Determining what exactly will happen under climate change scenarios,
and whether what is happening currently is attributable to climate change, is
extremely difficult but very interesting subject. Pounds et al in 1999 reported
a loss of an endemic toad species in Costa Rica (Bufo periglenes) due to an increase in sea surface temperatures,
the first species extinction attributed to climate change. Since then great
losses in population abundances have been linked to the increased stress
anthropogenic activity has placed on the planet.
The exact effects of climate change on species and proposed
extinction rates are inherently uncertain and difficult to predict due to the
many variables involved, and the attribution of losses to climate change is
problematic as all correlation/causation arguments are. One of the most
discussed effects of climate change is range shifts leading to declining population
abundances, in which species migrate to a different latitude to maintain their
environment at the right climate (Thomas et al, 2004). This recent paper from John
Wiens indicates that climate-related local extinctions have indeed been
happening in a number of species. Through an examination of studies focusing on
range shifts as a response to warming climate, Wiens finds climate-related
extinctions in 47% of the 976 species studied. Findings also showed more
frequent impacts of climate on biodiversity in tropical areas.
Minimum change scenarios for climate change predict extinctions
of 18% as a direct impact of climate change with maximum change scenarios leading
to extinction of ~35% (Thomas et al 2004), but as discussed the expected losses
in species populations are much greater than this. Whilst declines in
population abundances does not equal extinctions, it obviously makes them a lot
more likely.
Limited physiological tolerances to temperature is perhaps
one of the most obvious factors leading to species loss, but according to this
paper, very few studies suggest a direct relationship between temperature tolerance
and local extinction. So whilst Wiens details a worrying account of current local
extinctions, perhaps he is missing the bigger picture. Studies investigated by Cahill et al in 2012 indicate extinctions directly caused by species interactions leading to
a loss of prey and disease, and extinctions directly caused by stress of
abiotic factors leading to desiccation in trees and oxygen limitation in
aquatic species. As such it appears for
these assessments at least to not be a straightforward correlation between
temperatures and biodiversity loss as a result of climate change, more a
complex chain of events triggered by the alterations in global climate. 4
studies even reported extinctions being the result of ‘natural climatic
oscillations’, a phenomenon which occurs anyway without the influence of the
human race, but which could also be subject to change through anthropogenic
activity.
This range of extinction mechanisms causes only furthers the
unpredictability of climate change induced extinctions. The complex nature of
species interactions and ecosystem functions results in a wealth of different
trajectories for biodiversity. Furthermore, whilst extinctions may be caused by
human habitat modification, the presence of climate change as an exacerbating
variable can’t be ignored.
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