Abstract
Species distribution models are the tool of choice for large‐scale population monitoring, environmental association studies and predictions of range shifts under future environmental conditions. Available data and familiarity of the tools rather than the underlying population dynamics often dictate the choice of specific method – especially for the case of presence–absence data. Yet, for predictive purposes, the relationship between occupancy and abundance embodied in the models should reflect the actual population dynamics of the modelled species. To understand the relationship of occupancy and abundance in a heterogeneous landscape at the scale of local populations, we built a spatio‐temporal regression model of populations of the Glanville fritillary butterflyMelitaea cinxiain a Baltic Sea archipelago. Our data comprised nineteen years of habitat surveys and snapshot data of land use in the region. We used variance partitioning to quantify relative contributions of land use, habitat quality and metapopulation covariates. The model revealed a consistent and positive, but noisy relationship between average occupancy and mean abundance in local populations. Patterns of abundance were highly variable across years, with large uncorrelated random variation and strong local population stochasticity. In contrast, the spatio‐temporal random effect, habitat quality, population connectivity and patch size explained variation in occupancy, vindicating metapopulation theory as the basis for modelling occupancy patterns in fragmented landscapes. Previous abundance was an important predictor in the occupancy model, which points to a spillover of abundance into occupancy dynamics. While occupancy models can successfully model large‐scale population structure and average occupancy, extinction probability estimates for local populations derived from occupancy‐only models are overconfident, as extinction risk is dependent on actual, not average, abundance.
Highlights
Observed declines in insect abundance around the world remind us of the importance of understanding changes in species’ abundance in addition to looking at range shifts and population extinctions (Conrad et al 2006, Shortall et al 2009, Cameron et al 2011, Koh et al 2016, Hallmann et al 2017)
Suggested mechanisms that give rise to different, mostly positive, occupancy–abundance relationships both within and among species include for example sampling processes, metapopulation dynamics, species-specific environmental responses and spatial variation in habitat quality (Hanski 1991, Hanski and Gyllenberg 1997, Freckleton et al 2005, 2006)
We study how the spatial structure of habitat, population dynamics, habitat quality and the composition of the landscape contribute to occupancy and abundance and how these contributions differ at the level of local populations in the metapopulation
Summary
Observed declines in insect abundance around the world remind us of the importance of understanding changes in species’ abundance in addition to looking at range shifts and population extinctions (Conrad et al 2006, Shortall et al 2009, Cameron et al 2011, Koh et al 2016, Hallmann et al 2017). Occupancy and abundance patterns both arise from spatio-temporal variation in growth rates and dispersal. Ecological theory does not assert that distribution, occupancy and abundance respond to variation in environmental conditions, as they are different functions of those population dynamic processes. Spatio-temporal variation in the environment leads to variation in the dispersal and local growth rates, but the relationship between them. Where density-dependence affects growth and dispersal, this could lead to different relationships between occupancy, distribution and abundance. While ecological studies demonstrate that these population measures are often correlated (Gaston et al 2000, Cowley et al 2001), this does not imply that their relationship is fixed across different contexts and taxa (Holt et al 2002) or driven by the same processes. Suggested mechanisms that give rise to different, mostly positive, occupancy–abundance relationships both within and among species include for example sampling processes, metapopulation dynamics, species-specific environmental responses and spatial variation in habitat quality (Hanski 1991, Hanski and Gyllenberg 1997, Freckleton et al 2005, 2006)
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