By Zach Carter
Isolation and area are important quantities in island biogeography because they help estimate species richness on isolated natural communities. Geographically small and isolated communities are often hotspots of considerable endemism because immigration occurs at a low frequency, thereby allowing time for cladogenesis to occur. I have recently been interested in quantifying isolation because its application could extend beyond that of determining how species came in to being. Just as immigration is thought to be less extensive on islands that are isolated, so too should be the immigration of terrestrial mammalian pests. I have theorised that highly isolated islands hosting invasive pests are, therefore, good candidates for cost-effective eradication programmes.
While area is a concrete measure of geographic size, isolation has taken on a contrived definition since its scientific inception. In order to define this term, I compiled many of the most popular metrics describing isolation and reduced them using Principal Components Analysis (PCA). In doing so I could parsimoniously extract the most important information and graph isolation visually to better understand its inter-workings. The metrics I am using include: the shortest distance to the New Zealand mainland, the longest over-water distance from the mainland, the proportion of land surrounding each island within a target organisms maximum swimming distance (1km, 3km, and 5km, known as landscape-scale isolation), the summed distance to the nearest four life supporting islands (any landmass greater than 5ha in size), The path from the mainland that requires the least amount of energy expenditure (known as a least-cost path), and the amount of land covered with the least-cost path. An example of one metric, the least cost path, is provided below with a Grey Group Island off the coast of Great Barrier Island.
I conducted this analysis on (almost) every offshore New Zealand island at least one hectare in size. While very early in the analysis, the PCA determined that three components are necessary to describe isolation. A preliminary output featuring two principal components is provided below.
As can be seen via the red circle, islands that have already been eradicated of invasive mammal pests tend to cluster largely in the 3rd quantile. If the 3rd principal component is plotted, the clustering in a 3-dimensional space is even more apparent (though difficult to see with a photo). This indicates to me that many of the islands already free of invasive mammals exhibit many similarities from an isolation standpoint. With this mentality, islands within the red circle that have not been eradicated may be good option in the near future. This research is preliminary, though, and will require more analysis!