AQUATIC PLANT DIVERSITY IN RIVERINE WETLANDS - THE ROLE OF CONNECTIVITY

1. The hypothesis was tested that intermediate connectivity to a river results in propagule inputs to wetlands, whereas excessive connectivi ty impedes recruitment, and insufficient connectivity causes less comp etitive species to be eliminated, with no recruitment of new species. As a consequence, very low or very high nutrient levels should decreas e species richness by selecting specialized species, whereas intermedi ate nutrient levels should favour the co-occurrence of species with co ntrasting nutrient requirements. 2. Among cut-off channels with high s inuosity and which are infrequently flooded by the river (low flood sc ouring), one example possesses high species richness because most spec ies are saved from extinction by long-term isolation of the channel an d cold groundwater supplies. Other channels are poorly supplied with g roundwater and show a lower richness of species, because of low propag ule inputs and low recruitment potential. 3. Cut-off channels with low sinuosity and which are flooded at intermediate frequencies were divi ded into three groups. The first group was species-poor, being closely connected to the river through downstream backflows which maintain nu trient-rich and turbid waters, in keeping with the hypothesis. The sec ond group presents intermediate richness caused by: (i) lower river ba ckflows; and (ii) floods that partly scour substrate and plants, and a fford regeneration niches for transported propagules. The third group was species-poor because of excessive groundwater supplies, which prob ably acted as a limiting factor for species growth and recruitment. 4. The most frequently flooded channel shows the highest species richnes s, and occurrence of rare and fugitive species, because of floods whic h compensate competition by scouring sediments and plants, and afford regeneration niches for propagules. In this case, conservation of biod iversity necessitates propagule sources at the level of the river land scape.