2.4 Niche differentiation comparison
The ecological niche space occupied by a population, which depends on environmental variables influencing population occurrence, can be estimated using the n-dimensional ecological hypervolume (Lê et al., 2008). Using the R package hypervolume, we then estimated the four-dimensional hypervolumes for the EIOS and WPI populations based on the selected principal components. The size of the population’s niche space can be quantified by the values of hypervolume, a unitless measure. We used the R package BAT to calculate the niche differentiation between the EIOS and WPI populations. Total niche differentiation (βTotal) represents the degree of overlap or separation between two hypervolumes. It can be further divided into two parts: niche contraction/expansion, which indicates the net variations in how much space each hypervolume encompasses, and niche shift, which represents the replacement of the space between the hypervolumes inhabited by two populations (Carvalho & Cardoso, 2020; Mammola & Cardoso, 2020).