Evaluating alternative study designs for optimal sampling of species’ climatic niches

Ecologists have traditionally studied intraspecific variation by sampling
species across their geographic ranges. However, whether this classic
approach produces samples that accurately represent species’ climatic
niches is largely unknown. Alternative, niche-based study designs using
species’ climatic niches to inform sampling locations should more
efficiently and completely capture the breadth of the niche, but the
magnitude of this difference and how it may vary is unclear. Here we use
conifers as a model system to explore these issues and reach specific
recommendations for future sampling designs. Using an independent dataset
of high-quality species’ occurrences, we first show that recent
publications examining variation across geographic space do a poor job of
capturing the full breadth of species’ niches, such that on average, only
22% of species’ niche space was sampled. This was also true of a large
compiled database, the International Tree-Ring Data Bank (ITRDB), which
yielded average niche coverage of only 45%. Finally, we simulated common
sampling designs (i.e., random points, grids, and transects) in both
geographic and niche-based sampling frameworks. Using two sampling
metrics, niche coverage and niche undersampling, we measured how
completely and evenly these simulated studies characterized the niches of
64 North American conifers. Niche-based sampling better represented
species’ niches than geographic sampling, with the magnitude of this
difference depending on study design and sample size. Niche-based gridded
study designs achieved the most complete sampling at all but the smallest
sample sizes, covering ~15-25% more of a species’ niche than similar
designs implemented geographically. With fewer than 10 samples, however,
all study designs performed poorly, and niche-based transects achieved
slightly higher niche coverage. Consequently, when more than a handful of
samples are collected, we recommend that studies seeking to characterize
variation across a species’ niche consider using a gridded study design
implemented in a niche-based sampling framework.