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University of Queensland (2014)

Degraded or just dusty ? : 150 years of ecological change in inland eastern Australia

Silcock, Jennifer Lesley

Titre : Degraded or just dusty ? : 150 years of ecological change in inland eastern Australia

Auteur : Silcock, Jennifer Lesley

Université de soutenance : University of Queensland

Grade : Doctor of Philosophy (PhD) 2014

The ecological history of rangelands is often presented as a tale of devastation, where fragile drylands are irreversibly degraded through inappropriate land-use. There is confusion about how to recognise and measure degradation, especially in low productivity environments characterised by extreme natural variability and where abrupt management upheavals mean that there are few reference sites. These issues have important consequences for rangeland development and management programs, many of which are founded on a perception of serious and ongoing degradation from a former ‘natural’ state. In this thesis, I employ three approaches to assess degradation in inland eastern Australia, part of one of the largest desert landforms in the world and subject to recurring arguments about the cause and magnitude of landscape change since pastoral settlement 150 years ago : written historical records, grazing exclosures, and identification and surveys of rare and potentially sensitive elements of the flora.
From the 1840s, the journals of European explorers provide the first written descriptions of inland Australia. In Chapter 2, I use this record to test prevailing paradigms relating to five key themes of environmental change : vegetation structure, fire regimes, waterhole permanence, macropod abundance and medium-sized mammal assemblages. 4500 observations from fourteen journals spanning twelve expeditions between 1844 and 1919 were geo-referenced. Careful evaluation of the record suggests little change in broad vegetation structure or waterhole permanence, running counter to prevailing paradigms. The sparse observations of fire suggest burning was infrequent, while macropods were apparently uncommon in semi-arid areas where they are abundant today. Systematic evaluation of the explorer record for a region can provide ecological insights that are difficult to obtain by other means. However, there are limitations inherent in the historical record and findings are necessarily broad. In Chapter 3, I use long-term grazing exclosures to examine the impacts of cattle grazing on two widespread vegetation types. We measured herbaceous biomass and plant species richness and abundance at five 14-year-old exclosures in north-eastern South Australia. We did not detect any significant differences between grazed and ungrazed treatments in total species richness or abundance, life form richness or abundance, or herbaceous biomass. The dominance of ephemeral species confers resilience by limiting the development of strong feedbacks between grazing intensity and vegetation dynamics, meaning that the non-equilibrium paradigm best describes this grazing system. This chapter forms part of a series of three studies using exclosures to examine grazing impacts across three biogeographic regions. Exclosures encompass only a tiny area, meaning that rare or grazing-sensitive species may not be represented, or may have become locally extinct prior to the erection of exclosures. In Chapter 4, I identify rare and potentially sensitive elements of the western Queensland flora through a systematic examination of herbarium records and expert interviews. Five threat syndromes were identified, arising from the interaction of plant biology and threatening processes, and 60 potentially threatened species had been overlooked in the listing process. However, lack of data on distribution, abundance, population dynamics and threats precluded robust conservation assessments for most species. In particular, detecting genuine rarity and decline was confounded by extreme temporal variability, low collection effort spread over a vast area and poor understanding of threatening processes.
Chapter 5 examines patterns of rarity in the flora of a semi-arid mountain range, the Grey Range, with a high concentration of rare species and 150 years of elevated grazing pressure. Habitat specialisation, reproductive biology and biogeographic history interact to create observed patterns of rarity, and there is no evidence that any species have become rare or restricted as a result of grazing. Species confined to barren plateaux, sheltered habitats and gidgee toeslopes represent relictual populations, and the association of rare plants with larger plateaux suggests local extinctions were more probable on smaller plateaux during Pleistocene climatic fluctuations.
Chapter 6 presents the results of four years of targeted surveys for the candidate species identified in Chapter 4. Search effort and survey results were used to assess 91 species against international Red List criteria. One-third of species were widespread and abundant at least in certain seasons but had appeared rare due to sparse collections. The conservation status of 20 species, mostly newly-recognised species from restricted habitats, was upgraded and 14 remained listed due to having restricted areas of occupancy. The IUCN criterion that allows for listing of species due to extreme fluctuations (in combination with restricted and fragmented populations) is not justified for arid zones, where these fluctuations may actually confer resilience to grazing for short-lived forbs and geophytes. With the exception of 12 artesian spring species, continuing declines were documented for just six species.
In Chapter 7, I bring together my results and those of previous studies to provide a critical evaluation of the extent and magnitude of ecological change in inland eastern Australia. There is no evidence of unidirectional change in vegetation structure, irreversible degradation or loss of plant species, although some palatable species have declined at a landscape scale. It is apparent that some prevailing paradigms have become entrenched despite lack of empirical evidence. However, many medium-sized mammals have declined dramatically or become extinct since European settlement, while large macropod numbers have increased in the semi-arid zone. Management actions and areas requiring further research are discussed. The approach presented here, incorporating the historical record, comparison of sites with different management histories and targeted surveys for rare and potentially sensitive species, can be used to assess degradation in drylands with abrupt changes in management and contentious ecological narratives

Mots clés : Environmental degradation Rangelands Aridlands Explorers — Australia Vegetation change Rare species Threatened and endangered species Grazing Grazing — Queensland — Southwestern


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