Investigating the spread of an invasive tree in the Turks and Caicos Islands
About the project
In May and June 2011, I was lucky enough to spend eight weeks in the Turks and Caicos Islands, one of the UK Overseas Territories (UKOTs) in the Caribbean. I worked with the Kew UKOTs team and the TCI Department of Environment and Coastal Resources (DECR) carrying out fieldwork for my Masters thesis in Conservation Science. I was studying the invasive tree Casuarina equisetifolia, known in the Caribbean as Australian pine. C. equisetifolia is commonly known as she-oak in its original range in South-East Asia and Australia. In 2009, a previous MSc student, Chloe Hardman, predicted suitable habitat for this species in TCI using the Maxent software which models species distribution based on environmental variables. She found that native species richness was reduced in plots invaded by this tree. Building on this research, I set out to map the current distribution of the species and prioritise areas for its control.
Casuarina equisetifolia on Bambarra beach, Middle Caicos, stretching as far as the eye can see (Image: Alexandra Davey)
Determining the spread of Casuarina during the last ten years
Using aerial photographs of the islands from 2001 and 2007, in which patches of the invasive tree were clearly visible, I manually digitised the distribution of C. equisetifolia in each of these years using ArcMap GIS (Geographic Information Science) software, developed by ESRI. This allowed me to examine not only the current locations of C. equisetifolia but how its distribution has changed over a six year period. This invasive tree grows mostly along the northern and eastern coasts, which have the longest expanses of sandy beach, and along roadsides and in settlements. In the six years covered by the photographs the area of land occupied by the species had increased on both North and Middle Caicos.
Using the digitised distributions, I generated presence points in ArcMap. These points were then overlaid on environmental variables, such as distance to roads and elevation, in Maxent to produce a predictive model of suitable habitat for the species. Using digitised presence points avoided the bias often introduced by the difficulty of accessing some areas on the ground. The new model showed a greater area to be suitable for C.equisetifolia than the previous one had. The most important environmental predictor of suitable habitat was distance to the northern coast. On North and Middle Caicos areas close to the north coast are the most disturbed by human activity.
Fieldwork in Important Plant Areas
TCI endemic orchid Encyclia caicensis growing on Wild Cow Run, an Important Plant Area on Middle Caicos threatened by Casuarina equisetifolia (Image: Sara Green)
I selected two Important Plant Areas (as determined by another MSc student, Sophie Williams, in 2009) for further study and mapped the presence of C. equisetifolia in 2011. I then compared the trees’ distribution with that in 2007 and 2001. This showed the importance of disturbance caused by human activity in determining the spread of the species - land that had been cleared for development was very quickly colonised. On the island of North Caicos the majority of trees established themselves in the last 10 years. This area may be a good site to trial methods of controlling the tree.
These maps show the spread of the species on Horsestable Beach, Important Plant Area on North Caicos. Red squares show location of Casuarina equisetifolia in 2001, 2007 and 2011. (Image: Alexandra Davey)
Investigating control methods
In the final part of my study I looked at how the level of C. equisetifolia establishment affects native species. I found that native species richness decreased significantly with increasing height of the invasive Casuarina trees. I also performed a small baseline study into the current use of C. equisetifolia in making charcoal. This research contributes to the larger question of whether charcoal production could provide a future control strategy for this invasive species. One possible scheme would entail the funds raised from sale of licences to coppice C. equisetifolia in very heavily infested areas being used to pay for eradication schemes in newly invaded areas. Everyone involved in the study confirmed that Casuarina wood makes very good charcoal.
The principal recommendations arising from my work were to restrict development in the southern parts of North and Middle Caicos which as yet remain undisturbed and more or less free from Casuarina. Tight control of unnecessary or premature land clearance prior to development would also help to restrict the spread of the species.
The six months I spent on this project have been an excellent learning experience for me. I am very grateful to all those at Kew who have helped me and feel privileged to have first-hand experience of the work the UKOTs team carries out in the field. I could not have done the project without the support of the TCI DECR and in particular without the help of Bryan Naqqi Manco, Judnel Blaise and Sara Green in the field. Thank you!
- Alexandra -
Science Directory project page: Invasive Species in the UK Overseas Territories
Science Directory project page: Turks and Caicos Islands Pine Recovery Programme
Find out more about Geographic Information Science (GIS): Kew's GIS Unit
Imperial College MSc Conservation Science theses
Hardman, C. (2009) Invasive plants in the Turks and Caicos Islands (pdf)