As part of the initiative to restore the Carman Valley Watershed, in the summer of 1997 we began a survey of bat diversity and use of Knutson Meadows, the most severely impacted portion of the Watershed. The overall goal of this project is to provide baseline data for monitoring changes that may occur in bat populations, bat foraging behavior, and bat foraging intensity as a result of the meadow restoration initiative in the Knuthson Meadows of Carman Valley. The Carman Valley meadow restoration project provides an opportunity to track the time course and abundance changes of meadow-using species, bats included, as a result of restoration.
| Why survey bats? | |
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| How are bats surveyed? | ||
| Results to date | ||
| Carman Valley Bat Sampler see and hear some of the bats from Carman Valley | ||
Why survey bats?
Bats have very particular roost and foraging requirements,
and they reproduce slowly, having a single birth per year, typically bearing
a single "pup." Unfortunately, this specialization has resulted in
bat populations declines in many areas that have experienced disturbance. Because
of these strict requirements, however, they serve as useful indicators of ecosystem
health and management. In addition to abundance, bat species diversity is a
positive indicator of overall system diversity and robustness. Bat populations
can also indicate changes in the status of the vegetation communities because
they forage on the insects that depend upon that vegetation. They can thus also
serve as indicators of subtle hydrologic changes that impact vegetation communities. (back
to top)
How are bats surveyed?
Surveying bats necessitates resourceful techniques
because they can typically neither be seen or heard. Bats can be detected by
the echolocation calls they emit to navigate in the dark. For all but a few
bat species, though, these calls are ultrasonic, i.e. above the range of human
hearing, and must be electronically detected or transduced to humanly-audible
sounds.
Bat foraging activity can be correlated with the number of such detections made over time. Simple detectors pick up bats at approximately 20 m; a passing bat is heard to come and go, this event is counted as a "bat pass." By recording passes over time, a temporal activity pattern and an index of abundance is compiled. These data allow comparative activity to be assessed spatially and temporally. In addition, specialized call sequences that indicate insect pursuit are detectable, and these are tallied as an index of feeding activity.
Many bat species can be conveniently identified by their echolocation calls. However, unlike birds, which use their calls to announce themselves, bat calls serve a utility purpose and have no reason (i.e. selective pressure) to be different from other bats. Thus, some species are not easily distinguished by their calls alone and require capture for certain identification (see below). In this survey we used time-expanded full-spectrum audio analysis for acoustic bat identification because it provides the most acoustic information available from the recorded calls because it includes amplitude and harmonic components. Bat calls were acquired using devices that acquire calls at high digital sampling rates, then play them back in a x10 time expansion to render the calls audible; for example a 5 millisecond long 40 kHz call would be transduced to a 50 millisecond long 4 kHz call. We analyzed these time expanded calls using software that converts the audio signals to sonograms displaying time, frequency, and amplitude. We then compared unknown calls with known reference calls, many of which were acquired from light-tagged bats that we had previously captured.
We used mistnet capture of bats to confirm species using the meadow and to complement the acoustic survey. We employed mistnetting sparingly, however, as it is an invasive method that that interferes with bat's foraging activity. Furthermore, mistnetting is often only successful along riparian zones where bats fly low over the water. Mistnets cannot reach the airspace used by many bats, and has negligible success in the open areas around the willow and sage stands where it is more easily detectable by bats. (back to top)
Results to date
As of the October, 2003, we have observed fourteen
bat species in the Knuthson Meadow of Carman Valley (Table 1). This represents
more than half of the bat species in California, and reflects a high degree
of diversity.
| Table 1. Bats observed in Carman Valley as of October, 2003. Key to status: FSC: federal species of concern; CSC: CA species of concern; FSs: US Forest Service sensitive species, BLM: Bureau of Land Management sensitive species, WBWG: priority status as listed by the Western Bat Working Group. | ||
|
common name |
scientific name |
status |
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free-tailed bat |
Tadarida brasiliensis |
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spotted bat |
Euderma maculatum |
FSC, CSC, BLM, WBWG high |
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Townsend's big-eared bat |
Corynorhinus townsendii |
FSC, CSC, FSs, BLM, WBWG high |
|
pallid bat |
Antrozous pallidus |
CSC, FSs , BLM, WBWG high |
|
silver-haired bat |
Lasionycteris noctivagans |
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|
hoary bat |
Lasiurus cinereus |
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red bat |
Lasiurus blossevillii |
WBWG high |
|
big brown bat |
Eptesicus fuscus | |
little brown bat |
Myotis lucifugus | |
|
fringed myotis |
Myotis thysanodes |
FSC, BLM, WBWG high |
|
long-eared myotis |
Myotis evotis |
FSC, BLM |
|
hairy-winged myotis |
Myotis volans |
FSC, BLM, WBWG high |
|
California myotis |
Myotis californicus | |
|
Yuma myotis |
Myotis yumanensis | FSC, BLM |
Plotting a species accumulation curve provides a useful method for both quantifying species richness and assessing the thoroughness of a survey effort (Figure 1.) Species accumulation curves can be used to compare species richness between different communities and between different treatments. This data will help to assess any changes that may occur from the meadow restoration project
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| Figure 1. Bat species accumulation prior to restoration (1997-2001) within the combined existing creek channel and meadow in Knuthson Meadows of Carman Valley. The smooth curve is generated from a regression of randomizing the order of the sampling data, and fitting to a linear dependence model (J. Appl. Ecol. 2000 37:149-158). The initial slope of the curve relates to species richness, and the approach to an asymptote indicates survey completeness (98.4% for this data). |
However, nearly all of these observations were from the existing stream channel along the edge of the meadow, and, in places, through the adjacent ponderosa pines. The meadow restoration plans call for returning the stream flow to the center of the meadow. This would broaden the wetted area, and may increase the effective foraging area for bats. To follow any changes in bat foraging activity and intensity, we have monitored bat foraging activity along the existing creek channel and in the present meadow. These results are shown comparatively in Figure 2 below.
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| Figure 2. Comparative bat pass activity along the existing creek channel and among two vegetation types in the present meadow in Knuthson Meadows of Carman Valley, prior to restoration (1997-2001). Cumulative results of observations from June through October are displayed normalized to time of sunset. Error bars represent standard error of measurement. |
Initial Restoration Effects
Initial results from the first two seasons following completion of restoration work follow.
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| Figure 3. Bat species accumulation prior to restoration (1997-2001) compared with species accumulation following restoration work in Knuthson Meadows of Carman Valley. The steeper initial slope of the linear dependence model curve for the post-restoration data indicates an increased occurrence of diverse species from day to day. This indicates an increased use of the meadow by a variety of bat species on a regular basis following restoration. |
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| Figure 4. Bat activity at water sources prior to restoration (1997-2001) compared with bat pass activity following restoration work in Knuthson Meadows of Carman Valley (2002-2003). Although this data indicates greater relative activity following restoration, this may also result from bats concentrating at a reduced number of water sources following restoration. |
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| Figure 5. Bat activity over sage vegetation prior to restoration (1997-2001) compared with bat pass activity following restoration work in Knuthson Meadows of Carman Valley (2002-2003). This indicates an overall increased use of the meadow by a bats following restoration. These bats were likely flying over the sage vegetation enroute to other parts of the meadow as drier sage vegetation would not be expected to generate as much insect prey as other vegetation types in the meadow. |
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| Figure 6. Bat activity over willow vegetation prior to restoration (1997-2001) compared with bat pass activity following restoration work in Knuthson Meadows of Carman Valley (2002-2003). This indicates an overall increased use of the meadow by a bats following restoration. |
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