Our rivers – corridors for colonisation

Our river systems are undoubtedly one of the most diverse habitats found within the British Isles.  They provide us with numerous benefits including areas for relaxation and recreation, they harbour high levels of biological diversity, act as natural flood management, provide water for consumption and irrigation, and act as corridors for the movement of nutrients and species in an otherwise fragmented landscape.  However, our river systems are highly vulnerable habitats.  Seasonal variations in hydrological processes render riparian habitats prone to high levels of disturbance which aid the invasion and colonisation of invasive plant species.

RT.13.5.06.Devon.site.5 (5) Himalayan balsam monoculture on the banks of the River Torridge, North Devon, UK (CABI)

The Environment Agency (EA) has recently published the latest results of the River Habitat Survey (RHS) where they compare the original data set, collected between 1995-1997, to data collected during surveys in 2007-08.  Alarmingly, the report highlights over 40% of the river lengths in England and Wales are classed as severly modified and the spread of non-native plant species into our already stressed river network adds to their degradation by out-competing native species and reducing biological diversity.

Japanese knotweed (Fallopia japonica), giant hogweed (Heracleum mantegazzianum), floating penny-wort (Hydrocotyle vulgaris) and Himalayan balsam (Impatiens glandulifera), to name a few, are all present throughout our waterways and cause concern for conservationists and land managers on a national scale. The EA estimate that Himalayan balsam, now the most commonly occurring non-native plant species present in our riparian habitats, occurs along 13% of river lengths across England and Wales and its range expansion shows no signs of relenting. Currently, there are few rivers in the UK that have not been colonised by this species and most of us who venture out to enjoy our waterways will be only too familiar with river-banks awash with bright purple flowers and smelt their pungent aroma in the summer months.

The success of Himalayan balsam can, in part, be attributed to the plant’s prolific seed production (2,500 seeds per plant) coupled with the highly efficient explosive dispersal mechanisms.  The high seed production produces a dense seed bank which persists over winter and synchronously germination in the spring months to form fast growing monocultures. In addition, in the autumn, seeds are incorporated into the water-body and spread throughout the catchment.  Germination takes place on the bottom of the water body and then seeds are carried by the current to the riverbank where they obtain a foothold in disturbed ground.

The most significant negative impact Himalayan balsam has on native plant species in riparian habitats is the ability to shade out and displace species that assist in river bank stability. River banks densely colonized by Himalayan balsam have been shown to have reduced plant diversity by up to 25% (Hulme and Bremner, 2005). This in turn could lead to increased bank erosion and sediment intake into the water-system, which may impact on fish spawning grounds and invertebrate niches.  The EA report highlights the impact of silting into our rivers with the number of sites with extensive levels of silting has remained reasonably constant since the original 1990s survey.

No one can have missed the news on budget cuts which have dominated headlines in the UK over the last few weeks.  It is hoped that adequate financial resources will remain committed to conserving and up-keeping our natural resources throughout the UK, especially when consideration is given to the financial benefits we gain from ecosystem services from healthy ecosystems.  Environmental organisations in the UK, both governmental and NGO’s are all working together to upkeep our waterways and fulfil our European commitments, such as the Water Framework Directive, so our rivers can reach the required ‘good status’ required by 2015.

Further reading

Chittka L & Schürkens S (2001) Successful invasion of a floral market. Nature 411, 653

Hulme PE & Bremner ET (2005) Assessing the impact of Impatiens glandulifera on riparian habitats: partitioning diversity components following species removal. Journal of Applied Ecology 43, 43-50.

Prach, K. 1994. Seasonal dynamics of Impatiens glandulifera in two riparian habitats in central England. In: de Waal, L.C., Child, L.E., Wade, P.M. & Brock, J.H. (eds.), Ecology and Management of Invasive Riverside Plants, pp. 183-188. John Wiley & Sons Ltd, Chichester, UK.

Pyšek, P. & Prach, K. 1994. How important are rivers for supporting plant invasions. In: de Waal, L.C., Child, L.E., Wade, P.M. & Brock, J.H. (eds.), Ecology and Management of Invasive Riverside Plants, pp. 183-188. John Wiley & Sons Ltd, Chichester, UK.

Trewick, S. & Wade, P.M. 1986. The distribution and dispersal of two alien species of Impatiens, waterway weeds in the British Isles. Proc. 7th Int.Symp. on aquatic weeds. Pp. 351-356

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