What we can learn from mesocosms – and how to create your own little world

How can simulating little ecosystems be useful for assessing chemical risk? Read on to find out and make your own ecosystem in a jar.

Have you ever happened upon a spot out in the environment that seems completely unconnected to anything else in the world? Little patches of moss and lichen like tiny fairy cities, or a rock pool at low tide - systems with their own simple inputs and outputs that seem almost entirely independent of the complex processes of environments more familiar to us.

A mesocosm is a more scientifically controlled version of this. It is a simulated outdoor environment that is designed by scientists to determine the effects that different manipulations have on an ecosystem. The people doing the experiment can control what enters the mesocosm environment so that if they see an effect, they know what caused it.

At the EPA, scientists review research and evidence relating to the impact that activities, organisms, or chemicals might have on the environment. Mesocosm studies can sometimes provide evidence to help inform the rules and controls placed on substances to minimise any negative impact.

The word mesocosm comes from the Greek for medium (meso) and world (cosm). These medium worlds consist of living components, such as plants, animals and bacteria, and non-living components, like water, rocks and sand.

For a mesocosm to be comparable to real life, the components should work together in a way that simulates a real, self-sustaining ecosystem. This means the mesocosm should also ensure that energy and nutrients are available and waste by-products of biological activity can be recycled instead of building up. Once the system has proven to be stable, scientists can add things such as a chemical they are investigating or greenhouse gases, for example, and observe the effects this has on the mesocosm.

A photo of the ExStream mesocosm set-up, showing a series of white containers on a desk outside on grass, each of which is linked up to pipes. This image shows water filtering out the bottom of the white containers as it leaves the stream mesocosm

A series of mesocosms set up to simulate a stream environment. Water flows from a stream into the covered containers and out the bottom through a filter. Photo supplied by Sam Macaulay

Ecosystem in a jar

How to make your own mossy mesocosm (also known as a terrarium)

You will need:

A large jar with lid
Small stones/pebbles
Sphagnum moss, or foraged moss from the garden
Activated charcoal from your local garden store
Soil or potting mix (read the label – do not open bag inside or near face)
Small succulents or ferns

An illustration of an ecosystem in a jar, showing the layers of rocks, charcoal. moss, dirt and plants.

Instructions:

Add stones and pebbles to the bottom of the jar for drainage. Fill about a tenth of the jar with the pebble layer.
Add activated charcoal – this will help to control bacteria so they don’t have a party when you close the lid.
Add a layer of moss.
Add a thick layer of potting mix or soil, about ¼ of the jar. Be sure to open the bag outside away from your face. Read the label of the bag, and check out our guidance on gardening products below.
Add your plants. You can forage plants that like humidity from the garden such as ferns, or purchase succulents or specialised terrarium plants.
Optional: decorate with more stones, shells or other ornamental finishings. The jar shouldn’t be filled up past the halfway mark, as the mesocosm needs room for an atmosphere.
Add some water. The amount of water required for your system to be self-sustaining is specific to your mesocosm. Try adding enough to soak into everything, and then observe it over the next few days while it is establishing. If the soil seems dry you can add more water. You’ll know it is self-sustaining when you can close the lid and it doesn’t dry out over time.

If lots of condensation forms under the lid, there might have too much water in the jar. You can open the lid to let some evaporate for a day if this is the case. Scientists usually leave their mesocosms to establish for about three weeks before performing any experiments. Your mesocosm is not set up for experiments, so please don’t add any new variables once it has established.

Let's make a terrarium - Mermaid’s Purse website

Making mesocosms - BioNinja Ecology website

Information on gardening safely from our Safer Homes programme

Using mesocosms to assess chemical risk

When new chemicals are assessed to determine the risk they might pose to the environment, there are a number of tools ecotoxicologists use to ensure ecosystems are protected.

Not every new chemical will require examination in a mesocosm, because there might be other information available from lab experiments, which makes scientists confident that organisms won’t be harmed if exposed to reasonable amounts of the chemical (for example, information on the impact of the chemical on single species). But when more information is required, ecotoxicologists might design mesocosm experiments to supplement existing information about the chemical. When it exists, the EPA can request extra information like this to support decision-makers who are considering whether to approve a substance for a specific use (or uses).

An example experiment might simulate a freshwater environment with multiple algae, invertebrate, and plant species present, and determine if any toxic effects are observed when the chemical is introduced, and how much of the chemical needs to be present to observe the effect.

There is more information on the assessment (and reassessment) of hazardous substances on our website, including:

Hazardous substances and the EPA

Approval to import or manufacture a new substance for release

The reassessment process for hazardous substances

Multiple stressors

Mesocosms are often set up to examine the impact of just one change that is introduced at a time. In reality though, ecosystems are dealing with the input of many different things at once. They might be dealing with rising temperatures, nutrient enrichment, toxic pollutants and drought all at once, and the combined effect of all these different things could be more than the individual effects added together. Ecosystems in this situation are described as facing “multiple stressors” – the effects of which can be unpredictable.

Mesocosm is one of the tools that scientists can use to understand and untangle the impacts of multiple stressors on an ecosystem. A study from the University of Otago in 2012 used stream mesocosms to investigate the effect that three different stressors commonly associated with agricultural land use might have on ecosystem function.

After letting the mesocosms establish for 21 days, nitrate and phosphate chemicals were added to the systems to simulate nutrient runoff. At the same time, the temperature of the water was increased, and sediment was added to simulate the removal of riparian vegetation.

There were multiple channels of stream mesocosms with different combinations of stressors applied so the researchers could compare them. They found that the stressors did appear to work together to have a negative impact on the ecosystem they had set up – an impact greater than the sum of their separate effects.

This allowed them to isolate the mechanisms that are reducing water quality in a more realistic situation than a lab, emphasising where changes will be effective. They recommended having buffer or riparian strips along waterways to keep them cool, and reducing their exposure to excess nutrients and sediment.

A photo of the ExStream mesocosm set-up, showing a series of white containers on a desk outside on grass, each of which is linked up to pipes. This photo shows the white containers from above covered by hairnet-like hoods with black pipes entering them

ExStream system

Photos taken by Sam Macaulay during his ExStream System experiment for his PhD in Oct-Dec 2017.

IRL (In Real Life)

While it isn’t practical nor even possible to test the effect of all the different impacts that chemicals might have on ecosystems in combination with other stressors, it is helpful for us to acknowledge and think about what we might not know – indeed this is what fuels science along the path to innovations and discoveries. This can lead to regulators and scientists taking more caution and gathering more data. It can also influence our behaviour, so that we only use what we need and pay careful attention to how we use and dispose of hazardous substances.

Monitoring different properties of an ecosystem over time is a good way to stay in touch with it and detect any changes that might occur in response to things going on in the world around us. There are many ways that communities, iwi/hapū, schools, farmers and other land users can monitor their local waterways in Aotearoa New Zealand through citizen science and catchment group initiatives, including our eDNA programme, Wai Tuwhera o te Taiao.

Part of the beauty of the ecosystems is the intricate and delicate links between different processes, species and components. The dances between these variables are endlessly complicated and wild, and while scientists have developed some pretty great ways to observe and replicate them, there are more moves to learn.

This is why there are rules to follow when we use hazardous substances. When the rules are set, extra cautiousness is built in along the way to ensure the environment will be protected where there’s uncertainty, but they are set assuming the substances will be used appropriately.

Always follow the label instructions. If you’re not sure it’s safe for you or our environment think about alternatives or seek more information.