对于任何观察，常常可以识别出一个直接的原因，但这种因果解释往往是资料不足的，我们需要进一步探索，以达到更完全的掌握情况。即使是现象已经被“解释”了，更进一步和更深入的解释也是很好的，它允许我们看见更完全的情景。

   例：无线电标记发现鼹鼠已经停留3天，近似的解释是简单的，即它死了。进一步研究发现有大量的消化道寄生物，这可能是其死亡的原因。还有可能更进一步的研究，鼹鼠是否遗传上易于感染寄生虫病，或环境条件是否有利于寄生虫的存活等，或某些进一步的解释。[1]

   对我们周围的世界为什么会是绿色(Why is the World Green?)的这样一个现象？就有不同的解释［2］

首先：Definingthe question；

然后，各种解释：

Top-down? Experimental data

Top-down? Large-scale observational data

Bottom-up?

Bottom-up; not only poisons

More bottom-up; spatial processes andhiding places

What about microbes?

So bottom-up is the current favouredexplanation, but there are some problems

Top-down and bottom-up; the story so far

A sideways look; is it all just top-downand bottom-up?

Population biology and the sidewaysperspective

相关的问题：

Why is the soil brown?

小结：So why isthe world green?

The implication is clear: even in a world full of green energy, many/most herbivores cannot obtain enough requisite resources to grow, survive, or reproduce at high rates. Nutritional shortages regulate herbivore numbers, often limit their effects on plant biomass, and form one important reason why much of the world is green.

References

［1］A.麦肯齐等著，孙儒泳等译.生态学. 北京：科学出版社，2000

［2］Thomas N. Big Questions in Ecologyand Evolution. Oxford University Press，2009