Temperature accounts for the biodiversity of a hyperdiverse group of insects in urban Los Angeles

Abstract

The urban heat island effect is a worldwide phenomenon that has been linked to species distributions and abundances in cities. However, effects of urban heat on biotic communities are nearly impossible to disentangle from effects of land cover in most cases because hotter urban sites also have less vegetation and more impervious surfaces than cooler sites within cities. We sampled phorid flies, one of the largest, most biologically diverse families of true flies (Insecta: Diptera: Phoridae), at 30 sites distributed within the central Los Angeles Basin, where we found that temperature and the density of urban land cover are decoupled. Abundance, richness, and community composition of phorids inside urban Los Angeles were most parsimoniously accounted for by mean air temperature in the week preceding sampling. Sites with intermediate mean temperatures had more phorid fly individuals and higher richness. Communities were more even at urban sites with lower minimum temperatures and sites located further away from natural areas, suggesting that communities separated from natural source populations may be more homogenized. Species composition was best explained by minimum temperature. Inasmuch as warmer areas within cities can predict future effects of climate change, phorid fly communities are likely to shift nonlinearly under future climates in more natural areas. Exhaustive surveys of biotic communities within cities, such as the one we describe here, can provide baselines for determining the effects of urban and global climate warming as they intensify.

Keywords: climate change; phorid fly; species richness; urban heat island.

Figure 1.

Map of BioSCAN sites where phorid flies and climatic variables were sampled. The dot size represents the mean daily phorid fly species caught in traps, and colour represents mean annual air temperatures. (Online version in colour.)

Figure 2.

Thermal responses to landscape characteristics. The mean annual air temperature is not associated with (a) impervious surface, nor with (b) elevation. (Online version in colour.)

Figure 3.

Phorid fly abundance and richness responses to temperature. (a) Total abundance of phorid flies caught in each trap per sampling period, (b) abundance of the most prevalent species, M. agaraci, and (c) total species richness per sampling period. The x-axis represents the mean air temperature the week prior to sampling, and regression lines represent best fits. (Online version in colour.)

Figure 4.

NMDS of phorid fly communities from urban Los Angeles backyard sampling stations. The species are superimposed with environmental fit vectors for minimum air temperature (r² = 0.26, p = 0.02), normalized difference vegetation index within a 50 m radius of the site (NDVI; r² = 0.03, p = 0.65), distance to the nearest natural area (r² = 0.01, p = 0.92), relative humidity (r² = 0.04, p = 0.63), latitude (r² = 0.04, p = 0.61), and longitude (r² = 0.04, p = 0.56). Most common species names are plotted (with captures greater than 500 individuals). NMDS 2D stress = 0.13. (Online version in colour.)