Grazing as a management tool in dune grasslands: Evidence of soil and scale dependence of the effect of large herbivores on plant diversity
Introduction
Grazing by larger herbivores is widely used as a management tool to preserve and enhance plant diversity (Bakker, 1998, Van Diggelen and Marrs, 2003, Hodgson et al., 2005, Dorrough et al., 2006), particularly in productive environments (Pacala and Crawley, 1992, Grime et al., 1988). However, the effect of introducing large herbivores on plant community composition is still highly uncertain (Bakker and Londo, 1998). Available studies indeed yield contrasting results, showing positive to neutral or even negative effects on plant diversity (Mc Intyre et al., 2003, Landsberg et al., 2002, Dorrough et al., 2007). As hypothesized by Milchunas and Lauenroth, 1993, Olff and Ritchie, 1998, contrasting effects of herbivores on plant diversity patterns may be due to variations in soil conditions.
Herbivores can directly affect plant species richness through phytomass removal. On productive soils, with a high phytomass production, herbivores may alleviate plant competitive exclusion by tall, dominant plant species (Huisman and Olff, 1998, Kahmen et al., 2002). Small disturbances and a higher light intensity at the soil surface due to a more open canopy, create regeneration opportunities, a higher probability of successful germination and seedling establishment of less competitive species (Jutila and Grace, 2002). Trampling and dung can modify gap and resource availability resulting in changes of local competitive hierarchies. Grazing can also influence spatial heterogeneity in grassland composition by creating gradients in soil disturbance which enables species to coexist through niche differentiation (Olff and Ritchie, 1998, Jutila and Grace, 2002). Alternatively, grazing may negatively affect plant species richness, where soil resources limit the regrowth of plant species (Proulx and Mazumder, 1998).
Additionally, the effect of grazing on community structure and diversity strongly depends of the spatial scale (Olff and Ritchie, 1998, Spiegelberger et al., 2006). At a small scale that corresponds to individuals and their immediate neighbours grazing is predicted to result in an increase of diversity. This may result from both increased local colonization of species from larger scale (enhanced establishment possibilities through gap creation and enhanced dispersal through zoochory) and decreased local extinction by changed local competitive hierarchies and decreasing competitive exclusion by tall graminoid competitive species (Chaneton and Facelli, 1991). However, at the larger scale, a pattern of declining species richness is expected, due to selective grazing of grazing intolerant species. Alternatively, one can expect that at larger scales, plant interactions, which cause local extinctions, are likely to decrease in importance (Dorrough et al., 2007), and will at last lead to an increase in species richness. Whether the latter positive effect on plant richness is outweighed by negative effects of grazing on rare and high palatable, grazing intolerant plant species, remains to be investigated (Grubb, 1977, Olff and Ritchie, 1998).
In the late 1990s, different large herbivore species were introduced in several dune reserves along the Belgian and north-western French coast as a nature conservation tool, aiming at the prevention of succession towards coarse grassland with dominance of tall competitive grass species (largely Calamagrostis epigejos), with low plant species diversity (Provoost and Van Landuyt, 2001, Provoost et al., 2004). Soil conditions are a major source of environmental variation determining species composition and productivity of dune grasslands (Tahmasebi Kohyani et al., 2008). In dune soils, nutrient availability is strongly determined by soil acidity, with phosphorus limited growth in acid soils due to the immobilization of phosphorus by aluminium and iron, and a more balanced supply of nitrogen and phosphorus in soils with a higher pH (Kooijman et al., 1998, Kooijman and Smit, 2001). An independent survey by the authors confirmed that there was a significant correlation between aboveground biomass (as a proxy for nutrient availability) and soil pH (Table 1).
While most studies have focused on comparing the effects of grazing on plant diversity between different soil condition or spatial scales, only few have addressed specifically how plant species diversity is affected by their interactive effects (Dorrough et al., 2007). In addition, as Olff and Ritchie (1998) predicted, the loss of plant species richness at larger scales is likely to be attributable to rare species, but whether changes in plant species richness can be related to changes of richness of rare species still remains to be tested. This study assesses the impact of introduced grazing as a conservation tool on different aspects of plant species diversity (community heterogeneity and species richness) from dune grasslands at different scales along a soil acidity gradient. We aimed to address the following questions: (1) is introduction of larger herbivores essential for maintaining or enhancing plant diversity? (2) does the effect of grazing on plant diversity depend on soil acidity, scale or their interaction?
Section snippets
Data collection
Vegetation data were collected in four coastal dune reserves at the Belgian and north-western French coast (Dunes fossiles de Ghyvelde, Cabour dunes, Westhoek, Ter Yde) and one privately owned and managed grassland (Oostvoorduinen) in the summer of 2006. Depending on plant-compositional or environmental differences or on the extent of the location, two to three areas with grazing management were selected within each location. The investigated grasslands can largely be classified as
Results
In total, we found 132 plant species, 88 of which (66.66%) occurred in both grazed and ungrazed plots, 42 species (33%) were found only in grazed and 3 (0.023%) only in ungrazed plots. Overall, species richness responded positively to grazing and scale and there was no effect of soil acidity (Table 2, Table 3). However, we found significant interaction terms for both scale and pH with grazing (Table 3), which means that the response of plant species richness to grazing depended both on soil
Discussion
Exclusion of large grazers in coastal dunes due to agro-pastoral use abandonment is likely to be the main factor triggering secondary succession towards grass-encroached vegetation (Provoost et al., 2004). It has been hypothesized that fine-scale plant species richness is positively correlated with grazing (Olff and Ritchie, 1998). This is consistent with our results, demonstrating that species richness at a local scale (0.25 × 0.25 m2) increased with grazing, independent of soil conditions (Table
Acknowledgements
Soil analyses were conducted at the Research Institute for Nature and Forest (INBO). DB is a postdoctoral fellow of the Fund for Scientific Research, Flanders. We thank Edward Vercruysse for help with vegetation sampling and plant species identifications. We are grateful to the Agency for Nature and Forest of the Ministry of the Flemish Community and the Conservation du Littoral for their permission to do field research in their nature reserves.
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