Evaluating range-expansion models for calculating nonnative species' expansion rate

Area of Science:

• Ecology
• Biogeography
• Conservation Biology

Background:

• Species range shifts are accelerating due to environmental change and biological invasions.
• Accurate quantification of range expansion rates is vital for ecological and conservation studies.
• Methodology and sampling bias can significantly influence range expansion rate estimates.

Purpose of the Study:

• To compare different methods for estimating species range expansion rates.
• To assess the sensitivity of these methods to sampling bias.
• To identify the most robust method for range expansion rate estimation.

Main Methods:

• Compared distance-based range statistic models (mean, median, 95th gamma quantile, marginal mean, maximum, conditional maximum) with an area-based grid occupancy method.
• Utilized sampling simulations to evaluate method sensitivity to incomplete sampling.
• Analyzed range expansion rates for Roesel's bush-cricket (Metrioptera roeselii) in south-central Sweden.

Main Results:

• Range expansion estimates clustered into two groups: ~3 km/year from range margin statistics and ~1.5 km/year from central tendency and grid occupancy.
• Methods varied significantly in sensitivity to sampling effort, with grid occupancy and median being most sensitive.
• Incomplete sampling generally lowered estimates, except for the gamma quantile, which was slightly higher.

Conclusions:

• Care is needed when interpreting range expansion rates from partially sampled data.
• Methods using range margins yield higher estimates than those using central tendency.
• The gamma quantile method is recommended for its robustness against sampling bias when full distribution sampling is not feasible.