Are we nearing a 6th Mass Extinction?

Having already looked at the impacts of several alien species on their invaded environment, I think is important to examine past and future extinction rates.

So I thought we should delve into the past and examine whether or not we are entering into the Sixth Mass Extinction?

What are Mass Extinctions?

Mass extinctions are defined as the loss of  at least 75% of Earth's species in a geologically short time frame (Jablonski and Chaloner 1994). Palaeontologists recognize five major extinction events ("Big Five") in the past 540 million years, with the most recent mass extinction event ending 65 million years ago-Cretaceous (Barnosky et al. 2011).  The "Big Five" mass extinctions took place near the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous Period . As the number of known species which have become extinct over the past thousand years has risen, biologists suggest that a sixth mass extinction may now be under way.  It is expected that if all species currently deemed "threatened" become extinct in the next century, then future extinction rates will be 10 times recent rates ( Pimm et al.1995)-astonishing!!

Mass Extinctions and CO2 Levels

What are the main causes?

Observations suggest that humans are the main cause of this sixth mass extinction, through habitat fragmentation/ modification, introduction of non-native species, spreading pathogens and killing species directly (Barnosky et al. 2011).  All of these human activities are associated with climate change through global warming due to increasing levels of carbon dioxide, increasing levels of volcanism and ocean acidification. According to the graph above,  all major extinctions occurred when CO2 levels exceeded a thousand parts per million (ppm). Highlighting the important role human's play in determining the speed at which we meet the sixth extinction. 

The graph below, produced by Barnosky et al. (2011), compares extinction rates versus extinction magnitude across different geological time periods. It shows that in the future, if current extinction rates over the last 500 years were to continue it would produce a "Big Five" style magnitude mass extinction. This means that current extinction rates for species are either faster or just as fast as all rates that would have produced the "Big Five" extinctions. 

However, I must point out that you should take CAUTION when comparing extinction rates, as the fossils used and quality of their preservation varies greatly. For example, many of the earlier mass extinctions rates were solely based on marine organisms.

Extinction rates versus extinction magnitude

Today, the current rate of species extinctions has most definitely been enhanced by human activities as CO2 concentration levels increase. Although, recent species loss is definitely serious and dramatic, we can't yet call it a mass extinction. However, the number of species now classed as "critically endangered" suggests that a sixth mass extinction is soon to become a reality.

This makes me question as to what the future holds for us humans and when will we potentially become "extinct"?!?