Some consequences of diffuse competition in a desert ant community

Journal ArticleExploitative and interference competition are investigated in detail in a community of six coexisting species of granivorous desert ants . A linear model that includes both direct and indirect competitive interactions is used to predict positive or negative correlations in the abundances of competitors. Data on the abundances of the six ant species on 23 1/4-ha plots provide empirical support for the four predictions so generated. "Apparent facilitation." in the for of positive interspecific spatial associations of colonies, is detected between two competitors and interpreted as arising from indirect pathways of interspecific interaction. The results illustrate how indirect interactions among species at a single trophic level may play a significant role in organizing natural communities

Size variability in the worker caste of a social insect (veromessor pergandei mayr) as a function of the competitive environment

Journal ArticleWorker size polymorphism in colonies of Veromessor pergandei, a granivorous desert ant, is inversely related to the intensity of interspecific competition in the habitat for seven ant communities in the deserts of southern California and southern Arizona. Seed size preferences are positively correlated with worker body sizes, and diet breadth relative to seed size is probably enhanced by colony polymorphism . Niche shifts within colonies of these social insects may permit unusually fine adaptation to the resource environment. In contrast to solitary organisms, for which generalization of the diet may imply significant trade-offs, these ants may expand their utilization of resources where competitors are absent while retaining an efficient size match to the distribution of available resources

Foraging ecology and community organization in desert seed-eating ants

Journal ArticleGranivorous ants in the southwestern deserts of the United States are characterized by species-specific colony foraging behaviors that determine their efficiencies at utilizing seeds from different density distributions. Workers search for food either in groups or as individuals, and these feeding strategies represent specializations for high and low density resources respectively. While gathering experimental seeds, simultaneously supplied in clumped and dispersed distributions, feeding by group foragers focused on the high density resource patches, while individual foragers harvested seeds predominantly from the dispersed distribution, which required that prey be independently discovered. Collecting native seeds from unmanipulated environments, individual foragers tended to spend proportionately more time searching and experience lower foraging success than did group foragers in the same local habitat. During a period of high seed abundance, group foragers collected a much narrower range of prey types than did individual foragers. Colonies of Pogonomyrmex rugosus exhibited a mixed foraging strategy, with the most distinct feeding columns occurring during a period of peak seed abundance. Among group foragers, greatest activity coincided with periods of relatively high seed densities and low climatic stress, and these species utilized tactics such as seed storage, hibernation, and estivation to weather less favorable periods in a resting state. Individual foragers were active at intermediate levels during less favorable periods. Although high and low density seed resources are not renewed independently of one another, density specialization appears to promote coexistence between group and individual foragers. Their capacity to stably partition resources in this way should depend on (1) the degree of difference in their efficiencies at exploiting different density distributions of seeds and (2) the availability of seeds in the habitat as a function of seed density. In more mesic habitats, where seeds are renewed more frequently and probably also in larger pulses, group foraging species are proportionately more abundant

Experimental study of diffuse competition in harvester ants

Journal ArticleExperiments carried out over a 5-yr period in the Chihuahuan Desert support the a priori prediction of diffuse competition between two species of harvester ants. Despite dietary overlap between a large species {Pogonomyrmex rugosus) and a small species (Pheidole xerophila), the large species facilitates the small species indirectly by suppressing populations of an intermediate-size species (Pogonomyrmex desertorum). Although similar indirect interactions may occur frequently in natural communities and be predictable on the basis of body size, very long term studies may be necessary to demonstrate their effects experimentally

Species diversity and community organization in desert seed-eating ants

Journal ArticlePatterns of species diversity and community organization in desert seed-eating ants were studied in 10 habitats on a longitudinal gradient of increasing rainfall extending from southeastern California, through southern Arizona, and into southwestern New Mexico. Local communities of harvester ants include 2-7 common species, and at least 15 species from five genera of Myrmecines compose the total species pool in these deserts. Ant species diversity is highly correlated with mean annual precipitation, an index of productivity in arid regions. Communities are structured on the basis of competition for food, and interspecific differences in worker body sizes and colony foraging strategies represent important mechanisms of resource allocation. Seed size preferences, measured for native seeds and in food choice experiments with seeds of different size but uniform nutritional quality, are highly correlated with worker body sizes. Species of similar body size can coexist within local habitats if they differ in foraging strategy. Interspecific aggression and territorial defense and microhabitat partitioning all appear to be relatively unimportant in these ant communities. Patterns of species diversity and community organization in harvester ants are strikingly similar to those reported for communities of seed-eating rodents that occupy many of the same desert habitats. Separate regressions of within-habitat species diversity against the precipitation index of productivity for the two groups correspond closely in slope, intercept, and proportion of explained variation. Resource allocation on the basis of seed size characterizes local communities of both ants and rodents. Parallels between these two groups suggest that limits to specialization and overlap may be specified by parameters such as resource abundance and predictability that affect unrelated taxa similarly

Cecropia and its biotic defenses

Journal ArticleIn all the world, the genus Cecropia is unrivaled for the number of myrmecophytes, or true "antplants" counted among its species (McKey & Davidson, 1993). Based on the proportion of Cecropia species producing Mullerian bodies in at least some parts of their distribution, myrmecophytes comprise the vast majority (ca. 80%) of species in the genus; most nonmyrmecophytes occur at higher elevations and on islands, where their ants are missing (Wheeler, 1942). Geographically, myrmecophytic Cecropia occur throughout the latitudinal range of the genus, from southern Mexico to northern argentina. Given that association with ants is so widespread taxonomically and geographically, it is likely that relationships with ants have been highly influential in the evolutionary diversification of Cecropia

Ecological studies of neotropical ant-gardens

Journal ArticleIn a census taken in Peru's Manu National Park, 10 epiphytic angiosperms from seven plant families established principally on arboreal carton-ant nests. These "ant gardens" (AGs) were most often inhabited by parabiotic ants, Camponotus femoratus and Crematogaster cf. limata parabiotica, whose polygynous and polydomous colonies fissioned to form extensive AG aggregations. AGs tended by polydomous but probably monogynous Azteca cf. traili occurred on average in smaller isolates. All three ant species enriched nest gardens with vertebrate feces, but frequencies of occurrence of most AG epiphytes were lower on the less organic carton of Azteca AGs. Interspecific differences in epiphyte abundance and distribution were related to light requirements of plants and to colonizing abilities, as influenced by differences in allocational preferenda and life history. AG aggregations occupied 16-39% of five forest habitat types present and were especially common in frequently flooded habitats and areas of high light intensity. Patchy distribution was explained partly by overrepresentation on resource trees, such as Inga and Calyptranthes (parabiotic ants) and Cordia nodosa {Azteca). Habitat associations did not result from reduction of the terrestrial ant fauna in flooded forests. Other arboreal ants, but not terrestrial ants, were markedly lower in AG aggregations than in areas that lacked AGs, perhaps due to competition from aggressive and dominant AG ants. AGs formed principally by directed dispersal of epiphyte seeds to ant nests, where larvae fed on seed attachments without damaging seeds. AG ants also recognized and retrieved seeds of at least one AG epiphyte from feces of vertebrate fruit dispersers. The preference ranking of epiphyte seeds by Ca. femoratus was not correlated with either obvious differences in quality of seed appendages or long-term resource potential of plants. Seeds of AG epiphytes were rejected by three ants that do not tend AGs but were collected by a fourth such species. Seed attractiveness may depend in part on nonnutritional cues. Preadaptations of plants and ants appear to have been very important to the origin of AGs. Evidence for evolutionary specialization and coadaptation is circumstantial but suggestive

Experimental studies of species-specificity in cecropia-ant relationships

Journal ArticleStrict coevolution requires that interactions among organisms be speciesspecific. We assessed the relative roles of host- and habitat-specificity in determining the match between a genus of myrmecophytic trees and a guild of obligate plant-ants in the moist tropical forests of Madre de Dios, Peru. Four locally coexisting but habitat-restricted Cecropia species were cultivated in screen tents until all plants had developed myrmecophytic traits. Saplings were then placed within replicate blocks of each of two habitat types: riversides and small forest light gaps. Colonization events were recorded every 3 d between June and August of 1992, and queens were later removed from stem internodes for identification and brood censuses. A similar experiment, conducted in September through November of 1993, included just two species of Cecropia hosts. Effects of host species and habitat on queen colonization rates were evaluated by log-likelihood goodness-of-fit tests and contingency table tests. For three ant species, we also conducted queen preference experiments to compare queen behaviors across a range of host plants. Differences among ants in the extent of habitat-specificity vs. host-specificity provide evidence for multiple evolutionary routes to obligate association with Cecropia. Habitat- specificity exceeded host-specificity in Azteca ovaticeps (Dolichoderinae), for which queen preference experiments revealed no significant discrimination among hosts. This extreme riverside specialist is thought to have descended from generalist live-stem nesters in secondgrowth habitats. In Azteca australis, host-specificity was strong, and in this species only, directed toward hosts where brood production was most successful. Conflicting habitat associations in the two experiments indicated the weakness or absence of a consistent habitat affiliation in Azteca australis and suggested that colonization frequencies were influenced instead by proximity to foundress sources. Close relatives of A. australis live in exposed carton nests, which may have been positioned ancestrally on key resource plants, e.g., those producing lipid- and amino-acid-rich pearl bodies. Pachycondyla luteola (Ponerinae) exhibited both strong habitat and host associations and may have undergone pairwise coevolution with its forest-gap-dwelling primary host. Queens of Camponotus balzani (Formicinae), possibly a recent and secondary associate of Cecropia, were overrepresented in forest gap habitat but were host generalists, underrepresented only on a host with extremely small internodes. Apparently greater host-specificity in C. balzani at later stages of colony establishment may be due to differential post colonization mortality on the various hosts. Attack of ant queens by parasitoid wasps was strongly concentrated in the linear riverside habitat and weak to absent in the patchily distributed forest gap habitat. Due to lower rates of either parasitoid attack or other forms of queen mortality, Camponotus balzani experienced greater success in the forest gap habitat, where it was overrepresented in colonization experiments. Historical coincidences and preadaptations appear to have strongly influenced pairings between Cecropia species and their obligate plant-ants and account for much of the "apparent" niche partitioning observed in the system. Species-specificity seems to be determined mainly by coincident habitat affiliations of ants and plants ("coordinated dispersal") and by preadapted capacities of ants to distinguish among host-plant species. Multiple mechanisms for species-specificity may be characteristic of relationships in which associates disperse separately from one another (i.e., show horizontal transmission). Our results are consistent with the view that coadaptation and co-cladogenesis are more likely in systems where dispersal of associates is tightly coupled

Ant-plant symbioses in Africa and the neotropics : history, biogeography and diversity

Book ChapterSymbiotic ant-plant relationships afford an excellent opportunity to analyze the effects of both historical and ecological factors on the evolution of mutualisms. Occurring in tropical forests throughout the world, all myrmecophytic plants provide food and permanent housing to ants; the ants, in turn, are known or presumed to protect their hosts from herbivores or competition or to provision them with nutrients. Despite this underlying similarity, ant-plant symbioses differ in diversity and nature on different landmasses

Effects of trenching on growth and survival of planted Shorea parvifolia seedlings under pioneer stands in a logged-over forest

Journal ArticleEffects of trenching on growth and survival of planted Shorea parvifolia seedlings under pioneer stands in a logged-over forest. We conducted an experiment to determine the value of trenching in reducing potentially negative effects of root competition on growth of Shorea parvifolia seedlings planted in the understory of a 13-y-old logged-over forest