Will the ocean follow the land? Marine ecosystems at a tipping point to follow terrestrial defaunation

Figure courtesy of [1]

Figure courtesy of [1]

I’m late on this, but still an important story to discuss…a review in Science last week highlights the precipice on which we as a society perch regarding the potential massive loss of marine ecosystems.  The article reviews similar signs in the path that terrestrial mass extinctions have taken, and some of the same warning signs are popping up for marine life.  I’ve seen some brief overviews of the article, but I thought I’d go into a little more detail.

Extinctions have been happening on land, but can we really have such a large effect on ocean life?

We know the terrestrial ecosystems are in trouble.  We can see it with our eyes, report how climate change and the continued destruction of forest acreage is destroying habitats, plot changes in migration patterns, and so forth.  Our scientific tools to track these phenomena have been fine-tuned over centuries and we now have a clearer understanding of what our actions do to these ecosystem (hint: it’s not good).  These problems began tens of thousands of years ago as human civilization settled down and agriculture and buildings became standard ways of life.  The industrial revolution severely exacerbated these effects, dramatically speeding up the losses of diverse ecosystems due to anthropogenic climate change, over-consumption, pollution, and more.

The oceans, however, are more difficult for us to reach.  Because of this, even though humans began fishing tens of thousands of years ago, it is only in the last century that industrialized methods have penetrated marine ecosystems with the intensity required to have a dominant effect on their future trajectories.  Below we can see the terrestrial extinctions on a massive rise in the last century, but with a significant delay in marine extinctions:

Figure courtesy of [1]

Figure courtesy of [1]

So that’s hopeful, in a sense – marine life still has a low rate of extinction.  It’s also extremely sobering to think about what we’re doing to our land-locked life.  To give some numbers, there have been 514 terrestrial extinctions over the last five decades compared to NO marine extinctions (15 marine extinctions over last 500 years).  Part of this could be lack of information, since we’ve been able to catalog more land-based species, but this dramatic difference still highlights that we have not affected marine life as much…yet…

The oceans are also opaque to us – it’s harder for us to jump in and collect data.  This opaqueness, by the way, likely also eases our guilt as we dump waste into rivers and lakes, letting the trash, as it sinks below the surface, also sink out of our mind, drowning any voice our conscience may have.  But this lack of knowledge about how our actions – polluting, over-fishing, climate change – are actually affecting marine life can be seen in the following graph:

Figure courtesy of [1]

Figure courtesy of [1]

The graph separates different marine fauna – all those in the green box on left have some land interaction, whereas those in blue are only ocean-based.  Orange indicates percent of extinct species for each category, red is endangered, and gray and black signal data deficient or unreviewed.  Based on these categories,  look how much more we know about species that even spend just some of their time on land!  We know nothing about the endangered possibilities for marine ray-finned fishes (MRF) or marine invertebrates (MI), which both act as bedrocks of marine ecosystems.  So we definitely need more data…but moving on…

Where are the oceans heading?

The scary omens appear when we compare trends in current marine life extinctions to past terrestrial trends.  Extinction rates for marine life now look very similar to terrestrial rates immediately before the Industrial Revolution, when massive human expansion and environmental impact fomented the onset of higher extinction rates.  We are at the very same point with marine life, as the ‘oceanic’ industrial revolution began about 100 years ago.  So what changes are we now seeing in marine ecosystems to signal such a greater threat?

The main signal is seen by measuring different types of extinction instead just global trends.  These all serve as warning signs that a population is losing its stability:

1) Local extinction: more marine life is completely disappearing from specific geographic regions, even if their species as a whole still exists globally.  This is occurring for many marine fishes and invertebrates.

2) Ecological extinction: certain species have reduced enough (marine vertebrates by 22% and some whales up to 90%) that they no longer perform their ‘ecological duty’.  This has been seen terrestrially and causes an extreme disruption in the delicate balance of various species sharing the same habitat.  Such extinction types inevitably lead to drastic rearrangements of local populations that could lead to local extinctions.  This leads to empty corral reefs, empty bays, and so forth, where life has just vanished.

3) Commercial extinction: Species diminish so much that they can no longer be economically harvested.  Basically, not enough to make money off of them!  This has happened to grey whales in the last century and great whales in Antarctica.

The other major trends are seen through top-down or bottom-up processes.  In top-down processes, high-level predators disappear, leading to bursts of lower-level species that disrupt the ecosystem.  Conversely, loss of the bottom feeders will reduce reproduction rates and populations of predators.

But how does this affect us?

So let’s say could care less about the natural beauty of a diverse planet, full of well-balanced ecosystems with more species than we could ever possibly count  Let’s say you have a human-centric view, that whatever works best for our species is the best decision.  So what if all these other species die off just as long as we’re still around?  Should you care about this precipice upon which our civilization stands?

YES!  Even taking a self-centered point of view, you need this diversity.  Some reports indicate we eat 40 times more biomass from oceans than land.  Another reason to go vegetarian!

But it’s about more than just diet.  It’s about protection.  Cloud formations are stimulated by chemical releases from coral reefs.  Coral reefs also absorb huge amounts of energy from incoming waves that could damage coastal communities.

Over-fishing and pollution are horrible for marine ecosystems, but can climate change really be having an effect, too?

Most definitely and in several ways.  First, the ocean has a huge heat capacity and therefore it takes a lot of energy to change its temperature.  This means that marine life has not had to adapt to temperature changes and are ill-prepared, compared to terrestrial organisms, to deal with any change due to global warming.

The other major effect of climate change is to increase ocean acidification.  As more carbon dioxide enters the atmosphere, it is absorbed into the ocean where it converts into carbonic acid.  There is a tremendous, natural buffer system that occurs in the ocean that allows it to take up so much CO2 to prevent more terrestrial warming.  However, this slow build up of acid will leave new regions untenable for marine life.  This has already led to certain species leaving traditional locations and moving north, to colder, less acidic waters.  But if we keep releasing emissions, the acidic warm waters will continue to chase them.

This was a lot of doom and gloom – do we have no hope?

There is still hope!  Past devastation to specific ecosystems have demonstrated their incredible resilience.  Otters almost went extinct last century but are now flourished in many roles due to human protection.  Atomic bombs were released near coral reefs in the Pacific that have since recovered to health.  As much damage as we do, we also have the potential to put tremendous effort towards conservation and regrowth.  We just need to know what we need to save and how.

Also, marine life has significantly more mobility than terrestrial life to move away from poor environments due to the fluid medium in which they live.  Therefore, they may be more resilient – as seen below from the percent of given species with range contractions ( reduced living regions associated with local extinction):

Figure courtesy of [1]

Figure courtesy of [1]

Larger pelagic fishes (PF) show significant but variable range contraction, however fish and invertebrates (TFI) show some resilience.  Some of these signs suggest that marine life may not take the exact same path as terrestrial animals, giving us a little more time to manage and protect.

The funny thing about the Anthropocene era is that it’s not hurting all species.  So-called ‘ecological winners’ are seen due to our disruption, usually smaller species, like marine invertebrates, that reproduce fast and are more durable against fast environmental changes. Our species’ dominant effect right now seems to be reducing diversity, and unless we began assigning more resources to understanding and protecting marine life, a new generation of extinctions may be around the corner.




McCauley, D., Pinsky, M., Palumbi, S., Estes, J., Joyce, F., & Warner, R. (2015). Marine defaunation: Animal loss in the global ocean Science, 347 (6219), 1255641-1255641 DOI: 10.1126/science.1255641

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