We collected information on the following twelve variables using the taxonomy
of Frost (2004) throughout:
Geographic ranges (in km2) and aquatic life-stage data came from the GAA
(IUCN et al. 2004) database. A species was classified as aquatic life-stage if
it relied on freshwater for any stage (e.g. reproduction, egg deposition,
development) of its life cycle.
Mean snout-vent length (mm) was used as a measure of species body size,
and mean clutch size (eggs per clutch) was taken as a measure of a species’
speed of life history. Where we had more than one value of a variable for a
species, we used the mean. In order to investigate the role of a species’
environment on RD status, we obtained representative values for a species
using geographic distribution data (IUCN et al. 2004) and spatial datasets of
altitude (m) (Hijmans et al. 2005), annual actual evapotranspiration (AET)
(mm) (Willmott 2001), net primary productivity (NPP) (mm) (Willmott 2001),
isothermality (a measure of annual temperature consistency), maximum
temperature of the warmest month, precipitation seasonality, and precipitation
in the driest quarter (Hijmans et al. 2005). To examine whether human
impacts played a significant role in RDs, we used a map of human population
density (people/km2) (CIESIN. 2000). To extract the spatial data required
from the GIS data layers, we overlaid shape files of species geographic
ranges on a 30-second grid and calculated the median grid cell value within
the geographic range.
We transformed explanatory data to meet requirements of normality where
necessary: geographic range (km2), snout-vent length (mm), eggs per clutch,
altitude, and human population density were all log transformed, and
precipitation of the driest quarter was square-root transformed; other variables
remained untransformed.
Description of data collection and sources used
Data were collected from a range of sources, including peer-reviewed
literature, grey literature, field guides, web sites, and direct contact with GAA
Red List species assessors (a full list of sources used is included in appendix
B). When data were collected for a species in a given geographic location,
we collected data from species of a range of statuses (i.e. non threatened,
threatened, and RD) from that location in order to reduce the probability that
our results would merely reflect geographic heterogeneity in threat intensity.
We biased our data collection towards RD species in order to maximise power
to test our hypotheses. The percentage of RD species in the final dataset
(41.0%, 227/553) was therefore much higher than the percentage of RD
species in all anuran species (7.9%, 398/5066). Of the 227 RD species in the
dataset, 15 (6.6%) were over-exploited, 77 (33.9%) were threatened by
habitat reduction, and 134 (59.0%) were enigmatic decline species (c.f. 11%,
42.1%, and 47.6% respectively of RD species as a whole). One species was
threatened by both over-exploitation and enigmatic decline. Of the 326 nonRD species in the dataset, 194 (59.5%) were non-threatened, and 132
(40.5%) were threatened (c.f. 57.8% non-threatened, 42.2% threatened
respectively in anurans as a whole after exclusion of DD species).
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