1. Introduction
Currently, more than 1 million insect species have been described [
1]. They play an important role in ecosystems; for example, as elements of food webs. However, currently, we are observing a drastic decline in insect fauna on a large scale [
2,
3,
4,
5,
6]. There is a need to develop strategies and guidelines to counteract this process, but in order to do so we need to understand the reasons for the observed species decline and the response of insect populations to ecological species-protection measures.
Grazing is regularly used as an ecological method in order to protect or restore special biotope types that are important for species conservation [
7]; for example, protecting sedges and mosses in the Biebrzanski National Park in Poland using cattle and horse grazing [
8] or the recovery of
Calluna heather in The Netherlands [
9]. Carabid beetles are suitable for investigations on insect population trends because they are a species-rich group of insects that are ubiquitous in the majority of terrestrial ecosystems [
3]. They efficiently reflect environmental variation and bear indicator potential at various spatial scales [
10]. Carabid beetles react quickly to management practices in grasslands and forests, including grazing [
11]. Hence, carabid beetles can be useful indicators for the assessment of grazing as a species-conservation measure. Previous studies have shown that the degree of grazing intensity can variously impact insect populations [
12,
13,
14]. Heavy grazing by cattle and horses in Welsh peatland resulted in a strong influence on carabid assemblages whereas light sheep grazing had little impact [
12]. Researchers [
14] who investigated the response of carabid beetles to grazing in Cretan shrublands described them as good indicators of grazing pressure at assemblage level, rather than having direct impacts on specific species. Overgrazing resulted in lower species richness, and species richness and diversity were maximal under moderate to relatively high levels of grazing.
The European bison (
Bison bonasus (L.), also called wisent) is the largest living wild terrestrial animal in Europe. It is still an endangered species that requires comprehensive conservation efforts [
15]. For this reason, the research conducted so far has focused mainly on its biology and threats to species conservation and population development [
16,
17,
18,
19]. In turn, little research has been done that can explain the effect of the European bison on ecosystems [
20,
21]. To our knowledge, no research has been done on the effects of this species on carabid beetles. Because of European bison’s grazing activity, a strong impact on flora and fauna can be assumed. This is important because the population is still growing and new areas are being populated as a result of conservation activities [
22,
23]. Hence, due to increasing grazing pressure, an increasing population also impacts the landscape and populations of individual species, including various insect species. This increasing activity might pose some threats, but it also offers potential for implementing conservation measures through the use of active grazing.
The aim of the study was to assess the impact of European bison grazing on carabid fauna in forest and meadow ecosystems. This was carried out in two variants: (a) based on a free-ranging population (Augustowska Forest) and (b) based on a captive population (enclosures in the Kobiór Forest District and Niepołomice Forest District). Based on these two experiments, we wanted to test the hypotheses that (1) European bison grazing has an impact on the carabid beetle assemblages, and (2) the degree of this impact depends on the intensity of grazing by European bison.
4. Discussion
The rarefaction analyses suggested that the species numbers detected in the study for the individual study sites were sufficiently reliable. Our results did not show a notable influence on numbers of individuals of carabid beetles. However, there is an indication that high-intensity grazing may cause an increase in species numbers. Increased grazing intensity seems to have only a weak impact on species assemblage structure, but it has a stronger impact on the composition of functional traits in assemblages, as demonstrated in particular by the significant impact in the captive herds.
Our study revealed a more pronounced impact of other factors on the composition of the carabid beetle assemblages, which may mask the impact of grazing. Not surprisingly, a main factor was habitat type, but location also had a strong impact on the species assemblage structure. The year of the study did not have a significant impact. Researchers [
42] who have studied the impact of low-intensity cattle grazing detected a higher amount of variance that was explained by habitat and year than by grazing. However, grazing intensity is a notable factor. This result is in accordance with former studies [
12]. Researchers [
43] showed that on heather moorland in northeast Scotland the carabid distribution was strongly influenced by the organic content of soil and the height of
Calluna, but heavy grazing levels had a significant impact on the biomass, height and shoot structure of
Calluna and resulted in a significant change in carabid assemblages. In our study the relative impact of grazing intensity in relation to the other variables also increased with increasing grazing intensity. Mowing affects ecosystems in a similar way and has been proposed as a surrogate for grazing [
13]. It has been shown that mowing intensity is related to the degree of changes in carabid beetle assemblages [
44].
It is worth noting that the observed impact of grazing intensity in our study more strongly affected the composition of carabid functional traits compared to the species assemblage structure. This is an important detail, because functional traits are important with respect to the contribution of the species to ecosystem properties and services and the resilience of the ecosystem. The vulnerability of the ecosystem properties and benefits may be affected by changes in functional traits of the species [
45]. For subalpine grasslands, it was also shown that mammalian grazing not only altered the composition of carabid assemblages, but it also caused functional trait shifts [
46]. In differently managed grasslands in the Alps; however, no significant effect of cattle density on carabid species richness, assemblage structure or functional traits was detected [
47].
It should be noted that a more pronounced effect of the wisent’s influence was demonstrated in the enclosures. In the free-ranging population, despite the fact that meadows (the main food source for European bison) were analyzed, no such effect was found. The reason for this was probably a small herd that was released into the wild, the impact of which was imperceptible. Moreover, the intense grazing on meadows by the free-ranging population does not correspond to the intense grazing in the enclosures, where animal density is not natural. Nevertheless, an effect can also be expected in larger free-ranging populations. European bison do not use the entire area of forest complexes evenly [
48,
49], and herds of cows tend to routinely use some particular parts of the available space. It can, therefore, be speculated that such an effect may be more pronounced where animals congregate. The selection of habitats depends on the location of the population and can vary significantly [
50]. Attention should also be paid to the fact that the use of forest habitats does not necessarily have a nutritional dimension, as European bison may use sites for other purposes, e.g., for rumination [
51]. The structure of stands may also play a role in the intensity of grazing, as has been demonstrated for other grazers [
52,
53]. Therefore, capturing the effect of grazing on carabid fauna may be difficult to achieve.
Because only three European bison populations have been studied for a relatively short time, the validity of the results may be limited. In the Augustowska forest the population was studied in the first year after reintroduction of the bison (second year of grazing) and a longer time of grazing may result in more clear effects. Repeated studies on the study sites as well as additional study sites might help to corroborate our findings. Besides, pitfall traps, as used in our study, contain a bias because results depend on both abundance and activity of the beetles. As a consequence, the results may be skewed towards larger species [
54,
55].
As shown by studying carabid beetle assemblages in the olive agroecosystem [
56], agricultural practices’ interactions with the background of the respective ecosystem is a complex issue. This is also true when using grazing by European bison as a nature conservation measure. In this context, it should also to be considered that grazing may also have a negative impact. For example, studies have indicated that grazing has a negative impact on small mammals [
57]. The effects of grazing on organic content of the soil are also very complex and grazers might be managed differently to help mitigate greenhouse gas emissions [
58]. Therefore, different variables such as timing, duration, cessation and intensity of grazing have to be taken into account, because they greatly modify the structure of the respective grassland [
13]. According to researchers [
59] who studied grazing by Polish Konik horses, grazing intensity should be adjusted annually to balance vegetation development.