Enviro Perspectives on Ecological Principles with Dave Rushworth


Ecological Succession ? Grasses

ECOLOGICAL PRINCIPLES

Probably the most important things to understand, in the realm of 'matter', are the unchangeable principles or laws controlling the function of all physical systems on earth.

These various principles involve the function and passage of energy; the dynamics of growth and trophic (food) levels in individuals and populations; succession and recession within communities; the re-cycling processes of various forms of matter; controls and limits to growth; inter-actions and intra-actions between and within all forms of living and non-living matter and the stability of diversity.

All these and other principles are interlinked. Focus on the function of succession may assist with interpretation and identification of indicator species and assessment of veld conditions when planning remedial management.

SUCCESSION

Succession describes the progressive sequence in form and function of species within habitats - from lower to higher levels of energy and diversity. In a plain example - from desert to jungle. Species can be classified in this context by their physical adaptations to a particular set of environmental conditions.

Thus - long legs for deep wading or speed - thick coats for cold - sharp claws for climbing, digging or killing etc. Goats, ostrich and camels would be understood as 'low' succession animals while chimpanzees, crowned eagles and other forest species would be 'high' succession species. The same can be applied to water bodies and aquatic species - soils and subterranean species - grasslands, savannah and other habitats.

Succession can also apply to animal behaviour - as in the sequential feeding of raptors, vultures and other scavengers - the subsequent use of burrows constructed by digging species - the sequence of grazing species under different conditions - and so on.

Even in commercial business, humans practice 'succession' in the process of market surveys, production, advertising, distribution and back-up service. The correct sequence of progress produces what we term 'success'. There will be further opportunity to look at different categories of succession in various environments.

PRIMARY SUCCESSION

Primary succession involves the incorporation of 'life energy' into a non-living system - as in the production of soil - as opposed to pure sand. The only significant source of energy for life is from light - in our case, mainly from the sun. The only significant system for trapping this energy and converting it into food is photosynthesis, in the, mainly green, leaves of plants. Where there is no food, there will be no leafed plants - so the process starts with lichens and algae on bare (rock, bark) surfaces and in water.

In succession - lichens prepare conditions for mosses, which produce more food, hold more moisture and build up amounts of soil for (example) ferns. These develop conditions for higher plants such as salaginella, then forbs (weeds!), sedges and grasses. Meaningful animal life then becomes involved in the process of distribution and recycling to higher conditions. The highest, climatically controlled state is termed 'climax'.

SECONDARY SUCCESSION

Secondary succession starts with previously developed soil conditions. An example would be the natural development of a playing field, scraped and cleared of all existing vegetation.

Again, the progression would start with 'weeds' - then 'pioneer' grasses - to perennial grasses and shrubs and finally bush and larger trees. Animal life would succeed along with the plant life. Certain animals and plants would indicate to us the different stages of development - or various successional stages.

Tough pioneer plants, protected by thorns, taste and limited transpiration, will provide food, shade and increased moisture for 'better' plants. These, in turn, will increase conditions for growth of higher succession plants - and so on, up to climax within the climatic limits of the area.

GRASSES

Present focus will be on the general structure and sequence of grasses through successional stages. Grasses and sedges (grasses have nodes or joints - mainly absent in sedges) that have to survive in barren areas need to combat water loss, protect against predation and produce effective seed.

These 'pioneer plants' will thus have thin leaves with minimal transpiration and be unattractive to grazers. Their seeds will generally be pointed to stick into bare surfaces and dispersal will rely on wind (fluffy) or animals (burrs or spikes). Annual grasses that die off during the dry season will survive only in seed form until the next rains.

As conditions 'succeed', the grass species will appear with more succulent leaves and round seeds, capable of lodging in plant litter on the soil surface. Animal activity will increase the recycling process.

With improved soil and moisture conditions, grasses may survive the dry season and annual species will appear. These will compete successfully for light and shade out pioneer species, changing the species composition of the grassland.

Depending on soil and climatic conditions, grasslands will normally be invaded by woody species and progress to scrub or woodland. With the use of fire, conditions are often maintained at the successional level indicated by, what is commonly termed, 'thatching grass'.

This level, because it is held back by burning, is referred to as a 'fire sub-climax' state. The tall 'thatching grass' consists of many different species which are normally unpalatable to animals when mature, with only the new growth being grazed. Their seed cases are sharp and able to stick onto animals for dispersal.

MANAGEMENT CONSIDERATIONS

Overgrazed or too frequently burned grasslands will start to deteriorate - as will un-utilised grasslands, where shading and smothering will kill off grass clumps.

If not burned occasionally, these rank grasslands rely on heavy herds of elephant, buffalo or even cattle to tramp them down to allow in light for new growth. Grass requires some inter-action with animals at a sustainable level.

All plants require a sufficient rest period to allow them to replenish their root-stock and keep growing. Control of grazing pressure is not as important as control of the grazing period.

Under natural conditions there is a grazing succession where species move on to new areas once they have fed. Under confined conditions animals are unable to move on and, even in very small numbers, continue to nibble at re-growth over long periods.

Without leaves to photosynthesise the grasses die off. Recession sets in with good grass giving way to pioneer species. As cover diminishes the land is unable to hold rainfall, dries out and becomes eroded. Recession takes place relatively fast while succession back to good veld takes much longer. Sufficient rest period is the most important factor in veld management.

Reduce animal numbers to a sustainable level, with anticipation for years of poor rainfall. Place water points where they will not cause concentrated pressure on 'brittle' soils. Remove fences, where possible, to increase expansion areas - but resist the temptation to increase animal numbers.

VELD ASSESSMENT

Without getting too technical - there are easy ways of assessing the condition of veld - particularly grassland. Look at the scene through the eyes of a cow - or some herbivore. 'Good' grasses will generally have broad leaves and rounded seeds. 'Poor' grasses will have thin leaves and spiky seeds. Socks and clothing full of grass seeds after a walk through the veld will indicate 'poor' veld.

Look down into the grass - not across the area - to assess the true amount of ground cover. There should also be a good amount of litter (dead plant material) covering the ground between living plants to prevent erosion and provide shading to retain moisture.

The longer the soil stays moist, 7579the longer the growing period. Light should be able to reach the growth tips during the growing season but remember that desiccating, surface temperatures are also greatly reduced by shading.

Under-utilised grassland will be indicated by grey, moribund, standing material and die off in the centre of annual clumps, where growth has been shaded out.

Higher plant succession results in greater the diversity and stability. Better ground cover improves water infiltration and increases resistance to drought conditions. With our limited rainfall, much of the ?drought' and dropping water table is due to excessive run-off caused by over-grazing and poor land management.



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