Thursday, 24 January 2013

Why Druridge Bay needs Warthogs

Birds might seem the obvious carriers for seeds, spores and broken fragments of plants to get from pond to pond, but there may be other means of transport. One of my favourite academic papers* from 2012 about pond ecology is a fine example featuring some rather larger means of dispersal. I know that ”favourite” is not the sort of criterion science is supposed to use in these days of research metrics and impact factors, but let’s leave those cold measures behind and head to the warmth of Africa. Bram Vanshoenwinkel and his colleagues at the University of Leuven in Belgium and in Harare in Zimbabwe knew that many of the large mammals in the African bush needed to drink regularly, moving from pond to pond, often wallowing and caking themselves in mud. What if that mud held seeds of plants or eggs and spores of invertebrates? The buffalo, antelope, warthogs and elephants would be a superb means to get from pond to pond. All you’d have to do to find out was scrape the mud off an elephant. On second thoughts, maybe not. Let the elephant do the scraping for you. So Bram and co collected mud from the trunks of trees from around temporary ponds in the Zimbabwean bush where animals had rubbed and scratched.
Mud was collected from as high as 400 cm up the tree trunks. The mud was then rewetted in the lab, incubated and any animals that hatched out or plants that germinated were identified. The mud spawned a host of different creatures, mostly various water fleas (Cladocera), pea shrimps (Ostraocda) and other more exotic crustaceans of temporary pools such as tadpole shrimps (Triops). Also a few plants including a duckweed (Lemna). Best of all was the spread of mud up and down the tree trunks; there was a distinct pattern with a lot of mud at around 300 cm (elephant scratch height), another clump at 120 cm (rhino and buffalo) and a third at ~60 cm (warthog height). There will be some of you reading this humming songs from the Lion King by now. It is a superb mental picture though, all that rubbing, itching, scratching, wallowing and glooping. Elephants at Druridge is unlikely but cattle, sheep and horses are there in numbers. It is a fair bet that they make effective vectors too. * Vanschoenwinkel, B. et al. (2012) Freshwater Biology, Vol. 56, pp 1606-1619

Saturday, 19 January 2013

Just how fast can pondweeds travel?

The pondweeds that turned up in the field ponds, discovered at the end of the harvest when the ponds were accessible after a summer behind the thickets of oil seed rape (Blog, 12th Jan), were unexpected. I should not be surprised: much of my work on ponds has been teetering along that tricky line trying to work out if the animals and plants are where they are because of some reliable rules, a response to measurable, describable influences such as nutrient levels in the water or the presence of a predator... or perhaps they are where they are just as accidents of history. I am reassured ponds remain full of surprises. Just how fast can plants arrive in a pond or wetland? My favourite example, albeit just a one off anecdote, is from Aberlady Bay, on the south coast of the Firth of Forth east of Edinburgh. In 1989 a pond was scraped out of the sandy wet grasslands, intended to encourage wading birds. You can see the raw sand of the pond on the day it was dug.
The following day I was back on the reserve and walked up to the pond to take some more photographs. In the shallows were conspicuous rafts of feathers, and, tangled in amongst the preening debris, stems and clumps of aquatic plants. Not plants that grew around the edge of the pond which had been dug out in the middle of a grass sward, but new plants, plants i could not recall growing on teh site at all. The plants seemed to have been carried in by birds within twenty four hours. By the summer of 1990 the scrape was choked with a glorious mess of aquatic plants and teeming with invertebrates, even though the sand sides remained exposed and bare. None of the plants had been introduced by the reserve team. That is how fast plants can travel.

Saturday, 12 January 2013

Airborne pondweeds

The farm fields so rich in shallow subsidence ponds (9th Jan blog) are a snapshot of the ecology we seldom witness. When the oil seed rape was harvested several pools were exposed that had spent the summer behind an impenetrable thicket of brittle stems, shards of seed cases and broadsides of splinters. The ponds seemed caught out, their summer secrets revealed, their inhabitants easy pickings. Two ponds right in the middle of the arable fields were unvieled as home to clumps of fine leaved pondweeds, probably Potamogeton berchtoldii, rather meanly known as Small Pondweed, which is widespread along Druridge Bay, although I’ve not fished out a piece to check. The dense wefts of leaves wallow just below the surface of the shallow water, each leaf a thin, glistening, delicate frond, slightly translucent. The pondweed was not in the ponds in springtime nor had been there in the previous two autumns (although the ponds themselves are sometimes not entirely there, due to the ups and downs of water levels and ploughing). It seems the pondweeds must have arrived this summer. The ponds in summer are the private venues of ducks and seagulls, heron and swans, behind the wall of crops. The sudden establishment of the pondweed looks like an example of colonisation via a vector that has unwittingly given the plants a lift. I cannot be wholly sure; the plants are evidence of possible processes, but this is tricky to demonstrate definitively. I am reassured by the possibility of this hard-to-see ecology going on around us; pond plants have always been good at getting around the landscape. The sheer number of ponds create the opportunity for them to expand out from core sites suring wetter years. The winter wheat shoots are already greening across the field for this year's crop. The Small Pondweed may have found a new pond all to itself or settled in a doomed trap. I'll be keeping an eye on its fate.

Wednesday, 9 January 2013

A plague of ponds

Every couple of months I walk a circuit around Blakemoor Farm, just to the north east of Ellington and Cresswell villages. Takes a couple of days, partly stopping to admire the chocolate powder detail of summer ringlet butterflies or the winter echelons of geese as they honk and wail with displeasure at my presence in their fields. The purpose of the walk is to count and roughly map out the numbers of ponds. Satellite images and air photos might do the job but field walking reveals more than our sophisticated air borne familiars can properly detect. Some ponds are tucked away along hedges, under trees or in ruts. Even if air photographs could do the work I would want to be out in the fields, the fate of individual ponds surprisingly different as the seasons roll. The wet summer of 2012 picked out so many hollows and sags in the subsiding fields, in addition to the more permanent sites. The Google Earth view above is the fields around Ellington Caravan Park in May, as the rising saturation began to infect the whole of the summer. Red rings mark out the ponds, except for some of the smallest which do not show on the air image. The Oil Seed Rape's yellow pall is blotched by shallow field pools, most of them long re-current sites flooding in winter but seldom all through the summer. The Oil Seed was particularly grim in the rain; even walking around the edges the swell of stems would spray and drip water, enough to leave waterproofs sodden. Nonetheless the walking pays off. This is an extraordinary density of ponds, although making any such claims may be naive given how poorly we know the numbers of smaller ponds elsewhere in the British landscape.