Biodiversity Data Journal 11: e102897 OO)
doi: 10.3897/BDJ.11.e102897 pian ortet
Research Article

They live under our streets: ant nests

(Hymenoptera, Formicidae) in urban pavements

Louise Dijon?, Wouter Dekoninck§, Gilles Colinet!, Frédéric Francis*, Grégoire Noel*

+ Functional and Evolutionary Entomology, Gembloux Agro-Bio Tech — University of Liege, TERRA, Gembloux, Belgium
§ Royal Belgian Institute of Natural Sciences, Brussels, Belgium
| Soil-Water Plant Exchanges, University of Liege, Gembloux Agro-Bio Tech, Gembloux, Belgium

Corresponding author: Grégoire Noel (gregoire.noel@uliege.be)

Academic editor: Sebastian Salata

Received: 02 Mar 2023 | Accepted: 21 Apr 2023 | Published: 27 Apr 2023

Citation: Dijon L, Dekoninck W, Colinet G, Francis F, Noel G (2023) They live under our streets: ant nests

(Hymenoptera, Formicidae) in urban pavements. Biodiversity Data Journal 11: e€102897.
https://doi.org/10.3897/BDJ.11.e€102897

Abstract

In the context of global insect decline, the urbanisation process plays a key role. However,
urban pavements, which are considered to be impervious to biodiversity, can harbour
ground-nesting insects under certain conditions. Recent observations have revealed the
presence of Formicidae nests under urban pavements. The aim of this work is to determine
the species richness of Formicidae nesting under urban pavements in the Brussels-Capital
Region (Belgium) and to characterise their nest environment and soil texture. Seven ant
species were identified in 120 nesting sites: Lasius niger, Lasius brunneus, Lasius flavus,
Lasius fuliginosus, Tetramorium caespitum, Tetramorium impurum and Myrmica rugulosa.
Concrete slabs or natural stones with a sandy sub-layer are the main structures in which
ants nest. In addition, nests were mainly found under modular pavements with degraded
rigid joints. The results of this work highlight the capacity of urban structures to host part of
ant biodiversity in cities.

Keywords

urban ecology, urban conservation, ground-dwelling ant, Lasius niger, urban construction

© Dijon L et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY
4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are
credited.

2 Dijon L et al

Introduction

Since the last century, human populations have shown an increasing interest in urban life,
leading to the expansion of cities and the number of their inhabitants (Grimm et al. 2008).
The urbanisation process, or the conversion from natural areas to anthropogenic areas, is
responsible for the fragmentation, isolation and degradation of surrounding habitats (Clarke
et al. 2008). These disturbances reshape species assemblages and lead to the formation
of living communities adapted to urban ecosystems (Yamaguchi 2004, McDonald et al.
2019). Soil habitats are of fundamental importance for some species, especially ground-
nesting insects (e.g. in Antoine and Forrest (2021)). Recent studies focusing on the insect
biodiversity of urban areas revealed the presence of several Hymenoptera species which
can nest under urban pavements. These studies focused mainly on the nesting ecology of
solitary bees and digger wasps, but also recorded some ant species (Noél et al. 2021,
Weber 2022).

Ants play an important role, mainly as decomposers, in many ecosystems around the
world. The biomass of ground-dwelling ants exceeds the combined biomass of wild birds
and mammals and represents the fifth of the human biomass (Schultheiss et al. 2022).
Moreover, ants play an essential role in soil structure and energy flow; hence, some
consider them as the engineers of ecosystems (Folgarait 1998). Nutritional needs of each
species, nest density, species richness and community structure can vary greatly
depending on habitat space and available food resources (Wegnez et al. 2012, Feldhaar
2014). Site selection is crucial for colony development and several biotic and abiotic factors
appear to influence its density and distribution. Most ant colonies nest in the soil and are
sensitive to criteria, such as grain size composition and moisture that influence the
burrowing, stability and development of their colony (Diaz 1991, Kaspari et al. 2000, Wang
et al. 2001, Boulton et al. 2005, Clarke et al. 2008). The surrounding vegetation also plays
a significant role to ant communities. The richness, abundance and size of vegetation
patches influence ant species richness (Boulton et al. 2005, Uno et al. 2010, Cerda et al.
2013). Related to the presence of vegetation, another criterion that influences the location
of a colony is the availability of food resources near the nest. The range of movement
varies with species, colony density and food availability. This radius can vary from 50 cm to
sometimes 200 m (Carroll and Janzen 1973, Savolainen et al. 1988). Finally, foraging
activity is particularly influenced by climatic variations (Cros et al. 1997). Temperature
seems to be the first criterion that regulates the activity of ants. The optimal temperatures
for ant activity are between 10°C and 45°C (Hodlldobler and Wilson 1990), but some tropical
species can increase their critical thermal maximum (i.e. the maximum temeprature at
which an organism is unable to resist heat stress) over 50-55°C (Leong et al. 2022) which
is also associated with the diminution of their foraging activity (Cerda et al. 1998).

In general, studies relating to the ecology and diversity of ants in urban environments are
mainly concentrated in parks and green spaces (Yamaguchi 2004, Pacheco and
Vasconcelos 2007, Uno et al. 2010, Slipinski et al. 2012). These studies highlight a
negative relationship between species richness and proximity to urban development
(Sanford et al. 2009), such as green areas bordering streets (Slipinski et al. 2012) or in

They live under our streets: ant nests (Hymenoptera, Formicidae) in urban ... 3

isolated park areas (Santos et al. 2019), but this pattern is not always observed in every
study. Idiosyncrasies of other assessed cities may favour or maintain ant species richness
in urban areas (Guénard et al. 2015, Miguelena and Baker 2019, Perez and Diamond
2019). Ants are an integral part of urban ecosystems, but their ecology within urban
constructions is still rather poorly documented. Here, we hypothesised that urban areas
present thermophilic zones particularly attractive for ants. Hence, urban pavements could
constitute a suitable thermal refuge for their nest under certain conditions, such as
sufficient food resources, permeable pavements and favourable microclimatic conditions,
such as high temperatures and relatively high humidity.

The aims of this study are to determine and locate the specific richness of ant colonies
nesting under urban pavements in the city of Brussels-Capital Region (BCR; Belgium) and
to characterise the main factors that could influence their nesting site selection.

Materials and methods
Study site

The study focused on urban pavements throughout the municipalities of BCR. The surface
area of this region is 161.38 km? and pavements represent 3600 km in length (Bruxelles
Mobilite 2022). Fifty-four percent of the surface of BCR is non-built which corresponds to
green spaces, such as forests, private gardens or parks (Van de Voorde et al. 2010). There
is a contrast between the highly urbanised city centre and the periphery, which is like a
green belt. The main green areas are in the east and southeast of Brussels (Bruxelles
Environnement 2013). The remaining 46% are impervious surfaces (built-up), concentrated
more towards the city centre (Bruxelles Environnement 2015). The sampling period took
place in the spring from 12 April 2022 to 17 June 2022. BCR is characterised by temperate
oceanic climate, the 10°C ambient temperatures necessary for ant activity being reached
from April until October (Cerda et al. 2013, Bruxelles Environnement 2020).

Sampling methods and specimen preparation

As a first exploratory study on the diversity of Formicidae nesting under the pavements of
BCR, data were collected opportunistically. This sampling method allows for a larger
geographical scope due to the time saved at each site and will give a better chance to find
rare species. The locations from the dataset of Noél et al. (2021) were used as a basis for
the search for nests in order to cover as much territory as possible with a high probability of
observing ants. An anthill was located on a pavement, either thanks to the sand mound
between two paving stones or thanks to an ant followed to its anthill (Fig. 1A). The site was
then validated by detecting the activity of several workers outside the nest. Each site
assessment was conducted for 10-20 min. Fieldwork was performed on sunny days with
clear sky and a daily minimum temperature of 10°C between 09:00 and 17:00 h. Using a
brush previously soaked in 70% alcohol (Delabie 2001), about ten specimens were
collected for identification in the laboratory. All the specimens were identified using a
stereomicroscope with taxonomic keys of Seifert (1996), Wegnez et al. (2012), Blatrix et al.

4 Dijon L et al

(2013), and Seifert (2018). The identified specimens were cross-checked by reference
collections of the Royal Belgian Institute of Natural Sciences (RBINS, Belgium) and
vouchered to Gembloux Agro-Bio Tech (University of Liege, Belgium) and RBINS insect
collections.

ioe eit
Figure 1. EES]

Ant nest on pavements. Sandy mound (Uccle, Brussels) (A). Degraded rigid joints of concrete
slabs (Auderghem, Brussels) (B). Unbound joints of natural stone (Brussels) (C). Crack in a
concrete slab (Berchem-Sainte-Agathe, Brussels) (D).

Pavement characterisation

The width of the pavement (i.e. between the road and the constructed building) was
measured at each site. Then, the joint width was measured at the nest exit with a ruler
placed perpendicular to the joint length. The nature of the joint was encoded as a
qualitative variable: rigid joint (Fig. 1B) / flexible joint (Fig. 1C) / other (Fig. 1D). The nature
of the pavement was also considered as a nominal qualitative variable: asphalt / natural
stone (Fig. 1C) / concrete slab (Fig. 1B). Finally, the location of the nest in relation to the
pavement was also encoded: middle of the pavement / wall border / green border / street
border.

Granulometry

The soil excavated by the ants was collected at each site. It was not possible to extract the
substrate below the paving stones due to the logistic and administrative costs of this
practice. Consequently, a maximum of substrate taken out by the ants was collected. If
several mounds were present, the substrate was collected from all mounds. Substrate
samples were used to perform a particle size analysis. The soil texture definitions can vary
according to the spatial scale of the studied geographic areas and the nature of the
particles (Udden 1914, Wentworth 1922). We used the Bonnot-Courtois and Fournier's
granulometric definitions in Fournier et al. (2012). Each sample was weighed (1/100
accuracy) and then passed through a sieve (Haver and Boecker VWR) for 5 minutes at an
oscillation amplitude of 1 mm. Four sieves of mesh size 1 mm, 0.5 mm, 0.2 mm and 0.1
mm were used. It allowed us to differentiate coarse elements, coarse sands, medium
sands, fine sands and very fine sands including clays and silts (Fournier et al. 2012).

They live under our streets: ant nests (Hymenoptera, Formicidae) in urban ... 5

Rejects from each sieve were weighed and converted to relative mass to the total sample
mass to allow for comparisons between sites (Fournier et al. 2012).

Mapping and statistical analyses

Map analyses were performed using QGIS software - Version 3.18.1 (QGIS 2022). We
retrieved a map raster of high and low vegetation in Brussels from Bruxelles
Environnement (2022).

Statistical analyses and graphs were performed with R software v. 4.1.3 (RStudio Team
2022) and the following packages: ggplot2 (Wickham 2016) and corrplot (Wei and Simko
2021). To estimate the true diversity of our sampling, we performed Chao1 (abundance-
based) estimator of richness using the R package /NEXT (Hsieh et al. 2016)

After noting the dominant presence of the species Lasius niger (Linnaeus, 1758), the
statistical analyses focused on the comparison of this species compared to all the other
species according to the particle size fractions. Indeed, the abundance of the latter was too
low to study them individually. These comparisons were made using Student’s t test,
Wilcoxon-Mann-Whitney test and Welch’s t test, depending on the normal distribution of
the data and the homogeneity of variances.

Jet * |
ee... jette
_% ee.
Ganshoren
. Evere
Berchem-Sainte-Agathe (> PS,
oo Miasketbers Schaerbeek
Molenbeek-Saint-Jean Saint-Josse-ten-Noode
Bruxelles <
TiN id - Woluwe-Saint-Lambert
€ I
bed i pe Woluwe-Saint-Pierre
Anderlecht Saint-Gilles A Ast

Forest. >=
— ~Auderghem
Species
oh * Lasius brunneus
- ° Lasius flavus
Lasius fuliginosus
Uccle : Lasius niger
® Myrmica rugulosa
Tetramorium caespitum
Tetramorium impurum
® Tetramorium sp.
Vegetation
Impervious surface

@
e

Figure 2. EES

Ant nest location (coloured dots) in Brussels-Capital Region (Belgium). Green shade
corresponds to the vegetation cover and white shade corresponds to the impervious surface.

6 Dijon L et al

Results

Ant diversity

In 17 municipalities of BCR, 120 sites with active ants were sampled (Fig. 2). Seven taxa
were identified. These are grouped in two different subfamilies: Formicinae (4) and
Myrmicinae (3). Regarding the abundance of these species, L. niger is dominant,
occupying 92% of the sites. The six other species are present at a rate of 1 to 2% each
(Table 1). The species identification process was not possible for two specimens of the
genus Jetramorium which could be Tetramorium caespitum (Linnaeus, 1758) or T.
impurum (Foerster, 1850). Hence, we classified these two specimens in the Tetramornum
sp. complex.

Table 1.

Abundance of sampled species under urban pavements. Identification to species was not possible
for two individuals of the genus Tetramorium which could be Tetramorium caespitum (Linnaeus,
1758) or T. impurum (Foerster, 1850); that is why the species has not been specified for
Tetramorium.

Subfamilies Species Sites
Formicinae Lasius brunneus (Latreille, 1798) 2

Lasius flavus (Fabricius, 1782) 2

Lasius fuliginosus (Latreille, 1798) 1

Lasius niger (Linnaeus, 1758) 117
Myrmicinae Myrmica rugulosa Nylander, 1849 1

Tetramorium caespitum (Linnaeus, 1758) 1

Tetramorium impurum (Foerster, 1850) 1

Tetramorium sp. 2

The rarefaction curve and the Chao1 index estimated that the expected species richness of
nesting ants in pavements for BCR is around nine species with 95% of confidence interval
between 7.26 and 25.91 species (Fig. 3).

Pavement characterisation

The dominant type of pavement was concrete slabs (83.5%). Nests were also found under
natural stone pavement (15%) and at low frequency under asphalt (1.6%) (Fig. 4A). For the
nature of the joints, 56.7% are rigid and 41.7% are flexible. The remaining 1.6% represent
the other category which includes cracks in the asphalt (Fig. 4B). Finally, the position of the
nests was classified into four categories. The middle of the pavement was the most
represented with 37.8%. Some 30.7% were located at the border of green spaces, 22.8%
at the border of a wall and a smaller proportion was located at the border of the street
(8.7%) (Fig. 4C).

They live under our streets: ant nests (Hymenoptera, Formicidae) in urban ...

Species diversity

0 100 200 300 400
Number of colonies

Method = interpolated = * extrapolated Guides ad

Figure 3. EES

Rarefaction curve and its extrapolation from the 120 sampled sites. The green shade
corresponds to the 95% interval confidence.

OOO

3s , by Nest situation
Pavement composition Joint composition
yw Wall border

I) Aspnatt WD) otter

- ie) Street border
a Concrete slab | Rigid

, Green border
iw Natural stone ka Flexible

ita Middle of the pavement

Figure 4. EES]

Pavement ring charts for the 120 sampled sites for pavement composition (A), joint
composition (B) and nest location (C).

8 Dijon L et al

Granulometry

For the granulometric variables, the distribution of data is relatively similar between the two
groups. There is no significant difference between L. niger and other ant species. The grain
size class > 0.2 mm is present in the majority of samples with an average of more than
50% of the total mass. In addition, the class < 0.1 mm, which includes clays and silts, is
less than 10% of the total mass of the sample (Fig. 5).

Relative mass (%)

mS eas

Mesh size (mm)

Species HB Lasiusniger E63 Otmers

Figure 5. EES]

Boxplot per grain size class for Lasius niger (orange) and other ant species (green). The blue
dot corresponds to the boxplot mean of relative mass (%).

Discussion
Ant diversity

The ecology of ants in BCR pavements has been studied for the first time. Other studies
have reported their presence in Berlin and Oldenburg in Germany (Haeseler 1982, Weber
2022). Some species are common to all three cities, such as L. flavus, L. niger and _ T.
caespitum. Others were identified for the first time in our study, such as L. brunneus, L.
fuliginosus, M. rugulosa and T. impurum. Our study showed a similar species richness to
the Weber (2022) study in Berlin with seven different species, whereas the Haeseler
(1982) study in Oldenburg found only two species. These differences in richness can be
explained by the different sampling methods and effort between the studies. In BCR, a
larger area was surveyed in Brussels than in the other two studies, which focused on
predetermined smaller sites (Haeseler 1982, Weber 2022). Moreover, the 120 new ant
records complete the Formidabel database, which includes 535 records in BCR since the
beginning of the 20" century (Brosens et al. 2013).

They live under our streets: ant nests (Hymenoptera, Formicidae) in urban ... 9

Our sampling period occurred only during the spring, from April to June. Ants were located
primarily by excavated tumuli and worker activity outside the anthill. Therefore, some
species may have been missed if their worker activity is concentrated only underground
(i.e. the anthill composed of sand excavated for the nest prospection could not be
observed) and their swarming period is later. For example, species belonging to the
subgenus Chthonolasius, such as Lasius mixtus (Nylander, 1846) and Lasius umbratus
(Nylander, 1846), already recorded in Brussels, are subterranean and their swarming
period extends from July to September (Dekoninck et al. 2012, Blatrix et al. 2013).
Similarly, the Chao1 index, used to estimate the total number of species present in the
study area, is based on empirical data (Chao 1984).

Concerning the species in this study, L. niger, is largely dominant. Present in 92% of sites,
it seems to appreciate a wide range of conditions for its establishment. Indeed, L. niger is
considered as an _ ubiquitous species well Known in urban and more widely
anthropogenised environments (VWegnez et al. 2012, Blatrix et al. 2013). The other
species, much rarer in number, were present in the order of 1-2% each. L. brunneus, L.
flavus, T. caespitum and T. impurum are also characterised as ubiquitous and they are able
to establish in urban environments (Wegnez et al. 2012, Blatrix et al. 2013). One of them,
T. caespitum, is named the "pavement ant" (Kaufmann et al. 2012). Ants of the genus
Tetramorium particularly like thermophilic sites, whereas L. brunneus prefers environments
with at least a few trees. Both the 7etramorium species have a wide occurence range in
the Northern Hemisphere of New and Old World and show euryoecious abilities to live in
various habitats, mainly non-forested (Wagner et al. 2017), while 7. caespitum nests in
street medians of New-York City suggesting that this species is able to occupy a particular
ecological niche in highly crowded megacities (Pecarevic et al. 2010). Except for L. niger
and L. flavus, L. brunneus, L. fuliginosus and M. rugulosa have wide Palearctic distribution
according to the global biodiversity information facility (GBIF.org 2023). L. fuliginosus and
M. rugulosa are less frequent in urban environments. The first one is an oligothermal
species which prefers environments with tree or forest edges. The second prefers
thermophilic and sandy sites. In Wallonia and Brussels, the distribution of the latter species
is scattered and rather rare (Wegnez et al. 2012, Blatrix et al. 2013); however, in Flanders,
its can be a common species in sandy soils (Dekoninck et al. 2012).

In this study, data were collected in an opportunistic manner. They are described as
opportunistic when they were collected without a real standardised protocol and without an
experimental design ensuring the geographical representativeness of the sampled sites
(van Strien et al. 2013). This method was interesting for an initial investigation of the
territory. It allowed a larger area to be explored due to the time saved at each site (Brown
and Williams 2019). Indeed, implementing a standardised experimental design with
predefined transects would likely have missed some diversity due to low abundance of
some species. In addition, the use of participatory science in the study by Noél et al. (2021)
revealed spatiotemporal variation in survey effort. Due to the higher abundance of ants
under Brussels pavements compared to bees in the previous study, it was decided not to
use participatory science and to directly survey as many BCR municipalities as possible to
reduce this spatial variation.

10 Dijon L et al

Pavement characterisation

Regarding the composition of the pavements, ants are mostly found under concrete slabs
and, to a lesser extent, under natural stones. These proportions can also be explained by
the abundance of this type of material used for the construction of Brussels pavements
(Centre de Recherches Routieres 2012). The nature of the pavement may not be a major
criterion for ant colonies. In both cases, these materials have a high thermal conductivity
(Laurent and Vuillermoz 1993), allowing for warming of the anthill and brood in early spring
(Passera and Aron 2005). About the nature of the joints, most of them were rigid and,
therefore, theoretically impermeable, in contrast to the flexible joints made of granular and
permeable materials. However, many rigid joints were degraded and no longer constituted
a totally impermeable barrier. This was due to the age of the pavement and the vegetation
roots that contributed to its degradation. Finally, most of the nests were in the middle of
pavements, suggesting that the ants were not bothered by pedestrian traffic.

Granulometry

The results of the particle size analyses confirm the sandy composition of the pavement
subgrades. Indeed, the samples are composed of an average of more than 83% sand.
These observations are similar to data collected by Noél et al. (2021) regarding to solitary
bee and wasp nests. Sand is a frequently used component of the pavement sub-layer
(Centre de Recherches Routiéres 2012) and can be favourable for a wide variety of
Formicidae species including L. niger and T. caespitum (Vang et al. 2001, Maes et al.
2003, Boulton et al. 2005). It is important to note that the particle size analyses were
performed on soil particles excavated by ants and forming a dome above the nest.
Therefore, due to the modification of the soil nature caused by ants (Cammeraat and Risch
2008), these analyses may be biased and may not correctly represent the nature of the
substrate under the pavements. Indeed, Cammeraat and Risch (2008) showed that, due to
the displacement of soil particles by ants, the top of the domes consisted of an
accumulation of finer particles than the surrounding soil.

Conclusions

The ant species observed on the pavements preferentially nest under concrete slab or
natural stone with degraded or flexible joints, which are the main materials used for
pavements in the BCR. The sand most commonly used for the pavement sub-base is
suitable for the establishment of these species. The joints observed are not the most
recommended for the comfort of the pedestrian population. Currently, continuous asphalt,
concrete pavements or modular concrete pavements with narrow joints are preferred by
BCR urban planners to provide sufficient comfort for pedestrians. This study highlights the
capacity of urban environments to host biodiversity and, in particular, the pavements
considered as refuges for certain species.

They live under our streets: ant nests (Hymenoptera, Formicidae) in urban ... 11

Acknowledgements

We thank Jeannine Bortels for the help with the ant preparation for setting-up in collection.

Author contributions

Grégoire Noél contributed to the study conception and design. Material preparation, data
collection and analysis were performed by Louise Dijon and Grégoire Noél. The first draft
of the manuscript was written by Louise Dijon and all authors commented on previous
versions of the manuscript. Gregoire Noél edited and finalised all the last versions of the
manuscript. All authors read and approved the final manuscript.

Conflicts of interest

The authors have declared that no competing interests exist.

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