Our study shows the vulnerability of small, isolated populations to stochastic demographic events (Lande 1988; Roughgarden 1975; Shaffer 1981), and provides us with important insights into the population dynamics and connectivity of such small population stepping-stones of Eurasian lynx in a human-dominated landscape. Using data from repeated camera trapping surveys carried out between November 2014 and March 2019, we described the individual histories of nine individually distinguishable Eurasian lynx individuals in a small population stepping-stone in the North of the federal state of Hesse, central Germany. This stepping-stone emerged approximately 60 km southwest of the source population in the Harz Mountains and represented a promising first step for the successful dispersal of lynx in central Germany. However, during our 5-year study period, the Northern Hessian Subpopulation went through a complete turnover of individuals: none of the six individuals we detected at the onset of our study was still present when this study ended. A particularly serious event mainly responsible for this turnover was a population decline already in the first year of our study, when five of the initially six resident individuals either died (two individuals), dispersed (one individual), or disappeared for unknown reasons (two individuals). In the 4 years that followed, the population did not recover from this dramatic decline, mainly due to the absence of female dispersal into our study area.
Size, sex ratio, and stability of the Northern Hessian subpopulation
We presume the six individuals we detected at the beginning of our study are the minimum number of lynx individuals comprising the NHS at that time. Owing to the small size of the study population, we did not carry out capture-recapture analyses to estimate population size (Borchers and Efford 2008; Otis et al. 1978), yet with the exception of a potential disperser (L1010x) and an individual living predominantly north of our study area (B1061w), we detected all individuals at a highly regular basis (see the “Appendix” section), particularly since trapping season 2015–2016, suggesting that we detected all resident individuals within our study area. It is possible, however, that there might have been individuals living outside our study area, particularly at the beginning of our study, such that the NHS was, in fact, larger than described here.
Two of the six adult individuals we detected in trapping season 2014–2015 could be sexed as males, and two individuals could be sexed as females. The available evidence suggests that the remaining two individuals were likely also females. In early spring 2015, two detections could be sexed as females (one because the individual was photo-captured with offspring and one because genitals were visible), but in both cases, the individual identification of the animal was not possible. One of these detections occurred in an area where we previously detected B1035x, the other detection occurred approximately 13 km southeast, in an area where we regularly detected B1038x. Both photos did not show B1037w (which was easily recognizable due to its radio-collar), and both photos did most likely not show B1061w either (which had its territory mainly north of our study area). We conclude, therefore, that at the beginning of our study, the NHS consisted of at least two resident males, and at least three (more likely four) resident females. Yet despite this favorable sex ratio, our study population collapsed only a few months later due to the death or disappearance of all known and presumed females.
Population history and connectivity
In the 4 years after the population decline, until the end of our study, the NHS did not recover. During these 4 years, the number of resident individuals observed in our study area was never larger than two resident males. Moreover, while dispersing males from the Harz Mountains regularly reached our study area, during the past 9 years, not a single female was observed to overcome the approximately 60 km between the Harz Mountains and our study area. This result is perhaps not surprising, because dispersal is male-biased in the majority of mammals, i.e., males more frequently leave their natal group or territory than females, and/ or disperse over larger distances than females (Greenwood 1980; Lawson-Handley and Perrin 2007). With respect to dispersal distance, this is also true for Eurasian lynx. In Poland (Schmidt 1998), Switzerland (Zimmermann et al. 2005) and Scandinavia (Samelius et al. 2012), males dispersed over larger distances than females, though the male bias was not very pronounced in Switzerland (Zimmermann et al. 2005), yet despite sex-biased dispersal, in the Swiss Jura Mountains, one of the populations studied by Zimmermann et al. (2005), some females dispersed over distances larger than 60 km. In the Bialowieza region of Poland, one female was even observed to disperse 120 km. So why do females from the Harz Mountains not disperse to northern Hesse?
The answer to this question is likely to be found in the degree of habitat fragmentation, which is presumably much higher between the Harz Mountains and northern Hesse than within the Swiss Jura Mountains or within the Bialowieza region. It has been suggested previously that Eurasian lynx may have limited ability to cross unfavourable habitat (Samelius et al. 2012; Zimmermann et al. 2007), and our study suggests that this is particularly true for the sex less prone to dispersal, i.e. females. In line with this suggestion, in the more strongly fragmented Swiss Alps (Schnidrig et al. 2016), female lynx were not observed to disperse distances larger than 40 km (Zimmermann et al. 2005). Likewise, in cougars (Puma concolor) living in fragmented habitats, males did disperse over substantially larger distances than females (Sweanor et al. 2000), and in a tiger (Panthera tigris) population in Chitwan National Park, Nepal, only males have been observed to cross larger areas of unfavourable habitat (Smith 1993). Even if females occasionally disperse to northern Hesse, our study suggests that these long-distance dispersals are too rare to compensate adverse demographic events (female mortality, failed reproduction).
The decline of the Northern Hessian Subpopulation was further accelerated by the emigration of males from our study area. In several territorial mammals, dispersing individuals establish so-called transient home ranges (Beier 1995; Hinton et al. 2015; Smith 1993; Zimmermann et al. 2005), i.e., home ranges that are used for some time during dispersal, but that are later abandoned in search of a better home range. In Eurasian lynx, such transient home ranges are usually only maintained a few months (Zimmermann et al. 2005). Following Zimmermann et al. (2005), we thus classified both B1025m and B1039m as resident (rather than dispersing or transient) individuals, and their dispersal should consequently be classified as secondary dispersal. In contrast to natal dispersal, secondary dispersal is defined as dispersal occurring after the initial dispersal from the natal territory or social group (Lawson-Handley and Perrin 2007). Secondary dispersal is common in several species of primates (Pusey and Packer 1987), rodents, (Nunes 2007) and carnivores (Waser 1986). The majority of these species, however, are group living, and where males undergo secondary dispersal in these species, they usually do so in the attempt to improve their reproductive opportunities in the target group (Port et al. 2012; Van Horn et al. 2003). By contrast, male secondary dispersal is rare in solitary mammals, and we are unaware of any reports of secondary dispersal in Eurasian lynx (but see Schmidt 1998). We suggest, however, that the same proximate causes responsible for secondary dispersal in group living species, poor reproductive opportunities, might have also triggered B1025m’s and B1039m’s secondary dispersal.
Management recommendations
Population biologists and conservationists have long known that the viability of small animal populations is highly susceptible to stochastic demographic events (e.g., Boyce and Boyce 1988; Shaffer 1981), and our study provides an empirical example for this susceptibility. In addition, the collapse of the northern Hessian subpopulation provides a case study indicating that the expansion of lynx in Germany, and other central European countries might suffer from serious setbacks without active population management. It has already been discussed that founding few large populations might be insufficient to create continuously distributed, demographically and genetically stable lynx populations in central Europe (Kramer-Schadt et al. 2005; Schnidrig et al. 2016; Zimmermann et al. 2007). It has thus been suggested that, in addition to large-scale reintroduction programs, population stepping-stones of only a few individuals should be created to facilitate dispersal of individuals between (sub-)populations (Kramer-Schadt et al. 2011; Linnell et al. 2008; Schnidrig et al. 2016; Thiel-Bender and Heider 2017; Zimmermann et al. 2007). One possibility to create such stepping-stones is the translocation of individuals from established populations, a management action that has successfully been implemented to promote the expansion of lynx in the Alps (Schnidrig et al. 2016). In these cases, individuals of both sexes have been translocated, either to found a new subpopulation (Northeast Switzerland, Ryser et al. 2004) or to reinforce an already existing occurrence of lynx (Kalkalpen National Park, Austria, Schnidrig et al. 2016). We suggest making use of this management option also in Germany or other regions with isolated lynx occurrences. Moreover, our data suggest that in areas already populated by resident males (such as northern Hesse), or regularly visited by male dispersers, the translocation of few females might be sufficient to create a viable stepping-stone, because these females should induce male dispersers to settle in the area, or prevent males already present from leaving the area again (see also: Thiel-Bender and Heider 2017). Once established, genetic exchange between the stepping-stone and the source population, and potentially, other nearby (sub-)populations, may likely be maintained by the natural dispersal of males. We suggest that any translocation of individuals should be accompanied by close monitoring and the commitment to reinforce the stepping-stone if necessary. In this way, a possible collapse of a still growing stepping-stone population (as in northern Hesse) could be prevented, allowing it to grow to a demographically viable population size where further intervention is no longer needed.