Island Conservation recently welcomed Ted Haffner onto our Advisory Council – a wonderfully engaged, thoughtful and experienced thinker who brings a deep understanding of conservation to further guide our team. Ted’s vast experience connecting the natural world to the built environment through strategic thinking is ideal for an organization that strives to maximize global impact.

A crucial aspect of Island Conservation’s approach to ecosystem restoration is robust, science-based interventions. Ted has very kindly provided his expertise to summarize and explain the value of three significant journal articles and how their findings shape key decisions regarding invasive species and island restorations. We are so grateful!

Recently, Island Conservation compiled and released three different scientific journal articles containing research findings of paramount importance to the organization and its mission of protecting biodiversity and eradicating invasive species on islands. Though the papers revolve around the importance of islands and their role in species population dynamics, native and global biodiversity, and the impacts of invasive species on islands and their native species, they are vastly different in their research findings. This summary will compare and contrast this important scientific literature in non-scientific terms.

To unpack these related but different investigations, let’s start with three of the easiest differences: date of publication, terms and topic. Once we explore these differences and similarities, we can then turn to the important information and key takeaways from each.

The date of publication matters since information in the world of scientific literature generally builds upon itself to prove, or disprove, an idea. These studies were all published in different scientific journals between 2016 and 2022. Interestingly, the 2016 study has no shared authors with the other three, while the remaining three share many common authors. In this respect, information presented in the 2019 and 2022 studies will have similar sounding but likely different findings and quantifications of the data than the 2016 and 2017 publications. But a close look of each of the studies indicates that the increase in knowledge associated with the passage of time between publication is not a sufficient explanation into the differences between the publications.

Next, let’s explore the terminology of the publications. Close examination shows a rigorously correct use of terminology. This makes sense, as these articles are carefully written, edited and reviewed by other scientists with knowledge relevant to the article under review. What is confusing is the fact that the terms are used so frequently within each article. We see this repetition within the titles of each publication alone: invasive, vertebrates, islands, globally, eradication. Furthering this confusion is the fact that these terms are often used in conjunction with other terms which may or may not be similar. Though this leads to confusion when one is not familiar with scientific writing, it is not alone responsible for the differences between each journal article.

What really does differentiate the articles is that each has a different topic. As we have seen, this may be difficult to understand from a close reading of each of the titles and associated abstracts. The following pages will summarize each study and discuss the importance of the study findings to Island Conservation, presenting the studies in chronological order by date released.

Invasive Predators and Global Biodiversity Loss

This is the best place to begin, since this was the seminal study upon which all the other studies were undertaken. We can deduce this for two reasons: First, the authors of this original study do not appear in any of the bylines of the subsequent studies, while many of the authors of the studies performed between 2017 and 2022 contain many of the same authors. More importantly, however, we know this because of the citation list: reference note 17 of the 2017 publication entitled “Globally Threatened Vertebrates on Islands with Invasive Species” cites this original 2016 study, along with a host of other citations.

With this as the original piece of research, let’s dive into its central claim that invasive mammalian species negatively impact local populations of native wildlife (via outright extinction, or via threatening the indigenous populations of a species whereby its numbers dwindle below that population’s ability to reproduce itself in a genetically healthy way.) In the Results and Discussion section, we see this claim can be distilled to 738 native species of birds, mammals and reptiles that have been negatively impacted by just 30 species of invasives mammals, including cats, rats, mongooses, foxes and others.

Furthermore, these impacts result in 58 percent of all global extinctions, with 44 percent of impacts attributed to cats and rats. Mind you, this figure for cats and rats is a percentage of the whole and should not be read in proportion as causing half of the overall 58 percent of all global extinctions. Instead, when taken proportionally, we see that cats and rats combine for approximately 75 percent of the overall total of extinctions. While impacts from these non-indigenous species has a very lopsided result, it is important to note that these impacts on indigenous species is not always through direct predation but can instead be through the spread of virulent disease, the destruction of necessary living conditions and habitat, or by impacting food sources.

But that is not all this study finds. Importantly, this study clarifies that these results were found in “insular” species, which refers to environments that are cut off from other systems – otherwise known as islands. The remainder of the paper is dedicated to examining impacts in terms of global clusters of island geography, presenting both relevant and relative impacts to the various groupings of islands including, but not limited, to the Caribbean, Hawaii, the Pacific Islands (they label them Micro-/Mela-/Polynesia) Australia, Madagascar, and so on. While the data, species interactions and results varies by location, the results for each region are presented and the study indicates how the data amalgamates into the larger global figures presented above. Through close examination of this paper, we can see, and therefore rationalize, that the data for each cluster provided both barely sufficient and yet thoroughly incomplete results that both called for and served as a strong call for further study.

Globally Threatened Vertebrates on Islands with Invasive Species

This paper, released a year after the one above, offers updated facts and figures related to island biodiversity, predation impacts from invasive species, and rationales for these impacts. The main focus of the paper is a presentation of a “threatened island biodiversity database” as well as a call to action for further study. According to the article, the database documents which of the world’s islands support breeding populations of terrestrial vertebrates – such as amphibians, reptiles, birds and mammals – which are classified as “critically endangered” or “endangered” by the International Union for the Conservation of Nature (IUCN). Furthermore, the database also presents information regarding which islands with highly threatened vertebrates are colonized by invasive vertebrates such as rats, cats, pigs and mice.

While this database records this information for the first time, the paper also reveals other findings. Some of these findings are intuitive, such as there being a direct relationship between the size of the island and its likeliness to be inhabited by people, struggling native species and at least one type of colonizing invasive species. Smaller islands, in contrast, are less complex and offer a sweet spot for eradications. The paper also connects this work to important biodiversity goals for the first time, proving this work is not only worthy of financing, but also worthy of the risk to undertake eradication projects on islands with more complex conditions and multiple variables. These targets include the “Aichi Biodiversity Targets 9 and 12 of the UN Strategic Plan for Biodiversity 2020 and the UN Sustainable Development Goals 15.5 and 15.8 call to reduce the rate of extinction, particularly by reducing the impact of invasive species.”

Critically, the authors also spend a great deal of time investigating the following questions and answers:

 (i) How many highly threatened island vertebrate species and populations occur on islands? The authors found 1189 species that met this criteria and are presented on page two of the study.

(ii) What are the biogeographic and socioeconomic patterns in the distribution of these highly threatened vertebrates? Half of the 1288 islands investigated were uninhabited, while 130 had less than 100 people and 220 islands had a population greater than 10,000. Furthermore, invasives were found on 1030 islands with confirmed threatened species.

(iii) Where do invasive vertebrates co-occur with highly threatened vertebrates on islands? They occur on 1288 islands with varying degrees of sizes, socio-political conditions, and with varying degrees of complexity between native/invasive interactions. 

(iv) Which vertebrate groups co-occur most frequently with invasive vertebrates? Invasive vertebrates occurred on all of the breeding islands for 87 percent of highly threatened vertebrates by at least one invasive species, and impacts 97 percent of amphibian, 83 percent of reptile, 80 percent of bird, and 89 percent of mammal species on the most threatened islands.

While this paper does not offer all the answers, it nevertheless creates the database and serves as an important enabling condition for the next steps in island biodiversity research and conservation. As we will see, this paper also has an outsized impact to the remaining study and literature that has been undertaken since.

Globally Important Islands Where Eradicating Invasive Mammals Will Benefit Highly Threatened Vertebrates

This 2019 paper, published in Plos One, reveals several new findings and strategies to combat native species extinction from invasive species impacts on islands. The authors examined literature, data and existing conditions from almost half a million islands (approximately half of the islands in the world) to help understand, quantify, and prioritize world-wide eradication efforts of colonizing invasive species to minimize global extinctions of threatened and important wildlife species and populations.

This publication is packed with a host of wide-ranging results, but the most eye-popping is the indication that removing colonies of non-native species from just 169 islands can prevent 10 percent of global extinctions. Other intriguing results indicate that not only 75 percent of all bird, mammal, reptilian and amphibian extinctions since 1500 have taken place on islands, but are a direct result of island colonization by invasive species brought to islands by man. Nine species of non-native animals in particular have a devastating effect on local birds, lizards, snakes, turtles and endemic critter populations. These species primarily include cats, rats, dogs, goats and pigs. Furthermore, islands are currently home to over a third of the species found on global endangered and threatened species lists.

This study built on prior work and serves as an important publication that greatly enhances not just the field of knowledge in general, but also describing critical solutions and pathways to achieve the highest beneficial impacts at the lowest cost. Using prior academic studies, this effort examined a number of process applications that systematically examine, prioritize, and elevate the number of islands in order to ascertain/ascribe/derive a selection process for the most crucial work. The process examined and identified current and potential conservation risks to determine an overall eradication benefit. This basic investigation found that of almost 465,000 islands in the world, only 1,279 contained a correlation between endemic endangered species based on factors such as irreplaceability and severity of negative impact.

Of these 1,279 global islands, the investigation team found that qualifying criteria further identified 574 that had co-occurrences of both endangered native species and colonist invasive species. Through a series of further investigations the number of islands was further reduced through a process of interviews to determine socio-political viability for invasive species removal and eradication. These interview results further reduced the number of islands to a threshold of 169 originally described, with 107 of those islands able to undertake eradication work by 2020 and the remainder by 2030. Importantly, the study also highlights a number of other important considerations. First, while eradications are considered necessary to restore some of these island habitats, other work such as the restoration and assisted colonization of formerly native species may be required. While costs were discussed, they were not provided since costs are a factor of island size, human population numbers, and geographic factors such as remoteness of location and topographical issues and considerations. Finally, the study indicated that even for low-lying islands impacted by sea-level rise, eradication work can benefit resilience and help even low-lying islands buy necessary time.

This information, as well as studies undertaken since, allows us the opportunity to both understand impacts and identify specific islands to best direct precious resources to preserve our planet’s biodiversity. Critically, this multi-disciplinary effort includes team members from Island Conservation and supports the Island Ocean Connection Challenge (IOCC) by providing a direct link between knowledge and practice. This critical link provides Island Conservation with processes, information, and a network that did not exist before. Simultaneously, this paper outlines, perhaps for the first time, a clear path using a strategic framework and next steps for biodiversity conservation that meet globally recognized goals and targets such as Aichi Targets 9 and 12 under the Convention of Biological Diversity (CBD) and the United Nations Sustainable Development Goals. If this work is undertaken and financed, studies like this, and organizations like Island Conservation, have found what appears to be the most realistic, low-cost approach to preventing 10 percent of the world’s biodiversity loss to extinction.