Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - TEMPDEP (Ecological and evolutionary constraints on the temperature dependence of the carbon cycle)

Teaser

Forecasting and mitigating damaging levels of climate change is set to be the defining scientific challenge of our age. Central to this is an understanding of the mechanisms underpinning carbon (C) cycle feedbacks with the climate system. The C-cycle acts as a planetary life...

Summary

Forecasting and mitigating damaging levels of climate change is set to be the defining scientific challenge of our age. Central to this is an understanding of the mechanisms underpinning carbon (C) cycle feedbacks with the climate system. The C-cycle acts as a planetary life support system by maintaining conditions on Earth that are habitable for life, and has emerged through evolutionary feedbacks between organisms and their environment. The cycling of C between the biosphere and the atmosphere is mediated by its transformation and flux through ecosystems. Atmospheric CO2 is fixed by photosynthesis, exchanged between organisms via their interactions (e.g., predation, mutualism), and re-emitted to the atmosphere through the by-products of their metabolism (e.g., respiratory CO2 production). These metabolic processes are exponentially temperature dependent within the range of temperatures an organism typically experiences in their environment. Consequently a substantial body of work has highlighted the potential for elevated rates of respiration (CO2 production) in a warmer world to accelerate global warming. Despite a general consensus that warming is likely to increase CO2 emissions from the biosphere by enhancing rates of respiration, large uncertainties exist as to the magnitude and duration of any response, owing to limited understanding of the mechanisms that underpin the temperature response of metabolic fluxes at the scale of ecosystems.

The overarching objective of this project is to determine the extent to which ecological and evolutionary processes influence the temperature dependence of key ecosystem fluxes in the carbon cycle, specifically, gross primary production (GPP; e.g., the gross CO2 fixation of an ecosystem), and ecosystem respiration (ER; e.g., the total CO2 emission of the ecosystem). To this end we will tackle the following objectives using a carefully selected and designed set of experiments.

1. To determine the role of ecological interactions in shaping temperature dependence of ecosystem metabolism.

2. To quantify the magnitude of metabolic thermal adaptation under both in-situ and laboratory conditions.

3. To determine feedbacks between ecological and evolutionary processes in shaping the temperature dependence of ecosystem metabolism.

Work performed

We have completed 2 main projects thus far.

Project 1. Changes in temperature alter the relationship between biodiversity and ecosystem functioning.

We used high-throughput experiments with microbial communities to investigate how changes in temperature affect the relationship between biodiversity and ecosystem functioning. We found that changes in temperature systematically altered the relationship between biodiversity and ecosystem functioning. As temperatures departed from ambient conditions the exponent of the diversity-functioning relationship increased, meaning that more species were required to maintain ecosystem functioning under thermal stress. This key result was driven by two processes linked to variability in the thermal tolerance curves of taxa. First, more diverse communities had a greater chance of including species with thermal traits that enabled them to maintain productivity as temperatures shifted from ambient conditions. Second, we found a pronounced increase in the contribution of complementarity to the net biodiversity effect at high and low temperatures, indicating that changes in species interactions played a critical role in mediating the impacts of temperature change on the relationship between biodiversity and ecosystem functioning. Our results highlight that if biodiversity loss occurs independently of species’ thermal tolerance traits, then the additional impacts of environmental warming will result in sharp declines in ecosystem function.

Project 2. Scaling the temperature dependence of metabolism from individuals to ecosystems: the role of biotic interactions
It is now widely recognized that massive variation in the temperature dependence of metabolism (i.e. the activation energy, Ea) exists among species. If variation in Ea is prevalent then a simple linear scaling from individuals to ecosystems, assuming the ecosystem rate is the sum over all individuals, is unlikely to hold. We carried out a large scale experiment to understand how the interspecific differences in thermal responses and the nature of the ecological interactions (eg., competition, facilitation, predation) shape the overall temperature response of microbial ecosystems. We isolated 8 bacteria species from a naturally warmed aquatic environment in Iceland and characterized the temperature dependence of their metabolic rates and also the interspecific biological interactions. We then built an artificial microbial communities using the isolated species and further characterized the metabolic thermal response of the entire community. Our results show how it is possible to predict the response of the community to a rise in temperature by understanding the thermal traits and the nature of the interactions between the individuals. We found that facilitation amplifies the temperature dependence of microbial community metabolism.

Final results

We have broken new ground in linking evolutionary ecology with ecosystem ecology. This is a new and fertile area of ecology where the links between ecosystem science, community ecology and evolutionary biology remain grossly understudied. Thus far in this project we have shown how evolved variation in thermal traits shapes ecological dynamics under experimental warming and ultimately determines how community structure shapes ecosystem carbon biomass accumulation under scenarios of biodiversity loss and warming. In our second piece of work we have been exploring how the nature of ecological interactions determines the scaling of the temperature dependence of metabolic rate from individuals to whole ecosystems. It is often assumed that the temperature dependence of metabolic rate scales directly from individual physiology to carbon fluxes of whole ecosystems. We are finding that the nature of ecological interactions between species (e.g. competition, facilitation) can modulate the scaling of the temperature dependence at the ecosystem level. Our future work for the remainder of the project will explore how coevolution can change the nature of species interactions and thus the scaling of the temperature dependence of metabolic rate from organisms to ecosystems.