Climate change, volcanic eruptions, sea level rise, and changes to the atmospheric chemistry have contributed most significantly to

Below are the choices can be found elsewhere:

background extinction rates
historical mass extinctions
       increased threats to modern endangered species
       habitat fragmentation
        overexploitation of species

I think the answer is Historical mass extinctions

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Not only is it the purchase of about 40% of america, it also grounds to on-go our advancement as a civilization.