Causes and Consequences
of Species Extinctions
|4.||Consequences of extinctions|
The five largest mass die-offs in which 50–95% of species were eliminated occurred during the Ordovician [490–443 million years ago (mya)], Devonian (417–354 mya), Permian (299–250 mya), Triassic (251–200 mya), and Cretaceous (146–64 mya) periods. Most recently, human actions especially over the past two centuries have precipitated a global extinction crisis or the “sixth great extinction wave” comparable to the previous five. Increasing human populations over the last 50,000 years or so have left measurable negative footprints on biodiversity.
Allee effects. These factors cause a reduction in the growth rate of small populations as they decline (e.g., via reduced survival or reproductive success).
coextinction. Extinction of one species triggers the loss of another species.
extinction debt. This refers to the extinction of species or populations long after habitat alteration.
extinction vortex. As populations decline, an insidious mutual reinforcement occurs among biotic and abiotic processes driving population size downward to extinction.
extirpation. This refers to extinction of a population rather than of an entire species.
invasive species. These are nonindigenous species introduced to areas outside of their natural range that have become established and have spread.
megafauna. This refers to large-bodied (>44 kg) animals, commonly (but not exclusively) used to refer to the large mammal biota of the Pleistocene.
minimum viable population. This is the number of individuals in a population required to have a specified probability of persistence over a given period of time.
In the Americas, charismatic large-bodied animals (megafauna) such as saber-toothed cats (Smilodon spp.), mammoths (Mammuthus spp.), and giant ground sloths (Megalonyx jeffersonii) vanished following human arrival some 11,000–13,000 years ago. Similar losses occurred in Australia 45,000 years ago, and in many oceanic islands within a few hundred years of the arrival of humans. Classic examples of the loss of island endemics include the dodo (Raphus cucullatus) from Mauritius, moas (e.g., Dinornis maximus) from New Zealand, and elephantbirds (Aepyornis maximus) from Madagascar. Megafaunal collapse during the late Pleistocene can largely be traced to a variety of negative human impacts, such as overharvesting, biological invasions, and habitat transformation.
The rate and extent of human-mediated extinctions are debated, but there is general agreement that extinction rates have soared over the past few hundred years, largely as a result of accelerated habitat destruction following European colonialism and the subsequent global expansion of the human population during the twentieth century. Humans are implicated directly or indirectly in the 100- to 10,000-fold increase in the “natural” or “background” extinction rate that normally occurs as a consequence of gradual environmental change, newly established competitive