Saturday, 8 June 2013

More on the Norfolk Hawker

Photo by Ian Dawson 2012
The exciting, recent news that we have Norfolk Hawker dragonflies breeding on the Hayling Pit brings with it some responsibilities. This is a very rare animal and Paxton Pits is, as far as we know, the only UK site outside of Norfolk and Suffolk. It is a protected species and you need a licence to handle it. Fortunately, the site it has chosen at Paxton is already protected within the Site of Special Scientific Interest, but what does that mean?

The threats to Norfolk Hawkers are mainly to do with habitat. Water quality is a high priority and the main threats to this at Paxton come from floods and sewage leaks. Fortunately it would take a very extreme flood to reach the lake as the Little Meadow acts as a buffer between us and the river. However, sewage leaks have occurred in the past, but the ancient pipe was replaced this spring.

What attracted them to Paxton Pits rather than all the lakes between here and Norfolk? The answer is Water Soldier weed. This plant is native to East Anglia and virtually all of the known sites for the dragonfly in England have lots of it. Now, this was not always true. Long before the pits were dug at Paxton, in fact a century before, the Norfolk Hawker bred in the Cambridgeshire Fens and we do not think that Soldier Weed was ever abundant there.  In fact, where this dragonfly occurs on the Continent, it is often associated with densely weeded ditches, but not with Soldier Weed. Somehow along the way, at least in England, the dragonfly has become associated with this plant.

At Paxton Pits we are certain that Soldier weed was introduced and that is why there is no continuity in it's distribution between here and Norfolk. Anglers hate it of course, and they have tried quite hard to get it removed, but it is (thankfully, as it turns out) a very resilient plant with sharp, tough, glossy leaves. It also has the strange habit of sinking and then floating up again so it is difficult to attack with herbicides.

Anglers clear their swims of weeds early in the season and make piles of rotting vegetation on the bank.  We will have to make sure that anglers on the Hayling Pit are careful to stack their weed on the water's edge, long enough to allow invertebrates to crawl back into the water. Anglers and Norfolk Hawkers co-exist at a number of sites and I am sure we can get their co-operation.

The larvae seem to live for two years in the lake before emerging as adults, so it seems that this year's hatching originated from the sightings in 2011. Let's hope that this years adults lay again in the Soldier Weeds at Paxton Pits.

Should we consider spreading the weed to other lakes? I have never considered this, but, unless the population in the Hayling Pit was under threat, I would find it hard to justify. It is a very invasive plant.