Question

Part A: Barnacles (small sessile crustaceans) eat by filtering plankton (e.g. diatoms) out of the surrounding water. Barnacles compete for space on rocks in the intertidal zone with limpets, a kind of snail, and with mussels. Barnacles are preyed on by whelks that drill through their hard shell. Whelks have a commensalistic relationship with a polychaete worm that makes its home at the lip of their shell. Limpets and sea urchins graze on green algae and they are competing for this resource. Sea urchins are eaten by sea otters. Sea otters are sometimes infected by a trematode worm species that lays its eggs into the muscles of sea otters, where the young develop before burrowing out to disperse through the ocean water.

Lower levels of the food web are generally on the bottom; consumers are toward the top. Start with the plankton and green algae on the bottom. Use double-headed arrows between interacting pairs of species and put the relevant sign (-, + or 0 = cost, benefit, or no effect) next to each arrow head. Per convention, use horizontal arrows for competition, mutualism and commensalism and use vertical arrows for parasitism, herbivory and predation.

Part B: What do you expect to happen to the limpet population if barnacles become more efficient filter feeders and grow faster?

Part C: What do you think would happen to the green algae population if the trematode worms became more abundant and infected more otters?

Answer 1

Answer to part A

Answer to Part B

Since the relation between barnacles and limpets is that of competition, overnumbered barnacles may occupy more space and would lead to crisis for limpets. This may have a negative impact and drop the limlet count in account of low number to counteract the heavily populated barnacles.

Answer to part C

Trematode worms are parasites to sea otters and will affect only them as per the given food chain scenario. If they become abundant and infect more sea otters, the number of sea otters will reduce and predation of sea urchins will drop. That should not however effect directly or proportionally the algal population. The effect on algal population will be greatly affected with change in pattern of relationship between limlets or sea urchins or if more predators were to join them to gain energy from algae.