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Keywords:

  • Conservation planning;
  • European bison;
  • refugee species;
  • species distribution models

Abstract

  1. Top of page
  2. Abstract
  3. Acknowledgements
  4. References
  5. Biography

Understanding species' historical ranges can provide important information for conservation planning in the face of environmental change. Cromsigt et al. (this issue) comment on our recent European bison (Bison bonasus) range reconstruction, suggesting that bison were already 8000 years ago a refugee species (i.e. restricted to marginal habitat due to past human pressure) and that species distribution models (SDM) are generally of limited use for refugee species conservation. While we welcome this discussion, we find no evidence for the claim that human pressure prior to 8000 BP determined where bison occurred. More importantly, as human pressure is generally high and increasing, attempts to restore species across their former range may fail where the factors that relegated species into refugee status are still at play or where their optimal habitat has vanished. Identifying areas where human pressure is low and where refugee species have persisted over the last millennia is crucial, and SDM based on historical data are important for doing so. Refugee species suffer from the shifting baseline syndrome, but careful reality checks are needed and all available data should be considered before determining the baseline that should inform conservation planning.

Many large mammals are today confined to small, fragmented populations as a result of past habitat loss and overhunting (Morrison et al., 2007; Yackulic et al., 2011). Understanding the historical ranges and fundamental niches of these species can therefore inform conservation planners how to preserve them in the face of environmental change (Nogues-Bravo, 2009; Willis & Bhagwat, 2010). This may be especially important for refugee species (Kerley et al., 2011), which have been extirpated from the most suitable portions of their historical range and now only prevail in marginal habitat. Kerley et al. (2011) hypothesize that European bison (Bison bonasus) are an example of a refugee species and that the broad-leaved and mixed forests where they are commonly found today represent suboptimal habitat. Our recent analysis (Kuemmerle et al., 2012) of bison records and their habitat for the last 8000 years found little support for this hypothesis, and in response, Cromsigt et al. (2012) suggest that European bison were already a refugee species 8000 years ago. We thank Cromsigt et al. for their interesting contribution, and we welcome this discussion given the importance of this issue for bison conservation. However, we find Cromsigt et al.'s critique surprising given the substantial overlap between their argument and ours, and we strongly disagree with the dismissal of historical evidence to inform European bison conservation planning.

Cromsigt et al.'s critique is surprising given that our study was motivated by the same concern about potentially misleading distribution data from contemporary herds (Kuemmerle et al., 2011) that Cromsigt et al. (2012) list. We state all of Cromsigt et al.'s arguments about diet and evolutionary history in Kuemmerle et al. (2012) to highlight the need for range models based on historical data. Our range reconstruction for the last 8000 years shows that European bison were linked to both semi-open and closed forests and a range of forest types, and we found large, previously unreported areas of potential habitat at the north-eastern range edge. We thus agree that the fundamental niche of European bison is likely wider than previously appreciated, including more open habitat types – a point we have repeatedly made in the past (Kuemmerle et al., 2010, 2011). Cromsigt et al. also stress that they do not question the scientific validity of our work. In sum, although relying on different datasets and evidence, there is very little difference in the scientific facts that are reported in the papers by Kerley et al. (2011), Cromsigt et al. (2012) and us.

The disagreement lies in the interpretation of these facts and their conservation implications. We did not find evidence that European bison were widespread in Southern and Western Europe over the last 8000 years, nor that European bison preferentially selected grasslands during that time period. Cromsigt et al. concluded that bison must have already been a refugee species before 8000 BP and that human pressure had pushed bison out of Southern and Western Europe. Reconstructions of human population and farming expansion in Europe generally do not support this view, suggesting that human pressure in Southern and Western Europe was in fact lower than in Central Europe (Bramanti et al., 2009; Kaplan et al., 2009, 2011; Gaillard et al., 2010; Klein Goldewijk et al., 2010; Ellis, 2011). Also, no explanation is given by Cromsigt et al. as to why the bison's range would have been severely curtailed by humans prior to 8000 BP, but then maintained stable for the next 5000 years, although human pressure rose substantially during that time. No European bison remains from the early Holocene have been found in Western and Southern Europe, although many archaeozoological assemblages exist (Benecke, 1999, 2005), which include remains of other prey species of human hunters such as wild horses (Sommer et al., 2011). So while Cromsigt et al. assert that human pressure prior to 8000 BP largely determined where bison occurred, there are unfortunately no data to support this claim. Furthermore, Cromsigt et al. also suggest that European bison prefer open, grass-rich habitats, but the potential natural vegetation of Southern and Western Europe is dominated by either forests or sclerophyllous shrubs (Bohn et al., 2003). Steppe components were more widespread towards the South-East (Bohn et al., 2003), but none of the European bison occurrences we used occurred in steppe areas (Kuemmerle et al., 2012), despite the fact that grassland soils are often less acidic than forest soils and should thus promote the preservation of bones.

Be that as it may though, there is little value in repeating arguments that have been made numerous times before. As Cromsigt et al. (2012) state, both European bison habitat preference and Holocene vegetation patterns are heavily contested discussion topics (Svenning, 2002; Mitchell, 2005; Vera et al., 2006; Krasinska & Krasinski, 2007; Kerley et al., 2011; Kowalczyk et al., 2011). The question is what kind of scientific information should inform conservation action. Cromsigt et al. (2012) suggest that ‘in the case of refugee species, historic reconstructions using SDM cannot be used as a template for conservation management. Rather, experimental re-introduction programmes should provide us with population performance and life history data from a range of suboptimal to optimal habitats’. We respectfully disagree. Conservation should be based on all available evidence, and we cannot reject findings when they do not support a prior hypothesis.

Furthermore, it is not clear to us why experimental (re-)introduction would provide superior information for conservation decisions. Habitat selection and habitat quality depend on much more than just food resources (Kerley et al., 2011). Demographic trends in small-scale experimental reintroductions without predators and limited competition, and without natural disturbance processes such as fire, have limited value when the goal is to identify optimal habitat and to understand the demography of species that typically occupy large territories and have the ability to migrate.

Whatever mechanism is used to identify what constitutes optimal habitat, ultimately conservationists need to face the real world, and that is a world with many people. As such, we find it inconsistent that Cromsigt et al. (2012) argue, on the one hand, that humans relegated bison already into refugee species status in the early Holocene, but suggest, on the other hand, that current reintroduction efforts should focus on some of the most densely settled parts of the world in Southern and Western Europe, where conflicts with people and land use would be huge, where European bison have not been present during the last 8000 years and where conservation efforts incur high financial costs. Likewise, even if grasslands were the optimal habitat for European bison, it remains unclear why European bison reintroductions should focus on Southern and Western Europe, where natural grasslands are scarce and have mostly been converted to agriculture.

Given that the global population of European bison remains precipitously small, each additional herd would be a step forward. Experimental reintroductions in a range of habitat types may be important to improve our understanding of the species' habitat selection. Similarly, establishing new herds in densely population regions such as Western Europe may serve to create public awareness and raise support for efforts to preserve European bison within less-populated areas within their former range. However, our vision – and we assume Cromsigt et al.'s vision too – is a future in which European bison roam again in large, viable and connected herds, and are not limited to small herds, isolated protected areas and fenced enclosures. There is no doubt in our minds that the place where such a vision is most likely to come true is in Eastern Europe, the heart of European Bison's range for at least the last 8000 years, because that is where habitat abounds, and human pressure is both relatively low, and decreasing.

Refugee species prevail only in marginal habitat within their range, often due to past human pressure (Kerley et al., 2011). As human pressure is generally high and increasing in many regions, identifying areas where human pressure is low and where refugee species have persisted over the last millennia is crucial. Species distribution models based on historical data, particularly when coupled with mechanistic vegetation models, are important for doing so. Attempts to restore species across their former range may fail where the factors that relegated species into refugee status are still at play or where their optimal habitat has vanished. Refugee species suffer from the shifting baseline syndrome, but careful reality checks are needed and all available data should be considered before determining the baseline that should inform conservation planning.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Acknowledgements
  4. References
  5. Biography

We gratefully acknowledge support by the Einstein Foundation, the European Commission (ECOCHANGE, FP6-036866 & VOLANTE, FP7-265104), the NASA Biodiversity and NASA Land Cover and Land Use Change programs and the LOEWE initiative for scientific and economic excellence of the German federal state of Hesse. Two anonymous reviewers provided valuable comments, which greatly improved our manuscript.

References

  1. Top of page
  2. Abstract
  3. Acknowledgements
  4. References
  5. Biography
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Biography

  1. Top of page
  2. Abstract
  3. Acknowledgements
  4. References
  5. Biography
  • Tobias Kuemmerle is a junior research group leader and professor for Biogeography and Conservation Biology at the Geography Department of Humboldt-University Berlin. His research focuses on understanding dynamics in land systems, on exploring how land use and climate change affect biodiversity patterns and on assessing how to balance human resource use and biodiversity conservation.