What might a landscape driven by large herbivore grazing look like and would we want it?

Dr Keith Kirby, Visiting Researcher, Department of Biology, University of Oxford

Presentation

Frans Vera with his book Grazing and forest history threw down a challenge to the orthodoxy of a largely closed-canopy forest as the natural state for much of north-west Europe. His arguments have been disputed and I suggest that his original conclusions were overstated. Nevertheless he made us think again about the extent of openness in post-glacial landscapes and the role of large herbivores.

While the book was not about rewilding per se the ideas in it have inspired many of the practical attempts to rewild landscapes, such as on the Knepp Estate. There is a widespread belief that the use of free-ranging large herbivores and naturalistic grazing will lead to the development of mosaics of woodland, scrub and open habitats.

Vera himself has not (to my knowledge) ever been that specific about what a landscape developed under his hypothesis would look like. The case for the oft-quoted ‘half-open’ landscape is nowhere made – why not 10% or 90% open? Secondly, if we consider the different stages in Vera’s shifting mosaic hypothesis what would the grain-size of that mosaic be: where might there be lots of small openings interspersed by small patches of scrub and small patches of wooded grove; where a much coarser grain, with (say) the wooded areas all in one part of the landscape, the open areas in another, the scrub and regeneration in a third. Thirdly how long does a patch stay in a particular stage and what triggers the transition to the next?

These questions affect the patterns of biodiversity that might be generated: they matter if we are planning to use naturalistic grazing to achieve conservation aims.

For example James Fenton for example has argued a case that the largely treeless Scottish moorland (90% open) is the ‘rewilded’ state and that efforts to reduce deer numbers and to increase tree cover by fencing and planting (as by Trees for Life) could be seen as reducing naturalness. If only 10% openness is the more likely outcome we need to be cautious about using this approach where there is already extensive open heathland conservation interest.

If the grain-size of the mosaic is small then it may be maintained on quite small sites (a few tens of hectares); if it is likely to be large then a mosaic may be sustainable only on much larger sites.

If transitions between stages are more or less continuous then it is easier to predict how landscapes will change over time and to maintain the biodiversity associated with each stage, than if transitions are triggered by sudden and unpredictable events – for example if regeneration is episodic following rare sharp declines in grazing pressure.

Naturalistic grazing will not produce one outcome pattern: it will be situation-specific and managers will need to take their particular circumstances into account in assessing the likely outcomes, not rely on general statements as to what will happen.

It will be very rare for ‘rewilding’ to be completely laissez-faire with respect to herbivore management because of animal welfare, safety and economic reasons. This opens the door to adjusting grazing levels and distribution such that the mosaic pattern produced is more in line with what is desired/expected. This does mean that managers have to be clear as to their landscape objectives. The limits of acceptable variation may be much wider than under conventional conservation management, but there is usually still a target landscape pattern that is expected/desired.

In this way ‘rewilding’ may be as much a form of cultural landscape creation as restoring coppice or managing a hay meadow. It cannot claim to be some form of pure ‘value-free’ objective way of doing conservation that must therefore trump other approaches.