Ecological Studies, Vol. 164 Analysis and Synthesis Edited by I.T. Baldwin, Jena, Germany M.M. Caldwell, Logan, USA G. Heldmaier, Marburg, Germany 0.1. Lange, Würzburg, Germany H.A. Mooney, Stanford, USA E.-D. Schulze, Jena, Germany U. Sommer, Kiel, Germany Ecological Studies Volumes published since 1997 are listed at the end of this book. Springer-Verlag Berlin Heidelberg GmbH R. Wirth H. Herz R.T. Ryel W. Beyschlag B. Hölldobler Herbivory of Leaf-Cutting Ants A (ase Study on Atta colombica in the Tropical Rainforest of Panama With 82 Figures, 16 in Color, and 32 Tables " Springer Dr. Rainer Wirth Universität Kaiserslautern, Fachbereich Biologie, Lehrstuhl für Allgemeine Botanik, Postfach 3049, 67653 Kaiserlautern, Germany Dr. Hubert Herz Universität Würzburg, Theodor-Boveri-Institut für Biowissenschaften, Lehrstuhl für Verhaltensphysiologie und Soziobiologie (Zoologie 11), Am Hubland, 97074 Würzburg, Germany Prof. Dr. Ronald J. Ryel, Department of Forest, Range and Wildlife Sciences, Utah State University, 5230 Old Mai! Hili, Logan, Utah 84322-5230, USA Prof. Dr. Wolfram Beyschlag Universität Bielefeld W 4-107, Lehrstuhl für experimentelle Ökologie und Ökosystembiologie, Universitätsstr. 25, 33615 Bielefeld, Germany Prof. Dr. Bert Hölldobler Universität Würzburg, Theodor-Boveri-Institut für Biowissenschaften, Lehrstuhl für Verhaltensphysiologie und Soziobiologie (Zoologie 11), Am Hubland, 97074 Würzburg, Germany Cover illustration: by Dr. Damond Kyllo, University of Missouri, USA. The illustration is a visual homage to the leaf-cutting ants and their tropical rainforest environment as perceived by both the artist and authors after many years of living and working on Barro Colorado Island. ISSN 0070-8356 ISBN 978-3-642-07865-1 Library of Congress Cataloging-in-Publication Data Herbivory of leaf-cutting ants: a case study on Atta colombica in the tropical rainforest of Panama 1 R. Wirth ... [et al.l. p. cm. --(Ecological studies, ; v. 164) Inc1udes bibliographical references (p. ). ISBN 978-3-642-07865-1 ISBN 978-3-662-05259-4 (eBook) DOI 10.1007/978-3-662-05259-4 1. Atta colombica--Ecology--Panama--Barro Colorado Island. 2. Rain forest ecology--Panama--Barro Colorado Island. 3. Insect-plant relationships--Panama--Barro Colorado Island. I. Wirth, R. (Rain er), 1964-11. Series QL568.F7 H47 2003 595.79'6--dc211 2002030473 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permit - ted only under the provisions of the German Copyright Law of September 9,1965, in its current version, and per missions for use must always be obtained from Springer-Verlag Berlin Heidelberg GmbH. Violations are liable for prosecution under the German Copyright Law. http://www.springer.de © Springer-Verlag Berlin Heidelberg 2003 Originally published by Springer-Verlag Berlin Heidelberg New Yo rk in 2003 Softcover reprint ofthe hardcover 1st edition 2003 The copyright of the original illustrations (drawings or photographs ) remains with the authors. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: design & production GmbH, Heidelberg Typesetting: Kröner, Heidelberg SPIN 10691950 31/3150 YK - 5 4 3 2 1 0 - Printed on acid free paper For Edward O. Wilson, premier myrmecologist, naturalist, and ardent advocate for biodiversity research, in appreciation for his many pioneering contributions to our understandig of leaf-cutting ant societies. Preface The idea for this study was born at the University ofWürzburg, Germany, dur ing the time when two of the authors (W.B. and R.J.R.) were working on the effects of ne edle loss of dedining red spruce trees on the light microenviron ment within the forest canopy. In this system it could be shown that the par tial loss of foliage led to a significant increase in photosynthetically active radiation within the canopy which then benefited the remaining healthy foliage, particularly in dense canopies. Since Würzburg is one of the places in Germany where interdisciplinary biological research has a long tradition, these results were also discussed with zoologists. One of them (B.H.) became particularly interested because one of his major research interests was leaf cutting ants, an organism which creates substantial foliage losses in the canopies of Neotropical rainforests. Quickly it became obvious that by com bining the experience ofboth botanists and zoologists it would be possible to quantitatively assess the impact of these selective herbivores on their natural ecosystem, an important question which at that time had never been addressed before, although leaf-cutting ants, because of their importance as pests, certainly belonged to the most studied insect species of the Neotropics. As a starting hypothesis, it was formulated that leaf-cutting ants, by continu ously creating small gaps in arainforest canopy where light is typically a major limiting resource, would allow light to penetrate deeper into the canopy, thereby providing opportunities for species of the middle and lower canopy and, thus, perhaps contribute to the high biodiversity. Very quickly it became dear that testing this hypothesis was not an easy task to achieve, because such ant-induced changes in the canopy light dimate could, of course, not be analyzed without substantial quantitative information on rain forest canopy structure, distribution and activities of the ants, herbivory rates, and the response of the affected plants to this herbivory. Furthermore, after a doser look, it became dearer that leaf-cutting ants do affect much more than just the light resource, and leaf-cutting ant-induced disturbance occurs at multiple scales - from the single plant all the way up to the ecosys tem. Unfortunately, information on the spatiotemporal herbivory patterns of leaf-cutting ants was scarce and long-term studies on this subject were prac tically missing. Hence, more and more dimensions and scales had to be VIII Preface included into our considerations and finally we ended Up designing a major integrated and interdisciplinary ecosystem research project. Not surprisingly, after the fieldwork had started, unexpected results (for instance, the high fre quency of ant colony movements) added even more aspects, and what was ini tially planned as a 2-3-year study, became a major enterprise lasting a total of 8 years, including 5 years of fieldwork. Nevertheless, looking at the results, we can say that working on this project for such a long time not only answered quite a few of our questions and, thus, increased our knowledge of the ecological role of leaf-cutting ants, but most importantly also provided fundamental insights into the functioning of the rainforest ecosystem for each of uso We hope by writing this volume we were able to transport not only our scientific results and ideas (many of them orig inating from the doctoral theses of R.W. and H.H.), but also our excitement about these fascinating animals and their extraordinary habitat. This study would not have been possible without the help and the input of quite a few people and organizations. First, we would like to thank the Smith sonian Tropical Research Institute for permission to use their research station on Barro Colorado Island in Panama and we gratefully acknowledge the help of their administration in Balboa and the staff of the research station. In par ticular, we would like to thank the station's scientific coordinator Mrs. Oris Acevedo for her incredible support. We are grateful for all the help and stimu lation of the STRI staff scientists, particularly Dr. Egbert Leigh and Dr. Klaus Winter. Further, Mr. Eduardo Sierra t, Mr. Rolando Perez, and their colleagues contributed their valuable experience in determining plant species (particu larly from small harvested fragments). The following student workers and graduate students were involved in the fieldwork and contributed valuable data material: Jürgen Berger, Eva Cue-Bär, Querube D. Fuenmayor, Ralf Hömme, Dorkas Kaiser, Boris Leymann, Mir Rodriguez Lombardo, Andreas Meier, Stefanie Rottenberger, Gerold Schmidt, Antje Siegel, and Alexandra Weigelt. Many ideas would never have been created without the inspiring dis cussions and the valuable input of the following scientists: Dr. Elisabeth Coley, Dr. Cameron Currie, Dr. William Eberhard, Dr. Bettina Engelbrecht, Dr. Allen Herre, Dr. Jerry Howard, Dr. Tom Kursar, Dr. Egbert Leigh, Dr. Flavio Roces and Dr. Thomas Seeley (thanks to both for critically reading the natural history chapter), Dr. Thomas Steinlein, and Dr. Mary Jane West-Eberhard. Dr. Edward O. Wilson gave us permission to use some of the material from publi cations of which he was author or coauthor. Further, we thank Dr. Damon Kyllo who did a great job in designing the cover picture and Mr. Christian Ziegler for permission to publish some of his excellent photographs. Techni cal and administrative help in Germany was provided by Doris Faltenbacher Werner, Elke Furlkröger, Evelyn Hatzung, Ursula Herlth, Monika Noak, Chris tine Schlüter, and Dr. Hans Zellner. The project was funded by the Deutsche Forschungsgemeinschaft (SFB 251; Ökologie, Physiologie und Biochemie Preface IX pflanzlicher und tierischer Leistung unter Streß, University of Würzburg). Last, but not least, we would like to express our gratitude to Dr. Otto Ludwig Lange for inviting us to publish this mono graph in the Ecological Studies Series and for his continuous and stimulating support during the preparation of the manuscript. September 2002 The Authors Contents 1 About This Book 1 2 The Natural History of Leaf-Cutting Ants 5 2.1 The Significance of Sodal Insects .... . 5 2.2 The Fungus Growers ........... . 7 2.3 The Natural History of the Leaf-Cutting Ants of the Genus Atta ........ . 9 2.3.1 Colony Foundation in Atta . . . . . . 9 2.3.2 Division of Labor in Atta Colonies 14 2.3.3 The Maintenance of Fungal Hygiene 17 2.3.4 Communication and Foraging in Atta Colonies 20 2.3.5 Leaf-Cutting Behavior and Load-Size Determination 24 2.3.6 Communication Between the Fungus Garden and Workers in Atta Colonies ................ . 28 Color Plates 1-16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 31 3 The Study Area - Barro Colorado Island ........... 49 3.1 Location and History 49 3.2 Geology and Soils .. 51 3.3 Climatic Conditions . 51 3.4 Vegetation Structure and Composition 52 3.5 Fauna ........ 53 ........... 3.6 Research Locations 54 XII Contents 4 Species Composition of the Forest 55 4.1 Selection and Setup of the Study Plot 5S 4.2 Vegetation Inventory ........ . S6 4.3 Abundances and Diversity of Tree Species 57 5 Forest Light Regimes ................. . 61 5.1 Seasonal and Spatial Light Patterns in the Understory 61 5.1.1 Measuring Irradiance . . . . . . . . . 61 5.1.2 Horizontal Distribution of Irradiance 63 5.1.3 Seasonal Aspects ........ . 64 5.2 Vertical Light Variability .... . 66 5.2.1 Measuring Vertical Light Profiles 66 5.2.2 Vertical Distribution of Light 67 6 Canopy Structure of the Forest 71 6.1 Theory ............. . 71 6.2 Measuring LAI and Foliage Orientation 72 6.3 Vertical Distribution of Foliage Angles 73 6.4 Spatial and Temporal Characteristics of LAI Distribution 74 6.4.1 Vertical LAI Distribution . . . . . . . . . . . . . . . 74 6.4.2 Horizontal LAI Distribution and Seasonal Variation 77 6.5 LAI Measurements with the Li-Cor LAI-2000 .. 80 6.6 Comparison of the Study Plot with Other Parts of the BCI Forest ................ . 81 7 Colony Dynamics ......... . . . . . . . . . . . . . . . 83 7.1 Colony Distribution and Density 84 7.2 Colony Movements 88 7.3 Nest Size 89 7.4 Nest Trees 90 8 Harvest Dynamics 91 8.1 Annual Pattern of Foraging Activity 91 8.1.1 Measuring Foraging Activity . . . . 91