Population Consequences of Upper Respiratory Tract Disease on Gopher Tortoises

Document Type

Task Force Report

Publication Date



gopher tortoises, respiratory diseases


The research project had 5 objectives: (1) resurvey 10 gopher tortoise (Gopherus polyphemus) populations originally surveyed 8–10 years ago; (2) compare changes in the demographic well-being of populations at 4 sites that were known to have seropositive (for Mycoplasma agassizii antibodies) individuals with those of populations at 6 sites that were not known to have seropositive individuals; (3) document changes in the spatial arrangement and potential connectivity of individuals within populations and determine if the changes are related to habitat management practices; (4) test as many as 20 individuals from each population for the presence of M. agassizii; and (5) examine individuals from each population for signs of physiological stress.

Demographic well-being was assessed indirectly via burrow frequency, status, and size. Estimated numbers of active and inactive burrows, estimated numbers of abandoned burrows, and histograms of size distributions were compared among locations. Comparisons between surveys were made with maps of the areas of tortoise occupancy to reveal shifts in the spatial arrangement and potential connectivity of groups of individuals and by dividing transects into quartiles to determine if individuals had moved toward the peripheries of the various locations between surveys. Data on habitat structure (vegetation samples) and management history (distributions of tortoise habitat and burn histories) were searched for reasons underlying changes in demographic well-being and/or spatial arrangement. Blood samples were extracted from hand-captured individuals and analyzed for presence/absence of antibodies to M. agassizii, levels of stress hormones, and white blood cell counts.

Indicators of demographic well-being changed between surveys at most sites. Most of the changes, at most sites, indicate that the demographic well-being declined between surveys. Spatial arrangement of occupancy of the habitat changed only modestly, at best, between surveys at most sites. Habitat structure changed to varying degrees among sites between surveys and changes appear to be related, at least in part, to management history. A connection between the observed declines and reduction in habitat quality was indicated. Some combination of loss of herbaceous ground cover, increase in lower canopy cover, and increase in upper canopy cover between surveys appeared to be at least partially responsible for the declines, but our results do not reveal specific causes of the declines. Reductions in habitat quality were not expressed uniformly among sites with different amounts of habitat, but were expressed more strongly at relatively small sites. Habitat loss and fragmentation are likely to be exacerbated by concomitant habitat degeneration. No connection between the observed declines and the presence of M. agassizii was indicated, and the mycoplasma appears to be more widespread than previously suspected. Plasma cortisone levels could not be shown to be different among sites or between seropositive and seronegative individuals. The most readily detectable hematological response both to the presence of M. agassizii and to chronic stress was elevated heterophil/lymphocyte ratios. Our results tentatively establish a connection between chronic stress from habitat change, immune response, and demographic well-being.

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Citation / Publisher Attribution

Population Consequences of Upper Respiratory Trace Disease on Gopher Tortoises, NG98-005, 44 p.