Analyzing population trends of common Irish butterflies
An overview of the population trends of common butterflies (Lepidoptera) from Ireland between 2008 - 2012
Research Report
Author Wouter Staats Study Thirth year Applied Biology student from HAS Den Bosch University of Applied Sciences Company Supervisor Eugenie Regan Teaching Supervisor Sander van Huijzen Period of research February 18th 2013 – July 12th 2013 Staats, W. T., E. Regan & S. van Huijzen (2013). Analyzing population trends of common Irish Citation Butterflies. National Biodiversity Data Centre & HAS University of Applied Sciences: Waterford. Acknowledgements
My gratitude goes out to all the volunteers who have invested a lot of their time recording butterflies and who have contributed to the Irish Butterfly Monitoring Scheme. Without their overwhelming enthusiasm, this report couldn’t have been made and drastic changes in butterfly population sizes would have gone unnoticed. Secondly, I would like to give special thanks to Eugenie Regan for her supervision during the making of this report and for managing all the data of the Irish Butterfly Monitoring Scheme. I would also like to thank Sander van Huijzen for supervising and reviewing this report. Further thanks go out to Chris van Swaay, Arco van Strien and Wim Plantenga for explaining TRIM and giving advice on the trend results. Furthermore, I would like to thank Osama Almalik for further assistance with the statistics not related to TRIM. In addition, I would like to thank Catherine Bertrand for providing us with data from Northern Ireland between 2008 and 2012 and Marc Botham for providing us with data from Britain between 1976 and 2012. And last but not least, I would like to give my sincere appreciation towards the National Biodiversity Data Centre for their warm welcome and support during my five month internship.
Wouter Staats Waterford 2013
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Summary
In this study, research was conducted on the population trends of common Irish butterflies between 2008 and 2012. Population trends are vital to investigate changes in butterfly abundances and to assess the effects of climate change and conservation measures. These assessments are necessary, because recent studies have shown large declines in butterfly abundances on regional, national and European scales. The main objectives were to investigate changes that occurred in the abundances of the 15 most common Irish butterfly species and to investigate what factors influenced their changes. Data was used for five years (2008 – 2012) from the Irish Butterfly Monitoring Scheme, gathered by approximately 150 volunteers on a weekly basis between April and September of each year. TRIM, a statistic software package based on a log linear regression model, analysed the time series of counts for the Irish butterflies. Population trends and status (decline, increase, stable, uncertain) were produced using TRIM. Results were compared to the mean temperature and rainfall in Ireland. Other factors were also investigated including parasitism, food plant availability, habitat loss, et cetera.
This study found that the majority of the common butterflies in Ireland are in a decline within the five year time period. The total butterfly abundance peaked in 2010 and was at its lowest in 2012. Butterfly abundance followed a similar pattern as the mean temperature, although not significantly correlated. Therefore, temperature appears to be the over-riding factor influencing the butterfly abundance. Rainfall also influenced the abundance of some species. Parasitism may be affecting the abundance of butterfly species such as Holly Blue Holly Blue (Celastrina argiolus) and Large White (Pieris brassicae). No assumptions about the effects of climate change or habitat loss can be made for the five-year time period. Furthermore, the abundance of double-brooded butterfly species was more correlated with temperature than the abundance of single-brooded species. Double-brooded species are more depended on the temperature, because they get the opportunity to lay a second brood when temperatures are high early in the year, but also decline much faster than single- brooded species when temperatures drop significantly.
Five years of data gives an early insight into population changes, however ten years is recommended for more reliable trends. The indices (and therefore the trend) with a five-year dataset are generally reliable; however the standard errors should be taken into account. The five-year trends resulted in a high number of uncertain species, but addition of more years should result in less uncertain species. The butterfly species Holly Blue (C. argiolus), Orange Tip (Anthocharis cardamines) and Peacock (Inachis io) showed similar fluctuations with butterfly trends of Britain, which shares a similar geographical landscape and uses a long-term data set beginning 1976.
Most butterfly species are suitable for contribution of quality data for the butterfly climate change indicator, as used and assessed by the EU to hold biodiversity loss in 2020. However, the abundances of the butterfly species Holly Blue (C. argiolus) and Large White (P. brassicae) are mainly affected by parasitism and should be interpreted with caution when using their data in bioindicators that relate abundances with climate change. Both species could potentially underestimate the effect climate change has on butterfly abundance and should therefore be excluded from contribution from a climate change indicator.
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Contents
Acknowledgements ...... 2 Summary ...... 3 1. Introduction ...... 5 1.1 History of butterfly monitoring ...... 5 1.2 European Biodiversity Indicator ...... 6 1.3 Irish Butterfly Monitoring Scheme ...... 8 1.4 Study outline ...... 10 2. Methods ...... 11 2.1 Monitoring Scheme Methodology ...... 11 2.2 TRIM ...... 13 3. Results ...... 18 3.1 Butterfly trends ...... 18 3.2 Overall trend and temperature ...... 23 3.3 Voltinism ...... 25 3.4 Food plants …………………………………………………………………………………………………………………………….26 4. Discussion ...... 27 4.1 Factors influencing trends ...... 27 4.2 Reliability trends …………………………………………………………………………………………………………………….32 4.3 Bioindicators ……………………………………………………………………………………………… ...... 33 5. Recommendations ...... 35 6. References ...... 37 7.1 Appendix I ...... 43 7.2 Appendix II ...... 44 7.3 Appendix III ...... 45 7.4 Appendix IV ...... 49 7.5 Appendix V ...... 57 7.6 Appendix VI ...... 58 7.7 Appendix VII ...... 60
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1. Introduction
1.1 History of butterfly monitoring For centuries people have been studying, capturing and collecting butterflies. Over the years records and detailed maps of their whereabouts in Ireland were made to obtain more insight into their short- lived lives (Nash et al. 2012). Large-scale monitoring began in Britain in 1976 with the launch of the British Butterfly Monitoring Scheme (Fox et al. 2006). The scheme commenced due to a decline in butterfly numbers and to provide information on butterfly population trends (Butterfly Conservation 2013; Fox et al. 2006). Information for the scheme was gathered with the help of volunteers doing weekly recordings on a fixed route (transect). This transect monitoring method was developed by Dr. E. Pollard as a standard method to record butterfly numbers and became a great source of information on the status of butterflies and the effects of habitat management and climate change (Fox et al. 2006). Volunteers set up their own transect routes of approximatly 1-2 kilometers and walked it once a week between april and september if the weather conditions were optimal for butterfly monitoring (Van Swaay et al. 2012). Over the years more and more volunteers started to join the British scheme (Warren 2011).
A second scheme was set up in the Netherlands in 1990 under the name Dutch Butterfly Monitoring Scheme, which also grew exponentionally over the years (Van Swaay et al. 2002). Butterfly monitoring schemes are meant to run for a long period of time and are valuable after ten years, because after that time period population trends of butterflies species can be predicted with better accuracy (Van Strien et al. 1997). Trends are estimates of butterfly species abundance changes over a certain period of time (See figure 1.1). The main objective of using population trends in ecological research is to estimate the changes on a national and even regional scale (e.g. north, south, different vegetation) for common and rare butterflies (Van Swaay et al. 2002).
Figure 1.1: Population trends from the British Butterfly Monitoring Scheme for the Lulworth Skipper (Thymelicus acteon) (blue), Duke of Burgundy (Hamearis lucina) (red) and Wall Brown (Lasiommata megera) (pink). Population trends are showing an overall decline over a period of 12 years (Warren 2011).
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Butterfly recording began in Ireland during the time period of 1970-1980 by the Biological Records Centre of An Foras Forbartha, which produced an Atlas of butterfly distribution in 1980 (DNFC 2013). Several volunteers helped gather important data through extensive field work. Later in 1998, the Dublin Naturalists’ Field Club (DNFC) together with the Butterfly Conservation (UK) collected data from all over Ireland dating back to 1995 and published the Millenium Atlas of Butterflies in Britain and Ireland (DNFC 2013). With the help of a survey done between 2000-2004, an updated atlas was published under the name of the State of Butterflies of Britain and Ireland in 2006 (DNFC 2013; Fox et al. 2006). This updated atlas shows a distribution map for each butterfly species and trend analysis on British and, when there was sufficient data, on Irish butterflies (Fox et al. 2006). Trend analysis was done with the help of another method called sub-sampling. This method doesn’t use transect routes, but instead uses the distribution map of butterfly species and seperates them into 10 km squares (Fox et al. 2006). Both the DNFC and Butterfly Conservation came to the conclusion that the distribution data was insufficient for analysis on Ireland as a whole (Fox et al. 2006). Nonetheless, they were on the frontier of butterfly recording in Ireland and are still compiling distribution data of butterflies to this day (DNFC 2013).
1.2 European Biodiversity Indicator Eventually more European countries started to join by establishing their own butterfly monitoring schemes (BMS). There are currently fourteen European countries with active butterfly monitoring schemes and more are expected to join (See figure 1.2) (Van Swaay & Van Strien 2008). Of the fourteen countries, twelve have contributed their long-term datasets to help develop indicators which provides information on the status of butterflies on a European scale.
Figure 1.2: European countries with active butterfly monitoring scheme’s (green) and countries expected to join up in the future (blue) (Van Swaay & Van Strien 2008).
Such indicators are known as European Biodiversity Indicators and are compiled by the EU to monitor the progress of biodiversity within Europe (Van Swaay & Van Strien 2008) and to assess the goal of halting biodiversity loss by 2020 (EU Biodiversity Policy Development 2012). Currently, butterflies are
Analyzing population trends of common Irish butterflies | 6 - 61 one of 26 indicators capable of giving accurate estimates of biodiversity loss (Van Swaay & Van Strien 2008). Butterflies are good indicators because of their quick responses to environmental and habitat changes, simple identification, good representation of insect biodiversity as a whole (Thomas 2005), well-tested monitoring methodology (Pollard 1977; Pollard & Yates 1993) and overall appeal to the general public and decision makers (Regan & Fleischer 2011; Nash et al. 2012).
Essentially, indicators show (as comparable to trends) changes of species abundances over the years (Van Swaay & Van Strien 2008). However, they combine butterflies species into one group (e.g. grassland butterflies) and often take information such as temperature into account (e.g. climate change indicator). For example, there is a European Butterfly Climate Change Indicator that shows the effect of climate change on butterfly communities (Van Swaay et al. 2010b). There were several methods investigated for the effect of climate change on butterflies, but the method used by Devictor et al. (2008) was proven to be the most effective. Each butterfly species has a preference for a certain climate or temperature optimum. So, some species tend to stay in colder regions of Europe and others tend to stay in warmer regions. For each butterfly species the optimal temperature was calculated, which is known as the Species Temperature Index (STI) (Van Swaay et al. 2010b). Several butterfly species might have certain characteristics in common such as site, altitude or temperature.
Butterflies with corresponding characteristics are then grouped into communities of butterflies, and have all their STI’s averaged out into a Community Temperature Index (CTI). Therefore, a CTI shows the average optimum temperature for a specific butterfly community (Van Swaay et al. 2010b). When a CTI goes up then the butterfly community has more warmth-loving species and when it goes down than it has more cold-loving species. As shown in figure 1.3, the CTI across Europe has risen significantly over the past 15 years. This means that warmth-loving species in Europe are increasing and that cold-loving species are in a decline. However, this change in butterfly community is proven to be a slower process than the rise in temperature. In other words, the temperature is increasing faster than butterflies can adapt and therefore butterfly species in general are in a decline (Van Swaay et al. 2010b).
Figure 1.3: A graph of the Community Temperature Index (CTI) over the last 15 years, which shows an increase in warmth loving butterfly species across Europe (Van Swaay et al. 2010b).
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The decline in butterflies is also seen by another European Biodiversity Indicator, which only investigated butterflies that are characteristic of European grasslands. The study showed that the abundance of grassland butterflies declined by 60% since 1990 (Van Swaay & Van Strien 2008). Worryingly, decreasing numbers of butterflies have been widely reported on European (Van Swaay & Van Strien 2008; Van Swaay & Warren 1999; Van Swaay et al. 2010b), national and regional scales (Asher et al. 2001; Conrad et al. 2006: Warren et al. 2001; Warren 2011; Wolterbeek 2012). Declines in populations sizes are also seen for birds and vascular plants (Thomas et al. 2004b). These declines can be attributed due to loss of suitable habitats (Fox et al. 2006) and climate change (Van Swaay et al. 2010b). Butterfly conservation measures focus on these problems by preserving large diverse areas with large butterfly populations, by making connections between valuable habitats and reducing our emission of greenhouse gasses. Butterfly conservation measures can be made when by gathering data through already existing butterfly monitoring schemes or by setting up new schemes (Settele et al. 2008).
1.3 Irish Butterfly Monitoring Scheme The Irish Butterfly Monitoring Scheme started out on this same principal, that large quantities of data is needed for accurate predictions of butterfly population changes and to counter these changes with conservation measures (Regan & Fleischer 2011). The Irish scheme was established in 2007 by the National Biodiversity Data Centre with six volunteers monitoring their transect routes. Numbers grew steadily over the years with a total of 144 volunteers at the end of 2012 (See figure 1.4) (Regan 2012). Eventually, the objective is to maintain a network of 120 to 150 transect routes and to develop at least 25 transect routes per butterfly species (Regan & Fleischer 2011). Van Strien et al. (1997) found out that at least 25 transect routes per butterfly species is needed to make accurate trend predictions. Appendix I shows how many transect routes per species are available within the Irish Butterfly Monitoring Scheme for 2012.
148 137 144 150
120
90 69 60 39
Number of of Number volunteers 30 6 0 2007 2008 2009 2010 2011 2012
Figure 1.4: Number of transects monitored by volunteers in Ireland between 2007 and 2012 (Regan 2012).
Also, when making accurate trend predictions you have to account for butterfly species having one or two (or even more) generations per year (Van Swaay et al. 2002). Butterfly species are therefore called single or double-brooded. In short, broodedness is called voltinism and single-brooded species are called univoltine and double-brooded species bivoltine. Butterfly species with more than two generations per year are called multivoltine. During this study no distinction was made between
Analyzing population trends of common Irish butterflies | 8 - 61 bivoltine and multivoltine species, because of the difficulty to distinguish between overlapping second and third generations. Appendix II shows which butterfly species are single or double- brooded in Ireland. In addition, single and double-brooded species react differently to higher temperatures (Westgarth-Smith et al. 2012). When spring starts early with high temperatures, first generations of the double-brooded species lay their eggs sooner. This gives the second generation more time to lay their eggs, and overall accounts for more double-brooded species when temperatures are higher Single-brooded species are less affected by temperature changes as compared to double-brooded species.
Ireland currently has 33 species of butterflies, which is a small number compared to other European countries (Van Swaay et al. 2010a). A reason for this small number of butterfly species is due to the climatological history and habitat and food plant availability in Ireland (Regan et al. 2010). Most of the butterfly species in Ireland are widespread and common species throughout Europe (Regan et al. 2010). Nevertheless, research on common butterflies is as equally important as research on rare species (Conrad et al. 2002; Dunn 2002), because they are involved in many types of habitats and species interactions and therefore important within an ecosystem functioning (Conrad et al. 2006). This study focuses on the common butterflies (species shown in appendix I); because analyzing rare butterfly species doesn’t yield the benchmark of 25 transect routes per species.
Ireland has contributed data on nine butterfly species for the European Grassland Indicator and data on all butterfly species for the Butterfly Climate Change Indicator (See appendix III). Trends have been calculated before by Chris van Swaay (pers. comm.) for the nine butterfly species connected to the grassland indicator and are shown in appendix III. Particular terms are used to identify changes when a trend is in decline or increase. For instance, a decline or increase in butterfly abundance can be steep or moderate. In this context, steep means that the overall slope of a trend is significantly increasing or declining with 5% or more per year, which is comparable to doubling or halving the population abundance within 15 years. Moderate means that the overall slope of a trend is also significantly increasing or declining, but not significantly with 5% or more per year (Van Swaay & Van Strien 2008). A trend is stable if no significant increase or decline has been detected, but that the change in slope of the trend is certainly less than 5% per year. Lastly, a trend is called uncertain if no significant increase or decline has been detected and it is not certain if the change in slope of the trend is less than 5% per year (Van Swaay & Van Strien 2008).
To help identify trends, a statistical software programme called TRIM was used (Pannekoek & Van Strien 1997). TRIM is a software package, based on a log linear regression model, to help analyse time series of counts for butterflies and birds when a large amount of data is missing (Van Swaay et al. 2002). It has been used in a number of studies and has been proven to give accurate assumptions on population changes (Conrad et al. 2006; Van Strien et al. 2001; Van Swaay et al. 2008: Van Swaay & Van Strien et al. 2008). TRIM will give the change in population abundance (e.g. steep or moderate decline, stable) of each butterfly species in the output of the analysis (See appendix VI, green highlighting). Further details about TRIM can be found in paragraph 2.2.
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1.4 Study outline The primary focus of this study was investigating the population trends of common butterflies from Ireland. Our main objective was to evaluate which butterfly species are showing changes in population sizes and what the underlining factors were influencing the changes. Assessing change is important, because of the ongoing decline in butterfly species due to habitat fragmentation and climate change. Secondly, we validated the reliability of the Irish butterfly trends (five year trend) by looking at the rejection of the models, the number of uncertain species and by comparing results with long-term trends (more than 10 years) from Britain. Lastly, we gave recommendations to the Irish Butterfly Monitoring Scheme on the quality of their data and prospects for the future.
In summary, the research aims of this study were;
1. To evaluate which butterfly species are showing a changes in population size; 2. To determine which factors played a role in the changes in population size; 3. To validate the reliability of the Irish butterfly trends and 4. To give recommendations to the Irish Butterfly Monitoring Scheme.
Our overall hypothesis is that Irish butterfly populations are changing in a similar way to British butterfly populations in response to environmental change and that 2012 would show a major decline in butterfly species abundance due to a temperature drop.
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2. Methods
2.1 Monitoring Scheme Methodology When assessing butterfly population trends you need to know how many butterfly species there are and how large their population sizes are on a year-to-year basis. To assess this need for information, volunteers gather data by walking a transect route. Beforehand, volunteers set up their own transect route, which can be close to home or in any area that is familiar to them. Some rules have to be followed when setting up a transect route. A transect route is between one and two kilometres in length, which is divided into smaller sections. The total number of sections is between 5 and 15 and the length of each section may vary in size (See figure 2.1). Sections make it easier for the volunteer to have an overview of all the butterflies on the route. Sections are best identified by using landmarks (e. g. distinctive tree, poles, gates) or by using a GPS. Furthermore, a section is restricted to one type of habitat or land-use, but multiple types of habitats or land-uses are allowed within one transect route. For example, section 5 and 6 of figure 2.1 could have another type of habitat than the rest of the transect route. Once a volunteer starts walking their transect route then it is fixed. Alterations to the route can only be made if they make a new one (Eugenie Regan pers. comm.; Regan 2013; Van Swaay et al. 2012).
Figure 2.1: Example of a transect route divided into 11 sections (Van Swaay et al. 2012).
When a transect route is chosen and set up, volunteers walk their routes on a weekly basis (any day of the week) between the beginning of April and end of September (in total 26 times). In general, the first imagos (adult butterflies) start flying in April, peak in June and end around September (Nash et al. 2012). So, the time period for transect monitoring corresponds with their flight period. Weather conditions have to be right, because butterflies are influenced by temperature, rainfall and wind (Van Swaay et al. 2010b). Volunteers can monitor their transect routes between 10:45 am and 15:45 pm, but these may vary due to ever changing weather conditions in Ireland (Regan 2013). The transect route is only walked when the temperature is higher than 13 degrees Celsius or when the clouds above your head cover less than 50% of the sky when the temperature is between 13 and 17 degrees Celsius. Also, the wind shouldn’t be stronger than force 5 Beaufort, which is when moderately-sized branches begin to move (Van Swaay et al. 2012). In summary, butterfly monitoring is only done when it is nice weather outside (Eugenie Regan pers. comm.; Regan 2013; Van Swaay et al. 2012).
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Right before a volunteer begins to walk a transect route, they write down in a notebook or on a paper form temperature, wind speed, amount of sunshine, beginning time, location and any changes in habitat structure. The volunteer then continues to walk the route on a slow and constant pace. For each section, they write down the butterfly species and numbers within an imaginary box. This imaginary box covers 2.5 meters of either side and 5 meters in the front and above of the volunteer (See figure 2.2). Any butterfly that is outside of this imaginary box isn’t counted, but casual reports of, for instance a rare butterfly species, can be made outside of the transect route. During the walk volunteers may stop to identify a butterfly (Regan pers. comm.; Regan 2013; Van Swaay et al. 2012).
Figure 2.2: An imaginary box around a volunteer for monitoring butterflies on a transect route (Van Swaay et al. 2012).
Storing the data is done through three types of processes. The first process is done by sending the paper forms directly to the NBDC where the data is entered into the database by hand. The second process is done by putting in the data into a programme called Transect Walker, which manages all the data and helps by sending it to the NBDC (UKBMS 2013). It was originally developed by the British scheme for data storage and later used by Ireland, but it is no longer being used because it is out of date and needs upgrading (Eugenie Regan pers. comm.). The third process, and new, process is by submitting the data on the website: monitoring.biodiversityireland.ie. Here volunteers can upload their transect route on a map of Ireland by making their own accounts and later submit the data on a weekly basis (Regan 2013). The advantage of the third process is making the submission of data more reliable and faster. However, there are cases where volunteers don’t have access to an internet connection and then the first process is the better choice (Eugenie Regan pers. comm.; Regan 2013; Van Swaay et al. 2012). Moreover, in this study all data is stored in Transect Walker v. 2.5 and will be used for further data extraction.
As shown in appendix V, there are 144 transect routes being monitored in 2012. A shortcoming for butterfly monitoring schemes is that transect routes can get biased, because most walks are done within protected areas instead of being randomly distributed across the country (Fox et al. 2006). You could account for the biased transect walks through calculations called a weighting procedure. A weighting procedure divides the country into separate habitats and gives the habitat with the most transect routes a smaller number (or weight) than a habitat with less transect routes when calculating the population trends (Van Swaay et al. 2002). However, in this study we won’t account for the unequally distributed transect walks, because they seem to cover the whole basis of Ireland (not including Northern Ireland) (Eugenie Regan pers. comm.; Regan 2013; Van Swaay et al. 2012).
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2.2 TRIM TRIM (Trends and Indices for Monitoring data), as said before, is a statistical program based on a log linear regression model (Van Swaay et al. 2002). In this study TRIM version 3.53 is used to calculate the trends of common Irish butterflies between 2008 and 2012 (Pannekoek & Van Strien 1997). The year 2007 was left out of the trend analysis, because it only had 6 transect routes and would lessen the reliability of the trends.
Of 33 butterfly species annually found (Regan & Fleischer 2011), only 15 butterfly species with more than 25 transect routes are used for the trend analysis (Van Strien et al. 1997). Furthermore, there was no trend analysis done on the butterfly species Red Admiral (Vanessa atalanta) and Painted Lady (Vanessa cardui), because they are migratory species and don’t form site specific populations as the rest of the butterfly species. Data from Northern Ireland wasn’t used during the trend analysis because this study is only focussing on the Republic of Ireland and the numbers of transect routes only contributed significant more data to butterfly species with large numbers of transect routes. The main goal of trying to include data from Northern Ireland was to increase the number of transect routes for butterfly species to help get more species over the 25 transect route threshold. However, no butterfly species was increased over the threshold when Northern Irish data was included (See appendix I). The first generation of double-brooded species is used during trend analysis, because this accounts for greater accuracy (Van Strien et al. 1997). However, the second generation was included when the first generation had lesser number than the second generation. Appendix II shows for which species this has happened.
For TRIM analysis, data preparation was done by estimating the year counts per site for each butterfly species. Afterwards, prepared data was put into a Notepad file in a format TRIM can recognize. In short, the output from TRIM shows the change in population size per butterfly species over these five years and if this change is moderate, steep, stable or uncertain. Next two paragraphs explain how data preparation was done. Information came from personal communication with Eugenie Regan, Chris van Swaay, Arco van Strien and Wim Plantenga.
Calculating the year-count per site and weighted value Before TRIM was used we needed to convert the data into the appropriate format. All the data was first collected by volunteers doing their weekly transects. Over the years, weekly recordings were put into Transect Walker by volunteers or through forms sent to the NBDC. Eventually, all the data was stored in Transect Walker in the format shown in figure 2.3.
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s
Figure 2.3: Data of transect routes showing the amount of butterflies for individual species per week in Transect Walker v. 2.5 (Eugenie Regan pers. comm.). Each volunteer (and transect route) generated a dataset shown in figure 2.3, so there was a total of 141 datasets exported into Excel. This process was already done for the time period 2008 – 2011. An example of the Excel file format is shown in figure 2.4.
Figure 2.4: Excel format extracted from Transect Walker of recorded butterflies in 2011 done by volunteer Enda Kiernan (Eugenie Regan pers. comm.).
Eventually, data of each year was bundled into one Excel file. Within Excel a Pivot Table was used to calculate the total number of butterflies and visits for each year between 2008 and 2012 for each butterfly species (See figure 2.5). The number of visits represents the number of times that particular species of butterfly was recorded during all the transect walks for one year. For example, in figure 2.5 on transect route C02 in 2008 the total number of butterflies in that year was 13 and it was recorded on 4 different walks in 2008.
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Figure 2.5: Preparation for TRIM analysis of Common Blue (Polyommatus icarus) with the total number of butterflies per year and the number of visits and its weighted value (1/number of visits). The green highlighted part is what TRIM recognizes.
When a year is missing, say for instance on transect route C02 in 2011 and 2012 (See figure 2.5), a ‘ - 1’ is inserted as a missing value for that year. This is not to be confused with having found no butterflies at all, which is inserted as a ‘0’, as shown on transect route C02 in 2010. So a ‘ -1’ means that the transect route hasn’t been walked in a specific year and a ‘0’ means that it has been walked but that no butterflies for that species were found. A missing value is also given when a transect walk doens’t meet one of the criteria. The criteria are: (i) the time period between two visits shouldn’t be longer than half the flight period (with a max of 8 weeks missing) and (ii) a butterfly species is counted once during its flight peak (Van Swaay et al. 2002). So for example, Wood Whites’ (Leptidea juvernica) flight period was estimated to run the full length of the recording period (Week 13 till 39). When for instance 2008 had a gap of 9 weeks or higher between two visits or when Wood White (L. juvernica) wasn’t spotted during the month of its flight period peak, then 2008 got a ‘-1’ instead of the number of butterflies.
When for each year the total amount and number of visits was calculated, then the weighted value (not to be confused with weighting procedure) was calculated by dividing 1 with the total number of visits (1/number of visits). This is done to equalize the total number of butterflies for each year. For example, in figure 2.5 at transect route C03 in the year 2009 the total number of butterflies was 15 and in 2010 it was 37. However, the butterfly species was seen on 4 different walks in 2009 and 6 times in 2010. There is a possibility that this same butterfly species was recorded multiple times more in 2010 than 2009, because the number of visits is higher. So, the weighted value is added to account for the biased counting. In figure 2.5 the green highlighted part is in the right format for TRIM to recognize.
Calculating trends and indices From thereon, the green highlighted data in figure 2.5 was put into a Notepad file (See figure 2.6). This file was saved in the same folder where TRIM is installed, otherwise it wouldn’t be recognized. The first column contains the number code given to the transect route. The second column represents the year on which the transect route has been walked. The third column contains the total number of one butterfly species recorded in that year on that transect route. The fourth column contains the weighted value given to the total number of butterflies.
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Figure 2.6: Notepad file for TRIM analysis of the Common Blue (Polyommatus icarus) with in the first column the transect number, second column the year, third column total number of butterflies and fourth column the weighted value (1/number of visits).
Within TRIM the Notepad file was converted to a TCF file, which was used for the trend analysis. When starting the trend analysis TRIM gives you the option to choose between three models called: No Time Effects, Linear and Time Effects. For this study we used the Time Effects model, because it is used as the standard model for analyzing trends. For the analysis the boxes Serial Correlation and Over dispersion were ticked and the Base Time set to 2012 (5). Normally, in Base Time the numbers starts off at 2008 (1), because it compares the index from 2008 to the rest of the years. However, trend analysis in this study is more reliable when 2012 was used, because more data is available for 2012 and therefore makes smaller standard errors. An index (or multiple indices) are estimates of the annual abundance of one butterfly species and are calculated by fixing one year as the starting point and then compare the annual abundance of every year relatively to that fixed year (Conrad et al. 2006). Thus, indices represent the change butterfly species abundance goes through on a year-to- year basis. Indices are used to plot a graph and help visualize the change a butterfly population goes through over the years (See figure 2.7).
Population trend of Common Blue (Poloymmatus icarus) between 2008 - 2012 in Ireland 1.2
1
0.8
0.6 Index
0.4
0.2 Strong increase 0 2008 2009 2010 2011 2012
Figure 2.7: Population trend of Common Blue (Polyommatus icarus) between 2008 – 2012 in Ireland with the indices on the y-axis and five years on the x-axis. The green text represents the strong increase the butterfly species has gone through the last five years.
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The results TRIM gives as output of one species of butterfly is shown in appendix VI. The time indices (blue) can be used for making the trend graph on the population changes of butterfly abundance. The Overall Slope (recommended) (green) shows what the status of the butterfly is and the Multiplicative (yellow) underneath gives the overall change in abundance for each year. The status shows a (p<0.05)* or (p<0.01)** when stable, decreasing or declining or nothing when uncertain. The asterisk indicates how significant the difference between every year and the base year was. The (p<0.05)* stands for a significant difference in the population trend with a 95 % confidence interval and (p<0.01)** stands for a significant difference in the population trend with a 99 % confidence interval.
With the Multiplicative the total amount of increase or decline was calculated over the five-year time period. For example, Common Blue (Polyommatus icarus) had a Multiplicative of 1.1979. This means that for every year the population increased with +19.79 %. So over five years time, the population grew to a percentage of (1.1979^5=) 246.66 %. However, the difference between the starting and ending percentage needs to be calculated to determine the percentage it grew in five years. This was done by taking 100 % from 246.66 %, which resulted in an overall growth of +146.66 % in population size for Common Blue (P. icarus).
2.3 Remaining statistics The mean temperature for the months April - September were calculated for each year to get the graph of the mean temperature over the time period between 2008 and 2012 (See figure 3.6 and 3.7 (black dotted line)). The mean max temperatures of approximately 40 weather stations were used from Met Éireann to calculate the mean temperature for each month. A One-way Repeated Measures ANOVA was used to verify differences in mean temperature between the five years. A Linear Regression analysis was performed to look for a correlation between the overall butterfly abundance trend and the mean temperature in the five-year time period (See figure 3.5). Same analysis was performed for the correlation between the mean spring temperature and Orange Tip (Anthocharis cardamines) abundance (See figure 3.6).
Analyzing population trends of common Irish butterflies | 17 - 61
3. Results
3.1 Butterfly trends Of the 33 common butterfly species found in Ireland, only 15 species met the criteria for trend analysis (See appendix I). Their population abundance was categorized as being stable, uncertain, in steep or moderate increase, or in steep or moderate decline. Overall, the population over the five year time period has increased for three butterfly species (Common Blue (Polyommatus icarus), Meadow Brown (Maniola jurtina) and Small Tortoiseshell (Aglais urticae)), declined for six butterfly species (Holly Blue (Celastrina argiolus, Small White (Pieris rapae), Speckled Wood (Pararge aegeria), Wood White (Leptidea juvernica), Large White (Pieris brassicae) and Peacock (Inachis io)) and deemed uncertain for six butterfly species (Green-veined White (Pieris napi), Orange Tip (Anthocharis cardamines), Ringlet (Aphantopus hyperantus), Small Copper (Lycaena phlaeas), Small Heath (Coenonympha pamphilus) and Silver-washed Fritillary (Argynnis paphia)) (See table 3.1). The graphs for each butterfly species are presented in Appendix IV.
Table 3.1: The change in populations of the 15 most common Irish butterfly species as recorded by the Irish Butterfly Monitoring Scheme between 2008 and 2012. Three species are showing an increase, six species are showing a decline and the trends for the other six species are uncertain. Each section has the butterfly species in alphabetical order. (Significance of trends: *=(P<0.05); **=(P<0.01)).
Population trend Butterfly species Change Common Blue (Polyommatus icarus) ** Strong Increase Increase: 3 species Meadow Brown (Maniola jurtina) **
Small Tortoiseshell (Aglais urticae) * Moderate Increase
Holly Blue (Celastrina argiolus) *
Small White (Pieris rapae) * Moderate Decline Speckled Wood (Pararge aegeria) ** Decline: 6 species Wood White (Leptidea juvernica) **
Large White (Pieris brassicae) ** Strong Decline Peacock (Inachis io) *
Green-veined White (Pieris napi)
Orange Tip (Anthocharis cardamines)
Ringlet (Aphantopus hyperantus) Uncertain: 6 species Uncertain Small Copper (Lycaena phlaeas)
Small Heath (Coenonympha pamphilus)
Silver-washed Fritillary (Argynnis paphia)
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Of the three butterfly species that increased, Common Blue (P. icarus) and Meadow Brown (M. jurtina) showed a strong increase and Small Tortoiseshell (A. urticae) a moderate increase in population change (See figure 3.1). Over the time period of five years, Common Blue (P. Icarus) increased by +146.66 %, Meadow Brown (M. jurtina) +80.84 % and Small Tortoiseshell (A. urticae) +51.06 %. All three species showed a small population decline from 2008 to 2009 and increased in 2010. Then in 2011 the populations of all three species dropped down again and climbed back up in 2012. Common Blue (P. icarus) and Meadow Brown (M. jurtina) remained relatively stable between 2010 and 2012 whereas Small Tortoiseshell (A. urticae) showed a much higher peak in 2010 and dropped in greater numbers than the other two species. See table 3.2 for the means and standard errors for the trends of the increasing butterflies.
2
1.5
1 Index
0.5
Strong and moderate increase 0 2008 2009 2010 2011 2012 Common blue Meadow brown Small Tortoiseshell
Figure 3.1: The population trends of three common Irish butterflies that had an increasing population change between 2008 and 2012. Common blue (Polyommatus icarus) and Meadow Brown (Maniola jurtina) had a steep increase and Small Tortoiseshell (Aglais urticae) had a moderate increase.
Table 3.2: The means and standard errors for each year of the indices from Common blue (Polyommatus icarus), Meadow Brown (Maniola jurtina) and Small Tortoiseshell (Aglais urticae) in Ireland between 2008 and 2012.
2008 2009 2010 2011 2012 Common M= 0.6228 M= 0.396 M= 0.9612 M= 0.9348 M= 1 Blue SE= 0.0772 SE= 0.056 SE= 0.088 SE= 0.0887 SE= 0 Meadow M= 0.6469 M= 0.6504 M= 0.9707 M= 0.8903 M= 1 Brown SE= 0.0802 SE= 0.0635 SE= 0.0815 SE= 0.0811 SE= 0 Small M= 0.7278 M= 0.6189 M= 1.9217 M= 0.7483 M= 1 Tortoiseshell SE= 0.1048 SE= 0.077 SE= 0.1378 SE= 0.0706 SE= 0
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Four butterfly species showed a moderate decline between 2008 and 2012 (See figure 3.2). The species are Holly Blue (C. argiolus), Small White (P. rapae), Speckled Wood (P. aegeria) and Wood White (L. juvernica). Over the five year time period, Holly Blue (C. argiolus) declined by -36.49 %, Small White (P. rapae) -28.62 %, Speckled Wood (P. aegeria) -30.13 % and Wood White (L. juvernica) -42.64 %. Of four moderately declining species, three species (Small White (P. rapae), Speckled Wood (P. aegeria) and Wood White (L. juvernica)) had a similar pattern in population changes between 2008 and 2012. Their populations increased between 2008 and 2009, and 2009 and 2010 with the exception of Wood White, which had a slight decline between 2009 and 2010. Populations of all three declined between 2010 and 2011, and decreased more towards 2012 (Small White (P. rapae) and Speckled Wood (P aegeria)) or remained relatively stable towards 2012 (Wood White (L. juvernica)).
Holly Blue (C. argiolus) fluctuated in an entirely different pattern than the other three species. Holly Blues’ (C. argiolus) population declined between 2008 and 2009, and remained approximately the same in 2010. The populations then increased between 2010 and 2011, and declined towards 2012. See table 3.3 for the means and standard errors for the trends of the moderately declining butterflies.
2.5
2
1.5 Index 1
0.5 Moderate Decline 0 2008 2009 2010 2011 2012 Holly Blue Small White Speckled Wood Wood White
Figure 3.2: The population trends of four common Irish butterflies that had a moderately declining population change between 2008 and 2012. (Holly Blue (Celastrina argiolus), Small White (Pieris rapae), Speckled Wood (Pararge aegeria) and Wood White (Leptidea juvernica) are shown).
Table 3.3: The means and standard errors for each year of the indices from Holly Blue (Celastrina argiolus), Small White (Pieris rapae), Speckled Wood (Pararge aegeria) and Wood White (Leptidea juvernica) in Ireland between 2008 and 2012.
2008 2009 2010 2011 2012 M= 2.2289 M= 0.8158 M= 0.9103 M= 1.6349 M= 1 Holly Blue SE= 0.4107 SE= 0.1902 SE= 0.1949 SE= 0.2861 SE= 0 M= 1.2994 M= 1.5216 M= 2.1245 M= 1.3092 M= 1 Small White SE= 0.1824 SE= 0.1723 SE= 0.1838 SE= 0.1352 SE= 0 Speckled M= 1.134 M= 1.7599 M= 1.8038 M= 1.1054 M= 1 Wood SE= 0.0802 SE= 0.0989 SE= 0.0856 SE= 0.0608 SE= 0 M= 1.3238 M= 1.7282 M= 1.608 M= 0.997 M= 1 Wood White SE= 0.2478 SE= 0.2223 SE= 0.1885 SE= 0.1509 SE= 0
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Large White (P. brassicae) and Peacock (I. io) showed a strong decline in populations between 2008 and 2012 and seemed to follow a similar pattern (See figure 3.3). In the five year time period, Large White (P. brassicae) declined by -52.88 %. From 2008 until 2010 the population remained relatively stable and declined in 2011. Then it remained stable again between 2011 and 2012. For Peacock (I. io) the population declined with -40.62 %. Its population increased between 2008 and 2009, and went into a decline until 2011. The population remained stable between 2011 and 2012. See table 3.4 for the means and standard errors for the trends of the steeply declining butterflies.
2
1.5
1 Index Index
0.5
Steep Decline 0 2008 2009 2010 2011 2012 Large White Peacock
Figure 3.3: The population trends of two common Irish butterflies that had a steeply declining population change between 2008 and 2012. (Large White (Pieris brassicae) and Peacock (Inachis io) are shown).
Table 3.4: The means and standard errors for each year of the indices from Large White (Pieris brassicae) and Peacock (Inachis io) in Ireland between 2008 and 2012.
2008 2009 2010 2011 2012 Large M= 1.6521 M= 1.6946 M= 1.6558 M= 1.0273 M= 1 White SE= 0.1531 SE= 0.1353 SE= 0.1059 SE= 0.0826 SE= 0 M= 1.3074 M= 1.6469 M= 1.278 M= 0.9929 M= 1 Peacock SE= 0.1632 SE= 0.1636 SE= 0.1177 SE= 0.1037 SE= 0
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The butterfly species Green-veined White (P. napi), Orange Tip (A. cardamines), Ringlet (A. hyperantus), Silver-washed Fritillary (A. paphia), Small Copper (L. phlaeas) and Small Heath (C. pamphilus) had an uncertain population change (See figure 3.4). This means that the population had no significant increase or decline and that it is not certain if trends are less than 5% change per year. All six common butterfly species increased between 2008 and 2010. The species Green-veined White (P. napi), Silver-washed Fritillary (A. paphia), Small Copper (L. phlaeas) and Small Heath (C. pamphilus) had a peak in 2010 and kept declining until 2012. However, Ringlet (A. hyperantus) kept increasing until 2011 and declined in 2012. The same pattern is seen for Orange Tip (A. cardamines), which had an almost exponential growth up until 2011 and declined below 2008 numbers in 2012. See table 3.5 for the means and standard errors for the trends of the uncertain butterflies.
3.5
3
2.5
2
Index 1.5
1
0.5 Uncertain 0 2008 2009 2010 2011 2012 Orange Tip Small Copper Small Heath Silver-washed Fritillary Ringlet Green-veined White
Figure 3.4: The population trends of six common Irish butterflies that had an uncertain population change between 2008 and 2012. (Green- veined White (Pieris napi), Orange Tip (Anthocharis cardamines), Ringlet (Aphantopus hyperantus), Silver-washed Fritillary (Argynnis paphia), Small Copper (Lycaena phlaeas) and Small Heath (Coenonympha pamphilus) are shown).
Table 3.5: The means and standard errors for each year of the indices from Green-veined White (Pieris napi), Orange Tip (Anthocharis cardamines), Ringlet (Aphantopus hyperantus), Silver-washed Fritillary (Argynnis paphia), Small Copper (Lycaena phlaeas) and Small Heath (Coenonympha pamphilus) in Ireland between 2008 and 2012.
2008 2009 2010 2011 2012 Green-veined M= 1.264 M= 1.283 M= 1.6431 M= 1.3983 M= 1 White SE= 0.1339 SE= 0.1132 SE= 0.1081 SE= 0.1091 SE= 0 M= 1.1994 M= 1.3767 M= 1.9029 M= 3.0751 M= 1 Orange Tip SE= 0.1659 SE= 0.1466 SE= 0.1464 SE= 0.2425 SE= 0 M= 0.9571 M= 1.252 M= 1.4002 M= 1.4131 M= 1 Ringlet SE= 0.0969 SE= 0.0988 SE= 0.0902 SE= 0.0908 SE= 0 Silver-washed M= 1.099 M= 1.5253 M= 2.152 M= 1.3142 M= 1 Fritillary SE= 0.224 SE= 0.2449 SE= 0.2456 SE= 0.1837 SE= 0 M= 1.25 M= 1.5752 M= 2.2595 M= 1.5739 M= 1 Small Copper SE= 0.2133 SE= 0.2308 SE= 0.2705 SE= 0.2108 SE= 0 M= 0.958 M= 1.2944 M= 1.6018 M= 1.0895 M= 1 Small Heath SE= 0.1325 SE= 0.1527 SE= 0.1663 SE= 0.1389 SE= 0
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3.2 Overall trend and temperature The overarching population trend of all 15 common butterflies of Ireland between 2008 and 2012 is shown in figure 3.5. Overall, the abundance of all 15 butterflies taken together formed a stable trend with a peak in 2010. The mean temperature over five years was significantly higher in 2010 and at its lowest in 2012 compared to the rest of the years (P=0.031; F=3.321; One-way Repeated Measures ANOVA). So, both the population trend and mean temperature per year were at their highest in 2010. However, there isn’t a significant correlation between the two (P=0.255; F=1.970; R=0.630; Linear Regression). The means and standard errors of the indices and mean temperatures for each year are listed in table 3.6.
Out of 15 butterfly species, 9 showed a peak in 2010. They are in alphabetical order: Common Blue (P. icarus), Green-veined White (P. Napi), Meadow Brown (M. jurtina), Silver-washed Fritillary (A.s paphia), Small Copper (L. phlaeas), Small Heath (C. pamphilus), Small Tortoiseshell (A. urticae), Small White (P. rapae) and Speckled Wood (P. aegeria). However, 6 out of 15 butterfly species didn’t show a peak in 2010. They are in alphabetical order: Holly Blue (C. argiolus), Large White (P. brassicae), Orange Tip (A. cardamines), Peacock (I. io), Ringlet (A. hyperantus) and Wood White (L. juvernica).
1.6 17.5 Mean temperature April
1.4 17 1.2 16.5 1
0.8 16 Index Index - - September 0.6 15.5 0.4 15
0.2 (° Stable C)
0 14.5 2008 2009 2010 2011 2012
Figure 3.5: The population trend (big orange) of 15 most common Irish butterflies between 2008 – 2012, showing a stable trend and peak in 2010. On the right side the mean temperature between April – September over the same time period (black dotted line).
Table 3.6: The means and standard errors for each year of the indices from 15 most common butterflies and the mean temperatures in Ireland between 2008 and 2012.
2008 2009 2010 2011 2012 Total Butterfly M= 0.9925 Μ= 1.1471 Μ= 1.478 Μ= 1.202 Μ= 1 Abundance SE= 0.0348 SE= 0.0331 SE= 0.035 SE= 0.031 SE= 0
Mean Μ= 16.730 Μ= 16.858 Μ= 17.191 M= 16.561 M= 15.104 Temperature SE= 0.984 SE= 0.963 SE= 0.918 SE= 0.514 SE= 0.909
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3.5 15.5 temperature April springMean
3 15 2.5
2 14.5 Index Index 1.5 14 1 - May - 13.5 0.5 (°
Uncertain C) 0 13 2008 2009 2010 2011 2012
Figure 3.6: The population trend (dark green) of Orange Tip (Anthocharis cardamines) between 2008 – 2012, showing an uncertain population change with an exponential growth between 2008 and 2011 and steep decline between 2011 and 2012. On the right side the mean spring temperature between April – May over the same time period (black dotted line).
Table 3.7: The means and standard errors for each year of the indices from Orange Tip (Anthocharis cardamines) and the mean spring temperatures in Ireland between 2008 and 2012.
2008 2009 2010 2011 2012 M= 1.1994 M= 1.3767 M= 1.9029 M= 3.0751 M= 1 Orange Tip SE= 0.1659 SE= 0.1466 SE= 0.1464 SE= 0.2425 SE= 0 Mean Spring M= 14.760 M= 14.082 M= 14.543 M= 15.197 M= 13.151 Temperature SE= 2.658 SE= 1.056 SE= 0.918 SE= 0.461 SE= 1.965
The correlation between mean spring temperatures and Orange Tip (A. cardamines) indices from 2008 to 2012 is shown in figure 3.6. The two lines seem to follow a similar pattern with a peak in 2011, although no significant correlation was found (P=0.152; F=3.644; R=0.741; Linear Regression). However the R-value shows that a large part of the Orange Tip (A. cardamines) population was affected by the spring temperature. Both the mean spring temperatures and Orange Tip (A. cardamines) indices increased between 2009 and 2011, and declined from 2011 to 2012 to the lowest in five years. The means and standard errors of the indices of Orange Tip (A. cardamines) and mean spring temperatures for each year are listed in table 3.7.
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3.3 Voltinism More double-brooded species peaked in 2010 along with the temperature peak. Six of the nine species whose population peaked in 2010 were double-brooded compared to three of the six that didn’t show a peak. This was investigated further by comparing population trends of single-brooded species were three out of nine species showed a peak and three out of six that didn’t.
The population trends of single-brooded and double-brooded butterflies and mean temperatures between 2008 and 2012 in Ireland are shown in figure 3.7. For the double-brooded trend the following nine species of butterflies have been used: Common Blue (P. icarus), Green-veined White (P. napi), Holly Blue (C. argiolus), Large White (P. brassicae), Peacock (I. io), Small Copper (L. phlaeas), Small Tortoiseshell (A. urticae), Small White (P. rapae) and Speckled Wood (P. aegeria).
For the single-brooded trend the following six species have been used: Meadow Brown (M. jurtina), Orange Tip (A. cardamines), Ringlet (A. hyperantus), Silver-washed Fritillary (A. paphia), Small Heath (C. pamphilus) and Wood White (L. juvernica). All 15 common butterflies are listed with their broodedness in Appendix II. The means and standard errors for the single and double-brooded trends and mean temperatures for each year are shown in table 3.8.
1.8 17.5 MeanApril temperature 1.6 17 1.4
1.2 16.5 1 16 Index Index
0.8 - September ( 0.6 15.5 0.4 15 0.2 Single brooded: Double brooded: °C) Moderate increase Moderate Decline 0 14.5 2008 2009 2010 2011 2012
Figure 3.7: The population trends of single brooded butterflies (green – moderate increase) and double brooded butterflies (red – moderate decline) the time period 2008 – 2012 in Ireland. On the right side the mean temperature between April – September over the same time period (black dotted line).
Table 3.8: The means and standard errors for each year of the indices from single and double brooded butterflies and the mean temperature in Ireland.
Year 2008 2009 2010 2011 2012 Single Brooded M= 0.8707 M= 0.9877 M= 1.2884 M= 1.2929 M= 1 Index SE= 0.0514 SE= 0.0464 SE= 0.0504 SE= 0.0531 SE= 0 Double M= 1.1357 M= 1.3472 M= 1.6903 M= 1.1047 M= 15.104 Brooded Index SE= 0.0459 SE= 0.0461 SE= 0.0459 SE= 0.0354 SE= 0.909 Mean M= 16.730 M= 16.858 M= 17.191 M= 16.561 M= 15.104 Temperature SE= 0.984 SE= 0.963 SE= 0.918 SE= 0.514 SE= 0.909
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The index for double-brooded butterflies showed a moderate decline between 2008 and 2012 (P<0.01)**; Generalized Estimated Equations TRIM). The trend shows that the double-brooded butterflies increased from 2008 to 2009, peaked in 2010 and dropped below the 2008 index in 2011 and was at its lowest in 2012. The moderate decline in double brooded butterflies showed a correlation (R-value) with the mean temperature (P=0.154; F=3.594; R=0.738; Linear Regression), however not significant.
The index for the single-brooded butterflies showed a moderate increase between 2008 and 2012 (P<0.01)**; Generalized Estimated Equations TRIM). The trend shows that the single-brooded butterflies increased between 2008 and 2010. From 2010 onwards the number stayed the same for 2011 and dropped down in 2012 around the same level as it was in 2009. The moderate increase in single-brooded butterflies showed a lesser correlation with the mean temperature than double- brooded butterflies (P=0.630; F=0.303; R=0.303; Linear Regression).
Again, both tests show that there might not to be a correlation between the population trends and mean temperature. This might be due to a small sample size (N=5). However, the double-brooded butterflies were much more correlated with the temperature than the single-brooded butterflies.
3.4 Food plants Table 3.9 shows the 15 most common Irish butterflies grouped by food plant family. The colours green, red and grey represent the population changes according to table 3.1. The seven food plant families are: Aquifoliaceae, Polygonaceae, Violaceae, Fabaceae, Urticaceae, Brassicaceae and Poaceae. Population changes of butterflies appeared to have no correspondence with food plant family. For example, for the food plant family Poaceae, Meadow Brown (M. jurtina) had a strong increase and Speckled Wood (P. aegeria) a moderate decline. However, more research into the ecology and phenology of the individual food plants is needed to look for corresponding factors, which was not done during this study.
Table 3.9: The seven food plant families with their corresponding common Irish butterflies. Further included, their main food plant and voltinism. The colour indicates what their population change was between 2008 and 2012 according to table 3.1.
Aquifoliaceae Polygonaceae Violaceae Fabaceae Urticaceae Brassicaceae Poaceae
Common Blue Green-veined Peacock Meadow Brown (Bird’s foot White (Nettle; Double- (Grass; Single- Trefoil; Double- (Cuckoo Flower; brooded) brooded) brooded) Double-brooded) Silver-washed Holly Blue Small Copper Large White Ringlet Fritillary (Holly; Double- (Rumex; Single- (Brassica; Double- (Grass; Single- (Violet; Single- brooded) brooded) Small brooded) brooded) brooded) Wood White Tortoiseshell Orange Tip Small Heath (Vetch; Single- (Nettle; Double- (Cuckoo Flower; (Fine Grass; Single- brooded) brooded) Single-brooded brooded)
Small White Speckled Wood (Brassica; Double- (Grass; Double- brooded) brooded
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4. Discussion
The Irish Butterfly Monitoring Scheme was established in 2007 and the number of volunteers grew steadily over the years (Regan 2012). It started on the principal that information on butterfly populations should be gathered and assessed for conservation measures (Regan & Fleischer 2011), because decreasing numbers of butterflies have been reported on European (Van Swaay & Van Strien 2008; Van Swaay & Warren 1999; Van Swaay et al. 2010b), national and regional scales (Asher et al. 2001; Conrad et al. 2006: Warren et al. 2001; Warren 2011; Wolterbeek 2012). These declines can be attributed due to loss of suitable habitats (Fox et al. 2006) and climate change (Van Swaay et al. 2010b). Population trends are generally used to assess these changes in butterfly populations (Fox et al. 2006; Van Swaay et al. 2013). The main objective of this study was therefore to evaluate the population trends of the common butterflies of Ireland between 2008 and 2012. Secondly, we examined the fitness of our trend models and gave recommendations for quality data for the Irish Butterfly Monitoring Scheme. The trends were analyzed for 15 out of 33 species of butterfly.
This study found that the majority of the common butterflies in Ireland are in a decline within the five year time period. Total butterfly abundance peaked in 2010 and was at its lowest in 2012. The mean temperature followed a similar pattern as the total butterfly abundance, although not significantly correlated. Furthermore, the abundance of double-brooded butterfly species was more correlated with temperature than the abundance of single-brooded species. Temperature was the largest factor influencing abundance; with 9 out of 15 species having a peak in 2010 and decline in 2012. However, other factors such as rainfall and parasitism may also be affecting the short-term population changes of other species of butterfly. Butterfly species that had an association with the temperature or could potentially have are: Common Blue (P. icarus), Green-veined White (P. napi), Meadow Brown (M. jurtina), Peacock (I. io), Ringlet (A. hyperantus), Silver-washed Fritillary (A. paphia), Small Copper (L. phlaeas), Small Tortoiseshell (A. urticae), Small Heath (C. pamphilus), Small White (P. rapae) and Speckled Wood (P. aegeria). Data of these butterfly species will contribute quality data towards the butterfly climate change indicator.
4.1 Factors influencing trends Population fluctuations of butterflies are affected by many factors including:
. Temperature (inter alia, Asher et al. 2001; Bryant et al. 2002; Dennis 1993; Kocsis & Hufnagel 2010; Myers 1998; Pollard 1988; Roy et al. 2001; Sparks et al. 2007; Van Swaay et al. 2010b); . Rainfall (Asher et al. 2001; Dennis 1993; Myers 1998; Pollard 1988; Roy et al. 2001); . Parasitism (Berryman 1996; Morris 1959; Regan 2010; Varley et al. 1973); . Food plant availability (Guedes et al. 2000); . Pesticides (Longley & Sotherton 1997; Pollard 1988); . Atmospheric pollution (Barbour 1986; Kocsis & Hufnagel 2010); . Habitat change (Asher et al. 2001; Fox et al. 2006; Rackham 1986).
Hypotheses such as maternal effects (Rossiter 1994; Wellington 1965), disease (Myers 1993) and induced plant defences (Baltensweiler & Fischlin 1988; Haukioja 1988) are also suggested as explanations for population fluctuations. However, temperature and rainfall (often referred to as weather) are seen as the most influential factors affecting population changes (Myers 1998).
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Most of the common butterfly species in Ireland were in a decline between 2008 and 2012 or dropped between 2011 and 2012. These results are consistent with butterfly population trends from the British and Dutch Butterfly Monitoring Schemes (UKBMS & CEH 2013; Van Swaay et al. 2013). The declines in 2012 are linked to unfavourable weather conditions in both the Britain and the Netherlands, with the same pattern showing in Ireland. The mean temperature in Ireland had a significant drop in 2012 and followed a similar pattern as the total butterfly abundance, although it was not significantly correlated. Although no significant correlation is seen, effects of temperature on butterfly abundance have been thoroughly studied and proven and can’t therefore be ignored (See references above). Its effects are mostly on life cycle development, colonization, voltinism, and density and size of populations (Koscis & Hufnagel 2010). Butterflies are so responsive to temperature changes, because they are cold-blooded insects and have relatively short life cycles (Clavero et al. 2011; Thomas 2004a). With climate change raising the summer temperature with 1 °C over 25 years (Roy & Sparks 2000) and more new butterflies migrating northwards (Kiritani 2006; Mitikka et al. 2008; Parmesan et al. 1999; Parmesan 2006), changes in population abundances are clearly affected by temperature changes. However, climate change happens on a far longer time scale than this study examines. So only the correlation between temperature and butterfly abundance can be seen in the five-year trends.
Voltinism With temperature having such a profound effect on the total butterfly abundance, is its effect showing on other levels? Voltinism (broodedness of butterflies) is one such level of interest (Koscis & Hufnagel 2010). In this study, both single and double-brooded butterfly species followed the same pattern as the mean temperature. However, double-brooded species were more correlated to the mean temperature than single-brooded species. This result is consistent with Westgarth-Smith et al. (2012) which also had a higher correlation between double-brooded species and temperature than single-brooded species. They hypothesised that warmer weather favours double-brooded species, because the second brood can be laid earlier in the year and is therefore more likely to survive. Roy et al. (2001) also indicated a higher correlation between double-brooded species and the temperature and suggested that double-brooded species benefited more from the high temperatures in June than single-brooded species, because more stages of its life-cycle are in development during that period (egg>larvae>pupae>adult for double-brooded compared to only larvae>pupae for single-brooded). Additionally, single-brooded species such as Common Blue (Polyommatus icarus) are known to shift to double-broodedness when temperatures are high in Britain and Ireland (Asher et al. 2001). Overall, double-broodedness can be used as a factor deciding which species contribute quality data towards the butterfly climate change indicator.
The total butterfly abundance peaked in 2010 and was present in 9 out of 15 individual butterfly trends. A positive association with high temperatures was found with the majority of butterfly species (five out of seven) that had a peak in 2010 (Pollard 1988). However, six butterfly species didn’t show this peak in abundance. For three of those butterfly species no positive association were found with high temperatures (Pollard 1988). Therefore, more factors than only temperature are influencing the abundance of butterfly species.
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Increasing butterflies All increasing butterfly species had a peak in 2010. Pollard (1988) states that both Common Blue (P. icarus) and Meadow Brown (Maniola jurtina) have a positive association with higher temperatures. This study showed that both species had a similar pattern with the mean temperature. The temperature was therefore the main factor influencing their abundance. Small Tortoiseshell (Aglais urticae) fluctuated more than the other increasing species. Asher et al. (2001) stated that higher fluctuations for Small Tortoiseshell (A. urticae) are caused by the quality of food plants due to rainfall and their ability to quickly recover after a major decline. Small Tortoiseshell (A. urticae) larvae develop more quickly if the weather conditions (high temperatures) are right and adults cope with greater fluctuations in temperature than other butterfly species (Asher et al. 2001; Bryant et al. 2002). However, no explanations can be found for their increase between 2011 and 2012, because the mean temperature was significantly decreasing during that time period. Moreover, both British and Dutch trends had no similarities with the Irish trends for all three species (Marc Botham pers. comm.; Van Swaay et al. 2013).
Declining butterflies For six declining butterflies; only Small White (Pieris rapae) and Speckled Wood (Pararge aegeria) showed a peak in 2010. Although no studies speak of a correlation between Small White (P. rapae) abundance and temperature, such an association is seen in Ireland. The British trend for the Small White (P. rapae) dropped in 2012 and therefore suggests that there is a temperature influence (Marc Botham pers. comm.). The Dutch trend remained stable over the past six years and isn’t corresponding with the Irish trend (Van Swaay et al. 2013). However, geographical differences between the Netherlands and Ireland should be taken into account.
Speckled Wood (P. aegeria) had a smaller increase between 2009 and 2010 than Small White (P. rapae). More factors, than mainly temperature, are equally influencing the abundance of Speckled Wood (P. aegeria). Pollard (1988) and Roy et al. (2001) found that Speckled Wood (P. aegeria) has a positive association with high temperatures during its flight period, a positive association with rainfall in the year before its flight period and a negative association with high temperatures in the year before its flight period. However, with the rainfall being above average in May and June 2009, a higher peak in 2010 would have been expected. The peak in 2010 not reaching its full potential is explained by their negative association with higher temperatures in the year before their flight period.
In Ireland, two species of Wood White are recognized (Leptidea sinapis and Leptidea juvernica). In this study, only L. juvernica was analyzed because it constitutes the majority of Wood White found in Ireland (Nelson et al. 2011). The trend for Wood White (L. juvernica) seems similar to the trend of Speckled Wood (P. aegeria). Recommendation for a future study goes out for the association between Wood White (L. juvernica) abundance and weather factors. Although little information can be found about factors influencing the abundance of Wood White (L. juvernica), its conservation status is assessed under the IUCN Red List Criteria as of Least Concern and is relatively stable (Regan et al. 2010). Wood Whites species L. sinapis its stability stems from their ability to colonize new potential areas and endurance to live in intensively managed agriculture (Asher et al. 2001). However, the trend of Wood White (L. juvernica) shows a decline of approximately 40 percent between 2008 and 2012. The British trend of L. sinapis, having declined between 2010 and 2012, is
Analyzing population trends of common Irish butterflies | 29 - 61 suggesting a similar relationship with the temperature as this study found (Marc Botham pers. comm.). However, the overall decline in the British L. sinapis is believed to be influenced by the shading of woodland rides (Asher et al. 2001).
Holly Blue (Celastrina argiolus) doesn’t show any association with the mean temperature and only a negative association with the rainfall has been found in the month of July before their flight period (Pollard 1988). Parasitism by the ichnuemonine wasp Listrodomus nycthemerus has been found to be the major cause of their population fluctuations (Revel 2006; Thomas & Lewington 2010). Predictable cycles in their fluctuations of six to seven years are seen in the British trend, suggesting that a predictable cycle could almost be seen in the current Irish trend (Regan et al. 2010). The Dutch trend is also showing wild fluctuations, although no causes are mentioned (Van Swaay et al. 2013). The Irish trend of Holly Blue (C. argiolus) hints that a short-time trend of five years is able to show accurate trends.
The strongly declining butterfly species Large White (Pieris brassicae) and Peacock (Inachis io) aren’t showing any correlation with the mean temperature between 2008 and 2012. Nonetheless, Large White (P. brassicae) is associated with having a negative association with high temperatures during its flight period and Peacock (I. io) with having a negative association with rainfall during its flight period (Pollard 1988). Parasitism by the parasitic wasp Apanteles glomeratus is a major factor controlling their abundance and the cause of high year fluctuations seen in the British trend (Asher et al.2001). The cause of their indifference with the temperature could therefore be parasitism. Other factors contributing to their decline may be the usage of pesticides and intensification of agriculture (Asher et al. 2001). A similar decline is showing in both the British and Dutch trends of Large White (P. brassicae) (Marc Botham pers. comm.; Van Swaay et al. 2013).
Peacock (I. io) is largely affected by climatic influences, with an emphasis on temperature (Bryant et al. 2002) and rainfall during summers (Asher et al. 2001). However, this is contradicted by Pollard (1988) which states that Peacock (I. io) is only affected by rainfall during February of its flight period. In this study, the temperature wasn’t following the same pattern as the trend of Peacock (I. io) and is therefore concluded to have less of an effect on population abundance than on other species. The weather during the summer of 2009 was very wet and should have made 2009 their worst year, however their abundance was at its highest. No explanation for their fluctuations can therefore be conclusively stated. An association with the temperature could potentially show when more years are added to the trend analysis. The steep decline of Peacock (I. io) has been in reported in the Netherlands and Great Britain and both countries trends show the same peak of abundance in 2009 (Marc Botham pers. comm.; Van Swaay et al. 2013). The Netherlands give no mention as to why this happened, but Britain reports this might be driven by climate change (Asher et al. 2001).
Uncertain butterflies For six butterfly species that had an uncertain population change; four showed a peak in 2010 or had a decline in 2012. In this context uncertain means that the butterfly species had no significant increase or decline and that it is uncertain if their abundance changed with less than 5 % per year (Pannekoek & Van Strien 1997). In contrast to a stable population; wherein the difference lies that the abundance was certainly less than 5 % per year (Pannekoek & Van Strien 1997). Uncertain species who were affected by temperature are Small Copper (Lycaena phlaeas), Silver-washed
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Fritillary (Argynnis paphia), Green-veined White (Pieris napi) and Small Heath (Coenonympha pamphilus). The spring temperature affected the abundance of Orange Tip (Anthocharis cardamines) and an association with the temperature and the abundance of Ringlet (Aphantopus hyperantus) might be proven if more years are added.
The Small Copper (L. phlaeas) is known to be affected by extreme weather conditions (Asher et al. 2001) and has a positive association with high temperatures during its flight period (Pollard 1988). The Irish trend for Small Copper (L. phlaeas) showed a similar pattern as the mean temperature and had the highest peak in 2010 and steepest decline between 2010 and 2012. Therefore, the temperature seems to have had a large influence on the abundance of Small Copper L. phlaeas). The British trend showed a similar trend as the Irish trend (Marc Botham pers. comm.), suggesting that both landmasses undergo similar changes in abundance.
Silver-washed Fritillary (A. paphia) had an almost similar trend as Small Copper (L. phlaeas), which suggests that they might be similarly affected by extreme weather conditions. However, Pollard (1988) didn’t find such a positive correlation with high temperatures. Moreover, Silver-washed Fritillaries’ (A. paphia) population abundance is negatively associated with high temperatures during its flight period and with rainfall the year before. These negative associations stem from both the temperature and rainfall affecting the food plant availability and therefore indirectly affect the population size (Asher et al. 2001). Again, the British trend showed similarity with the Irish trend, suggesting that a short-term trend analysis could yield usable data.
Green-veined White (P. napi) had less extreme fluctuations as other butterfly species. The cause for their lesser extreme fluctuations may be their tendency to lay eggs separately instead of bundled which makes them less exposed to parasitism and their ability to live from a wide range of food plants (Asher et al. 2001). Green-veined White (P. napi) is positively affected by rainfall and negatively affected by high temperatures in the year before their flight period (Pollard 1988), because they have a preference for damp conditions in their habitat (Asher et al. 2001). With the rainfall being above average in 2009 and the temperatures having peaked in 2010, more extreme fluctuations would have been expected in both years. In contrast, Green-veined White (P. napi) had modest fluctuations as the British trend and seemed to be less affected by these weather extremes in both years.
Small Heath (C. pamphilus) showed a relationship with the temperature over the five year time period. According to Pollard (1988) Small Heath (C. pamphilus) has a positive association with high temperatures during their flight period and with rainfall in the year before. The peak in 2010 confirms this positive association with temperature and rainfall, because 2009 was a wet spring and 2010 a warm summer. Overall, their populations are declining in Britain and are stable in the Netherlands, although they had a massive drop in abundance between 1990 and 1992 (Marc Botham pers. comm.; Van Swaay et al. 2013). Reasons for their decline in Britain are unknown, but are suggested to come from weather changes and habitat loss (Asher et al. 2001).
Orange Tip (A. cardamines) had an almost exponential growth between 2008 and 2011 and had the steepest decline towards 2012 of all the Irish butterflies. Instead of the mean temperature, they showed a similar pattern with the mean spring temperature, because their flight period is generally
Analyzing population trends of common Irish butterflies | 31 - 61 between April and May (Harding 2008). They are positively associated with high temperatures and negatively associated with rainfall in the year before their flight period (Pollard 1988); however none of these effects are seen in the Irish trend. This sudden decline in abundance for 2012 is also seen in the British trend, but overall seems to be relatively stable during the last 37 years (Asher et al. 2001; Marc Botham pers. comm.).
Ringlet (A. hyperantus) didn’t have a peak in 2010, but had a decline between 2011 and 2012. They are positively associated with high temperatures and negatively associated with rainfall in the year before their flight period (Pollad 1988; Roy et al. 2001). The high rainfall in 2009 is likely the case of the low abundance in 2010. Overall, the Ringlet (A. hyperantus) is declining in both Britain and the Netherlands and the reasons range from air pollution, climate change, extreme drought to drainage of habitat (Asher et al. 2001). Although the effect of temperature on their abundance seems lesser in Ireland compared to Britain and the Netherlands, more years will give insight into how far this association really goes.
4.2 Reliability trends Predicting population changes in a six-year time period are known to be less accurate than longer running schemes (Roy et al. 2001), but will become accurate when the scheme has been running for ten years (Van Strien et al. 1997). This study showed on two levels that at least five more years should be added to increase its accuracy; (i) the high number of uncertain species and (ii) the rejection of the models. Despite having a minimum of 25 transects per butterfly species, 6 out of 15 species still had an uncertain population change in the five-year time period. The number of uncertain abundances is high compared to Dutch butterfly statuses which had been running for 23 years, where only 1 butterfly (Melitaea athalia) out of 52 species remained uncertain (Van Swaay et al. 2013). The prediction is that the number of uncertain butterflies will decrease over the running of the Irish scheme. Van Strien et al. (1997) found out that when comparing 20 and 10 year datasets, the amount of uncertain butterfly species didn’t increase by a large amount, but did up until the ten year threshold. So using a ten year data-set is a threshold for accurate trend analysis.
However, five year trends are still usable and shouldn’t be undervalued. Short-term trends give insight into what fluctuations and effects are already visible and what could be expected in the long run (Dennis 1993). Short-term data is often necessary to be assessed, because conservation management decisions should be made immediately if drastic declines are already happening (Van Strien et al. 1997). Extrapolation of Irish trends with British trends is possible for future trend analysis, because butterfly species such as Holly Blue (C. argiolus), Orange Tip (A. cardamines) and Peacock (I. io) show high similarities in both trends from Ireland and Britain.
Appendix VII shows the trends of all three butterfly species with trends from Ireland and Britain. Holly Blue (C. argiolus) showed high fluctuations in both trends, which has been thoroughly discussed in British data (Asher et al. 2001). However, they only showed similarities in their high fluctuations, not in their changes on a year-to-year basis. For example, Britain showed a high increase between 2009 and 2010 whereas Ireland remained relatively stable. Orange Tip (A. cardamines) showed a similar pattern in Ireland compared to Britain in abundance, with an increase between 2008 and 2011 and a decline until 2012. However, Ireland had higher fluctuations in abundance than Britain. Peacock (I. io) had the most corresponding abundance between Britain and Ireland of all Irish
Analyzing population trends of common Irish butterflies | 32 - 61 butterflies analysed. Both had a peak in 2009 and declined until 2011, with only a small difference wherein Ireland remained stable between 2011 and 2012 and Britain declined. Nevertheless, Peacock (I. io) shows that five-year trends are usable for immediate views of their abundance, but that their population status (steep decline) is only reliable after ten years.
Models for all fifteen butterfly species were rejected. Their Chi-square test and Likelihood Ratio/Deviance test were all below the threshold. All trends were analysed using a Time Effects model with serial correlation and overdispersion switched on (Pannekoek & Van Strien 1997). Switching off serial correlation was done for butterflies with less than 50 transect routes in 2012; otherwise the standard errors could be too high. However, differences between switching serial correlation on and off were small and didn’t affect the indices. The indices are generally reliable and rejection of a model is converted into bigger standard errors (Pannekoek & Van Strien 1997). Adding more years to your trend analysis will increase the fitness of your models, but will change your indices over time. However the changes in indices are comparatively small and within the boundaries of the standard errors (Pannekoek & Van Strien 1997). In summary, model rejection still makes good estimates of indices and the population status, but the standard errors have to be taking into account.
4.3 Bioindicators The EU Biodiversity Strategy currently uses indicators to assess biodiversity loss towards 2020 for a broad range of species, including butterflies and birds (Van Swaay & Warren 2012). This is a European response to the Convention of Biological Diversity that wants to halt biodiversity loss on an international scale (CBD 2013). There are currently two indicators developed for butterflies, named the European grassland butterfly indicator and climate change indicator (Van Swaay & Warren 2012). The European grassland butterfly indicator shows the trend of butterfly species which are associated with grassland habitat from 14 different countries that submitted data (Van Swaay & Van Strien 2008). The grassland indicator is primarily used to spot major declines in grassland butterfly species across Europe and to help assess conservation measures (Van Swaay & Van Strien 2008).
The butterfly climate change indicator is a bit different that the grassland indicator. The climate change indicator groups butterfly species into cold-loving and warmth-loving communities by looking at their optimal temperature, called a Community Temperature Index (CTI) (Van Swaay et al. 2010b; Van Swaay & Van Strien 2008). The CTI then shows the change over time each country and Europe went through in there amount of warmth- or cold-loving species. The number of warmth-loving species had significantly increased over the past 15 years, but with climate change rising in temperature faster than these butterflies can adapt (Van Swaay et al. 2010b). Overall, the number of butterflies is therefore declining.
Irish Butterfly Monitoring Scheme contributed data to both indicators, with nine butterfly species for the grassland butterfly indicator (Van Swaay & Van Strien 2008) and all butterflies species for the climate change indicator (Van Swaay & Warren 2012). However, data from some butterfly species should be treated with caution.
In this study, the butterfly species that showed a correlation between temperature and their abundance were: Common Blue (Polyommatus icarus), Green-veined White (Pieris napi), Meadow
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Brown (Maniola jurtina), Silver-washed Fritillary (Argynnis paphia), Small Copper (Lycaena phlaeas), Small Tortoiseshell (Aglais urticae), Small Heath (Coenonympha pamphilus), Small White (Pieris rapae) and Speckled Wood (Pararge aegeria). These nine species responded mainly to temperature changes and are therefore best suited for bioindicators, such as the grassland indicator or climate change indicator.
No strong association with the temperature was found for both Peacock (Inachis io) and Ringlet (Aphantopus hyperantus). This association might be strengthened when more years are added to the trend analysis. Other studies found or suggested a relationship for both Peacock (I. io) and Ringlet (A. hyperantus) with weather, so using their data for a bioindicator is still recommended (Asher et al. 2001; Pollard 1988). An association between the temperature and the abundance of Wood White (L. juvernica) was found in this study, but more information on the relationship between the abundance and other factors is still needed. Including data of Wood White (L. juvernica) in a bioindicators is still recommended, but could be changed when other overwriting factors are found.
Species that were influenced by other factors such as parasitism were Holly Blue (Celastrina argiolus) and Large White (Pieris brassicae). Data of Holly Blue (C. argiolus) should be excluded from a bioindicator, because the primary factor influencing their abundance is parasitism instead of the climate. Including their data could underestimate the effect climate change has on butterfly abundances and would therefore hamper our understanding. Large Whites’ (P. brassicae) decline and fluctuations are more associated with pesticide usage, intensification of agriculture and parasitism than with changes in temperature (Asher et al. 2001). Exclusion of data from Large White (P. brassicae) from a bioindicator is therefore recommended.
In summary, the abundances of butterfly species Holly Blue (C. argiolus) and Large White (P. brassicae) are mainly affected by parasitism and should be interpreted with caution when using their data in bioindicators that relate abundances with climate change. Both species could potentially underestimate the effect climate change has on butterfly abundance and should therefore be excluded from contribution from a climate change indicator.
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5. Conclusions and recommendations
This study found that the majority of the common butterflies in Ireland are in a decline over the five year time period or had a significant drop in abundance between 2011 and 2012. Similar declines in butterfly abundance were seen in both British and Dutch studies. The primary reason for their decline in Ireland appears to be linked to temperature changes, with a significant temperature drop in 2012. Both Britain and the Netherlands linked their declines towards climate change and habitat loss, but no such conclusions can be made in Ireland because of the five-year time period.
Total butterfly abundance peaked in 2010 and was at its lowest in 2012. The total butterfly abundance followed a similar pattern as the mean temperature, although not significantly correlated. However, multiple studies found a correlation between the butterfly abundance and temperature. In total, nine species showed an association with temperature over the five-year time period. Rainfall also influenced the intensity of fluctuations for some species, but would often contradict with conclusions from former studies. One species of butterfly (Orange Tip (Anthocharis cardamines)) had an association with the spring temperature and two species (Peacock (Inachis io) and Ringlet (Aphantopus hyperantus)) could potentially show an association with the temperature when more years are added. Butterfly species Holly Blue (Celastrina argiolus) and Large White (Pieris brassicae) may be more affected by parasitism than temperature. No major factor influenced the abundance of Wood White (Leptidea juvernica), but seemed to correspond with the trend of Speckled Wood (Pararge aegeria), which is influenced by weather conditions (temperature and rainfall).
In this study, the abundance of double-brooded butterfly species was more correlated with temperature than the abundance of single-brooded species. This is consistent with other studies that gave as explanation that double-brooded species are more depended on the temperature. Double- brooded species get the opportunity to lay a second brood when temperatures are high early in the year, but also decline much faster than single-brooded species when temperatures drop significantly.
For all butterfly species the models were rejected during the trend analysis. Switching serial correlation on and off didn’t affect the standard errors by any significant amount. However, rejection of the models still gives reliable indices and therefore population trends. Model rejection converts the standard errors into bigger numbers. The standard errors should be taken into account when making conclusions from the trends.
Using five years for trend analysis resulted in a high number of uncertain species (6 out of 15). Adding more years should result in less uncertain species. Ten years is seen as a benchmark for reliable trends, but trends for a five-year time period can still give early insight into the abundance of butterfly species. Irish trends for at least three butterfly species (Holly Blue (C. argiolus), Orange Tip (A. cardamines) and Peacock (I. io)) showed similar fluctuations with more reliable trends from Britain.
Most butterfly species’ abundances had an association with temperature and therefore contribute quality data for the butterfly climate change indicator, as used and assessed by the EU to hold biodiversity loss in 2020. However, the abundances of butterfly species Holly Blue (C. argiolus) and
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Large White (P. brassicae) are mainly affected by parasitism and should be interpreted with caution when using their data in bioindicators that relate abundances with climate change. Both species could potentially underestimate the effect climate change has on butterfly abundance and should therefore be excluded from contribution from a climate change indicator.
Our recommendations for future studies are:
1. Butterfly Monitoring Schemes should run for at least ten years to get reliable population trends and statuses, but a five-year time period is seen as reliable and can give an early insight into butterfly abundances; 2. At least 25 transect routes per butterfly species is needed to get good estimates for population trends; 3. The changes in abundances of butterfly species knowingly affected by parasitism should be interpreted with caution when used in bioindicators that use abundances as information. 4. Further research is needed on the effect of parasitism on butterfly species abundances, on Wood White (L. juvernica) with a focus on effects influencing their abundance and on the relationship between food plant availability and butterfly species abundances.
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7.1 Appendix I
The Irish butterfly species and number of sites at which they were recorded in the Irish Butterfly Monitoring Scheme (orange) and number of butterfly species recorded by Northern Ireland Butterfly Conservation (red) both in 2012. Every butterfly species below the benchmark of 25 transect routes is excluded from further trend analysis (grey). The butterfly species Red Admiral (Vanessa atalanta) and Painted Lady (Vanessa cardui) were excluded even though they were above the 25 transect limit, because they are migratory species and don’t form site specific populations as the rest of the butterfly species.
0 25 50 75 100 125 150
Speckled wood (Pararge aegeria) 130 12 Meadow brown (Maniola jurtina) 126 11 Green-veined white (Pieris napi) 124 13 Ringlet (Aphantopus hyperantus) 124 14 Large white (Pieris brassicae) 110 12 Orange-tip (Anthocharis cardamines) 106 11 Small tortoiseshell (Aglais utricae) 106 13 Peacock (Inachis io) 104 9 Small white (Pieris rapae) 104 11 Red admiral (Vanessa atalanta) 84 2 Common blue (Polymmatus icarus) 60 7 Small heath (Coenonympha pamphilus) 50 8 Small copper (Lycaena phlaeas) 43 6 Holly blue (Celastrina argiolus) 36 1 Silver-washed fritillary (Argynnis paphia) 34 2 Wood white (Leptidea juvernica) 34 5 Painted lady (Vanessa cardui) 31 1 Brimstone (Gonepteryx rhamni) 23 0 Dingy skipper (Erynnis tages) 15 1 Wall brown (Lasiommata megera) 15 0 Grayling (Hipparchia semele) 14 3 Green hairstreak (Callophrys rubi) 14 3 Gatekeeper (Pyronia tithonus) 12 0 Dark green fritillary (Argynnis aglaja) 11 5 Small blue (Cupido minimus) 9 0 Marsh fritillary (Euphydryas aurinia) 6 1 Large heath (Coenonympha tullia) 6 0 Comma (Polygonia c-album) 5 0 Pearl-bordered fritillary (Boloria euphrosyne) 5 0 Essex skipper (Thymelicus lineola) 2 0 Brown hairstreak (Thecla betulae) 1 0 Clouded yellow (Colias croceus) 1 0 Purple hairstreak (Quercusia quercus) 0
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7.2 Appendix II
All fifteen common Irish butterfly species that have been used for trend analysis with their corresponding flight period and if they are single-brooded (light orange) or double-brooded (dark orange).
Butterfly species Single or double Week of the year Commentary brooded (first Monday and ending Sunday) Common Blue Double brooded 13 - 29 (Polyommatus icarus) Green-veined White (Pieris Double brooded 13 - 25 napi) Holly Blue (Celastrina Double brooded 13 - 25 agriolus) Second generation Large White (Pieris included, because of the Double Brooded 13 - 39 brassicae) low numbers in the first generation Meadow Brown (Maniola Single brooded 13 - 39 jurtina) Orange Tip (Anthocharis Single brooded 13 - 39 cardamines) Second generation included, because of the Peacock (Inachis io) Double brooded 13 - 39 low numbers in the first generation Ringlet (Aphantopus Single brooded 13 - 39 hyperantus) Silver-washed Fritillary Single brooded 13 - 39 (Argynnis paphia) Second generation Small Copper (Lycaena included, because of the Double brooded 13 - 39 phlaeas) low numbers in the first generation Small Heath Single brooded 13 - 39 (Coenonympha pamphilus) Second generation Small Tortoiseshell (Aglais included, because of the Double brooded 13 - 29 urticae) low numbers in the first generation Second generation included, because of the Small White (Pieris rapae) Double brooded 13 - 39 low numbers in the first generation Second generation Speckled Wood (Pararge included, because of the Double brooded 13 - 29 aegeria) low numbers in the first generation Wood White (Leptidea Single brooded 13 - 39 juvernica)
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7.3 Appendix III
Trend analysis on nine species of butterflies in the time period of 2007-2011 for the European Biodiversity Indicator of Butterflies. This was done in preparation for the study of the Grassland Indicator (Van Swaay & Van Strien 2008). This functions as a short over few of what this study will gather in the near future and for what has already been done.
Species Trend in Ireland Common Blue Strong increase Dingy Skipper† Moderate decline Marsh Fritillary* Steep decline Meadow Brown Strong increase Orange Tip Strong increase Small Blue* Strong increase Small Copper Strong increase Small Heath† Strong increase Wall Brown* Steep decline
*Listed as threatened with extinction in the Red List. †Listed as near threatened in the Red List.
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Common Blue
Population trend: strong increase
4
3
2
1
0 2007 2008 2009 2010 2011
Dingy Skipper
Population trend: moderate decline
1.2 1 0.8 0.6 0.4 0.2 0 2007 2008 2009 2010 2011
Marsh Fritillary
Population trend: steep decline
1.4 1.2 1 0.8 0.6 0.4 0.2 0 2007 2008 2009 2010 2011
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Meadow Brown
Population trend: strong increase
6 5 4 3 2 1 0 2007 2008 2009 2010 2011
Orange Tip
Population trend: strong increase
2500
2000
1500
1000
500
0 2007 2008 2009 2010 2011
Small Blue
Population trend: strong increase
3.5 3 2.5 2 1.5 1 0.5 0 2007 2008 2009 2010 2011
Small Copper
Population trend: strong increase
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4
3
2
1
0 2007 2008 2009 2010 2011
Small Heath
Population trend: strong increase
2.5
2
1.5
1
0.5
0 2007 2008 2009 2010 2011
Wall Brown
Population trend: steep decline
1.2 1 0.8 0.6 0.4 0.2 0 2007 2008 2009 2010 2011
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7.4 Appendix IV
The population trends of 15 common Irish butterfly species over the time period of 2008 – 2012.
Population trend of Common Blue (Poloymmatus icarus) between 2008 - 2012 in Ireland 1.2
1
0.8
0.6 Index
0.4
0.2 Strong Increase 0 2008 2009 2010 2011 2012
Population trend of Meadow Brown (Maniola jurtina) between 2008 - 2012 in Ireland 1.2
1
0.8
0.6 Index
0.4
0.2 Strong Increase 0 2008 2009 2010 2011 2012
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Population trend of Small Tortoiseshell (Aglias urticae) between 2008 - 2012 in Ireland 2.5
2
1.5 Index 1
0.5 Moderate increase 0 2008 2009 2010 2011 2012
Population trend of Holly Blue (Celastrina agriolus) between 2008 - 2012 in Ireland 2.5
2
1.5 Index 1
0.5 Moderate Decline 0 2008 2009 2010 2011 2012
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Population trend of Small White (Pieris rapae) between 2008 - 2012 in Ireland 2.5
2
1.5 Index 1
0.5 Moderate Decline 0 2008 2009 2010 2011 2012
Population trend of Speckled Wood (Pararge aegeria) between 2008 - 2012 in Ireland 2 1.8 1.6 1.4
1.2 1 Index 0.8 0.6 0.4 0.2 Moderate Decline 0 2008 2009 2010 2011 2012
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Population trend of Wood White (Leptidea juvernica) between 2008 - 2012 in Ireland 2 1.8 1.6 1.4
1.2 1 Index 0.8 0.6 0.4 0.2 Moderate Decline 0 2008 2009 2010 2011 2012
Population trend of Large White (Pieris brassicae) between 2008 - 2012 in Ireland 2 1.8 1.6 1.4
1.2 1 Index 0.8 0.6 0.4 0.2 Steep Decline 0 2008 2009 2010 2011 2012
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Population trend of Peacock (Inachis io) between 2008 - 2012 in Ireland 1.8 1.6 1.4 1.2
1
Index 0.8 0.6 0.4
0.2 Steep Decline 0 2008 2009 2010 2011 2012
Population trend of Green-veined White (Pieris napi) between 2008 - 2012 in Ireland 1.8 1.6 1.4 1.2
1
Index 0.8 0.6 0.4
0.2 Uncertain 0 2008 2009 2010 2011 2012
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Population trend of Orange Tip (Anthocharis cardamines) between 2008 - 2012 in Ireland 3.5
3
2.5
2
Index 1.5
1
0.5 Uncertain 0 2008 2009 2010 2011 2012
Population trend of Ringlet (Aphantopus hyperantus) between 2008 - 2012 in Ireland 1.6 1.4 1.2 1
0.8 Index 0.6 0.4 0.2 Uncertain 0 2008 2009 2010 2011 2012
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Population trend of Small Copper (Lycaena phlaeas) between 2008 - 2012 in Ireland 2.5
2
1.5 Index 1
0.5 Uncertain 0 2008 2009 2010 2011 2012
Population trend of Small Heath (Coenonympha pamphilus) between 2008 - 2012 in Ireland 1.8 1.6 1.4 1.2
1
Index 0.8 0.6 0.4
0.2 Uncertain 0 2008 2009 2010 2011 2012
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Population trend of Silver-washed Fritillary (Argynnis paphia) between 2008 - 2012 in Ireland 2.5
2
1.5 Index 1
0.5 Uncertain 0 2008 2009 2010 2011 2012
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7.5 Appendix V
All of the 144 transect routes for multi-species monitoring done by volunteers in the Republic of Ireland in 2012 (Regan 2012).
All Species
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7.6 Appendix VI
The results TRIM gave as output after running the time effects model on data for the Common Blue (Polyommatus icarus). The time indices (blue) are used as graph input for the trend for each butterfly species. The Overall Slope (recommended) (green) states what the status of the butterfly is and the Multiplicative (yellow) underneath gives the annual change in abundance.
TRIM 3.53 : TRend analysis and Indices for Monitoring data STATISTICS NETHERLANDS
Date/Time: 14/03/2013 12:01:22
Title : COMMON BLUE.
The following 4 variables have been read from file: I:\TRIM\COMMON BLUE.txt
1. Site number of values: 109 2. Time number of values: 5 3. Count missing = -1 4. weight
Number of sites without positive counts (removed) 2
Number of observed zero counts 75 Number of observed positive counts 193 Total number of observed counts 268 Number of missing counts 277 Total number of counts 545
Total count 5217.0
Sites containing more than 10% of the total count Site Number Observed Total % 275 2105.0 40.3
Time Point Averages Weighted Weighted TimePoint Observations Average Index Average Index 2008 21 18.71 1.00 3.05 1.00 2009 33 8.18 0.44 1.53 0.50 2010 72 21.18 1.13 3.17 1.04 2011 68 22.79 1.22 3.19 1.05 2012 74 19.99 1.07 3.54 1.16
RESULTS FOR MODEL: Effects for Each Time Point ------
WEIGHTING = On
ESTIMATION METHOD = Generalised Estimating Equations
Total time used: 4.63 seconds
Estimated Overdispersion = 3.461 Estimated Serial Correlation = -0.206
GOODNESS OF FIT Chi-square 536.42, df 155, p 0.0000 Likelihood Ratio 599.37, df 155, p 0.0000 AIC (up to a constant) 289.37
WALD-TEST FOR SIGNIFICANCE OF DEVIATIONS FROM LINEAR TREND Wald-Test 32.04, df 3, p 0.0000
PARAMETER ESTIMATES
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Parameters for Each Time Point
Time Additive std.err. Multiplicative std.err. 2008 0 1 2009 -0.4622 0.1680 0.6299 0.1058 2010 0.4678 0.1140 1.5965 0.1820 2011 0.4224 0.1158 1.5256 0.1767 2012 0.5416 0.1178 1.7188 0.2024
Linear Trend + Deviations for Each Time
Additive std.err. Multiplicative std.err. Slope 0.1968 0.0255 1.2175 0.0311
Time Deviations 2008 0.1997 0.0677 1.2210 0.0826 2009 -0.4594 0.1006 0.6317 0.0635 2010 0.2739 0.0581 1.3151 0.0763 2011 0.0317 0.0497 1.0322 0.0513 2012 -0.0459 0.0395 0.9552 0.0377
Time INDICES, Base is Time 2012 Time Model std.err. Imputed std.err. 2008 0.5818 0.0685 0.6228 0.0772 2009 0.3665 0.0492 0.3960 0.0560 2010 0.9289 0.0735 0.9612 0.0880 2011 0.8876 0.0740 0.9348 0.0887 2012 1 1
TIME TOTALS Time Model std.err. Imputed std.err. 2008 254 27 255 27 2009 160 20 162 21 2010 405 26 394 27 2011 387 26 383 27 2012 436 28 410 30
OVERALL SLOPE MODEL: Strong increase (p<0.01) ** Additive std.err. Multiplicative std.err. 0.1968 0.0255 1.2175 0.0311
OVERALL SLOPE IMPUTED (recommended): Strong increase (p<0.01) ** Additive std.err. Multiplicative std.err. 0.1806 0.0268 1.1979 0.0321
Fitted and imputed values written to file: I:\TRIM\COMMON BLUE.F1 Slopes and indices written to file: I:\TRIM\COMMON BLUE.S1
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7.7 Appendix VII
The long-time trends of Britain and the short-time trends of Ireland were compared to look for similarities and to verify how the Irish trends are doing compared to long term datasets. At least three common Irish butterflies showed similarities in trends with Britain. The butterfly species are Holly Blue (Celastrina argiolus), Peacock (Inachis io) and Orange Tip (Anthocharis cardamines).
Holly Blue (C. argiolus) fluctuated in an entirely different pattern than the rest of the other 14 species analysed for Ireland. High fluctuations have also been reported in the British trend, with a focus on the time period between 1988 and 1994 (See figure 7.7.1). Both trends show similar high fluctuations, although they didn’t have a same pattern on a year-to-year basis. For example, both trends dropped between 2008 and 2009 in abundance and increased in numbers from thereon. Only Britain had its peak in 2010 and Ireland had its peak in 2011, showing that trends aren’t corresponding on a year-to-year basis but high fluctuations both occur in the trends.
3.5
3
2.5
2
Index 1.5
1
0.5
0 1976 1982 1988 1994 2000 2006 2012
Figure 7.7.1: The population trends of Holly Blue (Celastrina argiolus) from Britain (blue line) between 1976 and 2012 and Ireland (orange dotted line) between 2008 and 2012 (Mark Botham pers. comm.).
There are similarities between the Irish and British trend of Orange Tip (Anthocharis cardamines), although higher fluctuations are seen in the Irish trend compared to the British trend (See figure 7.7.2). Both trends were increasing between 2008 and 2011 and declined until 2012.
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Population trend of Orange Tip (Anthocharis cardamines) for Britain (1976-2012) and Ireland (2008 - 2012) 3.5
3
2.5
2
Index 1.5
1
0.5
0 1976 1982 1988 1994 2000 2006 2012
Figure 7.7.2: The population trends of Orange Tip (Anthocharis cardamines) from Britain (brown line) between 1976 and 2012 and Ireland (green dotted line) between 2008 and 2012 (Mark Botham pers. comm.).
The trends for Peacock (Inachis io) in both Britain and Ireland showed similarities in their fluctuations (See figure 7.7.3). Both increased between 2008 and 2009, with a peak in 2009, and then declined. The difference is that the Irish declined until 2011 and stayed stable between 2011 and 2012, whereas the British trend kept declining until 2012. However, both trends remain the best example for the suggestion of similarities between the butterfly abundances between Ireland and Britain.
2.5
2
1.5
Index 1
0.5
0 1976 1982 1988 1994 2000 2006 2012
Figure 7.7.3: The population trends of Peacock (Inachis io) from Britain (purple line) between 1976 and 2012 and Ireland (blue dotted line) between 2008 and 2012 (Mark Botham pers. comm.).
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