Many rainforest trees begin their life in the beak of a wompoo fruit dove. And wompoos find it hard to survive without rainforest. This partnership is among the latest in a long series of trysts between rainforest trees and fruit-eating birds. A fruit and its seeds may be just food to a bird. But to a plant, a seed is a hopeful bid for the future, and the fruit is a bribe to improve its chance of success.
The lives of fruit doves and rainforest trees have been entwined for at least 30 million years.[i] These and other plant-bird partnerships have allowed both trees and birds to move across the land and sea, and to establish new populations – and often new patches of rainforest – where suitable conditions exist.
The wompoo is a largish fruit-dove, sumptuously coloured. It has a regal purple breast and yellow belly, and is mantled olive-green above. The small, irregular yellow splashes on its bright green wings mimic flecks of sunlight on leaves. Its grey head is offset with a red and yellow beak, and an imperious red eye. This bird eats only rainforest fruits.[ii] Surprisingly agile in the treetops, it sometimes spends all day in a large tree, climbing out to even fine branchlets to pluck ripe fruit.[iii] Its call is deep and ghostly, but also rollicking: ‘wollock-a-woo’. Sometimes paraphrased by birdwatchers as ‘bollocks are blue’.[iv]
Once a fruit is swallowed by this – or any other – Ptilinopus fruit dove,[v] the flesh is massaged off by a gizzard that has no grit.[vi] Nothing to harm the precious seed, unlike the crushing beaks of parrots, or the gizzards of some pigeons that digest both fruit pulp and seed. After passage through a wompoo the seed emerges intact, and sometimes far away from the parent tree.
(Click to see whole picture)
Trees don’t move, but their seeds do. Seeds enable trees to migrate to new places or at least get away from their parents. This is not a trivial concern as seedlings germinate and grow better away from their own species, including their parent tree.[vii] Embedding those seeds in a fleshy-fruit parcel can increase the chances of moving further and landing in a suitable place for germination and growth.
Given the same amount of resources, a tree can either make lots of small seeds, or fewer large ones.[viii] Large seeds are packed with provisions for the first vulnerable stage of seedling growth. Small seeds, by comparison (to paraphrase Colin Tudge), are like children setting out on a wild adventure with nothing but high spirits and a bag of toffees.[ix] Most trees of closed forests – including rainforests – have large seeds,[x] while many trees of more open vegetation (e.g. eucalypts) have small seeds. Why should this be? Small seeds generally do best in well-lit places – all else being equal – as they have just enough stored energy to grow a few solar cells, and once this is done they can rely on sunlight for their ongoing energy needs. Large seeds often survive better than small seeds in shady places.[xi] In a rainforest, the greater energy stores in a large seed help it to grow leaves despite the shade, and push roots and shoots through the thick litter on the ground. But the drawback of being a large seed is that you won’t travel very far if you just fall off your parent tree, or rely on the wind to blow you further. This is where animals come in.
Worldwide, the majority of rainforest trees (over 70%) have fleshy-fruited seeds[xii] attractive to animal dispersers, as opposed to winged seeds for wind dispersal, or unadorned seeds that just fall to the ground. In the rainforests of south-east Queensland, 69% of woody plant species[xiii] have fleshy fruits attractive to animals, while another 3.5 % have fruits that are ‘enticing but unrewarding’, and which might still be dispersed by the gullible.[xiv] Dispersal takes the seed away from the parent tree, and also helps trees to colonise new areas. Some animals may also help to scatter fallen seeds, so that the chance of a seed germinating close to other seeds of its own kind is lower, which would enhance its survival and growth. Birds and mammals have been eating the fleshy fruits of flowering plants and dispersing their seeds for about 90 million years.[xv] In Australian rainforests, the main animal dispersers of fleshy-fruited seeds are birds and bats: unlike many other rainforests, there are no primates.
Many animals eat fruit in Australian wet rainforests, including possums, rats, bowerbirds, parrots, lizards and rat-kangaroos. But not all of these are good seed dispersers. Some digest the fruit as well as the seed. Some steal the fruit and leave the seed. And some move the seed only short distances. Fruit-doves are seed dispersers par excellence. They don’t harm the seeds and they fly – which means they sometimes move long distances between the fruit going in one end, and the seed coming out the other. However, the ability to fly does restrict the size of a bird, which limits the amount of fruit that can be eaten in a meal. The flightless southern cassowary, by comparison, can eat enough fruit in one sitting to put a kindergarten to shame, and produces whopping seed-filled droppings to prove it. But although cassowaries like to bathe, and are known to swim, their capacity to spread rainforest trees across the sea is probably low.
Dispersal options are limited for trees with very large fruits. Fruit doves tend to avoid fruits that are larger than their maximum gape width, except for large soft fruits with small seeds (e.g. figs) that can be eaten piecemeal. Bats can carry larger fruits for some distance before eating them, and possums and rats might do the same. Brush turkeys steal apples from camp grounds by spearing them with their beaks, and then run for cover into the rainforest. But I’m not sure if they do this when eating more natural foods. When a large-fruited rainforest tree loses its animal dispersers, this may diminish the rate of genetic mixing and lead to inbreeding and eventual extinction. Rainforest trees are long-lived and may persist for a long time. But not indefinitely, and especially not in times of more rapid change, such as now.
Much rainforest has been cleared in Australia, and in south-east Queensland the remnants are in mostly small, highly fragmented patches. Fruit-eating birds have the ability to bring the rainforest trees back into cleared parts of this landscape, and rainforest can be restored if conditions are right and certain threats are removed. But what if some important seed dispersing birds have declined along with the rainforest that was destroyed? Some rainforest birds have certainly declined, and others don’t like visiting small and degraded patches of rainforest. But the good news is that one plucky bird is both reasonably common, eats fruits of many different sizes, and frequents both regrowth and remnant rainforest patches, large and small. This is the figbird, a common visitor of gardens and often observed in inner-city Brisbane. Even some of the more specialist fruit-dove dispersers (e.g. the topknot pigeon) appear to tolerate some habitat fragmentation.[xvi] But there is no room for complacency: many seed-dispersing birds such as the wompoo fruit-dove have declined where rainforest has been fragmented. These declines are likely to reduce the volume and variety of native seeds reaching new and regrowth rainforest patches.[xvii] Animals and plants that mostly occur in rainforests may also depend on other vegetation types that form a matrix joining rainforest patches. They live in these places at certain stages of their lifecycle, during certain seasons and while travelling between rainforest patches. Even if most rainforest is now protected, clearing of this matrix vegetation continues in south-east Queensland and elsewhere, and this contributes to the declining health and status of plants and animals across the region. Protection of remnant rainforest and matrix vegetation, and restoration of cleared areas is essential if we are to conserve many of our native rainforest species, and especially in a time of climate change.[xviii]
Birds don’t choose fruit based on country of origin. So native birds also spread weeds, and especially those with fleshy fruits. This can be a curse when aggressive weeds out-compete native plants, but can also be a blessing. ‘Increaser’ bird species such as figbirds and silvereyes may change the trajectory of rainforest restoration by bringing in large numbers of weed seeds which swamp the native plant seeds.[xix] But some of the most-hated weeds such as lantana (Lantana camara) which smothers native vegetation, and is linked to eucalypt dieback, also provides food and shelter for birds and other creatures. The spread of the exotic, fruit-bearing tree camphor laurel (Cinnamomum camphora) is also a double-edged sword. Invasion of intact rainforest by camphor laurel is certainly not desirable. But in the highly-cleared landscapes of south-east Queensland and northern New South Wales this weedy tree is helping native rainforest species return to degraded lands. Camphor laurel can establish in open areas where many native rainforest trees find it difficult to get a foothold. Once there, camphor laurel trees attract native fruit-eating birds, who carry in and drop the seeds of native plant species.[xx] These native plants grow up underneath the camphor laurel trees where they are shaded, and protected from frosts and drying winds. The camphor laurels can then be killed by stem injection of herbicide, and left as dead trees to provide perches for fruit-eating birds, who can continue to drop seeds. As long as competing camphor laurel seedlings and other weeds are regularly controlled, this method can be highly successful in returning rainforest to oldfields, and is also much cheaper than clearing and replanting with natives.[xxi]
Rainforest trees and their bird gardeners have lived and travelled together in Australia for millions of years. They continue to do so today. They don’t ask too much of us. Just a place to grow, a perch to sit on, the removal of competing weeds, and enough connected habitat for both trees and birds to survive and thrive. Surely that’s the least we can give them, out of respect and admiration for such an enduring and fruitful partnership.
[i] Cibois et al. 2014 Phylogeny and biogeography of the fruit doves (Aves: Columbidae). Molecular Phylogenetics and Evolution 70:442-453
[ii] Crome 1975 The ecology of fruit pigeons in tropical northern Queensland. Aust.Wildl.Res. 2:155-85
[iii] Morcombe, M. 2003 Field guide to Australian birds. Steve Parish Publishing Pty Ltd, Archerfield.
[iv] Pizzey, G. and F. Knight 1999 A field guide to the birds of Australia. Harper Collins Publishers, Sydney.
[v] Ptilinopus fruit-doves that occur in Australia are the wompoo, superb and rose-crowned. The topknot pigeon (genus Lopholaimus) is the only other Australian rainforest pigeon that eats fruit without harming the seeds.
[vi] Crome 1975 The ecology of fruit pigeons in tropical northern Queensland. Aust.Wildl.Res. 2:155-85
[vii] Connell et al. 1984 Compensatory Recruitment, Growth, and Mortality as Factors Maintaining Rain Forest Tree Diversity. Ecol. Mon. 54:141-164; Harms et al. 2000 Pervasive density-dependent recruitment enhances seedling diversity in a tropical forest. Nature 404:493-495; Hubbell et al. 2001 Local neighborhood effects on long-term survival of individual trees in a neotropical forest. Ecol. Res. 16:859-875; Comita et al. 2014 Testing predictions of the Janzen–Connell hypothesis: a meta-analysis of experimental evidence for distance and density-dependent seed and seedling survival. J.Ecol. 102:845-856; Webb et al. 1967 A factor toxic to seedlings of the same species associated with living roots of the non-gregarious subtropical rain forest tree Grevillea robusta. Journal of Applied Ecology 4:13-25
[viii] Grubb et al. 2005 Comment on “A brief history of seed size”. Science 310:783.
[ix] Tudge, C. 2005 The secret life of trees. Allen Lane p. 75.
[x] Eriksson et al. 2000 Seed Size, Fruit Size, and Dispersal Systems in Angiosperms from the Early Cretaceous to the Late Tertiary. The American Naturalist, 156:47-58; Leishman et al. 2000. The Evolutionary Ecology of Seed Size, Ch. 2, pp:31-57, In: M. Fenner (ed) Seeds: The Ecology of Regeneration in Plant Communities, 2nd edition, CAB International; Butler et al 2007 Biogeography of seed-dispersal syndromes, life-forms and seed sizes among woody rain-forest plants in Australia’s subtropics. J.Biogeogr. 34:1736–1750
[xi] Leishman et al. 2000. The Evolutionary Ecology of Seed Size, Ch. 2, pp:31-57, In: M. Fenner (ed) Seeds: The Ecology of Regeneration in Plant Communities, 2nd edition, CAB International; Moles and Westoby 2004 Seedling survival and seed size: a synthesis of the literature. J.Ecol. 92:372–383
[xii] Willson et al. 1989 Vertebrate Dispersal Syndromes in Some Australian and New Zealand Plant Communities, with Geographic Comparisons. Biotropica 21:133-147
[xiii] trees, shrubs and vines
[xiv] Butler et al. 2007 Biogeography of seed-dispersal syndromes, life-forms and seed sizes among woody rain-forest plants in Australia’s subtropics. J.Biogeogr. 34:1736–1750
[xv] Fleming and Kress 2011 A brief history of fruits and frugivores. Acta Oecologica 37:521-530
[xvi] Moran, C., Catterall, C.P., Green, R.J. and M.F. Olsen 2004 Functional variation among frugivorous birds: implications for rainforest seed dispersal in a fragmented subtropical landscape. Oecologia 141: 584–595
[xvii] Moran, C., Catterall, C.P., Green, R.J. and M.F. Olsen 2004 Functional variation among frugivorous birds: implications for rainforest seed dispersal in a fragmented subtropical landscape. Oecologia 141: 584–595
[xix] Moran, C., Catterall, C.P., Green, R.J. and M.F. Olsen 2004 Functional variation among frugivorous birds: implications for rainforest seed dispersal in a fragmented subtropical landscape. Oecologia 141: 584–595
[xx] Neilan, W., Catterall, C.P., Kanowski, J. and S. McKenna 2006 Do frugivorous birds assist rainforest succession in weed dominated oldfield regrowth of subtropical Australia? Biological Conservation 129: 393–407
[xxi] Kanowski J, Catterall CP, Neilan W (2008a) Potential value of weedy regrowth for rainforest restoration. Ecological Management and Restoration 9, 88-99.