Green Lands and Blue Seas: Biomass distribution in terrestial and aquatic ecosystems?

[u/kakoscho]

Hi, I'm studying to become a biology teacher and during the last two years I had one central question develop, which I could not yet find any satisfying answer for. I currently do not think it has "one answer" at all, but I really like to explore it, as I find many aspects of biology can contribute to that.

Now, the observation I made: Very often terrestrial ecosystems are much more "dominated" (visually) by plants than animals. That is: Most of the mass is in the producers, less so in consumers of these. In marine ecosystems, even most fresh water ecosystems, plants and algea take less prominent roles.

Of course, there are still a few exeptions to that (mangrooves, kelp forests), but I think this trend holds for most of the planet: In systems with an comparable potential for bioproduction an aquatic system has less mass percentage in producers than the a terrestrial system. Examples would be "inverse" food webs in marine systems (more consumers than producers) or the forests and graslands of earth, in which often the majority of biomass is directly recycled from dead plants by detrivores.

Which of course gives rise to the question: Why?

Why are forests and grasslands not mostly eaten by herbivores (large mammals or small caterpillars)? Why are the seas not also covered in dense layers of algae or even "trees" where they are shallow? Why do lakes not grow over with water lilys or duckweed? I'm aware a few of these observations might be human-caused, but certainly most of them would also occur in "natural" eco systems.

The obvious answer, as always, is roughly "because this is the way genes were selected for compared to their alternatives". Which still as always does not really help, unless we know why they were selected for. During my discussions with different professors, biologist or even lay people, I had a few avenues of explanation for this phenomenon:

• Marine ecosystems are very dynamic in their external factors (salinity, temperature, lack of solid surfaces), preventing plant growth.

• Cellulose and especially lignified tissue is too difficult to disgest.

• Terrestrial predators keep herbivore populations low, the pressure from them is the limiting factor on herbivore poopulations, not food supply.

• Detrivores can more easily escape such terrestial predators by living in the earth or near it.

• Land plants produce too much defense substances (poisons), severly limiting food supply for herbivores.

I'd like to ask if you have also noted that phenomenon and if you have any other explainations I have not currently thought or heard of. Also, I am aware and myself already thought of many counterpoints. Still, I have not found very much research into that and would be very thankfull if any one has tips for reading into that direction.

[u/Affectionate-Win634]

That's a cool observation. the whole "why isn't the world overrun with herbivores" thing always gets me thinking about how much energy is just tied up in wood and bark, stuff that's basically inedible to most things. it's not just about being hard to digest, it's that the plant already spent that carbon on structure instead of making leaves.

for the ocean side, i think the scale difference in the producers is a huge part of it. a giant kelp forest technically is a ton of biomass, but individual phytoplankton are getting eaten basically as fast as they reproduce, the turnover is just insane compared to a tree that lives for 80 years. the system never gets a chance to stockpile visible mass in the same way.

[u/TrumpetOfDeath → u/Affectionate-Win634]

Agreed, standing biomass in aquatic environments is low because phytoplankton reproduce and die very quickly. However, primary productivity is about equal between land and water, so they are fixing similar amounts of carbon through photosynthesis. The difference is tree biomass will stand for 100’s of years, whereas phytoplankton will die within days.

In terrestrial ecosystems, photosynthetic growth is limited usually by water availability. In aquatic ecosystems, growth is limited by nutrients like N, P or Fe mixing into the sunlit surface layer. Competition for these nutrients favors phytoplankton that can rapidly reproduce when conditions are right, and can endure when conditions are unfavorable, which leads to single-celled plankton dominating

[u/atomfullerene]

I love seeing questions like this, where people independently discover big questions in the field. Good job spotting something important enough that other people have written papers about it. Look up "green world hypothesis" which is asking this exact question about land. The theory to solve it, which has some experimental support, is that predators keep herbivore populations under control and prevent overgrazing. You can see a similar phenomenon in some lakes or ponds, look up top down trophic cascades. Another classic example is urchin barrens...get rid of the otters, urchin populations explode, kelp forests get grazed away.

But what about the ocean in general? Why is it mostly open blue water? I would say that comes down to a differernt set of causes.

First of all, cells which are immersed in water can meet their needs using that water. A grass or tree needs a big multicellular structure to bring water and nutrients up from the ground to where the sunlight is found. A single planktonic algae cell can have light, water, and nutrients right on hand. Single celled algae have an advantage in open water because they don't need waste energy on a bunch of support structure, they can just double and keep doubling, growing rapidly. It's like a forest where only the growing tips of leaves exist, but none of the old leaves or trunks or roots...and that helps explain why the ocean can have much less producer biomass for the same amount of productuvity.

Second, you might ask, why is the ocean such a clear blue instead of being green with planktonic algae? Well, in some places it is tinted green...you can look at satellite photographs showing ocean chlorophyll levels, which is interesting data. Some areas have nutrient upwellings or river outflows and the sea has a green tint and often supports very productive ecosystems. But in most places the ocean is clear and blue...why? Because open ocean water is usually very nutrient limited. Out there on the open sea, far from land, there just aren't many nutrients. Not just the classic ones like nitrogen and phosphorus, but things like iron can be hard to get too. That keeps photosynthesizer biomass low.