Relative importance of density-dependent regulation and environmental stochasticity for butterfly population dynamics

Abstract

The relative contribution of density-dependent regulation and environmental stochasticity to the temporal dynamics of animal populations is one of the central issues of ecology. In insects, the primary role of the latter factor, typically represented by weather patterns, is widely accepted. We have evaluated the impact of density dependence as well as density-independent factors, including weather and mowing regime, on annual fluctuations of butterfly populations. As model species, we used Maculinea alcon and M. teleius living in sympatry and, consequently, we also analysed the effect of their potential competition. Density dependence alone explained 62 and 42% of the variation in the year-to-year trends of M. alcon and M. teleius, respectively. The cumulative Akaike weight of models with density dependence, which can be interpreted as the probability that this factor should be contained in the most appropriate population dynamics model, exceeded 0.97 for both species. In contrast, the impacts of inter-specific competition, mowing regime and weather were much weaker, with their cumulative weights being in the range of 0.08-0.21; in addition, each of these factors explained only 2-5% of additional variation in Maculinea population trends. Our results provide strong evidence for density-dependent regulation in Maculinea, while the influence of environmental stochasticity is rather minor. In the light of several recent studies on other butterflies that detected significant density-dependent effects, it would appear that density-dependent regulation may be more widespread in this group than previously thought, while the role of environmental stochasticity has probably been overestimated. We suggest that this misconception is the result of deficiencies in the design of most butterfly population studies in the past, including (1) a strong focus on adults and a neglect of the larval stage in which density-dependent effects are most likely to occur; (2) an almost exclusive reliance on transect count results that may confound the impact of environmental stochasticity on butterfly numbers with its impact on adult longevity.

What keeps ‘living dead’ alive: demography of a small and isolated population of Maculinea (= Phengaris) alcon

“…The foodplants are in very short supply in most of the locality, and even within its southern core fragment their density (ca. 300 gentians per ha) is well below the typical levels recorded at M. alcon sites (WallisDeVries 2004;Nowicki et al 2007Nowicki et al , 2009Radchuk et al 2012;Czekes et al 2014; but see; Maes et al 2004). It is worth pointing out that 1000 gentians per ha has been suggested as the threshold value, below which the foodplant density becomes a critical resource for M. alcon populations (WallisDeVries 2004).…”

“…All things concerned, it appears that the tiny size we recorded for the investigated population reflects its typical status rather than an occasional drop of a usually much larger but highly fluctuating population (cf. Nowicki et al 2009). The most obvious factor limiting our focal population is the low availability of G. pneumonanthe foodplants.…”

“…Unlike the foodplants of other Maculinea species, which tend to grow in high densities within relatively large meadow fragments (Bonelli et al 2013;Nowicki et al 2014), the marsh gentians are typically scarce and occur in highly localised places, and thus their availability constitutes a limiting factor for M. alcon abundances (Nowicki et al 2007). Consequently, the butterfly quite often exists in very small and spatially isolated local populations, although fairly large metapopulation systems with many interconnected populations can be found as well (WallisDeVries 2004;Nowicki et al 2007Nowicki et al , 2009). …”

The effect of temperature and habitat quality on abundance of the Glanville fritillary on the Isle of Wight: implications for conservation management in a warming climate

“…This result is further evidence that oviposition is temperature-dependent: in warm years a greater proportion of plantains are within the thermal envelope for oviposition (Weiss et al 1988;Roy & Thomas 2003), but in cold years females restrict their egg-laying to a small proportion of the available plantains. The most likely mechanism linking temperature to abundance is therefore the amount of hostplants within a suitably warm microclimate, but we cannot rule out the possibility of density-dependent larval mortality (Hanski et al 1996;Nowicki et al 2009) or that fecundity itself might be temperature dependent (Saastamoinen 2007;Ojanen et al 2013). …”

Adult longevity and its relationship with conservation status in European butterflies

“…In the case of studies spanning several years (e.g. Schtickzelle et al 2002;Nowicki et al 2009), we treated the data from each year separately. In making the calculations, we endeavoured to use data on life span and light period length from the same population and year.…”