Estimation and prediction of population sizes have been a challenge and a must of many ecological studies (Freckleton et al. 2006). There have been some endeavours to determine population growth, density regulation, and environmental variability (Hovestadt and Nowicki 2008). Moreover, in a world dominated by the need to protect vulnerable species (Crandall et al. 1999), the estimation of the population/species abundance became an important step towards the correct assessment of the need for rare species conservation.

Although the total number of cave species has been estimated to 50, 000-100, 000 (Culver and Holsinger 1992), few studies deal with estimations of cave populations and few mark-recapture experiments have been undertaken until present (Cabidoche 1966, Keith 1975, Peck 1975, Delay 1978, Hobbs 1978, Racovitza 1980, Hobbs 1981, Carchini et al. 1982, Tercafs and Brouwir 1991, Rusdea 1992, Carchini et al. 1994, Bernardini et al. 1996, Simon 1997, Knapp and Fong 1999, Willemart and Gnaspini 2004, Cooper and Cooper 2009, Venarsky et al. 2012). The aims of these studies were to census subterranean populations and to date the longevity of cave species. In most of them, seasonal variability has been documented and different explanations were proposed.

The use of the mark-recapture methods to evaluate subterranean beetle populations is problematic due to the difficult access to populations, and both long life and low reproduction rates of their representatives (Delay 1978). The estimated population of Aphaenops loubensis Jeannel, 1953 in one station of the Pierre-Saint Martin system (France) was of 100 to 400 individuals (Cabidoche 1966, Delay 1978). Speonomus infernus (Dieck, 1869) from theSaint Catherine Cave (France) has been estimated in one of the cave chambers at 300 individuals (Juberthie 1969, Delay 1978). Delay (1978) estimated the population of Speonomus longicornis (Saulcy, 1872) fromthe Pigailh Cave (France) at 50, 000 individuals. In Romania, the number of Pholeuon proserpinae glaciale Jeannel, 1923 in the Scărişoara Ice Cave was estimated at 33, 000 individuals (Racovitza 1980).

The aim of our study was to analyse the patterns of distribution for a beetle population inside one cave and in correlation to topoclimate variations, and to estimate the cave population size by using the mark-resight method. The results on both population size and dispersal behaviour of a subterranean beetle population are the starting point in further studies about the impact of the touristic infrastructure on cave inhabitants.

We chose Drimetous s. str. viehmanni (Ienistea, 1955) (Coleoptera, Leiodidae, Leptodirini) from the cave Peştera cu Apă din Valea Leşului for the mark-resight experiment. Drimeotus s. str. endemic for the Pădurea Craiului Mountains (North-Western Romania) is represented by 14 species most of them endemic for one cave system or a complex of caves in one valley (Moldovan 2000). Drimeotus viehmanni is abundant inside the cave Peştera cu Apă din Valea Leşului all year round, with variations from summer to winter months (Racoviţă and Şerban 1975). Previous researches found no correlation between population dynamics and the cave climate.

The temperature in the studied cave oscillated from 8 to 9.4°C, higher than during the period 1972–1974 (Racoviţă and Cocean 1977), when the temperature variations were between 7.6 and 8.4°C. This might suggest a warming of the cave climate with a possible ecological effect on the invertebrate populations from the Peştera cu Apă din Valea Leşului. The warming inside the cave follows the general raise of surface temperature in the last decades, knowing the fact that the mean cave temperature in deep zones is equal to the mean annual temperature at the surface.

The observed number of Drimeotus viehmanni was much higher during our study than during the period 1972–1974. In the previous study, Racoviţă and Şerban (1975) recorded the maximum of 410 individuals in the corresponding station IV, and 150 individuals in the corresponding station V. In our study, the maximum abundances were of 600 individuals at station IV, and of 1100 individuals at station V. At this stage of our research we are not able to provide a reliable explanation for such an increase. It can be due to the total growth of the population of Drimeotus viehmanni in the entire system of Peştera cu Apă din Valea Leşului due to the natural dynamics of the species, or it can be the result of increased migrations from the surface habitats caused by environmental changes in the general context of climate warming. Another possible explanation lies in the changes of the surface, intensively deforested in the last two decades, with a consequent drop of humidity in soil/subsurface habitats.

The fluctuation of the number of individuals presented the same seasonality as in the cases of Pigailh Cave (Delay 1978), Sainte Catherine Cave (Juberthie 1969), Scărişoara Ice Cave (Racovitza 1980) and previous study of the Peştera cu Apă din Valea Leşului (Racoviţă and Şerban 1975). A much higher number of individuals was recorded during the summer months compared to the winter ones. Drimeotus viehmanni is not influenced by temperature and air relative humidity because these parameters showed no significant difference between the two seasons in the same station, as was also suggested by Racovitza (1980). This author also suggested that the reproduction cycle cannot entirely explain the seasonal pattern of abundance. Therefore, we propose here another explanation that is the impact of surface seasons on the migrations between the voids network (including the cave) and the subsurface habitats.Colder months have high precipitation rates thus creating suitable conditions also in subsurface habitats, while during warmer months superficial habitats are drier and populations migrate towards deep habitats. Drimetous is known as a versatile genus and highly mobile in the entire subterranean environment, able to survive in subsurface humid habitats as well as in deep caves, probably preferring superficial habitats in wetter conditions.

The estimated population size presents high variability (between 5, 084 and 147, 764 individuals at station II and between 13, 845 and 533, 033 individuals at station III) inferred from the small number of recaptured beetles. The low resolution of this estimation is similar to that observed for Speonomus and Pholeuon representatives (Cabidoche 1966, Juberthie 1969, Delay 1978, Racovitza 1980). By considering the mean value for the entire cave population, we can estimate an abundance of 122, 464 individuals, with a lower number in station II (45, 135 individuals, similar to the estimation for other populations of Leptodirini; Delay 1978, Racovitza 1980), and higher in station III (199, 794 individuals). The mark-resight method is subjected to high errors and the correct method would be to mark the individuals on every occasion they are counted, but this might be of negative impact on the survival of individuals. There are some assumptions of the simple Lincoln-Petersen mark-recapture (resight) methods: (1) the subset of selected population is representative of the entire population in terms of sighting probabilities; this is the fundamental assumption of the mark-resight method (McClintock 2013). It is impossible to know the population distribution in the entire subterranean system and to be sure that the estimation of the population size is due only to the cave population or it is also the contribution of other populations that live in the system. Given the high number we obtained it probably reflects the size of the entire population at the level of the entire system. It must be also emphasized that Peştera cu Apă din Valea Leşului is a special case among Romanian caves for the extremely abundant population inside the cave; (2) the population is close to addition and deletion; (3) marks are not lost or overlooked; (4) there are equal chances of “catchability” of animals. We assume that the number of births equals the number of deaths in this population. The finding of marked individuals 2 years after the marking can be the proof of a lasting paint and of survival of the individuals.

The metapopulation concept provides explanations for how species can survive in fragmented landscapes, such as subterranean habitats (Moldovan et al. 2012). Movement of individuals between spatially separated populations ensures the survival of the entire metapopulation and re-colonisation of the patches where local populations have gone extinct (Hanski and Gilpin 1997, Hanski 2001). Animal movement and the factors that can affect it must be considered for the conservation of highly fragmented populations and management of show caves. The number of migrants between habitat patches is expected to decrease with increasing patch isolation because of the dilution effect associated with the spread of individuals in space (Ims 1995), the mortality of individuals in time (Hanski 1999) and the limited dispersal ability of the beetles (Baur et al. 2005). The obtained results suggest that the beetles from the Peştera cu Apă din Valea Leşului moved between neighbouring patches on a distance of 10 m over the same amount of time as on distances longer than 200 m. The subterranean stream was not a barrier for beetles and proves the connection between cave sub-populations of Drimeotus viehmanni. Individuals of this species are able to cover large areas in long periods of time. The season of dispersal is in winter and migration of the few cave beetles indicates a behaviour which is not continuous but triggered by environmental features and involves only a negligible part of the population.

Although dispersal is critical for the population persistence, particularly for small populations (Den Boer 1979) and in fragmented habitats (Tilman et al. 1994, 1997, Hanski 1999), it seems that cave beetle populations are stenotopic if there are no environmental constraints. If they find a proper place with more or less continuous input of nutrients, migrations are occasional. The differences between patches in a heterogeneous system such as the subterranean environment could be explained by differences in habitat quality, by presence/absence of other species, and by patch size. In the case of the studied population of Drimeotus viehmanni, whith individuals that are always concentrated in few sectors of the cave, the differences can be given by the points of nutrients input. The patches where this cave species is extremely numerous are near natural narrow pits with contact with the surface (station III), where diffuse flow of water from the surface brings food underground.

Data on dispersal of subterranean beetles are rare, despite the theoretical and practical importance for the conservation of these highly endemic taxa. Knowledge of the movement rules can predict the dispersal capacity and patchwise migration rates in complex landscapes (Baur et al. 2005), such as subterranean environments. At local scales, the lack of migration of subterranean populations would isolate some of them and processes of speciation would occur. However, such processes are known to occur only at higher spatial levels of the subterranean heterogeneous environment. Subterranean populations can persist only if the dispersal rate is sufficient to maintain genetic variability against random genetic drift (Baur et al. 2005). Genetic markers may confirm the described pattern for cave beetles and further researches should concentrate on such studies.