I would like to focus on some of the impacts the growing human population has had on the environment other than greenhouse gas emissions, or nitrogen addition. This week I want to take a look at how humans have in the past, and are now more than ever, affecting the other living things on our planet. This is important not only because species loss is bad in itself but because biodiversity is crucial to the productivity and sustainability of the Earth's systems (see Hooper et al. 2012 for more info). I don't want to make this post too long and boring so I'm going to concentrate on the role humans played in the past megafaunal extinctions and the evidence for an imminent 6th mass extinction in the Anthropocene as a result of human population growth and resource demands.
We know that humans have played a part in extinctions in the past. Barnosky et al. (2004) examine the evidence for the respective roles of climate and humans in the Pleistocene megafaunal extinctions and find that although there is a strong case backing climate variability, the effects were coupled with, and amplified by human action such as hunting. For example, many of the surviving animals were nocturnal, lived in dense, hight forest or even in the sea, so if climate was the only culprit, why did these habitat-specific species survive?
The authors find while comparing the timelines of the late Quaternary extinctions, human arrivals and climate change, that human arrival coincided with many extinction events and played a very significant role in the extinctions that occurred on the American and Australian continents (see below). Humans don't take all the blame of course, and they conclude that:
"A significant implication for conservation biology is that the coupling of marked
climatic change with direct human impacts on fauna is especially pernicious. Both
effects are under way today at unprecedented rates."
So I see this as a warning of what humans are capable of, and at that time there weren't very many of us! Think about today, with 7 billion of us! In fact this is exactly the line of reasoning many take to lead to the idea that in the Anthropocene, we could be very well moving towards a 6th mass extinction.
So given all that we know about the massive growth in human poopulation and activity, and that human populations clearly have an effect on extinction rates - through co-opting resources; fragmenting habitats; introducing non-native species; spreading pathogens; hunting; and changing the global climate - could we be heading towards the Earth's sixth mass extinction? We seem to hear about endangered species every day on the news and there is a general perception that our activities are causing more extinctions than is 'normal' - but what is the evidence? A paper published in 2011 by Barnosky et al. concludes that we aren't currently in a mass extinction (as compared to the previous five) but warns that if no measures are taken to protect endangered species, we could well end up in one.
The study compares the rate and magnitude of current extinctions with those of the past five mass extinctions, using the metric 'extinction per million species years' (E/MSY). They take two approaches: firstly if we assume the 'Big 5' all happened suddenly - say, over 500 years - what extinction rate would have been needed and how does that compare to the present rate? They find that that current rates are slower, but if we were to consider 'threatened' species as inevitably extinct then the rates would be comparable. For the second approach, they ask how long it would take to produce species loss equivalent to 'Big 5' magnitudes at current extinction rates. In fact if all 'threatened' species became extinct within a century and the rate remained constant, such magnitudes would be reached in 240-540 years.
This seems like a long time, but who is to say that extinction rates won't increase? The human population is growing, but also becoming wealthier. The rapid development of many Asian countries, especially China, means that there are way more people demanding the kind of lifestyle that requires activities that are harmful to biodiversity. Of course, you could argue the other way, that there is a lot of uncertainty about what 'normal' rates are and whether the results for the particular taxa they studied (due to better fossil records) can be extrapolated to other species in other places.
Nevertheless, the authors stress that fossil records show that species richness and evenness today are low compared to pre-Anthropogenic conditions, and I stress that one of the principal differences between present day and the pre-Anthropocene era is the size of the human population. Gaston (2005) writes that:
Nevertheless, the authors stress that fossil records show that species richness and evenness today are low compared to pre-Anthropogenic conditions, and I stress that one of the principal differences between present day and the pre-Anthropocene era is the size of the human population. Gaston (2005) writes that:
"The most important agent of change in the spatial patterns of much of biodiversity at present is ultimately the size, growth and resource demands of the human population. This is giving rise to [...] levels of species extinction largely unprecedented outside periods of mass extinction"
I obviously only touched on the issues of extinction and biodiversity as the main focus of this blog is overpopulation, but if you'd like the read more about them, here are some links to blogs on the topics:
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