Much of Mark Kurlansky’s introduction to World Without Fish summarizes the key ideas from Charles Darwin’s On the Origin of Species. It reveals the book’s focus: In the natural world, everything is connected, and if we don’t act soon, damage done to the oceans may have cumulative effects that completely alter the natural order of the planet, including humanity’s place in it. Darwin’s ideas about the struggle for survival are essential to Kurlansky because they are crucial to understanding the ecological implications of overfishing and form the scientific backbone of the book’s arguments.

The core idea is that as organisms struggle for survival, variance among them allows some species to survive while others go extinct. The traits and characteristics that allow them to survive are naturally emphasized over time, leading to variety among species (biodiversity), essential for the survival of all life on earth. This also means that species become tuned to take advantage of particular niches in an ecosystem, and any sudden changes in circumstances can easily upend the delicate natural balance that has emerged over very long periods. Suddenly removing a species of fish—whether it is completely extinct or drastically reduced in numbers—is the kind of change that can send ripples through the entire food chain.