Chapter 7. Synthesis and Discussion 318 CHAPTER 7. SYNTHESIS AND DISCUSSION 7.1 Introduction In Eurasia, the tahki (Equus ferus przewalskii) and descendants of the tarpan (Equus ferus ferus) (e.g. Konik polski breed) are being bred and re-introduced as ‘umbrella’ conservation species to play the role of ‘ecosystem engineers’ under the World Wide Fund Large Herbivore Initiative (LHI) (van Dierendonck and de Vries 1996; Bokdam et al. 2002; Michel 2008). The term ecosystem engineer is used to describe organisms that modify, create or define habitats by altering the habitat’s physical properties (Jones et al. 1994; Berke 2010). In the context of wetlands, peatlands and abandoned agricultural lands in Europe, the potential for horses to create or engineer richly structured landscapes is an efficient means of conserving and restoring biodiversity in entire ecosystems and providing habitat for declining bird species (Bunzel-Drűke 2001; Vulink 2001; Loucougaray et al. 2004). On continents with a long but discontinuous evolutionary history of horses, such as North America, opinions as to the conservation role of horses and other ungulates is controversial. In an initiative similar to the LHI, a group of conservation biologists proposed to populate western North America with African and Asian megafauna (termed Pleistocene re-wilding), including horses, lions, elephants, cheetahs, and camels, to restore some of the species assemblage lost (including equids) from North America during the Pleistocene extinctions (Donlan 2005; Donlan et al. 2006; Rubenstein et al. 2006; Caro 2007; Donlan 2007). However, in North America feral horses have also been identified as ecosystem engineers (e.g. Ostermann-Kelm et al. 2009), but with adverse effects on the biotic integrity of habitats and posing a threat to endangered or native plant and animal communities (Rogers 1994; Beever and Brussard 2000a, 2004; Beever and Herrick 2006; Ostermann-Kelm et al. 2009). In GFRNP, feral horses are suspected of causing significant environmental damage, and several reports have provided some preliminary evidence to that effect. Reports were limited by the scope of ecological attributes monitored and the spatial and temporal scale at which studies were conducted (Taylor 1995; Andreoni 1998; Jarman et al. 2003; Schott 2003). The environmental impact of horses in temperate and subhumid grasslands, with the exception of studies from Argentina (Zalba and Cozzani 2004; Loydi and Zalba 2009; Alejandro et al. 2010), has not been well documented. Thus, the adverse effects of feral horses as ecosystem engineers reported in the literature was evaluated in this thesis in Chapter 7. Synthesis and Discussion 319 respect to temperate–subtropical grassy woodland–open forest communities in GFRNP, where horse removal in the past has proven highly controversial. The potential for horses to influence the spatial distribution of macropods and supplant eastern grey kangaroos (Macropus giganteus) from their grazing niche is specific to Australia and had not been addressed prior to this study. The objectives of this chapter are to: 1) summarise the important results of each study chapter 2) provide a synthesis of all chapters in relation to the major questions posed by the literature 3) evaluate the outcomes of this research in the context of general theories pertaining to the effects of ungulate grazing 4) present the implications for management of feral horses in GFRNP arising from this research 5) provide recommendations for future research. 7.2 Summary of important results 7.2.1 Chapter 3: indirect effect of feral horses on small-scale habitat use of macropods This was the first study to monitor macropod dung in conjunction with the manipulated abundance of feral horses, thereby establishing a link between changes in macropod activity (effect) and horse removal (cause). Permanent dung transects were established in a 100 ha area around exclosure sites. At four of six sites, horse dung declined with progressive horse removals. A measure of horse abundance derived from the NPWS feral horse trap and removal program database confirmed resident horses, with stable home range and core use areas, were no longer present at the four sites. In the absence of feral horses, macropod dung increased after 4 months, possibly in less time at one site. The inverse relationship between horse and macropod dung was strongest in grazing habitat types preferred by both species according to the literature. Macropod dung was not present at the one site where feral horse dung and the number of horses trapped did not progressively decline, nor at the site directly adjacent where horse dung did not decline to nominal levels until the final sampling time. The majority (over 90%) of macropod dung was eastern grey kangaroo. Eastern grey kangaroos evidently altered their small-scale habitat use on Paddys Plateau in response to either the presence of feral horses or their dung or as a Chapter 7. Synthesis and Discussion 320 result of horse-induced changes to aspects of habitat quality, such as the removal of biomass. Feral horses may have also displaced macropods at a landscape scale from gorge country catchments. Macropods were absent from the catchment with the highest horse density, Bobs Creek, as essentially no dung was recorded on transects in a 4 × 1-km area centred on exclosures or at each of the ten exclosure sites along Bobs Creek and very little in the landscape-scale survey. Horse counts have historically been lower in Pargo Creek, where macropod dung was greater on spurs than hillslopes. The topography of spurs allows easier movement than hillslopes, and horses possibly deterred macropods from feeding in gorge catchments, restricting their use to travelling along spurs between non-horse catchments. Macropod dung was substantially greater and present on all gradsects in environmentally equivalent locations in catchments with no known population of feral horses since the 1920s. 7.2.2 Chapter 4: impact of feral horses on the groundstorey vegetation of a temperate grassy woodland–open forest plateau The exclosure design had the potential to compare plots grazed by horses (controls), plots grazed by macropods but not horses (horse exclosures), and ungrazed plots (complete exclusion). The removal of 241 horses prior to and over the course of the experiment weakened the ability of the study to detect the effect of horses on some plant variables. However, the removal of horses and results of the exclosure treatments further clarified the relationship between horses and eastern grey kangaroos. In the first year, horse dung in control plots and the number of horses trapped suggested that horses were relatively more abundant than at subsequent sampling times, whereas macropod dung was rare. Both the horse exclosure and complete exclusion treatments resulted in an increase in biomass and cover of sedges, and a reduction in the cover of bare ground. Horses were also associated with reductions in plant cover and concurrent increase in litter cover in addition to cover of bare ground. After the first year, horse dung decreased significantly and continued to decline at all but one site. Macropod dung progressively increased in controls and horse exclosures, coinciding with greater biomass under complete exclusion only. In the absence of horses, eastern grey kangaroos appeared to replace horses as the dominant consumer of herbaceous biomass. Reductions in kangaroo grass (Themeda australis) biomass and reproductive output also corresponded to greater counts of macropod dung in controls and horse exclosures, kangaroo grass being Chapter 7. Synthesis and Discussion 321 highly preferred by eastern grey kangaroos. Evidence was sufficient to conclude that after horses were removed, eastern grey kangaroos became the dominant grazer and interacted with site specific characteristics to regulate the accumulation of groundstorey vegetation biomass and the reproductive output of the competitive tall-tussock kangaroo grass. Exotic species richness declined under complete exclusion, but changes in all other richness measures were not related to grazing treatments. Richness measures, except shrub and tree richness, were greater than baseline through the course of the experiment, suggesting richness increased after horses were removed. The floristic composition of the groundstorey vegetation matched the putative pre-European conditions. Exotic species richness was notably low (<6.5% of total species richness) at the start of the experiment and declined thereafter, and exotic cover did not exceed 3.0% per exclosure quadrat at any sampling time. Most grazing lawns were dominated by kangaroo grass. Grassy woodland with kangaroo grass dominant in the groundstorey vegetation on Paddys Plateau in GFRNP has resisted more than a 100 years of livestock and horse grazing, suggesting that the intensity of livestock grazing over this period has been comparatively light. However, the general increase in plant cover and species richness measures, the progressive establishment of tussocky poa in sites in which it was absent at the start of the experiment due to historical higher cattle and horse grazing, and the lesser baseline cover of kangaroo grass in those same sites suggested horses contributed to minor retrogressive shifts in composition that were not detected in this experiment due to the reduction in horse grazing pressure. 7.2.3 Chapter 5: impact of feral horses on the groundstorey vegetation of grassy riparian flats in gorge country The