Habitat edges affect patterns of artificial nest predation along a wetland-meadow boundary

Acta Oecologica 59 (2014) 91e96

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Original article

Habitat edges affect patterns of artificial nest predation along a wetland-meadow boundary
a, *, Toma
s Albrecht b, c Petr Suvorov a, b, Jana Svobodova
129, 165 21 Prague 6, Czech Republic Department of Ecology, Faculty of Environmental Sciences, Czech University of Life Sciences, Kamýcka
7, 128 44 Prague, Czech Republic Department of Zoology, Faculty of Science, Charles University in Prague, Vinicna c
8, 603 65 Brno, Czech Republic Institute of Vertebrate Biology, Academy of Sciences of the Czech Republic, v. v. i., Kv etna a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 October 2013 Accepted 16 June 2014 Available online

Wetland habitats are among the most endangered ecosystems in the world. However, little is known about factors affecting the nesting success of birds in pristine grass-dominated wetlands. During three breeding periods we conducted an experiment with artificial ground nests to test two basic mechanisms (the matrix and ecotonal effects) that may result in edge effects on nest predation in grass-dominated wetland habitats. Whereas the matrix effect model supposes that predator penetrate from habitat of higher predator density to habitat of lower predator density, thus causing an edge effect in the latter, according to the ecotonal effect model predators preferentially use edge habitats over habitat interiors. In addition, we tested the edge effect in a wetland habitat using artificial shrub nests that simulated the real nests of small open-cup nesting passerines. In our study area, the lowest predation rates on ground nests were found in wetland interiors and were substantially higher along the edges of both wetland and meadow habitat. However, predation was not significantly different between meadow and wetland interiors, indicating that both mechanisms can be responsible for the edge effect in wetland edges. An increased predation rate along wetland edges was also observed for shrub nests, and resembled the predation pattern of real shrub nests in the same study area. Though we are not able to distinguish between the two mechanisms of the edge effect found, our results demonstrate that species nesting in wetland edges bordering arable land may be exposed to higher predation. Therefore, an increase in the size of wetland patches that would lead to a reduced proportion of edge areas might be a suitable management practice to protect wetland bird species in cultural European landscapes. © 2014 Elsevier Masson SAS. All rights reserved.

Keywords: Artificial nest Ecotonal effect Habitat fragmentation Nest predation Wetland meadow

1. Introduction Nest predation is the main factor driving nesting failure in birds, and may significantly influence the dynamics of avian populations (Ricklefs, 1969). Temporal and spatial variations in nest predation
n, 1992; Fisher and Wiebe, 2006; rates are well documented (Andre Lahti, 2001; Martin, 1993; Sieving and Willson, 1999), though they are not well understood or explained (Donovan et al., 1997;
et al., 2011). For example, nest predaGustafson, 2005; Koubova tion can be higher in habitat edges compared to habitat interiors (the edge effect; Gates and Gysel, 1978). Since the proportion of edges increases with habitat fragmentation, the edge effect can be * Corresponding author. Department of Ecology, Faculty of Environmental Sciences, Czech University of Life Sciences, Kamýck
a 1176, 165 21 Prague, Czech Republic. Tel.: þ420 224 383 852; fax: þ420 224 382 868. E-mail address: [email protected] (J. Svobodov
a). http://dx.doi.org/10.1016/j.actao.2014.06.003 1146-609X/© 2014 Elsevier Masson SAS. All rights reserved.

responsible for low nesting success and consequently for population declines of birds over large regions (Murcia, 1995). The existence of the edge effect has been very well documented in North American and Scandinavian studies (along arable landforest borders), but is less apparent in mosaic European land
ldi, 2004). Nevertheless, increased nest scapes (see Bat
ary and Ba predation rates in interior habitats compared to edge zones (an opposite edge effect) have also been demonstrated in some experiments (Marini et al., 1995; Storch, 1991). Such inconsistent results often depend on the predator community (Johnson et al., 1989; Lahti, 2001) and landscape context (Bayne and Hobson, 1997; Clurk and Nudds, 1991; Driscoll and Donovan, 2004). Moreover, it is evident that the edge effect is a dynamic process with temporal variation (Chalfoun et al., 2002; Pasitschniak-Arts and Messier, 1995; Stephens et al., 2003; Svobodov
a et al., 2011, 2012), and therefore research conducted over just 1e2 years may not be able to detect it.

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Many studies have investigated the edge effect, but mainly in
n, 1992, Bata
ry and agricultural landscapes with hard edges (Andre B
aldi, 2004; Bayne et al., 1997; Conner and Perkins, 2003; Donovan et al., 1997; Huhta et al., 1996; Lahti, 2001; Major and Kendal, 1996; Wilcove et al., 1986). Data from lower-contrast edges such as the transition zone between wetlands and meadows are less available (e.g. Pasinelli and Schiegg, 2006; Wallander et al., 2006), despite the fact that wetland habitats are among the most endangered ecosystems in the world (Zedler and Kercher, 2005). In Central Europe, research on nesting success in wetlands has been mostly
ry et al., restricted to reed habitats (e.g. B
aldi and Bat
ary, 2005; Bata
ldi, 2004, 2005; Schiegg et al., 2007; Trnka 2004; Bat
ary and Ba et al., 2009). Although open bogs and inundated meadows of Central Europe provide important breeding sites for particular threatened species such as waders (e.g. Common Snipe, G. gallinago; Common Redshank, Tringa totanus), Corn Crake (Crex crex), Hen Harrier (Circus cyaneus) or Black Grouse (Tetrao tetrix) (Hagemeijer and Blair, 1997), studies from grass-dominated wetlands are rare (Albrecht, 2004). Although there is no doubt that the edge effect occurs in many
ry and Ba
ldi, 2004), mechanisms leading to the habitats (Bata occurrence of habitat edges on avian nest predation have rarely
n and Angelstam, 1988; Svobodova
et al., been evaluated (Andre 2011, 2012). Edge effects on nest predation can be predicted based on the distribution of habitat-specific resources along the borders of two adjacent habitats and consequent patterns in predator occurrence and movements (Ries and Sisk, 2004, more in
et al., 2011). Basically, there are two models linking Koubova predator movements with elevated nest predation rates in habitat edges: (1) the matrix effect model supposes that predators penetrate from habitats of higher quality (for the potential nest predators) to habitats of lower quality, and cause an edge effect in the lower quality habitats (also termed the spillover model, sensu Lidicker, 1999) and/or (2) edges may contain complementary resources from both adjacent habitats (of the same or different quality) and/or can contain specific resources which can be specifically used by nest predators. In addition, some predator species may also focus their activity specifically around edge structures and re and Messier, 2000; use them as travel lines (Bider, 1968; Larivie a
lek et al., 2009). This leads to a higher predator density at the S border between adjacent habitats (the ecotonal effect model; Lidicker, 1999). The aim of our study was to analyse patterns of nest predation in a grass-dominated wetland surrounded by forests, pastures and harvested meadows. In our study area, Albrecht (2004) has already demonstrated higher predation on nests of a small shrub- and open-cup nesting passerine, the Scarlet Rosefinch (Carpodacus erythrinus), along wetland edges bordering arable land than in the interior of wetland habitat. However, the mechanism responsible for this edge effect was not evaluated because Rosefinch nests occurred only in the wetland habitat, i.e. not in whole transition zone between the wetland and meadow. Using artificial ground nests distributed along whole habitat gradient (i.e. the edge and interior of the wetland and harvested meadow respectively), we were able to test the basic mechanisms of the edge effect, i.e. the matrix versus the ecotonal effect models. We assumed that higher nest predation in habitat edges (i.e. in meadow edge and/or wetland edge) than in habitat interiors would support the ecotonal effect model, and lower nest predation in the wetland interior than in edge habitats and the meadow interior would correspond with the matrix model (also see Lidicker, 1999). We expected to find the matrix effect along the habitat gradient between wetland and meadow because a higher density of generalist nest predators usually occurs in the surrounding arable land (hay meadows and
n and Angelstam, 1988, also pastures in our study area; also Andre

see the discussion in Albrecht, 2004). In addition, we used artificial shrub nests to test whether the spatial distribution of their predation resembles the predation patterns of ground nests, and corroborate patterns of natural nest predation on shrub-nesting Rosefinches. 2. Materials and methods 2.1. Study site The study locality was situated in the Vltava River Valley of the  Sumava Mts. (Bohemian Forest) National Park (48 470 e48 530 N, 13 570 e13 510 E, 800 a. s. l., 24 km2), Czech Republic, which is one of very few areas of primary non-forested habitats in Central Europe
dlo and Bufkova
, 2002). The area was mainly composed by (Sa periodically inundated wetlands (25%) surrounded by coniferous or mixed forest (15%) and extensively used meadows (mainly harvested for hay) (60%). For the purpose of this study, we distinguished two main habitat types: (1) wetland; mostly created by a mosaic of shrub and humid herbal vegetation that is regularly flooded during the AprileMay period. The dominant species of this oligotrophic wetland ecosystem were Sphagnum, Spiraea salicifolia, Phalaroides sp., Glyceria sp., Carex sp., Eriophorum sp., and Filipendula ulmaria. (2) Surrounding hay meadows and pastures; composed mainly of meadow grasses (Poa sp., Festuca sp.) and other herbs (Taraxacum, Trifolium). Shrubs were completely absent in this habitat. Since the edge zone between wetland and meadow was relatively sharp (within 15 m of the habitat border), the vegetation structure does not significantly differ from the surrounding habitat. Hence, there were four types of nest locations (see below), i.e., wetland and meadow interiors, wetland edges (towards the meadows), and meadow edges (towards the wetland) (more details in Albrecht (2004). 2.2. Experimental design To test if nest predation is influenced by the distance of the nest from a habitat edge we used two types of artificial nests, i.e. ground nests and shrub nests. A ground nest was constructed as a small depression in the ground lined with a small amount of dry grass. The cup of a shrub nest was created from half of a tennis ball covered by soil and plant material, fixed by wire to a shrub branch. Since the rubber scent of tennis balls can discourage potential predators, the shrub nests were aired for 14 days (Davison and Bollinger, 2000). Whereas the ground nests may have resembled the nests of ground nesting bird species such as Corn Crake, Black Grouse, Common Quail (Coturnix coturnix), or Whinchat (Saxicola rubetra), the shrub nests represented nests of open-cup shrub nesting species such as the Scarlet Rosenfinch and Whitethroat (Sylvia communis). All these species regularly occur in our study area (Hora et al., 1997). The experiment was conducted from midMay till mid-June (i.e. 2005 May 20th, 2006 June 10th, 2007 June 1st), which is the average period of clutch laying for these species in the CR (Albrecht, 2004, 2011). Further, experiments were usually initiated at least four weeks after the spring floods because the presence of floodwater in the wetland could have had a major impact on predator activity (Lecomte et al., 2008). Each nest was baited with two quail eggs, and one of them was filled with wax for predator identification (hereafter wax eggs; Storch et al., 2005; Thompson and Burhans, 2004). In both nest types, wax eggs were anchored in the nest pits with a string and nail in order to prevent predators from carrying them away (Suvorov et al., 2012). In total, 360 ground nests were installed during the three breeding periods (2005e2007), i.e. 120 every year in different localities. Nests were randomly located (see below) along a

P. Suvorov et al. / Acta Oecologica 59 (2014) 91e96

wetland e meadow gradient: from the wetland interior across the wetland edge and meadow edge to the meadow interior (i.e. 30 nests in each of 4 habitats, wetland and meadow interiors, wetland and meadow edges). The shrub nests were installed in only two periods (2006e2007) and two habitat types, 31 nests in the wetland interior and 31 nests in the wetland-meadow edge every year (i.e. 124 in total). Similarly to ground nests, the shrub nests were placed in different localities every year. Whereas all interior nests (i.e. ground and shrub nests) were installed at least 100 m away from habitat borders (also see Albrecht, 2004 for the edge effect on real Rosefinch nests in our study area), all edge nests were placed at distances up to 15 m from habitat edges. To avoid pseudoreplication, the minimum distance between neighbouring nests in each habitat was 100 m (Gehring and Swihart, 2003). The nests were initially randomly distributed within the study area using an aerial map. However, these nest locations were then slightly adjusted so they would be in sites with similar vegetation cover, because vegetation cover is one from the main factors influencing
et al., 2004). nest success (e.g. Albrecht, 2004; Svobodova All nests were checked only once after a ten-day exposure to reduce the observer effect and to preserve nest concealment €rt, 2004). The nest was (Martin and Joron, 2003; Villard and Pa considered as depredated if at least one of the eggs was damaged or completely missing in the nest bowl. Red Fox (Vulpes vulpes), mustelids (Martes sp.), Wild Boar (Sus scrofa) and corvid birds (Corvidae) were expected as the most likely nest predators in our study area, because all these animals are able to handle quail eggs re, 1999; Montevecchi, 1976). On the by their bills or mouths (Larivie other hand, rodents were not considered to be likely nest predators of our artificial nests because they usually are not able to bite through the thick shells of quail eggs (DeGraaf et al., 1999; P€ art and Wretenberg, 2002). Nest predators were identified by beak or tooth marks left on the eggs and were divided into three categories: bird predator, mammalian predator, and unidentified. Due to irregular flood events we were not able to find some ground nests (2 in 2005, 1 in 2006, 3 in 2007) and shrub nests (6 in 2006). Therefore, only 354 ground nests and 118 shrub nests were included in the final analysis.

2.3. Statistical analysis The probability of nest predation was analysed by generalized linear models (GLM), where nest fate was the binary dependent variable with binomial distribution (predated ¼ 1, successful ¼ 0). Ground nests and shrub nests were analysed separately, so two models were created. Explanatory variables in the models were the effects of nest location (4 levels in the model for ground nests, 2 levels in the model for shrub nests) and year (3 levels in the model for ground nest, 2 levels in the model for shrub nests). The significance (p < 0.05) of a particular term and their two way interaction was based on the change in deviance between the full and reduced/ null models, with degrees of freedom equal to the difference in the degrees of freedom between the models with and without the term in question. All non-significant terms were removed using a backward stepwise procedure to reach the minimal adequate models, i.e. models with all terms significant (Crawley, 2002). All GLM models were performed in the software R.2.12.1 (R Development Core Team, 2008). According to the results of our previous study (Albrecht, 2004), we expected higher nest predation in wetland edges than in wetland interiors (i.e. a positive edge effect). The mechanisms leading to the emergence of the edge effect were assessed by a posteriori comparison of specific levels of habitat type using Tukey's HSD tests (package multcomp; Hothorn et al., 2008).

93

3. Results In ground nests, the total nest predation rate after 10 days reached 33.1% (n ¼ 39), 23.5% (n ¼ 28) and 34.2% (n ¼ 40) in 2005, 2006 and 2007, respectively. Unfortunately, we found only two reliable predator imprints, classified as a small carnivore and a bird. Both of them were found in the wetland interior. Nest predation on shrub nests varied substantially between years. Whereas in 2006 nest predation reached 57.1% (n ¼ 32), in 2007 it was only 6.5% (n ¼ 4). 3.1. Ground nests In the model for ground nests along the habitat gradient between wetland and meadow interiors (n ¼ 354), predation did not change among years. The only significant variable in the model was the effect of nest location (Table 1). The a posteriori comparisons revealed higher nest predation in the wetland edge (z ¼ 3.303, P ¼ 0.005) and meadow edge (z ¼ 3.030, P ¼ 0.013; Fig. 1) than in the wetland interior. Although nest predation was also higher in the meadow interior than the wetland interior (Fig. 1), this difference was not significant (z ¼ 0.376, P ¼ 0.195). 3.2. Shrub nests The results of the GLM (n ¼ 118 nests) showed the significant effects of nest location on nest predation (Table 1). Nest predation was significantly higher in wetland edges than in wetland interiors (Fig. 2). However, the interactive effect between the nest location and the year indicates that the difference between these habitats was significant only in 2006 but not in 2007, when the predation rate was very low (see above). 4. Discussion Our results showed higher nest predation along wetland edges on both ground and shrub nests. This supports the existence of an edge effect on nest predation in our study area, and corresponds with results of our previous study (Albrecht, 2004) that found evidence for an edge effect on open-cup nesting passerine, the Scarlet Rosefinch, in exactly the same area. There are some studies providing evidence for an edge effect in wetland habitats (e.g. Albrecht et al., 2006; Bat
ary et al., 2004; Pasitschniak-Arts and Messier, 1995), but there are just few studies evaluating the edge
ry and Ba
ldi, 2004), and none has effect in European wetlands (Bata yet been able to uncover mechanisms that lead to edge effects because predation risk has not been investigated throughout the entire edge gradient (i.e. in the habitat edge and both adjacent habitats).

Table 1 GLM results for the probability of predation on ground and shrub nests along the transition gradient between wetland and meadow (n ¼ 354). Model parameters

Estimate

SE

z-Value

P

Ground nests Intercept Wetland edge Meadow interior Meadow edge

1.651 1.105 0.743 1.199

0.292 0.365 0.376 0.363

5.660 3.030 1.977 3.303

<0.001 0.002 0.048 0.001

Shrub nests Intercept Wetland interior Year Year: habitat type

1.526 2.273 4.927 3.441

0.493 0.638 1.130 1.345

3.093 3.562 4.361 2.558

0.002 0.000 <0.001 0.011

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Fig. 1. Mean probability (±95% confidence intervals) of predation risk on ground nests along a wetland e meadow gradient (2005e2007) based on the GLM (n ¼ 354). Different letters above bars indicate significant differences between particular nest locations.

The simplest mechanism leading to an edge effect would be a predator penetrating from a habitat of higher quality (i.e. arable land) to a habitat of lower quality (i.e. the oligotrophic wetland ecosystem in our study area; Lidicker, 1999; Ries and Sisk, 2004). Previous studies have intuitively expected the matrix effect in the transition zone between extensively used meadows and wetland

Fig. 2. Mean probability (±95% confidence intervals) of predation risk on shrub nets in a wetland edge and wetland interior (2006e2007) based on the GLM (n ¼ 118).

meadows (i.e. the penetration of predators from meadow to wetland edges), because the cultivated surrounding matrix such as arable area usually provides more and higher quality resources, and hence maintains a high density of general predators (Albrecht, 2004; Albrecht et al., 2006; Verling, 2000). Alternatively, predators might avoid wetland interiors due to dense vegetation (Picman et al., 1993) that might impede their movement (Crabtree et al., 1989). However, our observations do not fully support this scenario because nest predation was not substantially higher in meadow interior and edge habitats than in wetland interior. The ecotonal effect model, when nest predation is elevated just along the edge and low in interiors, has been already documented
et al., 2011). That study on a forest-meadow continuum (Svobodova found increased carnivore activity along both forest and meadow edges. For such an increase in predation risk along edges, at least three not-mutually-exclusive explanations have been proposed. (1) Predator specialists typical for one habitat may mix with predators from another habitat at their interface, and compound the intensity of nest predation. (2) Some predators may preferentially use habitat edges because their prey is concentrated along the edge structure. Alternatively, an adjacent habitat may contain complementary resources (qualitatively different) (Ries and Sisk, 2004). Finally (3), some predator species may also focus their activity specifically around edge structures and use them as travel lines re and Messier, 2000; (Bider, 1968; Frey and Conover, 2006; Larivie Phillips et al., 2003). The last two scenarios are the most likely in Central Europe, since previous studies from the region have shown that important generalist nest predators such as small mustelids and martens (Weidinger, 2009) prefer habitat edges and tend to 
et al., 2011). In move along them (Cervinka et al., 2011; Svobodova our study area, we also found that nest predation in both edge habitats was substantially higher along habitat edges. Although nest predation was higher in the meadow interior than in the wetland interior, this difference was not significant. Our results therefore do not fully support either the ecotonal effect model or the matrix effect model. Actually, both processes may have been present. Whereas predators species like foxes and small mustelids may have preferentially used habitat edges in our study area, corvids may have penetrated from the arable matrix to wetland edges. Unfortunately, it is impossible to resolve this issue without reliable predator identification. We identified only two imprints because most depredated eggs completely disappeared from the nests. It is possible that intermediate-sized bird predators like corvids that tend to carry eggs away are responsible for the missing eggs (Suvorov et al., 2012). Further experiments, using video recording for example, would need to be conducted to better identify nest predators (Weidinger, 2009). Studies based on artificial nests have been criticized mainly because of the absence of parental defence and the different species composition of nest predators, both of which may bias the results € m, 1988; (Major and Kendal, 1996; Willebrand and Marcstro Zanette, 2000). However, results from observational studies (i.e. using natural nests) can be also problematic. First, natural nests detected by observers may be more visible than the average for the population (Martin and Geupel, 1993). Second, predation risk usually differs between bird species because clutch conspicuousness differs interspecifically due to nest placement, nest construction and parental behaviour (Flashpohler et al., 2001; Kreisinger and Albrecht, 2008; Weidinger, 2002). Hence, artificial nest experiments under certain condition are a useful tool in controlling these factors and can help provide data elucidating the matrix and ecotonal effects. In our current study, we used artificial shrub nests and compared the predation pattern found to results of our previous study on natural Scarlet Rosefinch nests. Although these studies were conducted in different time periods (real nests:

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1995e1998, artificial nests: 2006e2007), the increased predation on artificial nests along wetland edges resembled the predation pattern reported for real Rosefinch nests in the same area (Albrecht, 2004). As already mentioned above, Albrecht (2004) suggested that penetrating predators from surrounding managed meadows were responsible for that predation pattern (i.e. the matrix effect model). Unfortunately, our experimental design did not allow us to resolve this issue because we were not be able to monitor nest predation on shrub nests across the whole habitat gradient (shrubs are absent in meadows) although the results from the ground nests suggest that both mechanisms may be driving the edge effect in our study area. In contrast to ground nests, predation on artificial shrub nests varied from year to year. This between-year variation in the edge effect can be caused by temporal variations in population densities (and hence predation risk) of predators that prefer a particular ^ty et al., 2001; Wilson and Bromley, 2001), or by habitat type (e.g. Be the temporal variation in habitat preferences of individual predators (e.g. Lecomte et al., 2008; Zub et al., 2008). In conclusion, we found that predation pressure on artificial ground and shrub nests generally increased along wetlandmeadow edges. Although we are not able to completely explain the mechanism responsible for this pattern, our results indicate that both the ecotonal and matrix effect models could be applied. Our study can provide some clues for the protection of shrub and ground nesting bird species in wetland habitats. Results of our study demonstrate that birds breeding in wetland edges bordering on meadows may suffer from increased predation pressure. As shown previously, this may negatively affect populations of wetland breeding specialists, such as Scarlet Rosefinches (Albrecht, 2004). From the regional scale point-of-view, increasing the area of wetland habitats is a widely recommended management practice (e.g. Brown and Dinsmore, 1986), because larger areas host larger populations that are less vulnerable to extinction from stochastic reasons (Brown and Kordic-Brown, 1977). From the local scale point-of-view, we also recommend increasing the area of wetland patches, because properly designed patch shape leads to a decrease in the unfavourable edge/interior ratio and hence minimizes the edge effect on bird populations breeding in wetlands. Acknowledgement We thank Jan Schnitzer, Pavel Munclinger, Michal Vinkler and Jirí Bug
ar for their technical support. We are also grateful to the Editor-in-Chief Roger Arditi and four anonymous referees for critical comments to our manuscript. The study was supported by Grant Agency of Charles University (No. 191/2004/B-Bio), by Czech Science Foundation (No. 206/06/0851) and Grant Agency of Czech University of Life Sciences (IGA 20134222/2013). References Albrecht, T., 2004. Edge effect in wetland e arable land boundary determines nesting success of Scarlet Rosefinch (Carpodacus erythrinus) in the Czech Republic. Auk 121, 361e371.   (Eds.), ní. In: St'astný, Albrecht, T., 2011. Hýl rudý e Hnízde K., Hudec, K., Fauna, C.R. Scarlet Rosefinch eNesting. Academia Praha, p. 1189. Aves III/2 (in Czech). Albrecht, T., Hor
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