Temporary within multifunctional agriculture for the water shrew (Neomys fodiens) and tree ( arborea) in the Baakse Beek study area

Coen Westerduin Plant Production Systems Group, Wageningen University, Wageningen, the Netherlands

Various factors have contributed to the decline of many organisms, including the Eurasian water shrew Neomys fodiens and the European tree frog Hyla arborea, over the past decades. This literature study focuses on the requirements for these two , including water quality, food and vegetation, to investigate a re-introduction of these species in the Baakse Beek area in the east of the Netherlands, which is one of the subjects of study of the CARE (Climate Adaptation for Rural arEas) project. Developmental possibilities and timescales are investigated to find out if only temporary living areas for these animals could be accomplished, allowing for freedom of land use for farmers in the area. The varied diet, tolerance to pollution, relative independence of vegetation and lack of preference for different substrates or banks, complemented by its little affinity to certain areas and short lifespan, seem to make N. fodiens a suitable candidate for such schemes. Higher demands for water quality by H. arborea are the main objections against introduction of this species in an eutrophicated agricultural landscape. Dependence on vegetation and a habit of returning to older, no longer existing habitats provide further arguments against this practice. However, more research is needed on the optimum habitats of both animals, as reasons for their decline, and as such their requirements, are still ill- understood.

1. Introduction agricultural area in the Netherlands was involved Habitat loss, pollution and several other factors in nature conservation, well below the EU have resulted in a decline or even local extinction average of 25% in 2002 (Berentsen et al., 2007). of many plants and animals worldwide. For some Various inquiries have been carried out to find of these, conservation alone will not be enough to what motivated farmers to participate in forms of sustain them, but restoration and construction of nature conservation, or deterred them from doing suitable environments is necessary. so. Berentsen et al. (2007) found financial benefits As a country lacking large areas of nature, such to be the most important reason to participate, policies in the Netherlands often rely on and subsidies should increase to cover the participation from the agricultural sector. In agri- conservation costs. Governmental interference environmental schemes (AES), farmers are paid to should also be kept to a minimum, as this made modify their farming practise to provide farmers feel they were not in charge of their own environmental benefits (Kleijn and Sutherland, business. Extensive research by Jongeneel et al. 2003). The government coalitions Balkenende I (2008) showed similar lower participation in (2002) and Balkenende II (2003), as well as more nature conservation if farmers valued land recently Rutte I (2010), have chosen to purchase ownership highly. Another important factor was less and less agricultural land; instead further trust in the government: a result of the fact that increasing the role of private landowners and farmers have to rely on the government for farmers through AES in order to accomplish payments over a set period of years to compensate nature objectives (Jongeneel et al., 2012), and as for the loss of agricultural yields. Other reasons, such prevent the loss of biodiversity. as mentioned by Van Egmond and De Koeijer For these plans to work, however, cooperation of (2005) are a lack of expertise regarding the the farmers is required. In 2003, only 4% of the requirements of nature, as well as the fact that

1 (permanent) nature has no place in the business In addition, the plan was to provide information strategy. on the costs of construction for such habitats, and The CARE project (Climate Adaptation for Rural compare those to current compensation payments arEas) investigates the effects of climate change to see if the construction would be economically on agriculture, but also nature and water feasible; however, absence of suitable data management, as well as possible adjustments to prohibited this, although remarks on the overall these circumstances in order to create a resilient implementability of the adaptations will still be region. The study area of this project is located included. Finally, any benefits for farmers, around the Baakse Beek, a stream in the east of resulting from the presence of the animals or the Netherlands. Here, about 11% of the habitats, will be mentioned whenever applicable. agricultural area is involved in nature conservation (Karssing, 2012). In a recent CARE Initially, Scopus was the main source of suitable meeting, farmers from the area gave their reasons articles, using search terms as “multifunctional for participating (or not) in AES, which among agriculture” AND Netherlands, (“Neomys financial and time-related arguments included a fodiens” OR “water shrew”) AND habitat – or the wish for temporary adaptations. This literature similar (“Hyla arborea” OR “tree frog”) AND study will investigate if such transitory nature habitat. As time progressed, ‘habitat’ was replaced would actually provide benefits for biodiversity, by ‘food’, ‘lifespan’, ‘vegetation’ or ‘dispersal’, as represented by the Eurasian water shrew among others. Some additional terms were (Neomys fodiens) and the European tree frog occasionally added to limit large amounts of (Hyla arborea), both creatures that have shown a articles to only a few interesting ones, such as sharp decline in the Netherlands over the past ‘Eurasian’ to select just those about the Eurasian decades (H. arborea: Vos and Stumpel, 1995; N. water shrew, but as N. fodiens is commonly fodiens: Pellet et al. 2006; IUCN, 2012). referred to as just ‘water shrew’, this was not always preferable. When only a few dozen titles 2. Materials and Methods were left, visual inspection of the titles and/or In determining the scope of this research, several reading the summaries or abstracts helped select factors including time constraints, availability of those that were the most promising. data and use within the CARE project played a Along the way, more articles were obtained role. The latter two resulted in an investigation in through references in other articles. Part of a title the water shrew N. fodiens and tree frog H. (within double apostrophes) was inserted in arborea, as both of these were mentioned as Scopus and, if without result, Web of Science. important species in the project, and enough When neither resulted in the article sought, information was deemed to be available. Google Scholar was used as a last resort. If titles To find out whether temporary habitats could were very general or there was confusion on the provide an option for those two animals, the title and subtitle, an author’s name was usually following questions had to be answered: used, after which the specific article was found by year of publication. References also helped to find ~What are the environmental requirements for this (e.g. vegetation, food, humidity, size a number of interesting books or chapters of of area)? books, most of which were available from the Forum Library. Although peer-reviewed articles ~How long do these habitats take to develop? had a preference, some books provided important ~Do these habitats become more suitable as time progresses? key features of some animals not mentioned in the more technical articles (e.g. lifespan) or were ~What are the downsides for the temporary nature of the habitats for the animal (i.e. removal very often referred to in acquired literature (e.g. of housing)? Handoek natuurdoeltypen; Bal et al., 2001).

2 3. Results effect for the presence of some trees, although 3.1 Neomys fodiens relatively few shrews were found if the tree cover was dense. Bank height ranged from minimal to 3.1.1 Habitat and food 3m (shrews occurring more frequently with As its name suggests, the Eurasian water shrew N. banks >1.5m) and with inclinations anywhere fodiens has a preference for sites directly next to from 0° to 90°, with a positive effect for slopes of or at least nearby water, where it both lives and is >45°. active (Voesenek and Van Bemmel, 1984; Lardet, Castién and Gosálbez (1999) found N. fodiens in 1988; Bal et al., 2001). Rychlik (1997) caught 94% the Pyrenees at the edges of a wood (rich in of the water shrews at the water’s edge and the bramble patches and long grasses) and riverbanks, remaining 6% within 0.5 meter from water. Lack but not in moorlands or the beech forest itself. of fresh water has been suggested as the reason Heathlands are also mentioned by Greenwood et for the absence of N. fodiens on several islands in al. (2002) as being avoided by water shrews, Finland (Peltonen et al., 1989). Although poor while even built-up areas were inhabited. water quality has been thought to have a negative effect on the presence of N. fodiens (French et al., N. fodiens is a generalist in terms of food as well: 2001), either directly or by affecting prey a large amount of invertebrate groups (including, availability, Scott et al. (2012) more recently but not limited to: Diplopoda, Araneae, found them to be much more tolerant to Opiliones, adult Coleopterea, Diptera larvae, pollution than suggested, even the combined Trichoptera, Plecoptera) are on its diet, which it effects of phosphate, nitrate and pH being finds both in aquatic and various terrestrial minimal (this research was carried out in an environments (Rychlik, 1997; Castién and agricultural area in South-East England). The Gosálbez, 1999). ‘manual nature target types’ (Handboek When water shrews are found in very specific natuurdoeltypen, Bal et al., 2001) places water and highly localised areas, this is generally caused shrews in all kinds of wet environments, from by competition, in which case it chooses the mesotrophic ditches to lakes, both upstream and water niche, to which it is much better adapted downstream in rapid or still rivers and canals, than other shrews (Churchfield and Sheftel, 1994; even marshes and dune ponds. Castién and Gosálbez, 1999; Rychlik, 2005). Plans to re-introduce water shrews in the river While N. fodiens can opt for aquatic prey when Waal (Lenders et al., 1998) mention the terrestrial ones are rare (Castién and Gosálbez, possibility for a combination of floodplain forest 1999), and a normal diet can consist for well over and marshes, as well as side channels. Greenwood 50% of food from aquatic foraging (Rychlik, et al. (2002) found no preference for different 1997), high energetic costs of swimming and substrate types (rocks, gravel or mud), and also diving makes exclusive feeding in water less encountered the shrews over a wide variety of feasible (Churchfield, 1985). Scott et al. (2012) conditions: water depths from <0.25 to >1m, remarked that their tolerance for a large range of currents ranging from static to very rapid and sites could be caused by their opportunistic diet. widths of <1m to >5m, with no clear preference for any of these. The size of required habitats is unclear. Estimates N. fodiens lives in mountain streams with sandy for a core population (50 females or couples) or rocky banks and poor vegetation range from just 4 to 200 ha, depending on the (Spitzenberger, 1980), and Rychlik (1997) found quality, with reproductive units needing between no clear predilection for herbaceous cover on the 0.08 and 6 ha (Kalkhoven et al., 1995). Note that banks adjacent to the water (in lab experiments). Lenders et al. (1998) gives 100 individuals as the Greenwood et al. (2002), on the other hand, minimum population size, although it is not clear encountered water shrews less often in areas with if these are the total amount of animals (which none or little such vegetation, and saw a positive

3 would be 50 pairs) or the common method of some 2-4 km (Peltonen et al., 1989). Other than only listing the amount of females. Scott et al. that, no information on the dispersal of water (2012) mentions home ranges of some 60-80m2 shrews was found. Alternatively, animals could and densities of 3-5 per ha for these solitary and be caught and transported to another area, which territorial animals. As habitats of N. fodiens is probably also the most effective method of usually consist of narrow strips along water introducing the animals to the newly selected (Voesenek and Van Bemmel, 1984), these areas areas in the first place – note that any stress would be several hectometres long. caused will only be inflicted once on an individual, even if a certain area will only exist 3.1.2 Development for some 2 or 3 years, as shrews generally live for Assuming the streams and ditches necessary are only a single year, with the larger N. fodiens already present and no new ones are going to be reaching ages of 14 to 19 months (Churchfield, dug, few adaptations would be required. As 1990). mentioned, even polluted waters were used by water shrews without difficulty (Scott et al., 3.2 Hyla arborea 2012). Some vegetation may be preferred by the 3.2.1 Habitat and food animals when available (Greenwood et al., 2002), As is true for most , both aquatic and but even in the wild, water shrews are not always terrestrial environments must be considered for found near covered banks (Spitzenberger, 1980). the habitat of the tree frog. Rychlik (1997) commented on the little influence H. arborea is generally attracted to pools with of presence or absence of vegetation on the warmer water that are exposed to sunlight behaviour of N. fodiens. As such, planting of (Warren and Büttner, 2008), with a low water herbaceous vegetation is not a requisite. level and no eutrophication (Andersen et al., 2004), nor any with fish that predate on juvenile 3.1.3 Improvement with time (Brönmark and Edenhamn, 1994). Brackish Due to the inconsistency in data concerning the water is unsuitable for reproduction (Vos and exact effects of pollution (French et al. 2001 and Stumpel, 1995) and still water is preferred to Scott et al. 2012), it can for now be assumed that running water (Große and Nöllert, 1993). water shrews are able to live in water of poor Plant cover in the pools is generally, but not quality, but perhaps a smaller amount of aquatic always, preferred, although very dense vegetation prey is available in such a situation. Less may prevent the deposition of eggs on them eutrophication caused by farming activity may (Warren and Büttner, 2008) - such choking not be a requirement, but might result in a more vegetation can be caused by eutrophication suitable habitat. (Brönmark and Edenhamn, 1994). Vegetation can consist of Glyceria fluitans, Ranunculus aquatilis, 3.1.4 Drawbacks of temporality Ranunculus baudotti, Lemna minor, Zannichellia Water shrews are known to not stick in a specific palustris and Ceratophyllum submersum (Große site very long, but instead live a nomadic lifestyle and Nöllert, 1993; Vos and Stumpel, 1995). (Greenwood et al., 2002; Scott et al., 2012), Although there are many differences in the water possibly shifting sites depending on suitability. At types used by tree frogs, Große and Nöllert (1993) only 1 out of 26 study sites, Scott et al. (2012) found those in gravel or clay pits to be occupied found evidence of constant habitation on 7 very often, due to a high quality of groundwater consecutive samplings. As such, termination of a there. Cattle ponds and marshy areas in pastures special habitat (in as far as it exists) may not be a are also used by tree frogs to good effect in big problem. Shrews have been found on islands Germany, Denmark and Sweden, as these are several kilometres from the mainland of Finland, shallow and exposed to sun. Steep shores are which required swimming or walking over ice avoided by tree frogs (Große and Nöllert, 1993).

4 The area directly surrounding the water features A minimum area of 1km2 is deemed necessary for should ideally be kept clear of vegetation, so that a population of H. arborea, although the size of the water is exposed to sun for as long as possible the ponds does not seem to be of much influence (Große and Nöllert, 1993). Vos and Stumpel (Brönmark and Edenhamn 1994; Vos and (1995) found H. arborea to occur more often in Stumpel, 1995). Calling sites were even found in ponds with less vegetation very close to them, but garden ponds (Große and Nöllert, 1993). When high perennial herbs and shrubs at larger larger water features were avoided, this was distances (≥250m) had a strong positive effect on likely caused by the depth (Große and Nöllert, the presence of tree frogs, and these were 1993). considered to be part of the terrestrial habitat. Relatively little information was available on this Studies on food of H. arborea are mostly focussed subject, as most research seems to have been done on what is eaten during the reproductive season, on reproduction, and as such the aquatic habitats. but a year-long study on the feeding habits of tree The data that was found almost solely related to frogs showed significant variety during that year summer habitats, with only a few observations on (Kovács et al., 2007). Positive correlations were hibernation sites. found between H. arborea and the presence of In Sweden, terrestrial habitats consist of pastures Notonecta, Corixa and Zygoptrea larvae by Große with trees and bushes (Brönmark and Edenhamn, and Nöllert (1993). Kóvacs et al. (2007) found 1994), and high herbs, bushes and trees are Araeneida, Isopoda, Heteroptera, Homoptera, generally mentioned, with brambles (Rubus Coleoptera (both larvae and imago), Trichoptera fruticosa) often named specifically (Stumpel, (larvae), Brachycera (imago), Nematocera (imago) 1993). Planting of conifers is mentioned as very and Hymenoptera, in large quantities, with threatening to tree frogs (Corbett, 1989). several other groups of Arthropoda eaten to a Agricultural areas, marshy lands and forests (the lesser extent. Aquatic prey (Gastropoda, Isopoda, latter only in Sweden) are mentioned as possible Nematocera larvae) were just encountered in terrestrial habitats by Stumpel (1993), who March, and then only rarely. This variation of stresses the need for high vegetation in direct prey was thought to be caused by different sunlight, which the frog requires for its ecotypes occupied by H. arborea during the year, thermoregulation. Although H. arborea is often as well as seasonal availability caused by the life found in trees, this seems mostly in order to be as cycles of the prey. high (and as such: in as much sunlight) as possible. 3.2.2 Development Hibernation may sometimes occur in the summer Große and Nöllert (1993) commented on the use habitats, but tree frogs moving away from those of cattle drinking ponds by H. arborea, which regions at the end of the season have been would be ideal aquatic habitats if available in the observed (Fog, 1993). Bal et al. (2001) mentions Baakse Beek area. Also mentioned was a ponds and ditches both as places of reproduction preference for relatively new water areas, and as suitable hibernation sites. possibly caused by the absence of predators. On Reported distances between the aquatic and the other hand, amphibians, including H. terrestrial habitats vary from several hundreds of arborea, are known to continue to migrate to old metres to some kilometres (Fog, 1993; Stumpel, breeding sites, even when there has been a 1993). change in land use (Schuler et al., 2013). Human disturbance has little effect on tree frogs, Similarly, Warren and Büttner (2008) mention and breeding populations have been found in conflicting observations on the possible military training areas (Warren and Büttner, preference of tree frogs for temporary pools for 2008) reproduction. Construction of ponds has been mentioned in the CARE project as a strategy to

5 combat drought caused by climate change in the available habitats could prove to be a problem, as Baakse Beek area (Karssing, 2012). H. arborea is known to live 3-6, sometimes up to 9 years in the wild (Moravec, 1993; Pellet et al., A high ground water level, usually absent from 2006). Fog (1993), however, mentions little agricultural areas (Van Egmond and De Koeijer, affinity of tree frogs to certain ponds, with 2007) may be necessary for the tree frogs in their individuals switching between different breeding summer habitat, but perhaps only if no fresh areas, hundreds of meters apart, within a single water sites are available (Stumpel, 1993). Heavily season. Andersen et al. (2004) considers clusters cultivated lands are not suitable anyway due to of ponds within 4 kilometres of each other to be eutrophication of breeding ponds (Andersen et used by a single population of tree frogs, and a al., 2004) – this will also mean that ditches nearby study in Sweden showed barely any genetic agricultural lands, mentioned by Bal et al. (2001) variation in such a group. as breeding and hibernation areas, will often be too heavily polluted to be of use. 3.3. Similarities and differences Both N. fodiens and H. arborea are dependent on Although exact requirements for the terrestrial water, although the tree frog is clearly more habitats are as of yet unclear, high vegetation is a demanding in terms of water quality (Scott et al., common factor (Stumpel, 1993). Hedgerows 2012; Andersen et al. 2004). Shores for the frog defining the edges of property, reed next to ponds should be gentle (Große and Nöllert, 1993), while and lines of trees next to roads may suffice, and steeper banks are preferred by the water shrew trees are relatively widely available in the Baakse (Greenwood et al. 2002). Both species are Beek area (own observation). Brambles (R. insectivores with a generalist diet (Castién and fruticosa) could be planted, as these grow fast, are Gosálbez, 1999; Kóvacs, 2007). Terrestrial habitats commonly found near H. arborea (Stumpel, 1993) should ideally have some vegetation, although and the flowers could attract suitable prey, many this is not strictly necessary for the water shrew of which are pollinators. It should be noted that a (Spitzenberger, 1980), and dense trees are not large distance between aquatic and terrestrial preferred by this animal (Greenwood et al. 2002), areas, often the case in agricultural landscapes, while tree frogs are generally observed in areas results in high mortality during migrations (Vos with much and high vegetation (Stumpel, 1993). and Stumpel, 1995) and as such should ideally be Brambles are specifically mentioned for both avoided. (Castién and Gosálbez, 1999; Stumpel, 1993). Being solitary animals (Scott et al., 2012), N. 3.2.3 Improvement with time fodiens may need a larger area for a stable The aforementioned problems of eutrophication population than H. arborea, depending on habitat of waters could be countered by specific quality (Kalkhoven et al. 1995; Vos and Stumpel, measures, but Melman et al. (2005) give a 1995). Both animals are undeterred by human timescale of 8 years for the impoverishment of activity and disturbances (N. fodiens: Greenwood land (by mowing and removing products over the et al., 2002; H. arborea: Warren and Büttner, entire period), which is well outside the scope of 2008). temporary habitats. As such, ponds and ditches Finally, it should be noted that little competition unsuitable for H. arborea will not become between these two animals is likely to exist. They available in the short run. have different microhabitats: water, subsoil and surface for N. fodiens (Castién and Gosálbez, 3.2.4 Drawbacks of temporality 1999); H. arborea mainly high up in vegetation If tree frogs do indeed continue to return to old and aquatic only in the larval stadium, possibly breeding sites, even when these are no longer during hibernation (Große and Nöllert, 1993; available (Schuler et al., 2013), temporarily Stumpel, 1993). These different niches also result

6 in different prey caught, with the water shrew includes the slow process of natural succession of generally relying on large amounts of aquatic plants (Marshall and Moonen, 2002), as well as (Rychlik, 1997), while a tree frog migration of especially meadow birds and rarely feeds on aquatic prey, if at all (Kovács, butterflies (Bal et al., 2001). When suitable 2007). habitats are available for either of the two investigated creatures in the Baakse Beek area, 4. Conclusion and Discussion they should be released there to circumvent that The high tolerance of the Eurasian water shrew problem. Lack of natural succession and to a large variety of environmental conditions development may even lead to an absence of and the opportunistic diet, combined with fast predators and competitors, which could make the rate of life and dispersal qualities, could make this temporary habitats into relatively safe breeding animal well-suited to temporary nature schemes. areas for these declining species. Availability of suitable water features is the main On the other hand, Uthes and Matzdorf (2013) barrier for the European tree frog, and the warned that small isolated areas may result in apparent return to old breeding ponds combined ecological traps for animals, in which case the with a relatively long lifespan make this a less relatively limited dispersal abilities of these attractive species in such plans. creatures could be disastrous, and great care should be taken to avoid such situations. It should be noted that areas on the edges of It should be mentioned that most research on the arable land, including ditches, suffered much less exact needs of animals and the development of of eutrophication if a strip of only a few metres their habitats (e.g. Kalkhoven et al., 1995; Bal et adjacent to these was kept clear of fertilisation, al., 2001) concerns birds and butterflies, due to and most negative effects were found in the first the amount of available literature, their known half metre next to fertilised lands (Melman et al., indicative properties for the quality of an 2005). Measurements of pollution in ditches environment and their general appeal (Schaminee within the Baakse Beek area, as well as closer and Jansen, 1998). Not all questions asked by a.o. inspection of the exact practices of the farmers Borgula in 1993 on the decline of H. arborea have there, should be carried out to find if ponds and been answered yet, and the similar decrease in N. ditches are in fact suitable for especially the tree fodiens is difficult to explain when one considers frog. its varied diet and tolerance to environmental conditions. As long as such gaps in our Due to the relative novelty of agri-environmental knowledge on these species exist, defining the schemes, little information on the outcomes of optimal habitats is difficult at best. existing projects is available (Van Egmond and De Koeijer, 2007). Buffer strips adjacent to ponds, 5. Acknowledgements consisting of less than 4% of an arable area of First and foremost, I would like to thank Dr. some 800 ha, resulted in a three- to fivefold Pytrik Reidsma for her supervision, comments increase of some amphibians (crested newt and help on this research. Some additional Triturus cristatus and fire-bellied toad Bombina substantive comments were given by Dr. Claire bombina) in a 5-year period (Berger et al., 2006). Vos, for which I’m grateful. Thanks to the However, timescales for some desired results are department for allowing me to use their services estimated at well over 10 years, with >25 years and facilities, and finally to all who helped not exceptional for some types of landscapes, and motivate me to keep going. the former only true with an optimal initial situation (Van der Zee et al. 2004). On the other hand, it should be noted that these timescales are for the formation of entire ecosystems, which

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