The research team set out to answer a deceptively simple question: what do mammals do with these relict shrub habitats? Relict plants occupy a special position in Earth’s biodiversity. Because they are sedentary and often act as habitat foundations, their populations preserve ecological and evolutionary histories stretching back thousands of years, even through dramatic climatic upheavals such as the glaciations. Yet that same history makes them fragile. Climatic change narrows their ranges, and communities growing at the southern edge of their distribution, or at their altitudinal and climatic limits, are expected to act as early heralds of adverse global warming. Understanding which animals depend on them is therefore central to both conservation biology and climate monitoring.

The two studied habitats are tiny and intra-zonal, surrounded on all sides by the boreal coniferous forests that dominate the Rhodopes, formed mainly by spruce (Picea abies), fir (Abies alba) and Scots pine (Pinus sylvestris). The P. fruticosa thickets lie in the Beglika locality near Batak town at 1510–1530 meters above sea level, covering only about 1.2 hectares on gently inclined mountain slopes. They are dense and closed, with total vegetation cover of 90–100 percent, growing on Humic Cambosols. Nearly 60 species and subspecies of vascular plants share the community, with the families Rosaceae, Poaceae, Asteraceae, Apiaceae and Fabaceae each contributing four to seven species.

The S. salicifolia community occupies a different world within the same mountain belt. It forms dense, almost monodominant thickets along flattened river terraces of the Dospatska River near Dospat town, at 1210–1250 meters above sea level, spanning roughly 4.8 hectares. The soils there are Fluvisols with a thick humus layer and acidic reaction, and the dominant species covers more than 80 percent of the ground, leaving herbaceous plants of low abundance. About 40 vascular plant species belong to the community. The regional climate is typical of the boreal coniferous forest belt: humid, with mean annual temperatures of 7.3 °C at Batak and 8.1 °C at Dospat, and annual precipitation of 912 and 966 millimeters respectively. Both shrub species are listed in Annex 3 of the Bulgarian Biodiversity Act, and their territories have been declared protected sites and included in the Natura 2000 network.

Field surveys ran through the spring–autumn periods of 2021 and 2022. Over areas of about 0.5 to 1 hectare in each habitat, the team deployed Moultrie M-40 camera traps for medium and large mammals, two at the edge of each community, mounted on trees at roughly 50 centimeters height. The sampling effort was identical across habitats, averaging 167 trap-nights per habitat, with 142 in 2021 and 192 in 2022. For small mammals, forty Sherman live traps baited with oat nuts were set per habitat in lines about 10 meters apart. Trapping lasted a single night per shrub habitat to avoid animal disturbance, and every captured animal was released immediately after species determination at the place of capture. Relative abundance was estimated as individuals captured per trap-night, and the researchers also logged direct observations and traces of activity such as tracks.

The camera traps delivered a remarkable cast of characters. In the S. salicifolia community, they recorded roe deer (Capreolus capreolus) and red fox (Vulpes vulpes), both nocturnally and diurnally, along with the red squirrel (Sciurus vulgaris) and the yellow-necked mouse (Apodemus flavicollis) among the rodents. Around the community, tracks of roe deer, red fox, hedgehog (Erinaceus concolor), wild cat (Felis silvestris) and badger (Meles meles) were also documented. In the P. fruticosa thickets, cameras captured roe deer, wild boar (Sus scrofa), red fox, European hare (Lepus europaeus) and even domestic horses (Equus caballus). Most strikingly, two different individual pine martens were photographed in April and May of 2021 and 2022, active both during the night and the day.

The live traps told an equally important story about the small mammal base of these communities. The bank vole (Myodes glareolus) and the yellow-necked mouse represented the small mammal assemblage in both habitats. In the S. salicifolia community, the bank vole showed a relative abundance of 0.18–0.20 individuals per trap-night across the two years, while the yellow-necked mouse was captured only in 2022 at 0.20 individuals per trap-night. In the P. fruticosa habitat, the bank vole reached 0.25 individuals per trap-night, and the yellow-necked mouse 0.075, with the vole captured in both study years and the mouse only in 2021. These figures match the pattern reported by other authors in Central and Eastern European mountain forests, where one or the other of these two rodents dominates depending on habitat conditions and geographic position.

Why do rodents flock to these shrubs? Small mammals form the vital prey base for avian and mammalian predators, and many small vertebrates prefer to forage under dense plant cover where predators struggle to detect them, avoiding open ground or long distances between shelters. Shrub cover is considered to exert a strong influence on the small mammal species and communities that rely on it for safety. The studied communities are especially valuable because they sit between open space and forest, offering a transitional refuge. The data were not sufficient to establish microhabitat segregation between the two rodent species, but in other mountain studies the yellow-necked mouse appears more associated with forest edges, while the bank vole prefers areas within the forest with high tree and shrub cover. In Bulgaria, the bank vole is among the rodent species with the highest abundance in mountainous regions, and the team suggests it could serve as an indicator of climate change. Intriguingly, genetic studies have identified M. glareolus as a key species for understanding how European fauna responded to climate change after the Last Glacial Maximum, an example of a woodland mammal surviving in cryptic glacial refugia north of the Mediterranean.

The pine marten finding carries the greatest conservation weight. Martes martes is a habitat specialist mainly associated with forests, and the researchers assume that the two recorded individuals inhabit the spruce forests surrounding the P. fruticosa community but probably feed on the rodents living in the shrub. In this way, the relict thicket provides the marten with an alternative to the forest food base and hunting ground. The species is listed in the Red Data Book of Bulgaria, volume II, in the category of Endangered species, and in Appendices II and III of the Biological Diversity Law, while the IUCN Red List places it in the Least Concern category globally. It is monitored within Bulgaria’s National Biodiversity Monitoring System, and the study provides distribution data at the national level that could help clarify its habitat specialization.

Other mammals revealed unexpected uses of the thickets. From a camera trap video, the European hare appears to be pulling and chewing a twig of P. fruticosa, suggesting the shrubs serve as food. For herbivores, shrubs can be important food sources and are relatively resistant to grazing thanks to their deep root systems, multiple stems and height, although related Spiraea species are considered negligible in the diet of wild herbivores. Roe deer, red fox and hare were recorded both day and night, indicating the habitats probably also serve them as shelter. One emerging threat, however, is domestic: the horses present in the area could damage the shrubs through trampling, and the authors recommend monitoring and controlling the number of domestic animals around the communities. Both habitats are assessed as critically endangered at the national level, and their limited distribution makes a long-term monitoring program imperative. The researchers argue that mammal diversity monitoring must continue and expand, with more taxa and regions, before it is too late, so that the connection between these Ice Age refuges and the mammals that depend on them is not lost to a warming climate.

Subject of Research: Mammal use of boreal relict shrub habitats of Spiraea salicifolia and Potentilla fruticosa in the Western Rhodope Mountains, Bulgaria

Article Title: The use of boreal relict shrub habitats of willow-leaf meadow sweet ( Spiraea salicifolia ) and shrubby cinquefoil ( Potentilla fruticosa ) in Western Rhodope Mts. by mammal species

Article References: Spasova, V., Simeonovska-Nikolova, D., Dimitrov, K., Petrova, G., Pachedjieva, K., & Tzonev, R. (2023). The use of boreal relict shrub habitats of willow-leaf meadow sweet (Spiraea salicifolia) and shrubby cinquefoil (Potentilla fruticosa) in Western Rhodope Mts. by mammal species. BioRisk, 20, 139-152. https://doi.org/10.3897/biorisk.20.97604

Image Credits: AI Generated

DOI: 10.3897/biorisk.20.97604

Keywords: boreal relicts, pine marten, Rhodope Mountains, small mammals, camera traps, climate change, Natura 2000, conservation, boreal, relict, shrub, habitats