Competition can lead to unexpected patterns in tropical ant communities

Abstract

a b s t r a c tEcological communities are structured by competitive, predatory, mutualistic and parasitic interactions combined with chance events. Separating deterministic from stochastic processes is possible, but finding statistical evidence for specific biological interactions is challenging. We attempt to solve this problem for ant communities nesting in epiphytic bird's nest ferns (Asplenium nidus) in Borneo's lowland rainforest. By recording the frequencies with which each and every single ant species occurred together, we were able to test statistically for patterns associated with interspecific competition. We found evidence for competition, but the resulting co-occurrence pattern was the opposite of what we expected. Rather than detecting species segregationdthe classical hallmark of competitiondwe found species aggregation. Moreover, our approach of testing individual pairwise interactions mostly revealed spatially positive rather than negative associations. Significant negative interactions were only detected among large ants, and among species of the subfamily Ponerinae. Remarkably, the results from this study, and from a corroborating analysis of ant communities known to be structured by competition, suggest that competition within the ants leads to species aggregation rather than segregation. We believe this unexpected result is linked with the displacement of species following asymmetric competition. We conclude that analysing cooccurrence frequencies across complete species assemblages, separately for each species, and for each unique pairwise combination of species, represents a subtle yet powerful way of detecting structure and compartmentalisation in ecological communities.

The geographical variation of network structure is scale dependent: understanding the biotic specialization of host–parasitoid networks

“…Although supporting evidence is not unequivocal (Terborgh 2015, Ellwood et al 2016, Gainsbury and Meiri 2017, LaManna et al 2017), local dynamics in the tropics could exclude different consumers in different habitat patches, resulting in more specialized and less similar consumer communities across patches. Our results show that consumers segregated their dietary niches more in more constant environments (less consumer overlap) (Figure 3).…”

Community Sampling and Integrative Taxonomy Reveal New Species and Host Specificity in the Army Ant-Associated Beetle Genus Tetradonia (Coleoptera, Staphylinidae, Aleocharinae)

“…The proximate mechanisms underlying these varying tendencies of different Tetradonia species to co-occur in a given colony remain unknown. In general, co-occurrence patterns can be governed by various factors including interspecific competition [52], environmental filtering due to abiotic factors [53], host-encounter- and compatibility-filtering in the case of symbioses [18,54], as well as stochastic processes [55,56]. Studying potential proximate mechanisms in the future, such as the host-searching behavior of different myrmecophiles (encounter filtering; e.g.…”

Bird's nest ferns promote resource sharing by centipedes

“…While unexpected, these low levels of competition echo the remarkably low levels of competition observed between other invertebrates in bird's nest ferns, such as arthropod decomposers (Ellwood et al., ) and ant communities (Ellwood et al., ; Fayle, Eggleton, Manica, Yusah, & Foster, ). These previous studies concluded that competition must be less important in the harsh physical conditions of the high canopy (Ellwood et al., , ). However, we should continue to look at other biological explanations, such as resource partitioning, complementarity, or predation pressure.…”

“…Again the C ‐scores were not significant in terms of species co‐occurrence. Moreover, in the low canopy the observed C ‐score was lower than that of simulated random communities ( C obs = 3.97, C sim = 7.91, p = .38), suggesting species aggregation rather than segregation (Ellwood, Blüthgen, Fayle, Foster, & Menzel, ). In the high canopy, the observed C ‐score was higher than that of the random simulations, suggesting species segregation, although this result was not significant ( C obs = 8.05, C sim = 7.81, p = .085).…”

“…Centipedes are not social animals; they are typically aggressive predators, and thus, we would have expected them to show negative spatial associations with each other, in the same way that some large predatory ants exclude one another from bird's nest ferns (Ellwood et al., ). Yet we observed the opposite of what we expected, in that centipedes were coexisting within the ferns.…”