Diversity, Spacing & Body-Size Patterns in Ant Assemblages
Ants are fascinating organisms and exert huge influences on the environments they inhabit. Much of my early research focused on various aspects of ant ecology – spacing patterns of their nests, mutualistic associations, and latitudinal patterns in the diversity and body sizes of ant assemblages.
Spacing Patterns of Ant Nests
Spatial arrangements of ant colonies provide insight into the intra- and interspecific interactions operating within communities. A major factor contributing to the fitness of ant colonies is nest location, as site quality determines both the reproductive and foraging environment. Favorable foraging areas depend not only on the distribution of food but also on the presence of neighboring conspecific colonies. Uniform spacing of nests can reduce overlap of foraging areas, thereby minimizing intraspecific competition when colony density is high. While numerous studies have documented ant nest dispersion patterns, few have considered the impact of colony size and density on spacing patterns. Assessing the impact of colony size and density allows for more precise evaluation of spatial patterns and factors responsible for generating such patterns.
My collaborators and I studied the dispersion patterns of Formica altipetens in northern Arizona (Cushman, Martinsen and Mazeroll 1988). We present three lines of evidence which each suggest that intraspecific competition has significantly influenced the nest spacing patterns of this ant species. First, nearest-neighbor analysis of nest spacing patterns detected significant uniformity in six of eight plots. Second, there was a significant increase in the distance separating nearest neighbors as ant nest diameters increased. Third, ant nest density predicted substantial variation in the colony dispersion index, indicating the existence of a dispersion continuum at our study site. Collectively, these results suggest that intraspecific competition has been strong in this system and the uniform dispersal of ant colonies likely minimizes the degree of overlap in foraging areas and therefore the intensity of competition.
Latitudinal variation in diversity and body size European ant assemblages
A central problem in ecology is to identify and understand large-scale patterns in communities and regional biotas. Many ecological patterns occur along environmental gradients, such as those associated with latitude, and involve variation in species richness and body size. Body size is possibly the most fundamental trait of an animal. Every aspect of its life is influenced by body size – its interactions with the abiotic environment, the rates of its physiological processes, and the outcomes of its interactions with other organisms. Thus, body size should have considerable influence on the organization of ecological communities.
The study of variation in body size along latitudinal gradients has attracted considerable attention and debate. Much of this work has centered around Bergmann's rule, which states that body size in endotherms (within species or among closely related species) varies inversely with ambient temperature and thus increases with latitude. Despite its prevalence in the literature, the validity of Bergmann's rule has been severely criticized, largely because body-size patterns in endotherms are quite variable, and some ectotherms also exhibit larger body sizes at higher latitudes.
Using published distributions of 65 species from the British Isles and northern Europe, my collaborators and I showed that ant assemblages change with latitude in two ways (Cushman, Lawton and Manly 1993). First, as commonly found for many types of organisms, the number of ant species decreased significantly with increasing latitude. For Ireland and Great Britain, species richness also increased significantly with region area. Second, although rarely demonstrated for ectotherms (and especially invertebrates), the body size of ant species, as measured by worker length, increased significantly with increasing latitude. We found that this body-size pattern existed in the subfamily Formicinae and, to a lesser extent, in the Myrmicinae, which together comprised 95% of the ant species in our study area. There was a trend for formicines to increase in size with latitude faster than myrmicines. We also show that the pattern of increasing body size was due primarily to the ranges of ant species shifting to higher latitudes as their body sizes increased, with larger formicines becoming less represented at southerly latitudes and larger myrmicines becoming more represented at northerly latitudes. We conclude by discussing five potential mechanisms for generating the observed body-size patterns: the heat-conservation hypothesis, two hypotheses concerning phylogenetic history, the migration-ability hypothesis, and the starvation-resistance hypothesis.
Ants are fascinating organisms and exert huge influences on the environments they inhabit. Much of my early research focused on various aspects of ant ecology – spacing patterns of their nests, mutualistic associations, and latitudinal patterns in the diversity and body sizes of ant assemblages.
Spacing Patterns of Ant Nests
Spatial arrangements of ant colonies provide insight into the intra- and interspecific interactions operating within communities. A major factor contributing to the fitness of ant colonies is nest location, as site quality determines both the reproductive and foraging environment. Favorable foraging areas depend not only on the distribution of food but also on the presence of neighboring conspecific colonies. Uniform spacing of nests can reduce overlap of foraging areas, thereby minimizing intraspecific competition when colony density is high. While numerous studies have documented ant nest dispersion patterns, few have considered the impact of colony size and density on spacing patterns. Assessing the impact of colony size and density allows for more precise evaluation of spatial patterns and factors responsible for generating such patterns.
My collaborators and I studied the dispersion patterns of Formica altipetens in northern Arizona (Cushman, Martinsen and Mazeroll 1988). We present three lines of evidence which each suggest that intraspecific competition has significantly influenced the nest spacing patterns of this ant species. First, nearest-neighbor analysis of nest spacing patterns detected significant uniformity in six of eight plots. Second, there was a significant increase in the distance separating nearest neighbors as ant nest diameters increased. Third, ant nest density predicted substantial variation in the colony dispersion index, indicating the existence of a dispersion continuum at our study site. Collectively, these results suggest that intraspecific competition has been strong in this system and the uniform dispersal of ant colonies likely minimizes the degree of overlap in foraging areas and therefore the intensity of competition.
Latitudinal variation in diversity and body size European ant assemblages
A central problem in ecology is to identify and understand large-scale patterns in communities and regional biotas. Many ecological patterns occur along environmental gradients, such as those associated with latitude, and involve variation in species richness and body size. Body size is possibly the most fundamental trait of an animal. Every aspect of its life is influenced by body size – its interactions with the abiotic environment, the rates of its physiological processes, and the outcomes of its interactions with other organisms. Thus, body size should have considerable influence on the organization of ecological communities.
The study of variation in body size along latitudinal gradients has attracted considerable attention and debate. Much of this work has centered around Bergmann's rule, which states that body size in endotherms (within species or among closely related species) varies inversely with ambient temperature and thus increases with latitude. Despite its prevalence in the literature, the validity of Bergmann's rule has been severely criticized, largely because body-size patterns in endotherms are quite variable, and some ectotherms also exhibit larger body sizes at higher latitudes.
Using published distributions of 65 species from the British Isles and northern Europe, my collaborators and I showed that ant assemblages change with latitude in two ways (Cushman, Lawton and Manly 1993). First, as commonly found for many types of organisms, the number of ant species decreased significantly with increasing latitude. For Ireland and Great Britain, species richness also increased significantly with region area. Second, although rarely demonstrated for ectotherms (and especially invertebrates), the body size of ant species, as measured by worker length, increased significantly with increasing latitude. We found that this body-size pattern existed in the subfamily Formicinae and, to a lesser extent, in the Myrmicinae, which together comprised 95% of the ant species in our study area. There was a trend for formicines to increase in size with latitude faster than myrmicines. We also show that the pattern of increasing body size was due primarily to the ranges of ant species shifting to higher latitudes as their body sizes increased, with larger formicines becoming less represented at southerly latitudes and larger myrmicines becoming more represented at northerly latitudes. We conclude by discussing five potential mechanisms for generating the observed body-size patterns: the heat-conservation hypothesis, two hypotheses concerning phylogenetic history, the migration-ability hypothesis, and the starvation-resistance hypothesis.