Disentangling Ecosystem Functions: Our Imagination is the Limit

Post supplied by Tomas Roslin and Eleanor Slade (SPATIAL FOODWEB ECOLOGY GROUP, UNIVERSITY OF OXFORD & LANCASTER UNIVERSITY)

Studies of Action

Studies of ecosystem function are studies of action: of insects pollinating flowers, of predators killing pests – and in our case (well, more often than not) of beetles disposing of dung. To isolate the effects of the critters that we think will matter, we need to selectively include or exclude them. If we think a particular species or species group is responsible for a certain function, then we test this by keeping it in or out of enclosures. If we want to look at effects of species diversity, then we create communities of different species richness.

Research on dung beetles is far from boring. © Kari Heliövaara.

Research on dung beetles is far from boring. © Kari Heliövaara.

Depending on the target organism, this is sometimes easy and sometimes difficult. But it almost invariably proves to be fun! We enjoy the challenge of inventing new techniques for unravelling ecosystem functions sustained by insects. Working on dung beetles – as we tend to do – can be messy, but it’s definitely never boring.

In targeting ecosystem functions, the real trick is to make the experiments relevant. What we want to understand are the effects of changes occurring in the real world. All too often studies of ecosystem functions have been focused on artificial species pools in artificial settings. To see how we have solved this, we’ll give you a quick look at our dungy portfolio of approaches to date.

Diversity of Species Vs Diversity of Functional Groups

One intriguing question regarding ecosystem functioning is whether the diversity of species or of functional groups matter more. Among dung beetles, functional groups do different things: some just dwell in the dung pat, munching away on their home. Others try to secure their part of the cake by dragging parts of it into tunnels deep under the dropping. A third group goes even further, by rolling dung balls further away and burying them where they hope no one can find them. Finally, there are also parasitic dung beetles, which steal the balls of dung after they have been buried. And within each group, the species come in several different sizes.

The question is: which dung beetle faction matters the most? To answer this, we made the first experimental field manipulations to test the effects of functional diversity of a terrestrial animal group. We conducted this experiment in the rainforest of Danum Valley, Sabah, Borneo. To stop the rollers from doing their job, we surrounded a dung pat with a circle of mesh of different sizes. To target tunnelling beetles, we placed meshes underneath the dung pats instead. This worked amazingly well, and showed that functional group identity and species composition is important for ecosystem functioning.

Simple field experiments used to manipulate the subset of dung beetles able to access and process dung. © Eleanor Slade.

Simple field experiments used to manipulate the subset of dung beetles able to access and process dung. © Eleanor Slade.

National Ecosystem Services

On a larger scale, how might we disentangle the contributors to the nationwide service of getting rid of cattle dung, when we also want to know how it varies with climate and habitat changes? At the scale of a nation 1200km long, we could hardly do it ourselves. Setting up experiments at this scale would take more time and more money than we have ever had.

After a little thought we realized: it does not take a PhD to put out chicken wire! Pretty much every country is full of people who are happy to contribute to experiments if you only tell them how and why. With this new realization, we instructed youngsters across Finland to set up some simple exclusion devices to record the contribution of different invertebrate groups. The equipment was sent in the mail to close to one hundred young farmers and as a result, we managed to identify the contributions to an ecosystem service nationwide.

The scale and players of the ecosystem service of dung removal, as identified by volunteers implementing a joint experiment. The left-hand cube shows the total volume of cattle dung produced in Finland per year (ca 4 billion kg of dung; Statue of Liberty superimposed for comparison). Focusing on the dung deposited in pastures, we estimated the relative fractions (middle cubes) disappearing due to evaporation of water and microbial activity alone (top), decomposing through the action of invertebrates (centre), and remaining after two months (bottom), respectively. The lighter box refers to the situation in northernmost Finland, the darker cube to southernmost Finland. Of the dung decomposed by invertebrates, dor beetles account for 61%, (right-hand sub-cube, centre) ) earthworms for 28% (bottom) and dung dwelling Aphodius-species for 11% (top), respectively. (From: Kaartinen et al. Ecology 94: 2645–2652; © Riikka Kaartinen.)

The scale and players of the ecosystem service of dung removal, as identified by volunteers implementing a joint experiment. The left-hand cube shows the total volume of cattle dung produced in Finland per year (ca 4 billion kg of dung; Statue of Liberty superimposed for comparison). Focusing on the dung deposited in pastures, we estimated the relative fractions (middle cubes) disappearing due to evaporation of water and microbial activity alone (top), decomposing through the action of invertebrates (centre), and remaining after two months (bottom), respectively. The lighter box refers to the situation in northernmost Finland, the darker cube to southernmost Finland. Of the dung decomposed by invertebrates, dor beetles account for 61%, (right-hand sub-cube, centre) ) earthworms for 28% (bottom) and dung dwelling Aphodius-species for 11% (top), respectively. (From: Kaartinen et al. Ecology 94: 2645–2652; © Riikka Kaartinen.)

A mesocosm experiment exploring the effects of habitat fragmentation on decomposition rates. Here, each covered bucket sunk into the ground holds a dung pat and an artificially-assembled dung beetle community representing either the species composition of a fragmented or intact landscape. © Helena Rosenlew.

A mesocosm experiment exploring the effects of habitat fragmentation on decomposition rates. © Helena Rosenlew.

Following on from this, we wanted to know how habitat fragmentation affects ecosystem services delivered by dung beetles. This is hard to address straight away as, again, it calls for work at a massive scale. However, what we can do is to examine how species composition differs between landscapes at different levels of fragmentation, then create the same differences in species composition in small mesocosms.

Dung Beetles and Greenhouse Gases

Bess Hardwick measuring gas fluxes from cow pats. © Eleanor Slade.

Bess Hardwick measuring gas fluxes from cow pats. © Eleanor Slade.

Having shown how important dung beetles are for dung removal in both tropical rainforests and temperate pastures we then wondered, might dung beetles perform other services, too, like reducing the fluxes of greenhouse gases (GHGs) from cow pats. How could we study this though? Well, with some help from gas flux expert Terhi Riutta (University of Oxford), we enclosed dung pats within collars, corked them up with air tight lids and measured the accumulation of gases. Then voilá, we can isolate the effects of dung beetles on these GHGs. It turns out that they do indeed affect fluxes of important GHGs, such as methane, by the simple mechanisms of ventilating the pat interior with their tunnels. This means that the microbes that produce the methane within the pat can no longer survive.

Larger Enclosures for Insects

A MESOCOLSURE in the rain forest of the Southern hemisphere. © Gustavo “Tata” Schiffler.

A MESOCOLSURE in the rain forest of the Southern hemisphere. © Gustavo “Tata” Schiffler.

What unites all of the approaches so far is that they build on small exclosures or enclosures. However, we may then rightfully worry whether locking our species and functions up in such small chambers may affect their environment and behavior. Pondering this dilemma over a good Brazilian beer, Júlio Louzada and Jos Barlow then had a great idea: If we were working on large mammals, then we would simply fence out larger enclosures or exclosures, leaving more space for them to roam. So, why should this not work for insects, too? In fact, most of them seem to fly in rather simple trajectories and when faced with a barrier they usually do not understand to move straight up, then down, to circumvent it.

Plastic trays used to record beetles rattling into the MESOCLOSURE walls. © Gustavo “Tata” Schiffler.

Plastic trays used to record beetles rattling into the MESOCLOSURE walls. © Gustavo “Tata” Schiffler.

So, to try out their brave new idea, they teamed up with Gustavo “Tata” Schiffler, a researcher working at Unicamp, Sao Paulo, Brazil. Out into the rain forest they went to build large, open enclosures. To record the densities of beetles outside and inside of the enclosures, they placed water-filler trays, using the cage walls as flight intercept traps.

Good ideas tend to spread across the globe. This one travelled South to North, as we next decided to try them in the Northern hemisphere. And work they did, both in the grasslands of the temperate zone and the rainforest of the southern hemisphere! Beetles were both excluded from entering the cages and stayed inside them when put there. Thus serving as mixes of mesocosms, enclosures and exclosures, we later called our constructs MESOCLOSURES. Using these MESOCLOSURES we were able to examine ecosystem functions performed by particular beetle groups differing in size and attitude. Hence, we may not need to incarcerate our species and functions in as small boxes as we thought. There are other ways, if only we think outside of the box.

Building MESOCLOSURES in a Finnish pasture. ©:Saija Lähteenmäki.

Building MESOCLOSURES in a Finnish pasture. © Saija Lähteenmäki.

So studies of ecosystem function are (almost) always fun, and (definitely) never boring. To us, the most enjoyable part is dreaming up the crazy solutions to answer questions of both theoretical and practical importance. In designing ecosystem function experiments, our imagination is really the only limit. Let your brains run free, draw on anything you know about the behavior and habits of your target species and let science be as pioneering, inventive, and creative as it is supposed to be!

Follow the Spatial Food Webs Group and Eleanor Slade on Twitter: @foodwebs, @eleslade

To find out more about MESOCLOSURES, read the full article HERE.

This manuscript is being highlighted as part of our promotion of Southern Hemisphere authors coinciding with the 8th Southern Connection Congress in Chile (18-23 January 2016). Follow us on Twitter, Facebook and Google+ to see other great articles that we’re highlighting this week!

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