Responsible use of exotic tropical pasture cultivars — an ecological ...

334 J.G. McIvor and S. McIntyre
It is useful to examine this dilemma in an
ecological context.
Ecological model — factors controlling
vegetation structure
Both biotic and abiotic forces are involved and
there are a number of models of how these forces
combine to influence vegetation structure. One
proposed by Grime (1977; 1979; 1988) has
received the most attention. In his model, Grime
(1979) asserts there are three factors affecting the
composition of herbaceous vegetation — competition,
stress and disturbance. These factors have
resulted in the evolution of species with combinations
of characteristics, or strategies, to enable
them to survive in their particular environments.
Stress and disturbance limit plant biomass,
and combinations of them are associated with
different plant strategies — low stress and low
disturbance with the competitive (C) strategy;
low stress and high disturbance with the ruderal
(R) strategy; and high stress and low disturbance
with the stress-tolerant (S) strategy. No viable
strategy has evolved for environments with both
high stress and high disturbance.
These strategies are associated with particular
plant characteristics (Grime 1979). For example:
Competitive — high dense leaf canopy, extensive
lateral spread, low seed production relative
to biomass, and rapid growth rate;
Ruderal — rapid growth, high seed production
relative to biomass, small stature with limited
lateral spread, and high frequency of flowering;
and
Stress-tolerant — evergreens with long-lived
leaves, slow growth rates, low palatability, and
low seed production relative to biomass.
In addition to the primary strategies (competitor,
ruderal and stress-tolerator), there are
secondary strategies (competitive ruderals [C-R],
stress-tolerant competitors [C-S], stress-tolerant
ruderals [S-R] and “C-S-R plants”) which incorporate
the characteristics of two or all three of
the primary strategies. In an analysis of New
Zealand pasture cultivars, Campbell (1990)
showed them to be C-S-R or C-R plants, and
many Australian pasture cultivars are also likely
to be in these two groups.
The balance between competition, stress and
disturbance is a major determinant of vegetation
structure and species composition at any site.
Stress tolerators (low growth rates and low reproductive
effort) are favoured in undisturbed,
unproductive land; ruderals (high growth rate,
high reproductive effort) are favoured in disturbed,
productive land; and competitors (high
growth rate, low reproductive effort) are favoured
in undisturbed, productive situations. Pasture
development can involve considerable changes to
stress and disturbance regimes and competitive
balances, and native species adapted to the predevelopment
conditions are frequently not
adapted to the altered regimes and are lost from
the vegetation after development.
Species diversity is greatest at intermediate
levels of stress and/or disturbance (Grime 1979).
At high levels of stress or disturbance, only a few
species can tolerate the extreme conditions, so
diversity is low. At low levels of stress and disturbance,
the vegetation is dominated by a few
competitive species. It is only at intermediate
levels that many species can tolerate the stress/
disturbance regime and also not be competitively
excluded.
Ruderals feature highly in lists of agricultural
weeds, but it is instructive to look at the characteristics
of the pasture species listed as environmental
weeds in Humphries et al. (1991). Eleven
tropical pasture plants are listed — nine grasses
(Andropogon gayanus, Brachiaria decumbens, B.
mutica, Cenchrus ciliaris, Echinochloa polystachya,
Hymenachne amplexicaulis, Hyparrhenia
rufa, Melinis minutiflora and Pennisetum
polystachyon) and two legumes (Leucaena leucocephala
and Stylosanthes scabra). Under appropriate
conditions, these species are capable of
forming almost mono-specific swards. Although
some of these species (e.g. Pennisetum polystachyon)
have the high colonising ability characteristic
of ruderal species, the height and lateral
spread of the competitive strategy is the most
common feature. This enables these species with
the C strategy to dominate in ungrazed situations,
but if they lack tolerance of defoliation, they may
be lost from grazed pastures (e.g. Macroptilium
atropurpureum).
Geographic/ecological regions where exotic
cultivars should not be sown
Whether there are regions where exotic cultivars
should not be sown depends on the value systems
of the decision maker — these vary widely and it