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

Responsible use of exotic tropical pasture cultivars 335
is difficult to come to a general conclusion. However,
when considering the use of exotic plants,
the following considerations should be made to
minimise undesirable effects:
• What is the conservation value of the area?
Riparian zones and nearby areas are attractive
for development but are also key components
of landscapes for conservation. Grassy woodlands,
particularly those on fertile soils and
with gentle topography, are particularly suited
to agricultural development, which threatens
their survival. They are poorly represented in
reserve systems and attractive to sowing due
to their responsiveness. Unconstrained development
will lead to the loss of this vegetation
type, as has occurred in southern Australia.
• Exotic species should not be introduced to
new areas where they have not previously
been used; these areas should be maintained
free of exotics.
• What is the vulnerability of the existing
vegetation to change? At what scale should
this decision be made?
• Regional, catchment and whole-property perspectives
should be taken to planning pasture
sowings, not just the individual paddock, as
effects may extend off-site well beyond the
actual sown area. At the landscape scale, there
is a need to identify threshold levels of pasture
development beyond which ecosystem
processes, including the maintenance of biodiversity,
are likely to be adversely affected.
• Exotic species are not the solution to all
problems — they should be used only where
there is an economic gain, and their use
should be avoided in other areas.
Opportunities for developing improved
“environmentally friendly” cultivars
As mentioned earlier, the problem is the invasion
and colonisation of unsown (and often ungrazed)
areas by exotic species (typical of ruderal
species) and their subsequent dominance of the
vegetation (typical of competitive species). Both
colonising ability and the high yields associated
with dominance are valuable characteristics in a
pasture plant. It may be necessary to sacrifice
these to develop “environmentally friendly”
cultivars. This could involve selecting for:
• grazing dependence — plants with medium
palatability (high palatability could lead to
loss due to selective grazing and low palatability
to dominance as other species are
lost) and lower productivity (to reduce the
dominance associated with high yields). This
can be contrasted with palatable climbing
legumes (e.g. Macroptilium atropurpureum),
which have poor persistence in grazed
pastures but which can dominate ungrazed
vegetation.
• ability to co-exist — in many situations, diet
quality will be higher from a mixture of
species in a pasture as it is unlikely one
species will be superior at all times of the year
and in all conditions. This will involve less
emphasis on selecting for high yields as this
trait can lead to dominance and suppression of
other species.
• seed characteristics — a common feature of
ruderal plants is strong seed dormancy
mechanisms and often large soil seed banks.
The use of sterile plants (e.g. Leucaena
hybrids) or plants with no seed dormancy (as
in crop plants) would greatly reduce
weediness. Long-term persistence in such
species would depend on the survival of
established perennial plants rather than their
periodic replacement.
Opportunities for management
Since a number of exotic plants are causing
environmental damage, it will be necessary to
manage areas where this is a concern. Management
can minimise spread by reducing seed production
(e.g. with grazing, mowing, fire) and
transport of seed (water, wind, animals, humans).
These measures will not prevent invasion but
they can reduce the rate of invasion. Management
should also aim to prevent establishment and
growth of exotic species by minimising disturbance
(however, all ecosystems need some disturbance
for re-establishment) and encouraging
competition from established vegetation. This
will be difficult to do except for small areas, but
has a role in critical environments. As mentioned
in other sections, farm and regional planning of
pasture sowings to minimise off-site impacts, and
the development of “user-friendly” cultivars, are
needed to complement management designed to
minimise invasion and establishment.