Tuesday, May 19, 2009

The Interaction of Grey-headed Flying Foxes, Vegetation and Restoration on Bellingen Island.
            Grey-headed Flying Foxes (Pteropus poliocephalus) have had a breeding colony on Bellingen Island for about 50 years. The bats camp in a small remnant (1.8 ha) of subtropical rainforest belonging to the association Ficus spp .- Heritiera trifoliolata - Dendrocnide excelsa - Cryptocarya obovata. What effects have they had on the Island’s rainforest? And how do they effect its future? The following is a summary of effects observed between 1985 and 1999, during the course of an ongoing restoration project.
1.         The most obvious effect, and the one that gives rise to judgements about their destroying the rainforest, is that bats cause leaf and branch loss to canopy trees. The important factors are the species of bat (the resident Grey-headed Flying Foxes cause less defoliation per unit of area occupied than the infrequently visiting Little Red Flying Foxes); the size of the population in relation to the area of forest canopy; the time and duration of the stay; and the particular tree species occupied (Strangler Figs, say, seem to fare much better than Stinging Trees). In 1995-98 the increased canopy from restoration work and the slightly lower than usual numbers meant that defoliation had little visible effect. This was an important turning point in the remnant’s history, and hopefully the canopy will be able to accommodate greater bat numbers in future. There may however be long term changes in structure (e.g. canopy height) due to yearly defoliation of upper canopy.
2.            Defoliation has caused the slow death of trees and a lowering of canopy height. Weed competition from exotic vines, flooding and wind damage have also caused tree death. However there are no documented on site extinctions of tree species.
3.         As well as defoliation, bats may affect flowering and fruiting of resident trees. Though there is no evidence that the bats are responsible, at least 2 tree species (Litsea reticulata & Doryphora sassafras) and 2 vine species (Rauwenhoffia leichardtii & Legnephora moorei)- each represented by only single specimens - have failed to fruit during the last 13 years. Other species that are more common on the site have also failed to fruit. Initially, competition from exotic vines (Anredera cordifolia) was another factor effecting fruiting. Species of biogeographical significance - e.g. at their southern limit - have generally managed to fruit and produce seedlings.
4.         Though there are no documented extinctions of epiphytes, it is clear that they are poorly represented, and this is probably due to the bats. There are no Bird’s Nest Ferns in the space usually occupied by the bats, very few Staghorns, and only one individual epiphytic orchid (Dendrobium gracilicaule). There are only 5 species of epiphyte in all, represented by few individuals.
5.            Strangler Figs are epiphytic as juveniles and young Stranglers are rare. The population ecology of trees involves long time periods, but it seems the flying foxes may represent a significant hiccup in the history of the 4 species of Strangler Fig present on the Island.
6.            Defoliation of canopy increases the amount of light reaching the ground. This accelerates the growth of plants at ground level and encourages plants that respond most quickly to high light levels. Most native rainforest tree and vine species revel in these conditions, including the shade tolerant juveniles of canopy species. But the competition is fierce. Some species, including herbs like Cunjevoi, Scrub Nettle and Pollia crispata, trees like Stinging Trees, Sandpaper Figs, Red Cedars, and many vines are designed for these conditions. The herbs are well represented, even in unweeded areas; and the trees (i.e. their seedlings) are only common after weeding. Vine seedlings are far less common, probably due to the lack of adult vines on the Island, although the number of vine species represented by the seedlings has increased dramatically.
            The main weeds on the Island - Anredera cordifolia and Tradescantia fluminensis - out compete all the native species, under these high light conditions. Weeds are the most important factor in reducing the persistence of the native rainforest community. Along with flooding (the Island is well below the 1 in 20 year flood level) , bat defoliation reduces the resistance of the community to weed invasion.
            The relatively poor representation of vines and ground ferns - prior to restoration - was probably due to the history of vine invasion by Anredera and ground blanketing by Tradescantia. These weeds also stopped the recruitment of seedlings of all species. At the time of the commencement of the present restoration, seedlings of nearly all species were effectively non-existent. The relatively long life of trees, even under bat pressure, was all that kept the native rainforest community going, until restoration commenced.
7.         Bat droppings increase ground surface nutrients and sometimes damage and kill native seedlings, but this is probably not of major significance, because many seedlings survive. Certain tree species are more susceptible, so the species mix may be effected. High surface nutrients encourage species adapted to such conditions, particularly herbs in the family Commelinaceae. One of these is Tradescantia, but there are 3 native species on the Island - Commelina cyanea, Pollia crispata & Aneilema biflorum - and the last 2 of these do particularly well. They too can blanket the surface, and effect tree, vine and herb establishment but this is not a concern. They do not dominate large areas and so may be seen as contributing to overall diversity.
8.         Bats discourage birds and, in turn, this effects the kinds of seeds that are introduced onto the Island. Bats tend to introduce small seeded species, although they do also introduce large seeds (e.g. Queen Palms). Native laurels, an important component of this type of rainforest, are only introduced by birds; and as it turns out, laurels are fairly well represented among seedlings.
            The increased number of species underneath Strangler Figs is probably due to the large numbers of fruit eating birds that visit these trees between May and September (when bats are usually absent). Whatever the effect that the proportion of bat introductions to bird introductions has on the overall floristic mix, it is probably best regarded as a matter of the individuality of the Island’s plant community.
            Both bats and birds introduce weed species but none constitutes a management problem to rival Anredera or Tradescantia (both introduced by floods). Bats introduce Queen Palms - they carry the seeds in from trees in Bellingen - and though easily managed on the Island, this species may become a serious weed of less intensively managed rainforests.
9.         One way to assess the persistence of the rainforest community is to look at the history of on-site plant extinctions and recruitments. Regarding the following table: the blanks in the last row indicate that total numbers of natives and weeds were not calculated; but there have been at least 15 recruitments of new species, some of which may have come and gone (e.g. the climbing orchid Pseudovanilla foliata)

Structural Form
Total of Natives Including Recruitments
Native Recruitments since 1984
Trees and Shrubs
Epiphytes & Mistletoes
Ground Ferns
Perennial Herbs
            Firstly, extinctions. Strictly, the only way to tally extinctions is by the use of species lists. Of the species documented in the 1970s by Floyd there is only one confirmed extinction: Settler’s Flax (Gymnostachys anceps), a perennial herb. However, recruitments since restoration began, not only indicate the results of the work, they also give some clue to the scope and kinds of past extinctions. There have probably been extinctions of epiphytes, vines and ground flora. In the case of epiphytes, this can be inferred from the small numbers now present. In the case of vines, this can be inferred from the large percentage increase (+131%) in species numbers since the removal of weeds. Likewise with ground ferns, there has been a +56% increase in species numbers. Apart from the epiphytes these extinctions are probably largely due to weed competition. Past management has involved cutting native vines, a practice that would have assisted Anredera and led to loss of native species. Lawyer Palm (Calamus muelleri), which has appeared as a result of weeding, is very common in these subtropical rainforests and may have been actively discouraged by recreational users of the Island because of its vicious climbing fronds and prickles. There have probably been tree extinctions as well, but the percentage increase in tree species since weeding has only been +21%.
            The only structural class that has shown no new recruitments is the epiphyte group. Though epiphytes are likely to have been souvenired in the past, this indicates that epiphytes are probably the main structural type to have been directly effected by bats. The processes of tree and vine extinction are slow, and provided there are no weeds, populations are reasonably well maintained by seedling recruitments. There are changes in the mix of species, but with the exception of Strangler Figs, most of the distinctive tree species are being recruited. The doubling in the number of vine species indicates just how severely vines were effected by Anredera.
            Changes in community composition reflect many ecological factors, including the mix of bat and bird introductions, the historical presence of weeds, the process and the effects of their removal, the proximity of the Island to particular seed sources, the size of the community, fluctuations in canopy density, and choices made in plantings (Trees from on site seed sources are planted in large gaps and sunny edges, primarily to relieve pressure on existing bat roosts. There are, of course, many species, in all the structural classes, that occur in this type of community but are not on Bellingen Island. The only ones that has been planted to date are Black Apple (Planchonella australis) and Red Bopple Nut (Hicksbeachia pinnatifolia). Both are usually dispersed by rats and gravity and would be unlikely to colonise without human intervention. Some wind dispersed species - including ferns and orchids - have arrived from apparently quite distant sources.). These factors all contribute to the individuality of the community. Whether the result is degradation though is a matter up for judgement.
            Erosion is serious along the river bank, and Little Red Flying Foxes could well do some serious future damage to the canopy. However, barring these contingencies the main long term threat to the remnant remains, not flying foxes, but the threat of future cultural neglect.

No comments:

Post a Comment