By Tony Start, CALM WA
From Savanna Burning—Understanding and Using Fire in
Northern Australia, Tropical Savannas CRC, Darwin 2001
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Misteltoe Photo: Tony Start
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Mistletoes are a natural and diverse component of
Australia’s tropical savannas. Most species show some degree
of host preference; some grow only on eucalypts, others prefer
acacias.
In healthy landscapes, each species lives in harmony with its
hosts while contributing nectar to honeyeaters, fruit to the
mistletoe bird and palatable leaves to herbivores. They thus play
an important role in the whole ecosystem.
But many mistletoe species have a peculiar problem. If the host
is killed by fire, the mistletoe dies too; indeed most mistletoes
will die if they are scorched, even if the host can resprout. If
all the hosts in an area are burned, all the mistletoes die.
Mistletoes, then, are obligate seeders but, unlike other
obligate seeders they have no seed bank to initiate a new
generation. Their seeds have to be ‘planted’ on the
branch of a suitable host by a bird. After fire has killed a
population of mistletoes the only source of seed is another
population, usually one growing outside the burnt area. It becomes
even more drastic where the host is another obligate seeder, like
many of the acacias. Then the process of recolonisation cannot
begin until the hosts have grown up—usually adding several
years to the process.
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Mistletoe Photo:
Ian Partridge
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Frequent fires a threat
Frequent, extensive intense fires can eliminate mistletoes from
huge areas, and this has undoubtedly happened in parts of the
tropical savanna. No one knows what the effect has been for nectar
and fruit-eating birds or the other plants that shared their
services. However, we do know some herbivorous insects depend on
mistletoes. For example, the larvae of some of our most spectacular
butterflies feed on nothing else, so they too will have disappeared
from huge areas.
The converse is that the presence of a diverse array of
mistletoe species, including some that grow on obligate-seeder
hosts, can indicate that the area is long unburnt or, at least, has
had a regime of mostly mild and probably small fires. This can be a
useful indicator because there are few parts of the tropical
savanna where we have a long record of the fire regime. Being able
to identify areas where the regime has been mild may allow us to
examine many other aspects of the impact of fire over long periods
of time.
