Functionally rich dung beetle assemblages are required to provide multiple ecosystem services

Dung beetles provide numerous important functions in both natural and managed ecosystems. These beneficial functions are diverse, and include: the suppression of pathogens on fresh produce, promoting nutrient cycling, contributing to seed dispersal, and improving soil structure.

Dung beetles are hugely diverse, and the number and abundance of species will vary between different sites and seasons. Dung beetles are sensitive to disturbances like logging, or the use of veterinary anthelmintics – and when systems are disturbed, species may drop out of communities.  This may have consequences for the health of an ecosystem.

We typically evaluate how ‘effective’ a dung beetle community is through measuring ‘dung removal’. This is the quantity of dung either consumed directly, or buried by a particular collective of beetles. While this is a useful way of estimating baseline functioning, dung removal may not be best practice for truly understanding how the addition or loss of a particular species influences how an ecosystem functions.
Slide03The primary reason why dung removal may not be the best measure of functioning is because organisms use dung in very different ways. In the context of the UK dung beetle fauna, species use one of three principal strategies to exploit dung resources.We can use these life strategies to place beetles into categories known as ‘functional groups’. The majority of dung beetles are ‘lazy’, and don’t put in a whole lot of effort. They spend their whole life within the dung pat, and are known as endocoprids (Latin endo = within, copros = faeces), or dung-dwellers. This group can be further subdivided by making the distinction between dwellers that lay their eggs in dung (dung-ovipositing endocoprids), and ones that lay their eggs in soil (soil-ovipositing endocoprids).

In contrast to endocoprids,  species known as paracoprids (Latin para = adjacent, copros = dung): and excavate tunnels under dung, and partition small masses of excrement known as ‘dung sausages’ at the tunnels. Female beetles lay eggs singly in the dung mass, and the larvae will use this resource solely for development.

Our experiment looked at six different experimental conditions. Three conditions involved a single species of dung beetle each representing a different functional group. A fourth condition looked at a mixture of the three species in equal proportion. A fifth condition included dung but no beetles so we could quantify baseline levels of functioning by soil invertebrates. A sixth condition did not include dung or dung beetles.

Slide12
As beetle species are different sizes, we standardised each treatment by adding the same mass of beetles to each. We placed dung pats of a known mass into enclosures, covered by a fine mesh which prevented additional flying and crawling insects from colonising, but allowed for colonisation by soil fauna. We then took three measures related to functioning: dung removal, soil compaction, and the activity of soil fauna – measured through a method known as bait-lamina.

Dung removal is an important measure as it is directly related to freeing pasture forage to grow. If dung is not removed/consumed, it may limit pasture growth. Furthermore, dung beetle activity is thought to desiccate the dung pat preventing the spread of gastrointestinal parasites.

Compacted soils are problematic as they may be less capable of absorbing rainfall, meaning nutrients may leach offsite. There are also well established relationships demonstrating forage growth can be limited by compacted soils.

Soil feeding activity is important, for the freeing of plant available nutrients. Nitrogen within dead plant matter must be mineralised before it is able to be utilised again by plants. This process is carried out by soil microbes, but is sped-up by soil fauna. When these communities (composed of many tiny invertebrates: springtails, mites, earthworms etc) feed, digest and excrete plant matter, their excrement is more palatable/digestible for microbiota, speeding along this important process.

When we compared these different groups of dung beetles against our two beetle-free conditions, we found a few surprising and interesting results. Firstly, the differences in dung removal between groups were very small. Only the three species mixture, and our dung-ovipositing dweller removed more dung than background levels of colonised soil fauna. While these differences are statistically significant, it’s unlikely they have any relevant biological significance.

Slide18
Differences in the mass of dung remaining. Bars (means) which share no common letters, are statistically different from one another.

It was a different story when it came to soil compaction. The dung-ovipositing endocoprid was the only species that significantly reduced compaction at the soil surface compared to a beetle-free control. Surprisingly, our paracoprid species did not seem to promote any reductions in surface compaction. We suggest work that considers larger-bodied paracoprid species  could be important in improving soil structure at lower horizons, where compaction problems are known to be more difficult to address.

Slide20
Differences in soil hardness underlying dung pats. Bars (means) having no letters in common, are statistically different from one another.

Finally, we looked at feeding activity. As mentioned briefly, for this we used something called the bait lamina test. This is a standardised soil test that uses cellulose baits held within a thin plastic stick. You expose these sticks and baits to soil fauna, and after removing can test the difference in feeding activity amongst different conditions. This test was originally developed as a monitoring tool for measuring loss of function in polluted soils, but has now been used in a variety of different experimental contexts. Here we find that our paracoprid, despite having no appreciable contribution to dung removal – has a strong effect in stimulating higher rates of feeding activity in the underlying soil. When we look at relationships by depth, we see this is mostly driven by enhanced activity in the lower range test (6-8 cm depth). We also see the presence of dung stimulates high levels of feeding activity at the surface.

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A few interesting things can be gleaned from this research. Firstly, dung removal may not always be a measure sensitive enough to detect losses of functioning. By focusing solely on dung removal, we might miss the contribution of a species – like in the case of our paracoprid species stimulating heightened levels of soil fauna feeding activity.

Secondly, our results indicate that by having all three species present in a diverse community – we can maintain very similar levels of functioning to the most efficient single species assemblage. This means that finding ways of supporting or enhancing diversity could be important in supporting sustainable, and productive agroecosystems.

Thirdly, many aspects involved with intensifying agricultural landscapes can harm dung beetles: and loss of species or functional groups could have important negative consequences for ecosystem services. Thus to fully capitalise on the benefits of ecosystem services alongside traditional land management, there is great potential for scientists and farmers to collaborate in designing management plans to minimise harm.

This work has been published in the Journal: Agriculture, Ecosystems, and Environment. You can read it here through this freely accessible link until mid January, 2016. If you would like more information, please feel free to drop me a line. Thanks for reading.

References:

Manning, P., Slade, E.S., Beynon, S.A., Lewis, O.T. (2016). Functionally rich dung beetle assemblages are required to provide multiple ecosystem services. Agric Ecosyst Environ 218: 87-94. 

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