Consequenties van fragmentatie en het belang van landbruggen en oeverzones

Tom Buijse, Ralf Verdonschot, Peter van Puijenbroek & Piet Verdonschot

fragmentatie, oeverzone, bronpopulaties, isolement, connectiviteit

Fragmentatie heeft invloed op de habitatkwaliteit. Ecologische verbetering van beken en rivieren is daarom ook afhankelijk van het omringende land: natuurlijke oeverzones bieden schaduw en structuur en landbruggen verbinden bovenlopen. Een herwaardering van het denken in ecologische netwerken, op basis van de huidige kennis over migratiebehoeftes en habitateisen van soorten, onderschrijft de noodzaak voor additionele maatregelen.

The role of habitat quality, riparian zones and terrestrial connectivity for fragmented watersheds.

Tom Buijse, Ralf Verdonschot, Peter van Puijenbroek & Piet Verdonschot

habitat quality, fragmentation, source populations, migration barrier, connectivity

In rivers and stream migration barriers do not only fragment watersheds, but also significantly alter and impact habitat quality by impounding running waters over numerous kilometers in the gentle sloping Dutch landscape. Degraded riparian zones caused by agriculture right up to the banks of streams and rivers and reduced variability in landscape structure by large-scale land use both act as migration bottlenecks resulting from habitat alteration and simplification. Across Europe much restoration effort is directed towards improving fish migration by installing fish passes near barriers. We consider this scope too unbalanced and plead for improving habitat quality and migration pathways based on the  requirements for a wide range of aquatic species, Viz considering river basins as ecological networks do not only encompass whether species are able to migrate but also requires attention to the type and quality of habitats. Riparian zones affect light, temperature and coarse organic material in streams and are essential migration pathways for adult life stages of aquatic insects. Moreover, land bridges between the capillaries in watersheds facilitate the recolonization by air from source populations. Improving instream conditions, riparian zones and even the wider landscape around the upstream sources are required for aquatic biota e.g. fish and insects with an aquatic larval and terrestrial adult life stage depend on a suit of different habitats to fulfil their life cycle. Opportunities and requirements to restore or improve of these habitats may thus be situated outside rivers or streams. Wetlands along rivers and streams and in the upstream capillaries of watersheds are examples of such key habitats for improving the ecological status but receive too little attention as effective restoration measures. The main cause is these wetlands being situated outside the boundaries of for the EU Water Framework Directive designated water bodies which requires support and agreement with multiple stakeholders. We therefore plead for a complementary suit of restoration measures such as dam removal and wetland restoration along streams and small rivers for improving habitat conditions and migration over land and by air. A recent inventory identified 30 examples of dam removal in the Netherlands. One of the largest initiatives comprises the restoration of the mouths of numerous tributaries of the river Meuse for upstream migration and habitat improvement. Nature based solutions to improve riparian zones encompass amongst others sediment suppletion and wood addition to streams and have recently gained momentum. This reconnects the stream to its valley. On the other hand, increasing landscape structure variability around the headwaters from a stream ecological perspective is till now hardly practiced.

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