Abstract
In species distribution analyses, environmental predictors and distribution data for large spatial extents are often available in long-lat format, such as degree raster grids. Long-lat projections suffer from unequal cell sizes, as a degree of longitude decreases in length from approximately 110 km at the equator to 0 km at the poles. Here we investigate whether long-lat and equal-area projections yield similar model parameter estimates, or result in a consistent bias. We analyzed the environmental effects on the distribution of 12 ungulate species with a northern distribution, as models for these species should display the strongest effect of projectional distortion. Additionally we choose four species with entirely continental distributions to investigate the effect of incomplete cell coverage at the coast. We expected that including model weights proportional to the actual cell area should compensate for the observed bias in model coefficients, and similarly that using land coverage of a cell should decrease bias in species with coastal distribution. As anticipated, model coefficients were different between long-lat and equal-area projections. Having progressively smaller and a higher number of cells with increasing latitude influenced the importance of parameters in models, increased the sample size for the northernmost parts of species ranges, and reduced the subcell variability of those areas. However, this bias could be largely removed by weighting long-lat cells by the area they cover, and marginally by correcting for land coverage. Overall we found little effect of using long-lat rather than equal-area projections in our analysis. The fitted relationship between environmental parameters and occurrence probability differed only very little between the two projection types. We still recommend using equal-area projections to avoid possible bias. More importantly, our results suggest that the cell area and the proportion of a cell covered by land should be used as a weight when analyzing distribution of terrestrial species.
Highlights
Geographers have devised many different geographical projections to address the challenge of flattening the surface of our 3-dimensional world onto 2-dimensional maps
The question we address in this study is whether such change in cell size matters for the analysis of species distributions
In addition to the two projections, we investigate the use of cell area as regression weight to compensate different cell sizes in the long-lat projection
Summary
When representing data at continental to global scale, the main decision is whether to present a map with straight, parallel meridians and circles of latitude, intersecting each other perpendicularly (e.g., Mercator projection), or with curved lines. The former, “long-lat” projections yield world maps appealing to the human eye for their plane appearance; the areas they depict are distorted, and more so toward the poles (Mulcahy and Clarke 2001). The extreme opposite uses equal-area projections, for which every cm on the map covers the same area on the globe, but they do so at the expense of distorting circles of latitude and longitude (Fig. 1). Each projection can be transformed into each other, and the geographical coordinates associated, for example, with species locations can be displayed on any projection
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