Wisent in the Walled Garden: European Bison Alternated Between Refuge and Recovery

Abstract

European bison (Bison bonasus) represent one of conservation's more complex challenges. As the largest terrestrial mammal to survive the Pleistocene–Holocene extinction in Europe, they persisted while the mammoth, woolly rhinoceros, and cave bear vanished. Their survival positioned them as a potential keystone in postglacial landscapes, yet their present ecological status and its implications for future conservation remain unsettled.

The “refugee species” concept offers one explanation: that bison today occupy forests not out of preference, but because they were displaced from open, grassy habitats by long-term environmental change and human pressure (Kerley et al. 2012). This model anticipates reduced fitness in suboptimal environments and proposes that bison's original ecological niche could be tested and restored through targeted reintroductions. The genetic and isotopic synthesis by Llamas et al. (2025) now provides a long-term test of this concept, based on the analysis of 135 mitochondrial genomes spanning 50,000 years across the Eurasian continent.

Their analysis, including 75 newly sequenced specimens, reconstructs the spatial and ecological history of two European bison lineages (Bb1 and Bb2) and the extinct steppe bison (Bison priscus). With isotopic evidence, Llamas et al. conclude that European bison remain preferentially adapted to open environments. The detailed view provided by Llamas et al. shows that bison responded dynamically to periods of constraint and release, suggesting a complex relationship between environmental change and population persistence.

The extinction of steppe bison during the Pleistocene–Holocene transition (12–9 ka) meant the loss of a dominant grazer from Eurasian ecosystems. Meanwhile, European bison, smaller and partially hybridized with aurochs during the Last Interglacial (ca. 120 ka), faced expanding forests and intensifying human presence. Llamas et al. demonstrate that within B. bonasus, the two lineages realized different fates: Bb1 tracked open habitats northward into Scandinavia before vanishing in the early Holocene; Bb2 initially persisted in southern regions, then expanded across central and eastern Europe by the mid–Holocene.

Interestingly, this expansion of Bb2 coincides with the rise of Neolithic agriculture in Europe. At that time, practices like forest clearance, shifting cultivation, and extensive grazing had been reshaping landscapes into partly open mosaics of pasture, fallow, and scrub (Githumbi et al. 2022). The Neolithic transition across much of Europe is marked by a decline in the use of large wild herbivores by human populations, as domesticated livestock came to dominate faunal assemblages in early farming contexts. In southern Scandinavia, for instance, wild herbivore remains are nearly absent from Neolithic sites, apparently replaced by cattle and sheep whose isotopic signatures reflect grazing in clearings (Gron and Rowley-Conwy 2017). The timing of Bb2's recovery broadly aligns with these shifting subsistence strategies and land-use practices, inviting the hypothesis that cultural landscape change contributed to conditions that favored bison persistence and expansion.

Stable isotope data from bison fossils provide important context for these dynamics. Llamas et al. show that all three bison groups (steppe, Bb1, and Bb2) utilized similarly open habitats and had comparable dietary profiles during the Late Pleistocene (Hofman-Kamińska et al. 2019; Llamas et al. 2025). δ¹³C values in Bb2 decline in the Holocene, consistent with increased use of forested or mixed habitats, while δ¹⁵N values remain relatively stable. This contrasts with Bb1, which shows simultaneous declines in both δ¹³C and δ¹⁵N prior to its disappearance, potentially reflecting displacement into increasingly marginal environments. For Bb2, the stability of nitrogen signatures despite habitat shifts may indicate continued access to quality forage, whether in forest clearings, anthropogenic edges, or other patches.

These findings reveal complexity in the application of the refugee species model to European bison. Early isotopic evidence suggesting marginal habitat use was based on specimens belonging to Bb1 (Bocherens et al. 2015), a lineage that terminated in the early Holocene (Llamas et al. 2025). However, Llamas et al.'s expanded dataset suggests that Bb2 experienced different phases: apparent constraint when forests expanded in the early Holocene, followed by demographic recovery and range expansion during the mid to late Holocene, before experiencing refugial restriction in recent centuries. Ecological constraint for Bb2 appears to have been episodic, punctuated by intervals of opportunity. While the mechanisms driving these dynamics remain unresolved, the evidence points to a more dynamic history than implied by a static refuge model.

Climatic variability appears to have modulated these dynamics. Llamas et al. observe that increases in Bb2 abundance coincide with climate fluctuations. Specifically, decreases in Irish bog oak growth are associated with wetter conditions and shorter growing seasons (Turney et al. 2006). These intervals are linked to vegetation change and episodes of demographic stress in Europe (Turney et al. 2006), suggesting that climate forcing may have shaped both landscapes and opportunities available to bison. One possibility is that such shifts disrupted agriculture in marginal areas, creating openings that bison could exploit. The mechanisms remain uncertain, but the temporal alignment invites further consideration of how climate effects on land use may have altered the structure and availability of bison habitat.

Regardless, it is apparent that the relationships between climate, cultural regime, and ecological opportunity did not remain static. As agricultural systems expanded after the medieval period and land use became more spatially fixed, the heterogeneous landscapes that had likely buffered Bb2 populations began to decline. Forests and mixed landscapes were converted into more continuous cropland or enclosed pasture. By the late Holocene, bison were largely confined to isolated refugia in the Caucasus Mountains and Białowieża Forest, under protection but increasingly reliant on provisioning and vulnerable to demographic collapse.

Importantly, the genetic structure within B. bonasus indicates only recent isolation. Llamas et al. report “no obvious genetic structure” within the Bb2 lineage and demonstrate that modern lines (lowland and lowland-Caucasian) represent recent geographic isolation instead of long-term divergence. They emphasize that maintaining this artificial separation may further reduce already limited genetic diversity and constrain future adaptive potential (Tokarska et al. 2011).

The record assembled by Llamas et al. shows that European bison alternated between phases of restriction and release. The Bb2 lineage endured early Holocene constraint yet expanded again by the mid–Holocene. The timing of this recovery overlaps with the spread of extensive, patchy agricultural systems. Practices such as forest clearance, shifting cultivation, grazing, and fallow produced mosaics of openings and regrowth that altered the distribution of forage and cover (Feurdean et al. 2020; Githumbi et al. 2022; Gron and Rowley-Conwy 2017). Episodes of expansion coincided with climate fluctuations that disrupted vegetation patterns and stressed agricultural systems (Turney et al. 2006). In this sense, climate fluctuations perturbed the coupled system, exposing the constraints imposed by forest expansion and land-use intensification while revealing how heterogeneity buffered the system and supported bison.

Viewed across the Holocene, the refugee species model captures only part of the story. Kerley et al. (2012) describe bison as constrained to suboptimal habitats by postglacial forest expansion and sustained human pressure. While this diagnosis fits the tightly bounded landscapes of modern Europe, the synthesis by Llamas et al. reveals a more dynamic pattern wherein Bb2 populations experienced recurring episodes of release. Some of these openings coincided with cultural disturbance regimes that introduced or maintained heterogeneity at landscape scales. In such contexts, the net effect of human activity may have been to reduce constraint rather than impose it.

This history reframes conservation choices today. Strategies that confine small herds to fenced reserves risk replicating the isolation and homogeneous habitat that precede demographic decline. More durable approaches may involve restoring disturbance, reactivating metapopulation structure, and enabling dispersal across landscapes mixing forest, meadow, pasture, and low-intensity farmland (Bluhm et al. 2025). Rewilding, in this sense, is less about insulation from people than about re-establishing the shifting mosaics that once supported bison.

Several of these approaches are already yielding insights. Together, these directions point beyond a static notion of refugia toward a process-based understanding of persistence, laying the groundwork for a more dynamic understanding of how bison navigate coupled human–environment systems.

The refugee species framing (Kerley et al. 2012) captures only part of the European bison story. Llamas et al. show that Bb2 persisted through alternating phases of ecological constraint and demographic recovery. Some episodes coincided with Holocene climate fluctuations that reshaped vegetation and stressed agricultural systems, amplifying the effects of Neolithic forest clearance, grazing, and cultivation to produce shifting mosaics (Feurdean et al. 2020; Githumbi et al. 2022; Turney et al. 2006). These entangled cultural-climatic dynamics were not optimal in a simple sense, but their heterogeneity may have provided windows of opportunity that buffered populations.

Recognizing this history shifts emphasis from a static refugee status to episodic refugee conditions, in which constraint and opportunity alternated across centuries and landscapes. Kerley et al. treated “refugee species” as a reversible condition—Llamas et al. now show that such reversals did occur. European bison were not passive relicts, but active participants in ecocultural systems shaped by dispersal and disturbance, climate and land use. Today, restriction to fenced reserves and reliance on forage provisioning risks repeating the isolation that preceded demographic decline. The work of Llamas et al. suggests more durable strategies will involve restoring heterogeneity and reactivating metapopulation processes.

The author declares no conflicts of interest.

This article is a Invited Commentary on Llamas et al., https://doi.org/10.1111/gcb.70354.