Effect of sampling effort and species detectability on volunteer based anuran monitoring programs
Introduction
Reports of amphibian declines (e.g. Pechman et al., 1991; Wake, 1991) have persuaded both Canadian and American government agencies to implement programs to monitor amphibian populations. In recent years, geographically extensive anuran surveys have been used to determine if amphibian populations are declining (Bishop et al., 1997; Lepage et al., 1997). Many of these surveys are based primarily or solely upon volunteers to collect the data, and involve monitoring the calls of adult male anurans. Although much of the focus has centred on the use of these surveys for site occupancy, calling codes (an index of population size) and counts of calling males (a direct measure of population size) are generally also collected. Currently, many volunteer-based acoustic anuran surveys are active in Canada and the United States, most of which are funded, run by, or modeled after the North American Amphibian Monitoring Program (NAAMP), Environment Canada, and Frogwatch USA. Call surveys are also used in non-volunteer programs (e.g., Amphibian Research and Monitoring Initiative [ARMI]), as well as university funded projects.
Is the sampling effort of anuran call surveys sufficient to represent the anuran community? This question is central to the utility of acoustic surveys for monitoring population trends; unfortunately, instances of insufficient or inappropriate sampling effort have been well documented. Generally, monitoring programs are designed with the intent of sampling during peak calling times for anurans; nevertheless, success has been mixed. Bridges and Dorcas (2000) found that the typical timing of anuran surveys in South Carolina occurred when southern leopard frogs (Rana sphenocephala) had a small probability of calling. Crouch and Paton (2002) found that not all anuran species were detected using acoustic surveys, and that four sampling periods were necessary to monitor the seven species that were detected. Many acoustic survey programs have three sampling periods, not four, although the NAAMP protocol includes three seasonal, but also allows for an optional earlier sampling period to target wood frogs. Paszkowski et al. (2002) found that sampling effort of acoustic surveys was too low when they evaluated different survey techniques, as they sampled after the peak calling times for some species. Detection probabilities for many species can be low, which may underestimate site occupancy (Storfer, 2003). However, probability detection can be incorporated into models estimating temporal changes in anuran occupancy rates (MacKenzie et al., 2002). Although the problem of call detection is becoming well recognized (Bailey et al., 2002; Genet and Sargent, 2003; Weir et al., 2003), it is doubtful that monitoring programs have been designed with respect to this issue.
MacKenzie et al. (2002) described an approach to compare temporal changes in site occupancy when the probability of detection is as low as 30%. Currently, at least two amphibian monitoring programs are using this approach to examine differences in amphibian occupancy in light of suboptimal detection probabilities (Genet and Sargent, 2003; Weir et al., 2003). While we have no objections with statistically adjusting the data to account for detection probabilities of less than one, knowledge of detection rates does not aid in determining whether or not a species is present at any particular sites. Surveys can only accurately determine species presence or absence by assuring that the sampling effort is sufficiently high so that nondetection ceases to be a major issue.
We evaluated the sampling effort of two acoustic anuran monitoring programs: the Ontario Backyard Frog survey (1992–2001) by Environment Canada, and the Bait Frog (northern leopard frog; Rana pipiens) survey, 2001–2003 by the Ontario Ministry of Natural Resources (OMNR). Previous work has shown that volunteer observers generally correctly identify species by their call (Genet and Sargent, 2003; Shirose et al., 1997). Call intensity and count data are generally unsuitable for monitoring small areas or routes or for rarer species, because the combination of small sample size with large sampling error would reduce the precision of estimates of relative abundance (Shirose et al., In review), and so reduce power to unsuitable levels. Volunteers also had difficulty in estimating calling intensity (Genet and Sargent, 2003). Thus, we focused only on species richness and species detectability; specifically, we determined the sampling effort required for the Ontario Backyard Frog survey to adequately estimate species richness, and the relative detectability of different species. Lastly, we demonstrate using the Bait Frog survey of the OMNR how differences in detectability may give false significant differences in site occupancy among years.
Section snippets
Survey methods
The Ontario Backyard Frog survey is an atypical anuran monitoring program; volunteers surveyed only one location that they selected, usually near their residence, which was potentially sampled daily throughout the spring and summer months. All volunteers were familiarized with anuran calls through the use of audiotapes. Volunteers listened for three minutes, usually just after sunset, and recorded the species calling, the calling code (see below), the number of individuals calling, as well as
Species richness (Backyard Frog survey)
The proportion of species found varied among locations (F[31,60] = 30.02, p < 0.0001) and the number of survey nights (F[9,60] = 959.45, p < 0.0001). With only one sampling night, the average species richness was only 25.1% of the total, but reached an average of 80% of the total species richness by approximately 12 sampling nights, and 90% by approximately 24 sampling nights (Fig. 1). However, there was no interaction between the location and sample size (F[279,60] = 0.0014, p = 0.9958), indicating that
Discussion
Detectability affects estimates of anuran species richness and site occupancy for most, and potentially all, anuran acoustic surveys, and sampling effort is clearly an issue for numerous taxa regardless of the survey method used (Metcalfe-Smith et al., 2000; Patton et al., 2000; Walther and Martin, 2001). The Backyard Frog survey likely produces higher species detection probabilities compared to other acoustic surveys, as some Backyard volunteers sample for more than 100 nights in a given year
Acknowledgements
Many volunteers were invaluable for the collection of the Ontario Backyard Frog survey data, and we thank them for their time and diligence. Michael Berrill, Stephen Hecnar, and Frederick Schueler gave valuable advice in designing the Ontario Bait Frog survey, and Tana McDaniel and Neil Osborne aided in collecting the data.
References (32)
- Bailey, L.L., Simons, T.R., Pollock, K.H., 2002. Estimating temporary emigration and detectability in plethodon...
- et al.
An approach to estimate probability of presence and richness of fish species
Transactions of the American Fisheries Society
(2001) - Bishop, C.A., Pettit, K.E., Gartshore, M.E., MacLeod, D.A. 1997. Extensive monitoring of anuran populations using call...
- et al.
Estimating species richness: The importance of heterogeneity in species detectability
Ecology
(1998) - et al.
Temporal variation in anuran calling behavior: implications for surveys and monitoring programs
Copeia
(2000) - et al.
Line transect sampling of Karner blue butterflies (Lycaeides melissa samuelis)
Environmental and Ecological Statistics
(1998) - et al.
Using egg-mass counts to monitor wood frog populations
Wildlife Society Bulletin
(2000) - et al.
Assessing the use of call surveys to monitor breeding anurans in Rhode Island
Journal of Herpetology
(2002) - et al.
Evaluation of methods and data quality from a volunteer-based amphibian call survey
Wildlife Society Bulletin
(2003) - et al.
Perceptions of species abundance, distribution, and diversity: lessons from four decades of sampling on a government-managed reserve
Environmental Management
(1997)
Quantifying biodiversity: procedures and pitfalls in the measurement and comparison of species richness
Ecology Letters
On methods in the analysis of profile data
Psychometrika
Absent or undetected? Effects of non-detection of species occurrence on wildlife-habitat models
Biological Conservation
Sampling plethodontid salamanders: sources of variability
Journal of Wildlife Management
How should detection probability be incorporated into estimates of relative abundance?
Ecology
Cited by (88)
Improving sustainability of long-term amphibian monitoring: The value of collaboration and community science for indicator species management
2022, Ecological IndicatorsCitation Excerpt :With careful foresight, planning, and protocol development volunteer data rigor can approximate the quality obtained through expert data collection (Danielsen et al., 2005). Volunteer recruitment and retention across years is also a challenge (Cohn, 2008; de Solla et al., 2005; Dickinson et al., 2010; Fitzpatrick et al., 2009; Galloway et al., 2006). Projects should therefore also provide training that allows volunteers to feel confident and capable, offer an experience that is rewarding and enjoyable, be advertised through diverse networks, and remain relevant to interested groups to sustain attention and turnout (Miyoko et al., 2012).
Habitat use and spatial niche overlap of sympatric savannah tortoises at multiple spatial scales in South Sudan
2020, Journal of Arid EnvironmentsSurveying frogs from the bellies of their parasites: Invertebrate-derived DNA as a novel survey method for frogs
2020, Global Ecology and ConservationThe quality and contribution of volunteer collected animal vehicle collision data in ecological research
2019, Ecological Indicators