Networks of nature preserves are important for maintaining biodiversity. However, the fact remains that agricultural land will spread, increasing the risk of species loss.
Setting up land for both agriculture and conservation may help alleviate this problem. Unfortunately, a general model to predict conservation benefits from such practices, one that allows for incremental (as opposed to all-or-nothing) improvements over current practices, is not available.
One reason that a general model is not available is because data is often limited. Detailed data regarding plant height, crop type, etc, are not known, and vary from site to site.
Another reason for the lack of a general model is that researchers often investigate specific benefits offered at one site. Investigating the complimentary benefits of different sites would be more generally applicable.
Kai Chan and Gretchen Daily (Stanford University, California) have developed a general practices/benefits conservation model. They have used it to demonstrate that bird biodiversity can be enhanced in tropical farmland with minimal expense and complications to farmers
The questions.
The scientists sought to answer three general questions about bird conservation in tropical agricultural regions of Costa Rica:
Vegetation cover and composition is often the limiting factor in maintaining biodiversity. Hence, the scientists focused on these considerations, with the knowledge that other indirect impacts on biodiversity will each vary widely.
The model.
The scientists studied an approximately 700 square kilometer area in southern Costa Rica, centered on the Las Cruces Biological Station. This land consists of a mixture of habitats, as it was largely deforested back in the 1960s.
The scientists classified six types of plot habitats (such as residential and active pasture) and eight types of border habitat (such as shrubs and bare ground). The relative abundance of each plot habitat in the study area dictated sampling frequency.
Previous investigations noted bird frequency in each habitat within the study range. The dependence of a particular bird species on a particular habitat is not considered in this modeling study, reflective of the limited data available and rendering the results applicable to other agricultural regions.
The scientists' model is a variation of conventional network design algorithms, albeit with several important modifications to the parameters under consideration. Their model:
Model predictions.
The scientists' model predicts small changes in bird species richness with small changes in landscape plot or border habitat, as expected. However, border vegetation that is both tall and complex can impart a much greater impact on overall bird species abundance.
For example, a sprawling mixture of trees, shrubs, and vines is more than twice as diverse in bird abundance relative to either shrubs or trees alone. Although this was only true of border areas along agricultural land used either as pasture or for growing coffee, together they comprise 56% of the total agricultural land in this study area.
Bird species richness declines dramatically when natural habitat is converted to residential land. Eliminating tree habitat reduces bird species richness by roughly 45%.
The impact is typically far less when agricultural land is converted into residential land. Bird species richness is reduced by less than 5% in the case of land used for growing coffee.
A particularly useful prediction is that large changes in bird species richness are possible by simply planting rows of trees (forming windbreaks) in the border regions of agricultural lands. For example, a greater than 50% increase is observed in the case of pasture.
Since migrant and forest-specific birds make use of treed habitat more than agricultural habitat, the positive impact of this bird species richness enhancement observation is especially important.
Importance for bird conservation.
This study leads to one major conclusion as to how bird biodversity can be maintained in tropical agricultural lands. Planting strips of trees, and maintaining secondary forest growth, around the border regions can dramatically enhance bird species richness and abundance.
Although the effects may be piecemeal, conservation is enhanced overall. Importantly, the implementation of these measures is also piecemeal.
This suggests that farmers can implement these conservation measures at minimal cost. This increases the likelihood that, provided with the right information, farmers will implement them, to the benefit of biodiversity.
It is important to note that these results cannot be extended to other conservation efforts, such as to animals that require an extended, uninterrupted habitat. However, these results are a positive sign for birds, and provide a method for farmers to improve the conservation potential of land already used for agricultural purposes.
for more information:
Chan, K. M. A.; Daily, G. C.
The payoff of conservation investments in tropical countryside.
Proc. Natl. Acad. Sci. USA 2008, 105, 19342-19347.