The Animals That Are Meant to Keep the Forest Open

AWhen the ice sheets withdrew from northern Europe, the continent still carried a fauna that would now look African: straight-tusked elephants, two species of rhinoceros, aurochs, wild horse. Almost none of it survived to see the first cereal fields. Rewilding, in the sense the word has carried since the 1990s, proposes to put back what can be put back — hardy cattle and pony breeds standing in for animals that no longer exist — and then to interfere as little as possible afterwards. Three propositions sit behind that programme and are usually stated in one breath. The first is that the vegetation of pre-agricultural Europe was not an unbroken canopy but a shifting mosaic of grassland, thicket and grove. The second is that large grazing animals were what held the mosaic open. The third is that releasing their modern substitutes will bring the mosaic back, and with it the species it once fed. The first of the three now attracts little argument; the quarrel of the past twenty-five years has been over the other two.

BThe case for the second proposition was assembled most fully by Hendrik Boersma, who published counts from 41 peat cores taken across the lowlands between the Rhine and the Elbe. Boersma worked with the proportion of pollen belonging to light-demanding trees and shrubs — hazel, oak, blackthorn — reasoning that these do not flower under a closed canopy. In 33 of the 41 cores that proportion stood above twenty per cent for most of the period between 8,000 and 6,000 years ago, against under five per cent in the present-day closed woodland he sampled for comparison. Pollen, however, records what grew, not what kept it growing, and Boersma knew it. He therefore paired every core with counts of the spores of fungi that complete their life cycle only in herbivore dung, and showed that the two curves rose and fell together. The claim that grazing animals, rather than fire or storm, were what maintained the openness became known as the wood-pasture hypothesis.

CInes Marchetti's reanalysis, published in 2019, did not dispute a single count. Her objection was that the counts had been read as though every tree advertised itself equally. Hazel and oak scatter their pollen on the wind and produce it in enormous quantity; lime and elm, the trees that dominate a closed canopy on the same soils, are pollinated largely by insects and leave a faint trace. Applying a correction factor derived from modern flowering surveys, Marchetti recalculated Boersma's own material and found that the twenty per cent threshold was still cleared in only nine of the 41 cores. Those nine had one feature in common: every one of them lay within three kilometres of a river channel or of the contemporary coast. Marchetti called the result the riverine exception. Openness had been real, she accepted, but it looked like the work of floods, shifting channels and salt spray rather than of teeth.

DField evidence for the third proposition comes almost entirely from one place. The Kessel Marshes, 5,600 hectares of reclaimed floodplain, received forty konik ponies and thirty red cattle in 1996 under founding rules that forbade both the introduction of predators and supplementary feeding in winter. With nothing to check them, the herds grew to about 1,100 animals by 2015. In the hard winter of 2017–18 roughly a third of them died, and photographs of the carcasses reached the national press. The die-off was not really an accident of weather. It was an arithmetical consequence of the founding rules: a population that is neither hunted nor fed rises until the food available in the worst month sets its ceiling, and that ceiling is reached by starvation rather than by agreement. The vegetation, meanwhile, opened as predicted, and geese and lapwing arrived in numbers, while the reed and scrub birds that had colonised the site before 1996 largely left it.

EA separate objection concerns carbon. Tomas Ekelund fenced twenty pairs of plots in southern Sweden and compared them after twenty years. The grazed plots carried thirty-four per cent less carbon in wood and leaf than their fenced twins, which is what the suppression of saplings must do. Soil carbon, measured to a depth of 30 centimetres, ran about eleven per cent higher under grazing, and Ekelund's own summary states that the two figures cancel to a difference his design could not distinguish from zero. He was careful to add that the balance held only for the sandy soils he had worked on, and that on peat the grazed plots lost carbon outright. What has gone wrong since, in his view, is that each camp quotes whichever half of the result suits it, and neither quotes the condition attached to it.

FWhat has changed is not the science so much as the question. Nobody now denies that a sufficient weight of grazing animals will hold vegetation open; the disagreement is over the weight, and over who sets it. Reserves stocked lightly turn to scrub, reserves stocked heavily turn to lawn, and the mosaic both camps say they want appears somewhere between the two — at a density that has to be maintained deliberately, which is precisely what rewilding was meant to avoid. Nor is the choice only ecological. Culling is the ordinary means of holding a herd below its starvation ceiling, and it is the means the public most dislikes; the Kessel Marshes authority adopted a shooting programme in 2019, after twenty-three years of refusing one. An argument that began as a question about the distant past has become a question about how many animals a reserve should carry, and about whose decision that is.