View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by edoc Effects of experimental small-scale grassland fragmentation on the population dynamics of invertebrates Inauguraldissertation zur Erlangung der Würde eines Doktors der Philosophie vorgelegt der Philosophisch - Naturwissenschaftlichen Fakultät der Universität Basel von Brigitte Michèle Braschler aus Freienbach SZ Basel, 2005 Genemigt von der Philosophisch-Naturwissenschaftlichen Fakultät auf Antrag von Prof. Dr. Bruno Baur PD Dr. Andreas Erhardt Basel, den 21. Oktober 2003 Prof. Dr. Marcel Tanner Dekan - 2 - Table of contents Acknowledgements ............................................................................................................4 Summary ............................................................................................................................6 Deutsche Zusammenfassung ..............................................................................................8 General introduction...........................................................................................................10 Chapter I Short-term responses of plants and invertebrates to experimental small-scale grassland fragmentation...........................................................29 Chapter II Experimental small-scale grassland fragmentation alters aphid population dynamics...................................................................................62 Chapter III Effects of experimental small-scale grassland fragmentation on spatial distribution, density, and persistence of ant nests...........................86 Chapter IV Experimental small-scale grassland fragmentation alters competitive interactions among ant species ...............................................105 Chapter V Grain-dependent relationships between plant productivity and invertebrate species richness and biomass in calcareous grasslands ...................................................................................................132 General discussion and conclusions...................................................................................154 Contributions to each chapter.............................................................................................161 Curriculum vitae.................................................................................................................163 - 3 - Acknowledgements During my dissertation I had the occasion to work in three beautiful field sites in the Swiss Jura mountains. My thanks go to all those who made this possible. Numerous people contributed in important ways to this thesis. First and foremost, I would like to thank my supervisor Bruno Baur for his enduring support and always valuable advice. In particular, I would like to thank him for the chance to work in his habitat fragmentation project. His expertise greatly improved the papers in this thesis. I would also like to thank Samuel Zschokke for help with everything that consumes electricity (from tiny loggers to desktop computers) and Samuel Zschokke, Hans-Peter Rusterholz, Peter Stoll, Georg Armbruster and Andreas Erhardt for statistical and general advice and valuable comments on early drafts of my manuscripts. Eric Lüdin was always ready to discuss statistical problems. His advice greatly improved the statistics presented in this thesis. Claudine Dolt and Hans-Peter Rusterholz allowed me to use their data on plant species composition and plant productivity in the experimental plots to calculate Landolts indicator scores and examine the relationship between environmental factors and ant density. Michel Wurtz helped maintaining the study sites and helped with preparing some photographs for my conference contributions. Salome Schüpbach, Phillippe Leuenberger, Susanna Meyer, Nicole Minoretti and numerous other field assistants helped maintaining the field sites and assisted in data collection. In particular I want to thank all field assistants that were ready to work at rainy Sunday mornings to collect snails. Gerolf Lampel collaborated with me on the aphid project and made all aphid determinations. His expertise and advice are highly appreciated. Ludwika Sygnarski prepared the aphid specimens for determination. Samuel Zschokke was the main author of the article on short-term effects of the experimental fragmentation and Claudine Dolt, Hans-Peter Rusterholz, Heinrich Thommen, Peter Oggier, Eric Lüdin and Andreas Erhardt contributed to the manuscript. Samuel Zschokke helped to prepare the manuscript on the relationship between plant productivity and - 4 - invertebrate diversity to which Claudine Dolt, Heinrich Thommen and Peter Oggier also contributed data. Bernhard Seifert, Rainer Neumeyer and Cesare Baroni Urbani helped with ant determination. Advice from several anonymous referees improved the manuscripts that form this thesis. Their thorough work behind the scene is greatly appreciated. I would like to thank the other members of the habitat fragmentation group for discussions during seminars or cake breaks, namely Samuel Zschokke, Claudine Dolt, Marcel Goverde, Stephan Ledergerber, Peter Oggier, Heinrich Thommen and Andreas Erhardt. Evelyn Argast was always ready to help with administrative problems. The staff of the institute, in particular my roommates provided a very comfortable working climate. I want to thank Nicole Minoretti, Suresh Benjamin, Jovanne Mevi-Schütz, Ursula Monzeglio, Daniel Bloch, Katleen Beese, Rolf Locher, Nenad Bojat, Stefan Müller, Alexander Wacker, Christoph Jäggi and all others that worked in the rooms -12 to -14 and 21 to 35 during my time at the NLU. This thesis was part of the Swiss Priority Programme Environment supported by the Swiss National Science Foundation (grant 5001-44620 to B. Baur). The Reisefonds der Universität Basel awarded me travel grants to enable me to present results from my projects at conferences in Cambridge (UK) and Lund (Sweden). Last but not least I would like to thank my family and friends for continued support and for listening to all my tales about the dissertation project. Especially I would like to thank my parents Doris and Hermann Braschler for their enduring support, including the possibility to borrow their car or other things whenever necessary. - 5 - Summary One of the great questions in ecology is what determines and maintains biodiversity. This question is receiving increased attention as biodiversity is at risk. Species go extinct at such a high rate that some scientists speak of a man-made mass extinction. As biodiversity is related to ecosystem functioning also wide ranging consequences of the current species loss on ecosystem services can be expected. In addition to habitat loss, habitat fragmentation threatens biodiversity. Small and isolated fragments are expected to host less species than larger and better connected habitat patches. Fragmentation also reduces genetic diversity and disrupts interactions between species. Invertebrates, in particular insects, contribute considerably to species richness of a habitat. In the present thesis I focus on the effects of an experimental fragmentation on invertebrate diversity in calcareous grasslands. These species-rich, extensively used grasslands have been created by man and are today threatened by changes in agricultural practices and by fragmentation. In a long-term experiment at the University of Basel, fragments of different size (0.5 m2, 2.25 m2 and 20.25 m2) have been isolated and maintained by regularly moving the surrounding vegetation. Corresponding control plots were situated in adjacent continuous grassland. The experimental set-up consisted of 48 fragments and 48 control plots, distributed over three study sites in the northern Swiss Jura mountains. I examined effects of the experimental fragmentation on invertebrate population dynamics 3 to 6 years after the initiation of the fragmentation. Species richness of grasses increased in fragments while species richness of forbs, ants, aphids, gastropods and grasshoppers were not affected by the fragmentation. Only in butterflies, the most mobile animal group examined, a negative response to the fragmentation was found. The time frame used in the present experiment may have been to short to monitor extinction processes. However, the experimental fragmentation altered the abundance of single species and altered interactions between species. As predicted by theory, many common species were even more abundant in fragments than in control plots. Furthermore, aphids, a group of herbivorous insects, benefited from the fragmentation. However, the increase in aphid - 6 - density was not a result from reduced parasitation pressure, but rather a result of a higher degree of ant-tending and an increased plant productivity. The fragmentation also altered competitive interactions among ant species. With increasing density of the dominant ant species Lasius paralienus species richness and forager density of the other ant species decreased in fragments but not in control plots. The densities of foragers of the other species at natural and artificial sugar resources were not affected by L. paralienus forager density. This could be a result of an increased density of natural sugar resources in fragments and thus decreased competition for them. The fragmentation also affected the spatial distribution and persistence of ant nests. These findings were a result of altered abiotic conditions at the edge of fragments and were more pronounced for the dominant species