Drivers of arthropod biodiversity in an urban ecosystem

Abstract

Our world is becoming increasingly urbanized with a growing human population concentrated around cities. The expansion of urban areas has important consequences for biodiversity, yet the abiotic drivers of biodiversity in urban ecosystems have not been well characterized for the most diverse group of animals on the planet, arthropods. Given their great diversity, comparatively small home ranges, and ability to disperse, arthropods make an excellent model for studying which factors can most accurately predict urban biodiversity. We assessed the effects of (i) topography (distance to natural areas and to ocean) (ii) abiotic factors (mean annual temperature and diurnal range), and (iii) anthropogenic drivers (land value and amount of impervious surface) on the occurrence of six arthropod groups represented in Malaise trap collections run by the BioSCAN project across the Greater Los Angeles Area. We found striking heterogeneity in responses to all factors both within and between taxonomic groups. Diurnal temperature range had a consistently negative effect on occupancy but this effect was only significant in Phoridae. Anthropogenic drivers had mixed though mostly insignificant effects, as some groups and species were most diverse in highly urbanized areas, while other groups showed suppressed diversity. Only Phoridae was significantly affected by land value, where most species were more likely to occur in areas with lower land value. Los Angeles can support high regional arthropod diversity, but spatial community composition is highly dependent on the taxonomic group.

Figure 1

Spatial interpolation of diversity across the greater Los Angeles area. (a) Projected richness for all combined groups (n = 236 species) is shown using the continuous color map. (b–g) Projected richness is also shown for each individual taxonomic group modelled: Phoridae (n = 108 species), Syrphidae (n = 34 species), Tipuloidea (n = 20 species), Drosophilidae (n = 27 species), Myceotphilidae (n = 23 species), and Araneae (n = 24 species). BioSCAN sampling sites are overlaid with shapes displaying the collection phase (year of sampling). Across groups, total richness is not obviously driven by any single predictor but rather a result of complex group- and species-specific effects. The extent of the spatial map is based on (1) available land value data, which is confined to the Los Angeles and San Bernardino Counties and (2) the bounding of every predictor to 3 standard deviations away from the mean value present at BioSCAN sampling sites. Grid cells are at a 1 km resolution. R packages ggplot2, ggmap, and basemaps were used to create this map. Map tiles by Stamen Design, under CC BY 4.0. Data by OpenStreetMap, under ODbL.

Figure 2

Across-group predictors of occupancy across environmental variables. (a) Distance to Natural Area. (b) Distance to Ocean. (c) Temperature. (d) Diurnal Range. (e) Impervious Surface. (f) Land Value. The six colors displayed in the plots represent different arthropod groups: Araneae (dark blue), Drosophilidae (dark turquoise), Mycetophilidae (yellow), Phoridae (yellow–orange), Syrphidae (orange), and Tipuloidea (red). There are 24, 27, 23, 108, 34, and 20 species in Araneae, Drosophilidae, Mycetophilidae, Phoridae, Syrphidae, Tipuloidea, respectively. Each colored point represent a species from an arthropod group with the same color. The grey horizontal line at y = 0 represents no significant trend. The black point represents the average trend of an arthropod group. The black vertical line represents the 95% confidence interval of the average group trend. Stars above each group represent group-level 95% confidence intervals that do not overlap with zero. Note that the effects of land value and impervious surface are measured within a 0.25 km radius for Phoridae and Araneae, and 0.5 km for Drosophilidae, Syrphidae, Tipuloida and Mycetophilidae. Araneae and Mycetophilidae have different y-axis values for some predictors.

Figure 3

Within-group predictors of occupancy in Phoridae. 108 species are represented in this group. Each plot is showing the single effect of an environmental variable by keeping the other environmental variables in the same model at their average value from the sampling data. (a) Distance to Natural Area. (b) Distance to Ocean. (c) Temperature. (d) Diurnal Range. (e) Percentage of Impervious Surface. (f) Land Value. Each line represents the effect of the environmental variable on an individual species. Red lines indicate a statistically significant negative relationship between the occupancy of a species and an environmental variable, blue lines are statistically significant positive relationships, and grey lines indicate non-significant relationships.