Structuring of plant communities across agricultural landscape mosaics: The importance of connectivity and the scale of effect

Background: Plant communities of fragmented agricultural landscapes, are subject to patch isolation and scale-dependent effects. Variation in configuration, composition, and distance from one another affect biological processes of disturbance, productivity, and the movement ecology of species. However, connectivity and spatial structuring among these diverse communities are rarely considered together in the investigation of biological processes. Spatially optimised predictor variables that are based on informed measures of connectivity among communities, offer a solution to untangling multiple processes that drive biodiversity.

Results: To address the gap between theory and practice, a novel spatial optimisation method that incorporates hypotheses of community connectivity, was used to estimate the scale of effect of biotic and abiotic factors that distinguish plant communities. We tested: i) whether different hypotheses of connectivity among sites was important to measuring diversity and environmental variation among plant communities; and ii) whether spatially optimised variables of species relative abundance and the abiotic environment among communities were consistent with diversity parameters in distinguishing four habitat types; namely Crop, Edge, Oak, and Wasteland. The global estimates of spatial autocorrelation, which did not consider environmental variation among sites, indicated significant positive autocorrelation under four hypotheses of landscape connectivity. The spatially optimised approach indicated significant positive and negative autocorrelation of species relative abundance at fine and broad scales, which depended on the measure of connectivity and environmental variation among sites.

Conclusions: These findings showed that variation in community diversity parameters does not necessarily correspond to underlying spatial structuring of species relative abundance. The technique used to generate is extendible to incorporate multiple variables of interest along with a priori hypotheses of landscape connectivity. spatially-optimised variables with appropriate definitions of connectivity might be better than diversity parameters in explaining functional differences among communities.

Methods
We performed this study between July 2015 and June 2017 in the Vega del Tajo-Tajuña agricultural region of the Tagus River Basin, in the South-Central Plateau of the Iberian Peninsula (Fig. 1). We conducted 78 individual collections that included 23 sampling sites. Four sites each of Oak (n = 4 sites x 4 re-samples each) and Wasteland (n = 4 sites x 4 re-samples each) were visited with collections made in autumn and spring over two growing seasons. Edge (n = 2 sites x 6 re-samples + 2 sites x 5 re-samples) and Crop (n = 7 sites x 2 re-samples + 3 sites x 3 re-samples + 1 site x 1 re-sample) with four and eleven sites respectively, were visited in spring, summer, autumn. Eleven sites were chosen to better characterise the variation expected from the Crop communities as cultivated fields are subject to crop rotation and fallow periods. Crop collections comprised 4 fields of Cucumis melo (melon), 2 of Zea mays (maize), 2 of Brassica oleracea (cabbage and cauliflower), and 3 of Hordeum vulgare (barley), i.e., the major summer or winter crops in the area. In Oak and Wasteland sites, 25 m x 25 m quadrats were marked out and 150 samples per site systematically collected at each resampling. In Edge and Crop, 50 samples from a 25 m x 2 m area at each site were collected at each resampling. A boustrophedonic transect method (a line taken alternately from right to left and from left to right, and so on) was used in all instances except for Edge that have highly linear configurations. Depending on the habit of the species, a number of leaves from different parts of the individual were collected, each collection of leaves from the individual representing a single sample. The samples were harvested at fixed points along the transect. Individuals of each plant species were preserved at each collection and specimens assigned a provisional species, genus, or family rank prior to consultation with an herbarium for taxonomic assignments. The identifications were undertaken by Dr. Rosario Gavilán.