A "species" in a biological sense refers to male and female
organisms that can produce young. If organisms are living, this is easy
to observe. However, if the organisms are extinct we only have their
skeletal remains. In paleontology we compare similar fossil
morphology and refer to them as a single species. We record when a
new skeleton is observed and chart the range of that species. Sometimes
we can see slight changes that branch off into another species.
In the present we can see the evidence of why organisms became
extinct. The dodo bird is an example of a modern day extinction on
Mauritius Island in the Indian Ocean. This 50 lb. bird was flightless
with no enemies on the island. When humans came to the shores in the
early 1500's they ate the bird for meat. Humans also brought pigs, rats,
and monkeys that ate the dodo birdís eggs. Within 180 years after man
came to its shores, the dodo was extinct by 1681. So why is one species
important in the fossil record. Unfortunately when you take away one
species another species will soon follow.
In the case of the fruit-eating dodo, it was recently noted that
certain trees were no longer producing seedlings. There were 300 year
old trees with no younger trees. It was concluded that the dodo ate the
fruit and as the seeds passed through the gut, it prepared the seed for
germination. Without the dodo naturally eating the fruit the tree would
become extinct. To save the tree, turkeys are now being used to eat the
fruit to produce seedlings.