Rlh. Dennis et Pb. Hardy, Targeting squares far survey: predicting species richness and incidence ofspecies for a butterfly atlas, GLOBAL EC B, 8(6), 1999, pp. 443-454
1. Predictions on species richness and incidence of species are made using
data for three scales of mapping from the Greater Manchester butterfly atla
s; for the whole of the conurbation (2 km x 2 km scale) and for two sample
areas centred on the Mersey Valley (1 km x 1 km scale; 1 ha scale). Predict
ions are based on data for recording effort, altitude, biotopes, host-plant
s and nectar resources.
2. Data for Greater Manchester indicate that substantial shortfalls may occ
ur in recording butterfly species for atlases despite the fact that butterf
lies are generally easily identified and well supported with recorders. Sho
rtfalls lend to be larger for species with fewer records, indicating that s
ome species may be more easily overlooked than others.
3. The results demonstrate that targeting squares for re-survey is necessar
y and feasible. The predictions have other valuable research applications,
the most important of which is being able to assess the accuracy of distrib
ution maps, to correct them, and to make projections of distribution change
s.
4. Predictions may be enhanced by improvements to mapping in three ways: (i
) Collecting data on recording effort. Variation in recording effort typica
lly accounts for differences in species richness and incidence of species m
ore than any other variable; (ii) Collecting data on biotopes and specific
resources. The present results are promising and demonstrate that the colle
ction of environmental data linked to a suitable sampling Frame could facil
itate knowledge of the distribution of species over extensive areas that re
main under-recorded; and (iii) Distinguishing between breeding individuals
and vagrants. Vagrancy is a problem associated both with species and scale.
Although species vary substantially in their capacity to migrate beyond th
eir habitats, the effect of vagrancy on distribution maps becomes an increa
singly large problem as the grain of mapping (size of recording units) decr
eases. It is suggested that over-recording can be a problem, particularly w
hen mapping is fine-grained.