Beyond species list: Consequences of glacier retreat on biodiversity, ecosystem, functioning and species interactions.

Climate is rapidly warming due to the sharp increase in CO2 emissions. Glacier retreat is a symptomatic indicator of this recent warming. Glaciers have been melting since the end of the Little Ice Age, which coincided with the rise in atmospheric CO2. However, their retreat has accelerated dramatically since the 2000's, and future projections are alarming, with between 20% and 50% of glaciers threatened with disappearance by 2100. As glaciers retreat, newly ice-free terrains (hereafter: glacier forelands) become available for colonisation by a wide range of organisms — from microorganisms such as bacteria and fungi to mammals. The colonisation of glacier forelands has been widely studied, particularly for microorganisms and plants. Studies have shown that, initially after glacier retreat, biodiversity tends to increase rapidly until it reaches a plateau after approximately one hundred years, after which it may stabilise or decline. Another key finding is the rapid replacement of species: specialist species that thrive in glacier habitats, are gradually replaced by generalist species, which are not strictly dependent on glaciers. As a result, species that are tightly linked to glacier environments may disappear entirely, leading to local extinctions. Most existing studies, however, have focused mainly on species lists and inventories, even though biodiversity encompasses far more than a simple list of species. Biodiversity also includes aspects such as species interactions, functional traits, and ecosystem services. Neglecting these dimensions can lead to a serious underestimation of the consequences of biodiversity loss and to missing information that is crucial for predicting ecosystem and functional changes, such as those affecting nutrient cycling. In this thesis, I aim to integrate these different overlooked aspects of biodiversity by focusing on species traits, species interactions, and nature’s contributions to people. In the first chapter, four study glacier forelands were surveyed for plant communities. First, we examined the consequences of glacier retreat for human society. We conducted an extensive literature review to link each plant species with the contributions to people it provides, thereby developing the novel concept of “plant contributions to people”. In the second chapter, we then assessed both intra- and interspecific trait variability following glacier retreat, as well as the variation of individual traits along the retreat gradient. To this end, we measured a range of plant traits associated with the leaf economic spectrum. We then study species interactions following glacier retreat. In chapter three, we extensively sampled the glacier foreland of Mont-Miné, collecting plants and their associated microbial communities. Bulk soil, rhizosphere, and plant roots were sampled to understand how glacier retreat and plant presence influence microbial communities. In the fourth chapter, we further investigated plant–microbe interactions by focusing on rhizosphere and root-associated microbes, and calculated various network metrics to assess how interaction properties change with glacier retreat. Through these different approaches, I demonstrate that not only species inventories but all levels of biodiversity are important. By focusing solely on species lists, we miss a significant portion of biodiversity and underestimate the effects of climate change — in this case, glacier retreat — on ecosystems. Furthermore, I show that glacier retreat has large negative ecological impacts, leading to a homogenisation of the various facets of biodiversity assessed in this thesis. I also highlight the urgent need for more systematic biodiversity assessments and stronger legal frameworks to protect these highly sensitive glacier foreland habitats.