Plants have their predators; they are eaten by many things. Most obvious are the large animals, particularly mammals and birds (this process is generally known as grazing or browsing). Predation by smaller invertebrates, particularly insects, but also molluscs, is generally known as herbivory.
Vertebrate grazing and browsing
In pre-European times (pre-1800) grazing and browsing animals in the woodland would have included native macropods, kangaroos and wallabies. In the farming period (19th and 20th centuries) these would have been more or less replaced by introduced cattle, sheep and horses. Rabbits have been in the Australian environment since the 1860s, and though initially absent in the more open landscape of our woodland in 1988, have re-established there since 1990 as the groundcover has become denser. Wallaroos and Swamp Wallabies have established in the Australian Botanic Garden and have been periodic visitors to the woodland since the mid 1990s.
Rabbits and hares
Over the past decade, rabbits, digging for roots and using bared areas as latrines, have caused, and continue to cause, the most obvious vertebrate impacts on the woodland vegetation. Their activity is most evident during dry and drought periods. In general there appears to be very little discrimination for particular plant species, especially in their diggings, and we could never understand what, in particular, the animals preferred.
There does however appear to be selective grazing of some species, at least at certain times, as we noted particular browsing of plants of the yellow flowered daisy Calotis lappulacea
, such that plants were severely denuded, in comparison with some caged plants in the exclosure areas. Available plants a short distance away were not touched. Rabbit browsing may however explain the very low numbers of Calotis lappulacea
in the woodland.
Hares have been observed in the woodland, presumably browsing on plants, but they run too fast for us to find out what they eat!
For our research results see Monitoring impacting events - Rabbit impacts
Wallaroo and Swamp Wallaby (macropods) grazing appears to be more systematic than the rabbits, and concentrated particularly on grass tussocks, though we have not noted any species preferences. Again we know virtually nothing about the ecological impacts on the distribution of woodland plants. Their habits of creating bared areas where they camp may provide potential plant germination microhabitats, and it is possible they may also carry some seed long-distance by adherence to fur. However a future population build-up of any grazing animals in a relatively confined area has potential to cause serious deleterious grazing impacts.
Bared soil may provide suitable sites for germination of some plant species. In pre-European times, digging by bandicoots and native rats may have provided those conditions, but they have now disappeared; rabbit diggings may possibly be providing a similar microhabitat.
For example, at times there appear to be concentrations of seedlings of Solanum prinophyllum associated with rabbit latrines particularly during dry periods. Whether the rabbits are eating Solanum fruits and leaving them on the latrines in their scats, or whether soil conditions on the latrines are favourable for germination of Solanum seed already in the seedbank, we do not know.
Though insect herbivory is likely to have a major impact on the abundance and distribution of woodland plants, we know very little about the insect/plant interactions. Leaves, shoots, stems, flowers, fruits, seed and roots may be eaten. Here are a few interactions we’ve noted. See our invertebrate species lists and image galleries for more information.
Leaf and stem herbivory
Herbivory on the eucalypts is particularly significant, and includes many leaf eating insects, gall forming insects and stem boring insects. Periodic defoliation of Eucalypts (dieback) occurs when canopy eating insects build up in unusually large numbers due to environmental or climatic conditions, see plagues and diseases.
The Brown Leaf moth larvae Phallaria ophiusaria cleverly camouflaged as a twig, feeds on shrubs of Dodonaea viscosa (also reported to feed on Acacia and Hakea). The caterpillar larvae of Joseph’s coat moth Agarista agricola feed on leaves of the vine Cayratia clematidea. Crusader bug Mictis profana is a sap sucker on Hardenbergia violacea (and also on a range of garden plants). Larvae of the Botany Bay weevil Chrysolopus spectabilis (the first insect to be described from Australia) feed in the wood of wattles.
Milkweed bug Lygaeus kalmii is also found on *Gomphocarpus fruticosus and we have seen nymphal stages hiding in fruiting parts of the related natives Parsonsia straminea and Marsdenia rostrata. *Gomphocarpus fruticosus was introduced to Australia in the 1830s and had spread widely in western Sydney in the 1840s. It is the host plant for the caterpillars of the Lesser Wanderer Butterfly, which appears to have been an accidental introduction to Australia, though it needed the pre establishment of the Gomphocarpus. The bright colours of the caterpillars warn of their unpalatability due to chemicals taken from the host plant.
Most aphid species have been introduced to Australia with their host plants. In the woodland we have found Oleander Aphid *Aphis nerii on *Gomphocarpus fruticosus. It has also been recorded from species of Ascepiadaceae and Apocynaceae and could occur on native species such as Marsdenia and Parsonsia. We have also noted Aphids on Chloris ventricosa fruit.
Mature seed is eaten by insects such as ants, while developing seed may be eaten from within. Larvae of a Diptera midge eat Brunoniella australis seeds as they develop and in recent years all seed crops in the woodland appear to have been eaten. Interestingly, good seed collections of Brunoniella were made at Glenmore Park a few years after fires burned extensive areas in December 2001.
Seeds of Desmodium brachypodum are also eaten from within, by a different insect, though in this case not all seeds in a capsule are consumed.
Below ground herbivory
Root-eating insects include the large grub Abantiades, up to 8 cm long, which emerge from the ground in Autumn as the grey-brown Swift moth. Larvae of cicadas and many beetles also spend most of their lives underground, eating plant roots. The characteristic holes up to 2 cm diameter, that the cicadas and swift moths emerge from as adults, also allow water to penetrate deeply into the soil.
Other invertebrates in the woodland are snails including the Endangered Cumberland land snail Meridolum corneovirens which eats fungi, and the introduced Garden snail *Helix aspersa. It is likely that *Helix aspersa consumes seedling plants, both native and exotic, but although dead shells are common, it is rarely seen, and the extent of its predations are unknown. It is likely to impact on seedling recruitment during wet periods, when it might be selective for some species. The snails themselves are preyed upon by the Carnivorous Land Snail Austrorhytida capillacea and parasitized by the fly Amenia imperalis.
Do fewer predators impact on exotic species?
A factor in the success of many exotic plant species may be that they do not have many insect predators; these were left behind in their country of origin. For example exotic African Olives *Olea europea subsp. cuspidata in the woodland have healthy canopies with well-formed leaves that show little evidence of leaf eating insects. Compare these with the invariably chewed leaves of the eucalypts, or the stems of the native shrub Bursaria spinosa that always seem to carry some form of creature.
Scientific biological control programs have involved the introduction of controlling organisms from their county of origin. A well-known, and remarkably successful example was the introduction of the *Cactoblastus cactorum moth to control Prickly Pear (*Opuntia species) in eastern Australia in the 1920s. Biological control rarely involves the complete eradication of the target, and some plants will generally survive. In our woodland there are still scattered plants of *Opuntia stricta, but every now and then, under the right climatic conditions, a plant is parasitized by the *Cactoblastis moth and destroyed by its caterpillars which turn the cactus leaves to pulp.
Asterisk * indicates exotic species naturalised at the Australian Botanic Garden.