Dorn Silvia

Insects in a mosaic landscape: How heterogeneous land use influences species diversity and community structure

Project Number: CH-4826

Project Type:	Dissertation
Project Duration:	01/01/2002 - 08/31/2005 project completed
Funding Source:	other ,
Leading Institution:	ETH Zürich
Project Leader:	Prof. Silvia Dorn Institut für Agrarwissenschaften (IAS) ETH Zürich SOL G9 Sonneggstrasse 33 8092 Zürich Phone: +41 (0) 44 632 39 21 ; +41 (0) 44 632 11 11 FAX: +41 (0) 44 632 10 37 http://www.ias.ethz.ch/

related to this project.

for which the project has a relevance.

Research Areas:

Biodiversity

Disciplines:

zoology

Keywords:
"*" Biodiversity
"*" Wild bees
"*" Mosaic landscape
"*" Seasonal patterns
"*" Community similarity
"*" Species richness

Abstract:
"*" By direct netting over 2 seasons, as many as 247 bee species were recorded on 2 km2.
"*" None of the investigated taxa reflected the species numbers or community similarities of the other taxa well enough to qualify as as a general indicator for biodiversity (taxa studied: bees, grasshoppers, aculeate wasps).


The spatial distribution of organisms at landscape level is determined by the resource availability. Characteristics of landscape structure substantially influence this resource availability, particularly through the composition of habitat types, their spatial arrangement, and their temporal dynamics. Crucial for the conservation of biodiversity is a good understanding how organisms are affected by ecological processes operating at landscape level is. In agricultural landscapes, habitat composition and landscape structure are mainly determined by land use and agricultural practices. In the montane regions of Central Europe, diversified land use at a low intensity maintained heterogeneous landscapes.
In the present thesis, bees were chosen as indicators for biodiversity in a grassland ecosystem on a southern slope of the Swiss Alps. The principal goal was to investigate the influence of resource distribution and landscape structure on species diversity. Bees require flowering plants as pollen and nectar sources and specific structures as nesting sites. These two types of resources which are often located in different habitats have to lie within the flight range of the bee species. Hence, availability and distribution of these essential resources in space and time are crucial factors for the distribution of bees at landscape level.
A total of 247 bee species were recorded during two years of intensive sampling, and the total number of species present in the area was estimated to nearly 280 (chapter 4). The rank abundance distribution was characterized by very few abundant species and a high proportion of rare species. The analysis of ecological patterns in abundance classes revealed that more parasitic species than expected from random distribution were singletons, i.e. recorded with only one individual, and more primitively eusocial species than expected were observed in the individual-richest abundance class. Thus, as mainly rare species are missed with a low sampling effort, the ecological patterns in the obtained data set depend on sampling effort. On a temporal scale, the bee fauna showed a marked species turnover and a change of ecological patterns in the course of a season. The most pronounced seasonal effect was observed in nesting behaviour: more endogeic (ground-nesting) species than expected based on a constant proportion were recorded in April and May, while more hypergeic(above ground nesting) species than expected were recorded in June. In conclusion, a sampling protoeol that extends over the whole season is necessary to ensure a representative sample of the bee fauna. Of the 247 species collected during the two years of the study, 63 species (25.5%) were recorded in only one of the two years. Thus, assessing a bee fauna requires at least two years of sampling. Parasitic species were more often recorded in only one year than expected from the average, indicating that their assessment requires a particularly high sampling effort.
Bee diversity and species composition were related to selected environmental factors such as land use, resource distribution and small-scale landscape structure (chapter 5). Four study plots were chosen for each of seven land use types which covered the majority of the study area. Bees were collected on these plots following a standardized sampling scheme. Resource availability was measured as the flowering plant species and potential nesting sites present on the plots and in a belt around the plots. Furthermore, the composition and relative abundance of the land use types in the strip around the plots were mapped as a measure for landscape heterogeneity. Land use, resources and landscape heterogeneity were included in the analyses as explanatory variables. The number of bee species per plot was positively correlated with the amount of flowers on the plot. Land use did not significantly influence the number of species per plot. In contrast, species composition was most strongly influenced by land use and to a lesser extent by resource distribution and landscape structure. The mosaic character of the landscape is essential for the high bee diversity in the study area. The different land use types supplement each other by providing a temporal continuum of flower availability, and they complement each other as some offer plentiful pollen sources while others are rich in nesting sites. The analysis of ecological patterns among the plots revealed that endogeic and hypergeic bee species were not randomly distributed among land use types and responded differently to environmental factors. Numbers of endogeic species were correlated with the amount of flowers on the plots and in the surroundings of the plots, and they were more frequent on the hay meadows than expected from a random distribution. In contrast, numbers of hypergeic species were positively correlated with the number of land use types in the surroundings and the amount of stony nesting structures in the surroundings. Hypergeic species were more frequent than expected on fallow land and steppic grassland.
The use of indicator taxa as surrogates for biodiversity is only valid if the diversity of the indicator taxon is strongly correlated with total species diversity. We investigated the congruence in patterns of species numbers and community similarities among three insect taxa, i.e. bees, grasshoppers and aculeate wasps (chapter 6). The numbers of species per plot were only positively correlated between bees and aculeate wasps, and the community similarities between pairs of plots were only correlated between bees and grasshoppers. These results indicate that none of the investigated taxa closely reflects the species diversity or community similarities of the other taxa. The number of red-listed species per plot and the total number of species per plot were correlated in bees but not in grasshoppers. Thus, the conservation of threatened grasshopper species may require different measures than the conservation of overall grasshopper diversity. Land use did not influence the numer of species per plot in any of the three studied taxa. However, a significant influence of land use on the species composition was observed, which was strongest in grasshoppers and weakest in aculeate wasps.
This study provides valuable insights into the ecological processes that structure a bee community at landscape level. Implications for the conservation of bee diversity in agroecosystems were deducted from the results of this study. The study emphasizes the high value of heterogeneous agricultural landscapes and extensive land use for biodiversity.

Leading questions:
Insects in an agriculturally used grassland system on the Southern slope of the Alps:
"*" ecological and seasonal patterns of wild bees?
"*" cross-taxon congruence of species diversity and community similarity?

Publications:
Oertli, S., Müller, A. and Dorn, S., 2005. Ecological and seasonal patterns in the diversity of a species-rich bee assemblage (Hymenoptera: Apoidea: Apiformes). European Journal of Entomology 102: 53-63.
pdf article

Oertli, S., Müller, A., Steiner, D., Breitenstein, A. und Dorn, S., 2005. Cross-taxon congruence of species diversity and community similarity among three insect taxa in a mosaic landscape. Biological Conservation 126: 195-205.
pdf article


Last update: 7/18/17
Source of data: ProClim- Research InfoSystem (1993-2024)
Update the data of project: CH-4826

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