Animals and plants are “geomorphic agents”, shaping the landscape around them through their daily activities of feeding, building homes, reproducing, and seeking safety. A recent article in Reviews of Geophysics focuses on the burrowing activities of various invasive species found in aquatic environments, examining how they modify the landscape and increase the risk of erosion. Here, one of the authors gives an overview of how the presence of different species can change geomorphic and hydrological processes, and suggests where additional research is needed to better understand their impact.
How do plants and animals influence the landscape?
Wherever plants and animals exist on Earth, they influence natural processes and modify the environment around them. This happens on a variety of scales from the movement of individual grains of sediment – for example, as fish forage for food on a riverbed – to the transformation of landscapes – for example, as beavers fell trees to build dams and create ponds to live in.
The actions of different species in different types of environment can be ‘positive’ – in that they create or protect landforms, encourage the restoration of degraded environments, or increase biodiversity – or ‘negative’ – in that they disturb the landscape, break down landforms, destroy habitats, and reduce biodiversity.
Why are invasive species a particular concern?
The geomorphic activities of plants and animals in their native environments tend to be part of a well-balanced natural system. However, the introduction of non-native species can be quite disruptive to the natural landscape, and sometimes also causes damage to the economy and to human health.
Our review focuses on invasive species that make burrows in aquatic environments. Non-native species are introduced to new locations mainly through human activities; in the case of aquatic environments through commercial shipping, the aquarium and exotic pet trade, the fur trade, and aquaculture.
Many creatures excavate burrows to create space for reproduction or refuge. In aquatic environments – such as rivers, lakes, estuaries and saltmarshes, as well as artificial drainage channels and flood defense structures – burrowing activities can cause erosion, increase the risk of flood, and lead to habitat loss.
Which invasive species are of particular concern in different parts of the world?
Aquatic burrowing invaders include crustaceans, fishes, reptiles, and mammals. Our review looked at the distribution of 10 different species and found that over 120 countries, states and territories in the world have at least one of these invasive non-native populations.
As part our review, we searched multiple online invasive species databases.
The most globally widespread are the coypu (Myocastor coypus) and red swamp crayfish (Procambarus clarkii), which have established invasive populations in Africa, Asia, Europe and North America.
Other species, such as the isopod (Spharoma quoianum) are currently more geographically constrained in the United States of America but have the potential to spread.
Smaller animals excavate smaller burrows, but often occur in larger numbers and may also dig more burrows. This means that the impacts from smaller and less conspicuous animals such as aquatic invertebrates may rival those of larger mammals.
How does burrowing modify geomorphic and hydrological processes in aquatic environments?
The impacts of burrowing occur at different time and spatial scales. Consider, for example, a burrow in a muddy riverbank.
The excavation of an individual burrow will generate a relatively small input of sediment to the water body over a short time period. But multiple burrows across a larger area have the potential to generate more substantial changes to landforms and erosion rates over longer periods of time.
The creation, daily use, expansion, or even abandonment of burrows will alter the internal structure of the bank, likely weakening it, change the way water moves through the bank, and modify the flow of water around burrow entrances.
Additionally, the presence of a burrow and its occupant can modify the chemistry of the surrounding water and sediment which, in turn, can influence susceptibility to erosion.
Do we know how much damage burrowing species cause?
There are increasing reports of damage to aquatic environments, artificial drainage networks, flood defense infrastructure, and historic waterside landmarks, but there is a lack of research directly quantifying the impacts on instability and erosion. In part, this is due to the challenges of conducting such research. Erosion processes are highly variable over short distances and are episodic in nature so they are notoriously difficult to accurately quantify through field research.
What are some of the unresolved questions where additional research, data or modeling is needed?
It would be helpful if there were a model that could conceptualize the various geophysical effects of burrowing in an integrated way.
Our work brings together established models from soil science and fluid mechanics to hypothesize the range of effects that may be expected based on existing understanding of erosion processes in different environments. This provides a framework for future research.
Further research is needed to test the hypotheses set out in our conceptual model. In particular, we need a better understanding of how the size, shape and density of burrows created by different species influences the geotechnical, hydrological, and hydraulic processes that drive erosion. We need to understand how the impacts might vary for different sediment types and different types of aquatic environment.
Answering these questions will require a combination of computational modelling, laboratory experimentation and field research.