What does Gladwell mean when he says that biologists often talk about "the ecology" of an organism? How is this similar to "accumulative advantage"?

When the author is discussing "ecology," he means the environment—and in particular, the people—around an organism. Obviously, in the context of biology, this relationship often involves animals and plants; but in this case, the relation is to other individuals and the society surrounding these "outliers."
In the book, the concept of an "accumulative advantage" involves the idea that, in small groups, certain individuals are encouraged to be better than others similar to them. For instance, several major sports have a surprisingly large number of people from small populations—such as a local region that to produces many high quality players. In these instances, the environment surrounding these individuals encourages a small group (i.e., a microcosm) to become significantly better than average in order to give them a competitive advantage.
If you played soccer, for example, in an area that had significantly better players as well as significantly better coaches, your team would become much better than similar organizations that are not in this environment. This "ecology," which involves the superior coaches and players, gives the entire group an "accumulative advantage"—just like a specialized ecology that caters to the advantages of a certain organism will give that organism evolutionary advantages.

The "ecology" of an organism is the environment that surrounds it. Gladwell gives the example of an oak that's the tallest in the forest not only because of individual characteristics such as the acorn it grew from, but also because of what surrounded it. For example, it likely had rich soil and was not blocked by other trees.
The concept of the ecology of an organism is similar to the "accumulative advantage" of an individual, as people who start out with certain advantages are able to capitalize on these advantages to attain even more advantages. An example is a child who is born into a family with resources and whose parents read to him or her. Later, this child seems like a promising student in the early grades. This student is likely to receive more attention from his or her teachers and to improve his or her academic skills. In other words, the child can use early advantages to gain more advantages that build on his or her potential over time.