Managing Grasslands to Maximize Migratory Shorebird Use and Livestock Production☆
Introduction
Grasslands are used by both domestic and wild animals throughout the world (Gibson, 2009). Domestic animal husbandry frequently involves substantial modification to grasslands to maximize economic return of livestock production. Such practices may include spatiotemporal alterations of grazing livestock intensity, overseeding native grasslands with exotic grasses and applying fertilizers, burning, converting between row/grain crop agriculture and pastureland, and planting trees to provide shade for livestock, among others (Bailey, 2005). Understanding how these practices, as well as other natural characteristics of grasslands, affect wild animals is necessary for allowing wildlife and livestock managers to promote domestic animal husbandry practices that benefit both wild and domestic animals. Indeed, if done correctly, grassland management can provide positive environmental and economic benefits (e.g., Durant et al., 2008). However, most wildlife biodiversity studies have focused on conservation rather than balancing biodiversity and domestic animal production, constraining the possibilities of benefiting both wild and domestic animals (Neilly et al., 2016).
Migratory shorebirds (Charadriiformes) are important components of the biodiversity within the coastal grassland environment worldwide (Colwell and Dodd, 1997, Navedo et al., 2013). These species require coordinated conservation actions across countries that are part of either breeding, nonbreeding, or migration routes. Two species, the Buff-breasted Sandipiper (BBSA, Calidris subruficollis) and American golden-plover (AMGP, Pluvialis dominica), are insectivorous long-distance migrants that breed in the Arctic tundra in northern North America and overwinter in coastal areas of the Río de la Plata Grasslands (Lanctot et al., 2002, Lanctot et al., 2010, Isacch and Martínez, 2003a, Isacch and Martínez, 2003b, Blanco et al., 2004, Isacch et al., 2005, McCarty et al., 2017). Both species have been declared a global conservation priority due to loss of nonbreeding habitat, historic overhunting during migration, and ongoing population declines (Myers and Myers, 1979, Clay et al., 2010, Blanco et al., 2004, Lanctot et al., 2016, US Shorebird Conservation Plan Partnership, 2016). During the austral spring and summer, BBSA can be found primarily in the eastern flooding grasslands of the Argentinean Pampa, Southeastern Uruguay, and Southern Brazil (Soriano, 1991, Lanctot et al., 2002). The nonbreeding distribution of AMGP matches the area occupied by the BBSA but also includes portions of southern Paraguay (Clay et al., 2010, Johnson et al., 2018). For both species, the highest concentrations of birds appear to be in the coastal grasslands used for livestock production (Lanctot et al., 2002, Clay et al., 2010).
Several studies have shown that BBSAs and AMGPs are associated with intensively grazed, natural grasslands during the austral spring and summer, highlighting the importance of rangelands and cattle management for both species (Lanctot et al., 2002, Isacch and Martínez, 2003a, Blanco et al., 2004, Isacch and Cardoni, 2011). The reasons behind this selection for short grass heights are unknown. A greater understanding of this relationship would help better manage these rangeland ecosystems (McCracken and Bignal, 1998). In other grassland regions, short grass tends to have less arthropod biomass than tall grass (e.g., Gibson et al., 1992, Kruess and Tscharntke, 2002). If this pattern is true in the Southern Cone grasslands, then the short grass must be advantageous to the shorebirds in other ways, perhaps by allowing them to capture arthropod prey easier and/or detect avian predators (e.g., Falcons) earlier than in tall grass (Colwell and Dodd, 1997). Like many shorebird species, BBSAs and AMGPs are usually observed feeding in open areas, apparently so that they can detect predators early enough to escape. Other common features in the Rio de la Plata grasslands are tree belts and plantations that provide shade to livestock (Zalba and Villamil, 2002, Bilenca and Miñarro, 2004). Such plantings are used by avian predators such as Swainson’s Hawks (Buteo swainsoni) and other raptors as roosting and foraging perches (Sarasola and Negro, 2006). Wilson et al. (2014) showed that two grassland shorebird species in Scotland tended to avoid plantation forests, presumably because birds needed a clear view of approaching avian predators. However, the effect of trees on BBSAs and AMGPs has never been assessed on their southern nonbreeding grounds. In addition, it is unknown how shorebirds are affected by seeding of exotic grasses and applying fertilizers in native pastures (a practice called “grassland improvement,” Risso and Berretta, 1997, Ferreira et al., 2011). The limited data available suggest that several grassland insectivore specialists were negatively affected by grassland improvement (Barnett et al., 2004, Azpiroz and Blake, 2009, Fontana et al., 2016), potentially due to a drop in invertebrate biomass associated with an increase in grass blade density (Barnett et al., 2004).
In this paper we address three main questions related to habitat use and livestock production by overwintering BBSAs and AMGPs near Rocha lagoon, Uruguay. First, we assessed whether land use variables such as grass height, forest cover, and grassland type (improved or natural) were related to the presence and abundance of foraging BBSAs and AMGPs over a 7-yr period. Second, we examined the effect of arthropod biomass on the density of BBSAs and AMGPs in short grass paddocks near and away from Rocha lagoon. Third, we studied the relationship between grass height and arthropod biomass to shed light on the mechanisms behind short grass selection by overwintering BBSAs and AMGPs. Based on prior studies and the information provided earlier, we predicted that BBSAs and AMGPs would select short grass pastures, change their use of pastures over time so as to remain in short grass pastures, avoid pastures with vertical structures that might harbor or obstruct the detection of approaching avian predators, use improved pastures less than native pastures, and prefer pastures with more abundant arthropod resources.
Overall, understanding how habitat structure and food abundance affects the presence and abundance of BBSAs and AMGPs is important so that managers can determine what animal husbandry practices are compatible with grassland shorebird use. This knowledge will allow wildlife biologists and rangeland professionals to look for synergy when developing recommended practices for professionals in their respective fields and, in so doing, improve the prospects of these high-priority conservation species (US Shorebird Conservation Plan Partnership, 2016) while maintaining maximum economic return in grazing land management.
Section snippets
Study Site
We conducted our study at two ranches (La Rinconada, 4 500 ha, and La Laguna,1 300 ha) located adjacent to Rocha lagoon, Uruguay. These ranches include habitats representative of the area and use typical animal husbandry practices common in the area. These ranches are important wintering sites for BBSAs (Lanctot et al., 2002, Lanctot et al., 2010) and AMGPs (Clay et al., 2010) within the Rio de la Plata grasslands in southern South America (Fig. 1). Both species are typically present from
Effects of Rangeland Use Conditions on BBSA and AMGP Presence and Abundance
Grass height varied between an average of 3.3 cm and 28.6 cm throughout the 16 paddocks across the 4 survey yr. Buff-breasted Sandpipers (BBSAs) and AMGPs both used mostly short grass paddocks and only occasionally used tall grass paddocks above 10 cm in height (Fig. 2). Our hurdle model showed that grass height was the main driver of BBSA and AMGP probability of occurrence (Table 1, Table 2) and that AMGPs were able to use taller grasslands than BBSA. For example, for a probability of
Discussion
Our results support the tenet that grass height is the main driver behind the overwinter abundance and distribution of BBSAs and AMGPs in Rocha lagoon. The amount of forest cover was less important overall, but either alone or by interacting with grass height affected the presence or abundance of both shorebirds. Further, BBSA and AMGP densities were unrelated to arthropod biomass levels (i.e., both species were present in higher numbers in short grass paddocks with fewer arthropods). These
Implications
Rangeland professionals and wildlife managers must be cognizant of providing recommendations that help wildlife without hurting a rancher’s economic interests. We have shown that very short grass (2–5 cm) is important for these two grassland shorebirds and that small increases in this grass height results in reduced use (although AMGPs are more tolerant of taller grasses). Unfortunately, from the livestock production perspective, this grass height may not be optimal given that livestock gain
Acknowledgments
Héctor Caymaris, Verónica Correa and Iara Stinton helped us in the fieldwork. Celcito and Sandra offered housing and logistical support during fieldwork. Miguel Iendiowkov, Martín Segredo, and Fundación Amigos de las Lagunas Costeras de Rocha provided management information and access to the “La Rinconada” ranch. Enrique Zunini provided management information and access to “La Laguna” ranch. Laura del Puerto helped us in the design and realization of figures and provided valuable comments on an
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This work was supported by the National Fish and Wildlife Foundation and Manomet Center for Conservation Sciences (three Wetlands International projects); Neotropical Migratory Bird Conservation Act, U.S. Fish and Wildlife Service; International Program, U.S. Forest Service; BirdLife International; Southern Cone Grassland Alliance; Centro Universitario Regional del Este, Universidad de la República, Uruguay; Programa de Desarrollo de las Ciencias Básicas (PEDECIBA); and NGO Aves Uruguay.