Plant-frugivore interactions in a heterogeneous forest landscape of South Africa
Seed dispersal by animals is a very important ecological process, especially in tropical and sub-tropical forest landscapes where up to 90% of fleshy fruiting plant species depend on this process for the transportation of their propagules to suitable habitats. Frugivorous birds are among the most im...
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|Seed dispersal by animals is a very important ecological process, especially in tropical and sub-tropical forest landscapes where up to 90% of fleshy fruiting plant species depend on this process for the transportation of their propagules to suitable habitats. Frugivorous birds are among the most important vertebrates consuming fruits, thereby contributing to natural forest regeneration. However, changes in quality of forest habitats and structure of the surrounding matrix have been shown to disrupt the structure of single-pair seed dispersal interactions. Changes in forest habitats can also alter the functional diversity of dispersers within these seed dispersal communities and can have far reaching implications for natural regeneration processes of plants. To assess the overall effects of forest patch quality and matrix habitat on ecological processes and functioning, I compared (i) the structure of plant-frugivore interaction networks and (ii) functional diversity of frugivore communities within the studied networks among forest patches surrounded by dissimilar matrix habitats in and around Vernon Crookes Nature Reserve in South Africa. In addition, I experimentally tested the impact of seed ingestion and treatment in the digestive tracts of avian frugivores on germination to assess their legitimacy as seed dispersers. During the plant-frugivore networks study, I compared the network structure in a large scarp forest surrounded by a natural forest matrix with those of two small natural forest types, one surrounded by natural grassland and the other by sugarcane agriculture in relation to both forest patch quality and matrix habitat. Overall, I recorded a total of 54 avian frugivorous species involved in fruit removal activities on 31 fleshy fruiting plant species. Network structures across forest patches did not vary with matrix habitat. Instead, they were similarly robust and strongly influenced by forest patch quality. These findings suggest that the structure and stability of plant-frugivore interaction networks is rather determined by local forest patch quality than by surrounding matrix habitat. During the functional diversity study, I compiled data on three functional traits related to seed dispersal, namely body mass, gape width and degree of frugivory for the frugivore species observed in the seed dispersal networks. I then tested the effect of forest patch quality and matrix habitat on three measures of functional diversity as well as on species richness of frugivore communities. Results suggest that all forest types sustained a comparably high functional diversity among the seed disperser communities. Overall, functional diversity and shifts in functional composition were rather determined by forest patch quality than by matrix habitat. During the seed ingestion experiment, I collected fruits from sixteen woody plant species and I fed them to four avian frugivore species. Defecated seeds were planted and their seedling emergence and germination probabilities were compared with those of ripe manually depulped seeds and whole fruits. Results show no difference in seedling emergence and germination probability among the birds and remained similar across all treatments. The results suggest that the effect of frugivorous birds on natural forest regeneration is more on seed transportation, than enhancing germination of dispersed seeds per se. Overall, findings from the first two studies suggests that forest patch quality is an important driver of both network stability and functional diversity among frugivore communities. Therefore, given high habitat quality, forest patches surrounded by variable matrices can be of conservation importance in highly heterogeneous forest landscapes. Thus, as the seed ingestion experiment has shown that frugivorous birds are legitimate dispersers, the sustenance of seed dispersal processes will largely depend on maintenance of forest patches of high quality and with less restrictive matrix habitats.