Download Understanding the structure of interactions and the dynamics

Department of Biology / University of Fribourg, Switzerland
Understanding the structure of interactions and the dynamics of spider
populations in agricultural ecosystems
Odile Bruggisser
An understanding of the structure and dynamics of communities is an important long-term
objective of ecology for the maintenance of biodiversity. Spiders as generalist predators are
among the most abundant terrestrial arthropods and play a role in food webs in many
ecosystems. The global aim of this thesis was to investigate spiders and their natural prey and
predator species in agricultural landscapes to gain insight into the structure, dynamics and
stability of communities.
First we investigated experimentally the structure of a food web centered on the webbuilding spider Argiope bruennichi. We were interested in the bottom-up effect of vegetation
structure and plant diversity and in the top-down effect of a predator, the hornet Vespa crabro,
on the abundance of this spider species. We performed an experiment in wildflower-strips
differing in plant diversity and hence in vegetation structure. Our study system showed
evidence that the abundance of A. bruennichi is regulated by combined bottom-up and topdown effects. Three main factors were found to characterize this system: the strong negative
effect of a single predator, namely hornets and the positive effects of plant structure and of
plant diversity. Overall, these results show that the relative importance of different factors
affecting populations has to be considered simultaneously to understand the dynamics of
populations.
The analysis of a time-series of 103 ballooning spider species observed between 1994
and 2004, and their relation to climatic conditions and landscape modifications was the focus
of our second study. We found evidence for a significant shift in the ballooning phenology of
most species, with the summer peak in ballooning activity shifted earlier in the year. In the
long term, ground-living species decreased while tree-living spiders increased in abundance,
which can be explained by concomitant landscape changes in the study area. Ballooning
abundances decreased strongly in summer and fall 2003, when extreme meteorological
conditions were prevailing in Europe. The main results of this study show that spider
populations appear to be more at risk from the expected intensification of extreme climatic
events and changes in land use than from gradually rising temperatures.
In the third study, we aimed to develop a molecular method to track trophic links in
food webs. This method was needed to identify the prey community of the wasp Trypoxylon
figulus feeding in the larval stage on different spider species. Since larvae consume most of
the spider prey, the identification of prey remains to the species level is very difficult. With
the help of a mitochondrial cytochrome oxidase I (COI) fragments database of 46 different
spider species, we were able to identify the remains of spider tissue from T. figulus nests. Our
results highlight the advantages of genetic methods for a better understanding of otherwise
invisible trophic links in food webs by the identification of species from prey remains.
Jury:
Dr. Louis-Félix Bersier, thesis director, University of Fribourg
Prof. Dr. Soeren Toft, external expert, Aarhus University, Denmark
PD Dr. Sven Bacher, expert, University of Fribourg
Prof. Dr. Christian Lexer, expert, University of Fribourg
Prof. Dr. Heinz Müller-Schärer, jury president, University of Fribourg