Oh deer! Population genomics reveals high connectivity of invasive deer among islands

Invasive species are major threats to global biodiversity, especially on islands. Using Sitka black-tailed deer in Haida Gwaii, Canada, we demonstrate how population genomic data can inform management decisions that target ecological restoration.

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Ecologists and evolutionary biologists have long been fascinated by life on islands. While Darwin’s voyage to the Galapagos and beyond remains iconic for helping inspire the theory of evolution by natural selection, the work within Origin of Species also represents the important role that islands have had in shaping the distribution of life on our planet. Islands are often conveyed as wondrous places filled with unique biodiversity and culture; certainly, islands have much to offer in these regards, but unfortunately this means they also have much to lose. 

Although islands are home to disproportionally high levels of species richness, they also face elevated levels of species extinctions, largely driven by the introductions of non-native species. One such example is Sitka black-tailed deer (Odocoileus hemionus sitkensis) in Haida Gwaii, an archipelago off the western coast of Canada. Haida Gwaii has often been referred to as the ‘Canadian Galapagos’, due to its outstanding biodiversity and Indigenous culture, as the Haida Nation have long been stewards of the islands. Since the introduction of deer during western colonization in the late 1800s, the natural balance of life on Haida Gwaii has been disrupted, jeopardizing many of these relationships, as several of the deer’s favorite foods are also culturally important species (Fig. 1-3).

Fig. 1. Sitka black-tailed deer, the smallest sub-species of mule deer, but a big problem as an invasive species. Photo credit: Parks Canada.
Fig. 2. A Sitka black-tailed deer reaching up to feed on Sitka spruce on Ramsay Island. Photo credit: Andy Wright/Parks Canada.
Fig. 3. An exclosure constructed on Kunga Island, where Sitka black-tailed deer were prevented from entering the area on the left. The impacts of deer on the forest understorey can be seen on the right. Photo credit: Andy Wright/Parks Canada.

Islands are often characterized by their inherent isolation, which may lead you to believe that deer would face a tough time dispersing from island to island. In Haida Gwaii, however, it seems deer have aptly demonstrated the proverb “where there’s a will, there’s a way”, because they currently reside on dozens of islands. Management initiatives aiming to remove deer from islands of high ecological and cultural value have therefore been challenged by the deer’s resilience, and many questions have been raised from incomplete eradications. Most importantly, are there islands that could remain deer-free after successful eradications? 

We sought to answer this question, among others, in our recently published study using population genomic data from Sitka black-tailed deer harvested across Haida Gwaii, with an emphasis on deer connectivity within Gwaii Haanas National Park Reserve, National Marine Conservation Area, and Haida Heritage Site. Our findings confirmed previous anecdotal observations of deer movement among islands. In Juan Perez Sound especially, we identified several pairs of first and second order relatives harvested from different islands using a pairwise kinship analysis. In one example, a deer from Faraday was found to be the brother, father, or son of a deer from the Bischofs. Collectively, these inter-island relationships suggest plenty of deer movement among islands in Juan Perez Sound (Fig. 4).

Fig. 4. Inter-island close relatives identified between Sitka black-tailed deer. First order relative pairs are shown in orange (n = 9), while second order relative pairs (n = 16) are shown in blue. The number of pairs is designated above each line. Created using BioRender.
Throughout the archipelago, we found little evidence of genetic structure; even deer harvested from islands ~50 km apart were genetically similar. Of the 15 islands from which we genotyped deer, only one represented a significant exception. Deer harvested from SGang Gwaay consistently demonstrated a unique pattern of genetic isolation relative to the other islands across a suite of population genomic analyses that included a principal component analysis and Bayesian clustering (Fig. 5).
Fig. 5. Analyses investigating the population genomic structure of Sitka black-tailed deer in Haida Gwaii, where deer from SGang Gwaay can be observed as a unique genetic cluster.
So, what does this mean for the management of deer in Haida Gwaii moving forward? Well, the observed connectivity across the islands in this study indicates an undesirable reality for management; even if deer are completely removed from an island, it is unlikely to remain deer-free without additional measures put into place such as fencing and ongoing monitoring. The island of SGang Gwaay (Fig. 6), however, remains a possible eradication candidate but is an especially interesting (and challenging) case because deer here have been heavily culled in the past, which has almost certainly contributed to the observed patterns of genetic isolation. Even so, we did not detect contemporary migration to or from SGang Gwaay between the surrounding islands; thus, we cannot definitively say whether the island would remain deer-free following an eradication.
Fig. 6. Haida mortuary poles in SGang Gwaay Llnagaay (village), a UNESCO World Heritage Site. Photo credit: Parks Canada.

At a larger scale, our study further demonstrates the utility of population genomics for informing invasive species management on islands. By estimating population connectivity as part of management operations, managers are better equipped to efficiently allocate resources to meet conservation and restoration goals. With the status of many unique island species becoming increasingly imperilled, their persistence may very well depend upon our ability to make evidence-based decisions towards the removal of island invaders, protecting our most rich and wonderful systems for future generations. 

If you would like to learn more, please feel free to visit our open access paper in Communications Biology: https://doi.org/10.1038/s42003-022-03159-5

Brock T. Burgess

PhD Candidate, The University of British Columbia