Interactive effects of land-use and microclimate on drought resistance and their ecophysiological basis in insects

Background

Land use, such as mowing and clear-cutting and the creation of forest gaps, affects the microclimate by increasing temperature and drought stress. The absence of shade and humid micro-refugia increase temperature of available microhabitats.

Fig. 1: Schematic visualisation of different microclimates caused by land-use differences such as mowing and the creation of forest gaps

Higher temperatures are not only challenging in themselves, but especially because they increase drought stress:

With higher temperatures, evaporation increases (higher water vapour deficit).
Metabolic rate and respiration rate increase, facilitating water loss through respiration.
CHCs – the most important agent against desiccation – become more liquid and thus more permeable for water.

This explains why many insect taxa (e.g. hoverflies) often cease activity at higher temperatures, even if temperatures are far below their thermal limit. Drought resistance thus is a main driver of the ecological niche of insects, especially their tolerance to higher temperatures.

Cuticular hydrocarbons (CHCs) cover the body of all insects like a waxy layer and are the most important protection against desiccation in insects. Their composition varies strongly between insect species and determines the drought resistance of insect species. Differences in CHC profiles drive differences in drought resistance and may exclude species from habitats with higher levels of drought stress due to intense land use.

Fig. 2: Examples of CHC profiles of species adapted to different habitat types. The x-axis shows the chain length of the hydrocarbon molecules. The height of each bar represents the relative abundance of each CHC class (colour-coded) in the respective chain length. One can see that with in drier habitats, species tend to have less alkenes (blue), more n-alkanes (red), and/or longer CHC molecules.

Goals

Our goal is to study the link between land-use, drought resistance and CHC profile. The central question is: How do changes in microclimate induced by land-use affect the composition of insect communities, and how can this be predicted based on CHC profiles, CTmax and drought resistance?

Our final goal is to use CHC profiles as a proxy to predict species responses to land-use. We will do this for multiple insect taxa, in order to see how effects of climate and land-use differ across taxa with differing life-histories.

Furthermore, we want to assess whether drought-sensitive species can be supported by reduced land-use (e.g. reduced mowing), thus bringing together effects of climate change and land-use intensity.

Hypotheses

Land-use tolerant species are more drought resistant. Intense land-use acts as habitat filter that only drought-resistant species can pass.
Drought-resistant species differ in CHC profile from drought-sensitive species. They have a CHC profile with more n-alkanes, less alkenes and/or higher chain-lengths.
CHC profiles can predict the occurence of species in high or low land-use habitats.
Intense land-use leads to CHC homogenization. CHC diversity in insect communities with high land-use tolerance should be reduced compared to communities from sites with less intense land-use.
Different life stages of the same species should have similar drought resistance after correcting for body mass.

Methods

We will study six taxa:

Ants (Formicidae)
Hoverflies (Syrphidae)
Bees (Apidae, Halictidae, etc.)
Grasshoppers and locusts (Orthoptera)
True bugs (Heteroptera)
Dung beetles (Geotrupidae/Scarabaeidae)

We will study grasslands, forests, and arable fields, especially the REX/LUX and the FOX experiments. In each of the plots, we will catch insects using sweepnets, beating trays, or pitfall traps. This will be done for all three exploratories.

For each caught individual, we will record drought resistance and analyse their CHC profile using GC-MS. Together with our own abundance data, and previous monitoring data from BexIS, we will then calculate community-weighted means of drought resistance, and analyse how it is altered by land-use. Species-specific CHC profiles and community-level CHC diversity will be analysed for links to land-use and to local microclimates, which are strongly influenced by land-use.