McCoy Earl D, Mushinsky Henry R
Department of Biology, University of South Florida, Tampa, Florida 33620, USA.
Ecology. 2007 Jun;88(6):1401-7. doi: 10.1890/06-1188.
Minimum patch size for a viable population can be estimated in several ways. The density-area method estimates minimum patch size as the smallest area in which no new individuals are encountered as one extends the arbitrary boundaries of a study area outward. The density-area method eliminates the assumption of no variation in density with size of habitat area that accompanies other methods, but it is untested in situations in which habitat loss has confined populations to small areas. We used a variant of the density area method to study the minimum patch size for the gopher tortoise (Gopherus polyphemus) in Florida, USA, where this keystone species is being confined to ever smaller habitat fragments. The variant was based on the premise that individuals within populations are likely to occur at unusually high densities when confined to small areas, and it estimated minimum patch size as the smallest area beyond which density plateaus. The data for our study came from detailed surveys of 38 populations of the tortoise. For all 38 populations, the areas occupied were determined empirically, and for 19 of them, duplicate surveys were undertaken about a decade apart. We found that a consistent inverse density area relationship was present over smaller areas. The minimum patch size estimated from the density-area relationship was at least 100 ha, which is substantially larger than previous estimates. The relative abundance of juveniles was inversely related to population density for sites with relatively poor habitat quality, indicating that the estimated minimum patch size could represent an extinction threshold. We concluded that a negative density area relationship may be an inevitable consequence of excessive habitat loss. We also concluded that any detrimental effects of an inverse density area relationship may be exacerbated by the deterioration in habitat quality that often accompanies habitat loss. Finally, we concluded that the value of any estimate of minimum patch size as a conservation tool is compromised by excessive habitat loss.
可通过多种方法估算可行种群的最小斑块面积。密度 - 面积法将最小斑块面积估算为随着研究区域的任意边界向外扩展，不再遇到新个体的最小面积。密度 - 面积法摒弃了其他方法中关于栖息地面积大小与密度无变化的假设，但在栖息地丧失使种群局限于小区域的情况下尚未得到验证。我们使用密度面积法的一个变体来研究美国佛罗里达州穴小龟（Gopherus polyphemus）的最小斑块面积，在该地区这种关键物种正被局限于越来越小的栖息地片段中。该变体基于这样一个前提，即当种群个体被限制在小区域时，其密度可能会异常高，它将最小斑块面积估算为密度趋于平稳的最小面积之外的区域。我们研究的数据来自对38个穴小龟种群的详细调查。对于所有38个种群，实际确定了其占据的面积，其中19个种群在大约十年的间隔期内进行了重复调查。我们发现在较小区域存在一致的密度 - 面积反比关系。根据密度 - 面积关系估算出的最小斑块面积至少为100公顷，这比之前的估计值大得多。对于栖息地质量相对较差的地点，幼龟的相对丰度与种群密度呈负相关，这表明估算出的最小斑块面积可能代表一个灭绝阈值。我们得出结论，密度 - 面积负相关关系可能是栖息地过度丧失的必然结果。我们还得出结论，密度 - 面积反比关系的任何有害影响可能会因栖息地丧失常常伴随的栖息地质量恶化而加剧。最后，我们得出结论，由于栖息地过度丧失，作为一种保护工具的最小斑块面积估计值的价值受到了损害。
