In New Zealand, many would agree that fresh water is one of our most loved natural resources. We drink it, we swim in it, we use it to farm and to make a living, we even use it to generate our power! Unfortunately, especially in Canterbury after some major earthquakes, many of our streams and rivers are struggling. They look something like this:
In stream restoration, we want to return the features of a stream back to their original state, before things like urban development or introduced species affected the quality. This includes adding native plants, allowing fish to make their way out to sea or further upstream, and making sure farm animals can’t walk straight into the stream. All of these things and more can help us to make healthier waterways.
It does not always go to plan, with some hardy introduced species putting a spanner in the works and refusing to co-operate with careful scientific methods. Imagine a beautiful stream that’s been through tough times—pollution, habitat destruction, earthquakes you name it. People step in – scientists, council members, developers, maybe the general public, and they work hard to restore it, but here’s the kicker: sometimes, things just don’t bounce back like they should. Why? That’s exactly what a recent study by Issie Barrett and her team set out to uncover.
To understand why streams struggle to recover even after the most thorough restoration efforts, we need to understand a few key factors.
1. Species Interactions: In a healthy stream, different plants and animals interact in specific ways, such as some animals eating others or different plants competing for space. When a stream is damaged and then restored, these interactions might not work the same way anymore. This can make it harder for the original species to come back and thrive.
A particular species of snail, the New Zealand mud snail (P. antipodarum) is particularly good at living in these degraded streams, they thrive under pressure and limited food sources. These snails are perfect species to take over a degraded environment and reduce the recovery ability! So even when original species are introduced, such as the mayfly, the same food source now has double the competition, meaning a negative reaction – that habitat can’t provide that much food even in a restored state.
2. Negative Resistance: This is a big concept, which in essence means that even when the physical conditions of a stream improve (like cleaning up pollution or adding new habitats), the plants and animals in the stream don’t always come back as quickly or fully as hoped.
During the stream’s degradation years, new species like the mud snails might move in – kind of like uninvited guests crashing a party. Even after things are cleaned up, these newbies can stick around and hog resources, making it harder for the original gang to make a comeback. This is what they call “negative resistance.” This can happen because the habitat is too degraded for the ideal species to thrive even if they did before.
3. Resilience Mechanisms: This means the ability of a system to absorb and adapt to change, ultimately returning to the restored ideal. This is where our negative resistance comes into play. If the species or the system is already not functioning as it should, we are going to have a hard time creating a resilient system that can adapt to a changing environment and overcome any future issues.
For example, a high level of nitrogen could change the make-up of the riverbed so drastically that a species sensitive to nitrates may never repopulate that system. Understanding the relationship between negative resistance and resilience is important for predicting and enhancing any successful restoration efforts.
What can we do?
Look at the Big Picture: When restoring a stream, it’s not just about fixing what we can see. We need to think about how all the different plants and animals interact with each other. This includes what nutrients are in the water and what microscopic invertebrates might be living in that water.
Keep Checking In: It’s important to keep watching restored streams over time to make sure they’re getting better and to fix any problems that come up. If we don’t see an improvement in 5 or 10 years, there must be something else we can do.
Be Flexible: Sometimes, we might need to change our restoration plans based on what we learn from watching how the stream responds. As scientists we have to be okay with admitting our first idea didn’t work, and then be willing to help come up with a better solution for the future.
Why it matters
Overall, there are some pretty complex systems that are at play in stream restoration projects. It is not as simple as putting in some better plants and some bigger, cooler rocks and hoping it will all work out in 10 years. By paying attention to how plants, animals, and the environment all work together, perhaps we can work towards a deeper understanding of the best ways to help our New Zealand streams thrive for many more generations to come.
I think it would be pretty cool to keep swimming in our rivers and looking for fish in the summer, but next time you go to your local river, have a look and see what plants and other animals would really love to keep living there too.
This article was prepared by Postgraduate Diploma in Environmental Management student Tayla Cross as part of the ECOL608 Research Methods in Ecology course.