Biogeographical characterisation of Egypt based on environmental features and endemic vascular plants distribution
Introduction
The main objective of biogeography is to categorise and mapping the biota into meaningful and interpretable hierarchical homogeneous geographical units (Morrone, 2018; Rivas-Martínez, Rivas-Sáenz, & Penas-Merino, 2011). These units have been determined by present and past biological and physical forces and help to better understand the drivers for the spatial distribution of species. The so-called biogeographical regionalisation results in a hierarchical system that categorise geographical units in terms of their biota, in particular endemic taxa (Kreft & Jetz, 2010). Biogeographical definitions are powerful approaches not limited to the reduction of ecological data complexity (Kupfer, Gao, & Guo, 2012) but also for understanding spatial patterns of biodiversity and to explore the key elements influencing the historical and current distribution of species. In addition, it is an applicable implementation method for maximising the number of conserved species, due to its role in planning conservation strategies (Gao & Kupfer, 2018; Graham & Hijmans, 2006), attenuating difficulties to identify areas that should be protected without systematic mapping over large areas, such as multiple bioregions, climatic zones or political boundaries (Pressey et al., 2000).
For a long time, to do such biogeographical delineations, the qualitative data collection of experts and researchers has been considered, which directly or indirectly influenced biogeographical assessments (Gao & Kupfer, 2018). Nevertheless, the development of clustering algorithms, together with the availability of extensive environmental datasets and global species distribution, raised the interest of biogeographers and macroecologists to release and assess the biogeographical unit boundaries, from broad to fine scale, by the use of replicable methods (Blasi & Frondoni, 2011; Rivas-Martínez et al., 2011; Hattab et al., 2015; Kreft & Jetz, 2010; Mackey, Berry, & Brown, 2008). Delineating such units at small scale is crucial to define biodiversity areas that have been driven by micro-environmental factors and also provide a tool for filtering species and areas of priority, not only for the presence of endangered species, but also for the conservation of elements of biogeographical interest (e.g. Fenu, Fois, Cañadas, & Bacchetta, 2014; Rodrigues, Figueira, Vaz Pinto, Araújo, & Beja, 2015).
There is a great variety of previous studies on the subject that have based biogeographical regionalisation on different taxonomic levels and groups (Fenu et al., 2014; González-Orozco, Laffan, Knerr, & Miller, 2013; Kreft & Jetz, 2010; Rodrigues et al., 2015). To identify biogeographical areas, the spatial distribution of flora or fauna (Linder et al., 2012; Moreno Saiz, Donato, Katinas, Crisci, & Posadas, 2013) and, in particular, the co-occurrence of endemic taxa is especially used for conservational purposes (Cañadas et al., 2014; Escalante, Morrone, & Rodríguez-Tapia, 2013; Fenu et al., 2014; González-Orozco et al., 2013; Morrone, 2008, 2018). The main advantage of using endemic taxa in recognising biogeographic units is that their spatial distribution is not random and uneven through specific areas or habitat-type (Bradshaw, Colville, & Linder, 2015; Laffan & Crisp, 2003). In addition, endemics are often vulnerable because of their narrow distribution, distinctive evolutionary history and somewhat low population size (Huang et al., 2016; Orsenigo et al., 2018). Accordingly, their conservation is highly important in the global and local prioritisation efforts, and the recognition of areas with the highest endemic richness is the preliminary step for practical conservation policies (Moreno Saiz, Castro Parga, & Sainz Ollero, 1998; Orsenigo et al., 2018).
Several factors contribute to the distribution of endemic taxa, such as climate, altitudinal ranges, geographical barriers, human impacts, biotic interactions and stochastic events (Fenu et al., 2014; Fois, Fenu, Cañadas, & Bacchetta, 2017; Morrone, 2018); it is therefore common to include abiotic information for biogeographic classifications (e.g. Rivas-Martínez et al., 2011; Blasi & Frondoni, 2011; Cañadas et al., 2014; Escalante et al., 2013; Fenu et al., 2014), which is allowing the investigation of biogeographic dynamics related to environmental changes (Burns, 2016; Ferrier et al., 2006). Multivariate and clustering partitioning techniques, such as k-means algorithm (Mateo, Vanderpoorten, Muñoz, Laenen, & Désamoré, 2013; Razavi & Coulibaly, 2013; Rueda, Rodríguez, & Hawkins, 2010), unweighted pair-group method (Bradshaw et al., 2015; Dapporto, Ciolli, Dennis, Fox, & Shreeve, 2015; Hattab et al., 2015; Kreft & Jetz, 2010), Ward's clustering (Rodrigues et al., 2015; Wohlgemuth, 2006) and network clustering (Edler, Guedes, Zizka, Rosvall, & Antonelli, 2017; Vilhena & Antonelli, 2015) were used for regionalisation purposes in conjunction with spatial distribution of flora and/or fauna and have been succeeded in biogeographical delineations at different spatial scales. Examples are the regionalisation at global (Kreft & Jetz, 2010), European (Moreno Saiz et al., 2013; Rueda et al., 2010), sub-Saharan Africa (Linder et al., 2012), tropical Africa (Droissart et al., 2018), Sahara-Sahel (Brito et al., 2016) and Mediterranean Basin scales (Rivas-Martínez et al., 2011; Buira, Aedo, & Medina, 2017; Cañadas et al., 2014; Fenu et al., 2014). In order to depict a biogeographical unit, two means can be used separately or together: the occurrence of endemic taxa (flora or fauna) and the environmental conditions (e.g. Fenu et al., 2014; Rodrigues et al., 2015).
Egypt is an interesting country belonging to both the Saharo-Arabian and Mediterranean regions, and characterised by the presence of long coasts of both the Mediterranean and Red Sea, the Nile River, and by a high geological, environmental and climatic heterogeneity (Boulos, 2009; Zahran & Willis, 2009). Moreover, Egypt is the meeting point of four floristic regions: Africo-Zambezian, Irano-Turanian, Saharo-Arabian and Mediterranean (Rivas-Martínez et al., 2011; El-Hadidi, 2000). Hence, recognising the biogeographical units and endemic species-rich areas is useful for implementing effective conservation plans and measures in order to maximise the number of conserved species.
Despite many earlier studies have been defined the biogeographical territories of the entire Egypt (El Hadidi & Fayed, 1995; El-Hadidi, 2000; Hassib, 1951; Täckholm, 1974), all of them were based on native flora and physiognomy of vegetation without any special reference for environmental variables, species and/or endemism richness. In addition, these studies were based on expert-based delineations, which unable other authors to replicate, update or improve their works. On the other hand, biogeographical researches in Egypt considering both environmental variables, species and/or endemism richness were specific to particular areas (e.g. Abd El-Ghani, 2000; Abd El-Ghani & Abdel-Khalik, 2006; Abd El-Ghani, Salama, Salem, El-Hadidy & Abdel-Aleem, 2017; Le Houérou, 2001).
In this study, we strived to fill a knowledge gap in biogeographical units and endemic plant diversity patterns to underpin conservation planning efforts in Egypt. The main aim was to set out biogeographical units (sectors and subsectors) of Egypt based on a two-step procedure that first considers the regional environmental features together and then the spatial distribution of endemic vascular plant species.
Section snippets
Material and methods
Our study involved several steps concerning data sources and analyses (Fig. 1). A two-step procedure was applied to define the biogeographical units in Egypt. First, we classified Egypt into environmental clusters, according to a set of environmental variables, such as climatic, topographic, soil, and habitat heterogeneity. This step is important to assess the drivers that characterise the main environmental clusters. Moreover, this allowed to avoid areas without endemic species occurrences and
Environmental clusters
The maximum pseudo F-values were obtained for k = 15 (Fig. 3a), hence, 15 environmental clusters of Egypt were defined and mapped (Fig. 3b). The number of grid cells in each cluster as well as the R2 and the average value of characteristic variables are reported in supplementary material (Appendix C). Except for habitat heterogeneity variables (CV-EVI, evenness-EVI, range-EVI and Shannon-EVI), the rest of the variables were significantly different, as shown by Kruskal-Wallis test among clusters
Discussion
The present study is the first contribution for delineating biogeographical sectors and subsectors of Egypt depending on environmental features and spatial distribution of its endemic flora. It provides a replicable method to cluster biogeographical areas, using a reiterative approach which firstly considers abiotic factors and then ensemble them according to the presence/absence of relevant biotic elements. This approach was focused on the regionalisation of Egypt, by considering the
Conclusion
This is the first contribution towards delineating the biogeographical units in Egypt using advancement in quantitative approaches. Our results showed a distinctive biogeographical scheme including six sectors and nine subsectors, highlighting the importance of climatic-related variables, elevation and soil organic carbon in shaping the environmental clusters and endemism in Egypt. The importance of robust systems seeking to classify biogeographical patterns for their usefulness in conservation
Author contributions
MA, MF, GF and GB: Conceptualization; MA: Data collection; MA and MF: Data curation; and MA led the writing with assistance from MF, GF and GB. All authors gave final approval for publication.
Declaration of competing interest
None.
Acknowledgements
We would like to thank Prof. Kamal Shaltout (Plant Ecology and Flora, Tanta University, Egypt) and Prof. Ibrahim Mashaly (Plant Ecology and Flora, Mansoura University, Egypt) for their revision of the first draft of this manuscript. The first author is grateful to the Cagliari University that financially supported his research. We would also like to thank two reviewers for invaluable comments and suggestions that improved the earlier version of this manuscript.
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