Vineyards compared to natural vegetation maintain high arthropod species turnover but alter trait diversity and composition of assemblages
Introduction
Biotic homogenisation occurs where land-use change leads to the decline or extinction of localised specialists that are unable to adapt to changing conditions, and replacement by widespread generalists that tolerate conditions in transformed areas (Clavel et al., 2011). Biotic homogenisation increases the genetic, taxonomic, and functional similarity of regional biotas (Olden and Rooney, 2006). To mitigate this effect and for effective conservation management in an agricultural area, an understanding of the relationship between local and regional diversity dynamics, and mechanisms that drive changes in diversity from local to regional scales are essential (Kraft et al., 2011). In turn, conservation planning needs to identify areas which are more likely to be homogenised and to improve heterogeneity of already homogenised areas (Karp et al., 2012).
Regional patterns of diversity are the sum of within-patch (alpha) and among-patch (beta) diversities. However, species replacement may be a better indicator of overall biodiversity (Flohre et al., 2011), as it provides insight into factors that shape community structures (Stier et al., 2016). Partitioning beta diversity into its respective components of species replacement (changes in species identities among sites) and assemblage nestedness (species richness differences among sites) (Legendre, 2014) can reveal more complex patterns in metacommunities than using the overall coefficients alone (Cardoso et al., 2014). This is because high total beta diversity in transformed sites may be driven by localised species losses instead of true turnover, which leads to regional diversity declines (Socolar et al., 2016) and biotic homogenisation.
Local species declines are dependent on their traits (McGill et al., 2006). In turn, functional diversity quantifies the value and range of biological traits which interact with the abiotic and biotic environment (Wong et al., 2019), influencing organism performance and thus ecosystem function (Petchey and Gaston, 2006). Furthermore, functional diversity is a driver of ecosystem processes (Díaz et al., 2007) and can provide important information about responses to land-use change, including biotic homogenisation in terms of functional homogenisation. Human disturbances can decrease functional diversity by filtering out certain biological traits, making ecosystems more vulnerable to changes that could previously be absorbed, which leads to lower ecosystem resilience to disturbance (Folke et al., 2004). This means that trait-based approaches can provide a more mechanistic understanding of assemblage response to land-use change by identifying characteristics of species that are either vulnerable to landscape transformation, or those that benefit from it (Gámez-Virués et al., 2015). In general, higher trophic level species (Kruess and Tscharntke, 1994) with low dispersal abilities (Hendrickx et al., 2009), and highly specialised or endemic species (Kehinde and Samways, 2012) are predicted to be disproportionately vulnerable to land-use intensification (Gámez-Virués et al., 2015).
The Cape Floristic Region (CFR) of South Africa is a global biodiversity hotspot with exceptional plant diversity and endemism (Manning and Goldblatt, 2012). At larger spatial scales, the diversity of the CFR is extremely high, with near-complete species turnover between vegetation types and great floristic dissimilarity along transects ranging from hundreds of metres to hundreds of kilometres (Rebelo et al., 2006), which does not necessarily predict the turnover of flower-visiting insects (Simaika et al., 2018). However, within biodiversity hotspots, such as the CFR, biodiversity and ecosystem function remain under pressure due to habitat loss, fragmentation, and degradation through land-use change (Topp and Loos, 2019).
As the major crop in the CFR is wine grapes, it is important to assess the impacts of viticulture on biodiversity patterns at larger spatial scales in the CFR. However, there is a gap in the knowledge of arthropod distribution patterns in the CFR, as much of the previous work focussed on plant diversity and patterns of distribution (Manning and Goldblatt, 2012, Rebelo et al., 2006).
The diversity of certain arthropod groups in the CFR appears to parallel that of the plant diversity (Kemp and Ellis, 2017, Simaika et al., 2018). In fynbos, high beta diversity of herbivorous insects and gall-insects is associated with plant turnover at local scales, likely due to high hostplant-specificity (Kemp et al., 2017, Wright and Samways, 1998). However, even non-phytophagous arthropods, such as springtails, show this pattern of high beta diversity, suggesting additional biogeographical drivers of high arthropod species turnover (Janion‐Scheepers et al., 2020). Earlier work assessing the effect of vineyards on arthropod alpha (Gaigher et al., 2015, Gaigher et al., 2016, Geldenhuys et al., 2021, Kehinde and Samways, 2012, Kehinde et al., 2018) and beta diversity (Kehinde and Samways, 2014a, Kehinde and Samways, 2014b) suggest that, for certain taxa, the impacts are not as high as would be expected, likely due to the low-intensity management of most vineyards in the region. Nonetheless, considering the high biodiversity value of remnant vegetation, more in-depth assessments are needed on how different components of arthropod assemblages respond to landscape transformation.
Our aim here was to determine whether viticulture has a homogenising effect on the assemblage composition of spiders and beetles within the CFR. Our objectives were to compare overall alpha and beta diversity, and functional diversity of spider and beetle assemblages between natural fynbos and vineyards. We also determined whether spider and beetle assemblage composition differ between fynbos and vineyards, and which species traits were driving differences between assemblages in the two systems. This information enabled us to make a comprehensive assessment of the impact of viticulture on multiple facets of biodiversity at different spatial scales.
Section snippets
Study sites and sampling design
A total of 11 vineyards adjacent to natural fynbos vegetation were sampled across 11 commercial wine farms (located 6–79.6 km apart). Fynbos is dominated by fine-leaved sclerophyllous shrubs (53.2%), has a large number of geophytes (17.4%), very few tree species (2.4%), and a very low number of annuals (6.7%) (Goldblatt and Manning, 2002). Vineyards were selected to represent a range of management practices and localities throughout the Cape Winelands Biosphere Reserve (Fig. 1). Within a 1 km
Results
A total of 4796 individual spiders from 37 families comprising 106 genera and 148 morphospecies, and 9760 individual beetles from 36 families comprising 362 morphospecies were sampled.
Discussion
Fynbos has higher structural and compositional plant diversity, and lower levels of disturbance compared to cultivated areas, and was expected to have higher alpha diversity for both spiders and beetles (Djoudi et al., 2018). Although spider alpha diversity was lower in vineyards, generally vineyards were still relatively rich in terms of spider and beetle diversity. This could be because most farmers employ diverse cover crops in vineyard inter-rows while also following low-intensity pesticide
Conclusion
Throughout the study region, spider and beetle assemblages were relatively well-supported in vineyards, compared to assemblages in fynbos in terms of alpha, beta, and trait diversity. However, there were subtle differences in diversity measures, and large differences in assemblage composition between biotopes, the latter being influenced by differences in species traits associated with each biotope. The lower trait diversity of spider and beetle assemblages in vineyards may point to a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Mapula Trust and The Lewis Foundation as part of the GreenMatter Fellowship provided funding. We thank the landowners, farm managers, and viticulturalists for making their vineyards available. Many thanks to A. Dippenaar and F. Roets for the identification of spiders and beetles respectively, and A. Pieterse, M. Eckert, S. Witbooi, and L. van der Merwe for the sorting of field collected samples. Arthropod sampling was approved by CapeNature, permit number AAA007-00144-0056.
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