Patterns and drivers of biodiversity along a tropical elevation gradient: the interplay of energy and thermal limits

Climate change and biodiversity loss are among the most pressing and escalating global threats, underscoring the need to better understand the factors driving biodiversity. The most widely supported hypotheses explaining biodiversity patterns include energy availability and temperature as primary predictors. The steep elevational gradients of tropical mountains provide an ideal natural laboratory to investigate how organisms respond to climatic variation on relatively small spatial scales. Temperature increases metabolic, evolutionary, and ecological rates and is therefore hypothesized to positively influence biodiversity, but the underlying mechanism remains unclear. Depending on the thermoregulatory strategy, species might be limited by different variables. Particularly ectothermic groups are expected to be constrained by their upper and lower physiological thermal limits. In this dissertation, I examine the patterns and drivers of mammal and dung beetle diversity along a forested elevational gradient extending from the Amazonian lowlands to the high Andes. I also explore the underlying mechanisms by which temperature constrains insect diversity at its upper limits, focusing on thermal limits and protein stability. In chapter II, I show that mammal diversity decreased exponentially towards higher elevations, a pattern primarily driven by energy, i.e. net primary productivity (NPP), rather than by mean annual temperature (MAT). Since energy is difficult to measure within an ecosystem, most studies rely on remote-sensing proxies for NPP. I compared these proxies with detailed field-measurements of NPP and showed that these values accurately predicted mammal diversity, while satellite products did not correlate highly. In chapter III, I explored the mechanism of temperature shaping biodiversity by comparing the thermal limits across six major insect orders from elevational gradients in the neotropics and in the afrotropics. By predicting protein melting points and analyzing the genomic signal of heat tolerance, we suggested that thermal tolerance is a deeply conserved trait and based on the fundamental architecture of proteins. Furthermore, Amazonian lowland insect communities emerged as the most vulnerable group to climate change, with some current surface temperatures partly high enough to cause a coma from heat in less than one minute. In Chapter IV, I applied the previously established knowledge to dung beetles, an ideal model group that can be captured systematically along the elevational gradient with a well-defined energy resource. Dung beetle diversity increased exponentially from high to low elevations but exhibited a notable decline at the lowest elevation site. This overall diversity pattern was not primarily driven by energy availability, measured as mammalian biomass, but rather by mean annual temperature (MAT), with soil moisture playing a secondary role. In the lowlands, dung beetles’ thermal limits were closest to ambient temperatures, and surface temperatures even exceeded these limits, providing a likely mechanistic explanation for the observed decline in diversity at the lowest elevations. In chapter V, I explored the impact of a week-long cold wave on the Amazonian ecosystem. During this period of unusually low temperatures, reaching down to 10.5 °C, the activity of mammals declined, as did the biomass of flying and ground-dwelling insects. Most groups, however, recovered within a few months, except for dung beetles, which appeared to be more cold-sensitive. Insects tolerated even lower temperatures than experienced during this cold wave, but for a quarter of the insects, the thermal safety margin appears very narrow. As climate change progresses, increased temperature variability, including cold extremes, is expected to pose significant challenges to tropical animal communities, not only from heat stress but also from cold events. In this dissertation, I highlight temperature as a key driver of biodiversity along a tropical elevational gradient, while also demonstrating how high temperatures can constrain biodiversity and how future climates are likely to challenge tropical ecosystems. Diversity and abundance of endotherms along the gradient were primarily influenced by energy availability, which is itself regulated by temperature, whereas diversity gradients of ectotherms were directly driven by temperature. Insect thermal limits appeared to be phylogenetically conserved and lowland communities are exposed to temperatures which partly exceed thermal tolerance limits of many species. While current cold extremes in the Amazon do not necessarily cause lethal effects, the increasing intensity of climatic extremes, both hot and cold, will likely demand broader thermal tolerance ranges. To protect and sustain the Earth’s most biodiverse region, it is essential to maintain a well-connected, intact, and undisturbed forest ecosystem that supports high productivity and structural complexity, offering sufficient shade and microclimatic refuges for animals to escape thermal stress.