The impacts of an invasive herbivore (Camelus dromedaries) on arid zone freshwater pools: An experimental investigation of the effects of dung on macroinvertebrate colonisation
Introduction
Arid environments cover 47% of the earth (Kingsford and Thompson, 2006) and include a number of threatened ecosystems and associated flora and fauna (Byrne et al., 2008, Murphy et al., 2009, Anthelme et al., 2011). Arid waterbodies are considered “islands of water in a sea of dry land” (Faulks et al., 2010) but have been the focus of relatively few ecological studies (Sada et al., 2005, De Los Rios et al., 2010, Kirkman et al., 2012). Given their isolation within an inhospitable arid landscape, waterbodies are analogous to islands (Ward and Blaustein, 1994, March and Bass, 1995, Angeler and Alverez-Cobelas, 2005, Murphy et al., 2009) and in this sense are instructive for studying fundamental ecological processes such as assembly dynamics and the role of refugia (Davis et al., 2013). Waterbodies are particularly useful in assessing the interplay between local conditions, dispersal and colonisation (Jenkins and Buikema, 1998, Thompson and Townsend, 2006, Zickovich and Bohonak, 2007; Jeffries, 2011) and various aspects of Island Biogeography Theory (Ward and Blaustein, 1994, Bohonak and Jenkins, 2003). Invertebrate colonisation of waterbodies has been the focus of many studies because the site scale is relatively discrete, small and easy to comprehend, and the turnover rate of colonists is rapid enough to provide a feasible timeframe for research (Layton and Voshell, 1991, Clifford et al., 1992, Bohonak and Jenkins, 2003, Williams et al., 2008).
Degradation of aquatic ecosystems in arid regions is a major issue because of their biological, social and economic importance (Williams, 2000, Box et al., 2008, Kirkman et al., 2012). For example, springs in arid areas may provide the only reliable source of water (Sada et al., 2005), are geographically isolated (Box et al., 2008) and in many cases support an endemic aquatic fauna (Wilmer and Wilcox, 2007, Murphy et al., 2009). Many aquatic ecosystems in arid Australia are culturally significant to Aboriginal people, who have relied on permanent and temporary water to survive for thousands of years (Bayly, 1999). In central Australia, aquatic habitats provide a scarce and critical resource for flora and fauna (Davis et al., 1993; Box et al., 2008) and like other areas of the world are regarded as “precious jewels of the desert”, a status that reflects their importance to both aquatic and terrestrial biodiversity (Box et al., 2008). It is evident that the conservation of arid zone aquatic ecosystems is critical, particularly in the face of globally developing threats to water resources in arid regions (Vorosmarty et al., 2010).
Water resources in arid regions are under pressure from increasing human populations and the associated requirements of major industries such as mining and agriculture (Murphy et al., 2009). These factors in turn influence multiple and interacting stressors that include salinisation, weed invasion, pollution, aquifer drawdown and unmanaged feral herbivores (Sada et al., 2005, Box et al., 2008, Lindenmayer et al., 2010).
Australia has the largest feral population of dromedary camels (Camelus dromidarius) in the world. Their numbers increased since first being imported from their regions of origin (Afghanistan, Pakistan and northern India) between 1840 and 1907, with numbers reaching an estimated peak population of 1 million in 2008 (Dorges and Heucke, 1996, Edwards et al., 2010, Saalfeld and Edwards, 2010). Invasive herbivores such as the camel are a major problem in central Australia (Edwards et al., 2010). At high densities they have a large negative impact on environmental, cultural and economic resources by damaging infrastructure, vegetation and aquatic habitats (Edwards et al., 2010, Saalfeld and Edwards, 2010). While it is well established that various invasive herbivores pose a threat to aquatic ecosystems, there is a need to focus specifically on the impacts of camels due to their wide distribution and commonly unrestricted access to waterbodies throughout arid Australia. Camels can cause significant damage to waterbodies by trampling vegetation and eroding banks, by depletion of permanent springs, and by fouling through defecation (Brim-Box et al., 2010, Edwards et al., 2010), which can lead to eutrophication, and serious consequences for aquatic fauna (Smith et al., 1999).
Eutrophication, the excessive input of nitrogen and phosphorus into aquatic ecosystems, is one of the major threats to freshwater ecosystems globally (Smith and Schindler, 2009, Kneitel and Lessin, 2010, Teissier et al., 2012). The negative effects of eutrophication include major compositional and functional shifts in ecosystem components, changes in trophic levels, increases in invasive species, and an increased risk of emerging diseases (Smith and Schindler, 2009, Kneitel and Lessin, 2010). Consideration of eutrophication is important because it provides a basis to better understand ecological processes and interactions in seasonal ecosystems, information which is fundamental for effective management (Kneitel and Lessin, 2010).
The objective of this study was to investigate the impacts of camels on the water quality and invertebrate ecology of simulated arid zone waterbodies. We assessed the effects of camel dung on aquatic habitats by measuring changes in: i) physico-chemical variables and phytoplankton; ii) macroinvertebrate richness and abundance; and iii) macroinvertebrate community composition, over time in experimental mesocosms located in the central Australian arid zone.
Section snippets
Experimental mesocosms
Eighteen mesocosms (artificial pools), arranged in three groups of six, approximately 80 m apart, were placed in the Arid Zone Research Institute compound (133° 52′E, 23° 46′S) in Alice Springs, in the Northern Territory, Australia. The plastic 100 L, 1 m × 0.5 m pools were typical (in dimensions and volume) of small arid zone waterbodies. Within each group, three treatment mesocosms (with camel dung added) and three control mesocosms (without dung) were located 5 m apart in an alternating
Physico–chemical parameters
The average air temperature varied greatly over the eight-week experimental period, reaching a maximum of 38.7 °C and average water temperatures (recorded at the time of sampling each week) were high (∼32 °C) (Table 1). Little precipitation (7 mm) was recorded during the same period. Significant differences in conductivity and pH were recorded over time and between the treatment and control mesocosms (Table 2), however, the differences were not large (Table 1). The presence of dung resulted in
Discussion
Aquatic ecosystems are biologically important yet often overlooked features of arid landscapes that are useful for assessing anthropogenic disturbances (Williams, 2000, Box et al., 2008, Patten et al., 2008, Becerra Jurado et al., 2009, Davis et al., 2013). We sought to investigate how the impacts of camel dung affected the invertebrate colonisation of arid zone freshwater pools using experimental mesocosms. We assessed the effect of the addition of camel dung by measuring water quality,
Implications for natural waterbodies
This study provides experimental evidence of the negative impacts of camel dung on arid zone waterbodies. The effects, and interactions, of dung and water quality are likely to be exacerbated in natural pools, particularly in dry summers. As the landscape dries, aquatic sites evaporate and shrink in volume, which reduces habitat and concentrates solutes, especially nutrients (Williams, 2006). The deposition of dung is non-random at the landscape scale as feral camels move in from dry areas and
Acknowledgements
This work was supported by a National Climate Change Adaptation Research Facility (NCCARF) and National Water Commission (NWC) co-funded grant (FW1106) and an Australian Research Council (ARC) Discovery Project grant (DP120103010).
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