Nylsvley - A Conservation Imperative - Article on the Nyl Floodplain (excerpts) by Andrew Duthie and Warwick Tarboton
Two hundred kilometres north of Johannesburg, between the towns of Nylstroom and Naboomspruit, lies the largest and best conserved floodplain vlei in South Africa. A veritable paradise for wetland fauna and flora during years of extensive flooding, the lifeblood of the natural wonder of immense diversity and complexity is now being increasingly siphoned off. Ever increasing human demands for water, coupled with its injudicious use, cloud the future of this emerald in the crown of South Africa's Natural Heritage.
The headwaters of the Nyl River rise in the felsites and sand-stones of the Waterberg from where they spill down the catchments of the Klein Nyl, Groot Nyl, Olifantsspruit and Middelfonteinspruit to flood the Nyl floodplain.
Over the past 25 years the flood- plain has been inundated with water 18 times. About 16 000 hectares are flooded in the wettest years but only half this area in years of moderate flooding. When several years in succession have above average rainfall, the floodplain remains inundated for up to four years. In other flood years it tends to dry out during the following winter. In seven of the 25 years no flooding occurred at all and in some there was only a brief, unsustained flooding which rapidly dried up. So, based on the past 25 years' experience, one can expect flooding, on average, once every two years. In practice however, floods, like drought periods, are grouped together.
Flooding typically needs good rains 150 millimetres or more over a short period, say, within a month. This triggers off a surge of water which moves slowly down the flood-plain, inundating areas as it advances. Its timing is obviously dependent on the timing of the rain, and the size of the area flooded depends on how much rain follows. Usually flooding takes place in late December or January, but it has occurred as late as mid-March and as early as November. This initial surge of flood-water can dry up rapidly if there is no follow-up rain, or it can persist through to the next summer if above average rains continue to fall. Rainfall of at least ten per cent above average is needed to inundate the floodplain.
ECOLOGY
The flooding of the Nyl is synonymous with the cacophony of frogs. Various species which have been lying buried deep in the ground, sometimes hundreds of metres away from the Nyl, emerge and scramble down to the newly arrived water in their hundreds to find partners, mate and lay eggs.
Apart from the noise of the frogs, there is a vague sense of disappointment about the flooding; the surge of water that stretches across the floodplain like a lake does not appear to have brought much other life with it. Then surreptitiously the first grasses and sedges appear above the water and, with accelerating speed, transform the open lake into what looks like a waving green grassland - only from the air is the twinkle of water visible everywhere. Countless ribbons of frogs' eggs lie entwined among the grass stalks in the shallows. At road culverts, where the water surges through on its downward course, strings of tiny fish can be seen moving against the current.
While there is not a great variety of fish to be found in the Nyl system (16 species), what it lacks in diversity it compensates for in sheer numbers. It is estimated that between 300 and 600 tons of fish are produced on the floodplain, depending on the extent of flooding. Most are small species called minnows or "ghillieminkies" of the genera Barbus and Aplocheilichthys. The straightfin barb, Barbus paludinosus, is the most abundant species and makes up about 40 per cent of the total number of fish present. Like other species of minnow, it feeds on mosquitoes, midges, mayflies and their larvae, and shoals often concentrate where water currents suck dead floating insects below the surface hence the concentration at the road culverts. A near relative, the striped topminnow, A. katangae, is by contrast, one of the Transvaal's rarest fish species and, apart from occurring in the upper reaches of the Groot Nyl, is known from only two other catchment streams in the province.
There are two Labeo species in the Nyl, a small cichlid called the southern mouth brooder, two Tilapia species and two species of catfish or barbel. The common barbel Clarias gariepinus is the largest fish found in the Nyl, with some individuals growing to a metre in length and having a head as wide as a football. Barbel are great predators of other fish but they are also likely to eat anything they can overpower, including frogs, crabs, birds and insects. They might even scavenge on the carcass of a drowned animal or take fallen fruit. During floods they move into shallow flooded grass areas to spawn. Females lay as many as 500 000 eggs each and within a day or two these hatch into a myriad of tiny fry. Because the adults are so vulnerable to predators in the shallows, they waste no time in getting back into deeper water as soon as egg-laying is over.
As a result of the burgeoning food supply, the first waterbirds arrive. These are inevitably the ducks - either Whitefaced, Yellowbilled Duck or Redbilled Teal, or Southern Pochard, or all four together.
The first egrets appear and their flights upstream and downstream soon become a common sight. The great white, yellow-billed and little egrets are joined by other species of heron: Grey, Purple and Squacco, and Black Egrets. They disperse into scattered ones and twos across the floodplain to feed, some species wading and hunting, some still-hunting, standing in shallow water with crooked neck and head poised ready to strike. One, the black egret, fishes by throwing a black umbrella over the water to cut out reflection, then shuffles its yellow feet to stir up the mud.
At a few places along the floodplain where there are reedbeds growing over the main channel, the egrets and herons congregate to roost at night and to breed. In years when there is a high, sustained flood, these breeding colonies are an amazing spectacle with thousands of birds coming and going and nests everywhere at all stages of the breeding cycle. Eleven different species of heron and egret may be found breeding together in these colonies with reed cormorants, and sacred and glossy ibis. In such years the Nyl floodplain supports
the largest breeding populations of Great White Egret, Black Egret and Squacco heron known in South Africa.
There are three smaller heron species found on the Nyl floodplain which do not join the large mixed heronries in the reedbeds, all of them rare species in South Africa and listed in the South African Red Data Book - Birds. They are the Bittern, the Dwarf Bittern and the Rufousbellied Heron. The first is without question the most secretive bird on the floodplain. Were it not for its call it would go entirely undetected. In the evening and morning, and sometimes through the night, it can be heard booming from the extensive areas of flooded rice-grass, Oryza longistaminata, making a series of "oomph" sounds reminiscent of the sound of a distant lion. It is a deep, resonant noise, which carries for two or three kilometres. The bird is almost impossible to flush and it never seems to leave the depths of the marsh and show itself. It is altogether a mystery bird in South Africa, having been recorded from only a handful of localities in the past 30 years, nowhere permanently. During droughts it may go unrecorded for several successive years and then suddenly, when the rains come, it is back again. Somewhere, perhaps in the Zambian wetlands, there is a permanent reservoir of habitat, which supports the bitterns that find their way to the Nyl floodplain in flood years, arriving in December, breeding, and then leaving in April. In "good" years the Nyl floodplain
supports a hundred are more pairs of the Dwarf Bittern which nest solitarily in small, flooded trees and bushes along the fringes of the floodplain. Like the Dwarf Bittern, the Rufousbellied Heron arrives to breed on the floodplain in midsummer but its occurrence here is more erratic there are some good years, which pass without any sign of it. The Nil floodplain is the only locality in South Africa where breed knows this Species.
Another group of birds, known collectively as the Rallidae (or rails), move on to the floodplain en masse when the expanse of grass and sedge has reached a height that provides them with suitable habitat. They are a generally secretive and not easily observed family of birds, and their presence is usually detected by their sound rather than by sightings of them. The rapid clicking notes of the Lesser Gallinule, the -du du du du pumplike sound of the Lesser Moorhen, the snoring and wailing of the purple Gallinule and the sharp "kriks of the the Common Moorhen. These four Species make up most of their numbers. There are almost certainly far more Rallidae present on the floodplain in a wet summer than herons, but because 'they are so much less conspicuous, assessing their actual numbers is difficult. One attempt to do this involved locating all the active rail and crake nests, first in one hectare and then in a 16 hectare area, and then extrapolating this density to the whole floodplain. It led to an estimate of 43 000 crakes and rails being present that year. Add to this the 12 000 herons (17 species} that occur in a "good year and a further 19 000 ducks (17 species), plus the numbers from other groups such as cormorants, darters and spoonbills, and you will find that in a wet summer the Nyl floodplain's waterbird population probably exceeds 80 000 birds.
The Nyl floodplain is rich, not only in bird numbers, but also in diversity. So far 104 different species of waterbird have been recorded on the Floodplain. Eighty-seven of the 94 waterbird species known to breed in Southern Africa have occurred on the Nyl floodplain at one time or another. No other South African wetland can claim this. It also supports a large share of those waterbirds classified as Red Data Species in South Africa. More than three-quarters of the 23 currently listed waterbird species occur here. Eight of them breed on the flood plain and a few of these are not known to breed anywhere else in South Africa. Statistics like these demonstrate the Nyl floodplain's uniqueness in this country as a waterbird refuge. The nearest flood plains of comparative size and species diversity occur much further north in Botswana and Zambia.
Excerpt from Nylsvlei: A Conservation Imperative by Andrew Duthie, Wildlife Society.
