Managing Grasslands to Maximize Migratory Shorebird Use and Livestock Production

Abstract

Grasslands are important to domestic and wild animals. Migratory shorebirds are important components of coastal rangeland ecosystems. Buff-breasted Sandpiper (BBSA, Calidris subruficollis) and American golden-plover (AMGP, Pluvialis dominica) are two insectivorous, migrant shorebirds that rely on livestock-grazed grasslands in the Southern Cone of South America during their nonbreeding season, as well as on migration in North America. We studied habitat selection of these species and contrasted their needs with livestock requirements needed to develop recommendations for grazing management that benefit wildlife and livestock production. Short grass height was positively related to BBSA and AMGP abundance, with ideal grass heights from 2 to 5 cm. However, maximum livestock production is associated with grass height over 6 cm. The amount of forest cover, which is used to provide shade to livestock, was negatively related to the occurrence of both shorebird species, likely due to higher risks of predation. Grassland improvement did not affect BBSA but negatively affected AMGP abundance. Short grass habitat was selected by both shorebird species in spite of the higher arthropod biomass in taller grasslands, suggesting that other factors besides food abundance, such as the ability to detect prey and predators, are driving habitat selection. To enhance shorebird (and other wildlife) conservation and livestock production, we recommend managers adjust grazing intensity so that grass height is > 6 cm from mid-February to September, when the Nearctic migrant shorebirds are absent, and from 2 to 5 cm from October to early February when shorebirds are present. These austral summer adjustments should be restricted to paddocks with low forest cover so that livestock production in paddocks with high forest cover remains maximized. All adjustments should be evaluated by each farmer to ensure adequate economic returns are met.

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.