Cows grazing in a birch wood.
Cows, biodiversity, and ecosystem services
1 January 2009
How do grazing cows affect birch wood ecosystems? The answers are complex, as Adam Vanbergen and team have discovered.
Conserving biodiversity doesn't just serve ethical and aesthetic goals - it also underpins healthy ecosystems which supply services that support our society.
The Millennium Ecosystem Assessment (MA), published in 2005 under the auspices of the UN Environment Programme, provided an important synthesis of the state of global biodiversity. It highlighted biodiversity's crucial role in pollinating crops, controlling pests, recycling nutrients and water, and regulating greenhouse gas emissions.
The MA emphasised that a major threats to these ecosystem services is land-use change and the intensification of farming methods. It also stressed the need to mitigate biodiversity losses to maintain ecosystem services that can ultimately help develop sustainable economies.
Adam Vanbergen testing the soil.
At the Centre for Ecology & Hydrology (CEH), my colleagues and I are investigating the relationship between biodiversity, ecosystem services, and land use. This research started five years ago when I compared Scottish birch woodlands where farmers had grazed cattle for up to 70 years to nearby ungrazed woodlands.
A serendipitous early finding from this long-term experiment was that introducing cattle into these woodlands has influenced the degree to which a parasitoid wasp kills its host insect. Parasitoids are miniature monsters a few millimetres long that provide an ecosystem service by killing insect pests of agriculture. Like the Hollywood sci-fi Alien, the wasps lay their eggs in a living insect. The larvae then consume their host from the inside, and finally burst out of its body as new adult wasps.
A taste of nectar
Interestingly, even though cows and wasps do not directly interact, there were more parasitoid attacks where cows were grazing woodland. A clue lay in what happened to the woodland vegetation. Cattle transformed the habitat by trampling, munching on tough grasses, and fertilising the soil with their dung which led to increased plant diversity, particularly of flowering plants like buttercups. The increase in parasitism rates correlated with this increase in flowering plant diversity.
The precise mechanism linking flowering plants to parasitoid success remains unclear. The implication is that while searching for its hosts the adult parasitoid may take a sugary drink of flower nectar which may increase its lifespan, letting it find and kill more hosts. This means that by grazing cattle in woodlands, farmers are changing the natural ecological relationships between plants, insects and parasites. This is an excellent example of land-use affecting the provision of a potentially beneficial ecosystem service.
Parasitoids have a lifecycle so nightmarish that Charles Darwin saw them as proof the natural world wasn't created by a loving God - they lay their eggs inside living victims, which are then slowly devoured alive by the growing larvae. But parasitoid wasps are also a big ecological success story and play a vital role in keeping populations of host species under control. Without them, swarms of pests could make agriculture impossible.
This grazing in birch woods was beneficial to flowering plants and a parasitoid, but the effect on other species was unknown. To answer this question, we launched a study into the impact of this land use change upon the minute bacteria, worms and insects hidden from sight in the soil in 2007.
Early indications are that grazing in birch woods profoundly alters soil food webs: the number of nematode worms is higher than in ungrazed woods; but soil arthropod (collembola and mites) numbers and the proportion of predatory nematodes are lower. Soil biodiversity is important for the recycling of nutrients essential for plant growth, and work is continuing at CEH to tie the changes to soil biodiversity to the function of the soil.
From 2008 we are extending this research in birch woods to join up with research in Welsh grassland and English arable sites in a new multidisciplinary CEH research project. We want to see if changes in biodiversity - above and within the soil - provide the mechanism whereby land-use intensification alters ecosystem service provision. The diversity of insect pollinators, predators, plants and soil worms along with soil function is currently being measured in these woodland, grassland, and arable sites.
Establishing the links between diversity and ecosystem services across different levels of the food chain, and the ways they in turn affect ecosystem function, remains a scientific challenge. We hope our findings will have wider relevance, informing the development of agri-environment policies that will meet the aims of food production, biodiversity conservation, and ecosystem service provision.
Dr Adam Vanbergen is an invertebrate ecologist at Centre for Ecology & Hydrology in Edinburgh.
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