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

Spatial Sampling Designs
We established study sites based on random
and non-random criteria. We surveyed at 17
randomly located focal-point transects (Fig. 5.1;
see Chapter 1 for additional information). We
established the locations of all other surveys in
areas that we believed would have the highest
species richness or as a matter of convenience
(Figs. 5.1, 5.2). For all survey methods, we
collected data at individual stations or sections,
which we grouped into transects because of
convenience and efficiency. (An alternative
approach would have been to establish
individual stations or sections to maintain greater
independence, but travel time between stations
would have reduced the number of stations that
we were able to visit in a morning.) We placed
our non-random transects along riparian areas and
canyons in low-elevation areas (< 4000 ft); in all
other areas we established non-random transects
along trails because of accessibility and safety.
VCP Surveys
We used the variable circular-plot method (VCP;
Reynolds et al. 1980, Buckland et al. 2001)
to survey for diurnally active birds during the
breeding and spring migration seasons (mid April
through early July). Conceptually, these surveys
are similar to traditional “point counts” (Ralph
et. al 1995) during which an observer spends a
standardized length of time at one location (i.e.,
46
station) and records all birds seen or heard and
the distance to each bird or group of birds.
We used three types of VCP surveys
(Table 5.1). Methods differed primarily by
the sampling design used to establish their
location and by the number of visits (see Table
5.1 for additional information). The following
description of our survey protocol applies to all
VCP methods unless otherwise noted. We located
stations a minimum of 250 m apart to maintain
independence among observations. On each
successive visit to a transect we alternated the
order in which we surveyed stations to minimize
bias by time of day or direction of travel. We did
not survey when wind exceeded 15 km/h or when
precipitation exceeded an intermittent drizzle.
We attempted to begin surveys approximately 30
minutes before sunrise and conclude surveys no
later than three hours after sunrise.
We recorded a number of environmental
variables at the beginning of each transect: wind
speed (Beaufort scale), presence and severity of
rain (qualitative assessment), air temperature (ºF),
relative humidity (%), and cloud cover (%). After
arriving at a station, we waited one minute before
beginning the count to allow birds to resume
their normal activities. We identified to species
all birds seen or heard during an eight-minute
“active” period (5 minutes at reconnaissance
VCP stations). For each detection we recorded
the distance (in meters) the bird was from the
observer (measured with laser range finder
when possible), time of detection (measured in
Table 5.1. Characteristics of the three major VCP survey types for birds, Saguaro National Park, Rincon
Mountain District, 2001 and 2002.
VCP survey type
Repeat-visit
Characteristic Random (focal-point transects) Non-random Reconnaissance
Randomly located Yes No No
Number of visits per year 4 >2 1
Number of stations 4 variable variable
Count duration at each station 8 minutes 8 minutes 5 minutes
Advantages Scope of inference to larger area, Flexible, most complete abundance Maximum flexibility, allows for
vegetation data available data for areas with high species rapid inventories and larger spatial
richness, uncommon and rare species coverage, provides good distribution
are often accounted for
data
Disadvantages Inefficient for developing No spatial inference beyond transect Species lists are less complete,
complete species list, transects
because uncommon and rare
are often in areas of low species
richness
species may be missed