• Field sampling is biased against small‑ranged species of high conservation value: a case study on the sphingid moths of East Africa

      Beck, J; Takano, H; Ballesteros-Mejia, L; Kitching, I; McCain, CM (Springer Verlag, 2018-08-23)
      The range size of species co-occurring in local assemblages is a pivotal variable in assessments of a site’s conservation value. Assemblages featuring many small-ranged species are given more priority than assemblages consisting mainly of wide-ranging species. However, the assembly of relevant information can be challenging and local range size distributions of tropical invertebrates are rarely available for conservation planning. We present such data for sphingid moths in East Africa, a highly diverse region of high conservation value. We compare geographic range size distributions based on field samples with predictions from modelled range map data. Using this system as a case study, we provide evidence for a systematic sampling bias when inferring average local range sizes from field data. Unseen species (i.e., species present but missed in local sampling) are often those with small ranges (hence, of high conservation value). Using an elevational gradient, we illustrate how this bias can lead to false, counterintuitive assessments of environmental effects on local range size distributions. Furthermore, with particular reference to sphingid moths in the study region, we show that current protected areas appear unrelated to the spatial distribution of species richness or average geographic range sizes at a local scale. We discuss the need to treat field sampled data with caution and in concert with other data sources such as probabilistic models.
    • How climatic variability is linked to the spatial distribution of range sizes: seasonality versus climate change velocity in sphingid moths

      Grünig, M; Beerli, N; Ballesteros-Mejia, L; Kitching, I; Beck, J (Wiley, 2017-07-04)
      Aim To map the spatial variation of range sizes within sphingid moths, and to test hypotheses on its environmental control. In particular, we investigate effects of climate change velocity since the Pleistocene and the mid‐Holocene, temperature and precipitation seasonality, topography, Pleistocene ice cover, and available land area. Location Old World and Australasia, excluding smaller islands. Methods We used fine‐grained range maps (based on expert‐edited distribution modelling) for all 972 sphingid moth species in the research region and calculated, at a grain size of 100 km, the median of range sizes of all species that co‐occur in a pixel. Climate, topography and Pleistocene ice cover data were taken from publicly available sources. We calculated climate change velocities (CCV) for the last 21 kyr as well as 6 kyr. We compared the effects of seasonality and CCV on median range sizes with spatially explicit models while accounting for effects of elevation range, glaciation history and available land area. Results Range sizes show a clear spatial pattern, with highest median values in deserts and arctic regions and lowest values in isolated tropical regions. Range sizes were only weakly related to absolute latitude (predicted by Rapoport's effect), but there was a strong north‐south pattern of range size decline. Temperature seasonality emerged as the strongest environmental correlate of median range size, in univariate as well as multivariate models, whereas effects of CCV were weak and unstable for both time periods. These results were robust to variations in the parameters in alternative analyses, among them multivariate CCV. Main conclusions Temperature seasonality is a strong correlate of spatial range size variation, while effects of longer‐term temperature change, as captured by CCV, received much weaker support.
    • How has the environment shaped geographical patterns of insect body sizes? A test of hypotheses using sphingid moths.

      Beerli, N; Bärtschi, F; Kitching, IJ; Ballesteros-Mejia, L; Beck, J (Wiley, 2019-05-14)
      Aim: We mapped the geographical pattern of body sizes in sphingid moths and investigated latitudinal clines. We tested hypotheses concerning their possible environmental control, that is, effects of temperature (negative: temperature size rule or Bergmann's rule; positive: converse Bergmann rule), food availability, robustness to starvation during extreme weather and seasonality. Location: Old World and Australia/Pacific region. Methods: Body size data of 950 sphingid species were compiled and related to their distribution maps. Focusing on body length, we mapped the median and maximum size of all species occurring in 100 km grid cells. In a comparative approach, we tested the predictions from explanatory hypotheses by correlating species' size to the average environmental conditions encountered throughout their range, under univariate and multivariate models. We accounted for phylogeny by stepwise inclusion of phylogenetically informed taxonomic classifications into hierarchical random‐intercept mixed models. Results: Median body sizes showed a distinctive geographical pattern, with large species in the Middle East and the Asian tropics, and smaller species in temperate regions and the Afrotropics. Absolute latitude explained very little body size variation, but there was a latitudinal cline of maximum size. Species' median size was correlated with net primary productivity, supporting the food availability hypothesis, whereas support for other hypotheses was weak. Environmental correlations contributed much less (i.e. <10%) to explaining overall size variation than phylogeny (inclusion of which led to models explaining >70% of variability). Main conclusion: The intuitive impression of larger species in the tropics is shaped by larger size maxima. Median body sizes are only very weakly related to latitude. Most of the geographical variation in body size in sphingid moths is explained by their phylogenetic past. NPP and forest cover correlate positively with the body size, which supports the idea that food availability allowed the evolution of larger sizes.