|Ecological considerations of grassland management -- the role of the grass species in savannah and other grassland ecozones.
The ecology of Southern Africa's grasslands is affected by grazing, fire, rainfall etc. -- all these factors play a significant role in the species composition and health of a grassland.
If herbacious plants and trees (Dicots) are browsed or bitten by animals, the growing tip is destroyed and the plant reacts by putting out alternate shoots. This has the effect of causing the plant to 'coppice' and the plant produces more and denser growth. However, if grasses (Monocots) are bitten or grazed by animals, the growing part is not destroyed and it will continue growing from the base, with the bitten tips remaining a feature. The utilisation of grass species by grazers can thus easily be identified.
Through the process of 'photosynthesis', leaves produce food energy from sunlight, minerals and water. The energy to produce growth, however, is obtained from energy stored in the roots. As the grass plant grows so it withdraws from the roots until it has manufactured enough energy to restore that taken from the roots. If a plant is browsed immediately and frequently, as it tries to grow, it will be unable to replace root energy and the effect is that the rootstock will be weakened or diminished.
Even one animal, confined to an area, will nibble away, preventing any 'rest' period for the plants to grow and renew root vigour. If we started nibbling at growing vegetables in our garden, they would not grow into mature plants and certainly would not reach the stage of producing seed. Exactly the same happens with growing grasses -- they need a rest from grazing and have a time to grow.
The lack of rest and growing time is caused by what is known as 'constant nibble' and constant nibble can be effected by only one animal. Ecologically, rest time is the important factor and not numbers of grazers.
There are grass species adapted to almost every terrestrial habitat on earth. In arid 'pioneer' situations the grass forms tend to rely on seed production to carry them over dry periods. This a method used even by some fish (Nothobranchius sp.), whose eggs need to desiccate in the bottom dry pans before hatching in the next rains.
Most of the pioneer type grasses have narrow leaves, or some other method of cutting down transpiration, and sharp seeds that can lodge in bare ground that is often compacted by hooves. Pioneer grasses are unpalatable to most grazing animals, thus increasing their chance of survival. Seasonal grasses are termed annuals and they tend to indicate poor veld condition.
Certain other grasses rely on vegetative expansion. These 'creeping' grasses send 'runners' out from an established base. Should the runners encounter a suitable growing patch they will send down roots from the nearest 'node' and establish another plant. For example: Creeping Bristle Grass. Species of this growth form have low, lateral growth, often with the bulk of their mass underground. The leaves of these grasses are usually tough and hard to graze but the juicy root stock, in sandy soils, is utilised by warthogs, mole rats and springhares.
As conditions for growth improve the grass species tend to become broader leafed. The seeds become rounder, enabling them to drop between the increased ground cover. With these grasses becoming more attractive to grazers, animal droppings and hoof action further improve water infiltration, nutrition and other conditions for growth. Where grass plants survive through successive seasons in vegetative form they are termed 'perennial'.
The occurrence of perennial species indicates improved veld conditions. With increased competition for light, the grass species grow taller with more bulk and provide better soil protection. In many cases they become less palatable with inedible stems, like the grass used for thatching.
The leaves of many of these tall grasses are only accessible to certain ungulates with the necessary adaptations. The horns of both male and female sable antelope and roan antelope, as examples, help part the long grass stems to reveal the palatable leaves below. If the adapted species of antelope are missing, these grasslands may become unutilised and 'moribund'.
Too much shading from moribund material can smother plants and result in die off. Under-utilisation of grasses, while resulting in better soil cover, is nearly as detrimental as overutilisation. There is a very necessary relationship between grazers and grasses. In natural circumstances the 'coarse grazers' (like cape buffalo) will move through thick swathes of grass, opening it up for other species, which will shorten it further, making it suitable for the next species, and so on through the season.
The various species of ungulate assist each other with this type of interaction which results in ideal grassland management, producing a trampled mulch to retain moisture and permit light penetration. Where this doesn't occur the use of mowing may become necessary. Burning destroys the valuable mulch and sterilises much of the area. Grazing management. To return to the matter of time and numbers - many people consider veld management to involve solely the numbers of animals on a particular area.
This thinking comes from grassland / monospecies management. Under natural, subtropical conditions, animal populations would move around to the best grazing areas, unhindered by fences. Herds would tend to aggregate in large numbers at the end of the dry season, when they had to keep on the move to find food. It this way they acted like a natural mowing machine. When the rains arrived the large groups split up into their herds once more, with the benefit of 'new blood'. These natural systems enabled the grasses to receive the required rest periods for growth.
Furthermore, when one looks at the ecology of a grassland, there is little significance in numbers of animals at any one time but great significance in the amount of time spent on an area by even a few animals. If you walked one beast down a regular route each day of the year (x 365 animal days) there would be a well worn path at the end of the year. Even humans walking across a lawn often enough will form a distinct path!
Alternately, if you walked 365 animals down the same route on only one day of the year (also x 365 animal days), there would be no sign that they had been there at the end of the year. You could even pass 5000 animals through the area for a short period without affecting plant growth. The same thinking applies to grazing.
Regardless of the number of animals, moving herds leaves rest periods for the grass plants to grow. The damage of 'constant nibble' is caused by confined populations or animals 'anchored' to artificial and often badly sited water points. If the grasses are not getting the required rest during the growing period then veld management is faulted and the ecological balance of the grassland will suffer.
Common Grasses of Southern African Countries
Southern African Grass Species Checklist : Poaceae : Gramineae
Interesting facts about the Grass Family
Southern African Grasslands
African Savannah Biome