Consequences of Rainforest Fragmentation for Frugivorous Vertebrates and Seed Dispersal

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Catterall, Carla

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Seed dispersal strongly influences patterns of plant regeneration. Frugivorous (fruit eating) vertebrates disperse the seeds of between 70% and 90% of rainforest plant species. Forest fragmentation may affect the abundance and distribution of frugivore species. Consequently, patterns of seed dispersal and plant regeneration may vary between extensive forest and fragmented forest landscapes. This thesis assessed frugivorous vertebrates and seed dispersal in a rainforest landscape in subtropical Australia. First, this study quantitatively compared the distribution and abundance of frugivorous bird and bat species between fragmented and extensive rainforest. Second, the roles of these frugivore species in seed dispersal were evaluated based on their functional attributes and the plant species that they had been recorded consuming. Third, secondary consequences of forest fragmentation for seed dispersal were predicted from these results.

The field components of this study were conducted in the Sunshine Coast region of southern Queensland. Surveys of frugivorous bird and bat species were undertaken in a network of 48 study sites distributed throughout a 4 000 km² area. Sites comprised 16 replicates of each of three site types: extensive forest (>4 000 ha), rainforest remnants and patches of secondary regrowth. Extensive forest sites were stratified by altitude (low (<200 m above sea level (a.s.l.), medium (200-500 m a.s.l.), and high (>500 m a.s.l.).

Birds were surveyed using 40 minute area searches within a one hectare plot during the early morning. Each site was surveyed for birds four times: twice during summer and twice in winter. Forty-two frugivorous bird species were identified during surveys. Twenty-six of these species occurred frequently enough to quantitatively assess their abundance pattern in remnant and regrowth sites relative to extensive forest. There were five species that were recorded in much lower numbers in remnants and/or regrowth than in extensive forest (‘decreasers’), seven that showed higher abundance in remnants and/or regrowth than in extensive forest (‘increasers’) and 14 whose abundance did not vary substantially between the three habitat types (‘tolerant’ species). The decreasers included four rainforest pigeons (the wompoo, rose-crowned and superb fruit-doves Ptilinopus magnificus, P. regina and P. superbus and brown cuckoo-dove Macropygia amboinensis) and the green catbird Ailuroedus crassirostris. There was no evidence for the complete seasonal movement of frugivorous bird species between high and low altitudes.

A lack of understanding of the functional roles of frugivorous species has previously limited our capacity to predict specific consequences for seed dispersal of frugivore declines. A major dimension of functional variation among frugivore species is the suite of plant species that they disperse, which depends initially on their patterns of consumption of plant species. In this thesis, frugivorous bird species that were expected to have similar patterns of plant species consumption were assembled into ‘functional groups’. These groupings were based on the bird species’ gape width, degree of frugivory and their methods of seed treatment. For example, it was proposed that species with wide gapes would be able to consume large fruits, whereas those with narrow gapes could only consume small fruits. It was also expected that species with fruit-dominated diets (‘major frugivores’) may consume a different suite of plant species than species with mixed diets or with diets dominated by non-fruit (‘minor frugivores’). Species that crushed seeds were expected to disperse few viable seeds. Analyses showed that decreaser bird species were predominantly from functional groups that had the potential to disperse large-seeded plant species and may be the main dispersers of native laurels (Lauraceae). Consequently, it is likely that the dispersal of these plants may be reduced in fragmented forest.

Relationships between the functional attributes of frugivores and their actual patterns of plant species consumption were analysed using data on the plant species that each frugivore species was known to consume. Diet data were collated from 151 published sources as well as field observation and included records for 244 plant species. Major variation in patterns of plant species consumption corresponded with variation in frugivore species’ attributes. For example, the average size of fruits consumed by bird species increased with their gape width, although minor frugivores tended to consume fruits that were much smaller than their capacity. Statistical comparisons showed that highly frugivorous bird species consumed the highest number of plant species from the Lauraceae, whereas bird species with mixed diets consumed more arillate plant species from the Celastraceae, Sapindaceae, Mimosaceae and Elaeocarpaceae than other frugivore groups. Bird species from a range of functional groups consumed figs and small-fruited plants from families such as Euphorbiaceae and Solanaceae. Minor frugivores and a small number of major and mixed-diet bird species had species-poor diets that were dominated by these latter plant taxa.

In order to specifically assess the potential consequences of forest fragmentation for seed dispersal, patterns of plant species consumption were compared among decreaser, tolerant and increaser frugivore species. In particular, the potential for tolerant and increaser bird species to substitute for decreasers was evaluated. Analyses showed that dietary records for 12% of the 220 native plant species represented in the data set, including several from the Rubiaceae, were restricted to decreaser bird species. In addition, analyses showed that few non-decreaser species consumed numbers of native plant species with fruits wider than 10 mm, or from the Lauraceae, Myrtaceae, Meliaceae, Verbenaceae and Vitaceae that were comparable to decreaser bird species. Consequently, it is predicted that there is limited potential for functional substitution by other bird species for decreasers and, therefore, that the dispersal of these plant taxa may be substantially reduced in fragmented compared with extensive rainforest.

The potential for frugivorous bats to disperse seeds in fragmented forest was also assessed. Frugivorous bats were surveyed during summer in each of the 48 sites that had been surveyed for birds. Two observers conducted nocturnal, hour long searches along a 400-500 m transect. Two flying-fox species (grey-headed flying-fox Pteropus poliocephalus and black flying-fox P. alecto) and the eastern tube-nosed bat Nyctimene robinsoni were recorded during surveys. At the time of surveys, Pteropus spp. were most frequently recorded in regrowth, whereas N. robinsoni was detected more frequently in extensive forest and remnants than in regrowth. Decreaser bird species and N. robinsoni are rainforest and fruit specialists whereas tolerant and increaser bird species and Pteropus spp. have more generalist patterns of habitat and resource use. N. robinsoni has limited potential to substitute for decreaser bird species as a seed disperser in fragmented rainforest of the study region, because it is known to consume only a small number of plant species and because of its rarity in regrowth. In contrast, Pteropus spp. were widespread in fragmented forest and consumed approximately one-third of the plant species that were consumed by decreaser bird species. In fragmented landscapes, Pteropus spp. may potentially substitute for decreaser bird species as dispersers of large-fruited plant taxa and plants from the Myrtaceae, although they appear unlikely to disperse seeds >9 mm more than short distances away from parent plants.

The results of this study show that fragmented remnant and regrowth patches of rainforest do not adequately conserve the full complement of frugivorous vertebrate species in the subtropics of eastern Australia. Although the number of frugivore species that showed sensitivity to rainforest fragmentation was relatively small, this may have substantial functional consequences. These consequences are likely because decreaser species may be the sole or predominant dispersers of a substantial proportion of native plant species, which may consequently be susceptible to reduced dispersal away from parent plants in fragmented forest. Reduced dispersal may have a number of implications for plant regeneration. First, dispersal to recruitment sites within forest fragments is likely to be reduced, resulting in lower rates and clumped spatial patterns of recruitment. Second, dispersal of these species between rainforest fragments may be lower, leading to low rates of recolonisation following local extinctions. Third, short-distance dispersal to new habitats may be lower, resulting in low representation of susceptible plant species in regenerating forest on previously cleared land. Fourth, long distance dispersal of these plant taxa would be low, which would mean that they may have a limited capacity to shift their geographical range, for example in response to changing global climatic conditions.

Further clearing and fragmentation of rainforest would exacerbate the situation for decreaser frugivore species and may lead to the decline of additional frugivore species. It is recommended that remaining rainforest be protected from continued clearing. Restoration of forest areas based on the needs of decreaser frugivore species may help to re-establish them in fragmented landscapes. These actions could help to restore the regenerative capacity of many rainforest plant species and hence increase the long term integrity of fragmented rainforest ecosystems.

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Thesis (PhD Doctorate)

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Doctor of Philosophy (PhD)


Griffith School of Environment

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rainforest ecosystems

rainforest ecology

rainforest fragmentation

seed dispersal

plant regeneration

frugivorous birds

frugivorous bats

frugivorous vertebrates


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