Influence of landscape heterogeneity and meteorological features on small mammal abundance and richness in a coastal wetland system, NW Portugal

Seasonal patterns in habitat use by the harvest mouse (Micromys minutus) and other small mammals

The ecology of the harvest mouse (Micromys minutus) is poorly understood, partly because it is a difficult species to monitor. It is commonly associated with reedbeds, where evidence suggests that it experiences strong seasonal fluctuations in abundance. However, it is unknown whether these fluctuations are caused by real changes in population size, or by movement between habitats. This study investigated seasonal changes in population size and habitat use by harvest mice, and other small mammal species, by trapping the reedbed and three associated habitat types: woodland, pasture and arable land. A sampling effort of 9887 trap bouts across nine months, resulted in 70 captures of harvest mice, as well as wood mice (N = 1022), bank voles (N = 252), field voles (N = 9), common shrews (N = 86) and pygmy shrews (N = 7). The reedbed was the habitat with the most captures and highest diversity. Harvest mice were caught exclusively in the reedbed at the beginning of autumn. Wood mice and bank voles experienced fluctuations in population numbers and wood mice also showed seasonal variation in habitat use. Our study supports the idea that harvest mice undergo extreme seasonal fluctuations in abundance in reedbeds, but these do not appear to be related to changes in habitat use.

Local habitat specialization as an evolutionary response to interspecific competition between two sympatric shrews

Interspecific competition affects population dynamics, distributional ranges, and evolution of competing species. The competitive exclusion principle states that ecologically similar species cannot coexist unless they exhibit niche segregation. Herein, we assess whether niche segregation allows the coexistence of Crocidura russula and C. suaveolens in southwestern Iberia and whether segregation is the result of current (ecological effect) or past (evolutionary effect) competition. We performed an annual live-trapping cycle in the two main habitats of the Odiel Marshes Natural Reserve (OMNR), the tidal marsh and the Mediterranean forest, both in syntopic (i.e., where both species co-occur) and allotopic (where only one of the two species occurs) sites within this Reserve. We modeled the presence–absence of each species in both habitats and sites by generalized linear mixed models. The coexistence of both species was favored by spatial and temporal niche segregation. Crocidura suaveolens was restricted to tidal marsh and did not occupy Mediterranean forest, even when C. russula was absent. We interpret this to be the result of competition in the past triggering an evolutionary response in C. suaveolens towards its specialization in tidal marsh. Moreover, the specialist C. suaveolens currently is outcompeting C. russula in tidal marshes, reversing the dominance pattern observed elsewhere. The degree of co-occurrence between both species in syntopic sites was low, as they showed inverse dynamics of seasonal abundances. Interspecific competition leading to habitat specialization favors the coexistence of these ecologically similar species.

Resource partitioning confirmed by isotopic signatures allows small mammals to share seasonally flooded meadows

Meadows in river deltas are characterized by a high diversity and abundance of small mammals. However, neither their spatial arrangement nor differences in their use of microhabitat can necessarily explain the dense co-occurrence of sympatric species. We investigated how several small mammal species share a seasonally flooded meadow of limited size, testing predictions (P1) that herbivore, granivore, insectivore, and omnivore species are separated in time (dominant in different years), (P2) that sympatric species undergo isotopic partitioning, and (P3) that there are intraspecific differences in diet. Stable carbon and nitrogen isotope signatures in the hair of seven synantropic shrew, vole, and mice species were used as a proxy for their diet. We found that the three most abundant species in eight of the nine years were from different diet groups. However, based on the number of species in the functional groups, the state of small mammal community was considered unfavored in five out of the nine investigation years. In years with the greatest dominance of Apodemus agrarius, the small mammal community was characterized by decreased diversity and Micromys minutus was either in low abundance or absent. In 2014 and 2016, years of low abundance or absence of M. oeconomus, M. agrestis, and M. glareolus were both recorded in high numbers. Differences in the isotopic signatures of the three most abundant small mammal species in the community were clearly expressed and core areas in the isotopic space were separated, showing their dependence on different dietary resources. Intraspecific dietary separation between young and adult animals was observed only in M. oeconomus. Thus, the high species diversity of small mammals and the formation of their community in this investigated flooded meadow are maintained by isotopic partitioning (segregation in dietary space) and by changes in their number over time (shifting dominance).