title: Population and community responses of soil organisms after heat extremes
Creators: name: Martínez De León, Gerard
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 570 Life sciences; biology
subject: 580 Plants (Botany)
subject: 590 Animals (Zoology)

description: In the context of contemporary climate change, heat extremes are becoming more frequent and severe. Heat extremes can cause immediate ecological impacts, known as resistance, but populations and communities can recover beyond the extreme event, when thermal conditions return to normal. However, the underlying mechanisms behind various types of recovery after heat extremes remain largely unexplored, and importantly, loosely linked to mechanisms conferring resistance. This thesis examines the significance of various factors influencing ecological resistance and recovery, mainly with the use of experimental approaches. We tested these ideas using populations and communities of soil-dwelling Collembola, one of the most abundant and functionally important groups of invertebrates in terrestrial ecosystems. In the first chapter, we assessed how thermal effects on life-history traits affected population resistance and recovery after an extreme heat event, using populations of four Collembola species in monocultures. In the second chapter, we explored how various levels of population density at the onset of extreme heat events could determine their subsequent population responses. In the third chapter, we examined how natural soil communities from two distinct elevations respond to extreme heat events occurring at different seasons. In the fourth chapter, we synthesized the main mechanisms buffering the immediate biological impacts of heat extremes as well as their associated costs, and proposed a temporally-explicit conceptual framework describing the links between short- and long-term population or community responses to heat extremes.
Our results demonstrate that various kinds of relationships between resistance and recovery to heat extremes can be explained by the thermal sensitivity of vital rates, which varies among species and across different spatiotemporal contexts. In the first and second chapters, we show that higher thermal sensitivity of fecundity compared to survival can cause negative recovery despite negligible effects on resistance, especially in growing populations of cold-adapted species. Then, in the third chapter, we show that lowland communities are more vulnerable to heat extremes than high elevation communities, mainly in spring and summer seasons. While Collembola responded strongly in these contexts, we found that fungi remained generally stable to heat extremes, with notable exceptions in the case of fungal pathogens and saprotrophs (increased and reduced abundances, respectively). In addition, our findings from the second chapter suggest that density-dependent recovery processes are negligible in small
populations. Yet, in populations closer to carrying capacity, compensatory effects seem to facilitate recovery after heat extremes, as revealed from the responses of the lowland natural communities of the third chapter. This finding suggests that recruitment could counteract previous heat-induced mortality, owing to a relaxation of competitive pressure in shrunk populations. Finally, we propose that several mechanisms that immediately dampen the biological impacts of heat extremes, such as behavioral thermoregulation or the production of heat shock proteins, can have lagged costs known as ‘ecological debts’, that constrain the recovery of populations and communities in the long term. These ecological debts could accumulate as heat extremes become more severe and frequent, emphasizing the importance of the linkages between short- and long-term ecological responses to heat extremes. In conclusion, we show that recovery processes are fundamental to fully capture the whole range of ecological responses, including lagged effects. We demonstrate that ecological responses to heat extremes are strongly influenced by thermal effects on vital rates, as well as by various spatiotemporal contexts. These findings can contribute to enhance our mechanistic understanding of ecological responses to climate change, and accordingly devise policies and management strategies to halt climate-driven biodiversity declines.