Breaking down population density into different components to better understand its spatial variation

Abstract

Background Population size and densities are key parameters in both fundamental and applied ecology, as they affect population resilience to density-dependent processes, habitat changes and stochastic events. Efficient management measures or species conservation programs thus require accurate estimates of local population densities across time and space, especially for continuously distributed species. For social species living in groups, population density depends on different components, namely the number of groups and the group size, for which relative variations in space may originate from different environmental factors. Whether resulting spatial variations in density are mostly triggered by one component or the other remains poorly known. Here, we aimed at determining the magnitude of the spatial variation in population densities of a social, group-living species, i.e. the European badger Meles meles, in 13 different sites of around 50 km2 across France, to decipher whether sett density, group size or proportion of occupied sett variation is the main factor explaining density variation. Besides the intrinsic factors of density variation, we also assessed whether habitat characteristics such as habitat fragmentation, urbanisation, and resource availability, drove both the spatial variation of density components and local population densities. Results We proposed a new standardised approach combining use of multiple methods, namely distance sampling for estimating the density of occupied sett clusters, i.e. group density, and camera and hair trapping for genetic identification to determine the mean social group size. The density of adult badgers was on average 3.8 per km2 (range 1.7–7.9 per km2) and was positively correlated with the density of sett clusters. The density of adult badgers per site was less related to the social group size or to the proportion of occupied sett clusters. Landscape fragmentation also explained the spatial variation of adult badger density, with highly fragmented landscapes supporting lower adult densities. Density components were linked differently to environmental variables. Conclusions These results underline the need to break down population density estimates into several components in group-living species to better understand the pattern of temporal and spatial variation in population density, as different components may vary due to different ecological factors.

A Bayesian evolutionary model towards understanding wildlife contribution to F4-family Mycobacterium bovis transmission in the South-West of France

“…The average badger density in 13 study sites (including a 50 km 2 site situated in our study area) in France was estimated at 3.8 badgers per km 2 (range: 1.7–7.9), which is “relatively lower than those found in the UK and concordant with global estimates from Ireland” [ 45 ]. We inferred a badger-to-cattle transition rate 52 times greater than the cattle-to-badger rate.…”

Estimation of Bait Uptake by Badgers, Using Non-invasive Methods, in the Perspective of Oral Vaccination Against Bovine Tuberculosis in a French Infected Area

“…As suggested by the FAMD results at the social group and pasture levels, this contrast might be linked to the difference in the trapping intensity implemented in the last 9 years between area A and B (see Supplementary Material 1 ). Intense trapping in zone B resulted in reduced proportion of occupied main setts ( 28 ). It is also possible that a behavioral component occurred in area B: By being subjected to a more intense trapping activity, badgers from zone B may have been more sensitive to human disturbance than badgers from zone A.…”

“…The intensity of culling was, however, heterogenous among municipalities [source: local veterinary services and ( 27 )] ( Figure 1 ). At the north western boundary between the infected and the buffer area (called zone “A” thereafter), badgers' density [estimated by using a combination of distance sampling, camera trapping, and hair trapping for genetic identification, see ( 28 ) for details] was estimated in 2017 at 2.57 adults/km 2 , whereas, in the core of the infected area (called zone “B” thereafter), the badger density was evaluated in 2017, at 3.79 adults/km 2 ( 28 ). In both zones, the proportion of occupied setts decreased between 2012 and 2017 from 0.73 to 0.60 in zone A and from 0.79 to 0.55 in zone B ( 27 , 28 ).…”

“…At the north western boundary between the infected and the buffer area (called zone “A” thereafter), badgers' density [estimated by using a combination of distance sampling, camera trapping, and hair trapping for genetic identification, see ( 28 ) for details] was estimated in 2017 at 2.57 adults/km 2 , whereas, in the core of the infected area (called zone “B” thereafter), the badger density was evaluated in 2017, at 3.79 adults/km 2 ( 28 ). In both zones, the proportion of occupied setts decreased between 2012 and 2017 from 0.73 to 0.60 in zone A and from 0.79 to 0.55 in zone B ( 27 , 28 ). Regarding the density of adult badgers, it decreased in zone A by 26%, whereas it was multiplied by 1.5 in zone B (most likely because the peak of culling intensity was reached in 2012 and then decreased, as shown in Supplementary Material 1 ) ( 27 , 28 ).…”

“…In both zones, the proportion of occupied setts decreased between 2012 and 2017 from 0.73 to 0.60 in zone A and from 0.79 to 0.55 in zone B (27,28). Regarding the density of adult badgers, it decreased in zone A by 26%, whereas it was multiplied by 1.5 in zone B (most likely because the peak of culling intensity was reached in 2012 and then decreased, as shown in Supplementary Material 1) (27,28).…”