Saturday, October 31, 2015

Global nutrient transport in a world of giants

The past was a world of giants, with abundant whales in the sea and large animals roaming the land. However, that world came to an end followingmassive late-Quaternarymegafauna extinctions on land and widespread population reductions in greatwhale populations over the past few centuries. These losses are likely to have had important consequences for broad-scale nutrient cycling, because recent literature suggests that large animals disproportionately drive nutrient movement. We estimate that the capacity of animals to move nutrients away from concentration patches has decreased to about 8% of the preextinction value on land and about 5% of historic values in oceans. For phosphorus (P), a key nutrient, upward movement in the ocean by marinemammals is about 23%of its former capacity (previously about 340 million kg of P per year). Movements by seabirds and anadromous fish provide important transfer of nutrients from the sea to land, totalling ~150 million kg of P per year globally in the past, a transfer that has declined to less than 4% of this value as a result of the decimation of seabird colonies and anadromous fish populations. We propose that in the past, marine mammals, seabirds, anadromous fish, and terrestrial animals likely formed an interlinked system recycling nutrients from the ocean depths to the continental interiors, with marine mammals moving nutrients from the deep sea to surface waters, seabirds and anadromous fish moving nutrients from the ocean to land, and large animals moving nutrients away from hotspots into the continental interior.

There were giants in the world in those days.
Genesis 6:4, King James version


The past was a world of giants, with abundant whales in the oceans and terrestrial ecosystems teeming with large animals. However, most ecosystems lost their large animals, with around 150 mammal megafaunal (here, defined as ≥44 kg of body mass) species going extinct in the late Pleistocene and early Holocene (1, 2). These extinctions and range declines continued up through historical times and, in many cases, into the present (3). No global extinctions are known for any marine whales, but whale densities might have declined between 66% and 99% (4-6). Some of the largest species have experienced severe declines; for example, in the Southern Hemisphere, blue whales (Balaenoptera  musculus) have been reduced to 1% of their historical numbers as a result of commercial whaling (4). Much effort has been devoted to determining the cause of the extinctions and declines, with less effort focusing on the ecological impacts of the extinctions. Here, we focus on the ecological impacts, with a specific focus on how nutrient dynamics may have changed on land following the late-Quaternary megafauna extinctions, and in the sea and air following historical hunting pressures.


These are excerpts from the original pdf that can be found here


Footnotes

1. Sandom C, Faurby S, Sandel B, Svenning JC (2014) Global late Quaternary megafauna extinctions linked to humans, not climate change. Proc Biol Sci 281(1787):20133254.
2. Barnosky AD, Koch PL, Feranec RS, Wing SL, Shabel AB (2004) Assessing the causes of late Pleistocene extinctions on the continents. Science 306(5693):70–75.
3. Dirzo R, et al. (2014) Defaunation in the Anthropocene. Science 345(6195):401–406.
4. Christensen LB (2006) Marine mammal populations: Reconstructing historical abundances at the global scale. Fisheries Centre Research Reports 14(9):1–161.


Source: www.pnas.org

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