Long-term monitoring of rodent and shrew communities in a biodiversity hot-spot in Austria using barn owl (Tyto alba) pellets
Introduction
Rodent and shrew populations are an important component of many ecosystems around the world (Feldhamer, 2015). In the mammalian orders, rodents and shrews are the most diverse; for instance, rodents alone account for over 40% of mammalian species in the world (Barthelmess, 2016; Kay and Hoekstra, 2008; Wilson and Reeder, 2005). These small mammals are characterized by small home ranges, fluctuating population dynamics, high metabolic rates and populations vary from relatively stable to highly cyclic (Krebs, 2013). Rodent and shrew population dynamics are of central importance to the theoretical basis of population biology and regulation (Turchin, 2013). Following the r/K selection theory most rodents and shrews belong to the group of r-selected species (MacArthur and Wilson, 2001). They can reproduce several times per year, have high growth rates and produce many offspring; however, probability of surviving to adulthood is low. Given their high abundance at specific times of the year, they are an important part of the food chain for predators, as in our study, the barn owl (Tyto alba) (Goutner and Alivizatos, 2003; Paspali et al., 2013).
Understanding the mechanisms driving population decline and growth has proven difficult (Batzli, 1992; Krebs, 2013). Depending on the species, rodents can show three to six year cycles of population peaks and crashes (Krebs, 1996; Norrdahl, 1995). The phenomenon of these cycles is still one of the greatest mystery in animal ecology. The amplitudes can be very large, population densities at the peaks can be up to 1000 times higher than at the lows of the cycle (Lomnicki, 1995). Little is known about the factors influencing these cycles, such factors could include: weather, population density, their overwintering success, food availability, dispersal and predator prevalence (Boonstra et al., 1998; Myers, 2018; Radchuk et al., 2016). We need to gain insights into the dynamics of multiple rodent populations in a given area over multiple years, in order to understand the interplay between species abundance and ecological factors (Milana et al., 2016; Previtali et al., 2009). To facilitate such monitoring, researchers have turned to assess species abundance and composition by using the undigested food parts that birds of prey regurgitate in pellets (Obuch et al., 2016; Paspali et al., 2013; Torre et al., 2015). Barn owls are opportunistic feeders; they feed on all small mammals they can subdue (Moysi et al., 2018; Taylor, 2003; Tores et al., 2005). Thus, the results from examining pellets can be used to assess species composition and abundance in an area. In addition, pellets have been shown to be a more effective alternative for estimating a population compared to trapping (Andrade et al., 2016; Heisler et al., 2016; Torre et al., 2004).
In such pellets, beside rodents, another group of small mammals can be found frequently: shrews. They belong to the order of Soricomorpha and the family of Soricidae, in comparison to rodents they have an even higher metabolic demand. The population biology of shrews was subject of numerous studies (Churchfield et al., 1995; Wang and Grimm, 2007), however, despite the efforts, the factors driving their population dynamics are still poorly understood.
Barn owl pellets have been used successfully in several studies to decipher distribution and ecological preferences of rodents and shrews. For example, small mammal diversity seems to be higher in Tuscany compared to North-eastern Italy, which might be related to the extensive agriculture in the northeastern sampling region (Milana et al., 2016). In addition, pellet studies have been used to highlight decreasing small mammal diversity over the course of the last decades (Milana et al., 2018). Furthermore, it has been shown that pellets can also be used to analyze seasonal changes in populations, indicating, at least for southern Albania, that the ratio between rodents and insectivores remained stable across seasons (Paspali et al., 2013). Such data can facilitate identifying newly introduced species and update species distributions (Kiamos et al., 2019). For instance, pellet analysis revealed the unexpected absence of Savi's pine vole (Microtus savii) in southern Tuscany, an otherwise ubiquitous species in Italy (Battisti et al., 2019). In general, pellet data are valuable for evaluating differences in community composition and monitor species distributions and prevalence, i.e. to update red listed species (Heisler et al., 2016).
In the current study we use the pellets of the barn owl to gain insights into the rodent and shrew species composition and dynamics in the area east of Neusiedler See, we also investigated the effects of environmental parameters such as precipitation, temperature and season. We show that such parameters have an impact on rodent and shrew population cycles. Furthermore, we exemplify how rodent and shrew composition and dynamics can be monitored and used for faunistics by examining species remains in barn owl pellets.
Section snippets
The study area
Our study locations (Fig. 1) are all located east of the Neusiedler See (in the region Seewinkel), a lake in Austria, 47° 49′ 4″ N, 16° 44′ 55″ E, 115 m above the Adriatic. Extensive vineyards, meadows, agricultural land, emerging trees and periodic salt lakes dominate this region. The lake itself harbors a gigantic reed belt that builds up its own landscape (Fally, 2010). The combination of such diverse landscape offers an excellent habitat for mice, voles and shrews (Haberl and Kryštufek, 2003
Results
When all included species were analyzed in the same PCA, general trends were similar between Soricidae and Muroidea (Fig. 2A). We further split up the data in Soricidae and Muroidea in order to gain more detailed insights into the species co-occurrence in these two groups.
The PCA results for Muroidea showed a clear positive relationship of all Muroidea species with PC1, which means that all later analyses with PC1 as the dependent variable indicated an increase of Muroidea abundance with
Discussion
We show highly fluctuating rodent and shrew population dynamics, which are correlated with abiotic factors such as temperature and precipitation. Furthermore, we demonstrate that barn owl pellets, a non-invasive and cost-effective sampling method, can be used to quantify population dynamics; including monitoring threatened species and observing the arrival of new species in the area.
Although we have sampled and analyzed a wide array of rodent and shrew species, the general population trends
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We would like to thank Vinzenz Waba, a ranger of the National Park, for collecting barn owl pellets over so many years. We also thank Dr. Barbara Herzig-Straschil, Hans-Martin Berg, Sara-Maria Schnedl and two anonymous reviewers for their helpful comments.
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