The wood-pasture hypothesis (also known as the Vera hypothesis and the megaherbivore theory) is a scientific hypothesis positing that open and semi-open pastures and wood-pastures formed the predominant type of landscape in post-glacial temperate Europe, rather than the common belief of primeval forests. The hypothesis proposes that such a landscape would be formed and maintained by large wild herbivores. Although others, including landscape ecologist Oliver Rackham, had previously expressed similar ideas, it was the Dutch researcher Frans Vera, who, in his 2000 book Grazing Ecology and Forest History, first developed a comprehensive framework for such ideas and formulated them into a theory.
Vera's proposals, although controversial, came at a time when the role grazers played in woodlands was increasingly being reconsidered, and are credited with ushering in a period of increased reassessment and interdisciplinary research in European conservation theory and practice. Although Vera largely focused his research on the European situation, his findings could also be applied to other temperate ecological regions worldwide, especially the broadleaved ones.
Vera's ideas have met with both rejection and approval in the scientific community, and continue to lay an important foundation for the rewilding-movement. While his proposals for widespread semi-open savanna as the predominant landscape of temperate Europe in the early to mid-Holocene have at large been rejected, they do partially agree with the established wisdom about vegetation structure during previous interglacials. Moreover, modern research has shown that, under the current climate, free-roaming large grazers can indeed influence and even temporarily halt vegetation succession. Whether the Holocene prior to the rise of agriculture provides an adequate approximation to a state of "pristine nature" at all has also been questioned, since by that time anatomically modern humans had already been omnipresent in Europe for millennia, with in all likelihood profound effects on the environment.
The severe loss of megafauna at the end of the Pleistocene and beginning of the Holocene known as the Quaternary extinction event, which is frequently linked to human activities, did not leave Europe unscathed and brought about a profound change in the European large mammal assemblage and thus ecosystems as a whole, which probably also affected vegetation patterns. The assumption, however, that the pre-Neolithic represents pristine conditions is a prerequisite for both the "high forest theory" and the Vera hypothesis in their respective original forms. Whether or not the hypothesis is supported may thus further depend on whether or not the pre-Neolithic Holocene is accepted as a baseline for pristine nature, and thus also on whether the Quaternary extinction of megafauna is considered (primarily) natural or man-made.
Vera's hypothesis has important repercussions for nature conservation especially, because it advocates for a reorientation of emphasis away from the protection of old-growth forest (as per the competing high forest theory) and towards the conservation of open and semi-open grasslands and wood pastures, through extensive grazing. This aspect in particular has attracted considerable attention, and has made Vera's hypothesis an important point of reference for conservation grazing and rewilding initiatives. The wood-pasture hypothesis also has points of contact with traditional agricultural practices in Europe, which may conserve biodiversity in a similar way to wild herbivore herds.
Names and definitions
Frans Vera's hypothesis has many names, since Vera himself did not provide a distinguished name for it. Instead, he simply referred to it as the alternative hypothesis, alternative to the high-forest theory, which he called the null hypothesis. As a result, it has been called by many names over the years, including the wood-pasture hypothesis, the wooded pasture hypothesis, the Vera hypothesis, the temperate savanna hypothesis and the open woodland hypothesis. Especially in Continental Europe, it is commonly known as the megaherbivore hypothesis and literal translations of it.
Vera limited the geographic area of his ideas to Western and Central Europe between 45°N and 58°N latitude and 5°W and 25°E longitude. This includes most of the British Isles and everything between France (except the Southern third) and Poland and Southern Scandinavia to the Alps. Furthermore, he confined it to altitudes below . By extension, the North American East Coast is also addressed as an analogy with a comparable climate.
High-forest theory
Heinrich Cotta: high-forest theory In his 1817 work Anweisungen zum Waldbau (Directions for Silviculture), Heinrich Cotta posited that if humans abandoned his native Germany, in the space of 100 years it would be "covered with wood". This assumption laid the foundation for what is now called the high-forest theory, which assumes that deciduous forests are the naturally predominant ecosystem type in the temperate, broad-leaved regions.
Frederic Clements: linear succession
Later, this position was accompanied by Clements' formulation of the theory of linear succession, meaning that under the right conditions bare ground would, over time, invariably become colonised by a succession of plant communities eventually leading to closed stands dominated by the tallest plant species. Because in most of the temperate hemisphere the potentially tallest plants are trees, the final product would therefore chiefly be forest. Albeit with changes in conceptualisation and some modifications, this concept remains the one favoured by most, and provides the conceptual framework for many forest-related methods and customs in forestry and conservation. This includes the doctrine advocated by German forest-ecologist Knut Sturm, which highlights the importance of non-intervention and space of time for forest protection, as it is implemented in forest reserves such as Białowieża.
Further refinements
Clements' notion of stable climax communities was later challenged and refined by authorities such as Arthur Tansley, Alexander Watt and Robert Whittaker, who championed the inclusion of dynamic processes, like temporary collapse of canopy cover because of windthrow, fire or calamities, into Clements' framework. This, however, did not change anything about the status of the "high-forest theory" as the commonly accepted view; that without human intervention closed-canopy forest would dominate the global temperate regions as the potential natural vegetation. This is also the concept that was advocated by European plant experts like Heinz Ellenberg, Johannes Iversen and Franz Firbas.
The reconstruction of vegetation history
Apart from theoretical considerations, this concept has relied and continues to rely heavily on both field observations and, more recently, on findings from pollen analysis, which allow inferences about the vegetation structure of past epochs. For example, vegetation trends can be reconstructed from the ratio of tree pollen to pollen associated with grassland. Pollen analysis is the most widely used means of generating historic vegetation data and the analysis of pollen data has provided a solid database from which a predominance of forest throughout the early stages of the Holocene of temperate Europe, especially the Atlantic, is generally inferred, although the possibility of regional differences remains open. On that basis, the history of vegetation in Europe is generally reconstructed as a history of forest.
Pollen analysis, however, has been criticized for its inherent bias towards wind-pollinated plant species and, importantly, wind-pollinated trees, and has been shown to overestimate forest cover. To account for this bias, a corrective model (REVEALS) is used, whose application leads to results that differ substantially from those drawn from the traditional comparison of pollen percentages alone. Alternatively to or in combination with pollen, fossil indicator organisms – such as beetles and molluscs – can be used to reconstruct vegetation structure.
Large herbivores and high-forest theory
There is no general agreement on herbivores and their influence on succession in natural ecosystems in the temperate hemisphere. In the high-forest theory framework, wild herbivores are mostly considered as minor factors, derived from the assumption that the natural vegetation was forest. Therefore, wild herbivores were characterised by Tansley as followers of succession, not as actively influencing it, because otherwise Europe would not have been forested. From this assumption the principle was developed that the natural abundance of herbivores does not hinder forest succession, which means that herbivore numbers are necessarily considered too high once as they impede natural forest regeneration. For example, WWF Russia considers five to seven animals the optimal density of bison per 1000 ha (10 km2), because if the population exceeds 13 animals per 1000 ha, first signs of vegetation suppression are observed. Consequently, the bison population in Białowieża is controlled by culling. Similarly, it is widely believed that two to seven deer per is a sustainable number based on the assumption that if deer numbers exceed this bar, they start having a negative impact on woodland regeneration. Consequently, culling is commonly seen as necessary to reduce a perceived overabundance of deer to sustainable levels and mimic natural predation.
Others, however, have criticised this view. In a 2023 publication, Brice B. Hanberry and Edward K. Faison argued that in the eastern United States, where white-tailed deer are commonly considered overabundant due to the extirpation of wolves and cougars, there are currently no more deer than there were historically when these predators were present. Furthermore, they found that even at densities that are perceived as too high, the influence of deer may be ecologically beneficial. The assumption that population control through hunting is necessary in order to mimic the effect of natural predators is also not entirely supported by scientific analyses of natural predator-prey dynamics. Instead, the control of herbivore numbers in nature probably depends on other factors. A perhaps more important influence predators may have on prey animals is the landscape of fear their presence can create, promoting landscape heterogeneity. However, in the presence of megafauna over , which are largely immune to predation, even this ability is limited. Overall, how ungulate populations are controlled in nature is controversial, and food availability is an important constraint, even in the presence of apex predators.
In regions with relatively intact large-mammal assemblages in Africa and Asia, as well as in European rewilding areas where "naturalistic grazing" is practised, herbivore biomass exceeds the values commonly deemed appropriate for temperate forests many times over. Here, herbivore biomass reaches a maximum of per , while the mammoth steppe with an estimated per km2 falls within a similar range. The herbivore biomass of Britain during the Eemian interglacial has been estimated as more than per km2, which is equivalent to more than 2.5 fallow deer per ha. Hence, the ecologist Christopher Sandom and others have suggested that the comparatively high forest cover of the pre-Neolithic European Holocene may be a consequence of megaherbivore extinctions during the Quaternary extinction event, as compared to the last interglacial in Europe with a pristine megafauna, the Eemian, the early stages of the Holocene appear to have been much more forested. According to the authors, this is unlikely to be the result of the latter's only slightly cooler climate as compared to the Eemian. However, this is also subject to debate.
Background: grazers and browsers