The Interaction Between Habitat Conditions, Ecosystem Processes and Terrestrial Biodiversity - a Review

Australia: State of the Environment Second Technical Paper Series (Biodiversity), Series 2
M Doherty, A Kearns and G Barnett - CSIRO Wildlife and Ecology
A Sarre - Sarre and Associates Pty Ltd
D Hochuli, H Gibb and C Dickman - Institute of Wildlife Research University of Sydney
Department of the Environment and Heritage, 2000
ISBN 0 642 19490 4

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In June 1997, the State of Environment (SoE) Reporting Unit of Environment Australia commissioned a team consisting of staff from CSIRO Wildlife and Ecology and the Institute of Wildlife Research at Sydney University to review and critically assess the relationship between biodiversity, habitat complexity, habitat quality and ecosystem processes. A principal aim was to report on the search for surrogate or 'higher-order' indicators of biodiversity.

The literature search yielded approximately 1,700 references that were evaluated for relevance. The final database consisted of approximately 400 references. The focus of the review was terrestrial. Single species autecological studies were generally omitted; the emphasis was placed on studies that considered the interrelationships between more than one species or specifically studied the role of 'diversity' in ecosystems.

Debate surrounding the relationship between biological diversity and ecosystem function has been central to community ecology for many decades, although antecedents of the idea can be traced back to at least the 13th century. The debate is largely centred upon concepts of species redundancy and ecosystem stability, both concepts being characterised by a lack of empirical data and few well-designed field investigations. Mathematical models have produced inconclusive results showing that more diverse ecosystems can be either more or less stable than simpler ecosystems, depending on the ecosystem under investigation.

The relationship between habitat complexity and species diversity has also not been resolved. It has been known for decades that species diversity increases as the size of the sampling area increases: thus, larger areas contain more species. However, the relationship between this pattern and habitat complexity has not been adequately investigated.

There are numerous short-term studies of the effects of disturbance on biodiversity, but very few, if any, long-term studies that can unravel the complexity associated with multiple disturbances and inherent natural variability. Australian studies on the role of pastoral grazing on biodiversity consistently find that the presence of grazing reduces biological diversity, but there is greater ambiguity about the relationship between logging and biodiversity. There is a paucity of studies that examine the effects of multiple fires occurring in the same area over time. There is general agreement among scientists that habitat fragmentation is a major threat to biodiversity. Examples of such mechanisms include gross habitat loss, increased isolation of populations leading to reduced opportunities for recolonisation and increased vulnerability to stochastic events, edge effects, and increased disturbances.

Despite the large amount of literature discussing biodiversity, habitat conditions and ecosystem processes, there is scant empirical data to be able to prove more than simple linkages between specific elements. Essentially there is little known about the interactions between habitat conditions, ecosystem processes and biodiversity per se at all scales and what is known cannot be reliably generalised to other areas. However, an emerging trend in thinking involves the idea of biodiversity acting within functional groups as an 'insurance policy' or 'buffer' against major ecosystem change. The interaction of habitat conditions, ecosystem processes and biodiversity is therefore a two-way process and hence the effects of biodiversity on the environment are just as important as the effects of environment on biodiversity.

It is apparent that the importance of biodiversity cannot be assessed other than in relation to a specified function. In most, if not all cases, this will be in relation to some human use - even if it is only human appreciation of the natural world. However, science cannot determine the existential value of biodiversity per se since there is ultimately no end point in ecosystem development. All ecosystems are unique products of 3.5 billion years of evolution and environmental change, and their functionality will change over time irrespective of human inputs. Similarly, certain species will become locally and globally extinct over time as part of the ongoing evolution of life on earth.

A cautious approach to the implementation of biodiversity indicators for SoE reporting is therefore warranted. Given the lack of rigorous, scientific understanding of linkages between biodiversity and ecosystem function, broad level indicators such as the amount of vegetation cleared may serve a more useful purpose in the short term than more precise measurements of diversity at the species or gene levels. That is, an ecosystem-level approach will probably give best results overall for the measurement of the state of biodiversity until more detailed linkages are investigated, if ever. This approach is particularly pertinent at the national level of SoE reporting.

An experimental approach to this issue is crucial if any useful linkages are to be evaluated for SoE reporting. However, any such experiment must be clearly focussed and address specific aspects of ecosystem function that are measurable. Relevant fields that could contribute to and benefit from such scientific inquiry include conservation biology, agriculture, forestry, mining and the field of restoration ecology, where ecosystems are being rehabilitated, recreated or even artificially created. In these instances, structural and functional redundancies are critical issues that must be addressed if the aim of a long-term functioning ecosystem is to be achieved.

An expert workshop was held in March 1998 to review the draft of this manuscript and to develop an experimental approach, if feasible, to the development of surrogate indicators for monitoring the state of Australian biodiversity for national SoE reporting purposes. The way forward in the long term for national SoE reporting may be the integration of data from (i) existing long-term manipulative experiments and on-going management experiments with (ii) new data derived from a series of mensurative experiments conducted within Interim Biogeographic Regionalisation of Australia (IBRA) regions (Thackway and Cresswell 1995).