Understanding mechanisms of weed damage to ecosystems: para
grass, amphibians and reptiles
Exotic weeds invade ecosystems and cause reductions in plant and
animal biodiversity, alteration of food chains, reduction of
species abundances and alteration of assemblage structure of
existing flora and faunal communities. These negative effects can
severely reduce the ecosystem services provided by a previously
valuable area, and important functions of natural ecosystems (such
as maintenance of water quality, pollinator services, and
maintenance of local biodiversity) can be compromised.
Among the invasive weeds most likely to have negative effects on
ecosystems are grasses, because they compete successfully with
native species in a wide range of ecosystem types, and can alter
critical ecosystem processes, such as fire-proneness and nutrient
cycling, of the invaded ecosystem. Introduced pasture grasses
comprise a third of the worst environmental weeds in Australia.
Para grass ( Brachiaria mutica ) has become established in
waterways where it causes economic and ecological problems, such as
flow reduction and increases in sedimentation, as well as reduction
in the attractiveness of wetlands to water birds, and reduction in
fish diversity. Evidence from Australian wetlands invaded by para
grass suggests that it turns diverse wetland plant communities into
a monoculture, drastically reducing both floral and faunal
diversity. These changes are unlikely to be reversed simply by
removing vegetation. In this case, vegetation removal on its own
may simply degrade habitat further, rather than enhance it.
In this study, we examine the influence of para grass control,
on amphibians and reptiles in a northern Australian wetland –
the Townsville Town Common Conservation Park. We will also examine
uninvaded wetland areas adjacent to the para-grass-affected areas,
to quantify the reptile and amphibian assemblages present in
uninvaded wetlands. Finally, we will observe the habitat use of an
abundant predator (keelback snakes, Tropidonophis marii )
to predict the responses of native fauna to the removal of the
weed, and to understand some of the mechanisms responsible for
these responses.
Project update:
Surveys of amphibians and
reptiles in the woodland adjacent to the paragrass-dominated
floodplain reveal a high species richness and abundance in this
habitat. The woodland may be an important site for
recruitment of herpetofauna that utilize the Town Common
floodplain.
Pitfall and funnel trapping
conducted in the woodland during mid-March 2007 was used to gain
information regarding seasonal species composition, population
demographics, and movement patterns. The Green Striped
Burrowing frog (Cyclorana albogutta) and the Ornate
Burrowing frog (Opisthodon ornatus) dominated the
amphibian assemblage of 7 species encountered during this
period. The skinks Ctenotus robustus
and Carlia pectoralis, and the freshwater keelback snake
(Tropidonophis marii) were the dominant reptile
species. In total, nine species of skink, three
snakes, two dragons, and one goanna species were encountered during
March.
An early dry season trapping
session and additional incidental field observations in mid-April
added two species of arboreal gecko, 1 species of legless lizard,
and 2 species of frog, indicating a potential shift in the
assemblage. A winter trapping session planned for July
will be used to study this further, and it will provide data on
habitat use of species relative to burned, grazed, burned and
grazed and control plots.
The keelback snake is the
most abundant predator found on the Town Common.
During May, snakes were collected over 15 nights for 2-3 hours from
dusk along the road bisecting the woodland, and an additional six
snake species were identified. Over winter 2007, we
successfully tracked keelback snakes and determined details of
their habitat use in woodland and floodplain habitats.
Keelbacks moved almost exclusively in the daytime, and in the early
morning and evening were using shelter sites. On
the floodplain, snakes used para grass as shelter sites, whereas
snakes released close the border of woodland and para grass used
mostly burrows and logs, and occasionally clumps of other grasses
as shelter. Predation on snakes was highest on
the floodplain which suggests either that shelter is inadequate in
para grass, or that there is another reason why predators may more
successfully capture snakes in the para grass.
In spite of this, however, we found no evidence
that snakes released for tracking near the edge of para grass moved
into woodland habitats. Snakes were clearly able
to use para grass infested floodplain
habitats. We will conduct further
tracking during periods of greater activity (warmer months), and
will reveal movement patterns when snakes are feeding.
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