Questions about the Food Web...?

Question

1. Why is it beneficial that many predatory fish have larval and juvenile stages that feed at a low trophic level, while the adults feed at a tertiary or quaternary trophic level?

2.How does the location of each animal's position in relation to the producers contribute to their size?

3. Why do the plankton feeders are able to attain such large sizes compared to the carnivores?

Answer 1

1. Predators generally need to be larger than their prey, and they need some ability to trap or capture prey. Newly-emerged fish are small, they do not have good swimming skills, and since most fish do not have teeth, they can't eat anything larger than their mouth - not much food available with all of these limitations unless you can eat algae, plankton, or detritus.

2. Not sure there is actually a relationship between animal position, size and location of producers, except possibly for herbivores that have to be near their food. There are some very large herbivores that are sedentary - like hippos, and there are very large herbivores that migrate 100's of miles - like bison and caribou.

3. Not sure this is a general rule for plankton feeders, but it seems to be true for filter feeders, if you consider only the half-dozen largest marine species. The largest marine fish is the whale shark which feeds on plankton, but the largest animal in the ocean is the blue whale which is a predator that feeds on krill (mostly shrimp). The second largest fish in the ocean is the basking shark which is an omnivore. What these large marine species have in common is that they are filter feeders (not plankton feeders). Nobody knows for sure why these animals are so big, but one thing is certain, gigantic whales and sharks would have an extremely difficult time chasing fish and catching them individually -they would spend more energy chasing food than they took in. The only way a gigantic fish or mammal can get enough to eat is if they don't require much energy to obtain food. This illustrates one of the significant aspects of evolution - It just so happens that the most abundant food in the open ocean are krill and plankton, which are most effectively "harvested" by filtering instead of biting. Filter feeders can get a lot of food with little effort, which allows plenty of calories for growth. But filter feeders are an exception to the size rule. In general, most of the largest fish are predators - tuna, most sharks, halibut, ocean sunfish, etc.