1. An emerging research line in conservation ecology addresses how environmental change drivers may cause the biotic homogenization of ecological communities by shifts in species diversity and community composition. While the drivers have been explored in unmanaged ecosystems and managed agricultural systems, this issue has received limited attention in regards to a key soil bioindicator organisms, soil nematodes.

2. In this study, we evaluated the effect of land-use change and intensification on the diversity of plant-feeding nematodes (PFN) thought taxonomic and functional measures of alpha and beta diversity. We selected olive tree farms in southern Spain as the study system, given the wide distribution of wild forms in unmanaged systems and cultivated forms in agricultural systems, thus providing the opportunity to assess the effects of land use intensity.

3. Notably, our study revealed that the conversion from natural to agricultural systems and even moderate increases in land-use intensity caused a significant biotic homogenization by enhancing the functional similarities of PFN communities. Our study emphasizes the key role of body size in structuring nematode communities in response to land-use type and intensity. 

4.  Synthesis and applications. The importance of soil nematodes in soil processes is well known. We show that land use intensification reduces soil nematode diversity. Our study has important implications for the development of management strategies that foster soil biodiversity conservation such as no or minimal tillage and logging, vegetative covers and the maintenance of natural habitat.

The study systems included wild olive forests and cultivated olive areas in Andalusia, southern Spain, and covered an area of approximately 90,000 km2 (extent: 35.9377º to 38.7289º; -1.6272º to -7.5226º). A total of 246 plots were selected to obtain a representative sampling of the regional management within wild olive forest and commercial olive orchards that cover a wide gradient in land-use intensity. In wild olive forests, 123 plots of 100 m × 100 m were chosen to cover the overall surface of this ecosystem in the study area. In each plot, soil samples were collected from at least four wild olive trees, where possible, arbitrarily selected. In a similar manner, 123 plots where chosen in commercial olive orchards covering all types of management systems. In each commercial olive orchard, soil samples were taken from four, or five, olive trees that were arbitrarily chosen. 

Soil sampling was performed between 2014 and 2016 during spring, and they were taken from 5 to 50 cm depth using a pick in close vicinity to the active olive tree roots. e first few cm. All individual samples of a plot were mixed and homogenized to obtain a representative composite sample for each plot. The final soil sample was stored at a cool place before nematode extraction.

Nematode data

Nematodes were extracted from 500 cm3 of soil using the magnesium sulphate centrifugal-flotation method (Coolen 1979). PFN were identified at the species level using an integrative taxonomy. For each plot, nematode abundance was determined as the total number of nematodes per 500 cm3 of soil, and at least 100 nematodes were arbitrarily selected and identified. Nematode individual mass was estimated for identified species using the adjusted Andrássy´s formula, wherein nematode mass (μg) = L x D2 * 1,600,000-1; where L is nematode body length (in μm), and D is nematode maximum body width (in μm) (Andrássy 1956). Only adult specimens were used to obtain measurements, to avoid complications with ontogenetic character variation.

Details on the estimation of the land-use intensity (LUI) index for wild olive systems

Our land-use intensity measure (LUI) for wild olive forests was based on tree species composition, stand age and density, as well as the degree of naturalness. The land-use intensity for a given plot was the sum of the standarized measures of (1) the proportion of harvested tree volume (Iharv), (2) the proportion of trees that are not part of the natural forest community (Inonat), and (3) the degree of human intervention through the establishment of agriculture systems, clearing land, buildings, and roads (Intrs).

Details on the estimation of the land-use intensity (LUI) index for commercial olive orchards

In commercial olive orchards, the land-use intensity gradient ranged from extensive semi-natural to intensive commercial olive orchards, where high management inputs are required to maintain the systems. LUI for a given plot considered (1) the age of the plantation (Iplan), (2) the density of cultivation (Idens), and (3) the intensity of management within the commercial olive orchards (Iman).