Most people talk about biodiversity in terms of number of species, but you can also think about it on different scales, from small-scale genes to large-scale ecosystems. Conservation can target genes, populations, species, habitats and ecosystems. How does conservation biology change for different levels of biological organization?
Having protected areas like national parks is great, but knowing how big to make them is not always easy. After all, some animals travel long distances on a regular basis—elephants walk anywhere from a few miles to 50 miles a day, depending on food and water availability. Chimpanzees have large home territories that they defend as a group, and a protected area might not be able to include territories of two competing groups. Some animals, like sea turtles, travel the globe and return to one home beach to lay eggs. If that beach was turned into a resort, turtles won't be laying any eggs there.
The number of individuals in a population might not reflect the amount of genetic diversity in that population. Over time, small populations lose genetic diversity—the more generations, the lower the overall genetic variation. Conservation biologists use a measure called the effective population size, an estimate of the number of breeding individuals in a population. Effective population size is often lower than the total number of individuals because old and young individuals do not mate; sex ratios can be skewed, and in plant and fungi populations, some of the individuals may be underground in the seed bank (plants) or not reproductive (fungi).
Marine protected areas (MPAs) have developed from small-scale "no-take zones" or marine parks into networks of MPAs. Conservation biologists realized that fish and other marine animals do not stay in one place all the time. This is especially true of large marine animals that migrate or travel large distances looking for food and mates.