Local diversity of arable weeds increases with landscape complexity

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Abstract

Patterns of plant diversity are often related to local site conditions and to competitive interactions, but landscape context may also be important for local plant species richness. This is shown here by analysing the relationship between landscape complexity and local species richness of arable weeds in wheat fields. The fields were located in 18 landscapes characterised by a gradient in landscape complexity from structurally complex to structurally simple (39–94% arable land). We quantified local site conditions, field management intensity and landscape characteristics, and used principle component analyses to ordinate the environmental variables. The percentage of arable land was negatively correlated with perimeter–area ratio, habitat-type diversity and topographical heterogeneity, but landscape characteristics did not correlate with local site conditions and field management intensity. The number of plant species was mainly related to landscape characteristics and to a lesser extent to field management intensity (nitrogen fertilisation), whereas local soil characteristics did not contribute to the explanation of arable weed richness. In a geographic scale analysis using circular landscape sectors ranging from 1 km up to 5 km diameter, the predictive power of landscape complexity for local plant species richness was strongest at 2 km indicating a scale-dependent relationship between landscape context and plant species richness. Our results support the hypothesis that local plant species richness in arable fields is greatly influenced by processes operating at the landscape scale. Seed rain from ruderal source habitats and disturbed edges may be the most important underlying process.

Introduction

Patterns of plant diversity are traditionally explained by local abiotic conditions such as climate and soil properties, as well as disturbance regimes and competition for light, water and nutrients (Grime, 1979; Tilman, 1982; Ellenberg, 1988). Many arable weed studies have demonstrated the importance of local site characteristics and management practices for the occurrence of single species, community composition and species richness (Dale et al., 1992; Andersson and Milberg, 1998; Kay and Gregory, 1998, 1999; Hald, 1999; Hallgren et al., 1999; Swanton et al., 1999; Menalled et al., 2001; Hyvönen and Salonen, 2002; Walter et al., 2002; Hyvönen et al., 2003; Gerowitt, 2003; Lososova et al., 2004). In the last decades, the intensification in agricultural practices has caused major changes in the composition and species richness of weed communities in arable fields (e.g. Albrecht, 1995; Sutcliffe and Kay, 2000). Arable weeds are important components of agroecosystems (Marshall et al., 2003), as they constitute the base of food chains for herbivores and their natural enemies. Species-rich weed communities support many species of phytophagous and entomophagous insects and spiders (Mahn, 1992; Rypstra et al., 1999; Haughton et al., 2001), are pollen and nectar resources for pollinating insects, and enhance biological control (Heitzmann et al., 1992; Nentwig, 1994). They possess a number of specific life-history characteristics including an enormous seed production, seed dormancy, rapid development rates and short generation times. These traits enable populations to persist in crop fields, which are subject to high rates of disturbance such as annual soil cultivation, harvest of crops and crop rotation (Cousens and Mortimer, 1995). Such disturbance dynamics result in the requirement for weeds to re-establish annually from the local seed and bud bank and/or via propagule immigration from surrounding landscapes.

The landscape context is central to understanding local patterns of biodiversity (Kareiva and Wennergren, 1995; Gustafson, 1998; Wiegand et al., 1999), because ecological processes operate on a range of spatial scales larger than a single patch (Turner, 1989; Dunning et al., 1992). Recent studies in agroecosystems have demonstrated a relationship between landscape complexity and local diversity patterns of carabids, butterflies and solitary wild bees as well as on processes such as herbivory, parasitism and pollination (Thies and Tscharntke, 1999; Östman et al., 2001; Steffan-Dewenter et al., 2002; Krauss et al., 2003; Thies et al., 2003; Weibull et al., 2003). However, the potential importance of landscape context in driving local patterns has received limited attention in plant ecology (de Blois et al., 2002). A number of studies have examined regional plant diversity as a function of habitat-specific communities in a mosaic landscape (Wagner et al., 2000; Honnay et al., 2003), and thus the question arises as to whether local plant species richness may also be influenced by the landscape context (Kollmann and Schneider, 1999).

In this study, we analysed 18 landscape sectors along a gradient from structurally simple to structurally complex. In the centre of each landscape sector, a winter wheat field with homogenous local site characteristics and management intensity was selected to study local species richness of arable weeds in relation to landscape context at different spatial scales (landscape sectors ranging from 1 to 5 km diameter). We hypothesise that (i) local species richness of arable weeds increases with increasing landscape complexity, and that (ii) the predictive power of landscape complexity for local species richness of arable weeds differs between spatial scales.

Section snippets

Study area and landscape context

The study area was located around Göttingen, Germany. This region was characterised by intensive agricultural land use and patchily distributed fragments of semi-natural habitats such as grasslands, fallows, hedges and forests. We selected 18 landscape sectors, which represented a gradient of landscape complexity ranging from structurally simple landscapes with a high percentage of arable land (up to 94%) to structurally complex landscapes with a high percentage of non-arable land (up to 61%).

Local site characteristics and the landscape context at different spatial scales

Local site characteristics and the landscape context at five spatial scales are listed in Table 1. In PCA ordinating local and regional parameters simultaneously, at each spatial scale four PCs were extracted (Table 2a), which explained between 82.1% and 83.3% of the total variance. At the smallest spatial scale (landscape sectors of 1 km diameter), PC 1 was positively correlated with soil acidity (42.1% explained variance). PC 2 was positively correlated with the local and regional

Discussion

Our analyses showed that the number of arable weed species, and particularly the number of herb species, was mainly related to a factor complex characterising landscape complexity. Species richness increased with increasing perimeter–area ratio, habitat-type diversity and decreasing percentage of arable land in a landscape. These landscape characteristics were intercorrelated and indicated a higher diversity of arable weeds in structurally complex landscapes.

Our results suggest that diversity

Acknowledgements

The authors are grateful to Jan Bakker, Thomas Valone, Helene Wagner, Kerstin Wiegand, Joachim Saborowski, Martin Schmidt, Tatyana Rand and two anonymous reviewers for helpful comments on the manuscript. The authors thank Indra Roschewitz for advice and assistance in the field and Christof Bürger for GIS support. This study is part of the BIOPLEX-project (Biodiversity and spatial complexity in agricultural landscapes under global change) supported by the German Federal Ministry of Education and

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