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A Proposed Theoretical Study of the Nature of Amphibian Declines in Relation to Detectability.

Carolyn N.L. Seburn and David C. Seburn

Kemptville, Ontario, Canada, K0G 1J0

Rationale:
Before we finalize broadscale monitoring programmes it is important to ask what an amphibian decline would look like and therefore how will we recognize it when it occurs. Such a question may not be relevant to long-lived, stable, K selected populations but it is relevant to species whose natural fluctuations in population level are legendary, such as many anurans. Preliminary studies have shown that the statistical power of many population studies to detect declines is very low (Reed and Blaustein, 1995). In fact, it may take up to twenty years to detect a 10% per year decline in an intensively studied single population with an natural variability level (McDonald and Brooks, 1996). The population size by this time would average 14% of the original. This is obviously an unacceptable level of detectability. We argue that this approach may not be relevant to the issue of detectability by large scale monitoring programmes but other issues such as the expected shape of a decline function, changes in species distributions and metapopulation structure are relevant. The analysis by Mossman et al. (1995) suggests that for some species a 3% per year decline could be detected in 20 years (at which time the populations would average 54% of its original level). This analysis did not make use of the geographic information available from large scale monitoring programmes.

An Alternate Approach:
We argue based on a visual inspection of a highly fluctuating population function with a 10% per year decline, that the concept of a gradual decline at the population level is operationally meaningless. Because of the masking effects of variability any such decline (independent of cause) would appear catastrophic. That is, there is a finite probability that the population level at a given site can go from above average to zero from one year to the next. This, by the way is also true of anuran populations in the absence of a decline factor, although the probability is less. Since a decline factor increases the probability of catastrophic decline at the population level, decline at the regional level can be detected as an increase in the proportion of empty habitats (the sum of many small catastrophes). The regional pattern of decline would result from an interaction of: variability in habitat quality, metapopulation structure, the cause of decline and stochastic events. To then get the detected pattern of decline we must also superimpose the regional pattern of monitoring effort.

Request for Funding:
We are looking for funding in order develop a more rigorous theory based on this approach. There are several different aspects of this we would like to pursue including modelling the effects of metapopulation structure, variable habitat quality, different causes and monitoring effort. If you are interested in supporting this work and would like a more detailed proposal related to one of these aspects please contact us.

Acknowledgements:
We would like to thank the many participants of past DAPCAN conferences with whom we have discussed several of the concepts that have led to this formulation.

Literature Cited:

McDonald, Cam and Ronald J. Brooks, 1996. A real world look (i.e. by non-statisticians) at statistical power as it applies to amphibian declines. DAPCAN VI, October 5-7, Calgary, Canada.

Mossman, Mike, Paul Rasmussen, John Sauer, Same Droege and Lisa Hartman, 1995. Sample size estimation for amphibian calling surveys and some surprising trends from an 11-year analysis of Wisconsin frog and toad survey data. NAAMP II, September 27- 29, Burlington, Canada.

Reed, Michael J. and Andrew R. Blaustein, 1995. Assessment of "nondeclining" amphibian populations using power analysis. Conservation Biology 9(5):1299-1300.

U.S. Department of the Interior
U.S. Geological Survey
Patuxent Wildlife Research Center
Laurel, MD, USA 20708-4038
http://www.pwrc.usgs.gov/naamp3/naamp3.html
Contact: Sam Droege, email: Sam_Droege@usgs.gov
Last Modified: June 2002