Above- and below-ground effects of an ecosystem engineer ant in Mediterranean dry grasslands

Abstract

Within a local assemblage, ecosystem engineers can have major impacts on population dynamics, community composition and ecosystem functions by transforming or creating new habitats. They act as an ecological filter altering community composition through a set of environmental variables. The impact of ants on their environment has been widely studied, but their multi-component effects (both trophic and non-trophic) have been rarely addressed. We investigated the roles of Messor barbarus, one of the commonest harvester ant species in south-western European Mediterranean grasslands. We analysed soil physico-chemical parameters, above-ground vegetation (e.g. species richness, plant community, micro-local heterogeneity, plant biomass) and above- and below-ground fauna (macrofauna, Collembola, Acari and nematodes). A clear and strong local impact of M. barbarus on soil, vegetation and fauna compartments emerges. The environmental filter is altered by modifications to soil physico-chemical properties, and the biotic filter by changes to plant communities and altered above- and below-ground fauna abundance, occurrence and community structure. The engineering activity of M. barbarus affects not only these separate ecosystem components but also the trophic and non-trophic relationships between them. By altering ecological filters at a local scale, M. barbarus creates habitat heterogeneity that may in turn increase ecological niches in these highly diverse ecosystems.

Keywords: Messor barbarus; fauna; plant communities; soil; trophic and non-trophic relationships.

Figure 1.

Effect size (Cohen's d) and 95% confidence intervals for the effect of ants on vegetation parameters and above- and below-ground fauna abundances (macrofauna, mesofauna and microfauna). Soil fauna subgroups are in italics. Black triangles represent the mean effect size for variables with a significant difference between ant and ant-free patches. Dots represent the mean effect size for variables with no significant difference between ant and ant-free patches. Effect size > 0 indicates higher values in ant patches than in ant-free patches; effect size less than 0 indicates lower values in ant patches than in ant-free patches. (Online version in colour.)

Figure 2.

Path analysis models. Causal influences of vegetation heterogeneity and height, bare soil percentage, plant biomass, clay and N–NH₄ content on above-ground invertebrate abundance (epedaphic collembolans, Hemiptera, Araneae, plant feeder and predator Coleoptera) in ant patches (d.f. = 18, χ² = 18.83, p = 0.40, RMSEA = 0.04, CFI = 0.99) (a) and ant-free patches (d.f. = 18, χ² = 18.02, p = 0.46, RMSEA = 0, CFI = 1) (b). Causal influences of vegetation height, plant biomass, TOM and clay content on fauna abundance (microfauna PC and SC, mesofauna PC and SC, earthworms, macrofauna detritivores and Geophilomorpha) in ant patches (d.f. = 18, χ² = 17.78, p = 0.47, RMSEA = 0, CFI = 1) (c) and ant-free patches (d.f. = 18, χ² = 23.05, p = 0.19, RMSEA < 0.1, CFI > 0.9) (d). Numbers next to arrows show standardized parameter estimates (see electronic supplementary material, table S2 and table S3). Percentages of explained variance are shown with dependent variables. (Online version in colour.)