i- DISTURBANCE OF ANT COMMUNITY STRUCTURE IN CENTRAL TEXAS BY THE RED IMPORTED FIRE ANT, SOLENOPSIS INVICTA by GERARDO RAFAEL CAMILO, B.S. A THESIS IN ENTOMOLOGY Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Approved August, 1990 ^r^a ACKNOWLEDGEMENTS No C^) (* ^l express my deepest gratitude to Dr. Sherman A. Phillips, Jr., for giving me the opportunity to work under his direction. I am thankful also to Dr. Harlan G. Thorvilson and Dr. Michael R. Willig for serving on my thesis committee. Thanks are expressed to Drs. George C. and Jeanette N. Wheeler, Dr. James C. Trager, and Mr. James C. Cokendolpher for the identification of specimens. Thanks are also extended to Kathy Phillips and the graduate students of the Departments of Agronomy, Horticulture, and Entomology, and Biological Sciences, who provided help with the field work and helpful comments about my research. Many people helped me to reach this point. Dr. Bob Waide and Dr. Willig provided encouragement to continue graduate studies. Dr. Rip Phillips always knew when to pull the breaks on me, and other graduate students provided helpful comments on my research. I am deeply thankful to all of you. Finally, I want to thank my family, especially my mother, Josefina Rivera, for being so understanding, loving, and patient with me. They provided the extra encouragement needed. Thanks. 11 TABLE OF CONTENTS ACKNOWLEDGEMENTS ii ABSTRACT iv LIST OF TABLES v LIST OF FIGURES vi CHAPTER I. INTRODUCTION 1 Community and Competition 1 Dominance and Diversity 2 Ant Communities 3 Competitive Replacement by the Red Imported Fire Ant 5 Statement of Study Objectives 8 II. MATERIALS AND METHODS 9 Study Area 9 Sampling Techniques 9 Cluster Analysis 11 Principal Components Analysis 11 m. RESULTS AND DISCUSSION 13 IV. CONCLUSIONS 25 LITERATURE CITED 2 8 111 ABSTRACT The introduction of a competitor into an established community provides an excellent opportunity to study the factors that maintain the dominance, diversity, and interactions of that community. The introduction of the red imported fire ant, Solenopsis invicta Buren, into North America provides such an opportunity. A transect, with pitfall traps as sampling units, was established in central Texas, U.S.A., from areas uninfested to heavily infested by S.. invicta. Cluster analysis of the species composition at particular localities depicts four distinct ant assemblages, two without and two with S.. invicta. A fifth aggregation of species was found and consisted almost entirely of S.. invicta. Principal components analysis reveals that disturbance of the habitat, mostly by humans, is the main factor controlling ant diversity. More specifically, increasing densities of the red imported fire ant were negatively correlated with species diversity. Conomyrma insana (McCook) can coexist with S.. invicta. probably because direct competitors or predators have been eliminated by S.. invicta. Ants of the granivorous guild, mostly Pheidole spp., that occur with the red imported fire ant are being displaced, whereas more generalistic ants, like Monomorium minimum (Buckley) and Forelius pruinosus (Roger), coexist with S.. invicta. IV LIST OF TABLES 1. Species assemblages in central Texas, U.S.A., as detected by cluster analysis 17 2. Principal components analysis scores of plots, percent of variation accounted for by each component (R^), and cumulative variation (IR2). 21 3. Correlation of species importance values to densities of Solenopsis invicta 24 LIST OF FIGURES 1. Systematic affiliation and sample sizes of ant species collected from August 1986 to October 1987 14 2. Dendrogram of ant species assemblages located in Kerr and Kimble Counties, Texas, U. S. A. Plots K and L were excluded from the analysis. 16 3. Principal components analysis of plots by species. Squares in the 1-2 axes plane are the geometrical projections of the plots in the ordination space. Axis 1 is correlated with disturbance, axis 2 with temperature, and axis 4 with density of S.. invicta. 20 VI CHAPTER I INTRODUCTION Community and Competition The concept of community has generated a long series of arguments throughout the history of ecology. One school of thought is that the community is a "super-organism" in which the total is more than the sum of its parts (Clements 1916). Central to this theory is the view that the history of past interactions is the driving force that gives rise to the present communities (Odum 1971). The diversity of a community, that is, which species are present and in what numbers, is considered to be influenced mostly by competition. This community theory is based upon Cause's application of Malthusian population theory to interspecific ecological interactions (Hardin 1960). If two populations of different species, A and B, occupy the same niche and live in the same environment with limited resources, and A reproduces faster than B, then A will displace B, and B will become extinct (Miller 1967). This interpretation implies that interspecific competition in the past caused niche divergence, resulting in differences between species that are seen in contemporary communities (Schoener 1974). Yet other kinds of interactions, like predation and parasitism, were not considered as impo lant as competition in structuring communities (Seifert 1984). The other school of thought considers a community to be a group of species occupying a common area. The diversity of the community is a function of autecological rather than synecological processes (Strong et al. 1984). Such communities would not be holistic entities, but simply a collection of relatively autonomous populations occupying the same area at the same time. Community diversity is not affected just by competition but also by other biotic interactions and abiotic factors such as weather. Interactions like parasitism, predation and mutualism, can be the main factors responsible for the observed community structure (Strong 1984, Seifert 1984). The presumption that competition in the past is the general cause for species diversity and distribution in communities has been referred to as "the ghost of competition past" (Council 1980). Interspecific competition may not be common in communities because other natural factors (i.e., stochastic environmental fluctuations or parasites) frequently intervene to maintain densities below resource-defined equilibria (Strong 1984, Wiens 1977). Competition is more likely to affect species under moderately harsh environmental conditions, as opposed to either benign or extremely harsh environmental conditions (Council 1980). Under harsh conditions, such as droughts, populations of potential competitors may be reduced below the densities at which they would compete. Harshness, in this case, does not imply a reduction in resources. On the other hand, under benign conditions, predators, parasites, and pathogens may be more effective in maintaining population densities of prey or hosts below the level at which competition might occur. In addition, the intensity and duration of competition may not be enough as to be critical in the structuring of the community (Wiens 1977). Dominance and Diversity The dominance-diversity relationships in communities have long been of interest to ecologists (Brown & Gibson 1983, Ricklefs 1987). A dominant species in a community is one that largely controls energy flow and strongly affects the environment of other species (Odum 1971). Dominance of a species in a community may be expressed by high density, high frequency of occurrence, or appreciable influence on the densities and occurrence of other species (Werner 1984). The concept of diversity involves several other ecological concepts. Two of these concepts are species richness, or the number of species present in the community, and equitability, or the distribution of relative importance values among the species in the community (Peet 1974). The two terms. 3 dominance and diversity, are complements of each other. Dominance relationships are manifestations of the "sorting out" of species in the community, as dominance develops, diversity is established (Whittaker 1965). Ant Communities The high diversity and specialized behavior of ants make them important members of almost every terrestrial community (Carroll & Janzen 1973, Levings & Traniello 1981). Ants occupy all consumer trophic levels and exhibit all types of symbiotic relationships which have been demonstrated (Carroll & Janzen 1973). The structure of ant communities, as with any other community, depends on which ant species are present, in what numbers, and the type and intensity of the biotic interactions occurring at any given time (Levins et al. 1973, Heatwole & Levins 1972). However, the distribution and the number of species in a community are also a function of other factors. Species distributions have been found to be affected by climate, nutritional requirements, predators, and natural dispersal barriers (Culver 1974, Torres 1984a). Foraging ecology of ants, their highly aggressive behavior, and territoriality have a profound effect on the structure of ant communities. Levings and Traniello (1981) discussed the importance of spatial arrangement of ant nests and their territoriality in relation to the structure of ant communities. They found that ant species are regularly distributed with respect to conspecific and other sympatric species, and that these distributional phenomena are an outcome of high overlap of food utilization.