Factors limiting population size of the endangered Attwater's prairie chicken

Abstract

Factors limiting Attwater's prairie chicken (Tympanuchus cupido attwateri) numbers were investigated from 1991-93. The objectives of this study were to: (1) test accepted, yet untested, hypotheses regarding the influence of May and spring precipitation on Attwater's spring breeding numbers; (2) survey parasitism and infectious disease prevalence; and, (3) determine which demographic and reproductive variables most limit Attwater's population growth. The accepted hypotheses that greater than normal spring (Mar-Jun) or May precipitation leads to decreased Attwater's breeding numbers the following spring, while drought during this period leads to increased numbers, were not supported by the data. However, breeding success could account for breeding numbers the following spring. Analysis of the geographic incidence of prairie grouse helminthic endoparasitism predicted that Attwater's should maintain parasite diversity similar to greater prairie chicken (T. c. pinnatus) in Illinois, Kansas, and Missouri. Although I found no hemoparasites or specific antibody against 7 infectious diseases, evidence of Trichostrongylus tenuis was found in 5 of 6 samples collected from all but 1 Attwater's population. To delineate which life-history stages most limit Attwater's numbers, I tested the hypotheses that Attwater's clutch size, egg hatchability, nesting success, brood survivorship, chick survivorship, and juvenile to adult ratios were less than those of greater prairie chicken. The Attwater's ratio of juveniles per adult, nesting success, and number of chicks per brood prior to brood breakup were substantially less than those typically seen in greater prairie chicken. Simulations predicted Attwater's nesting success, brood survival, and chick survivorship must increase sufficiently to close approximately 85% of the difference between Attwater's and greater prairie chicken values before Attwater's numbers would increase. Results suggested that cold, humid conditions and the indirect effects of precipitation on Attwater's brood-rearing should be explored. Also, there is a need to determine if T. tenuis leads to decreased productivity in prairie chicken, as it does in other grouse species. Adult sex ratios should be accurately determined for all populations annually, and there is a need to determine why Attwater's nesting and brood-rearing success are comparatively low. Management strategies designed to reverse these conditions should be developed.