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Eradication methods on invasive species on islands; is there an effect on seabird populations?
Environment DepartmentUniversity of York, University Road, York YO10 5DD
AcknowledgmentsI would like to take this time to thank all of my lecturers in the Environmental Department for being supportive when Ive needed. Especially towards the end of the year. I would also like to thank my Uncle Boris, because without his trust in me, I would not have been able to write this. And as always, my parents, for still putting me and my brother before anything else in this World.
Abstract134 bird species have gone extinct in the last 500 years. 71 have been due to invasive species.11.8% are marine birds. Due to marine birds breeding on Islands and being ground nesters, they are more vulnerable. 80% of the worlds invasive species are found on islands. Islands are open niches for invasive species as there are no natural predators. 20 years ago eradication methods on these invasive species were developed. This study uses a systematic review and Meta analyse approach to investigate what these eradication methods have on seabird populations. Coefficient correlation (r) being the standardized measure.
Introduction Birds are the most diverse of the vertebrate family, having twice as many taxa as mammals and ten times more species that fly (Sekercioglu 2006). Over the last 500 years, 134 species of birds have gone extinct (Bird Life International 1). It is believed that the recent decline in bird populations is from invasions of non-native species, at least 71 of these extinctions is due to invasive species (Bird Life International 1). An invasive species is a non-native animal or plant that has a negative effect on the native species. They occur in ecosystems that are similar to their natural environment (Howald et al 2007). IUCN red list has agreed with this and it explains why 5.7% of terrestrial species, 2% of freshwater species (mostly birds) and 11.8% of marine species (again mostly birds) have become extinct recently. Out of the 21 recently extinct marine species, 11 of them have been marine bird species. (Gurevitch and Padilla 2004). Most of the marine birds that have become extinct, have been native island species. Island birds are more likely to be effected by alien invasive species because majority of the time island birds have no predators so have not had to evolve into fleeing when danger is near (Bird Life International 1 and 2).Of all the invasive species that have become a problem over recent years, rodents have become the most dangerous (Howald et al 2007). Unlike most other invasive mammals, rodents are omnivores so can affect all species on the island, plant or animal. 80% of the worlds major islands have invasive rodents and more are continuously invading the Worlds islands. Island ecosystems may be more prone to invasion because their species faced few strong competitors and predators, or because their distance from colonizing species populations makes them more likely to have "open" niches for invasion. Over 20 years ago systematic techniques were developed into eradicating the invasive rodents due to the negative impacts that were occurring (Howald et al 2007). Between n 1983 and 1984 the WWF poisoned and trapped Rattus rattus (Appendix; Table 2) on Cerro Pajas, Galapogas. This is because the once abundant, now endangered, dark Pterdroma phaeopugia (Appendix; Table 1) has been affected by the invasive R.rattus. The rat feeds on the eggs and chicks. Before the eradication methods began, only 31% of the eggs that were laid fledged. In 1983, 46% fledged (104 nests were checked) and in 1984, 72% fledged (100 nests were checked) (Cruz and Cruz 1996). To prevent extinction, eradications have become a very important tool (Howald et al 2007). Howe Scientists believe that the best way to prevent an invasion (defined as when none native species takes over an island) is to catch the alien species when it first arrives at the island and has not affected anything on the island yet. This can be achieved when an alien species has arrived on island but has not bred or taken over is known as incursion. This is the critical time for the eradication of the alien species (Russel et al 2006). A good example of an island that has managed to eradicate an invasive alien species problem is the Clipperton Islands, which is located 1000km South West of Manzanillo, Mexico. Feral pigs were introduced by settlers at the turn of the Centenary. Before this, the island was a sparsely vegetated atoll and was home to a high density of plant eating land crabs and 10,000s nesting marine birds. The pigs would feed on the crabs and the eggs of the nesting marine birds. Causing a massive decrease in marine birds. It was reported that only 150 Scula dactylatra and 500 Scula leycogasta (Appendix; Table 1) were now on the island. In 1958, all of the pigs were shot. By 1968, the breeding pairs of seabird population had increased to 25,000 (Birdlife International 3)Using a systematic review approach and a Meta analysis on independent and primary research studies on marine bird species after eradication of invasive species this study will discuss the impacts invasive mammals are having on island marine birds and how eradication techniques can help prevent declines in species population or eventual extinction in species. Systematic reviews and Meta analysis have been used in health care for a number of years now, however in recent years conservationists and ecologists have been using the methods. A systematic review is considered the best way to synthesis the findings of several studies that have investigated the same question. They are defined as an overview of scientific strategies that limit bias to the regular association, critical assessment and synthesis of all relevant studies on a specific topic. They are designed to locate, appraise and synthesize the evidence relating to the same scientific question to provide informative and evidence-based answers (Cook et al 1995; Dickson et al 2014). This type of review requires the following to be successful; definition of the question/problem, identification and critical assessment of the evidence thats available, synthesis of the findings and finally the drawing of a relevant conclusion (Dickson et al 2014). A Meta analysis is a quantitative overview of the systematic review. Statistical methods from primary and independent research are used to combine and summarize the results of several relevant studies all asking the same question. (Cook et al 1995). The aim of this study is to see if the eradication of predatory invasive species on islands has an effect on the native marine bird populations. This will be achieved by using a systematic review and a Meta analysis on previous primary and independent studies.
MethodsThere are nine steps to follow whilst doing a systematic review (Table 1). Fig 1 shows these steps as a flow chart, and how many studies were found/removed within reason during this study. Step 1 is to perform a scoping search to help determine the aim of the project. Once the aim of the project had been determined the inclusion criteria (Table 3) and search terms (Table 2) were developed to help with the following steps. Step 2 was achieved by using Web of Science (WoS) and Scopus were the search engines used to find the initial studies of this review (Over 22,000 studies). All of the studies were imported into an endnote software where the duplicates were removed.
Table 1: - Table showing what the 9 steps are when doing a systematic review (Edited from Dickson et al 2014)Step 1Performing scoping searches identifying the aim
Step 2Literature Searching and removing duplicates
Step 3Screening titles and abstracts
Step 4Obtaining studies
Step 5Selecting full text studies
Step 6Quality assessment
Step 7Data extraction
Step 8Analysis and synthesis
Step 9Writing up and editing
Once the duplicates had been removed, the inclusion criteria (established from the aim of the study; Table 3) was used to screen the abstract and titles of each of the remaining studies (over 10,000 studies), the studies that were not relevant to the aim were then removed. Successful eradication in this study was defined as the main invasive species of a study being removed from the island completely and no trace of the invasive species had be seen in over a year. 41 studies (Appendix; Table 3) remained after steps 1, 2 and 4 (Table 1) had been completed (Fig 1). The studies were then obtained for full text screening (Table 1) (Appenndix; Table 3). Full text screening of the text is the step in which appropriate data needs to be scoped for the Meta analyse software. The software used in this study was Meta Win 2.0 (Rosenberg et al 2000). 9 studies (Fig 1) had enough data that could be extracted for a quantitative analyse. However, some of these studies had different statistical methods or data was written in different formats. For a summary analyse to be performed, the data needs to be in the same statistical format. This study required the data to be measured in effect size. An effect size is an objective and standardized measure of the magnitude of observed effect between two or more variables. The observed effect for this study is the population changes for marine birds after successful eradication (Ferguson 2009). Effect size is ideal to use in a Meta analyse because different variables can be compared and different statistical data can be extracted and converted depending on the availability of data within a study. For this study coefficient correlation (r) was the measurement used for effect size as a correlation between seabird population changes and successful eradications needs to be established to reach the aim. Using the effect size calculator provided within the meta-win software and inpu