Maciá A, Bradshaw W E
Department of Biology, University of Oregon, Eugene, OR 97403-1210, USA e-mail:
Oecologia. 2000 Oct;125(1):55-65. doi: 10.1007/PL00008891.
The nutrient base of aquatic tree-hole communities is derived from leaf litter, benthic detritus, and water flowing down the tree trunk (stemflow water). Previous studies in eastern North America with the mosquito, Aedes triseriatus, have identified leaf litter as a major and stemflow water as a minor source of mosquito nutrition, but did not consider the role of the benthic detritus or how the aggregate or relative contribution of these sources of mosquito nutrition changed during the year. We use the leaf litter, benthic detritus, and stemflow water from tree holes in western Oregon (USA) to determine how these substrates affect mass at metamorphosis, biomass yield, and fitness (cohort replacement rate; R ) of the mosquito, Aedes sierrensis, through both natural and simulated winters, the normal growing season for larvae in tree holes. We found that fresh leaf litter constitutes the major determinant of mosquito fitness by a factor of >15:1 over any other substrate taken directly from tree holes in nature. The other substrates, including the benthic detritus, individually make only a meager contribution to mosquito fitness but, when added to the leaf litter, can sustain yield and improve fitness at high, limiting larval densities. Nutritional quality of tree-hole substrates declines by >90% from early (fall) to late (spring) in the larval growing season. At both times of year, the coarse or fine detritus provide minor resources, and stemflow water provides no detectable contribution to mosquito nutrition. The resources in the litter are not transported during the year to the benthic detritus; rather, these resources are either exploited by mosquitoes when they first become available, or they deteriorate and become progressively more unavailable to them. Growth and development of A. sierrensis feeding on dried and reconstituted tree-hole contents during a 6-month simulated winter in the laboratory showed: (1) the same relative contributions of leaf litter, benthic detritus, and stemflow water to mosquito nutrition, (2) that the winter deterioration of substrate quality is a direct consequence of microbial decomposition, and (3) that pre-emptive competition from pre-existing A. sierrensis greatly increases substrate deterioration. We conclude that the progressive winter deterioration of larval resources in combination with the dry summers of western North America are the most likely environmental factors that limit species diversity in tree holes and that have selected for early recruitment (autumnal hatching) of A. sierrensis and for its univoltine life cycle from Mexico to Canada.
水生树洞群落的营养基础源自落叶、底栖碎屑以及顺着树干流下的水（茎流水）。此前在北美东部针对三带伊蚊开展的研究已确定落叶是蚊子营养的主要来源，茎流水是次要来源，但未考虑底栖碎屑的作用，也未探究这些蚊子营养来源的总体或相对贡献在一年中是如何变化的。我们利用美国俄勒冈州西部树洞中的落叶、底栖碎屑和茎流水，来确定这些基质如何通过自然冬季和模拟冬季（树洞幼虫的正常生长季节）影响锡耶拉伊蚊变态时的质量、生物量产量和适合度（种群更替率；R）。我们发现，新鲜落叶对蚊子适合度的决定性作用比从自然界树洞直接获取的任何其他基质高出15倍以上。其他基质，包括底栖碎屑，单独对蚊子适合度的贡献微乎其微，但添加到底栖碎屑中时，在幼虫密度高且有限的情况下能够维持产量并提高适合度。在幼虫生长季节，从早期（秋季）到晚期（春季），树洞基质的营养质量下降超过90%。在一年中的这两个时期，粗碎屑或细碎屑提供的资源较少，茎流水对蚊子营养没有可检测到的贡献。落叶中的资源在一年中不会转移到底栖碎屑中；相反，这些资源要么在刚可用时就被蚊子利用，要么变质，逐渐变得无法被蚊子利用。在实验室为期6个月的模拟冬季中，以干燥并重新配制的树洞内容物为食的锡耶拉伊蚊的生长和发育表明：（1）落叶、底栖碎屑和茎流水对蚊子营养的相对贡献相同；（2）基质质量的冬季恶化是微生物分解的直接结果；（3）已有锡耶拉伊蚊的抢先竞争会大大加剧基质恶化。我们得出结论，幼虫资源的冬季逐渐恶化，再加上北美西部夏季干燥，是最有可能限制树洞物种多样性的环境因素，并且促使锡耶拉伊蚊提前繁殖（秋季孵化）并形成从墨西哥到加拿大的单化性生命周期。
