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Rivers at risk – links between climate variability and stream ecosystems

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Climate change is an ever-increasing reality, and it is vital for ecologists to understand how species will be affected to help prevent extinction. In collaboration with several colleagues, Dr Steven Thomas from the University of Nebraska is merging multiple theories to combat an important, yet understudied area – where are stream habitats most vulnerable to climate change? In the process, he and his colleagues hope to discover important new information about the biodiversity of animals that live in streams across the world.
As climate change looms, it is essential to identify areas that are most at threat. Stream ecosystems are incredibly important in the environment – a direct highway for water, sediments, and nutrients and a dramatic and diverse habitat for animals – but they are relatively understudied. In a large, interdisciplinary project spanning five years, Dr Steven Thomas and his colleagues have been collecting novel information from various tropical and temperate streams across the Americas. The team aim to reveal which are most at threat and predict how certain species will react to rapid changes in the environment.
One step ahead
By predicting how climate change will impact biodiversity, conservationists can take the most appropriate action to minimise damage. In some cases, environmental shifts will happen regardless of human changes, and by being one step ahead of the game we can protect environments or animals that are most at risk. By artificially accelerating some of the effects of climate change on a small scale, Dr Thomas and his fellow scientists can provide a window to the future and observe how the animals in tropical and temperate streams respond to changes in their thermal environment, or fail to do so. This tests previous theories of adaptation in temperate and tropical climates and provides insight into the relative vulnerability of species in these areas of the world.
Volcán Cayambe
So how do we know which habitats are most at risk? Dr Thomas’ project builds on early work by Daniel Janzen. His idea, known as the Climate Variability Hypothesis, proposed that organisms inhabiting thermally stable environments (e.g., the tropics) will struggle if there is a dramatic climatic shift because they have become thermal specialists. For example, insect species living in a tropical stream rarely experience temperature shifts outside differences between day and night. In contrast, temperate species must also deal with seasonal shifts in temperature requiring species to have broad thermal tolerance.

The team’s work will provide a window to the future and observe how the animals in tropical and temperate streams adapt, or fail to do soQuote_brain

Dr Alex Flecker and Dr Steve Thomas discussing sampling protocols at one of their Ecuadorian research sites.
However, Janzen’s theory went a little further than this. Because higher altitudes are also colder, thermal specialisation in the tropics creates barriers to dispersal resulting in a highly stratified distribution of species along tropical mountains. By extension, broad thermal tolerance in temperate species should also result in broad altitudinal ranges and less stratified communities in temperate mountains. This theory therefore suggests that animals in the tropics might not be able to tolerate (and survive) even small changes in temperature compared to their temperate counterparts who should be more tolerant to an altered temperature regime. Climate projections predict minor warming in the tropics and a progressive increase in the magnitude of warming leading many to conclude that polar species are much more at risk than their tropical counterparts. But those conclusions do not account for differences in the thermal biology of species.
An integrated study
However, as with most ecology, it is a little more complicated than this. Dr Thomas and his colleague’s work is testing, and building upon, this key idea by drawing upon other important environmental interactions. Currently, most climate change models focus on temperature, an obvious problem. However, there are many biotic and abiotic factors that will also come into play, either as a direct or indirect result of temperature and elevation change.
Disturbance is one main example, especially for freshwater life. Fires, flooding, and drought bring drastic changes to stream ecosystems as they are so dependent upon water flow (how much water runs downstream). Changing flow conditions influence stream nutrients, food availability and species survival. As with temperature, species living in stable versus highly variable flow conditions differ in their resilience to floods and/or droughts. Changing precipitation patterns and the resulting shifts in river flow is another way that climate change will impact streams and rivers by selecting for new organismal traits best suited to the future condition.

These tools will allow the team to forecast how species are likely to be impacted by various changing climate scenariosQuote_brain

Dr Thomas and his colleagues are sampling insects and amphibians in streams spanning more than a 2000-metre altitudinal gradient in Ecuador and Colorado. His group is looking particularly closely at physiological, genetic, and ecological traits of species collected along these gradients. More specifically, they are trying to identify which traits shape an animal’s ability to survive the various threats posed by climate change: a strategy they refer to as ‘An Integrative Traits-Based Approach’.
Streams of data
No doubt this has been a massive undertaking. Dr Thomas and his colleagues have been sampling streams in Ecuador (tropical) and Colorado (temperate) in a study spanning over five years. The project focuses on stream insects and amphibians – two largely understudied groups, particularly in the context of climate change. During this process, the team hopes to grow our knowledge of the biodiversity of these groups by uncovering ‘cryptic’ river species which cannot be identified by observation, instead requiring a closer look at their genetic code.

Dr Andrea Encalada (left) and Dr Juan Gauyasamin (centre) of the University of San Francisco de Quito collect aquatic insects in the Oyacachi river, Ecuador.
They are also rearing these animal groups under scientific conditioning to emulate the possible effects of climate change. Measuring their survival against extreme temperatures and low oxygen levels is particularly important. Dispersal is also cleverly measured using microsatellite markers which ‘track’ the range of these animals from a genetic perspective. This will allow Dr Thomas and the team to better define a species’ geographical range. These measurements will provide insights to the vulnerability of the organisms in each climate, and across altitudinal gradients, thus testing Janzen’s hypotheses.
Vulnerable hot spots
The final piece of the puzzle for Dr Thomas and his colleagues is to run statistical tests and form ecological models with their results. These tools will allow the team to forecast how species are likely to be impacted by various changing climate scenarios (changes in temperature, disturbance events) and potentially identify stream species and locations most at risk.
This ground-breaking study provides a targeted attempt at predicting the effects of climate change on biodiversity, whilst integrating multiple ecological factors. It may also serve to validate Janzen’s hypotheses which can be easily adapted to many more locations globally. Dr Thomas and the team have a huge undertaking ahead of them, but these results could be critical in protecting important stream ecosystems worldwide.
Is this model applicable to other aquatic ecosystems such as estuarine or marine?
Yes it should be. The Climate Variability Hypothesis tested in the EVOTRAC project should apply wherever gradients in temperature and its variation have persisted long enough for resident species to become locally adapted.
Do freshwater streams suffer from biodiversity vulnerability more than other ecosystems?
Yes, I think so. Aquatic species are disproportionately represented on threatened species lists across the globe. Streams and rivers provide essential ecosystem services and as a result, human populations are concentrated along river corridors. This arrangement has resulted in a wide array of impacts on these ecosystems including the direct harvest of species and indirect impacts on species through physical and chemical changes.
Although the importance of the model species within the stream ecosystem is well understood, are they important climate change indicators compared to other keystone species?
This remains to be seen; more research is needed to identify specific aquatic species that act as reliable and sensitive indicators of climate change in specific parts of the world.
A lot of the study work relating to this project has been carried out in Ecuador and Colorado. Would you expect to find similar results from other sites and if not, why?
The sites we’ve worked in all of these countries have minor local impacts. This is by design because we wanted to see how climate drivers impact these systems without the complicating effects of local human impacts such as forest clearing, channel alteration, or extreme fishing pressure. In many systems across the globe, these local activities have already compromised the ecological integrity of our streams and rivers.
Do you envisage that this will be a long-term project extending beyond the planned five years to establish a long-term dataset for more accurate modelling?
We hope so! The funding landscape for science has always been difficult to navigate and that has become increasingly so recently. Having said that, our team will be working hard to find new funds to continue this work because much remains uncertain. We are particularly interested in understanding whether there is differential vulnerability across species that serve specific functional roles relative to others (e.g., herbivores vs. detritivores) and what the implications of that may be for how streams and rivers function. We are also very interested in how the current surge in dam construction in the Andes will impact temperature and flow in the upper Amazon Basin.
Research Objectives
Dr Thomas’ research combines the fields of biology, ecology and hydrology. One of his current research interests is investigating how climate change is likely to affect the biodiversity and functioning of stream and river ecosystems.
National Science Foundation (NSF)
EvoTRAC Lead PI:

  • Dr LeRoy Poff, Colorado State University


  • Dr Alex Flecker and Dr Kelly Zamudio, Cornell University
  • Dr Cameron Ghalambor, Dr Chris Funk and Dr Boris Kondratieff, Colorado State University
  • Dr Andrea Encalada and Dr Juan Guayasamin, University of San Francisco de Quito

Dr Thomas has spent more than two decades conducting research in stream ecosystems. His specific areas of expertise include biogeochemistry, particulate carbon transport, ecosystem metabolism, isotope ecology, food web interactions and ecological-evolutionary interactions. He has been involved in several multi-investigator projects that have included ecosystem scale experimentation in streams.
Professor Steven Thomas
School of Natural Resources
University of Nebraska
Rm 403 Hardin Hall
3310 Holdrege St.
Lincoln, NE 68583-0974
E: [email protected]
T: +1 (402) 613 0043

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