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No big animals, no fertile Earth

A study recently published in the magazine Proceedings of the National Academy of Science (PNAS) reveals that animals play a key role in the transport of nutrients, but their contribution has been reduced due to the extinction or decline of many of the largest populations. In this post, we will review this study in order to understand the consequences of this fact.

INTRODUCTION

The ancient Earth was plenty of giant animals, with abundant whales in the oceans and large animals in the land. Nevertheless, their populations have been reduced for several reasons:

  • The massive extinction during the late-Quaternary: about 150 large mammal (more than 44 kg) species went extinct.
  • Recent and present extinctions.
  • Widespread population reductions in great whales due to hunting: some whale densities might have been reduced between 66% and 99%, like blue whale (Balaenoptera musculus).
  • Present environmental pressures: 27% of seabird species are threatened and anadromous fish’s populations have been reduced to less than 10% of their historical values (Pacific Northwest).

All this things are probable to have caused a shift in the global nutrient cycling. In concrete, Doughty et al. estimate a reduction to about 8% of the animals’ capability to spread nutrients in the land and about 5% in the oceans, compared with past values. Several animal groups have been identified to play a key role in this system.

  1. Terrestrial animals: accelerates cycling of nutrients from more resistant  forms to decomposing matter. Some animals transfer terrestrial nutrients to aquatic environments, while others do the contrary. Even some animals, like bears and eagles, transfer oceanic nutrients to land environments by feeding on anadromous fishes.

    Moose (Alces americanus) transfer aquatic-derived N to terrestrial systems.
    Moose (Alces americanus) transfer aquatic-derived N to terrestrial systems (Picture: BioLib).
  2. Anadromous fish, fish that travel from sea to rivers to spawn their eggs (such as salmon or striped bass) and seabirds transport nutrients from sea to land.

    Striped (Morone saxatilis) is an anadromous fish.
    Striped bass (Morone saxatilis) is an anadromous fish (Picture: Ethan Dropkin).
  3. Marine mammals, that include cetaceans, sirenians (with dugongs and manatees) and seals; have two functions in the cycling nutrient process: they transport nutrients vertically (from deep to surface waters via excrements and urine) and laterally due to migrations. [Read more about cetacean migration]

IS EARTH AS FERTILE AS IT WAS?

The answer to this question is “no”.

New findings reveal that the global nutrient distribution capacity on land has been reduced to 8% of its former value. Nevertheless, there is regional variation: most of the current capacity is on Africa because it is full of megafauna species, while in South America the capacity is at 1% of the past value. In the past, South America had the largest number of big herbivorous (more than 1,000 kg) but all of them went extinct. Nowadays, the largest animals in the continent weights about 300 kg. This difference explains the large reduction.

Arctotherium bonariense was a bear that live in South America during the late Plistocene (Picture:).
Arctotherium bonariense was a bear that live in South America during the late Pliocene (Picture: W.B. Scott, Creative Commons).

The current capacity of oceans is more than three times higher than for land. However, the reduction of its capacity is also important: 2% of its former value in the Southern Ocean and 14% in the Atlantic Ocean. Concerning the vertical transport of nutrients, the amount of phosphorus transported from deep to surface waters is nowadays a 23% of the original transport, with differences among oceans. Behind these reductions, hunting pressure to marine mammals is present. Nutrients that fall below the well-illuminated zone are considered to be lost. Marine mammals would have been responsible of returning nutrients to surface by feeding on the deep ocean and defecating and urinating on the sea surface.

Finally, seabirds transports 6.3 million Kg for square kilometre of phosphorous from sea to coastal environments each year (former values are not available) and anadromous fish capacity is 4% of the past value, possibly due to overfishing and habitat modification (such as damming of rivers).

Potential interlinked system of recycling nutrients (Picture: Doughty et al. 2015).
Potential interlinked system of recycling nutrients. Grey animals represent extinctions.  (Picture: Doughty et al. 2015).

HOW CAN WE RESTORE THIS?

Doughty and his colleagues give some solutions to restore this situation:

  • Future pastures may have less fences and more species to simulate natural pastures.
  • Restoration of free-ranging wild herbivores.
  • Restoration of whale populations.
  • Restoration of seabird colonies and anadromous fish populations.

REFERENCES

Difusió-anglès

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