sungei Buloh, SMO: Semakau old mangrove, SMN: Semakau restored mangrove). Phytophages and fungivores dominate the rain and swamp forests while predators are overrepresented in mangroves

Fig. 5

Habitat differentiation by insect guilds (3D NMDS plot of Bray-Curtis distances for habitats with >2 sites)