Only few studies have analysed the relationship between biodiversity and ecosystem function at the landscape scale although relationships and mechanisms known from experimental studies might be different in natural systems. We quantified bird diversity and seed removal from 38 wild cherry trees (Prunus avium) along a human land-use gradient from forest to structurally diverse to simple agricultural systems. High human land-use intensity led to low species richness and total abundance of the local bird community around wild cherry trees, as expected from previous studies. Nevertheless, trees in structurally simple agroecosystems were visited as frequently as trees in structurally complex landscapes and in forests. Furthermore, the number of seeds removed per tree did not decline with increasing human land-use intensity. Thus, ecosystem function was robust in spite of locally reduced bird diversity. The reason might be that movement behaviour and movement distances of birds changed along the human land-use gradient. It appears that birds moved longer distances to forage in fruiting cherry trees in structurally simple agroecosystems. This suggests that for systems where ecosystem function is mediated by highly mobile organisms, movement behaviour and distances are of considerable importance. Increases in movement distances with increasing human land-use intensity might also be common in other systems in which ecosystem function depends on mobile links.
Tropical agroecosystems cover an increasingly large proportion of the Earth's terrestrial surface. Yet, relatively little is known about the factors that influence their avifauna, especially in areas of high human population density. The potential of tropical farmland for sustaining bird biodiversity, including forest birds, can be influenced by habitat structure and the distance from the nearest forest. We investigated the effect of these two factors on the bird community in the farmland near Kakamega Forest, Kenya. Using point counts, we assessed the number of bird species and individuals on 56 study plots in distances up to about 2,100 m from the forest. We observed a total of 96 bird species in the farmland, 22 of which were forest, 58 shrub-land, and 16 open-country species. High vertical vegetation heterogeneity and a large number of woody plant individuals were related to high species richness of forest and shrub-land birds, whereas open-country birds avoided such areas. The species richness and total number of forest birds declined with increasing distance to the forest. A comparison with the bird community within Kakamega Forest indicated that only a fraction of the forest species could be sustained in the farmland. This suggests that agroecosystems with a diverse habitat structure can support a high diversity of birds, but have only a limited capacity to compensate for forest loss.
Land-use intensification is a major cause for the decline in species diversity in human-modified landscapes. The loss of functionally important species can reduce a variety of ecosystem functions, such as pollination and seed dispersal, but the intricate relationships between land-use intensity, biodiversity and ecosystem functioning are still contentious. Along a gradient from forest to intensively used farmland, we quantified bee species richness, visitation rates of bees and pollination success of wild cherry trees (Prunus avium). We analysed the effects of structural habitat diversity at a local scale and of the proportion of suitable habitat around each tree at a landscape scale. We compared these findings with those from previous studies of seed-dispersing birds and mammals in the same model system and along the same land-use gradient. Bee species richness and visitation rates were found to be highest in structurally simple habitats, whereas bird species richness--but not their visitation rates--were highest in structurally complex habitats. Mammal visitation rates were only influenced at the landscape scale. These results show that different functional groups of animals respond idiosyncratically to gradients in habitat and landscape structure. Despite strong effects on bees and birds, pollination success and bird seed removal did not differ along the land-use gradient at both spatial scales. These results suggest that mobile organisms, such as bees and birds, move over long distances in intensively used landscapes and thereby buffer pollination and seed-dispersal interactions. We conclude that measures of species richness and interaction frequencies are not sufficient on their own to understand the ultimate consequences of land-use intensification on ecosystem functioning.
Summary1. In Central Europe, many plant populations are patchily distributed in human-modified landscapes and depend on animal vectors for seed dispersal. To predict seed-dispersal distances and locations of seeds of wild cherry trees (Prunus avium L.) in forest and farmland habitats in a human-modified landscape, we integrate movement data and seed regurgitation times of the Common Blackbird (Turdus merula L.) in a simulation model. 2. We performed feeding trials with Common Blackbirds and wild cherries to determine the distribution of regurgitation times. We captured 32 male blackbirds and equipped them with radio tags to follow their movements in forest and farmland habitats. To simulate the movement of cherry seeds through the landscape, we combined the distribution of regurgitation times with bird movement data and modelled seed-dispersal distances and locations of seed deposition for forest and farmland birds. 3. According to our simulations, more cherry seeds were deposited under foraging trees in farmland (20.8%) than in forest populations (9.5%). Median seed-dispersal distances (50.8 vs. 68.2 m) and the proportion of long-distance dispersal events (distances > 100 m) were predicted to be lower in farmland (14.9%) than in forest populations (28.2%). 4. In the model, forest-dwelling blackbirds dispersed more cherry seeds into suitable habitat (98.7%) than farmland blackbirds (85.4%). In both habitats, seed deposition in suitable habitat was much higher than expected from the proportion of suitable habitat within blackbirds' home ranges, indicating directed dispersal. 5. To test whether differences in seed-dispersal locations were related to recruitment success, we recorded seedling densities in farmland and forest populations of P. avium and determined survival probabilities of seedlings. Seedling densities and survival were much lower in farmland than in forest populations, even accounting for strong environmental effects on seedling recruitment. 6. Synthesis: Our findings show that behavioural differences of animal seed dispersers between habitat types can result in substantial changes in seed-dispersal distances and locations in humanmodified habitats. These changes in seed-dispersal services for bird-dispersed plant species may be related to reduced seedling recruitment in farmland populations making such populations prone to extinction in the long term.
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