Ellenberg's indicator values support prediction of suitable habitat for pre-diapause larvae of endangered butterfly Euphydryas aurinia

Abstract

In spite of the great popularity of Ellenberg's Indicator Values (EIVs) in plant ecology, animal ecologists seldom use EIVs to address ecological questions. In this study we used EIVs to test their potential usefulness for the prediction of suitable habitat for pre-diapause larvae of the endangered butterfly species Euphydryas aurinia. Nine transects crossing grasslands in SW Poland with abundant populations of E. aurinia were designed. We sampled 76 vegetation plots along the transects. In addition, the presence of the larval webs of E. aurinia in sampled plots was also recorded. We then calculated the mean community EIVs of light, nitrogen, soil reaction, moisture and temperature for each sample plots. Generalized linear mixed-effects models (GLMMs) were used to assess which factors determine the local occurrence of larval webs of E. aurinia. We found the larval webs only in 12 plots, while the host plant was present in 39 of the examined plots. The presence of the host plant was the most important predictor in both models including all plots or including only plots with host plants. The other significant predictor was the mean EIV of light, and its importance increased in models considering all plots. We attributed the importance of the EIV of light to the site openness and density of the vegetation layer. A positive relationship between this predictor and the presence of larval webs indicates that sites with looser vegetation, a lower contribution of shrubs and tall herbs and better penetration of photosynthetically active radiation to lower vegetation layers are preferred by E. aurinia for oviposition. Moreover, the significance of EIV of light may be linked with management practices. Many light-demanding species decline after cessation of mowing as a result of litter accumulation and the dominance of tall herbs. An absence of light-demanding species decreases the community's mean EIV of light and thus indicates the influence of meadow abandonment.

Fig 1. Comparison of the relative variable importance (RVI) used to explain the presence of larval webs of Euphydryas aurinia in two data sets: Including all plots and only those with the presence of Succisa pratensis.

The RVI was computed as the sum of the AICc weights over all models including the explanatory variable.

Fig 2. Average parameter estimates and 95% confidence intervals (CIs) for all standardized variables in two data sets.

One group includes all plots (red color) and one only those with the presence of Succisa pratensis (green color). Parameters were averaged for the 95% confidence set of the models.

Fig 3. PCA ordination diagram of all 76 study plots of vegetation based on six explanatory variables, with differentiation of the three groups of plots presented with different colors and circled with 95% confidence interval ellipses.

Red points and ellipsoid represents plots with Euphydryas aurinia occurrence; green points and ellipsoid represents plots with Succisa pratensis occurrence and without E. aurinia; and blue points and ellipsoid represent plots without S. pratensis and E. aurinia. Arrows indicate the direction of the explanatory variables. Eigenvalues: PC1–2.397, PC2–1.514.