Connectivity of stormwater ponds impacts Odonata abundance and species richness

Abstract

Context: The successful dispersal of an animal depends, partly, on landscape connectivity. Urbanization poses risks to dispersal activities by increasing hostile land cover types.

Objectives: We investigated how connectivity of urban ponds impacted Odonata communities (dragonflies and damselflies), an order of semi-aquatic insects that actively disperse.

Methods: We sampled 41 constructed stormwater ponds and 8 natural ponds in a metropolitan area. The effect of connectivity and the quantity of available adjacent habitats was tested at different scales for dragonflies (900 m) and damselflies (300 m), determined by a literature analysis, to account for differences in suborder dispersal capabilities.

Results: Lower levels of connectivity and fewer nearest neighbours negatively impacted abundance, species richness, and composition of dragonflies (p values < 0.01, R² = 0.18-0.70). Adult dragonfly abundance had a stronger positive relationship with connectivity than species richness. In particular, the abundance of adult dragonfly Leucorrhinia frigida, found almost exclusively at natural ponds, had a positive relationship with connectivity. Connectivity and the number of nearest neighbours had no significant impact on damselflies apart from a slight negative relationship between connectivity and species richness (p value = 0.02, R² = 0.11). Natural ponds had significantly higher levels of connectivity when compared to stormwater ponds.

Conclusions: Our results suggest that dragonflies are positively affected by increased connectivity in an urban landscape, with no benefit of connectivity to damselflies at the scale measured. We recommend intentional planning of urban stormwater pond networks, where individual ponds can act as stepping stones, incorporated with strategic inclusion of beneficial land cover types.

Supplementary information: The online version contains supplementary material available at 10.1007/s10980-024-01817-z.

Keywords: Anisoptera; Circuitscape; Dispersal; Fragmentation; Urban ecology; Zygoptera.

Fig. 1

Left: Mean current density map of Ottawa, Ontario, surrounded by buffer with a randomized distribution of resistances and resolution of 10 m. Landcover data were based on a combination of aerial photography and LiDAR imagery (City of Ottawa 2011). Current (i.e., connectivity) was calculated using Circuitscape V 5.0, with 50 nodes in the pairwise, 8 neighbour mode. Higher current values indicate greater connectivity and imply potential for higher use of such habitats by pond odonates. Right: Study area map with satellite imagery. Satellite imagery retrieved from Google Maps on September 13, 2023. Location geocode used was “Ottawa, Ontario”. Both: Geographic coordinate system is WGS84 (epsg: 4326). Locations of natural ponds (n = 8) are denoted by circles (•) and the locations of stormwater ponds (n = 41) are denoted by triangles (▴). Shapes are hollow and overlapping to facilitate seeing sites that are close together, but no ponds are truly overlapping in situ. Figure created in R version 4.1.0 (R Core Team 2020)

Fig. 2

a Relationship between dragonfly (Anisoptera) estimated abundance and mean current density. b Relationship between dragonfly estimated abundance and number of surrounding habitats. c Relationship between dragonfly species richness and mean current density. d Relationship between dragonfly species richness and number of surrounding habitats. Lines and equations represent linear regression. All relationships were significant (p < 0.001). Mean current was calculated by averaging the pixel values within 900 m of every study site and number of nearest neighbours was calculated by tallying the number of other wetlands and stormwater ponds found within 900 m of every study site. Figure created in R version 4.1.0 (R Core Team 2020)

Fig. 3

Transformation based-redundancy analysis (tb-rda), using a Hellinger transformation, of the relationship between adult dragonfly species composition (black vectors without arrows) a mean current at a 900 m scale (mean.nine), b number of nearest neighbours at a 900 m scale (n.nine) at urban ponds (n = 49). Figure created in R version 4.1.0 (R Core Team 2020)

Fig. 4

a Relationship between damselfly (Zygoptera) Shannon diversity and mean current density. b Relationship between damselfy Shannon diversity and number of surrounding habitats. c Relationship between damselfly species richness and mean current density. d Relationship between damselfly species richness and number of surrounding habitats. Lines and equations represent linear regression for statistically significant relationships. Mean current was calculated by averaging the pixel values within 300 m of every study site and number of nearest neighbours was calculated by tallying the number of other wetlands and stormwater ponds found within 300 m of every study site. Figure created in R version 4.1.0 (R Core Team 2020)

Fig. 5

Comparisons between stormwater (n = 41) and natural ponds (n = 8) using a Wilcoxon rank-sum test for all statistically significant explanatory variables. The 900 m scale was used for dragonfly analysis and the 300 m scale was used for the damselfly analysis. Figure created in R version 4.1.0 (R Core Team 2020)