Disturbance in ecosystems and implications for conservation managers

Mark Hancock

PhD project (2003 onwards) in conjunction with the RSPB, Scotland

Scotland has lost 99% of its native pinewood. How should the surviving 1% be managed to best preserve its value? The largest surviving ancient native pinewood fragment in Scotland is at Abernethy Forest, an RSPB reserve near Aviemore in the Highlands (Amphlett 2003, Thompson and Summers, 2003). Abernethy is important for Capercaillie Tetrao urogallus – the world’s largest grouse – which is in danger of extinction in Scotland (Moss, 2001). But this is just one of many species in the remaining semi-natural forest.

There has been considerable debate in recent years over the extent to which conservation managers of forests should allow or re-create dynamic processes which disturb forests, such as fire, or trampling by herds of large herbivores (Angelstam, 1998, Svenning 2002). Disturbance may benefit some species, such as Capercaillie and Scots Pine Pinus sylvestris, but damage others. Can ‘natural’ disturbances be re-created by conservation managers and do they help us achieve conservation objectives?

To answer these questions, a series of five field experiments were set up at Abernethy, by the RSPB, with support from SNH, EU-LIFE and the Scottish Forest Alliance. At small experimental sites, in 2001-3, the ground vegetation was ‘disturbed’ – by burning, cutting or by a short episode of cattle browsing and trampling. A few results are already apparent:

• Fire and cattle can be safely managed within the ancient pinewood.
• Scorch damage to mature pine trees following low intensity ground vegetation fire has been minimal.
• Bilberry Vaccinium myrtillus, important for Capercaillie and other species, recovers rapidly following fire – but will Heather Calluna vulgaris re-assert its dominance?
• Cattle trampling creates bare ground and greater light penetration to the soil – but will this translate into greater pine seedling recruitment?
• A small-scale experiment showed that pine seedling establishment increases considerably after fire (roughly 4-fold). This was particularly marked after slow-moving (less than 0.03 m s^-1) fires, which produced a c10-fold increase in establishment rates (Hancock et al. in prep). These results are now being tested by a larger, management-scale experiment.

Diaz et al. 2003 recommend that ecologists exploit unintended “removal experiments,” often the result of management practices or experiments “in which certain functional types are removed or reduced in abundance or diversity. The ecosystem consequences of such practices are often not measured but....represent a rich source of information for both theoretical and practical reasons." The management experiments at Abernethy represent just such an opportunity – all use disturbance to reduce the biomass of the dominant field layer shrub, Heather. Invertebrate sampling is under way, with the principle aim of assessing effects of management on invertebrates of importance in the diet of Capercaillie chicks. In the spirit of Diaz et al.’s suggestion, responses of the invertebrate community of the pinewood field layer will be explored in more detail, and means of using these responses to test ecological theory will be developed. Suitable theories include those relating species diversity and disturbance – such as Connell’s Intermediate Disturbance Hypothesis (Connell, 1978) and Huston’s Dynamic Equilibrium Model (Huston 1979, 1994). Ultimately these analyses aim to contribute to our understanding of the role of disturbance in maintaining diverse ecological communities in natural habitats.

References

Anglestam, P. K. (1998) Maintaining and restoring biodiversity in European boreal forests by developing natural disturbance regimes. Journal of Vegetation Science 9, 593-602.

Amphlett, A. (2003) Contexts, developing ideas and emerging issues in the conservation management of RSPB Abernethy Forest Reserve. Botanical Journal of Scotland 55(1), 135 - 148

Connell, O. H. (1978) Diversity in tropical rainforests and coral reefs. Science 199: 1302-1310.

Diaz, S., Symstad, A. J., Chapin, F. S. III, Wardle, D. A., Huenneke, L. F. (2003) Functional diversity revealed by removal experiments. Trends Ecol Evol 18: 140-146.

Hancock, M. H., Egan, S., Summers, R. W., Cowie, N., Amphlett, A., Rao, S., and Hamilton, A. (in press) The effect of experimental prescribed fire on the establishment of Scots Pine Pinus sylvestris seedlings on Heather Calluna vulgaris moorland. Forest Ecology and Management.

Huston, M. A. (1979) A general hypothesis of species diversity. American Naturalist 113: 81-101.

Huston, M. A. (1994) Biological Diversity. Cambridge University Press, Cambridge.

Moss, R. (2001) A second extinction of Capercaillie (Tetrao urogallus) in Scotland? Biological Conservation 101: 255-257.

Svenning, J.-C. (2002) A review of natural vegetation openness in north-western Europe. Biological Conservation 104: 133-148 Thompson, M. & Summers, R. (2003) Restoring Abernethy Forest. Conservation Land Management 1: 4-6.