Botkin Environmental Science Earth as Living Planet 8th txtbk

274 CHAPTER 13 Wildlife, Fisheries, and Endangered Species
13.5 How a Species
Becomes Endangered
and Extinct
Extinction is the rule of nature (see the discussion of biological
evolution in Chapter 7). Local extinction means
that a species disappears from a part of its range but persists
elsewhere. Global extinction means a species can no
longer be found anywhere. Although extinction is the ultimate
fate of all species, the rate of extinctions has varied
greatly over geologic time and has accelerated since the
Industrial Revolution.
From 580 million years ago until the beginning of
the Industrial Revolution, about one species per year,
on average, became extinct. Over much of the history
of life on Earth, the rate of evolution of new species
equaled or slightly exceeded the rate of extinction. The
average longevity of a species has been about 10 million
years. 25 However, as discussed in Chapter 7, the
fossil record suggests that there have been periods of
catastrophic losses of species and other periods of rapid
evolution of new species (see Figures 13.13 and 13.14),
which some refer to as “punctuated extinctions.” About
250 million years ago, a mass extinction occurred in
which approximately 53% of marine animal species
disappeared; and about 65 million years ago, most of
the dinosaurs became extinct. Interspersed with the
episodes of mass extinctions, there seem to have been
periods of hundreds of thousands of years with comparatively
low rates of extinction.
Cenozoic
Era
2
Mesozoic
1
Period
Neogene
Cretaceous
Jurassic
Triassic
Permian
Age
(million years)
24
65
144
213
248
280
Epoch
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene
1.8
5
24
37
58
65
Nakedseed
plants
dominate
the land
Marine
reptile
The human family appears
First bats
Adaptive
radiation
of flowering
plant
Dinosaurs
First
monkeys
Adaptive radiation
of mammals
Pterosaurus
First reptiles
First whales
Mammals
First birds
Turtles
Carboniferous
320
Paleozoic
Devonian
Silurian
360
408
438
Widespread coal swamp
First true fishes
First insect
Vertebrates reach the land
Ordovician
505
Cambrian
600
Adaptive radiation of marine invertebrates with exoskeletons
(a)
Proterozoic
Archean
Precambrian
2.5
billion yrs.
4.6
billion yrs.
Adaptive radiation of marine
invertebrate animals
Prokaryotic life only
(bacteria)