Function of small habitat elements for enhancing plant diversity in different agricultural landscapes
Introduction
Land-use change leading to loss, isolation and degradation of habitats is often viewed as one of the main causes driving species decline at local, regional and global scales (Ewers and Didham, 2006, Kuussaari et al., 2009). The most severe change occurred during the last century, when traditionally low intensive farming was either abandoned or converted into intensified and specialized agriculture (Sutherland, 2002, Stoate et al., 2009). Both changes threaten biodiversity (Poschlod et al., 2005, Lindborg et al., 2008) by either highly exploited landscapes with vast crop fields and very little natural habitats, or abandoned farmlands dominated by commercial forests and overgrown fields and grasslands (Tschartke et al., 2012). Still, in either landscape there might be both small, so-called remnant, habitats left from the former traditional landscape and more recent habitats like road verges. To properly manage and protect species, habitats and ecosystems in any of these landscapes requires a better understanding of direct and indirect effects of the processes driving biodiversity decline.
Landscape composition is a factor steering biodiversity patterns at both local and landscape scale (Fahrig et al., 2011). The composition and quality of the landscape matrix, surrounding a focal patch, may influence species richness and abundance (Ricketts, 2001, Cook et al., 2002, Debinski, 2006, Öckinger et al., 2012). Landscapes with intermediate structural complexity have been found to support more species than homogenous landscapes (Tscharntke et al., 2005). Hence habitat patches in complex, more heterogeneous landscapes potentially receive a higher diversity of colonizing species from the surrounding species pool (Tschartke et al., 2012). Species-rich core areas with high habitat quality are essentially acting as source habitats for species survival at large scales (Bengtsson et al., 2003, Leibold et al., 2004, Rand et al., 2006), and the spill-over of organisms from these habitats to agricultural areas have been well documented (e.g. Thies and Tscharntke, 1999, Ricketts et al., 2008). Small marginal habitats found outside protected areas, such as road verges, shading trees, mid-field islets, and cropland set asides may also be important for biodiversity (Tscharntke et al., 2005, Kleijn and Baldi, 2005, Cousins, 2006), functioning as refugia or sources of dispersal (Dorrough et al., 2007, Cousins and Lindborg, 2008, Auffret and Cousins, 2011). Contributing to dispersal among fragments they enhance colonization and decease local extinctions (Fischer and Stöcklin, 1985), which may mitigate negative effects caused by fragmentation and isolation even in quite species-poor agricultural landscapes (McIntyre and Hobbs, 1999, Vandermeer and Perfecto, 2007).
Although many species react negatively to habitat deterioration, loss and isolation, there may be a range in their response as an effect of variation in life-history traits (Fischer and Stöcklin, 1985, Verheyen and Hermy, 2004). Dispersal ability is suggested to explain plant community patterns and the recolonization process (Vellend et al., 2007, Götzenberger et al., 2012). Hence, life-history traits such as canopy height (Cadotte et al., 2006, Thomson et al., 2011) or seed production, e.g. seed size (Jakobsson and Eriksson, 2000), seed dispersal attributes and vectors (Ozinga et al., 2004, Tremlová and Münzbergová, 2007) are often examined in a dispersal context. Empirical studies have proposed that directional animal dispersal might be a key trait increasing species robustness to habitat fragmentation. (Montoya et al., 2008, Sutton and Morgan, 2009). In contrast to dispersal related traits, plant longevity and clonal reproduction are important for plant survival in a long-term perspective (Marini et al., 2012). In grassland systems, long-lived plants, often with clonal propagation tend to build up remnant population systems in abandoned or isolated grasslands, where local populations may persist over periods long enough to bridge unfavourable phases of successional development (Eriksson, 1996, Honnay and Bossuyt, 2005, Lindborg, 2007). Such remnant populations may increase the persistence of a plant community and help to explain cases of high local species diversity, e.g. the semi-natural grasslands of Northern Europe.
In this study, we investigate if small habitat elements in rural landscapes can sustain a regional species pool for plant diversity usually associated with semi-natural grasslands. To estimate the importance of these small habitats for landscape diversity, we examined the species–area relationship by calculating species accumulation curves. As a measure of the variation in species identity between sites in a landscape, we calculated β-diversity that may link diversity measures across scales (Anderson et al., 2006). We examined two different types of small habitat elements: (1) mid-field islets, i.e. small semi-natural areas isolated in crop fields, and not managed today, and (2) road verges, i.e. linear areas connecting habitats in a landscape and are today managed for traffic safety. We specifically addressed the following questions: (i) how a large share of plant species richness associated with managed semi-natural grasslands can be preserved in small habitats depending on landscape fragmentation? (ii) is plant species richness (both total and grassland specialists) in small habitats affected by landscape matrix and distance to nearest semi-natural grassland? If so, (iii) is this also detected in β-diversity (species turnover) and species traits (dispersal or persistence related)?
Section snippets
Landscape selection
Three rural landscapes with different matrix configuration were selected in south-central Sweden: Selaön (59°24′N, 17°10′E); area 15 km2, Öllösa (59°8′N, 17°0′E) area 14 km2 and Nynäs (58°50′N, 17°24′E); area 10 km2. Selaön is a modern and fragmented agricultural landscape with a landscape matrix dominated by open agricultural fields and only little semi-natural grassland left (5%: Cousins and Eriksson, 2008). Öllösa is a landscape dominated by commercial forests and small-scale farming with
Species richness
Ninety-six out of 261 plant species found across all three landscapes were grassland specialists. Semi-natural grasslands did not differ significantly in total richness among the three landscapes, but grassland specialist richness in the modern forest landscape was markedly lower than in the traditional and modern agricultural landscape (Table 1, Fig. 1). However, there was a significant difference in total richness in the midfield-islets habitat, where the modern forest landscape had highest
Effect on the species pool
The total richness and richness of grassland specialists found in the small habitat elements was dependent on both type of element, distance to semi-natural grasslands and type of landscape matrix surrounding these habitats, but in significantly different ways. Species richness declined with distance to grassland habitat in the modern agricultural landscape, which was not detected in the other, more heterogeneous, landscapes. In addition, the grassland specialists were positively related to the
Acknowledgements
The authors thank J. Lindgren and S. Malmgren for vegetation sampling on the semi-natural grasslands in Nynäs and Öllösa; S. Malmgren is acknowledged for the GIS support. J.P. held an open-postdoctoral scholarship granted to S.C. by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS). This study was partially funded by the EkoKlim project at Stockholm University and SAPES at Lund University.
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2021, Biological ConservationCitation Excerpt :Smaller grassland fragments, comprising road verges, old-field hedges and complex forest borders are often scattered throughout the landscape, many being remnants of previously larger grassland areas (Cousins, 2006; Auffret and Lindgren, 2020). These smaller fragments –though potentially lower in habitat quality– have the potential to form a network of Green Infrastructure (GI) around large semi-natural grasslands, and facilitate functioning species' metacommunities that harbour more diversity at the landscape scale (Lindborg et al., 2014; Fahrig, 2017). Green Infrastructure – ‘networks of natural and semi-natural areas with other environmental features’– is a key concept in European Union policy, for example, designed to enhance biodiversity and ecosystem services (European Commission, 2013).