Environmental heterogeneity is an important driver of ecological communities. Here, we assessed the effects of local and landscape spatial environmental heterogeneity on ant community structure in temperate semi-natural upland grasslands of Central Germany. We surveyed 33 grassland sites representing a gradient in elevation and landscape composition. Local environmental heterogeneity was measured in terms of variability of temperature and moisture within and between grassland sites. Grassland management type (pasture vs. meadows) was additionally included as a local environmental heterogeneity measure. The complexity of habitat types in the surroundings of grassland sites was used as a measure of landscape environmental heterogeneity. As descriptors of ant community structure, we considered species composition, community evenness, and functional response traits. We found that extensively grazed pastures and within-site heterogeneity in soil moisture at local scale, and a high diversity of land cover types at the landscape scale affected ant species composition by promoting nest densities. Ant community evenness was high in wetter grasslands with low within-site variability in soil moisture and surrounded by a less diverse landscape. Fourth-corner models revealed that ant community structure response to environmental heterogeneity was mediated mainly by worker size, colony size, and life history traits related with colony reproduction and foundation. We discuss how withinsite local variability in soil moisture and low-intensity grazing promote ant species densities, and highlight the role of habitat temperature and humidity in affecting community evenness. We hypothesize that a higher diversity of land cover types in a forest-dominated landscape buffers less favorable environmental conditions for ant species establishment and dispersal between grasslands. We conclude that spatial environmental heterogeneity at local and landscape scales plays an important role as a deterministic force in filtering ant species and, along with neutral processes (e.g. stochastic colonization), in shaping ant community structure in temperate semi-natural upland grasslands.

Ant survey and nest density calculation was based on “Seifert, B. (2017). The ecology of Central European non-arboreal ants – 37 years of a broad-spectrum analysis under permanent taxonomic control. Soil Organisms, 89(1), pp. 1–67. Dataset collection is provided in detail in the article's main text and appendix. 

Ant survey: The procedure consists of direct localization of workers and nests within a spatially nested scheme covering three levels of search effort in a specific area: an intensive scrutiny (S-) search performed on soil and vegetation within a smaller area (S- sampling area); a quick (Q-) search on ground surface performed within a larger area (Q- sampling area); and a spot inspection (SI-) in the most promising habitats for nests in the surroundings of the Q-areas. The S- search aims to detect nests of small species with hidden nests and small territories, while the Q- search reflects realistic nest densities of larger species with lower nest densities but larger territories. The SI sampling allows discovery of nests of rare species such as social parasites of Lasius or Formica genera. We employed fixed dimensions of 64 m² for S- sampling areas and 400 m² for Q- sampling areas, while SI areas covered c. 900 ± 82 m². The combination of these three levels constituted a sample unit referred to hereafter as Seifert-plot. Time expenditure for ant searching in S- sampling areas varied from 30 to 60 minutes depending on the vegetation structure, and up to 180 minutes for the entire Seifert-plot. Recording of foraging workers and nests was performed sequentially from S- to Q- and SI-areas, and up to 10 workers per nest were collected after finishing each sampling area. Depending on the grassland size and accessibility, one to three Seifert-plots were established per grassland site. All Seifert-plots were searched between 08:00 and 18:00 hrs local time in August 2017.

Nest density calculation (Integrated nest density calculation): Nest abundance from each Seifert-plot component (S-, Q-, and SI-) was combined into a final integrated species-specific density (ISSD) which represents the nest density of a species within 100 m² (Seifert 2017). The ISSD per species is calculated as the sum of nests found in the S-, Q- and SI- sampling areas divided by the pseudo-area of the “recording group” (RG) to which a particular species belongs (Eq. 1). The RG is a generalization of how perceptible a nest is accordance with the ant species biology. The assignment of a species into a given RG describes the probability of finding a nest in each sampling level (S-, Q-, and SI-), and such probability is determined by nest's position, type, size and density. Based on almost four decades of research on Central European ants, Seifert (2017) defined five RGs (i-v) ranking from lowest to largest perceptibility. The pseudo-area is calculated for each RG separately and provides a measure of the total intensity of investigation on a Seifert-plot per RG. This parameter may be understood as the area equivalent needed in a specific RG to find the sum of nests recorded by S-, Q- and SI-search. The pseudo-area is defined as the sum of all nests detected in all search levels divided by a fixed value per RG, the recording-group-specific total density or FRSD (Eq. 2). The FRSD is the number of nests of a specific RG expected to occur in 100 m², based on the total number of nest of all species occurring in such RG and weighted by the sampling area (Eq. 3). The sampling area employed in Eq. 3 is subedited to the RG (See Seifert 2017 for more details).

As a short example, let us say that we are interested in calculating nest densities of M. rubra per grassland site. Our estimations would be focused on the RG iii (common grassland species; Seifert 2017), and FRSD and pseudo-area would be based on S- sampling area (64 m²). Thus, in an hypothetical community of five species and 11 nests resulting from a Seifert-plot with S= 3 nests (2 M. rubra), Q= 4 nests (1 M. rubra) and SI= 4 nests (0 M. rubra), the FRSD and pseudo-area will be 4.688 nests/100 m² and 2.347 m² respectively, and the integrated species-specific density (ISSD) for M. rubra will be 1.278 nests/100 m².

Microsoft Excel - LibreOffice Calc