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Associations Among Plants, Birds and Insects
The following article is based on a lecture given to the North West Group (Tasmanian Region) in September 1985.
I am a trained zoologist with a special interest in birds. I grow plants, especially Australian natives, primarily because they provide a home for our native birds. I study insects because I teach entomology and because they provide food for birds.
Birds are associated with plants and feed on insects - insects feed on plants and are preyed upon by birds. Before talking about these two way associations I want to talk about an intriguing three way association
among birds, plants and insects.
I was fortunate enough to spend 3 years of my life studying bower birds in sub-tropical rainforest in northern New South Wales. The green catbird, a monogamous bowerbird, lived and reproduced in the rainforest. It turned out that this catbird was highly dependent for its food on one plant species, the strangler fig; Ficus watkinsiana. This fig forms an emergent tree that towers above the rainforest canopy. Strangler figs were randomly distributed throughout my rainforest study area and produced an abundant crop of ripe figs regularly each year for several months. Adult green catbirds are predominantly fruit eating birds and strangler fig fruits formed the principal food item of their diet. The diet of nestling catbirds also consisted largely of fruit, mainly figs. Nearly all catbird territories contained at least one fig tree. The seeds of strangler figs germinate on the trunks and branches of host rainforest trees. The fig roots and branches form an interlocking lacework that envelops the host tree and eventually kills it, hence the name strangler fig.
But how are the seeds deposited high up in the canopy? This is where the catbird plays a part. Catbirds use their strong bills to detach fig fruits. Then they carry them in flight beneath the forest canopy and eat them on sites such as branches and crevices of trees. The catbirds help to disperse the seeds of figs. This is of mutual benefit to fig plants and catbirds. However, the story doesn't end here.
This stangler fig, Ficus watkinsiana, has a firm grip on a brush box (Lophostemon confertus). Select the thumbnail image or plant name for a higher resolution image (46k).
Figs are unusual in that fig flowers are enclosed inside the developing fruit or receptacle so how are they pollinated? It turns out that the figs and their pollinators are interdependent. Fig flowers are pollinated by tiny agaonid wasps and these wasps can only develop inside the developing fruit or receptacle. A female wasp carrying pollen. enters a young receptacle through an opening and pollinates female flowers, each of which develops and produces a seed. The female wasps also lay eggs in other flowers and the developing larvae form "gall flowers". The emerging wasps copulate and the female collects pollen before leaving the fig receptacle to search for and pollinate other fig flowers. There are many other interesting examples of these sorts of associations but now I want to briefly discuss associations between birds and plants.
Many birds depend on plants for their survival and reproduction. What do birds need to survive and reproduce and how do plants help? First, birds live in a habitat. Plants make up this habitat. Ornithologists classify bird habitats in terms of vegetation; examples of habitats are wetlands, grasslands, deserts, forests and oceans. Terrestrial habitats can be classified on the basis of structure and floristics (plant species composition).
In Australia forest habitats can be rainforest (tropical, sub-tropical, temperate) wet sclerophyll, dry sclerophyll or woodlands. These habitats can be further subdivided in terms of plant communities or associations. The plant habitat provides birds with a number of necessities, as indicated in the table.
|Food||The parts of plants eaten by birds are nectar, flowers, fruit, seeds, leaves and succulent stems. Plants also provide a home for insects and their allies and these are preyed upon by birds.|
|Shelter||Birds need plant cover to escape from enemies and avoid predation.|
|Water||Birds need water for drinking and bathing. In addition to creeks and streams water is also found in rock pools and tree crevices. Birds also use wet foliage for bathing.|
|Perches||Birds use perches not only for foraging but also for preening and resting. High perches such as dead twigs and ,branches are used by birds as vantage sites for detecting predators, intruders and for singing. Song functions to attract mates and repel intruders.|
|Nest Sites||Birds use plants to gather nesting material such as bark, leaves stems and roots. Birds build their nests in grasses, sedges shrubs and trees. Old trees like eucalypts have hollows which birds use as nest sites.|
Attracting Birds to Gardens
We can use our knowledge of bird behaviour to attract birds to gardens. Birds have an annual cycle consisting of a breeding season and a non-breeding season. During the breeding season birds are more resident
and territorial and need space and food to rear their offspring. A home garden is usually too small for many breeding bird territories. However, during the non-breeding season (autumn-winter), birds tend to range over a larger area since their food is less abundant. So we can capitalise on this behaviour to attract birds. Birds can be attracted to gardens by providing shelter, water, perches and food plants; (e.g. autumn/winter flowering grevilleas and banksias for nectar feeding birds and Eucalypts for pardalotes and some honeyeaters).
I now want to turn to associations between insects and plants.
1. Plants as Insect Food
Plants provide food for an enormous assemblage of insects. Insect species may feed on a wide range of plant species, a few species are highly specific and feed on only one species of food plant. Insects have modified mouthparts for chewing or sucking. Depending on what plant parts they eat insects can be classified as leaf eaters, sap feeders, wood and bark feeders, gall inspects and seed eaters.
- Leaf eating insects
Some insects are great defoliators and may reach plague proportions - e.g. stick insects (Phasmids) and plague locusts. Many kinds of insects defoliate eucalypts;
- caterpillars of moths such as autumn gum moth and gum leaf skeletoniser moth
- adults and larvae of beetles such as chrysomelids and scarabs;
- sawfly larvae - e.g. pear and cherry slug, large green sawfly.
Sawfly larvae, making a meal out of the foliage of a callistemon. Select the thumbnail image or highlighted name for a higher resolution image (20k).
Insect attack on plants is not always detrimental and may be beneficial. e.g. cactoblastis larvae eating prickly pear weevils and their larvae feeding on Salvinia.
Wood and bark feeding insects
Some insect larvae that bore into wood are those of longicorn beetles and cossid moths.
Sap feeding insects
e.g. aphids, scale, bugs, psyllids.
Scale insects, conceal themselves beneath a waxy shell and often occur in large colonies. Select the thumbnail image or highlighted name for a higher resolution image (24k).
2. Plants provide shelter for insects
Plants provide shelter for insects and so that helps the insects escape from enemies. There is intense predation pressure on insects by birds and insect parasites. Insects have counteracted this predation by developing extraordinary defensive and offensive behaviour to avoid and minimise predation. Some of these defence mechanisms are:
- cryptic coloration or camouflage that enables insects to be less conspicuous against their background
- posturing so that insects resemble twigs, leave and flowers
- bold patterns such as eye spots to alarm predators
- nocturnal feeding
- release of chemicals
- feigning death
- production of unpalatable and distasteful chemicals
- warning coloration - e.g. bright colours such as yellow and black stripes
- mimicry - e.g. the coloration of distasteful lycid beetles are mimicked by other beetles and a moth
3. Predation of Insects
Predators of insects include birds, spiders, other insects such as predatory beetles and bugs, parasitic wasps and flies. These predators help control insect populations. Many bird species consume vast numbers of insects. For example, white-eyes (silvereyes) mainly feed on insects (e.g. aphids) so they are welcome visitors to gardens. Yellow-tailed black cockatoos excavate wood boring insects such as the larvae of longicorn beetles and cossid moths. Forest ravens feed on cockchafer grubs in the soil. There are many more examples of insectivorous birds.
Insects too are beneficial and help control garden pests such as caterpillars and aphids. What insects are beneficial and how they can be recognised? We grow plants to attract birds but how many people grow plants to attract beneficial insects.
Beneficial insects include - ladybird beetles and their larvae - lacewings and their larvae - predatory beetles, bugs and mites - preying mantids - parasitic wasps and flies - spiders.
We have seen that insects use plants for their survival (food and shelter) and reproduction (egg laying sites, larval food, and pupa attachment), so what do plants get out of their association with insects?
Insects pollinate plants and thus play an essential role in plant reproduction. Flower structure has undergone extraordinary development to ensure that cross pollination occurs. Some flowers even mimic insects. For example, one orchid species emits a chemical similar to the odour of a female wasp. The male wasp is attracted to the scent, copulates with the orchid flower and deposits pollen. Both flowers and insects benefit from their association. Insects are attracted to flowers - flowers reward insects with nectar and pollen (protein). In return insects cross pollinate flowers, What other animals pollinate plants?
Australian myrtaceous plants provide abundant blossom for nectar feeding birds such as honeyeaters so it is not surprising that birds pollinate plants. Mammals such as possums also pollinate plants.
What insects are pollinators?
The introduced honeybee is a conspicuous and useful pollinator of garden ornamentals and fruit and vegetables. The Australian flora evolved long before the honeybee arrived, so what pollinators evolved together with Australian plants? The heathlands support a vast number of plant species and many heath flowers are pollinated by native Trigona bees. These bees are solitary and the females forage and compete with honeybees for flowers. In addition to bees and wasps, other insect pollinators are beetles, flies, moths and butterflies. Scarab beetles are mainly plant feeders and many of these feed on nectar and pollen - e.g. Phyllotocus and Diphucephala. Beetles tend to feed on open flat flowers such as Leptospermum and related genera. Pollinating flies, including tachinids, hover flies and bee flies, also feed on flat open flowers - e.g. Hibbertia. The food plants that attract beneficial insects to gardens include acacias, leptospermums, hibbertias and Bursaria.
Hibbertia stellaris, shows the flat, "buttercup"-shaped flower which is typical of the genus. Select the thumbnail image or plant name for a higher resolution image (36k).
This article began with an example of a 3 way association between catbirds, fig trees and fig pollinating wasps. In conclusion, I would like to mention another intriguing 3 way association between the mistletoe bird that feed on mistletoe fruits and disperses its seeds and the insects associated with mistletoe plants. An account of this association is illustrated and discussed in the magazine, GEO (June - August 1985. Vol. 7 No. 2).
From the June 1986 issue of the Newsletter of the Tasmanian Region of SGAP..
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