Introduction
Urbanization in developing countries has for long been identified as a
major process with multiple consequences at the global scale (Cohen,
2006; Henderson & Turner, 2020). This process comprises both the
emergence of megacities and the rapid growth of small and medium cities
(Cohen, 2006; United Nations, Department of Economic and Social Affairs,
Population Division 2018). As a result, urban areas are projected to
house 60 per cent of people globally by 2030, with almost 90 per cent of
this growth taking place in Asia and Africa
(un.org).
The corresponding rise of the “indoor biome” raises new
eco-evolutionary questions regarding species, and species communities,
associated with this expanding environment (Martin et al., 2015;
Hulme-Beaman et al., 2016). Long composed mainly of native species,
these communities (like others from more natural biomes) have been
progressively colonized by introduced species benefitting from
human-caused global changes (Vitousek et al., 1997).
Among other groups of living organisms, rodents comprise species that
are especially prone to take advantage of the modification of habitats
by human activities, being core species in an urbanization context
(Capizzi et al., 2014; Rothenburger et al., 2017). Their impacts are
diverse and multidimensional, including notably biodiversity loss
(Doherty et al. 2016), threats to food security (Singleton et al.,
2021), disease transmission (Han et al., 2015), economic burdens (Dossou
et al., 2020; Diagne et al., 2023) and societal decay (Colombe et al.,
2019). Some of these rodents are well-known as major invasive alien
species (hereafter ‘invasive rodents’) worldwide (Lowe et al., 2000;
Capizzi et al., 2014). This is the case of Rattus rattus , the
black rat, and Mus musculus , the house mouse, which are both
listed amongst “100 of the world’s worst invasive species” of the
planet (Lowe et al., 2000). The ongoing expansion of those invasive
rodents in several parts of the world (see for instance Dalecky et al.,
2015; Zeng et al., 2018; Yu et al., 2022) leads to multispecific
assemblages of small mammals (mainly rodents) that combine invasive and
native species in a variety of ecological and evolutionary contexts. One
of these contexts is represented by commensalism in anthropogenic
environments, where the species concerned literally “live within
houses”, in close proximity with humans (Hulme-Beaman et al., 2016).
There, despite ever-increasing studies on the distribution (including
invasion history of invasive small mammals), impacts and dynamics of
individual species over space and time, the multispecific assemblages of
these rodent-dominated communities has rarely been studied from a
community ecology perspective.
Species sampling in anthropogenic habitats is often complicated because
it involves going into people’s homes or industrial or commercial
buildings. Moreover, even if rodent communities in such habitats are
often depauperate, diversity is generally not taken into account because
one focus on a (or a pair of) target species in relation with specific
questions raised by it/them. This is the case in the review by Feng &
Himsworth (2014) on R. norvegicus and R. rattus , in the
studies focusing on the impact of urban characteristics on the genetic
structure of rodent populations in different cities (R.
norvegicus in American cities: Combs et al. 2018; M. musculus in
Dakar (Senegal): Stragier et al. 2020), or in experiments on species
cohabitation and interspecific competition involving M. musculusin SW Argentina (Castillo et al., 2003; Gomez et al., 2008). However,
some studies have already considered more complete communities. For
instance, Panti-May et al. (2012) measured data on abundance, population
and habitat use parameters of M. musculus and R. rattusamong their native counterparts in households of a rural area of Mexico
as part of a study on zoonotic disease transmission. Masi et al. (2010)
evaluated the respective importance of socioeconomic and environmental
risk factors for urban rodent (including R. rattus , R.
norvegicus and M. musculus ) infestation in Sao Paulo, Brazil.
Cavia et al. (2009) analyzed the relation between rodent community
composition and diversity and the landscape structure in the city of
Buenos Aires, showing a clear trend of habitat partitioning between
invasive R. rattus, R. norvegicus and/or M. musculus(dominant in parklands, shantytowns or industrial–residential
neighborhoods) and native species (dominant in a natural reserve but
also present in parklands). In Africa, Olaseha et al. (1994) presented
general considerations on the importance of housing and sanitation on
the presence of rats (R. rattus and R. norvegicus ) and
mice (M. musculus ) based on questionnaires completed by
interviews in towns and villages of a rural area in southwestern
Nigeria. Demby et al. (2001), followed by Fichet et al. (2005, 2009)
provided information on small mammal distribution in urban as well as in
rural areas of Guinea, in relation to Lassa virus distribution and
prevalence. Taylor et al. (2008) gave a few elements of urban
distribution of rodents in Durban (South Africa) in a small mammal
community largely dominated by R. norvegicus . Monadjem et al.
(2011) compared movement patterns and possible interactions ofMastomys natalensis (a native rodent species) and R.
rattus in distant sites of Tanzania, Malawi and Namibia using telemetry
and Rhodamine B marker. In the capital city of Niger, Niamey, Garba et
al. (2014) analysed the distribution of native and invasive rodents in a
series of sites corresponding to habitation districts, cultivated
gardens and industrial zones. They showed the dominance of the nativeM. natalensis over the invasive R. rattus and M.
musculus and spatial segregation between them, that they interpreted as
the result of an ongoing native-to-invasive species turn over. Hima et
al. (2019) assembled an important dataset on commensal small mammal
distribution in a series of localities along the Benin-Niger
“corridor”, between Cotonou and Niamey. They showed the dominance of
either invasive R. rattus in Cotonou (see also Houemenou et al.,
2014) or native M. natalensis in Niamey, with segregation
patterns between Rattus spp. and M. natalensis , and a very
regular and important presence of Crocidura spp. (incl. C.
olivieri ), especially at lower latitudes. None of these studies has
nonetheless addressed in detail the co-distribution and coexistence of a
set of species (both native and invasive) belonging to a whole small
mammal community in human-made environments, especially at a fine
spatial scale. At best, they considered co-occurrence patterns at the
scale of a country, a region or a whole city, but never at the level of
the housing units or the buildings, where inter-individual (be they
intra- or interspecific) interactions actually occur. Yet it is
precisely at this fine scale that the ecological interactions take place
which probably determine the trajectory of the communities in terms of
their distribution in space and time.
Niche / ressource partitioning represents a way to manage coexistence
among competing species within habitats (Pianka, 1973; Chesson, 2000).
Indeed, the complex and interactive effects of species niche overlap,
niche breadth and environmental heterogeneity on species co-occurrence
patterns have been highlighted repeatedly (see a synthesis in Bar
Massada, 2015). In the particular case of commensal small-mammal
communities, information on and analyses of species co-distribution and
co-existence, habitat partitioning (if any), and interspecific
interactions between species (including invasive ones) are lacking,
being however of paramount importance to better understand: i) the way
invasive species spread at the microhabitat scale; ii) the consequences
of this spread on the distribution of native species at the microhabitat
scale ; iii) the actual associations between species likely to represent
zoonotic disease reservoirs at the very contact with humans.
In Africa, both R. rattus and M. musculus colonized most
countries via boats of European or Arab settlers, often centuries ago
(Happold, 2013). Long confined to coastal areas and larger cities, they
have been spreading continuously over inland areas thanks to the
development of infrastructures and associated human exchanges (e.g.
movements of goods and people) that accompanies the ongoing urbanization
of rural areas. This is the case in Senegal (West Africa) where bothR. rattus and M. musculus have experienced recent range
expansion eastward from the western Atlantic coastal areas (Duplantier
et al., 1991; Dalecky et al., 2015; Konečný et al., 2013). Being
exclusively commensal in this country, they encounter native species
that inhabit human settlements, leading to inevitable interactions which
ultimately determine the patterns of cohabitation between them. To
describe this coexistence and try understand the underlying
interactions, we sampled communities of commensal small mammals from
localities of various size within the southern half of Senegal
(corresponding to the distribution range of R. rattus in the
country) within a 3-year time period. The sites sampled were widely
invaded by the black rat and/or the domestic mouse, which most of the
time cohabited with a wide spectrum of native rodent and shrew species.
We aim at providing novel insights from the following questions: i)
which invasive and native species compose the small mammal community
across the different localities targeted? ii) what are the preferred
habitat types and ecological niches of each of these species? iii) do
these species show particular interspecific associations (segregation or
aggregation) globally and/or locally? To answer these questions, we
investigated here the composition, geographic distribution,
micro-habitat use, ecological niche breadth and overlap, and species
co-occurrence within the target small mammal community at various
spatial scales.