乾旱對哈盆溪大型無脊椎動物群聚組成之影響

Abstract

摘要 乾旱是內陸水域生態系重要的干擾因子，對其中動、植物的組成有顯著影響。雖然過去已有許多研究在探討乾旱對溪流生態系的影響，但多在乾、濕季分明的溫帶或乾燥氣候區；且這些研究均無將乾旱干擾定量，本研究以研究期間樣站河床無水日數比總記錄日(稱之為乾旱比)來定量乾旱干擾。於哈盆溪間歇河段選取乾旱比不同的八個樣點，比較樣站間大型無脊椎動物群聚組成之差異。自2003年9月至2004年7月，每兩個星期以Surber’s sampler (面積 = 50 x 50 cm2; 網目 = 250μm)在樣站採樣一次。結果發現各樣站間大型無脊椎動物的種類數(taxa)、個體數、Shannon多樣性指數與Pielou均勻度指數均有顯著差異(ANOVA, p < 0.05)；且這四項群聚參數均隨乾旱時間比例之增加顯著下降。此結果與中度干擾假說不符，推測是由於在溪流環境中，除了乾旱外，高流量亦是溪流中重要的干擾因子，因本研究也發現有些樣站的大型無脊椎動物受高流量的影響而顯著減少。除了乾旱與洪水干擾的影響外，主成分分析的結果發現遮蔽度、酸鹼值、與磷酸濃度也對大型無脊椎動物的組成有顯著的影響。此外，乾旱後各樣站大型無脊椎動物組成均能迅速恢復，推測是由於大型無脊椎動物具快速拓殖的能力，而非對乾旱的抵抗能力。

Abstract Drought in inland waters is one of the most important disturbances that affect community compositions of macroinvertebrates in these water ecosystems. Although there had been many researches on the effect of drought on lotic ecosystems, most of them were studied in temperate or arid region with an obviously wet and dry season in a year. But there has been no study that has quantified the drought disturbance and been carried out in subtropical areas. In this study, the drought disturbance was quantified by the ratio of the number of days that streambeds were dry to the total number of days in the sampling period. Eight sampling sites with different drought ratios were selected along Hapen Creek located in northeastern Taiwan. Macroinvertebrates were collected at each site every two weeks from September 2003 to July 2004 by a Surber’s sampler (area = 50 x 50 cm2; mesh size = 250μm). There were significant differences in taxa richness, number of individuals, Shannon diversity index, and Pielou evenness index among the 8 sites (ANOVA, p < 0.05). All 4 community parameters decreased with an increase in drought ratio. This result was contradictory to the intermediate disturbance hypothesis. This may be due to a high discharge, which was also an important disturbance in the stream ecosystem. It was found that the macroinvertebrates decreased significantly after a high discharge. In the principal component analysis (PCA), the first component was highly correlated with water depth, current velocity, and discharge, suggesting that drought and high discharge were the important disturbances that influenced macroinvertebate assemblages. Besides the effects of drought and high discharge on macroinvertebrate assemblages, PCA also identified the canopy cover, pH values, and the concentration of phosphate that were important factors affecting the macroinvertebrate assemblages. In this study, it was also found that the macroinvertebrates could recover in a few days after a drought. This was because they could colonize a new patch from refugia very fast, but not resist the drought phase.