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Not only did the point-stop transects return more calls, but also “higher quality” calls that were more confidently ID’d to species.
Over 125 total bat passes recorded during four driving transects. (Blue bars represent transects that began in the farmlands and ended in the pine forest (Red bars represent transects that began in the forest and ended in the farmlands)
At our California “acoustic monitoring” workshop in 2010 we tested the efficiency nd results of using both the continuous, and the point-stop method during a straight-line transect from our lodge in an agricultural area, thru Lava Beds National monument where we expected a great deal of bat roosting and commuting behavior, and into the Modoc National Forest where we expected to find bat foraging areas. The entire length of the transect was 20-miles. We had enough equipment to deploy four vehicles, two starting at the lodge and two starting in the forest. One car in each location drove continuously at 20mph while the other stopped at 10 pre-determined locations and monitored for 6 minutes, for a total of 1-hour of monitoring time. Each vehicle was outfitted with two AnaBat SD1 units and two Pettersson D240x units recording to Zoom MP3 players.
Each type of transect has advantages and disadvantages.
Point Counts, where the area is sampled at discrete points along a transect and no monitoring is done inbetween selected points.
These are easier to do with walking transects where trails or overland routes do not lend themselves well to both walking and paying attention to making bat recordings.
Monitoring foraging areas can be carried out using two basic techniques, continuous counts of bat passes along randomly placed transect lines of fixed or variable length, or counts of bat passes for a discrete time period at a fixed number of spots spaced systematically along rand omly placed transect lines.
Some of the folks that have done the most acoustic monitoring are the Brits. They’ve spent years perfecting their protocols and have a great set of long-term data. They have a “National Bat Monitoring Programme” using acoustic, capture, and roost count inventories. The NBMP employs both methods. A 1-km2 area is the basic sampling unit for NBMP field surveys.
This is because 1-km2 areas are easily surveyed within a single evening, and they integrate with a land classification scheme developed by the Institute of Terrestrial Ecology.
A transect should cover as much variability in habitat as the area allows, but it should not be too long that temporal variation in bat activity over the night affects “captures.”
Very long straight line transects can be conducted simultaneously with observers starting at each end and working towards each other to help adjust for this. Some researchers select for straight-line transects, but consider the efficiency of a loop, especially for driving transects.
One problem with bat detector use is the relative expense of the technique compared to the amount of area it can cover. One way to maximize area covered per dollar spent is to move the detectors through time and space utilizing mobile transect. Two sampling methods have been employed to infer habitat use by bats.
Different habitat types can be sampled actively as an observer moves along a transect. To account for temporal variation in bat activity, each transect is sampled during different periods of the night. However, the majority of the studies designed to examine habitat use have employed passive monitoring.
In these studies a detector is randomly placed in each habitat type being sampled, and activity is compared among habitat types to infer habitat use. Due to considerable among-night variation in activity, sampling should be done simultaneously to increase comparability.
A “transect” is simply an arbitrary line throughout a habitat where continuous or point-count measurements of species occupancy can be conducted. Using bat detectors on transects allow us to estimate and monitor bat abundance and/or species abundance at single locations and multiple locations throughout a habitat. This is of interest to us, because it allows us to do a lot of sampling, with a lot less effort than physical capture would take in the same habitat. By moving continuously through a habitat and recording bat calls we can be assured that each call recorded represents a different individual, since it is unlikely in the extreme that a single bat will be following us along the entire transect, or portions of a transect. By stopping periodically on our transect line and monitoring for a pre-determined amount of time, we can cover a lot of ground and perhaps intercept bats and record longer file sequences that could be more useful for species ID.
Before we plan our transect, we have to decide what types of questions about our habitat we would like to answer. Does a given species use one study area more than another? Are there relatively more individuals of a given species in specific habitats? Does the relative abundance of a species using a study area differ over time?
Bat Acoustic Monitoring:
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