The range map in a field guide looks authoritative, but it describes where a species was recorded during a past compilation. Distributions are moving faster than maps are redrawn.
Maps are compiled from records, not surveys
Most published ranges are assembled from accumulated observations, museum specimens and survey reports rather than from systematic coverage of every area.
The boundary drawn is therefore an interpolation between known points, smoothed into a continuous shape that implies more certainty than the underlying data supports.
Gaps in the map often indicate places nobody has looked rather than places the species is absent. Effort is unevenly distributed toward roads, refuges and university field stations, and the map inherits that pattern.
Detection at a new edge takes years
A species expanding into new country arrives at low density, and low-density populations are hard to detect without targeted effort.
The first records tend to come from areas with many observers, which biases the apparent direction of expansion toward populated regions.
By the time enough records accumulate to justify redrawing a boundary, the leading edge has usually moved further. Compilers also wait for confirmation of breeding rather than mere presence, which adds seasons to the delay.
Contraction is even slower to register
Proving a species has left an area requires repeated surveys finding nothing, which is expensive and produces results nobody is eager to publish.
Range maps consequently retain trailing edges long after populations there have thinned or disappeared, overstating current distribution.
This asymmetry means maps tend to show ranges as larger and more continuous than the animals occupy. A shaded block can conceal a distribution that has broken into isolated fragments with no connection between them.
Regulation inherits the lag
Permitting, survey requirements and habitat designations frequently reference published range maps to decide where a species must be considered.
A project outside the mapped range may face no survey requirement even where the species has recently moved in, and the reverse also occurs.
Updating official maps involves administrative process, so the regulatory version lags the scientific one, which already lags reality.
Modeled distributions carry different problems
Statistical models predict suitable conditions from occurrence records and environmental variables, producing maps that can be updated as conditions change.
These show where a species could live rather than where it does, and suitability does not account for barriers, competitors or whether animals have arrived.
Used carefully the two approaches complement each other, with models suggesting where to look and records confirming what is there.