Boyer Katharyn E, Fong Peggy
Department of Ecology and Evolutionary Biology, University of California, 621 Charles E. Young Drive, Los Angeles, CA 90095-1606, USA.
Oecologia. 2005 May;143(4):619-28. doi: 10.1007/s00442-005-0015-6. Epub 2005 Mar 24.
The roles of co-occurring herbivores that modify habitat structure and ecosystem processes have seldom been examined in manipulative experiments or explored in early successional communities. In a created marsh in southern California (USA), we tested the individual and combined effects of two epibenthic invertebrates on nutrient and biomass pools, community structure, and physical habitat features. We manipulated snail (Cerithidea californica) and crab (Pachygrapsus crassipes) presence in field enclosures planted with pickleweed (Salicornia virginica) at elevations matching the plant's lower extent in an adjacent natural marsh. In the 4-month experiment, C. californica altered habitat structure by reducing sediment surface heterogeneity and shear strength (a measure of sediment stability) markedly throughout the enclosures. Both invertebrates had strong negative effects on a group of correlated sediment physicochemical characteristics, including nitrogen and organic matter concentrations and soil moisture. In addition, both invertebrates greatly reduced benthic chlorophyll a, a proxy for biomass of microphytobenthos. Compared to controls, macroalgal cover was up to sixfold lower with crabs present, while snails increased cover at low elevations of enclosures. Unexpectedly, macroalgal cover was eliminated with both species present, perhaps through P. crassipes consumption of larger thalli and C. californica reduction in cover of recruits. Neither species influenced the S. virginica canopy (quantified with an index of branch length and number); however, at the lower elevation of enclosures, the two species together negatively impacted the plant canopy. The two invertebrates' modifications to our experimental marshes led to distinct suites of biotic and physicochemical features depending on their presence or co-occurrence, with the latter producing several unexpected results. We propose that the roles and interactions of habitat-modifying fauna deserve further attention, particularly in the context of efforts to conserve and restore the processes found in natural systems.
在操纵性实验中，很少研究同时出现的食草动物对栖息地结构和生态系统过程的影响，在早期演替群落中也很少进行相关探索。在美国加利福尼亚州南部一个人工创建的沼泽地，我们测试了两种底栖无脊椎动物对养分和生物量库、群落结构以及物理栖息地特征的单独和综合影响。我们在种植了海蓬子（Salicornia virginica）的野外围隔中控制蜗牛（加利福尼亚滨螺，Cerithidea californica）和螃蟹（厚蟹，Pachygrapsus crassipes）的存在情况，围隔的海拔高度与相邻天然沼泽中海蓬子分布的下限相匹配。在为期4个月的实验中，加利福尼亚滨螺通过显著降低整个围隔内沉积物表面的异质性和抗剪强度（沉积物稳定性的一种度量）来改变栖息地结构。两种无脊椎动物对一组相关的沉积物物理化学特征都有强烈的负面影响，包括氮和有机质浓度以及土壤湿度。此外，两种无脊椎动物都极大地降低了底栖叶绿素a的含量，底栖叶绿素a是微型底栖植物生物量的一个替代指标。与对照组相比，有螃蟹存在时大型藻类的覆盖率降低了多达六倍，而蜗牛在围隔的低海拔区域增加了覆盖率。出乎意料的是，两种物种同时存在时大型藻类的覆盖率被消除了，这可能是由于厚蟹吃掉了较大的藻体，以及加利福尼亚滨螺减少了新藻体的覆盖率。两种物种都没有影响海蓬子的冠层（用分支长度和数量指数来量化）；然而，在围隔的较低海拔区域，两种物种共同对植物冠层产生了负面影响。这两种无脊椎动物对我们实验沼泽地的改变导致了不同的生物和物理化学特征组合，这取决于它们的存在或同时出现，后者产生了一些意想不到的结果。我们认为，改变栖息地的动物的作用和相互作用值得进一步关注，特别是在保护和恢复自然系统中发现的过程的背景下。
