Commensal small mammal community composition and distribution
A previous analysis of commensal small mammal communities at the scale of Senegal has been presented by Dalecky et al. (2015). While it was primarily focused on the house mouse distribution, this work depicted data gathered between 1983 and early 2014 on the expansion of both exotic M. musculus and R. rattus vs. all native species taken as a whole. The present study extends the effort of Dalecky et al. (2015) both temporally (to September 2015) and spatially by adding large localities such as Tambacounda and Rufisque, and new areas such as the one north of Kidira at the Senegal-Mauritania eastern border. Using data from 49 localities in the southern half of the country (i.e. covering the Senegalese distribution of R. rattus ), it also details the patterns of occurrence / co-occurrence of all the small mammal species encountered.
Commensal species can be classified precisely following Hulme-Beaman et al. (2016) who provided a series of definitions concerning the type of relationship that species can have with anthropogenic environments. According to this terminology, we have here a mixture of ‘obligate commensals’ represented by the exotic invasive species M. musculus and R. rattus that can only survive in the study area because of their ability to occupy houses, and of ‘occasional commensals’ that occur both within houses and in outdoor habitats (all the native species). Among the latter, M. natalensis tends however to be an obligate commensal in Senegal (Duplantier & Granjon, 1988), even if the species is known to occur outdoors elsewhere in Africa (Leirs, 2013).
In the sample gathered here, exotic invasive species outnumbered native ones from nearly all points of view. Indeed, M. musculus andR. rattus represented more than 55% of all the small mammals captured (1749 / 3160). They were also found dominant in the largest number of localities (16 and 14, respectively), even though they are not present in the majority of them. These data testify for the success of these invasive species in Senegal, where the trend toward a rapid west-to-east expansion (i.e. from coastal areas where they were first introduced, to inland) has been spectacular over the last decades (see Fig. 1 in Dalecky et al., 2015). In other words, once these species colonize a new place, they can rapidly become dominant over native ones. This seems to be especially the case for M. musculus which is dominant in the majority of the localities where it is present (16 / 21). This potential to rapidly invade a small mammal community and extirpate the native species previously present has been documented in a number of localities of northern Senegal over the last two decades (Dalecky et al., 2015; Diagne et al., 2020, 2021). It even seems that this recent expansion of M. musculus has come at the expense ofR. rattus , as suggested by the comparison of the data presented in Duplantier et al. (1991) and ours. One may bet that this situation of dominance of invasive species over native ones in commensal small mammal communities is going to become the rule in a number of regions / countries all over Africa. As an example, R. rattus appears as often dominant in Benin’s localities in the survey of Hima et al. (2019) along a Benin-Niger axis. In Guinea, M. musculus was only found in the coastal region by Demby et al. (2001), and especially in the city of Kindia where its abundance decreased from the centre to the periphery. Later, Fichet-Calvet et al. (2005) found R. rattus as the dominant commensal species in smaller villages of the coastal region. Interestingly, nearly 30 years ago, the study by Olaseha et al. (1994) suggested that Rattus spp and M. musculus were already the main commensal species in the urban and rural areas they studied in south-western Nigeria. This may also be the trend in the New World where, even in rural areas, invasive rodents already constitute the bulk of small mammals found within houses. For instance, 74% of the rodents caught in various urban habitats of the city of Rio Cuarto, province of Cordoba, Argentina (Castillo et al., 2003) were of (mainly)M. musculus and Rattus spp.; similarly, 92% of the captures indoors and in the yards surrounding the houses were ofM. musculus and R. rattus in a rural area of Yucatan State, Mexico (Panti-May et al., 2012).
The only native rodent species that stays dominant wherever present isM. natalensis . However, it has to be underlined that this species, restricted to the southeastern part of Senegal (Duplantier & Granjon, 1988), is only co-occurring with an exotic invasive species (here R. rattus ) in one locality, namely Kédougou, this locality constituting the invasion front of the species in this part of the country. There, M. natalensis has apparently resisted to the arrival of R. rattus , that occurred at the end of the 1990’s (Bâ, 2002), since its dominance in this city has continued until this period (unpubl. data). This situation also occurs in villages of Upper Guinea where M. natalensis was found as the main commensal species (Fichet-Calvet et al., 2009), as well as in a number of localities of Niger, including the majority of its capital city (Niamey) districts (Garba et al., 2014; Hima et al., 2019).
In our dataset, the other native rodent species which stay dominant in only a small number of localities are M. erythroleucus andP. daltoni in the extreme East of the study area, where M. musculus is apparently progressing and is expected to replace them in a near future. As found by Hima et al. (2019) at lower latitudes, the native small mammal species which finally stays as the most regularly present, and often co-dominant with invasive rodents is the shrewC. olivieri . This species proves here that it can behave as a true commensal species even if rarely presented as such (Churchfield & Hutterer, 2013).