Comment by DesignerPangolin on 02/11/2023 at 02:23 UTC

45 upvotes, 1 direct replies (showing 1)

View submission: Are there more predatory fish or non-predatory fish? If there is one, why the discrepancy?

Your intuitive observation is for the most part correct.

In most (all?) terrestrial ecosystems, you tend to have a pyramid of biomass: The greatest biomass is in plants, there's less biomass in herbivores, less biomass still in predators, and less biomass still in the predators that eat the predators. Think of the mass of grass on the Serengeti compared to the total mass of lions.... it's huge in comparison!

• *In aquatic and marine ecosystems, you often observe the exact opposite.** Algae biomass is low. Bugs that eat algae are more, insectivorous fish even more biomass, and the most biomass in apex predators. This is called an inverted biomass pyramid. In order to sustain this food web structure, you need the lower steps on the pyramid to reproduce very rapidly to make up for the fact that they're always getting eaten, and the upper steps to reproduce progressively more slowly. So, in many aquatic ecosystems there's actually very little algae, but what little there is grows quickly enough that there's always a constant low-level supply of food for herbivores. And those bugs (and small fish) reproduce quickly enough that there's a low but constant food supply for the predators that eat the bugs. In many lakes, coral reefs, kelp forests, this inverted biomass pyramid is the rule rather than the exception. Here's a good paper that examines the general phenomenon in marine systems: https://www.nature.com/articles/s41467-017-02450-y[1][2]

1: https://www.nature.com/articles/s41467-017-02450-y

2: https://www.nature.com/articles/s41467-017-02450-y

In terrestrial ecosystem ecology, there's a strong focus on measuring "primary production" the amount of plant biomass produced, as an indicator of the total energy flows through the ecosystem. By contrast, in aquatic ecosystems, there is often a very strong focus on "secondary production", the amount of biomass of bugs that eat algae, as an indicator of total energy flows, because the algal biomass is very small but the turnover of algae in that biomass pool is huge.

Edit: removed jargon

Replies

Comment by ven_geci at 06/11/2023 at 12:45 UTC

2 upvotes, 1 direct replies

This is awesome! But the causality has to be obviously the other way around? That is, algae came first and reproduced really quickly. Herbivores came and reproduced quickly on algae, but they could not eat the algae into extinction as they reproduced really quickly. And so on. So there is something in water that pushes things towards fast reproduction?