Vascular Plant and Vertebrate Inventory of Saguaro ... - USGS

features. We based extensive surveys on visual
encounters (Crump and Scott 1994) and, in
contrast to intensive surveys, did not constrain
surveys by area or time. We focused extensive
surveys during mornings and also surveyed
during evenings and nights in low-elevation areas
when detectability of snakes and amphibians is
often highest (Ivanyi et al. 2000), and during midday
at higher elevations.
Field Methods
We selected areas randomly and non-randomly
(Table 4.1). We placed random survey areas
within approximately 1 to 2 km of focal point
transects, and surveyed each area once. We
selected non-random areas by using topographic
maps and prior knowledge of the district. We
relied upon visual detection and often looked
under objects and illuminated cracks to detect
hidden individuals. We surveyed in spring (4
April – 24 May) and summer (25 June – 20
September) of 2001 and 2002. One, two, or three
observers searched each area simultaneously
and recorded data separately. Total duration of
surveys among all observers combined averaged
5.5 ± 0.4 (± SE) hours per survey (range = 1.2
- 20.4 hours). We recorded data using similar
methods as intensive surveys and noted UTM
coordinates and elevation at the start and end
points of each survey.
Effort
We surveyed 85 areas in 2001 and 2002 (Fig.
4.2), 94.1% of which were surveyed in 2001
(Table 4.2). Total survey effort was 465.2 hours,
81% of which was in non-random areas. Survey
effort was roughly three times greater than
for other methods and focused mainly during
daylight except at lower elevations where we also
surveyed during late evenings and nights. We did
not survey higher elevation areas in late evenings
and at night because detectability declined
markedly with elevation.
Analysis
We calculated relative abundance for each area
as the number of individuals detected for each
species or all species combined per 10 hours of
effort. For surveys completed by >1 observer
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per area, we summed survey times and detection
data for all surveyors when calculating effort and
relative abundance for an area. Although some
locations were surveyed multiple times, survey
routes often varied and we therefore considered
each survey an independent sample despite some
spatial overlap. To describe general patterns of
relative abundance for species groups (lizards,
snakes, and amphibians) and species richness
across the district, we post-stratified survey
areas by elevation (low = 6,000 feet) using the
median elevation of all animal observations for
each survey. We then tested for variation among
strata using one-, two-, or multi-way ANOVA.
Because relative abundance and species richness
varied between day and night and no areas
were surveyed during night at middle and high
elevations, we limited comparisons only to
days. To describe patterns of relative abundance
of individual species across elevation, we used
multiple linear regression. We transformed
relative abundance values when necessary
using log(x) or log(x + 1) to improve normality.
Because patterns of relative abundance often
varied with relative humidity (or cloud cover),
season, and time of day, we adjusted for these
factors when they explained variation (P ≤
0.10) in relative abundance. To describe cloud
cover, relative humidity, and temperature for
each area, we averaged measurements taken at
the beginning and end of each survey. To adjust
for temporal variation in relative abundance
and richness across time of day, we considered
three time periods: day, late evening or night, or
surveys that spanned portions of both periods
(day equaled reference level). We considered
20-min before local sunset time as the cut-point
between day and late evening or night surveys.
To adjust for seasonal variation in relative
abundance and richness, we considered two
seasons, spring and summer (spring equaled
reference level). Because relative humidity and
cloud cover were strongly correlated (r = 0.76,
P < 0.0001) we only adjusted for the factor that
explained the most variation in responses.