Sunday, 11 July 2010

Scrapes - a rant!

Vange Marsh - a scrape in good condition.

So why is it that most reserves can’t seem to manage a decent wader scrape over a period of time? They start off good but then fade away. We have all seen muddy pools that pull in loads of birds and probably all lamented the lack of birds on some established reserve’s wader scrape. So what is it that makes a scrape perform or not?

Food abundance and availability are the key issues, and these are affected by water regime, nutrient levels and predators. New wetlands are generally highly productive in the first few years of existence as early colonising invertebrates benefit from high nutrient levels and reach high abundance levels. Food! As time passes, nutrients become locked up in the system in mud and decomposing material and invertebrate predators increase as a more balanced system develops. Less food!

So the key is to dry out shallow water bodies to allow nutrients to be released to the system, to kill off predators and then to re-set the clock by allowing re-colonisation. But how often should this be done? Some ‘food’ invertebrates may peak in year 1, others up to years 4-5. The trouble is that birders then moan when a scrape is dry for a season and there are no birds. Many reserves fail to manage scrapes correctly for fear of a visitor backlash when they are dry, but then end up with a visitor backlash for having few birds.

This week I visited a few scrapes, most were crap! Rainham had a very few birds, (including the White-tailed Plover) but notice they rarely stay. The best was at Vange Marsh ; 125 Black-tailed Godwits, 7 Spotted Reds, 5 Greenshank and good numbers of Green and Common Sands.

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