畜牧管理與劣化莽原之隨機復健研究

Grazing Management and Rehabilitation of Degraded Rangeland under Uncertainty

西澳洲乾燥地區的莽原長期以來因牧養過多的綿羊牲口而導致生態環境惡化，土地生產力也因而降低。補救這種生態惡化的方法很有限，本文利用隨機最適控制理論對各種不同惡化程度的莽原探討與其相對應之最適牲口數目的畜牧政策，並應用馬可夫鏈理論評估這些畜牧政策的長期經濟利益以及其對莽原生態所造成的衝擊效果。研究結果顯示定畜或空畜政策是必要的，輪畜在目前的市場情況下不具經濟性。最適的綿羊牲口數目與莽原內週年生的成年植物樹木及其生質量呈正比，但與植物幼苗之數目呈反比。在追求長期平均淨收益最大的決策目標之下，輕微惡化的莽原經由最適畜牧政策，復健仍有可能；但中度至高度惡化的莽原復健則不太可能會成功。至於在追求長期淨收益現值最大的決策目標之下，若折現率為5％，一般而言，最適的畜牧政策雖能維持經濟的超額利潤，郤未能確保生態的永續性。因此惡化的莽原若仍然為維持私人利潤而使用的話，莽原的資源將逐漸失去其更新再生的能力。如果生態的永續發展是社會的一種規範，那麼惡化莽原的管理勢必不能只顧及經濟的理性，短期的利益必須放棄，同時也必須從事其它莽原更新的投資工作，以便維持莽原的長期生產力。

Rangeland in the arid zone of Western Australia has been degraded by sheep overstocking since the turn of the century. The strategies available for restoring the productivity of such degraded rangelands are limited. In this paper optimal economic policies with respect to the choice of stocking rates and grazing patterns were derived for various starting states under a stochastic optimal control framework. Evaluation of these optimal policies was carried out by comparing their long run economic and ecological impacts on grassland. All optimal policies call for a strategy of set stocking or destocking only. Rotational grazing is not economically viable at the specified cost levels. Optimal stocking rate increases with the level of forsge biomass and adult plants, but decreases with the level of either young or old seedlings at the degraded range condition. The results indicate that under the criterion of maximisation of the long run average return, for slightly degraded range, rehabilitation through grazing management alone is possible. For range which is moderately to severely degraded, rehabilitation to fair or good condition is not technically practical. On the other hand, under the criterion of maximisation of the net present value at a discount rate of 5 percent, the consequence of the long run impact on the range resource following the optimal policy is ecologically unsustsinsble, though economically viable. These findings suggest that if degraded ranges continue to be exploited for private profit, the range resource may be driven beyond its capability to renew. If ecological sustainability is an Australian social norm, economic rationality should subside in the decision criterion, and range management will require some willingness to forego short term profits for the sake of long term rewards or investment in regeneration measures.