最优化设计连续的自然保护区

生境破碎是导致生物多样性损失的重要原因之一,避免生境破碎的一个有效方式是建立连续的自然保护区使物种可在保护区内自由移动。不加选择地把大片土地都转为保护区是实现连续的一个途径,但资源是有限的,应当以最优的方式分配。如何最优化设计生态上和经济上都有效的保护区成为生物保护领域一个重要议题。从一组备选地块中选择一部分组成自然保护区,这样的问题主要有两种解法:启发式方法和最优化方法。启发式方法虽然灵活且运算速度快但不能保证最优解因而可能导致稀缺资源的浪费,最优化方法保证得到的解是最优的但建模和运算存在困难。建立一个线性整数规划模型用于设计一个最小的连续保护区,用Dantzig剪切法消除循环确保形成一个连续的树,对应一个连续的保护区,检验了模型的计算效率。结果显示,模型可在合理时间内解决一个包含100个备选地块和30个物种的连续保护区设计问题,计算效率显著优于同类目的的其它方法。以美国伊利诺伊州Cache河流域11种濒危鸟类的保护区设计为例说明了该方法的应用,设计了两种情况下连续的保护区。讨论了模型的局限和数据问题。

The optimal design of a connected nature reserve network

Habitat fragmentation is considered one of the critical reasons for the biodiversity loss. Establishing a connected reserve network which accommodates the free movement of protected species in the reserve is an effective way to reduce habit fragmentation. One possible method to ensure a connected reserve is to convert a whole large area to a conservation reserve. However, the resources devoted to conservation effort have always been scarce, and it is important that these resources are allocated in an optimal way. The optimal design of a reserve network which is effective both ecologically and economically has become an important research topic in the reserve design literature. The problem of optimal selection of a subset from a larger group of potential habitat sites is often solved using either heuristic or formal optimization methods. The heuristic methods, although flexible and computationally fast, can not guarantee the solution is optimal therefore may lead to scarce resources being used in an ineffective way. The formal optimization methods, on the other hand, guarantees that the solution is optimal. But it has been argued that it would be difficult to model site selection process using optimization models, especially when spatial attributes of the reserve have to be taken into account. This paper presents a linear integer programming model for the design of a minimal connected reserve network. Dantzig cut approach is employed to prevent loops that may occur in the solution to ensure the selected sites assemble a connected tree, corresponding to a connected reserve. Computational performance of the model is tested using datasets randomly generated by the software GAMS. Results show that the model can solve a connected reserve design problem which includes 100 potential sites and 30 species in a reasonable period of time, and the model significantly outperforms the representative method in the literature in terms of computational efficiency. As the number of potential sites increases, the processing time needed to solve the problem is expected to grow up quickly. As an empirical study, the model is applied to the protection of endangered and threatened bird species in the Cache River basin area in Illinois, USA. 11 bird species and 56 potential sites are included in the empirical study. Two minimal connected reserve networks are designed. One is to protect each species at least once and the other twice where applicable. The model presented in this paper is deterministic in terms of the occurrence of species in potential sites, and static in terms of the time when selection decision is made. More studies are needed to combine uncertainty and dynamics into the optimization model. The successful application of any site selection model has to be based on complete and accurate data. However, the data itself may have some associated problems such as incompleteness, out-of-date, inaccurate, etc. Data issue is critical for a successful design of an effective reserve network and it should get more attention especially in China.