what would you expect to happen if sea otters were removed from this food web? check all that apply. the population of elephant seals would decrease. the population of kelp crabs would increase. the population of kelp would increase. the population of sea urchins would decrease.

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# Brief Explanations:
1. Analyze the food web: Sea otters eat kelp crabs. If sea otters are removed, kelp crab predators decrease, so kelp crab population should increase.
2. For elephant seals: Their predator (horn shark) - sea otters' removal doesn't directly affect elephant seals' food source (horn shark's other prey? No, the web shows elephant seal is prey of horn shark? Wait, no, the arrows: horn shark eats sea urchin and sea otter? Wait, the arrow from horn shark to elephant seal? Wait, no, the diagram: elephant seal has an arrow up to horn shark? Wait, no, the labels: elephant seal, horn shark (arrow from horn shark to elephant seal? No, the arrow is from elephant seal up to horn shark? Wait, no, the arrow direction: usually, arrow points from prey to predator. So elephant seal is prey of horn shark? Wait, the diagram: elephant seal, horn shark (arrow from elephant seal to horn shark? No, the arrow is from horn shark to elephant seal? No, the user's diagram: "Elephant seal" with an arrow up to "Horn shark" – so horn shark eats elephant seal? Wait, no, maybe I got the arrow direction wrong. Wait, the other arrows: sea urchin to horn shark (arrow up), kelp to sea urchin (arrow left, so sea urchin eats kelp), kelp to kelp crab (arrow right, kelp crab eats kelp), kelp crab to sea otter (arrow up, sea otter eats kelp crab), horn shark to sea otter (arrow left, horn shark eats sea otter), and horn shark to elephant seal (arrow up, horn shark eats elephant seal)? Wait, this is confusing. But the question is about sea otters removed. Let's re-express the food web:

- Kelp is eaten by sea urchin and kelp crab.
- Sea urchin is eaten by horn shark.
- Kelp crab is eaten by sea otter.
- Sea otter is eaten by horn shark.
- Horn shark is eaten by... no, elephant seal: arrow from elephant seal to horn shark? So horn shark eats elephant seal? Wait, maybe the arrows are prey → predator. So:

- Kelp → Sea urchin (sea urchin eats kelp)
- Kelp → Kelp crab (kelp crab eats kelp)
- Sea urchin → Horn shark (horn shark eats sea urchin)
- Kelp crab → Sea otter (sea otter eats kelp crab)
- Sea otter → Horn shark (horn shark eats sea otter)
- Elephant seal → Horn shark (horn shark eats elephant seal)

Now, if sea otters are removed:

- Sea otter's prey (kelp crab) has no predator (sea otter), so kelp crab population increases (so "The population of kelp crabs would increase" is correct).
- Sea otter's predator (horn shark) now has one less prey (sea otter), so horn shark might eat more of its other prey: sea urchin or elephant seal? But if horn shark eats more elephant seal, elephant seal population would decrease? Wait, but the first option: "The population of elephant seals would decrease." Let's check: If sea otters are gone, horn shark's food from sea otters is gone, so horn shark will hunt more elephant seals (if that's its prey) or sea urchins. If horn shark eats more elephant seals, elephant seal population decreases. So that could be correct. Wait, but maybe my initial analysis of the food web is wrong. Alternatively, maybe the arrow from elephant seal to horn shark is the other way: horn shark is prey of elephant seal? No, that's unlikely. Alternatively, maybe the food web is:

- Kelp (producer)
- Sea urchin eats kelp
- Kelp crab eats kelp
- Sea otter eats kelp crab
- Horn shark eats sea otter and sea urchin
- Elephant seal eats horn shark? No, the arrow is from elephant seal to horn shark, so horn shark eats elephant seal.

This is getting too complicated. Let's focus on the options:

Option 1: Elephant seal population decreases. If sea otters are removed, horn shark loses a prey (sea otter), so it might eat more elephant seals, so elephant seal population decreases. Possible.

Option 2: Kelp crab population increases. Sea otter eats kelp crab; remove sea otter, kelp crab has no predator (from sea otter), so population increases. Correct.

Option 3: Kelp population increases. Kelp is eaten by sea urchin and kelp crab. If kelp crab population increases (because sea otters are gone), kelp crab eats more kelp, so kelp population would decrease, not increase. So option 3 is wrong.

Option 4: Sea urchin population decreases. Sea urchin is eaten by horn shark. If sea otters are gone, horn shark might eat more sea urchin (since it lost sea otter as prey), so sea urchin population would decrease? Wait, but earlier I thought kelp crab increases, which eats kelp, but sea urchin also eats kelp. If horn shark eats more sea urchin, sea urchin population decreases. But let's re-express:

Food web:

- Kelp → Sea urchin (prey → predator: sea urchin eats kelp)
- Kelp → Kelp crab (kelp crab eats kelp)
- Sea urchin → Horn shark (horn shark eats sea urchin)
- Kelp crab → Sea otter (sea otter eats kelp crab)
- Sea otter → Horn shark (horn shark eats sea otter)
- Elephant seal → Horn shark (horn shark eats elephant seal)

So when sea otters are removed:

- Sea otter (predator of kelp crab) is gone → kelp crab population increases (option 2 correct).
- Horn shark (predator of sea otter, sea urchin, elephant seal) loses sea otter as prey → so it will hunt more sea urchin and elephant seal.
  - Hunting more elephant seal → elephant seal population decreases (option 1 correct).
  - Hunting more sea urchin → sea urchin population decreases (option 4: "The population of sea urchins would decrease" – is that correct? But sea urchin eats kelp, and kelp crab also eats kelp. If sea urchin population decreases, kelp is eaten less by sea urchin, but kelp crab is eating more kelp (because kelp crab population increased). So kelp population: sea urchin eats less, kelp crab eats more. Net effect? Not sure, but the option 4 says sea urchin population decreases. If horn shark eats more sea urchin (because it lost sea otter), then sea urchin population decreases. So option 4 could be correct? But earlier analysis of option 2: kelp crab increases (correct), option 1: elephant seal decreases (possible), option 4: sea urchin decreases (possible). But let's check standard keystone species: sea otters are keystone, eat urchins? Wait, maybe I got the food web wrong. Maybe sea otters eat sea urchins, not kelp crabs. Oh! That's a common mistake. In real life, sea otters eat sea urchins, which eat kelp. So maybe the diagram has a mistake, or I misread the arrows. Let's re-express: maybe the arrow from sea urchin to sea otter (sea otter eats sea urchin), not kelp crab. Then:

- Kelp → Sea urchin (sea urchin eats kelp)
- Kelp → Kelp crab (kelp crab eats kelp)
- Sea urchin → Sea otter (sea otter eats sea urchin)
- Kelp crab → Horn shark (horn shark eats kelp crab)
- Sea otter → Horn shark (horn shark eats sea otter)
- Elephant seal → Horn shark (horn shark eats elephant seal)

Ah, this makes more sense (real-life sea otter - urchin - kelp relationship). So if sea otters are removed:

- Sea urchin population increases (because sea otter eats them; remove sea otter, urchin has no predator), so urchin eats more kelp, kelp population decreases.
- Kelp crab: horn shark eats kelp crab; sea otter's removal doesn't affect kelp crab (since horn shark still eats them), so kelp crab population stays same or... no, this is conflicting.

But the diagram provided by the user shows "Kelp crab" with an arrow up to "Sea otter" – so sea otter eats kelp crab. So back to the user's diagram:

- Kelp → Sea urchin (arrow left: sea urchin eats kelp)
- Kelp → Kelp crab (arrow right: kelp crab eats kelp)
- Kelp crab → Sea otter (arrow up: sea otter eats kelp crab)
- Sea urchin → Horn shark (arrow up: horn shark eats sea urchin)
- Sea otter → Horn shark (arrow left: horn shark eats sea otter)
- Elephant seal → Horn shark (arrow up: horn shark eats elephant seal)

So:

- Sea otter's prey: kelp crab.
- Sea otter's predator: horn shark.
- Horn shark's prey: sea urchin, sea otter, elephant seal.
- Sea urchin's prey: kelp.
- Kelp crab's prey: kelp.

Now, remove sea otter:

1. Kelp crab: no predator (from sea otter), so population increases (option 2 correct).
2. Horn shark: loses sea otter as prey, so it will eat more sea urchin and elephant seal.
   - Eating more elephant seal → elephant seal population decreases (option 1 correct).
   - Eating more sea urchin → sea urchin population decreases (option 4: "sea urchins would decrease" – correct? But sea urchin eats kelp, and kelp crab (now increased) also eats kelp. So kelp is eaten by both; if sea urchin decreases, kelp eaten by sea urchin decreases, but kelp crab eats more, so kelp population: let's see, option 3: kelp increases – no, because kelp crab is eating more. So option 3 is wrong.

But the options:

- Option 1: Elephant seal decreases (correct, because horn shark eats more of them).
- Option 2: Kelp crab increases (correct, because sea otter is gone, so kelp crab has no predator from sea otter).
- Option 3: Kelp increases (wrong, because kelp crab eats more kelp).
- Option 4: Sea urchin decreases (correct, because horn shark eats more of them).

Wait, but the original diagram might have different arrow directions. This is very confusing. But based on the diagram provided (kelp crab → sea otter), so sea otter eats kelp crab. So removing sea otter, kelp crab population increases (option 2 correct).

Also, horn shark eats sea otter and elephant seal (if arrow is from elephant seal to horn shark, meaning horn shark eats elephant seal). So removing sea otter, horn shark eats more elephant seal, so elephant seal population decreases (option 1 correct).

So the correct options are:

- The population of elephant seals would decrease.
- The population of kelp crabs would increase.

Wait, but let's check again. If sea otters are removed, kelp crab (prey of sea otter) has no predator (from sea otter), so kelp crab population increases (option 2 correct).

Horn shark (predator of sea otter and elephant seal) loses sea otter as prey, so it will hunt more elephant seals, so elephant seal population decreases (option 1 correct).

Sea urchin: horn shark also eats sea urchin. If horn shark eats more elephant seals and sea urchins, sea urchin population decreases (option 4: "sea urchins would decrease" – correct? But sea urchin eats kelp, and kelp crab (increased) eats kelp. So kelp is eaten by both; if sea urchin decreases, kelp eaten by sea urchin decreases, but kelp crab eats more, so kelp population: let's see, option 3: kelp increases – no, because kelp crab is eating more. So option 3 is wrong.

But the problem says "check all that apply". So possible correct options:

- The population of elephant seals would decrease.
- The population of kelp crabs would increase.

Wait, but maybe the arrow from horn shark to elephant seal is the other way (elephant seal eats horn shark), but that's unlikely. Given the diagram, the most plausible correct options are:

- The population of elephant seals would decrease (A)
- The population of kelp crabs would increase (B)

But let's confirm with the diagram:

- Sea otter eats kelp crab (arrow from kelp crab to sea otter) → remove sea otter, kelp crab population increases (B correct).
- Horn shark eats sea otter and elephant seal (arrow from elephant seal to horn shark) → remove sea otter, horn shark eats more elephant seals → elephant seal population decreases (A correct).

So the correct options are A and B.
# Answer:
A. The population of elephant seals would decrease, B. The population of kelp crabs would increase

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