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Plant/animal interactions in the Sahelian zone

Pierre GRANIER *

(*) P. Granier, Chef Service Agrostologie, laboratoire d'Elevage, B.P. 485, Niamey, Niger.


Introduction
Influence of biotic factors on the recovery of the herbaceous vegetation stratum
Influence of biotic factors on the evolution of the woody stratum
Conclusion
Bibliography


SUMMARY

The drought which has ravaged the Sahel has had a significant effect on natural selection. Because of it, processes have become apparent which in normal times would have been difficult to discover.

The evolution of vegetative associations, the disappearance of certain species, and the overrunning of the ground by pioneer species have revealed adaptive capabilities which were hardly perceptible when the distribution of vegetation was basically related to inter-species competition.

Biological studies of species, carried out on the land and in the laboratory, have provided an explanation of the direction taken in the rapid evolution of vegetation due to the drought, and have made it possible to draw up a diagrammatic estimation of the slow evolution which substituted for climax vegetation the species now produced.

Introduction

The biomass is directly influenced by ecological physical factors, and it has been possible to study correlations between productivity, amount of rainfall, temperature and evapotranspiration. However, the biomass may undergo extensive modification when man or livestock appears, because it depends indirectly on seed stocks and on the hardiness of the fresh growth of perennials, the potential productive yield of which is linked to the time and intensity of grazing during the preceding period of growth.

Although feed problems for livestock occur during the dry season, it should not be forgotten that the evolution of vegetation depends on the pressure of grazing during the rainy season, when abundant supply causes the breeders to cease to manage and control the use of the pasture.

The influence of livestock is apparent at different levels:

- trampling of young germinations,
- uprooting of young germinations,
- trampling of spikelets and the liberation of seeds, which counteracts the protection provided by the envelopes,
- dispersal of certain diaspores,
- successive occurrence of defoliation.

It is possible to make a laboratory study of the impact of trampling by comparing germination, at different depths, of unprotected seeds and whole spikelets.

The impact of over-grazing on the woody stratum may only be studied in an area periodically protected from grazing.

Influence of biotic factors on the recovery of the herbaceous vegetation stratum

1. Perennial species

One consequence of the drought was the almost total disappearance in certain zones of perennials, the most important of which are: Andropogon gayanus, Cymbopogon proximus and Cyperus conglomeratus.

Biological study of these species on the land and in the laboratory has revealed the causes of their disappearance.

A prerequisite factor of an ecosystem is an essential biological factor: the ability to reproduce, to disseminate diaspores, and the capability of the diaspores to germinate.

The seeds of perennials have been allowed to germinate under laboratory conditions, and their germinating capacity has been tested (rate and vigour of germination) in comparison with that of the seeds of annuals.

The results are presented in the annexed graphs:

- yield is independent of the depths of the seeds;

- the percentage of yield is markedly greater for unprotected seeds; germination is restricted by seed envelopes; and

- the most significant element is the comparison of rates of germination.

As early as the 5th day after the first rain, 85 percent germination of perennials and only 6 percent germination of annuals has occurred. On the 12th day this figure stands at a maximum of 98 percent for perennials, whereas for annuals it does not exceed the rate of 46 percent. The " germination curve" for perennials is smooth and therefore reflects a homogeneous group, whereas the " annual's curve" goes up in steps as if composed of genetically different populations.

Hence only one early fall of rain is required for 85 percent of the seeds of perennials to germinate. If this initial rain is followed by a dry period, which frequently occurs, all the immature plants die and the supply of seeds is exhausted.

In conclusion, this method of reproduction is hazardous, requiring as it does climatic conditions which only rarely occur naturally.

The spread of these perennials may only be considered for a climate with a higher rainfall, in which the rains are more suitably distributed in time. They occur at the moment in the Sahelo-Soudanian zone, and were probably established in this zone during a more humid period.

Impact of Livestock

If there is no sexual reproduction, pressure of grazing on existing clumps of vegetation becomes intense. All the parts of the plants in which photosynthesis occurs are destroyed by the teeth of livestock, and this defoliation, causing the release of reserves contained in the underground parts, ultimately dries them up. The plant cannot grow long roots, a very important factor in ensuring supply of water in sandy soils. If there is a prolonged lack of water, the plant, incapable of renewing its cellular tissue, dies. It is quickly attacked by termites, and then a small tumulus is all that is left of a clump of perennials.

The brutal evolution caused by the drought enables us to understand how, in the course of time, vegetation consisting of species of perennials could gradually deteriorate, and how the better adapted annuals could spread and replace the perennials.

Perennials + overgrazing + lack of water annuals
® annuals

Importance of perennials

The disappearance of perennials has had repercussions at different levels of the ecosystem:

- Considerable decrease in productivity. The biomass of an association of perennials might produce as much as 2.5 t/ha dry matter, while on average, a group of therophytes rarely exceeds 1 t/ha.

- The productivity of the perennials extends over a period of time because of the occurrence of a second growth at the beginning of the dry season and new growth beginning when the moisture content of the air increases in the spring.

- With regard to nutritional value, the second growth of perennials produces, as well as energy, nitrogenous matter and carotenes, which are totally lacking in annual stover vegetation.

- The survival of stumps permits the enrichment of the soil in organic matter through the reconstitution in situ of the dead matter.

- A certain amount of activity at the level of the flora and fauna of the soil is maintained over the whole year, whereas sandy soils bearing annuals become practically devoid of living organisms.

- Wind erosion is reduced by the survival of stumps, which hold in place the surface soil layer of loose ground.

- Finally, perennials are exploited by man for purposes in addition to animal feed. In the Sahelian zone straw is used to retain the sand in pits and wells, and its disappearance has required breeders to abandon some pastureland which is no longer usable because of this, and to concentrate themselves on productive pasture - with predictably disastrous results.

2. Annual species

The regrowth of pasture of annuals can only be achieved, in normal production conditions, if the process of fructification is not disturbed. The only significant effect of seed brought in from elsewhere, by wind or animal, is in reestablishment by pioneer groups in areas where there has been a period of deterioration into a desert state because of drought or prolonged over-use.

Therophytes generally have a short vegetative cycle, particularly where water is a limiting factor. Grazing will thus have an impact on the production and dispersal of seeds according to whether it takes place before, during, or after fructification of the graminaceae.

Where pastureland is forward, trampling and chewing affects clumps of vegetation which have not taken root very securely in loose ground; and density is considerably reduced because of the pulling up and destruction of many germinations.

In turn, the loss of all the photosynthesizing parts at the leafy stage results in a reduction in the creation of reserves in the underground parts, which then causes a reduction in root growth and thus the supply of water.

Grazing during the growing season reduces the number of new flower-bearing shoots and the number of seeds produced.

Certain species develop no protection during fructification, and the livestock ingest a large amount of immature seeds. (Ripe seeds rejected by the digestive tract cannot retain their capacity to germinate until the following year's rainy season). This is the case with Chloridae, Panicae, Eragrostae...

However, certain species such as Cenchrus biflorus, and certain Aristida have diaspores which quickly become wounding, thus providing protection and ensuring their dispersal. This is a favourable factor in the consideration of the wider use of Cenchrus.

In an ecosystem where there is equilibrium between livestock and forage yield potential, rotation occurs naturally. Given the large areas of land, and the variations in time and space of rainfall it only exceptionally occurs that pastureland must support a high rate of stocking without being allowed to rest from time to time, as the grazier moves his herd to more productive pasture.

The process of deterioration begins when the rate of stocking is too great when areas which some graziers have judged to have been grazed heavily enough are occupied by others who are less concerned about maintaining a balance of the vegetation, or when the convenient location or an area of pastureland causes an excessive concentration of livestock.

Associations of annuals + overgrazing ® associations with early germination and short cycle = decrease in primary production ® overgrazing ® decrease in reseeding potential ® deterioration.

Influence of biotic factors on the evolution of the woody stratum

In the dry season, the totally dry herbaceous layer no longer contains carotenes, and the total nitrogenous matter content is around 1 percent of the dry matter. It is acknowledged that in certain zones, since digestibility is not good, the only source of digestible nitrogenous matter is to be found in the woody stratum (trees and bushes).

a) The importance of the woody stratum in animal nutrition

In the framework of a study of livestock behaviour in the Sahelian zone, the length of grazing period, rumination, movement and rest of two herds were compared; both herds were grazed in the same zone, and were watered at the same source, but " woody " pasture was available to one herd, while the other was kept in an area with no woody stratum.

Comparison of the duration of principal activities

 

Woody pasture

Non-woody pasture

Grazing

5 hrs. 30 mins.

6 hrs. 30 mins.

Movement

3 hrs. 40 mins.

8 hrs. 30 mins.

Rumination

4 hrs. 30 mins.

3 hrs. 30 mins.

Rest

10 hrs. 15 mins.

5 furs. 20 mins.

Watering

05 mins.

10 mins.

Observations

Grazing in woody pasture occurs in the daytime only, whereas in the non-woody pasture grazing at night is significantly more important than grazing in the daytime.

In the case of the non-woody pasture, the amount of time taken for movement is too great, and for rest and rumination too little.

It is noted from observation that many animals are in a very bad state, losses due to cachexy occur, and Vitamin A deficiencies are apparent. Some animals afflicted with blindness must be slaughtered.

No losses whatsoever were reported in the herd which ate the leaves of trees.

b) Importance of grazing on the structure of the woody stratum

The structure of vegetation has been studied using a cross-section composed of low-lying land, alluvial plain and dune in a heavily stocked area (cattle, goats, sheep).

The area of the cross-section was enclosed, in order to follow the evolution of the different strata. In the dry season, at the time the land was taken out of grazing, a sampling was made to determine the specific contribution of individual specimens, their manner of regeneration, their height and diameter. (The herbaceous stratum was non-existent.) The first finding is that the total number among the woody types, if all the shoots and germinations are taken into consideration, is distinctly greater than the number normally found in samples concerned only with " adult " trees. In the low-lying land, per hectare, the total reaches 703 individual specimens, only 47 passing the height of 3 metres.

Certainly, in an environment which has 703 individual woody growths per hectare, if deterioration-causing factors can be removed, the stratum will develop towards a closed formation.

Measurement of growth is calculated per 20 cm, and it is noted that a very high proportion of woody types do not exceed a height of 1.20 m, which is the average height at which small ruminants are able to graze.

Distribution of " woody " types according to accessibility to small ruminants for grazing

Species

Heights

% of woody growths

< 10 cm

< 120 cm

> 120 cm

< 120 cm

Balanites

30

60

54

52.6

Ziziphus

63

260

84

75.5

Maerua

10

15

4

78.9

Acacia

21

29

9

76.3

The distribution of individual types within the population, which should normally follow a hyperbola, is disturbed by a factor which in this case is, without any possible doubt, the action of livestock. It is noted that with regard to the most sought-after species, such as Maerua crassifolia, the proportion of individuals less than 1.20 m in height reaches almost 79 percent, which could be taken as an objective test of edibility.

Regeneration occurs by germination (84 percent) or by sprouting (16 percent). This high percentage of germinations is attributable to the abundance of " woody " types per hectare and illustrates the potential of these groupings as well as their vitality and their adaptation to an environment which is normally considered unfavourable.

In conclusion, it may be stated that if it is possible to control the influence of livestock, woody types would evolve towards a closed formation, with a productive yield far greater than the present level; and that resting the land by rotation is the most realistic method of preventing the deterioration of the environment, natural regeneration guaranteeing a complement to the diet during the dry season.

c) The importance for pasture of periodic protection from grazing animals

The importance of "rest periods " is undeniable from the psychological point of view, when one realises that the graziers have lost the concept of ecological balance, because they almost never have the opportunity of seeing climax vegetation in the Sahelian zone. Comparison between periodically protected areas and deteriorated areas, which clearly shows the considerable increase in productive yield, is the most effective way of reawakening a sense of responsibility in graziers

In addition, the renewal of the vegetational environment enables techniques such as rotation and the reserve stocking of feed to be introduced. Physical planning can only be considered if the ecosystem is developed to allow man to rediscover the traditional rhythm of rotation between areas of pasture in the rainy season and areas held in reserve for the dry season.

Finally there can only be progress in research if it is organised in such a way that areas both of experiment and practical application are not fragmented but integrated in the ecosystem.

Conclusion

The drought which has ravaged the Sahelian zone has abruptly brought into focus the process of desert encroachment; and there is a tendency to attribute the imbalance of the ecosystem to this one factor alone. Closer study of this phenomenon, based on analysis of aerial photographs and comparison of samples of vegetation reveals that the imbalance was begun a long time ago, and that the drought has only served to speed up a process which would undoubtedly have caused the same changes, although its full effect would have become evident much later.

The effects of natural selection have been accentuated and have revealed adaptive mechanisms which explain how over a period of time the most hardy types (annuals) have been able to take hold among climax formations and replace them.

The study of the influence of grazing on the structure of the woody stratum and of the comparative biology of herbaceous vegetation enables us:

1. To propose a structure for climax vegetation before man provokes regressive evolution. The Sahel should be a savannah of perennials, including woody species, with mimosas being proportionally fewer in number, the lower lying areas supporting closed woody formations, annuals occurring in newly established (fallen earth, young dunes, alluvial deposits) or skeletal soils.

2. To propose a means of making rational use of the Sahel if we want to stop regressive evolution and the decrease in primary productivity. The most important of the measures to be undertaken is either to appropriate the pasturelands or to assign responsibilities regarding the use of complex pastureland and water sources. The reduction of the stocking rates per hectare and attention paid to rotation and transhumance will only be possible if the authorities and technical services are able to establish who is to take full responsibility.

Bibliography

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