DOI: 10.5061/dryad.47d7wm3ds
title: Agriculture causes homogenization of plant-feeding nematode communities at the regional scale

description: 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.
description: 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. <em>Nematode
 data</em> 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.
 <em>Details on the estimation of the land-use intensity
 (LUI) index for wild olive system</em>s 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). <i>Details on
 the estimation of the land-use intensity (LUI) index for commercial olive
 orchards</i> 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).