Small-scale resource heterogeneity in semi-arid landscapes

Abstract

Patchy distribution of plant populations is a hallmark of arid and semi-arid ecosystems. This has been attributed to the patchy distribution of scarce or limiting resources across the landscape and within the soil itself. Behind these descriptive properties are a range of processes which are the causal mechanisms of resource allocation, conservation and utilization within the landscape. Terrain-controlled mechanisms have been previously described in respect of groved mulga (Acacia aneura) communities. This paper describes a set of resource regulation mechanisms which are largely controlled by plants and plant communities and which are effective at fine scales. The actual mechanisms are inferred from field observations, and validated by looking for the net effects of defined processes acting over time. Plant-mediated resource control is inherently more sensitive to grazing pressure than terrain-controlled processes, because herbivores are able to quickly and drastically alter the density and basal cover of plants, and so change the effectiveness of the control processes. This may lead to a long-term change in system function. This paper examines the generality of these propositions in a series of contrasting landscape types, and proposes a framework by which landscape degradation can be assessed by examining the modes of basic resource regulation.