Effects of habitat isolation on pollinator communities and seed set |
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Authors: | I Steffan-Dewenter Teja Tscharntke |
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Institution: | Agroecology, University of G?ttingen, Waldweg 26, D-37073 G?ttingen, Germany e-mail: isteffa@gwdg.de Tel.: +49-551-398807, Fax: +49-551-398806, DE
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Abstract: | Destruction and fragmentation of natural habitats is the major reason for the decreasing biodiversity in the agricultural
landscape. Loss of populations may negatively affect biotic interactions and ecosystem stability. Here we tested the hypothesis
that habitat fragmentation affects bee populations and thereby disrupts plant-pollinator interactions. We experimentally established
small ”habitat islands” of two self-incompatible, annual crucifers on eight calcareous grasslands and in the intensively managed
agricultural landscape at increasing distances (up to 1000 m) from these species-rich grasslands to measure effects of isolation
on both pollinator guilds and seed set, independently from patch size and density, resource availability and genetic erosion
of plant populations. Each habitat island consisted of four pots each with one plant of mustard (Sinapis arvensis) and radish (Raphanus sativus). Increasing isolation of the small habitat islands resulted in both decreased abundance and species richness of flower-visiting
bees (Hymenoptera: Apoidea). Mean body size of flower-visiting wild bees was larger on isolated than on nonisolated habitat
islands emphasizing the positive correlation of body size and foraging distance. Abundance of flower-visiting honeybees depended
on the distance from the nearest apiary. Abundance of other flower visitors such as hover flies did not change with increasing
isolation. Number of seeds per fruit and per plant decreased significantly with increasing distance from the nearest grassland
for both mustard and radish. Mean seed set per plant was halved at a distance of approximately 1000 m for mustard and at 250
m for radish. In accordance with expectations, seed set per plant was positively correlated with the number of flower-visiting
bees. We found no evidence for resource limitation in the case of mustard and only marginal effects for radish. We conclude
that habitat connectivity is essential to maintain not only abundant and diverse bee communities, but also plant-pollinator
interactions in economically important crops and endangered wild plants.
Received: 7 May 1999 / Accepted: 19 July 1999 |
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Keywords: | Habitat fragmentation Pollination Resource limitation Plant reproduction Foraging radius |
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