Long‐term demographic surveys reveal a consistent relationship between average occupancy and abundance within local populations of a butterfly metapopulation

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 methodespecially 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 butterfly Melitaea cinxia in 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. Long-term demographic surveys reveal a consistent relationship between average occupancy and abundance within local populations of a butterfly metapopulation Torsti Schulz, Jarno Vanhatalo and Marjo Saastamoinen T. Schulz (https://orcid.org/0000-0001-8940-9483) ✉ (torsti.schulz@helsinki.fi), J. Vanhatalo (https://orcid.org/0000-0002-6831-0211) and M. Saastamoinen (https://orcid.org/0000-0001-7009-2527),

Narrow oviposition preference of an insect herbivore risks survival under conditions of severe drought

“…Previous studies have shown that food scarcity and high levels of desiccated host plants at the end of the summer may lead to reduced body mass and even result in starvation of the larvae, which may consequently reduce overwinter survival and cause local extinctions (Kuussaari et al, 2004;Nieminen et al, 2004). Long-term monitoring of the metapopulation has led to a good understanding of the determinants of overall patch quality on the occupancy and abundance, however, little is known about within-patch variation (but see Schulz et al, 2019).…”

“…This method is increasingly being used in ecological studies as an efficient way to model species occurrences and population dynamics while accounting for spatial and temporal dependencies in the data (e.g. : Myer, Campbell, & Johnston, 2017;Schulz et al, 2019;Ward et al, 2015). In brief, spatial dependency of observations are accounted for using a latent Gaussian random field, which we constructed using a two-dimensional irregular grid (mesh) based on the geographic coordinates of cell centroids.…”

“…We capitalize on the systematic long-term monitoring data of the metapopulation (e.g. Hanski et al, 2017;Schulz, Vanhatalo, & Saastamoinen, 2019) and combine it with detailed field assessments to ask whether oviposition preferences vary across years and whether mothers generally choose locations that enhance their offspring's overwinter survival. As Finland is the northern range limit of these butterfly species, with relatively short time window for larval development, and based on our recent findings that larval performance may be increased by feeding on droughtexposed host plants (Rosa, Minard, Lindholm, & Saastamoinen, 2019;Salgado & Saastamoinen, 2019), we hypothesize that drier microhabitats are generally favoured by the females but may be disadvantageous for the larvae during particularly warm and dry years.…”

The effect of summer drought on the predictability of local extinctions in a butterfly metapopulation

“…Survey data have been used to demonstrate that the long-term persistence and population dynamics (local extinctions and recolonizations) of the metapopulation largely depend on the number of available habitat patches, their area, and their spatial configuration (i.e., connectivity) (Hanski & Ovaskainen 2000;Dallas et al 2019). Other key factors affecting local extinctions and population dynamics are habitat quality (e.g., number of host plants) (Harrison et al 2011;Schulz et al 2019) and genetic characteristics of individuals (Saccheri et al 1998;Niitepõld & Saastamoinen 2017).…”

Summer drought decreases the predictability of local extinctions in a butterfly metapopulation

“…Other key factors impacting local extinctions and population dynamics are the habitat quality (e.g. amount of host plants; Harrison et al 2011;Schulz et al 2019) and genetic characteristics of the individuals (Saccheri et al 1998;Niitepõld & Saastamoinen 2017).…”