Seabed exposure and ecological disturbance on Australia's continental shelf: Potential surrogates for marine biodiversity

Disturbances characterise many natural environments - on land, a forest fire that removes a patch of old-growth trees is an example. The trees that first colonise the vacant patch may be a different species to the surrounding old-growth forest and hence, taken together, the disturbed and undisturbed forest has a higher biodiversity than the original undisturbed forest. This simple example demonstrates the intermediate disturbance hypothesis (IDH) that has applications in many natural environments. The application of IDH is significant for managers tasked with managing and conserving the biodiversity that exists in a given area.

In this report we have used models of seabed sediment mobilisation to examine IDH for Australia's continental shelf environment. Although other disturbance processes may occur (eg. biological, temperature, salinity, anthropogenic, etc.) our study addresses only the physical disturbance of the seabed by waves and currents. Our study has shown that it is feasible to model the frequency and magnitude of seabed disturbance in relation to the dominant energy source (wave-dominated shelf, tide-dominated shelf or tropical cyclone dominated shelf). We focussed our attention on high-energy, patch-clearing events defined as exceeding the Shields parameter value of 0.25. Based on what is known about rates of ecological succession for different substrate types (gravel, sand, mud) we derive maps predicting the spatial distribution of a dimensionless ecological disturbance index (ED).

Only a small portion of the shelf (perhaps ~10%) is characterised by a disturbance regime as defined here. Within these areas, the recurrence interval of disturbance events is comparable to the rate of ecological succession and meets our defined criteria for a disturbance regime. To our knowledge, this is the first time such an analysis has been attempted for any continental shelf on the earth.

干扰是诸多自然环境的共性特征——在陆地上，一场焚毁小片原始老龄林树木的森林火灾便是典型一例。率先定植于这片裸露空地的树木物种，可能与周边原始老龄林的树种存在差异；如此一来，受干扰区域与未受干扰区域的森林整体生物多样性，将高于原始未受干扰的森林。这个简单案例阐释了中间干扰假说（intermediate disturbance hypothesis, IDH），该假说在诸多自然环境中均有应用价值。中间干扰假说的应用，对于负责特定区域生物多样性管理与保护的管理者而言意义重大。

本报告借助海底沉积物运移模型，针对澳大利亚大陆架环境展开了中间干扰假说的验证研究。尽管海底可能存在其他干扰过程（如生物、温度、盐度、人为干扰等），但本研究仅聚焦于波浪与水流对海底产生的物理扰动。研究证实，基于主导能源类型（波浪主导型大陆架、潮汐主导型大陆架或热带气旋主导型大陆架），对海底扰动的频率与强度进行建模是可行的。本研究将高能量、清除斑块的扰动事件定义为希尔兹参数（Shields parameter）超过0.25的事件，并将分析重点置于此类事件之上。基于不同底质类型（砾石、砂质、泥质）的已知生态演替速率数据，我们推导得到了可预测无量纲生态干扰指数（ED）空间分布的地图。

仅有极小一部分大陆架区域（约占10%左右）符合本文定义的干扰体系特征。在这些区域内，扰动事件的重现间隔与生态演替速率相当，且满足我们为干扰体系设定的判定标准。据我们所知，这是全球范围内首次针对任何大陆架开展此类分析研究。