Cumulative impacts of multiple stressors: improving temporal and biological realism

Project Details

Key Questions

How do staggered anthropogenic stressors alter aquatic food webs?


Understanding and managing the impacts of environmental change is one of society’s greatest challenges in the 21st Century. Human population growth and economic development are combining with long-term anthropogenic impacts to impose increasing pressure on the planet’s ecosystems, often through multiple stressor events. Freshwater ecosystems are at risk, with average population declines in vertebrates since 1970 estimated to be >80% (WWF Living Planet Report 2018). The stressors that cause these changes almost never occur in isolation, yet our knowledge of their combined effects remains limited to simple unrealistic scenarios which lack temporal and biological realism. Non-additive stressor combinations and food web interactions can generate ‘ecological surprises (Jackson et al 2016, Global Change Biology) yet recent research has shown these unexpected effects are actually widespread. For instance, stressors can mitigate or amplify the effects of others and, therefore, the combined effect is not the sum of the parts. However, many studies that come to this conclusion use simple additive models and unrealistic experimental scenarios that do not reflect the complexity of the real world. Here you will use a combination of experiments and field work to quantify how the sequence of heatwaves and pollution events alters their combined impacts on biodiversity and food webs. 

Aims of the Project

To advance our understanding of how multiple stressor interactions shape freshwater ecosystems by improving both temporal and biological realism 

Project Description

The overarching objective of this project is to advance understanding of how multiple stressors interact and shape biological systems by going beyond the status quo of ignoring species interactions and using unrealistic stressor scenarios. Using a unique set up of >200 mesocosms (as well as field work at one of our labs regular field sites: see more details here:, you will establish how previous exposure to heatwaves and pollution events alters responses to further stressor scenarios. We hypothesise that previous exposure history is an important driver in how populations and communities respond to multiple stressor scenarios. For instance, Brans et al. (2017, Global Change Biology) found that Daphnia from urban environments coped better with elevated temperatures by maturing at smaller sizes and developing higher levels of haemoglobin. This will alter how we predict multiple stressor impacts, even if the past stress has disappeared.  


You will work alongside a team of 2 postdocs in the Jackson Lab working on the same project. The focus of the PhD student will be on food webs. Characterising food web structure is an ideal method to make more general comparisons across different systems and regions. Important here is our explicit consideration of how the properties of food webs will affect ecosystem processes (e.g. primary production) and the goods and service we gain from the natural world. However, much of the research in global change ecology to date has focused on individuals or (but rarely and) populations (i.e. the nodes in a food web), ignoring interactions (i.e. the links in a food web). This is a huge limitation because when a stressor, such as a heatwave, affects one node, its effects can cascade to all of the other species which that node interactions with in the food web. As such, the effects of stressors will be transmitted both directly and indirectly in potentially non-intuitive ways through the food web, and keystone species can potentially buffer or amplify these impacts. You will use a whole food web approach to quantify how stressors ripple through natural communities to alter important ecosystem processes, including decomposition, primary production and whole ecosystem respiration. 

Methods to be used

Mesocosm experiments, field work, stable isotope analysis, gut content analysis, eDNA techniques

Specialised skills required

Insect identification 

Use of R 

Food web analysis 

Molecular laboratory skills 

Please contact Michelle Jackson on  if you are interested in this project