Life at the Edge: The Limits of Occurrence of Insect Larvae

Extreme environments are found all over the world. They are extreme because of high or low temperatures, lack of water, acid or alkaline environment, UV-radiation, or just harsh in any other aspect. Most of these habitats are inhabited by plants and animals, usually taxa adapted to just this specific environment.But every specific environment has limits, special limits where the environment changes in a way that one species declines and others take over. Lets talk about specialized animals: Specialization allows maximum exploitation of resources at low competition but reduces success if resources become limited. Consequently niches for specialists are narrow. We may characterize the borders of a niche by comparing animals from the peak of their occurrence up to those from their limits of occurrence. These ecological niche widths provide us with an understanding of species requirements, preferences, or limits for survival. Chironomid larvae (larvae of non-biting midges) comprise an almost endless list of species each being assigned to a specific environment. They receive considerable interest in ecology as they are indicative for climate change and water quality assessment. Only few species conquer extreme environments like high mountain rivers, they are frequently found up to the mouth of glaciers. The research question around this study is inspired by ecological studies in high mountain areas. Which difference would we expect between individuals from the harshest spots compared to less extreme places? Are they bigger or smaller, are they well fed or not, are they more or less abundant? In fact, each of these effects may be observed depending on the species and subspecies. But these findings clearly indicate that metabolic responses to the environment may be observed, possibly pointing to limiting factors for survival. We hypothesize, that there are species differences and intraspecific differences in the response to harshness. In this project all aspects allowing for adaptation or survival will be analyzed. Are there genetic differences? Which genes are activated? Which responses of the organism are observed? All together this project will shed some light on the limits of occurrence of species and the mechanisms defining the niche requirements for each species. Are these larvae living at the edge of their occurrence?

The waters in the high mountains are certainly an extreme habitat. In this habitat, temperatures close to the freezing point prevail all year round. Depending on the season and the weather, there are very strong currents with a high load of sediment, but also low water levels with a risk of drying out. Overall, the productivity of these waters is low, so that organisms living there have to reckon with a low food supply. The organisms living there have to adapt to these living conditions. However, this extreme habitat also offers some advantages. Even if the resources are few, competition for food is less than in lower areas. The biggest advantage, however, is probably the lower pressure from predators, as most predators depend on more moderate conditions. The larvae of chironomids (non-biting midges) colonize almost every moist habitat, including the streams in the high mountains up to the edge of the glaciers. Of course, the number of species is relatively low, but the question still remains: how do these animals manage it? Within the chironomids, the genus Diamesa is a mountain specialist. Only a few species of this genus ascend to the most extreme locations, with species differences playing a lesser role. One species, Diamesa steinböcki, is a specialist for the most extreme locations up to the edge of glaciers. The energy reserves, glycogen and lipid, in the animals' bodies are a measure of the nutritional status of the animals. Measurements in the animals showed surprisingly low concentrations. The populations of insect larvae in high mountain streams are obviously limited by the availability of food. The larvae of chironomid midges are widely considered to be grazers, they graze on the surface algae from the stones. However, they also feed on animal material, even to the point of cannibalism, as well as foreign material brought in by wind and weather. The animals' response to shortages is primarily a reduced metabolism, which helps them survive times when food is scarce. This also results in a variable development time: the development from egg to adult insect capable of flight is not tied to a timetable. Rather, the duration of development depends on the resources available and can probably take from a few months to several years. How will these populations develop in the event of future global warming? The retreat of glaciers will expand the possible habitat upwards. The boundaries for predators of chironomid larvae will also shift upwards. This does not only affect fish, other insect larvae such as stoneflies also migrate with them. This shifts the entire habitat. Passing the summits, only the chitin remains of the larvae will remain in the sediments as evidence of past populations.