How Humanity is Changing the Course of Evolution Part 1: Under the Sea

Species must be able to adapt to changed circumstances unless they're to go extinct. There's not really much question that humanity has changed the circumstances for just about every single species on the planet- and as such, species are being forced to adapt to us. We might be driving as much as 10% of the total species on the planet extinct within the next century- but those that survive are likely going to be changed irrevocably.

An Atlantic cod, one of the species that has seen significant changes in response to human action. [Image source]

We're going to start off where life started: In the oceans. Some of our most severe environmental impacts have occurred there- I've actually got a 14 part series on it for anyone who is interested. These impacts have been huge, and are resulting in rapid changes in our oceans.

First of all we have the impacts done by our fishing industry, by far the most severe changes in the oceans. We've wiped out 90% of the large bony fish biomass in the oceans thanks to fishing. This has resulted in massive changes in our oceans. Fish size is the first change- the fish we're pulling from our oceans have grown steadily smaller and smaller over the years, and it's not just that we're catching all of the big ones- it's that we're forcing the whole species to become smaller. The average size of fish targeted by our fishing industries has declined by 20 percent. Research has shown that it's quite likely to get even more severe- in research programmes carried out in captivity, selectively targeting larger fish has been able to reduce the average size of fish by nearly half.

Fishermen engaging in longline fishing. Longline fishing is considerably better for the environment in many ways than net fishing- for one thing, it doesn't specifically target larger fish, but equally targets all sizes of fish, reducing evolutionary pressure against larger fish. [Image source]

The maximum size of fish hasn't just become smaller, either- the fish are reaching maturity sooner and at smaller sizes. There is some good news, however- this is possibly reversible if we act quickly. A similar effect happened to pike in Britain during the Second World War, as they were overfished during wartime scarcity. After the war, however, when the overfishing let up, they gradually returned to their pre-war size. This isn't a guarantee, of course- they'll only return to their larger size if enough of the genes from the larger fish survive and if conditions in the oceans continue to advantage the shrunken species to return to their larger sizes- something that's not guaranteed.

Another major impact fishing is liable to have is on species habitat choice. In fish, this is most likely to change the depth at which they primarily live. This is already being seen in Atlantic cod. As shallow dwelling cod are easier to catch, most of the focus of cod fisheries have been on them, rather than deep dwelling cod. And while deep dwelling cod come to shallow waters to spawn, cod fishing is banned during spawning time, further biasing evolution towards driving cod deeper. Similar effects are likely to occur for seamount dwelling fish like the orange roughy that have been heavily targeted by deep sea fishing. As fish flee deeper, fishing is liable to get more and more expensive.

Tuna, one of the mightiest predators in the sea, are being severely impacted by human activity. Several species of tuna are likely candidates for extinction, and all of them are evolving to reach full growth at smaller and smaller sizes. [Image source]

There are some palliatives for these changes, however. As mentioned above, switching fishing techniques can reduce or alter the evolutionary pressures on these fish. In addition, establishing larger marine wildlife sanctuaries has the effect of creating genetic reserves and wellsprings- populations will restore themselves in the sanctuaries, then spread out into neighboring seas.

It's important, however, to remember that evolution isn't just a matter of genetic changes in a single species, but also a matter of the changes in the species and ecosystems around them, and the rest of the oceans are changing radically around fish. One of the most important of these changes involves coral reefs- simply put, most of them are dying off, thanks to increasing ocean temperatures and acidity. Some will survive- Red Sea corals seem more resistant to higher temperatures and acidity than most other corals- but the ones that don't will cause intense changes in oceanic ecosystems. Coral reefs really are the rainforests of the sea in quite a large number of ways, and their loss will be felt sorely.

Coral reef losses are already severe- as many as a third of the coral in the Great Barrier Reef have already been lost to bleaching events and other environmental damage- and the global losses are comparably large. These losses are going to irrevocably alter evolution and ecosystems within the ocean. [Image source]

Take parrot fish, for instance. Parrot fish eat coral- or, more specifically, the algae growing off coral, but they swallow large amounts of coral every day. Their digestive system spits out sand from their other end- they might be the source of as much as one third of the world's beach and ocean floor sand. (Hawaii's white sand beaches are almost entirely parrotfish poop!) As the reefs die off, there will be less sand produced by parrotfish grazing the reefs. This loss of sand in these various ecosystems will result in changes that could be potentially catastrophic for many sand-dwelling species, and could force others to adapt in radical ways.

Numerous other ocean ecosystems are under threat, but that doesn't mean the whole ocean is getting ready to die- other organisms are already preparing to claw their way to the top- most notably, sea jellies (jellyfish) and squid. Sea jellies and squid are both notable for producing immense amounts of young. The only reasons they don't dominate the oceans is that they both make such attractive meals for fish. Both are essentially just lumps of pure protein without bones- a tasty meal indeed. This is probably why they reproduce in such large numbers- in order to keep from being eaten to death by fish. As large bony fish (and marine mammals) are wiped out, squid and sea jellies begin to gain an edge on their competition. It also helps that they're more tolerant to the toxins we're dumping into the ocean.

The Pacific Sea Nettle, a species of sea jelly. Sea jellies are looking to be one of the winners in the evolutionary arms race being provoked by human damage to the environment. [Image source]

On land, this wouldn't necessarily result in a permanent change in the same way that it would in the ocean, but land based food chains tend to be much simpler than oceanic ones. On land, you can generally just point to two species and say that one eats the other. In the ocean, both species are liable to prey upon each other at various points in their lives, even when they have huge size differentials once fully grown. Tuna might be some of the biggest, baddest predators in the ocean, but plenty of smaller fish eat tuna while they're young, even though they're tuna food once the tuna grow a bit more. This drastically increases the probability that oceanic ecosystems will alter in both drastic and stable manners- things are very unlikely to go back to the way they were before.

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Bibliography:

• The Ocean of Life, by Callum Roberts
• The Sixth Extinction, by Elizabeth Kolbert
• The Reef, by Iain McCalman
• Spineless, by Juli Berwald
• Squid Empire, by Danna Staaf
• https://www.newscientist.com/article/dn20420-unnatural-selection-fish-growing-up-fast/
• http://www.newsweek.com/many-white-sand-beaches-are-basically-fish-poop-369494
• https://en.wikipedia.org/wiki/Tuna
• https://en.wikipedia.org/wiki/Longline_fishing