Biedma, L. , Román, J. , Godoy, J.A. and Calzada, J. (2019), Using owl pellets to infer habitat associations and clarify the regional distribution of a cryptic shrew. Journal of Zoology, vol. 308, pp. 139-148. https://doi.org/10.1111/jzo.12660
It is obvious that a species cannot be conserved if its distribution is unknown. An accurate delimitation of the distribution of a species is also key to understanding the ecological factors that determine its survival. Unfortunately, the geographical distributions of the majority of species remain poorly understood or contain many information gaps, a problem that has been called the Wallacean shortfall.
Getting accurate knowledge of their distribution is specially challenging for rare and cryptic species, which is unfortunate because rare species are prone to extinction, and the number of species becoming rare is increasing. The low abundances and/or small distribution ranges that characterize rare species demand huge sampling efforts by expert field scientists, and the possibility of obtaining false negatives (not detecting the species when it is present) is always a big concern. These difficulties are even greater when such species are sympatric with morphologically similar species which can lead to identification errors.
In this paper we show how the analysis of owl pellets, a common non-invasive method for studying small mammals, can overcome many of the difficulties associated with the detection of rare and morphologically similar species. For instance, owl pellets allow the detection of more species than other standard methods of sampling small mammals. Moreover, owl pellets can often be collected with relatively low effort, which is especially important when the sampling is carried out over large geographical areas. Furthermore, most survey methods are incapable of reliably distinguishing morphologically similar species. By contrast, owl pellet analysis allows the identification of morphologically similar species based on the distinctive osteological characteristics of the species. Owl pellet analysis is also more cost-effective a technique than other small mammal survey methods wherever owl roosts are abundant and well distributed in the target area.
In this study, we used barn owl (Tyto alba) pellets to study potential habitat associations of the lesser white-toothed shrew (Crocidura suaveolens) in the Gulf of Cádiz, southwestern Iberia, and to clarify the distribution of the species in this region. In the Gulf of Cádiz, C. suaveolens is a rare species that lives in sympatry with its congener, the greater white-toothed shrew (Crocidura russula), a much more abundant and extremely similar species. Before carrying out this study, the distribution of C. suaveolens in the Gulf of Cádiz had never been adequately clarified and was a controversial issue due to doubtful records of the species throughout the region.
The results obtained were striking and unexpected. Through several indirect lines of evidence we inferred a possible association of C. suaveolens with tidal marsh habitat. This was a very surprising finding since C. suaveolens is a habitat generalist throughout its global distribution range. In addition, although there are eleven tidal marshes in the Gulf of Cádiz, C. suaveolens presented a highly restricted distribution, occurring only in four of them.
Based on these results, two questions arose: why is C. suaveolens associated with tidal marshes in the Gulf of Cádiz? And why is C. suaveolens only present in some tidal marshes of the region and not all of them? The causes of the limited distribution of C. suaveolens in the Gulf of Cádiz remain poorly understood to date, but our results were consistent with the hypothesis that C. suaveolens was displaced from tidal marshes through competitive exclusion by C. russula. However, this hypothesis of competitive displacement was not sufficient to explain the absence of C. suaveolens in many of the tidal marshes of the region. Our hypothesis needed to be completed.
So, we argue that the geomorphological history of the tidal marshes of the Gulf of Cádiz may also explain the current regional distribution of C. suaveolens and the phylogeographical patterns recently described in the region. Basically, C. suaveolens occupied the tidal marshes that existed at the time of the arrival of C. russula to the Iberian Peninsula from Africa, and has been unable to colonize more recent marshes, probably because of the unhospitable landscape already colonized by C. russula that separates them. Finally, some conservation implications for C. suaveolens are exposed. In this regard, our study constitutes an important first step for the conservation of C. suaveolens in the Gulf of Cádiz by filling the knowledge gap on its current distribution.
This study demonstrates the usefulness of a simple and effective tool to assess the distribution of cryptic small mammals, where new populations could remain undetected due to their rarity and morphological similarity with other species. In this context, owls proved to be really inexpensive, dedicated and efficient field technicians for sampling small mammals.