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Journal of Insect Conservation

, Volume 21, Issue 3, pp 477–485 | Cite as

Effect of host species, host nest density and nest size on the occurrence of the shining guest ant Formicoxenus nitidulus (Hymenoptera: Formicidae)

  • Salla K. HärkönenEmail author
  • Jouni Sorvari
ORIGINAL PAPER

Abstract

Understanding habitat requirements of species is important in conservation. As an obligate ant nest associate, the survival of the globally vulnerable shining guest ant, Formicoxenus nitidulus, is strictly tied to that of its hosts (mound building Formica ants). We investigated how host species, nest density, inter-nest distance and nest mound size relate to the occurrence of F. nitidulus. In total, 166 red wood ant nests were surveyed in SW Finland (120 Formica polyctena, 25 F. rufa, 14 F. aquilonia, 5 F. pratensis, and 2 F. lugubris). Overall, F. nitidulus was found in 60% of the nests. For the actual analysis, only F. polyctena and F. rufa nests were included due to the small number of other nests. F. nitidulus was more likely to be found among F. polyctena than F. rufa. Also, while inter-nest distance was not important, a high nest density, commonly found in polydomous (multi-nest) wood ant colonies, was beneficial for F. nitidulus. The guest ant was also more likely to be found in large host nests than small nests. Thus, our results show that the best habitat for the guest ant is a dense population of host nest mounds with a high proportion of large mounds. Conservation efforts should be directed at keeping the quality of the red wood ant habitats high to preserve their current populations and to increase colonization. This will not only benefit the guest ant, but also a plethora of other species, and help in maintaining the biodiversity of forests.

Keywords

Formicoxenus nitidulus Formica Social parasite Xenobiosis Conservation Metapopulation 

Introduction

Human induced habitat loss and fragmentation are a serious threat to the persistence of numerous species (e.g. Tscharntke et al. 2002; Fahrig 2003; Van Swaay et al. 2006; Van Dyck et al. 2009). Moreover, inappropriate management of existing habitats can be the cause of further population declines (Balmer and Erhardt 2000; Waring 2001; Mabelis and Korczyńska 2016). To effectively direct conservation efforts, it is essential to understand the habitat requirements of species (Thomas et al. 2009). Special care must be taken when the species under consideration are narrowly specialized.

Parasites are expected to decline or go extinct when the host population size decreases below a critical threshold density (Altizer et al. 2007). They can also be more affected by area loss and increased isolation than their hosts, as shown in host-parasitoid systems (e.g. van Nouhuys and Hanski 1999). Furthermore, in the special case of social parasitism (here referred to as a parasite-host relationship between two social insect species), parasitic cuckoo bumblebees were more vulnerable to extinction than their host bumblebee species (Suhonen et al. 2015). Most social parasites are rare and often occupy only small parts of the range of the host species (e.g. Hölldobler and Wilson 1990; Zamora-Muñoz et al. 2003; Buschinger 2009).

Social parasitism is especially manifold among ants where four basic types of obligatory parasitic associations can be recognised: xenobiosis, temporary parasitism (occurs during colony foundation), permanent parasitism with slavery (dulosis) and without slavery (inquilinism) (Buschinger 2009). Ants of the genus Formicoxenus (Hymenoptera, Formicidae, Myrmicinae) are xenobionts, so called guest ants, which live freely within the host colony and, unlike other social parasites, care for their own brood (Buschinger 2009). Social parasites often rely on chemical mimicry to blend into the host colony, and are thus, also highly host specific (Errard et al. 1997; Lenoir et al. 1997). However, the shining guest ant, Formicoxenus nitidulus (Nylander 1846), employs an entirely different strategy, which allows it to invade the nests of at least nine different host ant species of the genus Formica (F. rufa Linnaeus 1761, F. polyctena Förster 1850, F. aquilonia Yarrow 1955, F. pratensis Retzius 1783, F. lugubris Zetterstedt 1838, F. truncorum Fabricius 1804, F. uralensis Ruzsky 1895, F. exsecta Nylander 1846, and F. pressilabris Nylander 1846) (Francoeur et al. 1985; Busch 2001; Czechowski et al. 2002). Due to chemical deterrents in its cuticle (Martin et al. 2007), the guest ant is mostly treated with indifference by its hosts. On the rare occasion a guest ant is grabbed by a host worker, it is immediately dropped (Robinson 2005; Martin et al. 2007).

We studied the occurrence of F. nitidulus in Finland in the nests of five species of red wood ants (Formica rufa group): F. rufa, F. polyctena, F. aquilonia, F. lugubris and F. pratensis; which are dominant insects in the boreal forests of Eurasia (e.g. Rosengren and Pamilo 1983). They build large long-lived mound nests consisting of forest litter and resin and are considered as keystone species in the forest ecosystem with ecological effects extending over several trophic levels (see Sorvari et al. 2011). Furthermore, they have an important role as host species to a wide array of other invertebrates, i.e. myrmecophiles, in addition to the guest ants (e.g. Härkönen and Sorvari 2014; Parmentier et al. 2015, Robinson et al. 2016).

The number of queens and nests in a red wood ant colony varies considerably within and between species (Ellis and Robinson 2014). In Finland, most populations of F. rufa, F. lugubris and F. pratensis are monogynous (one queen) and monodomous (single nest colony), while F. polyctena and F. aquilonia are polygynous (several queens per colony) and polydomous (multi-nest colonies) (Rosengren and Pamilo 1983). Red wood ants also differ in their dispersal strategies. Consequently, Formica polyctena thrives in areas where there are large continuous forests while F. rufa is better at dispersing to small and isolated woodland patches and, thus, is better adapted to habitat fragmentation (Rosengren et al. 1993; Punttila 1996; Sundström et al. 2005; Mabelis and Korczyńska 2016).

Formicoxenus nitidulus has a wide distribution ranging throughout most of Europe and into Eastern Siberia (Collingwood 1979; Agosti and Collingwood 1987; Czechowski et al. 2002). Although the guest ant is quite common in Finland (Rassi et al. 2010), globally it has been classified as vulnerable according to the IUCN Red List (IUCN 2015). Naturally, the survival of the guest ant is intimately tied to the survival of its hosts. However, not all of the potential host nests are occupied by the guest ant. The nests of the red wood ants can be seen as suitable habitat patches surrounded by uninhabitable landscape for various obligate associates, including guest ants. Patch occupancy can be predicted by different parameters such as patch size, patch isolation and habitat quality (e.g. Kindvall and Ahlén 1992; Hanski 1999; Thomas et al. 2001; Eichel and Fartmann 2008). Based on metapopulation theory, small and isolated patches (or nest mounds) are expected to have a higher risk of extinction due to smaller carrying capacities and fewer chances of colonization when empty (Hanski 1999). Previous guest ant studies have mostly focused on F. nitidulus occurrence in regards to the prevailing host nest conditions, and show the guest ants preferring larger and more evenly built nest mounds with a higher mean temperature (Dietrich 1997; Ölzant 2001).

Also, the different colony structures of the host species likely play a role in the guest ant occurrence. Young F. nitidulus queens disperse in late summer flying or walking, as in the case of wingless intermorphs (intermediate forms between regular workers and winged queens). Formicoxenus nitidulus is able to use the scent trails left by its host for orientation (Elgert and Rosengren 1977) and, thus, the trails connecting nests in a polydomous colony provide easy pathways for them to follow while dispersing. This would increase the chance of survival for the local guest ant population in a polydomous colony.

In this study, we investigated how host species, host nest density, inter-nest distance, and nest size, relate to the occurrence of the guest ant F. nitidulus. As nests tend to be larger and nest density higher among polydomous red wood ants (Punttila and Kilpeläinen 2009), we expect their nests to be more likely occupied by the guest ant than the nests of monodomous hosts. We also discuss our results in the context of conservation of this species, while considering the differences in the host species.

Materials and methods

Study species

The shining guest ant F. nitidulus is a tiny (~2.8–3.4 mm) Myrmecine ant easily identified from their much larger hosts (Fig. 1). They are most easily detected from late summer to autumn (Robinson 1999; Van Hengel 2011) after mating has occurred on top of the nest mound, and most often it is the males that are seen. Instead of dying shortly after mating, the males continue to come to the surface of the nest for the rest of the season. Unlike in most other ant species, F. nitidulus males are wingless and very worker-like in appearance though they have 12 antennal segments while workers and queens have 11 (Fig. 2; key in Collingwood 1979). Workers are seen more rarely as they tend to stay hidden within the nest mound.

Fig. 1

Formicoxenus nitidulus (left) and Formica rufa (right)

(Photo by S. K. Härkönen)

Fig. 2

Formicoxenus nitidulus a male, b regular worker (without ocelli), and c winged queen

(Photos by Veikko Rinne)

Field work

The field work was carried out in Turku, SW Finland (60°25′N, 22°09′E), in June–September 2014. We surveyed 166 red wood ant nests (120 Formica polyctena, 25 F. rufa, 14 F. aquilonia, 5 F. pratensis, and 2 F. lugubris) in 25 sites for the presence of Formicoxenus nitidulus. The sites were mostly in conifer and mixed forests, but there were a couple of sites in herb-rich oak forests. Both sites with high nest density and sites with low nest density were chosen for this study, as well as couple of sites with single isolated nests. Twelve sites were occupied by F. polyctena, five by F. rufa, one by F. pratensis, and one by F. aquilonia. On the remaining six sites, F. polyctena occurred together with F. rufa, F. pratensis, and/or F. lugubris.

Nest mounds were systematically observed during August–September as this is the best time for detecting the guest ants. Each nest was observed a maximum of 10 min before moving on to the next one. This has been found to be a sufficient time to detect the guest ant when they are present (Robinson 1998; Green and Westwood 2006). Despite their small size, the shiny appearance and rapid movements of these ants make them relatively easy to spot against the matte background of the nest mound.

Most of the nests in this study were also sampled (N = 135). Sampling of the nests started in June, before the systematic observations. In a few cases the guest ants were seen on top of the nests already in July while on a sampling round. July 3 marked the first such occasion when a mating occurrence was observed on one of the F. polyctena nests. For sampling, nests were divided into seven size classes based on the above ground nest volumes (<250 l, 250–750 l, 750–1,250 l, …, 2,250–2,750 l, 2,750 l<). The guest ants are not likely to be equally distributed within the nest mounds and with increasing nest size the amount of guest ant free space is also likely to increase making it less likely for the guest ants to end up in a sample. Thus, in an effort to counteract this, we increased the amount of nest material taken from the nests based on their size class (1–7 × 0.5 l). Samples were taken about 5 cm beneath the outer layer of the mound and then sieved with a 2.5 mm sieve. The coarse material left on top of the sieve was looked through in the field and returned to the nest while the fine material was brought into the laboratory for later examination.

Two variables were used to describe mound isolation: nest density and nearest neighbour distance. To measure nest density, we counted the number of all potential host nests within a 100 m radius of each observed nest. Inter-nest distances were calculated between all nest locations from all sites based on their coordinates. The distance to the nearest neighbouring nest (of a potential host species) was recorded for each nest. If there were no nests within 100 m, the search was continued until a nest was found. Seven nests had a longer than 150 m distance to the nearest neighbour. For these nests, the recorded distance might be inaccurate as the scanning of the environment was more cursory after that distance, so there might have been closer nests which were not found.

Above ground nest volumes were estimated by first measuring the height and diameter of each nest mound and then using the equation for a half ellipsoid: V = (4/3 πabc)/2, where a, b and c are the lengths of the semi-axes of the ellipsoid. The surrounding habitat of each nest was described as either forest edge or forest interior (≤5 and >5 m from the edge respectively). Weather on each day was classified as either mostly sunny or mostly cloudy.

Statistical analyses

Only Formica polyctena (N = 120) and F. rufa (N = 25) nests were included in the statistical analyses, as there were so few of the other species’ nests. Also, as the sampling method proved very inefficient, we focused only on observational data. All statistical analyses were made with statistical software SAS version 9.3.

We used the generalized linear mixed model in the GLIMMIX procedure with binomial distribution and logit link function to determine how the host species, nest density, distance to the neighbouring nest, and mound size relate to the occurring probability of the guest ant. The occurrence of F. nitidulus was used as a dependent variable (presence = 1, absence = 0) and host species, nest density (measured as number of nests within 100 m radius from the focal nest), mound size, distance to the neighbouring nest, and their interactions as fixed effects. In addition, observation date and site were included as random factors with Kenward–Roger approximation method for the degrees of freedom. Since inter-nest distance and nest density were correlated (Pearson’s r = −0.45, P < 0.0001, Spearman’s ρ = −0.69, P < 0.0001), they were placed in two separate models. All interactions were non-significant and were excluded from the models. As the models were unable to estimate AIC values, the best model was selected by comparing the Pearson’s correlation coefficients (r) between the predicted values of the competing models and the explanatory variable by which the models differed (distance and nest density). Since the variables were not normally distributed, the Spearman’s non-parametric correlations were also compared.

We used the MIXED procedure to see whether the surrounding habitat of the nests was connected to nest volume and nest density. Thus, two models were made, where surrounding habitat of the nest (forest edge/interior) was used as an explanatory variable with either nest volume or nest density as a dependent variable.

Since nest volume and nest density were strongly dependent on the surroundings of the nest (forest edge/interior), and weather (sunny/cloudy) was connected to date, a separate model (GLIMMIX: binomial distribution, logit link function) was used to test the effects of nest surrounding habitat and weather on guest ant occurrence, with F. nitidulus occurrence as the dependent variable and nest habitat and weather as fixed effects.

Results

Of the 166 red wood ant mounds we surveyed, ca. 60% were inhabited by F. nitidulus (Table 1). Mostly when the guest ants were found to be present, they were seen within the first few minutes of observation. We detected no guest ants in the studied nests of F. pratensis and F. lugubris.

Table 1

Number of observed Formica host ant nests and the mean, SD, minimum, maximum, and median values of parameters for Formica nests: nest mound volume (l), nest density (number of nests/100 m radius of the focal nest), and distance to the nearest neighbouring nest (m); the number and percentage of observed nests in which the guest ant Formicoxenus nitidulus was found are indicated within parentheses

Host ant

Observed nests (F. nitidulus present)

Parameters

Mean

SD

Min

Max

Median

F. polyctena

120 (90, 75.0%)

Volume (l)

529.7

504.2

8.4

2915.4

366.2

  

Nest density

7.1

5.8

0

24

5

  

Distance (m)

29.7

32.8

5.9

195.3

16.5

F. rufa

25 (7, 28.0%)

Volume (l)

217.9

264.7

4.7

1169.6

113.1

  

Nest density

2.5

2.1

0

7

2

  

Distance (m)

76.0

94.7

10.0

417.9

47.7

F. aquilonia

14 (3, 21.4%)

Volume (l)

514.4

608.6

35.3

2312.2

278.0

  

Nest density

13.4

0.9

12

14

14

  

Distance (m)

9.2

8.9

1.1

31.2

5.1

F. pratensis

5 (–, –)

Volume (l)

165.7

57.2

108.4

237.5

141.8

  

Nest density

0.4

0.5

0

1

0

  

Distance (m)

82.7

65.1

12.2

145.1

122.1

F. lugubris

2 (–, –)

Volume (l)

319.6

260.7

135.3

504.0

 
  

Nest density

0.5

0.7

0

1

 
  

Distance (m)

131.6

90.2

67.8

195.3

 

Total

166 (100, 60.2%)

      

Maximum daytime temperature during observation days ranged from 14 to 26 °C (mean = 20 °C, SD = 3.2), which is well within the temperature range at which the guest ants (males) can be found on the nest mounds (Van Hengel 2011). The guest ants may be observed on the nest mounds throughout the day; some reports indicating that mornings are best and others vouching for afternoons (Ölzant 2001; Van Hengel 2011). In this study, observations were mostly made in the afternoons starting around 13:00 and lasting until 15:00–16:00. There were a couple of days when observations were started earlier at around 11. Exact times were not recorded but based on the order in which nests were visited on each site, there was no apparent pattern in detection success within a day of observation.

Overall, the sampling method used was very inefficient in detecting the guest ants, as it gave a positive result in only 23% of the sampled nests. Also, sampling was successful in only ~39% of the cases where guest ants were found by observation. There were also no nests where the guest ants were found only by sampling. Failure to get the guest ants in a sample could be the result of not sampling deep enough, as they might be more concentrated deeper in the nest. Alternatively, trying to focus the sampling at nest openings might also be helpful, as Busch (2001) found workers lurking just within when looking with a flashlight. However, it is not always easy to see clear nest openings, especially in nests with a coarser surface structure.

All the extremely large nests belonged to either F. polyctena or F. aquilonia. The nest density within 100 m radius of the focal nest ranged from 0 to 24 nests, obviously being highest with the polydomous species (Table 1). Correspondingly, inter-nest distances were generally lower with the polydomous species (Table 1). There was also a lot of variation in nest volumes, ranging from 4.7 to 2,915.4 l (Table 1).

Though the proportion of nests occupied by F. nitidulus was generally higher with high nest density and large nests (Fig. 3), the guest ants were also found in isolated and very small nests (smallest ~20 l). According to both Pearson’s and Spearman’s correlation coefficients, the better of the two competing models was the one with nest density instead of distance (Table 2). F. nitidulus was more likely to occur with F. polyctena than with F. rufa (Table 2; Fig. 4). There was a significant positive relationship between host nest density and the presence of the guest ant (Table 2; Fig. 4a). Also, nest volume was significantly and positively correlated with the occurrence of the guest ant (Table 2; Fig. 4b).

Fig. 3

The number of observed Formica polyctena (Fpoly) and F. rufa (Frufa) nests at varying a nest densities (number of nests within 100 m radius of the focal nest), and b nest volumes (l); the darker bottom sections of the bars indicate the number of nests where Formicoxenus nitidulus was found

Table 2

Results of the two competing GLIMMIX models showing the effect of red wood ant host species (F. polyctena, F. rufa), host nest density (number of nests/100 m radius of the focal nest), distance (m) to nearest nest, and host nest volume (l) on the occurrence of Formicoxenus nitidulus

 

DF

F

P

Model 1*

 Host species

1, 76.45

4.87

0.030

 Nest density

1, 141

5.47

0.021

 Volume (l)

1, 141

4.86

0.029

Model 2

 Host species

1, 72.41

8.24

0.005

 Distance (m)

1, 138.1

0.16

0.693

 Volume (l)

1, 141

4.77

0.031

*Indicates the better model according to both Pearson’s and Spearman’s correlation coefficients (model 1 predicted occurring probability × nests density: Pearson’s r = 0.65, P < 0.0001, Spearman’s ρ = 0.73, P < 0.0001; model 2 predicted occurring probability × distance: Pearson’s r = −0.27, P = 0.0009, Spearman’s ρ = −0.20, P = 0.0167)

Fig. 4

Probability of Formicoxenus nitidulus occurrence (mean ± 95% CL) among red wood ants Formica polyctena (black) and Formica rufa (grey) at varying a nest densities (number of nests within 100 m radius of the focal nest) and b nest volumes (l). Original occurrence data (0/1) is also included: triangle pointing up for F. polyctena nests and triangle pointing down for F. rufa nests

Nest mounds were smaller (F1, 143 = 5.02, P = 0.027) and nest density lower (F1, 143 = 20.81, P < 0.0001) along forest edges than inside the forests. The guest ants were more likely to be found in nests that were inside the forest than on the edges (F1, 142 = 8.07, P = 0.005). The majority of the nests (108) were observed in mostly sunny weather (with scattered clouds) and the guest ants were found on 63% of these. There were 37 nests which were observed in partly cloudy to cloudy weather, of which 78% were found to be occupied by the guest ant. Nevertheless, there was no significant difference in guest ant occurrence between sunny and cloudy days (F 1, 142 = 1.33, P = 0.251).

Discussion

We found the guest ant F. nitidulus to be quite common in the study area in SW Finland. The red wood ant F. polyctena was by far the most common of the host species in the area. With 75% of its nests being occupied by the guest ants, it was also the most likely host. In contrast, the guest ants were found in barely a third of the F. rufa nests. The guest ants were also more likely to be found in well-connected nests (i.e. nests surrounded by high nest density) as well as large nests. As these characteristics tend to be more usual among polydomous and polygynous species (such as F. polyctena) than monodomous and monogynous species (such as F. rufa) (e.g. Czechowski et al. 2002; Punttila and Kilpeläinen 2009), colony structure of the host species plays an important role in the guest ant occurring probability. Particularly, the trails increasing connectivity between the nests in a polydomous colony seem to contribute to the high rate of occupancy among F. polyctena. Generally in ant social parasites, Buschinger (2009) estimates the rate of parasitism to be much lower, somewhere between 3 and 10% of colonies parasitized within patches where the parasite is present.

Our results are consistent with the predictions of metapopulation theory (Hanski 1999). The guest ant was more likely to be found in nests surrounded by a high nest density, as opposed to more isolated nests. A similar result, where isolation explains patch occupancy, has been observed in several other studies in insects (e.g. Kindvall and Ahlén 1992; Thomas and Harrison 1992; Appelt and Poethke 1997; Thomas et al. 2001; Carlsson and Kindvall 2001; Eichel and Fartmann 2008). Though published data are scarce, socially parasitic ants usually occur in more or less isolated patches within the host range, and the patches are characterized by a high density of the host species (Buschinger 2009). Also, previous studies on myrmecophiles in red wood ant nests have reported a negative correlation between myrmecophile diversity and host mound isolation (Päivinen et al. 2004; Härkönen and Sorvari 2014; Parmentier et al. 2015). Patch networks have to be sufficiently linked by dispersing individuals to ensure the survival of species within them (Fahrig and Merriam 1985; Adler and Nuernberger 1994; Hanski 1999; Bowne and Bowers 2004). When local populations become extinct, recolonization relies on the amount of dispersing individuals and the ease of movement within the landscape (Kindlmann and Burel 2008). A high nest density will facilitate the dispersal to a new host nest.

Moreover, in a polydomous red wood ant colony dispersal can further be aided by trails connecting the nests. Nests of monodomous red wood ants, on the other hand, might be harder to find, even when inter-nest distances are relatively short, as they are not similarly connected to other nests. Our results support this theory. On a F. rufa site where five nests were within 77 m of each other, and each had ≤27 m to a nearest neighbouring nest, only one nest was found occupied by F. nitidulus. On another site, where F. rufa nests were relatively close to several F. polyctena nests inhabited by F. nitidulus, the F. rufa nests were all without the guest ant. The same was true for F. pratensis and F. lugubris nests even when they were fairly close to guest ant inhabited F. polyctena nests. Similarly, Van Hengel (2011) reported that F. nitidulus could be found in nearly all the nests in one F. polyctena super-colony while the species was absent from all but one of the nearby F. rufa and F. pratensis nests. These observations suggest that F. nitidulus prefers dispersing along the connecting trails, which might be especially true for the wingless intermorphic females. Winged females, on the other hand, are likely in a key position when dispersing to more isolated nests. For intermorphic females, leaving a nest that is not directly connected to another nest might be much riskier. In such cases, it seems it would be more prudent for the flightless queens to remain in the same nest, though whether this is the case requires further study.

We confirm the previous finding of F. nitidulus being more likely to occur in larger nests than smaller ones (Dietrich 1997; Ölzant 2001). Populations in large nests (patches) are less likely to go extinct due to larger carrying capacities (Hanski 1999). Similarly, the diversity of myrmecophilous beetles has also been found to be higher in large red wood ant nests (Päivinen et al. 2004). Not only do the large nests provide the guest ants, as well as other guest species, with more resources (Päivinen et al. 2004), larger nest mounds are also better able to buffer against weather fluctuations and keep the inner temperature optimal, and thus have a more stable microclimate (Hölldobler and Wilson 1990). Since ants are ectotherms, their growth and reproduction is affected by the temperature of their habitat (Ratte 1984; Atkinson 1994; Chown and Nicolson 2004). Large nests are also usually older and have been around longer for the guest species to find and end up in by chance.

As the nests were observed only once during this study, it is possible that the guest ant was not detected in all the nests where it is present. The probability of detecting the guest ants is likely to be affected by their population size in the host nest mound. Thus, the possible false-negative observations could come from nest mounds with only few guest ant inhabitants.

Conservation perspectives

The Red List status of the guest species is based on the assessment of the IUCN Social Insects Specialist Group from 1996 and requires revision. Recently, intensified ant inventories have resulted in several new records of the species in Belgium and France after decades of no observations (Wegnez et al. 2011). Also, in the UK intensified searches have resulted in new records of the species (UK Biodiversity Group 1999; Green 2009). Compared to many other ant species, F. nitidulus is much harder to find and may thus be underrepresented in surveys unless special attention is paid to the habits of the species. Due to its elusive lifestyle, to maximise the chances of finding this guest ant, surveys should be made from late summer to autumn with the best time usually being from August to September when the males are most likely to be seen on top of the nest mounds (Ölzant 2001; Van Hengel 2011; Wegnez et al. 2011).

According to the IUCN Red List (IUCN 2015), most of the host species of F. nitidulus are near threatened (NT). This is due to the loss of suitable scrub and forest habitats for the host species caused by agricultural clearing and inappropriate forest management practices. Wood ants seem to be vulnerable even when modern forest management practices are used (Sorvari and Hakkarainen 2007). Though clear-felling may temporarily increase nest mound density due to the frequent establishment of new bud nests (Rosengren and Pamilo 1978; Rosengren et al. 1979; Sorvari and Hakkarainen 2005), most nests, both old and new, will be abandoned by the wood ants within a few years of clear-cutting (Sorvari and Hakkarainen 2007). One crucial factor causing nest abandonment is the distance of the nest mound to the remaining forest, i.e., nests that are relatively close to the forest edge have a better chance to survive (Sorvari 2013). Therefore, small size clearings may not be deleterious for red wood ant colonies, and their associates.

Formicoxenus nitidulus is strictly dependent on its host species for survival, and like the socially parasitic cuckoo bumblebees (Suhonen et al. 2015), it is probably more vulnerable to extinction than its hosts. For F. nitidulus, as well as for other obligate ant nest associates, the best way to protect them is to ensure the survival of their hosts. Ants of the Formica rufa group, which are the main hosts of the shining guest ant, are protected by law in many European countries (IUCN 2015; Sorvari 2016). Per our results, the best habitat for the shining guest ant is a dense population of mounds, with a high proportion of large mounds. These parameters are more easily satisfied among polydomous host colonies, where trails further increase connectivity. However, while large nest mounds are most optimal for the shining guest ant, the small and medium sized nest mounds ensure the continuum of large nests in a population also in the future.

Maintaining healthy populations of polydomous red wood ants requires the management of sufficiently large (at least ≥25 ha) forest areas (Mabelis and Korczyńska 2016). However, in many fragmented areas, most woodland patches are smaller, and thus better suited to monogynous species (Punttila 1996). Habitat quality is one of the crucial factors affecting species persistence (e.g. Dennis and Eales 1997; Thomas et al. 2001). In Central Europe, the quality of small woodland patches bordering agricultural land may be deteriorated increasing the chances of extinction for wood ant populations (Mabelis and Korczyńska 2016). Thus, it is essential to keep the quality of the woodland patches high to preserve the wood ants and their various guest species. Red wood ants prefer to build their nests in sunny and open areas within forests and along the edges (Mabelis and Korczyńska 2016). To increase the chances of colonization for red wood ants in managed forests, which are often dense, small open areas could be created. This will create habitats for many other forest species as well. Thus, maintaining a varied forest structure could help maintain or even increase the biodiversity of forests.

Notes

Acknowledgements

This study was funded by the Jenny and Antti Wihuri Foundation, Turku University Foundation, and Vuokon Luonnonsuojelusäätiö (Vuokko Foundation for Nature Conservation). Fieldwork in Ruissalo was conducted under a permit obtained from the Centre for Economic Development, Transport and the Environment of SW Finland (ELY Centre; VARELY/1106/07.01/2014). We are grateful to the anonymous reviewers for their constructive comments.

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© Springer International Publishing Switzerland 2017

Authors and Affiliations

  1. 1.Department of BiologyUniversity of TurkuTurkuFinland
  2. 2.Department of Environmental and Biological SciencesUniversity of Eastern FinlandKuopioFinland

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