Tracing human impacts on the islands of Cabo Verde: Palaeoecology for the conservation of island ecosystems in southern Macaronesia

Abstract

Oceanic island ecosystems are exceptional due to their isolation; island species typically exhibit high levels of endemism and small population sizes, which make them especially vulnerable to anthropogenic pressures. Loss of biodiversity has been especially severe on islands during the last several centuries, often associated with human-driven biological invasions and direct impact via land use. In this context, the conservation and restoration of island ecosystems is urgent, yet historical information on long-term environmental trends remain scarce. Palaeoecological evidence can provide insights on how island ecosystems responded to diverse natural and human- driven disturbances through centuries and millennia. In this PhD thesis, long-term environmental changes in the highlands of three islands of Cabo Verde islands (Santo Antão, São Nicolau, Brava) are investigated to determine how vegetation (pollen, phytoliths, leaf wax n-alkanes) and ferns and fungal communities (non-pollen palynomorphs) varied over time in response to the occurrence of fires (charcoal concentration), depositions of volcanic ash (tephra) and erosion pulses (grain sizes and elemental composition).

In Santo Antão and São Nicolau, pre-human ecosystems consisted of grasslands and woody scrublands and showed limited variation in composition linked to naturally occurring disturbances such as wildfires and erosion pulses between 2,000 and 500 cal yr BP. São Nicolau’s highlands held abundant woody vegetation incuding Euphorbia tuckeyana, Ficus and Dracaena draco subsp. cavoberdeana. In Brava, the regional drying trend after 4,000 cal yr BP was associated with increased local erosion but had limited impacts on highland grassland vegetation. In contrast, the expansion of fern-rich woody scrubland was likely triggered by volcanic ash deposition after 1,800 and 650 cal yr BP. Human impacts after Portuguese settlement (1460 CE) generated multiple disturbances in all studied islands, including increases in fires, deforestation and the introduction of exogenous species. This led to chronic and persistent disturbances, such as soil degradation and erosion, and the decrease of native vegetation cover in favour of introduced species. The resulting island histories of environmental change described in this thesis are used as guidelines to set historically contextualised management goals. These new palaeoenvironmental discoveries can contribute to the restoration of scrublands and woodlands, soil management, fire prevention and the monitoring and eradication of introduced species.

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Identifiers

ORCID for Sandra Nogué: orcid.org/0000-0003-0093-4252

ORCID for Mary Edwards: orcid.org/0000-0002-3490-6682

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