Woodland warfare in a warming world
12 October 2012
Could woodlice grazing on soil fungi limit the effects of climate change? Don A'Bear and Tom Crowther think so.
Woodlands store almost half the world's terrestrial carbon - 1240 petagrammes (Pg) of the 2860Pg total. Fungi are the main agents of decomposition in these ecosystems so they are responsible for storing and producing this woodland-soil carbon.
Apart from the occasional mushroom, these fungi go largely unnoticed, as most of their growth occurs where the soil meets the leaf-litter layer. These fungi grow as networks of 'mycelium' - intricate webs of filaments, like the root systems of plants. Underground battles for supremacy between competing fungi, and interactions with grazing invertebrates, shape the world as we know it.
Studying interactions between these organisms is difficult, which is why soil has been traditionally seen as a 'black box'. Soil communities are hidden, highly complex and more diverse than you'd ever imagine. Known as 'the poor man's tropical rainforest', this unseen majority consists of thousands of animal and microbial species living side by side, in conflict or in harmony, just below our feet. These organisms are nature's recyclers - breaking down dead organic matter and releasing the nutrients locked within.
Global climate change presents some of the biggest scientific and political challenges of the 21st century. Perhaps the greatest is the need to understand better the mechanisms regulating how carbon is exchanged between land, ocean and atmosphere. On land, plants are responsible for taking up carbon dioxide (CO2) from the atmosphere. Over time this leads to substantial amounts of carbon being stored in soil and vegetation. Decomposition of dead plant material eventually releases it back into the atmosphere.
The authors digging soil turves.
This delicate balance between carbon uptake and release is very sensitive to climate change. If a warming climate means terrestrial ecosystems produce more carbon overall, then increased atmospheric CO2 concentrations will lead to further climate warming. We know climate change is likely to accelerate CO2 uptake by plants, but how will it influence the rate at which they decompose litter and release this carbon back into the atmosphere? The answer lies in the soil, where the unrivalled diversity of microbes and soil animals regulate these important ecosystem processes.
To predict how climate change will influence this activity, we first need to understand how these organisms interact with each other. We use simplified models of the woodland floor to study specific interactions within such complex systems. Soil is compacted and flattened in trays to ensure that fungi grow across the surface, where they can be seen. This lets us visualise and measure the effects of soil-dwelling invertebrates on decomposer fungi, as well as estimating individual species' contribution to wood decomposition rates.
Our own work has shown that a wide range of soil animals - worms, springtails, mites, millipedes and woodlice - graze on these nutritious fungi, limiting their growth and decomposition. But the identity of these grazing species matters! Different grazers have different effects on the fungi they eat, depending on their energetic needs, feeding strategy and body size.
Tiny grazers, big effect
Larger invertebrates, such as woodlice, have the strongest impacts. The 'grazing pressure' they exert can determine the outcome of battles for dominance between fungal species. When they eat more of the strongest fungal competitor, woodlice save important but less competitive species from being wiped out - maintaining microbial diversity and ensuring that forest floor nutrients are processed efficiently.
The different effects of soil invertebrates on fungal growth and decomposition are important because climate change is predicted to alter their communities. This will indirectly affect the composition of fungal communities, and how quickly they can break down organic matter. Now that we understand more about how these organisms interact, we can begin to consider the impacts of climate change.
Decomposition rates partly depend on the balance between how fast the fungus grows and how quickly invertebrates remove it. Global warming is likely to mean both faster fungal growth and more grazers - so what does this mean for decomposition? We investigated the impact of a 3°C temperature increase on this growth and grazing balance. In general, fungi grew and decomposed wood more rapidly in warmer conditions.
Two fungi interacting.
Yet grazing by growing soil invertebrate populations stopped fungi from expanding into new territory any faster. This may have restricted the increase in decomposition rates that would have happened if the invertebrates had been absent. The increased grazing also meant temperature didn't change the fungal community composition. These interactions may, therefore, be particularly important in the years to come - they may limit the effects of global warming on microbial communities and the rates of carbon production from soil.
Different invertebrates like to eat different fungi. So the identities of both fungus and grazer - and therefore the composition and diversity of the decomposer community - are important in determining how the whole system responds to warming. For example, springtails grazing on their favourite fungi showed the greatest increase in population size due to warming. This ended up halting the fungal colony's expansion, as the larger grazer populations consumed more fungus.
The species-specific nature of these interactions could also shed light on an old question in soil ecology - why are soil communities so diverse? Scientists have traditionally assumed these ecosystems have a large degree of functional redundancy - lots of co-existing organisms all playing similar roles. But if this is the case why did such diversity evolve? The answer may lie in tiny differences in feeding strategy - invertebrates feeding on different fungal food sources - and tolerance for different environmental conditions.
We are now investigating whether our findings in these model systems reflect what is happening in nature, with all its variability and complexity. To do this, we took soil 'turves' from established woodland ecosystems, containing the full diversity of microbes and animals. We brought these small versions of real systems into the laboratory and are studying them under controlled climatic conditions.
This is an important step in bridging the gap between purely laboratory- and field-based studies. As in the microcosm models, larger soil fauna, such as woodlice, have the most important effects. The next step will be to conduct experiments in the field, using natural environmental gradients from colder to warmer places to simulate climate-change scenarios.
Ultimately we want to understand how climate and soil communities interact to regulate ecosystem function. So far, our results suggest that the impacts of global warming on decomposition and carbon feedback depend on the composition of the decomposer community. So maintaining soil biodiversity seems to be crucial if we're to keep our woodland ecosystems working efficiently and maximise their resilience to climate change.
Don A'Bear is a PhD student at Cardiff University funded by a NERC studentship. Tom Crowther recently completed his NERC-funded PhD in the same group, and is now continuing his research at Yale University on a Yale Climate & Energy Institute fellowship. Email:ABearAD1@Cardiff.ac.uk, Thomas.Crowther@Yale.edu
Adaptation & mitigation,
Interesting? Spread the word using the 'share' menu on the top right.