Plant Species Richness on Islands Over a Century of Primary Succession: Lake Hjalmaren

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Abstract
Between 1882 and 1886 the water level of Lake Hjalmaren, Sweden, was lowered °130 cm. As a consequence, a large number of new islands arose. Lists of vascular plant species have been obtained for such islands of size range 50 m2 to 2.5 ha in 1886, 1892, 1903—1904, 1927—1928, and 1984—1985. Species—area relationships were slightly better described by the exponential S = C + z log A than with the power model S = CAz (fitted with nonlinear regression) in all stages of succession. There were no clear trends in the shape of the species—area curve over time. Except for the 1886 survey all slopes of the power model included the theoretically suggested value of 0.26 in their confidence intervals, indicating that this value has no obvious significance in relation to equilibrium or succession. On large islands (>0.3 ha) species number had reached at equilibrium at °95 species with immigration of °30 species balancing extinction between the last two surveys. Indications of interactions that led to decreases in species richness at late successional stages are weak. Small islands still continue to accumulate species, mainly due to substrate stabilization, which leads to formation of habitats for plant establishment. A stepwise regression indicates over the whole successional sequence log area as more important for variation in species number than descriptors of island shape, habitat diversity, or location. However, for individual islands the residuals in the species—area regression could be explained by special habitat features.
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