The phenology of the Cabbage White Butterfly the effect of climate change

Transcription

1 Cygnus (2012) 1:31-44 DOI , , , RESEARCH ARTICLE The phenology of the Cabbage White Butterfly the effect of climate change Ebonine Lacey Sital Bassan Thien Dai Vo Kate Ryan Received: 23 May 2012 / Accepted: 28 May 2012 Abstract Pieris rapae, or the Cabbage White Butterfly (CWB) is an indicator species listed by the ClimateWatch organisation. Events in the lifecycle of the species, such as oviposition (egg laying), are monitored by citizen scientists in an attempt to create a large data set. These observations, if accurate, may eventually detail the impact that climate change is having on the timing of the species lifecycle. This article hypothesised that an increase in temperature, especially during oviposition, will cause an increase in population of CWB in specific areas and cause earlier sightings. Both historical findings and current ClimateWatch data was compared, and was found to support the hypothesis. However due to the lack of variety and detail in the current data available, especially data that spanned the last decade, it is believed that as the database is expanded in proceeding years, future conclusions will be more reliable. A larger data set will more accureatly ascertain the role of climate change in these processes, rather than potentially, short-term temperature anomolies. Keywords phenology, climate change, indicator species, spatial distribution, citizen science, Cabbage White Butterfly 31

2 C per decade since the 1970 s, with the last decade being the warmest of the last millennium (Hulme and Sheard, 1999) 1 Introduction Climate change and the effect it may have on different species is a topical issue in present day societies. Global mean temperatures have increased at a rate of approximately

3 This study will utilise the available historical data, collected by previous studies of repute, as points of comparison to current data collated by ClimateWatch. ClimateWatch is a citizen science data collection program that uses data submitted by volunteers who do not necessarily have scientific training. This provides the basis for a wide spanning data set which includes numerous biological species and covers a greater geographical area than would be viably possible if the data was collected solely by scientists. Historical data, primarily gathered from peer reviewed journals and university studies of repute, will encompass historical timing of peak oviposition, spatial distribution densities and the typical number of eggs laid per plant. Although the ClimateWatch data-set is not as encompassing, the inclusion of broad historical data in this study provides scope for future comprehensive studies. Temperature trends corresponding to the timescales of the historical and present day data will be compared to draw conclusions about the causality of deviations over timing of life cycle events, if any, relative to changes in temperature. Since the ClimateWatch data set is restricted in its timescale, the findings from available peer-reviewed literature will be utilised to deduce whether climate change alters the Cabbage White Butterfly s seasonally timed events. It is hypothesised that if temperatures during life cycle milestone events for the Pieris rapae have increased, changes in spatial population distributions, species emergence and species reproduction will result. These changes include increases in spatial populations and their distributions, earlier emergence and reproduction, and earlier peak oviposition dates in conjunction with higher egg laying rates. It is believed that higher temperatures for longer durations provide the P. rapae a more favourable environment for egg laying, ideal oviposition timing, and more abundant suitable habitat for reproduction to occur. The realisation of these variables will account for demonstrated expanding population numbers through increased sightings. 2 Materials and Methods Global and Local Temperature Data A wide spanning data set was utilised for analysis with the purposes of identifying and establishing any evident trends in mean temperatures for both South Western Australia and globally. Available data from the Australian Bureau of Meteorology (BOM) was analysed in graphical form. Research into alternate climatic measurements was conducted to derive a comparison metric for the BOM data through analysis of NASA Goddard Institute for Space Studies (GISS) measurements presented by waclimate.net (2012). Global climatic data was collected from the Climate Research Unit, UK (2012), with further findings relating to hemisphere specific climatic data derived from the research of Jones et. al ( ). 33

4 Historic Data Acquisition Despite the presence of ClimateWatch s geographically specific data, for the purposes of this study further analysis was conducted of past research into the phenology of the P. rapae. This analysis includes results spanning several sets from previous years that can be used to historically compare and contrast current data, in order to extrapolate a knowledgeable conclusion. Data was utilised from studies conducted in several locations around the world. The primary justification for comparison of global findings to local findings is the widespread global distribution of the species. Accordingly it is appropriate to selectively employ Spanish, American and East Australian data for this study due to the similarities of the Mediterranean and West Australian climates along with the available South West Australian data. The use of global comparison data has been approved by ClimateWatch. Methods for recent data analysis The results are obtained from raw data, mostly collected in Australia between October 2009 and February 2012, which has been collated on the ClimateWatch website. This data was provided by citizen scientists; volunteers who often have no scientific qualifications or experience with indicator species and data collection. Although the data collected by nonscientist volunteers may result in the occurrence of few outliers, the geographically encompassing nature of the data sets provides value for this study. Important information such as population numbers, density, location and egg laying has been recorded on a monthly basis and these are the figures used to test the hypothesis. Data on spatial distribution is acquired through the recorded location of the specie. In order to investigate the effect of increasing temperature on P. rapae, it is essential to have data on mean temperature at a corresponding time and place to the specie data. Therefore monthly mean temperatures between October 2009 and February 2012 recorded from Perth Airport weather station have been obtained from the Bureau of Meteorology webpage. In order to make accurate observations on the two data-sets, the data on mean temperatures and on the Cabbage White Butterfly are plotted to produce graphs. For the spatial distribution of the butterfly, the data has been made into a map which can be use to compare with the map of Trend in Mean Temperature. The findings from the graphs and the maps are then used to write the Current Findings part of this Journal. 34

5 3 Results Historical findings - climatic changes The mean annual temperatures of the Perth metropolitan region exhibit and upwards trend indicating a steadily rising climate. The mean temperature maximums year on year (Fig. 1) displays a trend of higher mean temperatures compared to the previous year indicating steadily increasing temperatures from Annual Mean Temperature ( C) Year y = x R² = "Mean Annual Temperature" Fig. 1. A Trend of Increasing Mean Temperatures Present, Perth Airport Weather Station (BOM, 2012) Historical findings spatial distribution The historical average species density for P. rapae in the metropolitan South West Australian region has been 17.1 individuals per km in autumn and 21.6 individuals per km in Spring- Summer, accounting for new births (Williams, 2009). The probability of sighting the Cabbage White Butterfly in South Western Australia has been historically highest in the October period (Table 1), coinciding with the expected initial emergence of adults prior to commencement of breeding. Table 1. Estimated species detectability (p) of each species in each sampling period. Blank entries indicate p = 0 (Williams, 2009) Detectability Within Each Time Period Species Year Sept 1-15 Oct Oct 1-15 Nov Nov 1-16 Dec 27 Feb - 14 Mar 15 Mar - 17 Apr Pieris rapae 2002/ / /

6 Historical findings oviposition timing The historical expected timing of the two peak ovipositions was determined to occur in the November December and January February periods (Fig. 2) during the 1980 s by Jones et al. (1987). Across the 10 sample sites, expected peak oviposition times of November December and January February were observed to be accurate over the span of the 3 year study. A trend that is a useful historical metric with mean peak oviposition rates of between 6-25 eggs laid per plant per day was established. An outlier group was excluded (the Black Mountain group) due to unexplained predation in the site resulting in a high variability in eggs counted deeming it statistically invalid for use. Fig. 2. Average oviposition rates during expected peak months in 1986 (from Jones et al., 1987) Correlation between Climate and Phenology Gordo et al. (2010) determined in their Spanish study, that temperature and altitudinal spatial gradients account for the majority of spatial variation in the phenology of the P. rapae. Vegetation productivity and land use were found to have a low relevance to the phenology of the species. The earlier emerging adults recorded in each breeding season were found in areas that had warmer springs and dry summers, with a correlation between increasing warmth in spring with dry summers and earlier initial adult numbers recorded ultimately resulting in earlier and prolonged breeding (Fig. 3). These areas are the southern coastal areas with climatic similarity to South Western Australia. The distribution of the phenological records 36

7 across Spain depicts two distinct climatic zones: the warmer southern areas and the cooler northern mountainous regions. The northern areas exhibit later appearances of adults in comparison to the southern, and a delay of almost one month was observed between egg laying in the southern coastal areas and the northern mountainous regions (Fig. 4). The northern region yielded lower adult and egg sightings (Fig. 5). Fig. 2. (B): P. rapae appearance by minimum Spring temperature and date (C): P. rapae sightings by Climate Zone, higher latitude indicate the cooler climate region and (D): Appearance by precipitation. Solid lines depict the best polynomial fitted model. The method of mapping used is the Universal Transverse Mercator (UTM) method. This method has proven to be an accurate mapping measurement in geographically narrow zones (from Gordo et al., 2010). 37

8 Fig. 3. (B): Maps of the distribution of phenological records of the appearance dates of Pieris rapae Spain. Each square represents a UTM with phenological records, and its colour depicts the mean date. Scale colour bar is in Julian days (eg. 1 = 1 January). The total number of records, localities, and UTM together with the mean value and the standard deviation (SD) of all records are also specified. A histogram with the distribution of mean dates per UTM is also shown (scale of x-axis in Julian days). (Gordo et al., 2010) Fig. 4. Map of the appearance dates for P. rapae eggs in Spain according to the best complete models. Scale colour bar in Julian days (Gordo et al., 2010) Current findings - climatic change The mean temperatures recorded from Perth Airport weather station over the period from Sep 2009 to Feb 2012 fluctuates seasonally (Fig. 5). The warmest month was January or February with highest mean temperature of 35 C. The lowest mean temperature was recorded as 18.3 C in June on July. Overall, the temperature during the period investigated, appears stable and typical to Australian climate. 38

9 Oct-09 Dec-09 Feb-10 Apr-10 Jun-10 Aug-10 Oct-10 Dec-10 Feb-11 Apr-11 Jun-11 Aug-11 Oct-11 Dec-11 Feb-12 Mean Temperature (oc) Mean Temperature (oc) Months Fig. 5. Mean temperatures from Sep 2009 to Feb 2012 from Perth Airport weather station. Current findings - temporal distribution The analysis of recent results shows that the population of CWB rises significantly toward the end of the year, particularly from September to December. When comparing data for climate change against data for number of butterflies present (Fig. 7), it can be observed that the increase in population of P. rapae corresponds with the months October to February in Australia. For example, there was a peak in population, within the three-year data set ( ), of around 800 in October 2011, which had the mean temperature of 24.4 C. Moreover, there is another slight increase in the population numbers of CWB in January 2011 which had the mean temperature of 33.7 C. During June to August 2010 and April to July 2011, which have mean temperatures as low as 18.3 C, there was no presence recorded for the specie. Mean Temperature (oc) Number of present Mean Temperature (oc) Number of present 0 0 Oct-09 Jan-10 Apr-10 Jul-10 Oct-10 Jan-11 Apr-11 Jul-11 Oct-11 Jan-12 Months Fig.7. Presence of P. rapae vs. Mean temperatures from Sep 2009 to Feb 2012 from Perth Airport weather station. 39

10 Current findings - spatial distribution The map of distribution, (Fig. 8), demonstrates areas where the Cabbage White Butterfly was sighted and believed to inhabit. It shows that a large portion of the population is distributed along the coast; mostly on the South West and Eastern coastline of Australia, correlating with capital cities or areas with a large human populace. However, there are also some CWB populations located in the central Northern Territory and north Western Australia, inland of Exmouth and Carnarvon. The areas where the P. rapae are distributed have an average increase in mean temperature trend of C/10 years (Fig 9). Fig.8. Spatial distribution of P. rapae in Australia (Atlas of Living Australia) Fig. 9. Trend in mean Temperature ( C/10yrs) (Australian Bureau of Meteorology) 40

11 Current findings oviposition Overall oviposition records were quite low, with a maximum of only 10 egg laying butterflies sighted in January and September 2011 (figure 9). Figure 8 suggests that the oviposition date of CWB is in January 2011 with 10 egg laying recorded, which corresponds with the increase in temperature at that time. The next time of oviposition was recorded in September 2011with 10 egg laying records as well, with a mean temperature reaching 20.5 C. The oviposition data supplied only covers one year Mean Temperature (oc) Egg laying Mean Temperature (oc) 0 0 Oct-09 Dec-09 Feb-10 Apr-10 Jun-10 Aug-10 Oct-10 Dec-10 Feb-11 Months Apr-11 Jun-11 Aug-11 Oct-11 Dec-11 Feb-12 Fig. 10. Egg laying rate of P.rapae vs mean temperatures from Sep 2009 to Feb 2012 from Perth Airport weather station Egg laying Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 2011 Months Fig. 9. Oviposition dates in 2010 and

12 4 Discussion The depth and range of data analysed by Gordo et al. (2010) provides insight into the phonological and biogeographical responses to climate in Spain. The Mediterranean southern coastal areas are similar in terms of climate to South Western Australia. Accordingly, their findings provide a point of comparison and an expected outcome for the study of climatic response in South West Australian P. rapae populations. The study s results imply that Southern Spain s earlier warmer conditions with longer durations result in earlier and longer breeding during each oviposition period in comparison to consistently cooler Northern climates, which did not exhibit significant phenological variation over the study period. Williams (2009) study identifies patterns of emergence for the Cabbage White Butterfly. The sightings exhibit decline approaching April, then again peak in October for the subsequent breeding period. It can therefore be deduced that historic peak breeding periods in South Western Australia occur typically in October. Jones et al. (1987) determined that peak oviposition occurred during the November- December and January-February time periods. Their determination of mean peak oviposition rates of between 6-25 eggs laid per plant per day is a useful comparison that can be utilised by future studies to determine present day mean peak oviposition data with the aim of determining causality of any variations if present. Present day data showing that the presence of CWB peaks in September/October 2011 (Figures 5, 8 & 9) is inconsistent with the Jones et al. (1987) expected peak during the 1980 s of November/ December. The peak in October 2011 follows a warmer than usual end to the 2011 winter where both maximum and minimum temperatures were higher than average (BOM, 2012). The present data is therefore consistent with the Spanish findings, whereby warmer conditions can initiate the onset of earlier sightings, and it s notable that the changes in appearances can be attributed to the changes in climate. While the use of citizen science data is not commonplace, the range of data collected extends beyond that normally attained in studies limited by geography, funding and time. Questions remain whether citizen science data, collected by amateur or non-professional scientists, yields accurate observations compared to data collected by experts, however the benefits outweigh the potential for minor inaccuracies. Programmes such as ClimateWatch provide detailed guidelines to help minimise inaccurate data recordings, and ultimately the data is still analysed by experts in the field. Although ClimateWatch has provided potential indicator data that can be utilised, this study has identified some shortcomings in the ClimateWatch data set, namely the breadth of data observed. If there were further points of comparison to historical data and broader fields of observation, a more definite conclusion could be derived regarding the influence of climate change on this species. Conclusively, despite the setbacks resulting from the citizen science approach to data collection, the changes in climate have affected the phenology of the Cabbage White 42

13 Butterfly. Specific findings from the present day data support our hypothesis that temperature increases during life cycle milestone events for the Pieris rapae will result in changes to spatial population distributions, species emergence and species reproduction. The primary finding in support of our hypothesis has been the determination that species emergence has an earlier onset in recent years with increased temperatures as evidenced through data analyses. It is our belief that although the effects of increased temperature on the P. rapae have been documented by Gordo et al. (2010) in the Spanish study, further accumulation of ClimateWatch data will allow for a more substantial data set providing scope for further future studies relating to local populations. Acknowledgements - The authors of this article would like to acknowledge Maggie Triska for donating her time for initial proof reading and feedback in the development of this study as well as the constructive comments received during a peer review by four BIOL1130 students and the overall editor Emile van Lieshout. ClimateWatch, the Atlas of Living Australia and the Australian Bureau of Meteorology must also be acknowledged as valuable sources of information. References Beaumont, L. J. & Hughes, L Potential changes in the distributions of latitudinally restricted Australian butterfly species in response to climate change. Global Change Biology, 8, Bom Annual Mean Temperature Anomaly - Western Australia. In: TMEAN_WA_0112_30303.PNG (ed.). Australian Bureau of Meteorology. Burt, J., Hoffman, H., Hardie, D. & Golzar, H Garden Note. In: FOOD, D. O. A. A. (ed.). Western Australia. Gordo, O., SANZ, J. J. & LOBO, J. M Determining the environmental factors underlying the spatial variability of insect appearance phenology for the honey bee, Apis mellifera, and the small white, Pieris rapae. Journal of Insect Science, 10. Ferguson, A. M White Butterfly Life Cycle [Online]. New Zealand: The Horticulture and Food Research Institute of New Zealand. Available: [Accessed 25 March ]. Field, R Cabbage White Butterfly Pieris rapae [Online]. Museum Victoria. Available: [Accessed 22 March ]. Franklin Brassica, Butterflies and Caterpillars. Bryn Mawr University. Hoffman, A. A. & Sgrò, C. M Climate change and evolutionary adaptation. Nature, 470, 7. Hulme, M. & Sheard, N Climate Change Scenarios for Australia. In: UNIT, C. R. (ed.). Norwich, UK: UEA. Jones, P. & Salmon, M Climatic Research Unit: Global Temperature. Climatic Research Unit, UK. Jones, P. D., Lister, D. H., Osborn, T. J., Harpham, C., Salmon, M. & Morice, C. P Hemispheric and large-scale land surface air temperature variations: an extensive revision and an update to Journal of Geophysical Research, 117. Jones, R. E., Nealis, V. G., Ives, P. M. & Scheermeyer, E Seasonal and Spatial Variation in Juvenile Survival of the Cabbage Butterly Pieris rapae: Evidence for Patchy Density-Dependence. Journal of Animal Ecology, 56, Merriam-Webster Phenology - Definition and More. The Merriam Webster Dictionary. Waclimate.Net Comparison of BOM and GISS Temperature Data. In: PERTH-AIRPORT- BOMHQ-GISS.JPG (ed.). waclimate.net. Williams, M. R Butterflies and Day-flying Moths in a Fragmented Urban Landscape, South-West Western Australia: Patterns of Species Richness. Pacific Conservation Biology, 15,