University of Halmstad School of Business and Engineering

Anuran colonization of newly constructed ponds: The importance of time and distance to source populations

Jimmy Almhagen

Master thesis in Applied Ecology: 30 ECTS Supervisor: Jon Loman 2007-09-14

Abstract

Dispersal is an important factor in animal ecology. Anurans (frogs and toads) are often philopatric (home loving) but some specimens in a population usually have the capacity to disperse relatively long distances. In this study I investigated the colonization of newly constructed ponds in the southwest of Sweden by three anuran species: The common toad (Bufo bufo ), the moor frog ( Rana arvalis ) and the common frog ( Rana temporaria ). The ponds were constructed between two and five years ago and were now as frequently occupied as older source ponds in the area. For the common toad and the common frog there was no correlation between distance to source populations and degree of colonization. The moor frog was more common in ponds that were situated in the vicinity of older source ponds with ample populations. The main impression was that these species rapidly colonize newly constructed ponds, at least within moderate distances from source populations. There were some differences between the species though and it seems like the moor frog have more limited dispersal abilities than the other two species.

1 Introduction

This work is about anuran (frog and toad) dispersal. I introduce the subject with a rather broad review. With the review my purpose is too explain what dispersal really is, to look at different causes and effects of dispersal, and to look at different methods of studying the phenomena. Further I want to look at different aspects of anuran dispersal such as distances, sex bias, barriers and corridors. I conclude with a field study that concerns anuran colonization of newly constructed ponds. The main questions I wanted to answer with my field study were, as the title implies, how important are time and distance to source populations for anuran dispersal? I also looked at the importance of pond area.

Mini review

Dispersal

Definitions of dispersal and related words

Many animals exhibit different kinds of movements. Dispersal and migration are two words for movements sometimes used interchangeably (Beerli & Felsenstein 1999; Taylor 1986; Jehle et al. 2005; Fouquet & Measey 2006).

Dispersal is most often defined as the spreading of individuals away from others whereas migration is defined as the (approximately) simultaneous point to point directional movement done by many members of the same species (Begon et al. 2006). Classical examples of migration are the intercontinental journeys of birds and the movement of locust swarms (Begon et al. 2006). Another classical example is the migration of amphibians from their hibernation sites to their breeding grounds (Sinsch 1990). Sometimes migration is used for the broad concept of moving from one space to another (Johnson & Gaines 1990).

Another definition of dispersal is “intergenerational movements” (Bullock et al. 2002). The same authors mention the Oxford English Dictionary definition of migration which includes change in distribution of a species. Dispersal has also been defined as individuals leaving their home ranges, permanently or at least to not return in the short- term (Stenseth & Lidicker 1992). Endler (1977) uses the world dispersal to account for the practically random and non directional movements mostly made by single individuals and as a part of their daily activities.

A more restricted meaning of the word dispersal is when individuals spread outside the range of the species to expand the distribution area (Udvardy 1969). In Howard’s (1960) definition of dispersal this word does not only mean the spreading of an individual but also points out that the dispersed animal should or would have reproduced if it had survived to do so. The movement of an individual from a population to another and successfully breeding there has been called “genetic dispersal” (Johnson & Gaines 1990).

2 Some researchers use the expression “genetic migration” in the same sense and distinguish it from “ecological migration” that is simply the movement from one locality to another (Bull et al. 1987). Dispersal of individuals whether or not they reproduce is called “gross dispersal” to distinguish from “effective dispersal” when reproduction follows dispersal (Greenwood 1980).

Natal dispersal is referred to as the movement of pre reproductive animals from their site of birth to the place where they later reproduce. In contrast, breeding dispersal is when adult individuals move from one breeding site (or group) to another (Greenwood & Harvey 1982). Sometimes the dispersal of individuals from one group to another is named “transfer” (Greenwood & Harvey 1982).

Even if defined as two different concepts, it is sometimes difficult to distinguish between dispersal and migration. Probably, in many natural systems at least some migrating animals are also dispersers. I think it is a pity that migration is being used in a broader sense since it makes it even more confusing. For the pond breeding anurans in my study I suggest a definition of dispersal as the movement of an individual from either the place of birth or from a place were this particular individual has been a) reproducing or b) observed during the breeding season. The movement should be directed to a different breeding locality used in a different breeding season. This individual should thereafter be observed reproducing or at least be present at the new breeding locality at the time of breeding.

Causes and effects of dispersal

There can be different reason for an individual to disperse. Intraspecific competition can cause density dependent dispersal (Bullock et al. 2002) and there is abundant evidence from different groups of organisms that dispersal rates are positively correlated with increasing competition for limited resources, an example is in the goldenrod beetle (Trirhabda virgata ) (Clobert et al. 2003). A special case of intraspecific competion is sibling (kin) competition in which the subordinate sibling sometimes have to departure from the natal territory and this has been observed in the gray jay ( Perisoreus canadensis ) (Clobert et al. 2003). Inbreeding can result in loss of fitness and also in phenotypic abnormalities and one reason for dispersal can be inbreeding avoidance (Clobert et al. 2003). Interspecific effects such as predation and parasitism can be causes of dispersal (Clobert et al. 2003). An example of predator induced dispersal has been found in spider mites that dispersed in the presence of predatory mites (Clobert et al. 2003). There can also be physical causes for dispersal such as the transience of habitats (Bullock et al. 2002).

Dispersal can have profound effects on population dynamics and population genetics: “From the population ecological point of view, dispersal rates determine to what degree different populations will function as independent demographic units. From the population genetic point of view, dispersal determines the rate of genetic differentiation of the semi isolated populations.” (Palo et al. 2004).

3 Methods used to study dispersal

There are different ways of studying dispersal. One method that is often used is mark and recapture (Clobert et al. 2003). The animals can be marked directly or equipped with various kinds of tags (Bullock et al. 2002). Genetic techniques can also be used to study dispersal. Introducing novel genetic markers and see how they spread in a population or performing parentage analysis are two genetic methods (Bullock et al. 2002). Introduction of a species to a new locality can provide a way studying dispersal such as in the pacific treefrog ( Hyla regilla ; Reimchen 1990). Also creation of new habitat is a method that can be used (Baker & Halliday 1999).

Since pond breeding amphibians such as the common frog ( Rana temporaria ) most often deposit their spawn clumps in water (Savage 1961) looking for eggs in newly constructed ponds is a way of studying dispersal in pond breeding amphibians. This approach will be used in the field study that concludes this work.

Anuran dispersal

Many amphibians have often been regarded as having low dispersal abilities (Smith & Green 2005; Seppä & Laurila 1999; Marsh & Trentham 2001). Peter (2001) wrote that amphibians rarely move more than five kilometers and Petranka & Holbrook (2006) that amphibians seldom disperse more than 2-3 kilometers. Different distances have been mentioned as critical and from which beyond dispersal should be limited and could result in isolation, one kilometer has often been set as a limit (Smith & Green 2005).

Sometimes amphibians are highly genetically differentiated at moderate distances. Populations of cascades frog ( Rana cascadae ) showed strong genetic differentiation separated by 50 kilometers (Monsen & Blouin 2004) and Lampert et al. (2003) found significant genetic differentiation between sites at a distance of only 3.8 kilometers in túngara frogs ( Physalaemus pustulosus ). Not all anuran species are great dispersers and the saffron frog ( Geocrinia lutea ) is even more extreme and has shown genetic differentiation between populations that were separated by only 1.25 kilometers (Driscoll 1998).

Dispersal abilities are not always limited though, and the wood frog ( Rana sylvatica ) showed only a slight differentiation at a distance of three kilometers (Newman & Squire 2001). Allozyme data of the sand frog ( Heleioporus psammaophilus ), an endemic species of Western Australia indicate that individuals of this species regularly disperse several kilometers (Berry 2001). There was low genetic differentiation between common frogs and common toads ( Bufo bufo ) inhabiting islands in the Baltic Sea separated by several kilometers, suggesting high dispersal abilities and gene flow. However, a different explanation could be recent colonization (Seppä & Laurila 1999).

In a review, where articles recording maximum distances moved by amphibians were analyzed, it turned out that 7 % of the anuran species had the capacity to disperse more than 10 kilometers and 44 % more than one kilometer (Smith & Green 2005). Smith &

4 Green (2005) found the average maximum distance moved by 53 species of amphibians was 2.02 kilometers. According to Smith & Green (2005) at least one individual in a population of anurans should be able to move 11- 13 kilometers and larger study areas should discover longer amphibian movements in the future. The fowler’s toad ( Bufo fowleri ) has been reported to disperse frequently enough over distances up to 10 kilometers to keep populations connected (Smith & Green 2005).

Long distance dispersal is difficult to detect and normally underestimated by mark and recapture studies and even though long distance dispersers may be rare they are important as colonizers (Marsh & Trentham 2001). Some amphibian can disperse rather impressive distances and a juvenile Fowler’s toad has been marked and later recaptured 34 kilometers away. However, a possible explanation was passive aquatic transport (Smith & Green 2006). Other anurans with exceptional dispersal abilities are the cane toad ( Bufo marinus ), the red legged frog ( Rana aurora ) and the Rio Grande leopard frog ( Rana berlandieri ). Distances recorded for these species are 35, 24 and 16 kilometers (Smith & Green 2005).

Also European species are able to disperse and the pool frog ( Rana lessonae ) has been told to have dispersal abilities of 15 kilometers (Smith & Green 2005). In the Netherlands treefrogs ( Hyla arborea ) have been reported to disperse as far as 12 kilometers (Vos et al. 2000). The fire bellied toad ( Bombina bombina ) may move 11 kilometers (Marsh & Trentham 2001) and also natterjack toads ( Bufo calamita ) are known to disperse several kilometers (Stevens et al. 2006). The common toad and the moor frog ( Rana arvalis ) are able to disperse at least 3.6 and 7.6 kilometers respectively (Smith & Green 2005) and the common frog has been able to colonize new ponds at distances up to 950 meters (Baker & Halliday 1999).

Philopatry

Amphibians have also often been considered to display high site fidelity (Smith & Green 2005; Seppä & Laurila 1999; Marsh & Trentham 2001). Temperate anurans tend to be philopatric (breed in the same ponds year after year) (Smith & Green 2006) and in a study of the wood frog about 11000 adults were marked but none was caught in a different pond than the one were they had first been marked (Berven & Grudzien 1990). In a study of the common spadefoot ( Pelobates fuscus ) in Denmark animals were marked and recaptured. Only 1.09 % of the specimens were caught in more than one pond (Hels 2002). The natterjack toad has often been reported to be philopatric (Sinsch 1992; Sinsch & Seidel 1995; Husté et al. 2006) and when considering breeding areas and not breeding ponds overall exchange of male toads was only 2.2 % (Sinsch & Seidel 1995). In England and in Sweden adult common toads were found to bee highly philopatric (Reading et al. 1991). Also water frogs in Switzerland and treefrogs in the Netherlands have turned out to be philopatric (Peter 2001; Vos et al. 2000). Philopatry seems to be the rule in the white bellied frog ( Geocrinia alba ) and the yellow bellied frog ( Geocrinia vitellina ), two endangered frog species from Australia. 90- 97 % of the individuals were displaced less than 20 meters between breeding seasons (Driscoll 1997). Further in a within year experiment (using artificial ponds) 91 % of the male túngara frogs stayed

5 within 50 meters of a pond between the 7 th of July and the 13 th of August (Marsh et al. 2000).

Wood frogs are not always philopatric and change breeding sites at least when inter pond distances are shorter than 80 meters (Petranka & Holbrook 2006). Artifical ponds were colonized by gray treefrogs ( Hyla versicolor ) within a breeding season showing that philopatry is not total in this species (Johnson & Semlitsch 2003). Some individuals in a population always seem to disperse and at a higher ratio in some species than others but even though it seems like philopatry is a common phenomena within the anura order. Anurans are more dependent on moisture and more vulnerable to desiccation than many other terrestrial animals (Duellman & Trueb 1994). To disperse, at least through arid landscapes can thus be dangerous and philopatry seems, when possible, a good option.

Juvenile dispersal

Many species of frogs are philopatric as adults and juvenile dispersal is thus an important factor in anuran population dynamics (Berven & Grudzien 1990). Individual anurans dispersing the greatest distances have often been juveniles (Smith & Green 2006). There are many examples from both Europe and North America in which juveniles disperse at a higher degree (Sinsch 1992; Sjögren- Gulve 1998; Reading et al. 1991; Berven & Grudzien 1990; Dole 1971; Breden 1987; Breden 1988; Funk, C. et al. 2005). An example is the mark and recapture study by Breden 1988 of the fowler’s toad, where 48 % of the metamorphs (juveniles) dispersed whereas only 12 % to 17 % of the older life stages did do so.

However, it is not always juvenile that is the main dispersal life stage and in a study by Smith & Green (2006) of the same fowler’s toad that had earlier been reported to have a juvenile skewed dispersal curve, juveniles moved neither faster nor farther than adults. Relatively few studies have been done with juveniles and reasons for this could be they are relatively small and difficult to mark (Breden 1988).

Sex bias

There have been various studies with the purpose of finding out whether there exists a sex bias regarding dispersal (or philopatry). Different patterns have been found for different species and in the common frog it seems that females disperse at a higher degree (Palo et al. 2004). Also in the bullfrog ( Rana catesbeiana ) females tend to be the more dispersing sex (Austin et al. 2003). There was also female bias in the columbia spotted frog in a study using direct methods (Palo et al. 2004). Sinsch (1992) studied the natterjack toad and found females to be the most dispersive sex. There was also a sex bias in both England and Sweden looking at the common toad. Females were less philopatric than males (Reading et al. 1991).

In the túngara frog genetic data suggest that there might be a male biased dispersal in this species (Lampert et al. 2003). Also in the treefrog there was a male biased dispersal, this

6 in the Netherlands and in spite of a predicted female biased pattern (Smith & Green 2006).

Not all studies have found any indications of sex biased dispersal (Berven & Grudzien 1990; Smith & Green 2006) and in an Australian study of two species of the genera Geocrinia there were no significant difference between sexes regarding philopatry (Driscoll 1997). A possible explanation for the female bias in the common frog dispersal, versus lack of bias in the wood frog, in spite of similar ecology could be that in the wood frog both sexes mature at the same age and that in the common frog females mature later than males (Palo et al. 2004).

At least some anuran males can reproduce at low cost and repeatedly over a season (Austin et al. 2003). In contrast many anuran females produce only one or two egg clutches a year (Arnold & Ovenden 2004). Possibly inbreeding avoidance is therefore more important for females and if so it is possible that they should disperse at a higher degree.

Colonization of new habitats

Extinctions and colonization seem to be common occurrences in amphibian population dynamics (Skelly et al. 1999; Sjögren- Gulve 1994). In Oregon, USA six newly constructed ponds had been colonized by western toads ( Bufo boreas ) within nine months after construction (Pearl & Bowerman 2006). Other temperate anurans such as the american toad (Bufo americanus) and natterjack toad have also been reported to colonize new ponds (Pearl & Bowerman 2006). Also in Italy creation of new ponds (and restoration of old ponds) has been successful and the ponds were spontaneously colonized by various species including the common toad (Gentilli et al. 2002). In Jutland, Denmark where amphibian populations have been severely reduced, ponds have been constructed since 1985 and a survey of 83 of these ponds was done in 1999. Amphibians were reproducing in 98 % of the ponds. Breeding common toads were found in 8 % of the localities and common frogs were found in 40 % of them (Henriksen 2000).

Various Anuran species have been successfully introduced and have after introduction been able to expand their range. The marsh frog ( Rana ridibunda ) has been successfully introduced from Hungary to Britain and has there been able to disperse (Zeisset & Beebee 2003). Also other introductions of frogs to areas outside their normal range have showed that that they have dispersal abilities. Introductions of the pacific treefrog have shown that Anurans can disperse and expand their distribution at rates of 1.9 kilometers per year. The pacific treefrog had colonized 26 % (2600 km 2) of the Queen Charlotte islands since a first introduction in the early 1930´s (Reimchen 1990).

Translocation can also bee successful within areas of anurans natural range and translocations of tadpoles of the italian agile frog ( Rana latastei ) and the common spadefoot have worked out well (Gentilli et al. 2002). The common toad has been considered to be highly philopatric but when toads were transferred to a new possible

7 breeding site more than 70 % had accepted the new pond after two years (Schlupp & Podloucky 1994).

Some of the introduced species are regarded as invasive like the african clawed frog (Xenopus laevis ) and the cane toad. The african clawed frog is a South African species that has been accidentally introduced to different parts of the world with mediterranean (and at least in France oceanic) climate. It has been used as a laboratory animal in France and there was one known release of the frog in the 1980´s. This species is mostly aquatic and can disperse through rivers showing that there are different ways of amphibian dispersal (Fouquet & Measey 2006). Cane toads have been introduced to more than 30 countries and were brought to Australia in 1935. The cane toad has since dispersed over a range of more than a million square kilometers (Brown et al. 2006). The african clawed frog can have a negative impact on the native fauna as a predator of both fish and amphibians (Fouquet & Measey 2006) and the cane toad produce skin toxins that can affect both aquatic and terrestrial predators (Seabrook & Dettmann 1996). The cane toad can further reach high densities and affect wildlife by competition (Seabrook & Dettmann 1996).

Barriers and corridors

Roads have been shown to be effective barriers for migrating amphibians (Hitchings & Beebee 1997; Fahrig et al. 1995). In a study of migrating common toads it was estimated that 24- 40 cars per hour killed half of the moving animals and another study showed that 26 cars per hour could kill all toads crossing the road (Fahrig et al. 1995). Fahrig et al. (1995) showed that the number of dead anurans increased with increasing traffic intensity. They also found that the proportion of frogs killed was positively correlated with traffic intensity and that chorus intensity was negatively correlated with high traffic intensity. Roads and railways explained more of the variation between moor frogs than geographical distance (Vos et al. 2001) indicating that infrastructure can have negative effects on amphibian connectivity.

Habitat fragmentation can lead to limited dispersal and hence reduction of gene flow between populations (Vos et al. 2001). In a fragmented landscape dispersal sometimes has to occur between suitable patches and the matrix between can provide a barrier (Stevens et al. 2004). Urban common frogs have been shown to exhibit lower genetic diversity than rural conspecifics and genetic distances in urban ponds separated by less than 4.4 kilometers where twice those for rural ones separated by 41.3 kilometers (Hitchings & Beebee 1997). Knutson et al. (1999) found a negative association of anurans with presence of urban land further indicating that urban areas could act as dispersal barriers. Avoidance of forest clear cuts in different species of amphibians and especially juvenile wood frogs indicate that these environments can have a negative effect on amphibian dispersal (Patrick et al. 2006). In a colonization experiment using artificial ponds in Tasmania, ponds placed in logged forest needed twice the time needed by ponds placed in unlogged forest to be colonized (Lauck 2005).

8 Barriers can also be natural and mountain habitats promote genetic isolation among Anuran populations (Monsen & Blouin 2004). In a study of the Columbia spotted frog there was a positive correlation between genetic differentiation and elevation, and mountain ridges seem to be physical barriers (Funk, W. et al. 2005). In Panama túngara frogs separated by Chagres River showed a higher degree of differentiation at the same distance than those on the same side, an indication that a river can be a barrier too (Lampert et al. 2003). There were some gene flow across the river though and explanations apart from swimming could be frogs crossing a bridge or unintentional human transport (Lampert et al. 2003). In Australia, genetic differentiation in the sand frog shows that river catchments can be partial barriers of gene flow and thus dispersal (Berry 2001).

Corridors can facilitate amphibian movements (Vos et al. 2000). The invasive cane toad in Australia uses roads to disperse and avoids heavily vegetated habitat (Brown et al. 2006). Cane toads using roads is especially frequent in areas with forest vegetation and in a study 76 % of the distance traveled in forest environments was conducted on roads, compared to agricultural landscapes where the figures were 45 % (Seabrook & Dettmann 1996). In an experimental study of the natterjack toad using juveniles (the main dispersive life stage) the animals preferred to move on bare environments like cement (Stevens et al. 2006). Juvenile wood frogs and American toads selected road habitats in a study in New England and forest roads are no barrier for these species (deMaynadier & Hunter 2000). It seems like roads can be both detrimental and beneficial for frogs. But when traffic is more than sporadic the effects are probably mostly negative.

Intermittent streambeds can facilitate pickerel frog ( Rana palustris ) movements (Gibbs 1998). In a hostile environment, movement and dispersal can be facilitated by ditches (Mazerolle 2004). Bog ponds connected by ditches recorded higher abundance of Green frogs than those that were not and ditches in agricultural landscapes can have positive effects at least on common frogs (Mazerolle 2004). Also road underpasses and other facilities that help frogs cross the road can be considered corridors. Underpasses have been suggested to keep anurans (and other animals) from the road but one concern is that predation could be facilitated (Fahrig et al. 1995). Temporary fences, road closures and substitute breeding sites have also been suggested to rescue amphibians from road mortality (Schlupp & Podloucky 1994).

Field study

In my field study I wanted to look at anuran dispersal. Many amphibian species are threatened and declining in numbers both in Sweden (Gärdenfors 2005) and abroad (Alford & Richards 1999; Husté et al. 2006) and knowledge of their dispersal abilities can provide essential information for conservation. The anuran species inhabiting Laholm, the region were I conducted my field study are all pond breeders (Ahlén et al. 1995). I took advantage of the fact that a set of newly constructed ponds was available and indirectly I studied their colonization by anurans. Because density of potential source ponds varied among these ponds I could look at the importance of source ponds for

9 anuran dispersal (colonization). The ponds had been constructed over five years and I could therefore also look at the importance of time for anuran dispersal (colonization).

None of the species monitored in my field study: The common toad, the moor frog and the common frog seems to be threatened in the area right now (Ahlén et al. 1995) but could be so in the future. Species with similar ecology could also be threatened elsewhere and the same is true for species depending on anurans as a source of food. Pond creation has been successfully used as a tool for amphibian conservation in Italy (Gentilli et al. 2002) and thus knowledge of the ability of the Swedish anurans to colonize ponds in Laholm can prove to be valuable.

Methods

The field study was conducted in 2007. It started the 28 th of March and was finished the 18 th of April. A total of 51 ponds were investigated. They were all located in the municipality of Laholm in the southwest of Sweden. 21 were newly constructed ponds, created between 2001 and 2005 (here called experimental ponds) and the rest were all ponds within a distance of 700 meters from the experimental ponds (here called source ponds). The experimental ponds were selected among those presented in a catalogue (Österling 2006) and chosen to get an approximately even distribution of time since construction. They had been constructed mainly to reduce the eutrophication of the Laholm bay (Österling 2006).

The common toad was monitored by counting adult male toads observed at the shore, the bottom or floating on the surface of the ponds. This was done mainly at night. Values used in the analysis are the maximum number of males recorded at a particular pond and at a particular inventory. Chorus (play) was also scored for the common toad.

The common frog and the moor frog were censused by counting spawn clumps. Sometimes spawn clumps were so numerous and aggregated that they could not be counted directly. In those cases I first calculated the area covered by spawn mass at the breeding site and then calculated the number of spawn clumps by using a correction factor of 215 clumps/ m 2 for the moor frog and 140/ m 2 for the common frog (Loman & Andersson 2007).

GPS was used to measure the area of the ponds. Some of the variables had to be log transformed to fit a normal distribution. Variables that were log transformed are the following: Pond area, number of spawn clumps (for both the moor and the common frog), and source population within 250 meters for the moor frog. Not all of the ponds did contain any spawn and before log transformation I had to add +1 to all values. Some variables did not fit a normal distribution even after log transformation. In these cases the initial values were used and a non parametric test performed.

I made comparisons of experimental vs. source ponds. Also I analyzed importance of pond area, source populations and pond age. When analyzing experimental vs. source ponds, pond area and pond age I looked at number of species, occurrence of each species

10 and abundance of each species. Importance of source populations was analyzed by looking at source populations within 250 and 700 meters.

Results

Experimental vs. source ponds

Number of species

Experimental ponds did have more species than source ponds (Mann-Whitney U-test: U=201.0, P=0.018, N=51; Table 1.).

Table 1. Differences between old and experimental ponds regarding number of species.

Zero One Two Three Mean Experimental 0 2 5 14 2.57 Source 1 7 12 10 2.03

Occurrence

The common toad was present more often in the experimental than in the source ponds (Table 2.). There also seemed to be a trend for the moor frog to occur more often in the experimental ponds although this was not the fact for the common frog (Table 2.).

Table 2. χ2 test of association for occurrence in experimental and source ponds, and percentage of ponds occupied.

χ2 P Experimental Source ponds % ponds % (of 21) (of 30) Common toad 5.437 0.020 61.9 53.3 Moor frog 3.062 0.080 90.5 70.0 Common frog 0.003 0.955 90.5 90.0

Abundance

The common toad was also more abundant in the experimental than the source ponds (Table 3.). This what not true for the two species of brown frogs (Table 3.).

Table 3. ANOVA and Mann-Whitney U-test results for abundance in experimental (N=21) vs. source ponds (N=30). (Abundance was measured as maximum number of toad males and total number of spawn clumps for the moor and the common frog).

F U P Mean Mean Median Median (Experimental) (Source) (Experimental) (Source) Common - 218.5 0.043 - - 2 0 toad Moor frog 0.029 - 0.866 29.3 47.3 - - Common 1.828 - 0.183 75.3 37.5 - - frog

11 Experimental ponds tended to have a larger area than source ponds (Mann- Whitney, U=131.0, P=0.001, N=51; Fig. 2.).

Pond area

Number of species

There was a positive correlation between number of species and area of pond for experimental ponds but not for source ponds (Table 4.).

Table 4. Spearman-rank results looking at correlation between pond area and number of species.

rs P N Experimental ponds 0.512 0.018 21 Source ponds 0.199 0.292 30

Occurrence

The common toad was more often present in the larger of the experimental ponds but the two species of frogs showed no such preference, although there was a tendency for the common frog (Table 5.). For source ponds there was no relationship for any species (Table 5.).

Table 5. Mann-Whitney U-test results looking at pond area (m 2) and occurrence of the different species.

Experimental (N=21) Source (N=30) U P Median area Median area U P Median area Median area (occupied) (unoccupied) (occupied) (unoccupied) Common 16.0 0.050 3213 965 69.0 0.086 891 434 toad Moor frog 14.0 0.610 2171 2173 80.0 0.533 712 504 Common 4.0 0.086 2352 897 16.0 0.100 504 975 frog

Abundance

There was a positive correlation between number of male toads (Fig. 2a.) and area of experimental pond (Table 6.). There was no such correlation for number of spawn clumps of either frog species (Table 6.). For source ponds there was no correlation for any of the species (Table 6.)

12 Table 6. Pearson and Spearman-rank correlations. Anuran abundance correlated with area (m 2) of pond.

Experimental Source (N=21) (N=30) r rs P r rs P Common 0.798 - 0.001 - 0.233 0.215 toad Moor frog -0.166 - 0.471 0.194 - 0.304 Common 0.142 - 0.538 0.067 - 0.723 frog a) b)

80 25 70

20 60 50 15 40 10 30

5 20 10 Max. Number ofMax. males Max. Number ofMax. males 0 0 0 5000 10000 15000 1 10 100 1000 10000 100000 Pond area (m2) Pond area (m2)

Fig. 2a+2b. Common toad abundance plotted against size of pond. a) experimental ponds and b) source ponds.

Importance of source populations

There was no correlation between the number of common toad males recorded at an experimental pond and number of males recorded at source ponds within 250 or 700 meters (other experimental ponds excluded; Table 7.). For the moor frog there was a correlation within 250 meters and there was a weak correlation looking at source ponds within 700 meters (Table 7.). For the common frog there was no correlation looking at the same distances (Table 7.).

Table 7. Test of the correlation between abundance of anurans in experimental ponds (N=20) and their isolation, as measured by (the inverse of) corresponding total abundance in all ponds within 250 and 700 meters respectively.

250 (m) 700 (m) r P r rs P Common toad (max. males) -0.104 0.661 - 0.051 0.832 Moor frog (spawn clumps) 0.479 0.033 0.429 - 0.059 Common frog (spawn clumps) 0.211 0.371 0.297 - 0.203

13 Pond age

Number of species

There was no correlation between number of species and time since construction of the experimental ponds: r s=-0.341, P=0.130, N=21.

Occurrence

There was no relationship between time since construction and presence of neither species (Table 8.).

Table 8. Mann-Whitney U-test results looking for relationship between occurence and time since construction (months).

U P Median age Median age (Occupied) (Unoccupied) Common toad 35.0 0.719 39 62 Moor frog 7.0 0.190 39 64 Common frog 3.0 0.057 39 68

Abundance

Time did not seem to be an important factor for anuran abundance (Pearson correlation, P≥0.264). There was no correlation between number of males and age of the experimental ponds. This was true for the moor frog and common frog too as there was no correlation between age of experimental pond and number of spawn clumps for neither species.

Discussion

Anurans were relatively common in the area. Indeed, they were found in all but one pond. All three species seem to have the ability to colonize ponds rather efficiently. Actually, the youngest pond, a pond constructed in 2005 (18 months old at the inventory) had been colonized by all three species! Also, one pond that contained all three species did not have any source populations at all within 700 meters. This suggests dispersal abilities exceeding distances studied for all three species here. Previous studies of dispersal abilities have shown similar results for the common toad and the common frog in Britain (Baker & Halliday 1999) and also the moor frog has been reported to have dispersal abilities exceeding 700 meters (Smith & Green 2005).

Experimental vs. source ponds

Experimental ponds contained more species than source ponds and this could partly be due to the common toad being more frequent in the former ponds. The common toad has earlier been reported to prefer younger ponds (Baker & Halliday 1999; Henriksen 2000) and in Denmark it was breeding in 20 % of ponds of age 1-2 years but only 3-14 % of

14 ponds older than six years (Henriksen 2000). Also a higher abundance of the common toad in the experimental ponds than in the source ponds in Laholm is an indication of a preference of younger ponds. Another (hypothetical) explanation for the reason why experimental ponds contained more species could be that experimental ponds were located in pond dense areas and that source ponds were often located in the periphery. This is only speculation though, since I have not surveyed the area around source ponds.

Pond area

Species richness and common toad presence was positively correlated with surface area of the pond looking at experimental ponds but not so when looking at source ponds. A hypothetical explanation for this is that larger ponds more easily attract colonizers (Cook et al. 2002) but that after sufficient time equilibrium is reached and the suitable localities have all been colonized.

Importance of source populations

The moor frog was the species most dependent on source populations, showing a positive response to ample source populations within 250 meters (and also a weak response to those within 700 meters). Increasing number of spawn clumps for this species with increasing source population but not so for the common frog suggests the latter is a better disperser. The fact that the common frog was also present at more localities is further reinforcing that impression. There have been studies on common frog dispersal before (Henriksen 2000; Baker & Halliday 1999) but to my knowledge there have not been any studies comparing moor frog and common frog dispersal.

Pond age

Looking at the experimental ponds time since construction did not affect species richness. Maybe the results would have been different if I had had the possibility to look at younger ponds (more ponds constructed 2005 and also ponds constructed 2006). Time did not affect presence or abundance of any of the three species in the experimental ponds. These are unexpected results and at least I expected the common frog to be more common in the older of the experimental ponds. Looking at constructed ponds in Denmark, the common frog was most common in ponds that were between five and 10 years old (Henriksen 2000) and also in Britain the common frog seem to prefer older ponds (Baker & Halliday 1999). It seems reasonable that it should take some time to build up a population and therefore I expected the older ponds to have a higher abundance. The result suggest, in contrast to my expectations, that these frogs (at least the common toad and the common frog) are quite fast colonizers, already in a few years all suitable ponds in the present landscape were colonized.

The common toad was as I have mentioned more common in the experimental ponds. All three species normally assemble in quite large numbers during breeding season (Arnold & Ovenden 2004) and it is not unusual that hundreds of common toad males are present in a pond during that time (Reading et al. 1991). The common toad was never so

15 numerous in my experimental ponds and maybe the colonization of this species (although it was present in 13 of 21 experimental ponds) does not have to mean a very successful colonization. A way to find out whether or not the colonization turn out to be really successful would be to monitor the common toad population over an extended period of time to see if there is stability regarding population size or an increase of individuals.

My study shows that different species seem to have different dispersal abilities. The moor frog but neither the common frog nor the common toad exhibited source population dependence at a distance of 250 meters. In my study area, ponds were relatively numerous and dense and it would be interesting to study ponds at greater distances from known source ponds of these species to better understand their dispersal abilities.

Lessons for conservation

Studies on anuran colonization have been done before but often looking only at occurrence (Baker & Halliday 1999; Henriksen 2000; Eterovick & Fernandes 2002) and to really understand dispersal I think it is indispensable to look at abundance as well. Dispersal is partly a random process, and especially in a landscape rich in ponds, species occurrence does not have to mean that a pond or surrounding habitat is exceptionally good. But if the population grows large there is a greater chance that the pond is valuable for anurans

Even if the anurans in my field study are not threatened now they could be so in the future and therefore it is important to learn more about them. Also, experience from these species could to some extent be applicable for other frogs with similar ecology. An example could be the agile frog ( Rana dalmatina ). This species is threatened in Sweden (Gärdenfors 2005) and it is closely related to the moor and the common frog. Even if it has different requirements regarding choice of habitats (Ahlén et al. 1995) it is possible that it could exhibit similar dispersal characteristics as the moor and the common frog.

Other animals than frogs can also be benefited by conservation work that have positive effects on the anurans in my study. For many predators frogs are an important part of the diet (Duellman & Trueb 1994) and the white stork ( Ciconia ciconia ) was formerly nesting in Sweden but disappeared (Tjernberg & Svensson 2007). Efforts are made to make it nest once again (Tjernberg & Svensson 2007) and if frogs are benefited it seems reasonable that the white stork should be so too.

In northern Italy pond creation has previously been successfully used as a tool for amphibian conservation for various species, among them the common toad (Gentilli et al. 2002). Sweden is situated in a different part of the common toad range and 20 of the 21 experimental ponds in Laholm had been colonized by anurans. Not only the common toad but also the moor frog and the common frog had been able to colonize the ponds in Laholm so pond construction seems to be an effective conservation strategy at least for amphibians.

16 Acknowledgements

Jon Loman has been my supervisor and his guidance, knowledge and his passion for anurans have been indispensable. He has given me important inspiration. Geraldine Thiere has not been involved directly in the process that has resulted in this work but she has earlier introduced me to important concepts of ecology as well as statistics. I wish to give her my appreciation. There are more people that have been supportive and even if I do not mention their names (because I would certainly forget someone) I hope that they are aware of my appreciation.

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