Weed flora of South Africa 2 : power shifts in the veld

This paper, the second in a series analysing data from the National Weed List, concentrates on weedy changes that affect the indigenous flora. The incidence of weediness and of threatened species is used as an indication of success or failure of families under prevailing conditions of disturbance. The resulting power shifts between indigenous families and the impact of exotic weeds on the situation are reviewed. Many power shifts are taking place between indigenous species in the veld. Superficially it seems as though a relatively few invasive species are displacing a larger number of threatened species, but evidence from Natal points to more species increasing than decreasing under conditions of over-utilization. Amongst small families power shifts are almost as prevalent and important as at species level. Seventeen small families have over 20% indigenous weed species. Thirty small families have over 20% threatened species, and 21 small families have had their species numbers bolstered by more than 20% by exotic weeds. A very few large and medium-sized families contain over 50% of our weeds and our threatened species. It is mostly temperate (Cape) families that are under pressure relative to more tropical families. This trend is echoed at sub-family level within the Fabaceae.


INTRODUCTION
The veld, that 'ragged-brown carpet, vast and bare', that has served South Africa so well for so long is now becoming threadbare, leaving the way open to invasive weeds (Fig. 1).
Three main types of invasion by indigenous species take place (Wells & Stirton, 1981): -On its higher, drier margins the retreating grassland is being invaded by small, xerophytic 'karroid' shrubs.This is referred to as the spread of the karoo.
-On its lower, moister margins grassland is being invaded by trees (followed by shrubs and herbs) from the adjacent wooded communities.This is referred to locally as 'bush encroachment'.

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In the heart of the grassland, particularly on the higher, wetter ridges, unpalatable grassland species, including tussocks and poisonous plants are on the increase.These power shifts between physiognomically distinct communities are individually well known (Acocks, 1975), although it is doubtful if there is sufficient appreciation of their cumulative effect on the vulnerable grassland biome.
Power shifts between taxa, between exotics and indigenous plants, and between plants with different life cycles often take place within physiognomically homogeneous communities.Unless the plants involved are very distinctive, e.g.Aloe species increasing at the expense of grasses, these changes tend to go unnoticed.A case in point is the gradual infiltration of streambank communities by woody exotics, which has resulted in the almost total replacement of indigenous woody, streambank species in many places (Wells et al., 1980;Duggan et al., 1981).
In this paper we will try to throw more light on changes being brought about in the South African flora as a result of weed invasion, and to answer the questions: -'What power shifts are taking place between indigenous vascular plant taxa?' and -'How are exotics affecting the situation?'To answer these questions we need to know not only what taxa are showing expansionist tendencies, but also which are giving way.In the absence of specific and wholly compatible data on the subject, we have taken: -inclusion in the National Weed List (Balsinhas et al., in press) as an indication of maintained or increasing success; and -inclusion in lists of threatened species (Hall et al., 1980;Arnold et al., unpubl.)as an indication of lack of or decreasing success.

POWER SHIFTS BETWEEN TAXA
Power shifts have been analysed at species and at family level.

Indigenous weed species
Some of the weeds of natural pastures have been so classified because they are poisonous or carry burs, rather than because they are invasive and contribute to power shifts.
There is even one species, Harpagophytum procumbens (the grapple plant), a classic weed, that could soon be on the threatened plant list, as a result of over-utilization for medicinal purposes.However, most of the indigenous weeds of natural pastures are woody, unpalatable species that are increasing their range and numbers as a result of disturbance.
In some cases a single species, such as the unpalatable Aristida junciformis (ngongoni) replaces not only the previous dominant (Themeda triandra), but supresses most other indigenous plants in the area, forming almost pure stands over thousands of hectares (Edwards et al., 1980).In this particular case, the power shift is serious because it is difficult to reverse by means of management techniques.
In other cases e.g.Chrysocoma tenuifolia (the bitter karoo bush) the invasive species are regarded as weeds because they are unpalatable, or even poisonous, but they spread by colonizing bare areas rather than by suppressing other species.They therefore also have a caretaker role, providing shade and shelter, binding the soil and acting as a buffer against degeneration (Acocks, 1971).Most power shifts involving caretaker species can be reversed fairly easily using management techniques, provided that the environmental conditions have not degene rated too far.

Threatened species
Some of these species are naturally rare or localized and even those whose numbers are decreasing are not all the victims of plant invaders.However, the vigour and adaptability of other plants in the vicinity (even caretaker species) must contribute to the competitive pressures exerted on those that are less resilient.
At present far more indigenous species are considered to be threatened (Hall et al., 1980;Arnold et al., 1980) than are considered to be weeds (Balsinhas et al., 1982).Superficially, it would appear that a relatively few vigorous, invasive, indigenous species are strengthening their positions at the expense of a much greater number of species that either are or will become threatened.In view of this, the lack of knowledge about so many species, their success rate and status, gives cause for concern.

The species inbetween
Information about the success or failure rate of species that are neither considered to be weeds, nor threatened, is very fragmentary.Foran et al., (1978) have developed a method for assessing conditions in grassland which measures decreaser species that decline in abundance with bad management, increaser-I species which increase with under utilization and increaser-II species which increase with over-utilization.Tainton etal., 1978 suggest the addition of an increaser-III group to include species encouraged by selective grazing.Westfall (1981) has expanded the method to include woody vegetation, and there is a good prospect that increaser and decreaser species will be recorded on a countrywide basis during the course of a natural resources survey which is being carried out by the state Department of Agriculture & Fisheries (Edwards, 1981).
It is of interest that Foran et al. (I.e.), recorded more increaser than decreaser species under conditions of over-utilization and the overall effect of power shifts between indigenous species is far from clear.

Small families
The 156 small families contain an average of only 6,4 indigenous species each, and include 46 families with only a single indigenous species.In such small families the success or failure of a single species or a very few species has a profound effect on the fortunes of the family.Power shifts are almost as prevalent and important as they are at species level.There are 17 small families that contain over 20% indigenous weed species.They include the waterplant families: Lemnaceae, Hydrocharitaceae, Potamogetonaceae and Aponogetonaceae, whose spe cies are taking advantage of habitats provided by man-made dams as well as the enriched conditions in natural water bodies.

Large and medium-sized families
A very few families supply most of our weeds and threatened species (see Table 1).
Of the 8 largest families that supply most of our species, only 4 (Poaceae, Asteraceae, Fabaceae and Liliaceae) together with one medium-sized family (Cyperaceae) supply over 50% of our indigenous weed species.Apart from Liliaceae (most of whose weed species are poisonous rather than invasive), these families appear to be strengthening their position relative to the remaining large families Mesembryanthemaceae, Iridaceae, Ericaceae and Asclepiadaceae.
If we ignore the vast areas of crop and wasteland covered by agrestal and ruderal weeds, and concentrate on the remaining areas of veld, we find that 50% of the weed species of natural pastures are supplied by the same five families that supplied most weeds of all kinds, plus one medium-sized family Rubiaceae.A further change is that the weedy role of indigenous Poaceae is much lower in natural pastures than in adjacent crop and wastelands.This may well reflect vegetation changes shown in Fig. 1, where grass-dominated communities are giving way to karroid and arborescent communities in which most species are drawn from Asteraceae and Fabaceae.
Most indigenous families, even the weedy ones, contain more threatened species than weeds.The threatened species are more evenly distributed, 50% of them being supplied by 9 families: 6 large families (Iridaceae, Asteraceae, Liliaceae, Asclepiadaceae, Ericaceae and Fabaceae), together with 3 medium sized families (Proteaceae, Orchidaceae and Santalaceae).The overall tendency seems to be for the large families to maintain or strengthen their positions relative to the smaller families.
As with indigenous weeds, the exotic weeds are concentrated in very few families.Practically 50% of all exotic weed species, and more than 50% of exotic species naturalized in the veld are provided by only 4 families: 3 large families -Poaceae, Asteraceae and Fabaceae, and one medium-sized family Solanaceae.
Three different patterns of success or failure within large and medium-sized families emerge from Table 1.
Pattern 1: large families containing many weeds and many threatened species, with the weedy side of the family being re-inforced by many exotics (Asteraceae, Poaceae and Fabaceae).Amaranthaceae, not shown in Table 1, exhibits a very similar pattern.
Pattern 3: medium-sized families with very few indigenous weeds and threatened species, being supplemented by many exotic weed species (Solana ceae).Other families not included in Table 1, that show this pattern, are: Apiaceae and Polygonaceae (and many small families).
A fourth pattern, not represented here, is that of the 10 exotic families which between them supply 21 weed species, all of which are naturalized in the veld.
There are many variations of and intermediates between these patterns, but those shown here illustrate the main power shifts i.e. away from families showing pattern 2, (mostly centred in the Cape) and towards those showing patterns 1, 3 and 4 (mostly centred in the more tropical parts of the country).

Sub-family level
An analysis of Fabaceae, one of the large families with many weeds as well as many threatened species, shows interesting differences at sub-family level (Table 2).
If the percentages in columns C, D and E are added (column G) it is seen that there is far less stress and expansion (9%) in the large sub-family Papilionoideae than in the other sub-families (Caesalpinioideae 45%, Mimosoideae 72%).In Papilionoideae the ratio of threatened species to weeds C + E is also very different (0,7), compared to 3,8 in caesalpinioideae and 3,3 in Mimosoideae.There would appear to be a strong shift away from the more temperate, herbaceous Papilionoideae in favour of the other two more tropical and woody sub-families.

CONCLUSIONS
Many power shifts are taking place between indigenous species in the veld.Superficially, it seems as though a relatively few invasive species are displacing a larger number of threatened species, but evidence from Natal points to more species increasing than decreasing under conditions of over-utilization.Amongst small families power shifts are almost as prevalent and important as at species level.Seventeen small families have over 20% indigenous weed species.Thirty small families have over 20% threatened species, and 21 small families have had their species numbers bolstered by more than 20% by exotic weeds.
A very few large and medium-sized families contain over 50% of our weeds and our threatened species.It is mostly temperate (Cape) families that are under pressure relative to most tropical families.This trend is echoed at sub-family level within the Fabaceae.
. 1.-D ia g r a m m a tic re p re se n ta tio n o f th e th ree m a in types o f in v a sio n o f the grassland.

TABLE 2 .
-The distribution of weeds and threatened species in the three sub-families of the Fabaceae