Is there an effect of nearby land on stream communities?

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The OTS biology graduate course is like the “Project Runway” of ecology. We get introduced to a new place, walk around for a day, and then we have 3 days to come up with a question, design an experiment, and present our data and conclusions. Below is what my project was at Las Cruces Biological Station.

More and more land is being converted into reserves or protected areas around the world. But where is this newly protected land coming from? A majority of the land that is being converted was formerly used by people for a variety of purposes, like farming or pasture. Now, intuition tells us this land has to have been impacted by years of hard use. Let’s focus on pasture land, for example. Cows have a large effect on the land where they graze. They compact the soil with their hooves, eat much of the grass, and these two impacts together can cause the land to lose a lot of its nutrient rich topsoil. Topsoil is the thin layer of soil that covers the land. The topsoil is typically very nutrient rich, which is why most plants have their roots in this soil.

But as I mentioned, grazing by cattle can cause this topsoil to be lost. Where does it go? Well, typically the topsoil is swept into rivers by rainwater. When the topsoil hits the streams, it is a huge energy boost for the system. Usually healthy streams don’t have a lot of nutrients, which is why you are able to see clear to the bottom. However, when there is an overload of nutrients, the algae take advantage of all that food and go nuts! But sooner or later, the algae will die and begin to breakdown. As the algae decomposes, it uses up a lot of the oxygen within the water. That means there’s less oxygen for the organisms, large and small, that call the stream home.

So, who cares if some tiny stream organisms die? Actually, lots of animals care. Some of these tiny organisms that are sensitive to changes in the oxygen level play a very important role in the stream ecosystem. They help break down nutrients, like falling leaves, into smaller pieces so other living things can use those nutrients to live. Without these oxygen sensitive organisms, the entire nutrient cycle within streams would be disrupted, which would mean no clean drinking water for both us and other animals. These small, oxygen sensitive organisms fall into the category of “macroinvertebrate.” A macroinvertebrate is anything without a backbone (“invertebrate”) that can be seen with the naked eye (“macro”). Usually this means a crustacean, like a crab, or insect.

Macroinvertebrates are a very diverse and abundant group, both in water and on land. What I wanted to do, as I am an a water ecologist, is look at whether the macroinvertebrate community changes when looking at streams within different land types such as protected forest vs. pasture that has become protected forest. To measure where there was a difference, I had to compare macroinvertebrate communities in streams in both forest and former pasture. Las Cruces Biological Station in southern Costa Rica is a great place to do this study because there’s land that has always been forested and land that was pasture less than 10 years ago. I thought I would see a less diverse macroinvertebrate community at streams in former pasture because the organisms that are sensitive to less oxygen would be gone, but I had to prove it.

Taking measurements of the streams was the fun part! I walked to my sites in rubber boots for protection against potential venomous snakes on the trail. Once I found the stream I wanted to sample, I sometimes had to fight through plants to find a good stretch of stream to measure. I recorded the physical traits of the stream, like width and flow speed, but I also sampled the macroinvertebrate community. I did this using the “kick method”, which is where you put a mesh net downstream of where you are standing and basically kick up sediment and rocks for a set amount of time. As you kick, the macroinvertebrates are dislodged and you catch them in your net. Then you dump them into a container and identify and count everything. Some of the macroinvertebrates were really cool and moved in surprising ways. If you want to see my methods first hand, and see some awesome macroinvertebrates, check out the video!

After two days of intensive sampling, I was ready to look at my data and see if there was a difference between macroinvertebrates in streams in former pasture vs. forests. Unfortunately, based on my analysis, I didn’t see an effect of land use on the macroinvertebrate community. This could mean I was wrong: streams in former pasture areas don’t have a less diverse macroinvertebrate community than streams in forested areas. This result is sort of boring though…

Or I could have found something really exciting! Perhaps the reason I didn’t see an impact of land type on the macroinvertebrate community is because the stream has already righted itself to pre-pasture conditions. Maybe once land surrounding streams is conserved, the stream community can return, in less than 10 years, to a healthy ecosystem without lots of nutrients and lowered oxygen levels. If it were true that streams could recover in less than 10 years, it could have big impacts on the idea of conservation. It would mean when you protect land that is currently pasture, it doesn’t take long for streams to be healthy and providing quality living space for macroinvertebrates again. And although this idea sounds great, because this is science, it doesn’t mean much until you have data to support your ideas. Back to the forest…

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