Original articleLow habitat overlap at landscape scale between wild camelids and feral donkeys in the Chilean desert
Graphical abstract
Introduction
The spread of alien species as an outcome of human activities and their naturalisation in ecosystems is regarded as one of the greatest threats to biodiversity. Such introduced species may alter the ecosystems through such processes as competition, predation, the spread of disease or the alteration of the physical environment (Meffe and Carroll, 1997, Primack, 1998). As a result, some native species may decline, sometimes to the extent of local or global extinction. The first species to disappear are frequently specialists and those that occur at low population densities (Davies et al., 2004). A cascade of effects is frequently set in train afterwards via networks of interspecific interactions, with a variety of often poorly predictable collateral consequences (Traveset and Richardson, 2006). By this means diverse ecosystems see the replacement of indigenous species by those that are more tolerant of human activities, leading towards the biotic homogeneity that is currently one of the principal concerns of conservation biology (Olden et al., 2004, Olden, 2006).
The raising of grazing livestock is among the human-led processes that most modify ecosystems, on account of its extent and the variety of the effects generated. In the first instance, the primary objective of livestock introduction is to channel primary productivity towards human consumption, which invariably involves the occupation of the habitats of wild herbivores and a reduction of the resources available to the latter (Bagchi et al., 2004, Suryawanshi et al., 2010). Furthermore, there are deliberate human influences on livestock-grazed ecosystems that may modify vegetation to facilitate grazing (e.g. use of fire) and that may involve predator control or the forced displacement of wild herbivores from their preferred habitats in order to minimize competition (Michalski et al., 2006, Kissui, 2008).
Arid and semi-arid zones represent an extreme case of the use of ecosystems for extensive livestock raising and of potential for competition between native and exotic ungulates. Given the lack of water, such ecosystems are only exploitable for agriculture or intensive livestock raising in the vicinity of the few rivers and they are most frequently exploited for low-density grazing over extensive areas. However, the scarcity of resources also has an impact on wild herbivore populations, increasing the likelihood of adverse effects of competition with domestic herbivores (du Toit and Cumming, 1999, see however Homewood et al., 2001). This circumstance is exacerbated by the interannual variability of rainfall and availability of forage, given that herders will do their best to feed and water their animals when resources are scarce. As a result, the wild herbivore populations of arid zones may suffer greater deprivation during scarcity periods and may display fluctuations in abundance that are greater than those characteristic of their natural population dynamics (Marshal et al., 2008). The establishment of feral populations of livestock species is another collateral effect of extensive grazing in arid zones, arising from the minimal management of livestock and because such feral populations may represent a supplementary resource to humans during periods of scarcity.
The donkey is a paradigmatic animal in this context within the ecosystems of arid zones, but little is known about its effects on wild herbivores and their shared ecosystem. The donkey is a native of arid regions from Africa that has successfully colonised the American and Australian deserts following its introduction there by humans as a pack animal. It was introduced to the Americas in the 16th century and seems to have become established in the wild during the 19th century (McKnight, 1958, Grinder et al., 2006). From the human perspective, the donkey is indispensible as a pack animal in many arid regions and it is often the preferred choice given its resistance to adversity, its low forage requirements and its tolerance of water shortage (Smith and Pearson, 2005). In addition, its feral populations are often exploited by local people, both as a source of pack animals and for food in times of need (Attum and Mahmoud, 2012). The ecological effects of these donkeys are not well known but they may compete with other herbivores for food, water and shade (Choquenot, 1991, Marshal et al., 2008, Suryawanshi et al., 2010, Attum and Mahmoud, 2012), transmit parasites (Ferede et al., 2010) or damage vegetation (Abella, 2008, Malo et al., 2011). However, in some cases the donkeys show sufficient habitat segregation from native herbivores for the possibility of interspecific competition to be discarded (Marshal et al., 2012).
Populations of feral donkeys are relatively frequent in the deserts and semi-deserts of South America, from lowlands up to altitudes of nearly 4000 m (Iriarte, 2007), although they have received little attention. They coexist with wild camelids: the guanaco Lama guanicoe and the vicuña Vicugna vicugna, with which they may compete for food (Borgnia et al., 2008, Reus et al., 2014, Wurstten et al., 2014). The principal guanaco populations are on the steppes of Patagonia and Tierra del Fuego, and there are only small and fluctuating populations in desert areas (Baldi et al., 2008). Some of these latter occur at such low densities as to promote hybridisation with their domestic congener, the llama Lama glama (Kadwell et al., 2001). The vicuña, in contrast, inhabits semi-desert regions above 3700 m where it depends on montane meadows and swampy habitats -vegas (Franklin, 2011). The vicuña was in danger of extinction in the 1960s but its populations on the northern Chilean altiplano have increased and now they fluctuate in relation to plant productivity and other local environmental conditions (Lichtenstein et al., 2008, Shaw et al., 2012). Poaching and competition with livestock are regarded as significant threats to both species despite they are listed as of Least Concern by IUCN (Baldi et al., 2008, Lichtenstein et al., 2008). There are few studies offering parallel data on feral donkeys and camelids and all of them have been from areas where donkeys are scarce. Such studies note that donkeys may compete with camelids for food but that they show some degree of difference in habitat selection at the landscape scale (Ovejero et al., 2011, Acebes et al., 2012).
In this context our principal objective is to evaluate potential competition between feral donkeys and wild camelids in desert and semi-desert areas of the Atacama Desert by examining their overlap or segregation in habitat use at the landscape scale. It is expected that both donkeys and wild herbivores will coincide within the most productive habitats, given the low productivity of the region, and in the areas of coincidence it is expected that wild herbivores will shift towards less preferred habitats where their fitness could be reduced. Conversely, if habitat selection by donkeys and wild camelids was very different, competition for resources due to habitat overlap could not arise and negative effects on fitness would not be expected. As secondary objectives of the paper we present data on abundance and habitat selection by donkeys and guanaco/llama hybrids in a South American desert, given the existing lack of such information.
Section snippets
Study area
The study area embraces all land above 2500 m a.s.l. in the Tarapacá region, Chile, an area of approximately 1,680,790 ha. Tarapacá is in the heart of the Atacama Desert, with areas in which precipitation is virtually absent and too unpredictable to allow the establishment of ungulate populations. Nevertheless, the altiplano and pre-cordillera areas here receive annual rains of 10–200 mm derived from humid tropical air that crosses the Andes during the Austral summer (Moreira-Muñoz, 2011). As a
Habitat selection by ungulates
In total, 331 animal groups were detected during the course of the two sampling periods with very similar features in both seasons. In all, they comprised 221 sightings of vicuñas (average of 515 individuals by season), 77 of donkeys (161.5 individuals by season), 25 of guanacos (28 individuals by season) and 8 of guanaco x llama hybrids (20 individuals by season).
The multivariate GDA analysis permitted differentiation between the two types of observation (ungulate species and controls)
Discussion
The results show that habitat selection by camelids differs from that of wild donkeys and support the absence of both habitat displacement away from preferred sites and a decrease in herd size or reproductive output in camelid groups living close to donkeys. These results point to a low potential level of competition of donkey with both camelid species although local negative impacts on camelid populations cannot be ruled out. The value of this finding is reinforced by the large extent of the
Conclusion
In conclusion, all the large herbivores of the area actively select their habitat and the donkey occupies an intermediate position in habitat selection within the camelid community that makes it unlikely that intense competition for food exists between them. Such competition could nevertheless occur locally and a definitive conclusion in such cases would depend on a comparative analysis of the herbivores' diet, and on a fuller understanding of the factors limiting local populations of the wild
Authors' contribution
JEM, BAG and CFE conceived and designed the study; JEM, BAG, AV, DSD and NF carried out fieldwork; JEM, BAG DSD and CME did the analysis and built up the GIS. All authors contributed to manuscript writing and correction.
Acknowledgements
We thank the Servicio Agrícola Ganadero (SAG) of Tarapacá and the Corporación Nacional Forestal for their institutional assistance, and especially Vinko Malinarich and Jorge Valenzuela. Ignacia Nuñez, Inao Vasquez, Daniel Valencia, Oscar Chacón and Claudia López assisted with the fieldwork, and Paz Acuña and Jaime Hernández (GEP, Universidad de Chile) with GIS building. This paper forms part of the dissemination material from the project “Diagnóstico de la ecología poblacional de los ungulados
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