is labor intensive. The largest atlases so far attempted have covered Australia,
6 countries of Southern Africa and Western Europe. A sampling approach would
make possible the mapping of large areas of the species-rich tropics, even where
the density of potential recorders is not high.
An example of an atlas based on sampling has been produced for the
United States (Price, Droege, and Price 1995). In this case, the fieldwork was not
designed to produce an atlas but was the Breeding Bird Survey, which is designed
for population monitoring. Sample units consist of routes of 20 roadside point
counts. The distribution of survey routes is designed within a bioregional frame-
work but also allows for differing numbers of contributors in different parts
of the country. Distribution maps showing relative abundance were generated by
interpolation. There are several ways of inferring distributions from individual
locality records or from the rough maps in field guides and knowledge of
habitats. At atlas of the distribution of larger African mammals is an excellent
example (Boitani et al. 1999).
1.7 Estimating species richness
Species diversity is a common focus of study. Notwithstanding the ranges of
indices available, the primary dimension of species diversity is the single figure
of the number of species occurring in an area—species richness. The critical
question is how to tell when to stop looking for more. At a first visit, all the
species listed will be new. Over time, the number of new ones gradually dimin-
ishes. A plot of total species seen against accumulated effort will rise at an ever-
decreasing rate until reaches an asymptote. The full list is practically impossible
to achieve. Even after a 100 years of very intensive searching, the list of birds that
occur in Britain is not complete. One or two new ones are still added in most
years though of course these are vagrants of highly irregular occurrence and
10 |Bird diversity survey methods
Table 1.3Examples of the range of units used to standardize effort in different
atlas projects
Southern Africa Checklists for any time period between 1 day and 1 month
(Harrison et al. 1997) (but mainly 1 day) within a quarter degree cell
UK (Gibbons, Reid, and Species list from 2 h in a 2 km^2. At least 8 of the 25 such
Chapman 1993) squares were targeted within a 10 km^2
Australia 20 min counts within 2 ha (preferred method) or: up to
http://www.birdsaustralia. 5 km from a central point in a time over 20 min but
com.au/atlas/index.html no more than 1 week