AUTHORS: Nazareno Sobral Zotta, Lucía I. Rodriguez Planes, Gabriela Nicosia, Laboratory of Eco-Epidemiology, Department of Ecology, Genetics and Evolution, Universidad de Buenos Aires-IEGEBA (CONICET-UBA); Aristóbulo Maranta, Parque Nacional El Palmar, Administración de Parques Nacionales; Ricardo E. Gürtler, Laboratory of Eco-Epidemiology, Department of Ecology, Genetics and Evolution, Universidad de Buenos Aires-IEGEBA (CONICET-UBA)
ABSTRACT: Exotic ungulates play a major role in the current biodiversity crisis [1]. Axis deer (Axis axis) are expanding its range and causing increasing concern in multiple regions, though its negative impacts have only been described in a few places [e.g. 2]. A long-term management program of wild boar (Sus scrofa) and axis deer in a protected area in north-eastern Argentina (El Palmar National Park, covering 8,500 ha) documented an unexpected increasing trend in deer numbers despite lethal removal through controlled still shooting performed by recreational hunters over a 12-year period [3,4]. In order to assess the effects of the overabundant axis deer population and contribute to the park’s management goals, our ongoing work there seeks to describe deer habitat use and the prevalence of tree damage by fraying and bark-stripping across a gradient of distance to water courses and associated refuge availability. We used a systematic sampling design consisting of 28 equidistant strip transects measuring 1000 x 1 m, each one subdivided in three blocks (200 x 1 m), deployed across the park’s northern zone (approximately half the park’s area) open for public use. On each block we counted deer fecal pellet groups, trees and woody shrubs, and recorded for each individual its diameter at breast height (DBH), presence and type of damage, and wound area [5]. At each block we recorded a set of environmental variables every 50 m: degree of invasion by exotic trees, overstory and understory cover, average height of grasses, and type of habitat. Deer fecal pellet groups were found in all transects and habitats, and at most blocks (73 of 81, 90.1%), but its spatial distribution was not uniform. The density of deer pellets steadily increased starting at 4 km from the Uruguay River (eastern limit of the park) up to the coast (Figure 1). Deer pellet groups reached maximal density in the riverine forest, which is densely invaded by seven exotic species of trees and shrubs, and is probably key for deer activities [6]. We registered 444 trees or woody shrubs of which 113 (25.5%) presented some type of debarking attributable to axis deer (Figure 2). Among native trees with more than 10 individuals examined, the most frequently affected species were Myrcianthes cisplatensis (55.0%), Allophylus edulis (34.3%), Acacia caven (8.7%), and other 2 low-density species (Cordia boissieri and Daphnopsis racemosa); the relative frequency of affected specimens differed highly significantly among these species (p < 0.001). We detected five types of debarking affecting 9 species in a widely variable manner. The exotic shrub Pyracantha atalantoides was frequently (39.3%) affected with signs attributable to axis deer, presenting bark stripping by feeding in 83% of the specimens showing some damage. Regarding the severity of damage, nearly half (48.6%) of all individuals with fraying damage occurred in specimens with DBH values below 9 cm, potentially causing lethal effects. Girdling (the most severe type of damage) occurred very frequently in M. cisplatensis (22% of specimens displaying any sign of damage), a relatively common native tree species. Deer presence through the park reflected in a spectrum of levels of damage to several native trees and exotics woody species. Our preliminary results suggest a positive association between axis deer habitat use and the increasing expansion of exotic trees and woody shrubs. Additional studies are needed to assess deer habitat use over different seasons in the context of culling actions, and their impacts on forest regeneration and trees growth rates.
