Literature Summary #3
Excellent, Ed. 10/10
Ed Pritchard
Balbach, Harold, Guyer, Craig, Styrsky, Jennifer N., and Turkmen, Asuman 2010. The Relationship
Between Burrow Abundance and Area as a Predictor of Gopher Tortoise Population Size. Journal:
Herpetelogica (2011) Vol. 66, 403-410.
H. Balbach is with US Army Engineer Research and Development Center, Construction
Engineering Research Lab in Champaign, IL, USA. C. Guyer and J. N. Styrsky are with
Auburn University’s Department of Biological Sciences in Auburn, AL, USA. A.
Turkmen is also with Auburn University in Auburn, AL, USA but works in the
Department of Mathematics and Statistics.
1) Research problem or question: The particular interest of this study is to
determine how many individuals constitute a population of gopher tortoises and
what sized areas are required by such populations so that animals currently
threatened might be moved to designated reserve areas.
2) Background knowledge leading up to study: Gopher tortoises (Gopherus
polyphemus) are regarded as endangered in South Carolina and Mississippi,
protected in Georgia, and are listed as a federally threatened species in Alabama,
Louisiana, and Florida. Several dangers threaten the survival of the tortoise such
as habitat degradation and destruction from urbanization, mining, and careless use
of herbicides and pesticides. Refined conservation measures are needed as
number of gopher tortoises continue to dwindle and public passion and policy
spreads.
3) Methods used by investigators: Researchers used surveys of burrows of gopher
tortoises from a large sample of areas of differing sizes to test for a relaxation of
the linear relationship between area and number of burrows expected at the edge
of a population of gopher tortoises. Surveys were conducted at 21 study sites in
AL, GA, and MS where burrows were present. For measures of burrow
abundance, some of the data are from sites on which estimates were based on
transect samples; on other sites, complete counts were made.
4) Results: Overall, tortoise burrow abundance increased significantly with area
surveyed. Results included a segmented regression analysis which revealed
among sites surveyed by a comprehensive count of all burrows, a threshold point
occurred at 755.1 ha, indicating a change from a positive linear slope to a slope
that was indistinguishable from zero. This plateau occurred when the population
size equaled 444 burrows, or approximately 240 tortoises (burrow-to-tortoise
correction factor equaled 0.54). Threshold point was not detected in the analysis
of site surveyed by line-transects.
Relationship between area and burrow abundance for sites surveyed by transects (A) and by
comprehensive counts of all burrows (B).
5) Weakness of study: The results of the study highlighted a bias in survey method.
Abundance estimates obtained from transect surveys were greater per unit area
relative to estimates obtained from comprehensive surveys. Furthermore, the
overall relationship between burrow abundance and area was incongruent for the
two survey methods. Burrow abundance estimated by transect surveys increased
linearly with area; in contrast, abundance estimated by comprehensive surveys
increased with area at smaller sites but was independent from area at larger sites.
6) What was learned: Conservation methods and site selection for reserves must be
re-evaluated and revised as many of the areas sampled were smaller than the
threshold area (areas on the order of 145-155 ha) that the results of this study
indicate is needed to support a healthy tortoise population.