Wednesday, 27 November 2013

Flood, drought ... and now the plough

 The modestly named Small pond weed, Potamogeton berchtoldii, (or, less modestly, Berchtold’s pond weed, because that is what the Latin says) is nonetheless a canny plant, turning up in ponds across Druridge Bay, including unlikely sites in the middle of arable fields. The delicate, filigree strands of stems and leaves seem ideal to fragment, stick to birds and be carried between sites. This little pondweed also seems able to tough it out in exposed shallows or ponds with slightly higher salinity, which seems to come up from underground springs. Although it is a flowering plant this species relies on cloning to spread using ramets; individual plants that, together, form the whole vegetative colony. To over-winter the pondweed sets turions, essentially a bud, but a bud adapted to tough it out during the lean times, functionally akin to a seed or bulb  in other species. Just how tough this overlooked pondweed can be is under severe test in some of the subsidence ponds at Blakemoor Farm. The warm summer has seen water levels fall far enough for several subsidence ponds to be ploughed through, including the wide but shallow site featured on the blog as it began to dry out.

The cracked, dried soil from the sequence earlier in the year (2nd September blog, compare the drying sequence in the photos above to the latest ploughed state of the pond at the top of the blog) is now tilled and turned, the pond which was once the summer hang out for avocets , teal and black headed gulls is barely detectable. Whether ramets and turions cope well with ploughing I do not know, although I suspect the plant will recolonise once the pond refills. On the 20th November the site was just the bare, ploughed ridges but by the 27th a silver flash of water had puddle across the lowest part of the shallow basin. Potamogeton berchtoldii is not a rare plant: the map shows the national records from the NBN gateway data base since 1980. If anything it may be overlooked given the small size and identification challenges

Wednesday, 20 November 2013

The water beetles' sad end as a teenage heron's snack

The predatory beetles of the last two entries look spectacularly fearsome through a hand lens; pointed jaws which pierce prey and along which the digested innards of their victims are sucked out, clusters of simple eyes spots which suggest a very different perspective on their world. However if you are a 1 metre tall Grey Heron, Ardea cinerea, these beetles probably pass for a light snack. The experimental ponds at Hauxley are a frequent hang out for the young herons that have left the nest. They loiter, preen and wander round with no obvious purpose in a perfect echo of teenagers. They also throw up pellets, like the one in the photo above. Heron pellet contents vary greatly with time and place, as they seem to hone their hunting to fine grained variations in the available prey. Mammal remains such as voles and moles occur frequently, birds too, especially chicks, and the tough wing cases (elytra) of water beetles. Three large, ridged elytra, edged with a distinct yellow border are in this pellet, the remains of Great Diving beetles (Dytiscus) based on the size and colour. The victims were female beetles, the ridges a give-away of gender compared to the smooth wing cases of the males. The rarity of fish bones, even their apparent absence, from heron pellets has attracted comment in many more detailed studies. Why they should be missing is not clear, since plenty of other fish eating birds regurgitate bones and otter spraints are essentially fish bones in Earl Grey tea scented oil. The beetles’ wing cases are tough. Beetles body parts found in ancient soils and peat can be so intact that precise species identification is still possible allowing local habitat conditions to be worked out based on where the species live these days. Quite where these two female Dytiscus came from I cannot tell. Dytiscus beetles have never been found in the experimental ponds, so the herons must have travelled further afield. In true teen fashion they last leave the debris lying around when they’ve finished.

Saturday, 2 November 2013

Acilius, the elegant plankton hunter

The Acilius larvae of the previous entry repay closer inspection. Two species of Lesser Diving Beetle are found in the UK, Acilius sulcatus and the scarcer Acilius canaliculatus. The “Lesser” is the curse of sharing the same ponds with Great diving Beetles, Dytiscus species, but the Acilius have their own strangeness and charm, less of the brute force of the Dytiscus larvae or clockwork toy look of adult Great Diving Beetles. Adult Acilius resemble submarine almonds in shape and size. They are apparently the fastest swimming of the diving beetles. However it is the larvae that are most startling. They are nektonic, meaning they live out in the open water column. Legs fringed richly with fine hairs make powerful paddles, but they often hang, stationary, waiting for prey to come in reach. Whilst they will take a variety of invertebrates they are particularly effective predators of water fleas, perhaps voracious enough to skew the distribution of these prey throughout a pond and also ferociously efficient hunters of mosquito larvae. Juvenile mosquitoes have to come to the surface occasional to replenish oxygen supplies, relying on a snorkel-like siphon to break though the water’s surface to the air above. Swimming to the surface, or even static but silhouetted against the light above makes them vulnerable to specialist hunters. Whilst Dytiscus larvae routinely capture large prey such as tadpoles (or each other) the Acilius’ delicate, elongated thorax and small head make for an altogether more refined but  equally deadly hunter. (Adult photo from Old Billuck and larvae from Biodiversity Heritage Library, Creative commons, Flickr).