B10) Light - a limiting resource

Light - a limiting resource for diurnal butterflies in forests

Thomas Gottschalk
Doctoral researcher: Simon Heitzler (since 2022)

University of Rottenburg, Chair of Landscape Ecology and Regional Management

Background

Light availability is one of the main limiting resources for diurnal butterflies. Within forests, one important reason for the decline of many butterfly species is the lack of former traditional forest management, like wood pasture, coppice, coppice-with-standards, lack of terminal and decay stages in trees, and absences of large herbivores. Most forest butterflies use different habitats within forests and even outside depending on their life cycle.

However, as for light availability, a more generalized approach to assess the general role of landscape heterogeneity has not been attempted yet. In order to link heterogeneity to mechanisms of species movement/ dispersal, traits like flight capabilities are extremely valuable. So, the influence of the landscape matrix and the distance between suitable larval and adult habitats will become much clearer if the distance forest butterflies are able to fly will be studied better.

The proposed project will address the knowledge gaps stated above: the dispersal ability of butterflies within forests, and the relationship between light heterogeneity and butterfly occurrence and abundance.

Research questions and hypotheses

The key research questions include:

• How is the abundance of the different stages of diurnal forest butterfly species and their larval survival rates related to light conditions along gradients of structural complexity caused by retention measures? Which of the forest butterfly species are supported by retention forestry?
• How does the distance between larval and adult habitats influence the occurrence and abundance of forest butterflies? How is the presence of butterflies within ConFoBi plots related to light supporting structures outside the plots (landscape matrix) like clearings, wind throws, drought disturbances and broad forest roads?

The key hypotheses include:

• Occurrence and abundance of selected forest butterfly species (adult stage) increase with increasing light heterogeneity and decreasing shading at the understory. The number of individuals (preimaginal stages) increases with increasing abundance of specific host plants and with specific climate variables, e.g. increasing air humidity.
• Increasing distance to open, sunny rides, glades or clearings reduces the occurrence probability and abundance of selected forest butterfly species in the ConFoBi plots.

Approach, methods and linkages

Sampling will take place on ConFoBi plots as well as on the surrounding landscape matrix to record the distance between larval and adult habitats. To do so, host plants and preimaginal stages will be searched following the methods of (Hermann 1999, 2007) and existing data on host plant occurrence by ConFoBi will be used. Movements and resource selection of selected butterfly species will be recorded with two techniques: 1) capture-mark-recapture (CMR) method (Gall 1984) and 2) radio-tracking using NanoPin transmitters (0.12 g weight). Resource selection of the tracked butterflies can be analysed using flight pathes, occupancy pattern as well as the Minimum Convex Polygon (MCP) and kernel density (KD). Furthermore, preimaginal stages of selected butterfly species will be searched by systematic sampling of their food plants, e.g. Viola sp. for Argynnis paphia, Lonicera sp. for Limenitis camilla, Betula pendula and Salix sp. for Nymphalis antiopa, Salix alba for Apatura iris.