Impact of human settlement on island ecosystems

Oceanic island ecosystems are unique and often contain species that are limited to specific islands or island groups. They are also vulnerable to disturbance.

To provide a timeline of how humans changed these territories over the centuries, a team led by the University of Southampton, studied multiple indicators of landscape change buried in sediments deposited over periods of up to ten thousand years. The team examined samples including fossilised pollen, spores of dung-decomposing fungi that indicate the presence of sizable herbivores, fragments of charcoal indicating use of fires as well as the composition of the sediment itself.

Their findings, published in the journal PNAS, showed that while forests on the islands changed naturally over thousands of years, human arrival on the Canary Islands, around 2000 years ago, and Cabo Verde, 500 years ago, led to an increase in fires and rates of soil erosion, the latter associated with the introduction of non-native livestock such as goats and pigs. One particular type of forest typical of Macaronesia, known as thermophilous forest, and characterized by iconic species such as the dragon tree, was most impacted. In Cabo Verde, data suggest that island vegetation is suffering a process of homogenisation due to human pressures and the uniqueness of individual island ecosystems is being lost.

The team also found that the first use of Canarian forests by aboriginal settlers appeared to have limited impact on the native vegetation, such as the laurel forest of the island of la Gomera. This is possibly due to smaller populations, typically interacting and trading with other nearby islands. By contrast, colonial era settlers who arrived in the 15th Century, adopted much more aggressive acts of deforestation, change in land use, and introduction of non-native species due to much wider trading networks which had a much greater affect.

Dr Sandra Nogué Bosch, Lecturer in Palaeoenvironmental Science at the University of Southampton said, "Contrasting the long-term history of different ecosystems, such as island forests, helps put in perspective the transformative force that humankind is unleashing all around the world."

Prof Mary Edwards, Professor of Physical Geography at the University of Southampton said, "we hope that local and international institutions tackling environmental challenges in the region can use new knowledge about past ecosystem composition and variability to restore natural parks and other parts of island landscapes."

Dr Alvaro Castilla-Beltrán, who completed his PhD at the University of Southampton explained, "This evidence about past environments provides valuable evidence on how forests responded to human actions and how best to go about restoring these landscapes, which have in some cases suffered severe transformations and species loss."

The team plans to keep using cutting-edge methodologies to be able to answer these questions, unleashing the potential of geochemical tools and searching for ancient DNA preserved in in sediments.

Having analysed the consequences of human impacts on the Canary Islands and Cabo Verde, the team plan to continue their studies on other archipelagos to delve further into how island habitats have evolved.

"Other important research questions remain open, for example, what was the role of a changing climate in these processes in the past, and how will global warming affect future ecosystems? What was the local impact of extreme natural phenomena such as volcanic eruptions -- like the one currently active in La Palma -- and how did island life change the cultures of those people that settled them? We will also keep up the work on other archipelagos to provide a new perspective of the human footprint in these fascinating territories," Dr Nogué Bosch concluded.