Breeding birds and the success of floodplain rehabilitation

Posted on March 28, 2013

On river floodplains, ecological processes are often disrupted by human activities such as agriculture, flood protection, excavation and channelization, which typically result in modified landscapes and disconnection of the floodplain from the river.  Floodplain rehabilitation programs attempt to reestablish ecological function by creating diverse river landscapes with marshes, grasslands, shrubs and riparian forests.  A good example of such a program is work carried out since the early 1990s to rehabilitate over 8000 ha of floodplain in the Rhine and Meuse catchments in the Netherlands.  How successful have these efforts been?  This question has been answered by examining historical trends in the numbers of breeding birds, which are good indicators of landscape quality and the richness of floodplain ecosystems.  Breeding data on all Dutch bird species have been collected by a national bird monitoring program since 1984.  Program evaluation focussed on the two main aims of successful floodplain rehabilitation:  first, the progressive replacement of short grassland by marshes, tall herbs, shrubs and woodland; and second, an increase in the frequency, duration and depth of inundation.  Vegetational changes should provide new nest locations for breeding birds, while the changes in water regime should favour bird species that cope with the risk of flooding by migrating or by producing multiple clutches.  Regression analysis on data from 1989-2007 revealed that 35 out of 93 bird species showed a significant increase in the number of breeding territories in response to floodplain rehabilitation, while only eight species showed a significant negative response.  On average, bird numbers increased by 5.1% per year in the first decade after rehabilitation.  Nest location was the best predictor variable, explaining  63 and 23% of the variation in breeding numbers after 5 and 10 years respectively.  Species showing the strongest positive response to rehabilitation were those nesting on or near bare ground or water (after five years), and herbs, shrubs and trees (after ten years).  Migratory behaviour explained 27 and 8% of the breeding variation after 5 and 10 years respectively.   Reproductive investment (the number of clutches) explained 12% of the variation after ten years.  These results suggest that the positive effects of floodplain rehabilitation are linked most strongly to traits that allow birds to take advantage of recovering vegetation rather than those that help them counteract the risk of flooding.  This may be connected with the fact that in temperate seasonal floodplain rivers, links between life-history features and the flood pulse tend to be relatively weak.

Reference:  Van Turnhout, C.A.M. et al.  2012.  Ecological strategies successfully predict the effects of river rehabilitation on breeding birds.   River Research and Applications 28, 269-282.