4 Endangered Species
Whatever the time period, the common thread is a widespread
degradation of habitats based on human activities and the loss
of biotic diversity leading to endangered and extinct species.
A conservative estimate predicts that of the fi ve to nine million
animal species inhabiting the planet, the extinction rate is likely
between 11,000 and 58,000 species annually. Vertebrates have
experienced a mean decline of 28 percent, and of identifi ed in-
vertebrate species, about 40 percent are threatened (Dirzo et al.
2014, 401–402).
But species always have gone extinct. What about the di-
nosaurs? Fossil evidence shows that at least fi ve mass extinc-
tions have occurred in the last 540 million years or so when
the Earth lost more than three-quarters of its species in a geo-
logically short interval. Th e diff erence between these fi ve ex-
tinctions and the Anthropocene is the rate of extinction. Not
since the dinosaurs have so many species and populations
died out so fast on both land and in the oceans (Barnosky
et al. 2014). Species extinctions occurring now far outstrip
“background” rates derived from historic extinction rates as
calculated by extinctions per million species-years (Pimm
et al. 1995). In a conservative estimate, the number of spe-
cies that have gone extinct in the past 100 years would have
taken, depending on the taxonomic group, between 800 and
10,000 years to disappear (Ceballos et al. 2015). Tom Lovejoy
(2000), a prominent scientist who studies biodiversity, ex-
pressed deep concern about the biological trouble and extinc-
tions unequalled since the dinosaurs. He warns: “Th e rate at
which species disappear is about 1,000 to 10,000 times nor-
mal, and a quarter or more of all species could vanish within
a couple of decades . ”
Not all scientists accept the idea of a “sixth mass extinction.”
Th ey argue that current extinction rates have been overesti-
mated and species extinctions during the past 500 years have
occurred at a much slower pace (Briggs 2016). Th e methods
used to determine extinction rates may not be robust enough
for reliable estimates, and the lack of fossils for comparisons are
