[throwaway5752]

Nothing that was talked about in the parent post is unique to marine ecosystems, is it?

The terrestrial food chains information is just incorrect. Fungi are absent in that description. Insects are eaten by countless prey species, apex predators have multiple trophic levels below them. There are countless insect, fungal, and animal carrion feeders.

A post that is so factually inaccurate should not be so upvoted. We shouldn't meta comment, but the rapid decrease in submission and comment quality on Hacker News is really a problem.

And the fact that land animals evolve to marine lifestyles is just expected, oceans are more than 70% of Earth's surface. The lack of marine, pelagic animals is notable in that sense.

[DanCarvajal]

Well I did say that I was simplifying somethings, which I think is useful as this is kind of a classic shorthand way of comparing example food chains for the layperson. My goal was to not lose the forest for the trees.

Come to think of it, I talk to a lot of five years olds which has definitely impacted how I approach topics.

[kuhewa]

The replies to your comment were wild to read through, strong vibes of 'how dare you simplify something and retain the essence of a valid point but fail to include this one tangental aspect I thought of'

[throwaway5752]

I feel like have to address some issues I have with your comment point by point. I am not even an expert specifically on this topic.

First, sea life bounced back faster than expected, not faster than other life. Sea life is believed to have been disportionately impacted by the P-Tr extinction phases. Plenty of terrestrial life survived the extinction period. Unless you or anyone else knows better, the worst impacted was on marine life that relied on calcium carbonate. Lots of life survived this extinction event, terrestrial and marine. This paper is not comparing terrestrial and marine recovery rates.

Second, the statement about terrestrial food chains is not simplifying, it is reductive to the point of being incorrect. What does an earthworm eat (or other annelids)? How many trophic layers are present in soil microbiomes? Terrestrial food chains are wonderfully deep and complex, even if marine ones are moreso. You don't have to detract from one emphasize the other. That reduces peoples' understanding.

Third, life originally evolved in marine environment, and migrated to land. Life evolves back and forth between the marine and terrestrial over time. Both are great, and life particularly likes the boundary. Saying "land animals keep going back to the ocean" ignores parts of the story, and also reduces peoples' understanding by giving an incomplete picture.

A good example of "simplify[ing] something and retain[ing] the essence of a valid point" is a wonderful thing, but I have have sincere issues with the degree of simplification and retention here.

[kuhewa]

1. The comment wasn't about the new findings of the study though, it was just a note about the length of food chains in the ocean.

2. I replied to the comment to point out that it was a slight exaggeration but directionally is correct. I took it to indicate exactly that, a comparison between length of food chains in either domain. And mentioning complexity among sediments and annelids among them doesn't change that because ocean has very similar and complex communities (and annelids assemblages). I doubt anyone came away from that message thinking that there really are no predators of predators on land but I could be wrong. Really, after intially replying about the shortcoming of the answer, after reading replies I was then left wondering what the commenter did wrong.

I dunno, I'm an ecologist and reasonably familiar with trophic ecology so I suppose it's possible the comment comes across as more misleading than I think to others, (maybe I'm filling in the gaps without realising it)

3. Life does keep coming back to the ocean. Sure there were separate emergences from it, but take tetrapods - I am pretty sure the current understanding is just one event close to 400 million years ago? OTOH dolphins, seals, and manatees all arose from separate ancestors much more recently.

[throwaway5752]

1. Fair, but I think it's reasonable to think people will contextualize it based on the submission like I did.

2. My biggest problem was with the level of reduction of terrestrial, not directionality.

3. That is a fair point, and I know you could have listed even more examples than just tetrapods. But the gills to lungs is also a much bigger step, and none of those examples have reacquired gills. So it comes back to the ocean, but with an asterisk.

My thanks for the stimulating discussion.

[kuhewa]

1. Maybe, I still haven't clicked on the article to be honest.

2. Yeah, since my first take before reading replies to point out it was exaggerated I reckon we don't fundamentally disagree. It has occurred to me since, the reason I probably saw this comment as useful is because while things are really way, way more complex as you've mentioned, when we seek to understand it, we usually do simplify things back down as far as possible while still capturing the essence of whatever aspect of the complexity. For example, simply calculating mean trophic level and simplifying organisms to a single number based on the average trophic level of what they eat [1] which enables modelling key aspects of the system [2]. Then you can do something like.. compare commonalities between a soil ecosystem, which may indeed have a three levels of consumers, to an African savannah. But you are right that it is important to consider a web and not a chain as the fundamental topology, but again if you want to look at one aspect, max lengths of the web, there's a good reason to pull out a single chain. For someone that can drown in complexity, I feel a debt of gratitude for people that have demonstrated when highly simplified models can capture aspects of trophic ecology.

Regarding point three, that's an interesting point. You might be interested in this recent study [3] that suggests that the purpose of gills was originally electrolyte balance, not gas exchange for respiration. If that's the case, terrestrial animals might not have the impetus to 're-evolve' gills. There's no right way to make a living in the ocean though, I reckon as soon as your lifecycle has no obligate terrestrial phases, you've made it.

Until next time, 10 y.o. 'throwaway' account!

[1] https://doi.org/10.1006/jmsc.1997.0280 [2] https://doi.org/10.1093/icesjms/fsr062 [3] https://www.nature.com/articles/s41586-022-05331-7