When Europeans first paddled a
long boat up Derbarl Yerrigan (Noongar for the Swan River), Perth was a very
different place. The coastal plain was covered in Banksia woodland, and the
hills were blanketed in virgin forest of giant jarrah and marri trees. Inland
of the Darling Range stretched an enormous woodland of wandoo, weerluk (salmon
gum) and other eucalypts. Today this has been replaced by a vast area of
agricultural land which is known as the Wheatbelt, but, as a result of salinity
and climate change, is fast becoming a barren wasteland.
Felling trees on a landscape
scale, replacing them with shallow-rooted cereal crops, spreading many
introduced grasses, and installing thousands of bores and dams altered southern
inland WA forever. There are still pockets of bushland, but the wildlife these
areas (and the broader region) can support is a shadow of its former self.
However, not all animals were negatively impacted by land clearing. Some have
benefited enormously from the landscape changes, and changes in their
population has an affect on Perth bushland. How?
The Galah (Eolophus roseicapillus; also called the ‘pink-and-grey’) once only
occurred far inland, more than 500 km to the north and east of Perth. It began
to visit this region in the early 1900’s, but only occasionally. When
land-clearing went full steam ahead between 1930 and 1960, its population
boomed, colonising the south-west in a relatively rapid, widespread expansion. This
was a result of a huge increase in its main food (the seeds of grasses,
including wheat), and the supply of permanent water on farms. It was not until
the 1980’s that this bird established at Armadale. Such information is only available
because a few dedicated ornithologists carried out survey expeditions and kept
track of changes happening across the state. The information is stored in the
WA Museum, highlighting the importance of wildlife study and record-keeping
institutions in our society.
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| The extent of galah chewing on this Marri tree is evident |
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| Bleeding and re-sprouting shoots are signs of stress. |
Galahs are now very common in the
south-west, including the Perth suburbs. Like many birds, they breed in tree
hollows, being the only cockatoo species to use other material (gum leaves) to
build a nest inside. They also happen to be a very aggressive guardian of nest
hollows, so once galahs move into a tree, they chase other species away,
deterring many smaller birds from breeding. They have even been recorded
driving out larger, endangered species like Red-tailed Black Cockatoos.
Their long-term attachment to a
nest tree also means galahs like to advertise that ‘this is our place’. To do
this they chew huge patches of bark off the trunk, and wipe powder from their
feathers onto the bare patch, leaving a chalk-like appearance. It’s thought
that ‘painting’ this smooth surface white makes it highly visible to other galahs
(and perhaps other parrots), which probably see an extra vivid mark by seeing
in the ultra-violet spectrum. The blue patches just below the cheek on male Budgerigars
don’t look particularly vivid to us, but to a female budgie, who can see
ultra-violet light, they glow very brightly.
It has been suggested that this Galah
chewing habit aims to stop nest predation by climbing animals such as goannas,
which climb trees to raid nests. However, there is no real evidence to support
this: no goannas have been known to lose their footing on smooth bark, and the
scars are not made right around the tree, just on one face on the same side as
the nest entrance. There were also early theories that Galahs aim to kill trees
and speed up the hollowing process, thus creating future nest sites, but, as
Ian Rowley says in his early publications on Galahs, no animal is known to be
capable of such a degree of forward thinking.
Whichever the case, Galahs have
become so common in a region where they previously didn’t occur, and this isn’t
a good thing. In many patches of bush around Perth, all the large,
hollow-bearing trees have a resident pair, defending their home from most other
birds and giving them ‘Galah scars’. Scarring causes massive stress to trees,
and consequently the health of many bushland areas suffers. Keeping in mind our
bush already has enough to put up with as habitat fragmentation and changing
climate bring increasingly dry conditions.
The Galah story gives us an
example of how changes to a landscape in one area can cause a native species to
become over-abundant, and have large impacts to vegetation and creatures living
in another. In bush around Perth now, you will see Galahs occupying many tree
hollows, introduced Rainbow Lorikeets taking over the others, and the
occasional native Australian Ringneck or ‘28’ squeezing in where they can. Other
natives like Red-capped Parrots, Purple-crowned Lorikeets, Elegant and Regent Parrots,
as well as a whole suite of LOCAL native birds and mammals, are either totally
gone or severely reduced in number. Overall we see a loss of biodiversity, an ‘ironing
out’ of the landscape which once had many different shaped and sized ‘bumps’.
Each bump is a different plant or animal, responsible for a different task in
the ecosystem, and the ‘job losses’ which result from diminished biodiversity are
negative impacts to the landscape.
Nature works in weird and
sometimes not-so-wonderful ways.
