Approximately a year ago I read the research publication discussing the population growth in the highly endangered Cantabrian brown bears (Uzal, A. et al., 2022).
The study stated a rather logical conclusion which I also find to be supported, namely, that 1) increased proximity between the reproductive dens and 2) decreased proximity to landscape elements that are considered suboptimal to bears, e.g., lower altitudes, anthropogenic disturbance attest to a growing population density and occupancy of the best habitats.
It was found that the distance between breeding areas (females with the cubs of the year) decreased locally (between neighbours) while the general distance between breeding areas increased (due to population expansion and females settling further away from the population core areas).
I agree with the conclusion that this is demonstrative of a growing population (density) and optimal habitat saturation, mainly because of the increased occupancy of suboptimal habitats (which occurs when the optimal habitats are already taken and thereby suggests that the population has reached a certain threshold density needing to expand).
However, more recently I have read two publications from the Fennoscandinavian region (Støen, O.-G., 2005 and Olerjarz, A. et al., 2022) that discuss the tendency of the natally philopatric female bears to settle close to their mothers.
As a result, multigenerational female matrilinear clusters are formed in a population and the overlap between the ranges of any two related females is greater than that between unrelated females.
The Swedish-Norwegian study does not state the population density status and the Finnish study analyzes data from two regions – in the eastern region bear population density is higher than in the central region and the matrilineal clustering was less pronounced.
However, in eastern Finland there are other factors of influence such as immigration from Russia and hunting pressure by humans that can destabilize the local population increasing turnover of individuals.
Thus, it is difficult to state with certainty whether matrilineal clustering would be indicative of low or moderate or high population density but it is likely indicative of a relatively stable population.
Of course, matrilineal clusters cannot really form in newly colonized areas where not enough females have been recruited.
After having acquired this new knowledge of brown bear ecology, I recalled the Cantabrian study and I wondered if the shorter distances between the female breeding areas on a local level was not due to the effect of matrilineal clustering – i.e., related females tolerate one another closer to their core range and, as a result, the distances between their dens and core areas decrease.
Thus, by itself, the locally increased density (when expressed as decreased distances between female breeding areas) might not be a reliable indicator of increased population density but rather of increased local density that results from matrilineal clustering (related females staying close to each other).
It is not know precisely what benefits are involved in staying proximate to mother.
If the benefits are not that impressive, clustering might occur only under conditions of certain population saturation – i.e., when it is no longer easy to claim new ranges that do not overlap with that of mother’s.
If the benefits are greater, daughters might choose to remain close to mother even if additional and suitable habitat is available.
If the latter were true, this might create an effect of locally increased population density and habitat saturation even though the decreased distance between female breeding areas suggestive of their relatedness rather than contraction of ranges due to conspecific pressure.
As mentioned before, I believe that the Cantabrian brown bear population is increasing and expanding.
However, it might be important to further study the mechanisms of matrilineal clustering because it could confound the population density and habitat saturation estimates.
For example, if average population density was determined on a larger scale, the locally clustered ranges of the related females might bias the results and, in truth, vaster areas between clusters might have very low densities.
Also, if female kinship clusters exhibited any strategies of preventing immigration (as propositioned by Støen, O.-G., 2005), this could alter female bear distribution.
For example, matrilinear clusters might take up the best habitats and non-related females might find it difficult to establish between these clusters because males would be attracted to greater densities of females and other females who were left outside of the clusters might not be able to ensure mating opportunities.
If finding a mate led to intruding into the ranges of matrilinear females, this could be too dangerous for the non-related female.
As a result, non-related females might be prompted to use habitats that are not proximate to the clusters but rather distant to them.
It might happen that these females selected suboptimal habitats further away from these clusters over optimal habitats that are too heavily influenced by the matrilineal clusters due to proximity.
Thus, the regular population growth dynamic would be modified and assumptions regarding optimal habitat saturation as well as population density might be flawed.
Additionally, if there was a tendency for such female kinship areas to form, this could affect the genetic diversity of the population.
Population viability is sometimes quantified as a number of reproductively capable bears per area.
However, under the conditions of matrilineal clustering, larger areas might arise containing genetically similar females.
As a result, any such unit might have to be treated not as ‘a number of breeders per area’ but as a type of ‘subpopulation’ and the genetic viability of the entire population should then be estimated in accordance with the number of subpopulations so that there was plentiful genetic diversity among females and so that there was a sufficient number of males produced who could immigrate into these different clusters.
Notes by Ieva 