Using multiple palaeoecological indicators to guide biodiversity conservation in tropical dry islands: The case of São Nicolau, Cabo Verde
Graphical abstract
Introduction
Anthropogenic activities have been a major driver of environmental degradation in oceanic islands worldwide (Whittaker and Fernández-Palacios, 2007; Whittaker et al., 2017). Tropical dry islands are also especially vulnerable to extreme climatic events such as drought and tropical storms (Hamann, 2004; Holmgren et al., 2006), which are projected to increase in the next decades (Cai et al., 2014). Studying long-term ecological change in semi-arid environments is challenging due to poor micro-fossil preservation and irregular sediment deposition (Brunelle et al., 2018). However, the understanding of long-term trajectories (i.e. trends of environmental change) is relevant for the management of vulnerable small island populations, especially in the light of increasing anthropogenic impacts and projected global climate change (Holmgren et al., 2006). In this context, palaeoecological studies can provide information to determine ecological baselines (e.g. Nogué et al., 2017), past vegetation distributions (e.g. de Nascimento et al., 2009), changes in composition and structure (e.g. Nogué et al., 2013), fire regimes (e.g. McWethy et al., 2009), and long-term ecosystem variability (Froyd and Willis, 2008; Wilmshurst et al., 2013). There are several examples of the application of palaeoecological data to assess relevant conservation strategies in island ecosystems (e.g. Boessenkool et al., 2013; Burney and Burney, 2007). For instance, palaeoecological studies carried out in the Galápagos Islands have also demonstrated the importance of taking a long-term perspective when developing criteria for the designation of non-native/native status of plant taxa (van Leeuwen et al., 2008; Coffey et al., 2011).
Cabo Verde is the only tropical archipelago of the biogeographical region of Macaronesia (the Azores, Madeira, the Salvages, the Canary Islands, and Cabo Verde). Due to the scarcity of historical documents and archaeological research (Evans et al., 2017), little is known about the past abundance, distribution, and variability of endemic vegetation in the archipelago and for example the potential impacts of the drying trend that took place in NW Africa (5500 BP onwards) after the end of the African Humid Period (ca. 9000–5500 BP) (deMenocal et al., 2000). Understanding past ecological change in relation to drying and extreme climatic events is relevant to reduce the uncertainty of island species responses in a changing climate. It is also uncertain how human settlement affected the islands' ecosystems (Romeiras et al., 2014; Castilla-Beltrán et al., 2019). There is currently no evidence of human settlement in Cabo Verde before the arrival of the Portuguese in 1460 CE (490 BP). The first two islands to be settled by Europeans were Santiago and Fogo (Green, 2012). In the case of São Nicolau, historical documents (administrative censuses) suggest that a permanent community was already established by 1580 CE (370 BP) (Patterson, 1988). The presented study site is located in Monte Gordo Natural Park within the highlands of São Nicolau Island. Monte Gordo Natural Park contains the largest remaining fragments of the endemic Euphorbia tuckeyana scrubland to be found in the archipelago, as well as endemic species such as Dracaena draco subsp. caboverdeana and Sideroxylon marginatum. In terms of number of endemic species and its size, it is one of the most important of the nine Natural Parks in the archipelago. The Park management plan (MAA/DGA 2008) highlights that the main threats impacting the native vegetation in the park are: the introduction of exotic plants (e.g. Lantana camara, Pinus, Eucalyptus), agricultural practices and grazing, deforestation, and the fragmentation of natural vegetation (MAA/DGA 2008). Because of these impacts, it is believed that taxa such as the endemic Euphorbia tuckeyana have been experiencing continuous population decline since human settlement, and in the last decades have been increasingly replaced by invasive species, introduced conifers and Eucalyptus (MAA/DCA 2008). However, it is not clear how fast endemic plant populations have declined or which drivers of landscape degradation were at play.
This study will analyse human impact on a dry tropical island and the specific drivers of ecological change associated with anthropogenic activities. Specifically, one of the remaining key knowledge gaps related to the conservation of Cabo Verdean biodiversity and in particular of the Monte Gordo Natural Park is to determine pre-human baselines. Baselines help describing the past diversity of endemic and native plant species for the Natural Park and improve the understanding of plant responses to ecological disturbances in pre- and post-human settlement times. We use a palaeoecological approach to fill this gap and provide new information on past vegetation distribution, composition, and long-term biodiversity variability that will be relevant to inform conservation actions and restoration initiatives for Monte Gordo Natural Park and the native vegetation of the highlands of Cabo Verde (Fig. 1).
In this study, we set out to assess the long-term vegetation dynamics using an integrated multiple-approach analysis: fossil pollen, non-pollen palynomorphs (NPPs), charcoal particles, sediment grain size distributions and elemental composition, and leaf-wax n-alkane biomarkers, from a 190-cm stratigraphic profile excavated within a volcanic caldera from Monte Gordo Natural Park (1000 m asl). Our specific aims are to: 1) characterize biodiversity baselines in the highlands and to determine ecosystem responses to disturbances before the first human arrivals; 2) assess the impacts of land-use on São Nicolau's highland environments, identifying the main drivers of environmental change after human settlement; and 3) suggest potential directions for informed management of Monte Gordo Natural Park in particular, and 4) demonstrate how palaeoecological methods can help to manage dry islands and their natural heritage.
Section snippets
Environmental change in Cabo Verde
Cabo Verde is an archipelago formed by ten volcanic islands situated 600 km west of the African mainland (14–17°N, 22.5–25.5° W). It has a tropical dry climate (mean annual temperatures between 20 and 24 °C) marked by low and irregular levels of rainfall (1200–1600 mm in the highlands) which have a monsoon origin (August–October), and Saharan winds (Harmattan) that regularly deposit dust on the islands (Duarte et al., 2008). The archipelago supports a varied flora with high levels of endemism
Sediment composition and n-alkanes
The sediments of Calderinha are dominated by silts, with an average content of 63%, reaching a minimum of 45% in level 180 cm. The main elements/compounds identified in the sediment were silicon dioxide (SiO2, average 24%), silicon (Si, 11%), iron (Fe, 9%), and aluminium (Al, 4%). The concentration of n-alkanes varied throughout the profile, from a maximum of 1.72 μg in zone Cld-3 to a minimum of 0.13 μg/gds in Cld-4. n-Alkanes had chain lengths ranging from n-C23 to n-C31, and were always
Highland vegetation composition and the drivers of ecological change
Reconstructions of past vegetation are a first step towards setting conservation and restoration goals (Boessenkool et al., 2013; Barnosky et al., 2017), and in combination with other palaeoecological indicators can fill further knowledge gaps in the long-term ecology of dry tropical islands. For Cabo Verde, we identified two main knowledge gaps related to the management of island biodiversity that can benefit from palaeoenvironmental data: the long-term response of ecosystems to fire and
Conclusions
This study shows how palaeoecological methods, including multiple lines of evidence (e.g. changes in vegetation, fungal communities, fire and erosion), can reveal long-term information about landscape change in dry tropical islands that can be useful for conservation and restoration. The combination of classical proxies such as pollen, which can be subject to uneven preservation, with molecular and sedimentological information, has the potential to reveal important information about vegetation
Author statement
ACB, SN, LdN, ME, RW and JFP designed the study; ACB and ID collected the soil samples (June 2018). ACB analysed the sedimentary core under the supervision of: SN, TF, LdN, CM, MJE, AC. All authors made substantial contributions to the writing of the article.
CRediT authorship contribution statement
Alvaro Castilla-Beltrán:Conceptualization, Investigation, Formal analysis, Writing - original draft.Ivani Duarte:Investigation, Writing - original draft.Lea de Nascimento:Conceptualization, Writing - original draft.José María Fernández-Palacios:Conceptualization, Writing - original draft.Maria Romeiras:Writing - original draft.Robert J. Whittaker:Conceptualization, Writing - original draft.Margarita Jambrina-Enríquez:Writing - original draft, Investigation.Carolina Mallol:Writing - original
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
We would like to thank The Explorers Club for funding of fieldwork (TEC EXP 2018), the Quaternary Research Association (QRA-14Chrono 2018), and the NERC radiocarbon support, which funded six RC dates (2143.1018). This research was enabled by the Geography and the Environment +3 Postgraduate Research Scholarship from the University of Southampton awarded to Alvaro Castilla-Betrán (2017–2020, WRJB1B). We would like say thanks for the kind assistance in Monte Gordo provided by Francisco de Deus
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