Comparative Population Ecology in Moor Frogs with Particular Reference to Acidity

Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 172

FREDRIK SÖDERMAN

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This thesis is based on the following papers, which will be referred in the text by their roman numerals.

I Söderman, F., Räsänen, K., Laurila, A. and Merilä, J. Compara- tive ecology of the moor frog (Rana arvalis) – acid versus neutral pH habitats. Manuscript

II Merilä, J., Söderman, F., O'Hara, R., Räsänen, K. and Laurila, A. 2004. Local adaptation and genetics of acid-stress tolerance in the moor frog, Rana arvalis Conservation Genetics, 5: 513-527.

III Räsänen, K., Söderman, F., Laurila, A. and Merilä J. Life-history evolution in stressful environments: effects of acidification on maternal investment in Rana arvalis. Submitted manuscript.

IV Söderman, F., Räsänen, K., Castanet, J., Petersson, E., Laurila, A. and Merilä, J. Sex specific responses to environmental stress, shape growth strategies in Rana arvalis. Manuscript.

V Söderman, F., van Dongen, S., Pakkasmaa, S. and Merilä, J. En- vironmental stress increases skeletal fluctuating asymmetry in the moor frog Rana arvalis. Submitted manuscript.

Paper II is reprinted with kind permission of Springer Science and Business Media.

Contents

Introduction...... 7 Objectives ...... 10 Material and Methods ...... 11 Study species...... 11 Study areas ...... 11 Morphological measurements ...... 12 Quantitative genetic design ...... 12 Age estimation...... 13 Fluctuating asymmetry...... 13 Results and Discussion ...... 15 Acid vs. neutral pH habitats – comparative ecology of the moor frog (I)15 Genetics of acid stress tolerance in early life stages (II) ...... 16 Maternal investment correlates with environmental acidity (III)...... 17 Among-population variation in age and growth in the moor frog (IV)....18 Developmental stability and acidity in moor frog populations (V)...... 20 Acknowledgements...... 22 References...... 23 Livet som sur groda ...... 27

Introduction

The evolution of life histories is a central theme in evolutionary biology, and diverse abiotic and biotic environmental factors have been identified as selective agents driving life history evolution (Roff 1992, Stearns 1992). Ad- aptations to environments are often compromised by trade offs where the individual have to be optimized in several traits and where the benefit is often connected to a certain cost. Adaptation to a local environment is one of the major evolutionary forces where spatial environmental heterogeneity (e.g. Endler 1986, Merilä & Crnokrak 2001, Gandon et al. 1996) and environmental stress in its various forms (1997, Linhart & Grant 1996, Reznick & Ghalamor 2001) have been identified as important selective forces creating local adaptation. Major traits showing adaptive variation are for example, growth and the development rate of individuals with the direct effects on fecundity, mating success (Andersson, 1994) and age at maturity (Roff, 1992), and maternal investment with the trade off between the fitness of the female and the fitness of her offspring (e.g. Smith & Fretwell 1974, Roff 1992, Hendry et al. 2001). Environmental stress caused by anthropogenic changes to the environment is a major contributor to contemporary evolution (Palumbi 2001, Reznick & Ghalambor 2001). Example of such worldwide environmental stressors in- clude pollution (e.g. Forbes & Calow 1997) and climatic change (e.g. Ka- reiva et al. 1993, Bürger & Lynch 1997, Walther et al. 2002), and develop- ing an understanding of how natural animal populations will respond to an- thropogenically caused environmental changes has become one of the major challenges in conservation and evolutionary biology. For example, while there are many examples of evolutionary response to environmental stress, its effects on life history evolution are often poorly known. Studies on large- scale climatic variation show that temperature and seasonal time constraints may cause strong selection on maternal investment. Altitudinal and latitudinal clines in egg size and fecundity is found in many ectotherms (e.g. Ber- ven & Gill 1983, Fleming & Gross 1990, Armbruster et al. 2001, Johnston & Leggett 2002). However, human activities are depleting atmospheric ozone levels. Pollutants are accumulating in the natural systems, weather patterns are changing and wildlife diseases are spreading. Such gradual but funda- mental changes can be expected to have an effect on the ecosystems. Several life-history traits and other factors are likely to vary with the environments harshness and carrying capacity and to be sources of variation in

7 body size adaptation. For example, Bergmann’s rule, originally developed for endotherms, implies body size to be larger at higher latitudes and altitudes due to the decreased temperature loss with a larger size at a colder climate. Several studies have showed similar results for ectotherms with body size increasing towards higher latitudes and altitudes, conforming a Bergmann’s cline (Lindsey 1966, Van Voorhies 1996, Atkinson & Sibly 1997) but the opposite has also often been found (Mousseau 1997, Ashton 2002, Ashton & Feldman 2003). Another way to look at the environments stressfulness for the organism is to study fluctuating asymmetry (FA: small random deviations from perfect symmetry, Møller & Swaddle 1997). However fluctuating asymmetry is a controversial indicator of stress. There is a wealth of studies showing an increase in FA with stress, but many others fail to find any response (reviews in: Leung & Forbes 1996, Møller & Swaddle 1997, Møller & Alatalo 1999, Lens et al. 2001). Although very simple in principle, the general use of FA as a bio monitoring tool is hampered by the lack of knowledge about the factors that predict if and when an association between FA and stress can be expected. Over the last 50 years, many species of amphibians (frogs, toads, salaman- ders and newts) throughout the world have declined markedly in numbers and some species have become extinct (Alford & Richards 1999, Stuart et al. 2004). In many instances, these declines are attributable to adverse human influences acting locally, such as deforestation, draining of wetlands, and pollution. There is also evidence that there are one or more global factors that are adversely affecting amphibians. Possible candidates for such influences are climatic and atmospheric changes, such as increased UV-B radia- tion, widespread pollution and diseases. Acidification, derived from both natural and anthropogenic sources, exposes a wide range of organisms to stressful conditions. The negative effects of low pH on survival, growth and development have been demonstrated in a diversity of organisms (Schindler 1988, Rusek & Marshall 2000). Longterm acidification is hence likely to induce significant directional selection, and there is some evidence of local adaptation to acidity in the aquatic fauna (reviewed in Räsänen et al. 2003b). Acidification as a consequence of acid rain has had a strong impact on freshwater ecosystems in the northern hemi- sphere and Fennoscandia in particular (Bertills & Hanneberg 1995). Due to acidification, many local fish populations in Scandinavia have gone extinct (e.g. Hesthagen et al. 1999), and also amphibian populations have suffered to some - but often unknown – degree (Böhmer & Rahmann 1990). Neverthe- less, there are also indications that some frog populations have been able to adapt to acidity in their environment, moor frogs (Rana arvalis Nilsson) from certain heavily acidified areas in Sweden have evolved a better tolerance towards low pH than populations from less acidified areas (Räsänen et

8 al. 2003a,b). However, little is known about the impacts of acidity on adult life histories in amphibians. Studies on amphibians inhabiting environments that vary in acidity provide a good model system for studies of life-history evolution in stressful environments. Natural environments vary greatly in acidity, hence exposing populations to different selective pressures. Acid environments are stressful as manifested in negative effects of low pH on survival, growth and development rates in a variety of organisms, including amphibians (e.g. Haines 1981, Rusek & Marshall 2000, Rowe & Freda 2000). In amphibians, acid stress can have strong physiological negative effects both on terrestrial adults (e.g. Brodkin et al. 2003) and the aquatic embryos and larvae (reviewed in e.g. Pierce 1985, Rowe & Freda 2000).

9 Objectives

In this thesis I studied life history variation in the moor frog with a special emphasis on the effects of acidification. The main objectives of this thesis were to investigate: 1. The variations in adult life history traits in Swedish moor frog populations. (Paper I). 2. The genetic basis of acid stress tolerance in moor frog embryos (II). 3. How female life history and maternal investment are shaped by acidification (III). 4. How age, size and growth of females and males vary in response to acidity and latitudinal gradients with special emphasis on sexual dimorphism (IV). 5. The usefulness of using fluctuating asymmetry in estimating levels of environmental stress in response to acidification (V).

10 Material and Methods

Study species The moor frog is a small anuran species with a broad distribution in northern and eastern Europe and Asia (Gasc et al. 1997, Kuzmin, 1999). In Sweden it occurs as far north as 66° N, and breeds in small lakes, ponds and bogs in agricultural and forested low altitude areas (Elmberg 1984, Gislén & Kauri 1959). It is known to be relatively tolerant to acidity (Leuven et al. 1986, Andrén et al. 1988) and can show local adaptation to acidity (Räsänen et al. 2003a,b). Moor frogs are explosive spring breeders, and breeding starts in late March or early April when water temperature has reached about + 10°C (Kuzmin 1999).

Study areas The studied Swedish moor frog populations are situated along a 1100 km latitudinal gradient from southern Scania (55°30’’N) to northern Västerbotten (65°15’’N; Fig. 1). In paper I, all locations were visited several times during the peak breeding period to get reliable estimates of female population size. Envi- ronmental variables recorded for each location were: altitude, latitude, water acidity, population size and pond size. The growth season (defined as the number of days with a daily mean temperature exceeding 5°C, data from SMHI, Swedish Figure 1. Map of Sweden showing the Meteorological and Hydrological location of the study populations. For the Institute) varied between 215 abbreviations see paper I. days in the south and 135 days in

11 the northernmost populations. Acidity was estimated with a Ross sure flow electrode 8165BN and an Orion 210A pH meter. The estimate is an average of four measures at 10 cm depth, 50 cm from the pond shore at the most northern, eastern, western and southern part of each pond. Measurements were done in the afternoon between 12.00 and 20.00. Among the localities, pH varied between 4.0 and 8.0, and altitude between 15 and 341 m a.s.l.

Morphological measurements From each adult, we noted the sex, measured the snout-vent-length (SVL) from the nose tip to the end of the urostyle (with a digital calliper to the nearest 0.1 mm), and recorded the fresh body weight (with an electronic balance to nearest 0.1 g). In paper II, III and IV, for each female, the number of eggs were counted, and egg size measured from photographic images. For the egg size measurements, 20-40 eggs per female (covered with water and illuminated from below) were photographed. A black-and-white negative image was then scanned with Agfa DuoScan (1000 dpi) and saved for analyses in the NIH image program (version 1.61, http://rsb.info.nih.gov/nih-image). Egg size was measured as the minimum and maximum diameter for each egg to the nearest 0.05 mm, which was averaged to get the egg diameter. For the assessment of FA in morphological traits (paper V), I used weights of bilateral bones instead of linear measurements because weights are likely to be less prone to measurement error than linear measurements (Karvonen et al. 2003). To prepare the skeletons, all individuals were skinned and cleaned by dermestid beetles, after which the bones were rinsed for four hours in 1% solution of Neutras® (Novozymes A/S), and then rinsed in water before drying. All bone elements were individually weighed with a AND ER-60A microbalance to the nearest 0.0001 g. To increase the accuracy of the weight estimates, weightings were made three times for each bone, ex- cept for the radio-ulna, which was weighted five times due to its small size, and hence, relatively low accuracy of weightings. All weightings were made blindly with respect to the individual identity, and repeated measurements of the same bones were temporarily separated. The bone elements where han- dled with forceps to avoid increasing variability in weights due to attached fat and moisture from fingers.

Quantitative genetic design To perform the controlled crosses required for the separation of different genetic and environmental components of variation in embryonic and hatch-

12 ling traits, adult frogs for the parental generation were collected from spawn- ing sites with dip-nets, and kept in 4qC until used in artificial fertilizations. Crosses were performed in a framework of North Carolina II design (Kearsey & Pooni 1996, Lynch and Walsh 1998). In brief, within each of the two populations we crossed three males with three females, and repeated this design using different sets of individuals (paper II), three times per locality.

Age estimation Skeletotochronology (Kleinenberg & Smirina, 1969, Castanet et al. , 1977), which is based on the cyclic growth pattern of bones, was used for age de- termination (III,IV). The bones were first decalcified in 2% nitric acid solution under constant stirring for about three hours. The cross sections were made with a Leica cryotom to 12µm slices. The sections were stained in Herlich haematoxilin for 6 minutes and rinsed in water three times before mounted on ordinary object glasses. The sections were photographed with a digital camera fitted to a microscope and age was determined from the digital images. A growth layer in amphibians consists of a dark band of bone tissue (MSG) bordered by a light resting line (LAG; Castanet et al., 1977). Next to the annual resting line, a dark line is also formed just after metamorphosis (Schroeder and Baskett, 1968). However, resorption of the periosteal bone can lead to a complete loss of the innermost LAG(s), and the method may, hence, underestimate age of individuals with LAG resorption. The number of missing resting lines was estimated for each individual with resorption using methods described in Hemelaar (1985) with some modifications (See paper IV).

Fluctuating asymmetry Individual asymmetry values for each trait were obtained from mixed regression models as described in van Dongen et al. (1999) and van Dongen (2000). These estimates are corrected for measurement error (ME) and directional asymmetry (DA). The individual asymmetry values are corrected for this apparent DA by subtracting the average degree of asymmetry from all individual values. We explicitly assume that for these traits, the mean degree of asymmetry reflects the optimal state, like perfect symmetry does for the traits without any directional component. Individual and trait-specific FA values were obtained as the empirical Bayes-estimates of the random slopes of the mixed regression models (see van Dongen et al., 1999 and van Don-

13 gen, 2000 for details). Correlations in signed and unsigned FA between traits were estimated. It has been suggested that individual FA values only crudely reflect the underlying developmental instability (Whitlock, 1996). Patterns in FA can be transformed into patterns in developmental instability using the so-called hypothetical repeatability. We followed Whitlock (1998) to achieve this.

14 Results and Discussion

Acid vs. neutral pH habitats – comparative ecology of the moor frog (I) In general, moor frogs were more sexually dimorphic in more neutral habitats (Fig. 2a) and significantly larger towards higher latitudes (Fig. 2b). Males were on average larger (mean SVL 55.04 ± 0.58 (SE)) than females (53.36 ± 0.60). 3b). The negative effects of acidification on amphibian embryonic and larval stages are well documented (Pierce 1993, Rowe & Freda Figure 2. Relationship between body size (SVL in 2000, Merilä et al. mm) of moor frogs and (a) pH and (b) latitude. Body 2004), but whether size is presented as mean square values of residuals from generalized mixed linear models with the actual these effects are also predictor removed from the model. In figure a. open reflected on adult life circles refers to females and filled to males. histories and population dynamics remain controversial. I found that males were gener- ally larger than females, but the sexual dimorphism decreased in acidified localities, suggesting less competition among males in the acidified environment. Fecundity, as reflected in the number Figure 3. Relationship between pond pH and egg size of eggs laid by individ- (a) and clutch size (b). Population mean values are ual females trades off shown. with egg size and the

15 results found are easiest explained as an adaptation to acid stress. These results are consistent with the interpretation that the negative effects of acidity observed on embryos and larvae are also reflected in adult life histories. The increased body size with latitude could be a result of either direct benefits of larger size or for example increased survival rate at higher latitudes, giving an increased gain with postponing first reproduction.

Genetics of acid stress tolerance in early life stages (II) Embryos from the population of acidic origin (AOP) were more tolerant to low pH than embryos from the neutral origin (NOP) (Fig. 4a), but there was no variation in developmental stability (Fig. 4b) or developmental rate (Fig. 4c). The additive genetic contribution to the variation was small while the maternal effects described a large part of the variation in all traits (Fig. 5).

Figure 4. Effects of the pH treatments on (a) survival probability of embryos from day 3 post-fertilisation to stage 25, (b) probability of being anomalous and (c) developmental time from fertilization to stage 25. All plotted values are posterior modes (±SD).

As is typical for traits closely associated with fitness (e.g. Houle 1992, Crnokrak & Roff 1995, Merilä and Sheldon 1999, 2000, Kruuk et al. 2000), all traits in both populations in this study had low heritabilities and high dominance contributions. Maternal effects have been recognized as being an important source of phenotypic variation in various traits in many taxa, (Mousseau & Fox 1998) including amphibians (review in Kaplan 1998). Here, we found that the influence of maternal effects were always larger than the additive genetic influences, and most of the time, larger or similar in magnitude as dominance effects. The results show that moor frog embryos from the AOP had a better tolerance to low pH in terms of survival than the embryos from the NOP.

16 Figure 5. Effects of pH treatments on (a) survival probability of embryos from fertilization to stage 25 b) survival probability of embryos from day 3 post- fertilization to stage 25, (c) probability of being anomalous, (d) developmental time from fertilization to stage 25, (e) mean size at stage 25 and (f) mean size at stage 25 corrected for developmental time in NOP and AOP. All plotted values are posterior modes (±SD). This suggests local adaptation to low pH, and in line with the earlier demon- strations, that acid origin embryos of amphibians (e.g. Gosner & Black 1957, Picker et al. 1993, but see: Pierce & Harvey 1987), including the moor frog (Andrén et al. 1989, Räsänen et al. 2003a,b), tolerate low pH better than the ones from neutral origin.

Maternal investment correlates with environmental acidity (III) When controlling for female body size and age, egg size was on average larger among females from acid (LS means ± SE: 1.230 ± 0.032) than from neutral (1.1730 ± 0.033) environments (Fig. 6). Females from acid environments produced on average fewer eggs than neutral originated females (acid: 956 ± 68, neutral: 1251 ± 70; Fig. 6). Overall, fecundity increased with fe-

17 male size, whereas female age did not contribute significantly to fecundity. Egg number increased with female age in the acidified populations, whereas it decreased with female age in the neutral populations. Female size was the main determinant of the total reproductive output, whereas age had only a marginally positive effect on this trait. The relatively larger egg size in the acid environment is most likely an adaptation for the females to maximize their fitness as large egg sizes has positive effects during larval development (Räsenen et al. 2005). Overall, females from the acid environment had a lower total reproductive output, indicating that the acid environment is more physiologically stressful (Wyman et al. 1987, Horne & Dunson 1994, Vat- nick et al. 1999; Fig. 7).

Figure 6. Mean (± S.E.) egg number Figure 7. Mean (± S.E.) total reproduc- and egg size in four acid (open squares) tive output in four acid (open bars) and and four neutral (solid circles) origin four neutral (solid bars) origin popula- populations of R. arvalis. arvalis. Val- tions of R. ues above error bars refer to sample size.

Among-population variation in age and growth in the moor frog (IV) Age of breeding individuals ranged from two to six years for both sexes; however, the mean age of the sexes interacted with acidity. On average, females from the acid and of the southern neutral populations were younger than males, whereas the opposite was found for the neutral northern populations (Fig. 8a). Individuals from the acid environments were smaller and less sexually dimorphic in body size (Fig. 8b). I also found a significant cohort effect suggesting that early growth conditions might have long-lasting effects on individual performance. Sex had a strong effect on growth, with males having a higher growth rate. A non-significant trend for acidity effect

18 Figure 8. Mean age (years ± SE) (a)., mean body size (SVL ± SE) (b). and mean individual LAG size residuals from the common growth curve (c). of female (grey diamonds) and male (black circles) moor frogs in relation to breeding pond pH. suggested reduced growth in acidified populations. The higher growth rate of males was more pronounced in the neutral than in the acid populations. There was a negative relationship on early growth and age at capture for both sexes and within both acid and neutral populations (Fig. 9), suggesting that individuals that grew fast rarely reached an old age. Male moor frog’s relatively larger size could be a result of intense sexual selection favouring large males (Hedengren 1987, Andersson 1994). The variation in sexual dimorphism could then easiest be explained as a result of sexual selection in Figure 9. Mean (± S.E.) LAG (see text) perimeter size for males versus natural female (grey diamonds) and male (black circles) moor selection in females frogs (a) at acid localities (pH < 5.0) and (b) neutral lo- where the largest calities (pH > 6.0). dimorphism is found in areas with the most intense male competition. The results suggests a lower growth rate in acidified areas probably because the physiological stressfulness of the environment. The lower survival rate of fast growing individuals can be seen as a demonstration of a cost of high growth rate in natural populations.

19 Developmental stability and acidity in moor frog populations (V) All measured traits showed a significant sexual dimorphism, with the bones of males being heavier. This sexual dimorphism was strongest in the arms, i.e. the humerus and the radio-ulna, as indicated by the proportional differences in the weights. Fluctuating asymmetry was associated with age, with older individuals being less asymmetric. When correcting for age we found that frogs from the acidified populations were more asymmetric (Fig. 10). According to univariate analyses, significant differences among populations were observed for SVL, growth rate, age, body weight and mean size- corrected asymmetry. The results corroborate the earlier studies indicating that low pH constitutes a strong environmental stressor that influences the development and growth of amphibian embryos and tadpoles (e.g. Pierce 1985, Böhmer & Rahmann 1990, Räsänen et al. 2003a, b, 2005). More im- portantly, our results indicate that the stress effects induced by low pH are also visible at later (adult) life stages once the individuals have changed from an aquatic (larvae) to a mostly terrestrial (adults) life. These effects were very clear in both trait means and FA, suggesting that FA per se can be a useful indicator of the degree of environmental stress experienced by amphibians. Increased FA among individuals exposed to acid stress has been observed in fishes (Jagoe & Haines 1985), but our results provide the first test in amphibians. Given the alarming evidence for global decline of amphibian populations (Alford & Richards 1999, Stuart et al. 2004), FA could provide a useful metric for identifying populations under stress when direct population size estimates are difficult or impossible to obtain.

20 Figure 10. Overview of patterns in unsigned asymmetry values in Rana arvalis uncorrected (left panels) and corrected (right panels) for trait size.

21 Acknowledgements

First of all I would like to thank Germán Orìzaola, Beatrice Lindgren, Jacob Höglund and Anssi Laurila for reading and commenting earlier versions of this “kappa” and Steve Danielsson for his valuable help with writing the Swedish summary. Then I thank my family for supporting me in different ways to continue with this project; Anna, Eskil, Rasmus, Truls, Lilleman, Malena, Jessica, Maggan, Lasse, Nicolina, Freja, Julia, Mattis, Alfons, Stor- Julia, Fanny, Uffe-Utter, Örjan, Anneli, Krister and Lena. And last I also would like to thank all the people at the department, some special frog col- leagues and other loosely people who have been to special help with this work: Jonas Andersson, Claes Andrén, Cano Arias, Roger Arvidsson, Sven- Åke Berglind, Martin Carlsson, Jacques Castanet, Emma Dahl, Johan Dan- newitz, Xavier Eekhout, Sönke Eggers, Robert Ekblom, Jan Ekman, Gunilla Engström, Henri Engström, Marcus Fornbacke, Torsten Gislén, Michael Griesser, Peter Halvarsson, Maarit Haataja, Marianne Hei- jkenskjöld, Ronny Isaksson, Pär Jacobsson, Markus Johansson, Hans Kauri, Theresa Knopp, Simon Kärvemo, Björn Lardner, Jobs Karl Larsson, Karin Lindström, Jon Loman, Arne Lundberg, Jan Lundström, Geir Löe, Kari Löe, Ged Malsher, Cim Matsuba, Juha Merilä, Erik Nettelbladt, Emil Nils- son, Torbjörn Nilsson, Johan Nilsson, Magdalena Nystrand, Susanna Pak- kasmaa, Thomas Persson, Jarmo Perälä, Henna Piha, Maria Quintela, Alex Richter Boix, Björn Rogell, Andreas Rudh, Katja Räsänen, Tobias Sahlman, Jonas Sahlsten, Martina Schäfer, Dirk Schmeller, Lisa Shorey, Per Sjögren-Gulve, Ella Smirina, Mattias Sterner, Magnus Svensson, Ce- line Teplitsky, Carl-Gustaf Thulin, Ane Timenes Laugen, , Tanja Viio, Antti Vähäkari and Johan Wallin The project was supported by grants from stiftelsen Oscar och Lilli Lamms minne, NordForsk, Zoologisk stiftelse, Helge Axsson Johnsons stiftelse and Swedish Institute,

22 References

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26 Livet som sur groda

Den här avhandlingen beskriver hur åkergrodan (Rana arvalis) påverkas och anpassar sig till olika miljöer. Studien granskar populationer från miljöer med varierande klimat och surhet. Groddjur är mycket känsliga för föränd- ringar i livsmiljön, varför markutnyttjande och miljöföroreningar ofta får allvarliga konsekvenser.

Olika grodpopulationer har mer och mer isolerats från varandra genom exempelvis utdikning av småvatten och anläggande av allt större sammanhäng- ande åkermarker. På senare tid har dessutom forskare över hela världen uppmärksammat att grodpopulationer till synes utan anledning hastigt mins- kat eller till och med dött ut. Det finns tänkbara naturliga förklaringar till detta, men troligast är att minskningarna beror på mänskliga aktiviteter i form av miljögifter och utsläpp. Stora delar av norra Europa är utsatta för luftburna miljöföroreningar i form av surt regn. Detta skapas vid förbränning av fossila bränslen som leder till utsläpp av svavel- och kväveoxider.

Åkergrodan förekommer i hela Sverige utom i fjällmark och de allra nordli- gaste delarna. Detta är den svenska groda som har bäst tolerans för låga pHvärden och deras lekvatten varierar i pH (surhet) mellan 4.0 och 8.5. Stu- dien har granskat grodornas livshistoria, det vill säga de egenskaper som påverkar hur väl en individ klarar att föröka sig under sin livstid. Detta inne- fattar exempelvis livslängd, tillväxt samt hur många och hur stora ägg honor har.

Studien har huvudsakligen testat två variabler som inverkar på grodornas tillväxt och storlek; klimat och livsmiljöns surhet. Åkergrodor som lever i södra Sverige växer fortare men är mindre än de som lever längre norrut. Grodor från försurade områden har en sämre tillväxt och blir mindre. Studi- en har också undersökt skillnaden i storlek mellan honor och hanar, så kallad könsdimorfi. För hannar är det en fördel att vara stor i miljöer där det är konkurrens om honorna, medan vinsten för stora honor är att de kan bära fler eller större ägg. Skillnaden mellan könen är mindre i sura miljöer och i nord- liga populationer medan de är större i neutrala miljöer och i sydliga populationer. Grodyngel med ett ursprung från en försurad miljö överlevde bättre i surt vatten än andra yngel. I undersökningen har grodornas ålder och tillväxt bestämts genom en analysmetod som kallas skelettkronologi. Metoden kan

27 liknas vid sättet att bestämma träds ålder och tillväxt genom att räkna års- ringar.

Naturvårdsforskningen behöver mer detaljinformation om hur organismer påverkas av miljöförstöring, bland annat för att kunna förutsäga hur föränd- ringar i miljön påverkar olika arters framtid. Slutsatsen i denna studie är att åkergrodor har en viss förmåga att anpassa sig till förändringar av miljön men att försurningen leder till försämrad tillväxt och att de anpassningar som sker indirekt kommer att leda till försämringar i reproduktiviteten.

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