If the same figures were for cats breeding at two years of age, the pair of cats would reach 100,000 in a little over 30 years. Obviously something must happen to limit those populations before they get so large. On the other hand, a green turtle that takes 40 years before being old enough to lay a hundred eggs of which only 2 survive to grow up will just succeed in replacing itself if it is lucky, and just a few adult turtles killed would mean the collapse and disappearance of the population. Since most plants and animals produce many offspring, it is clear that few of those young survive to adulthood, yet some must succeed if the population is to continue. It is those that survive to reproduce that count, not the number that die along the way.
Clearly reproduction is the most vulnerable stage for the survival of a population or a species.
Those that survive are generally the strongest or best suited to the environment, and the process permits the selection for the best qualities in the population. You can use the same principle to find and maintain the best varieties of crop plants or races of domesticated animals. Food chains One of the essential relationships between species is eating and being eaten. All of the food in island environments starts with plants, which use energy from the sun in order to grow. This plant life, whether in forests, grasslands or food gardens on the land, or mangroves, seagrasses, algae or plankton in the sea, is the starting point and first level of productivity for all island ecosystems.