The feeding rate of predators can be expressed in terms of clearance rate (volume of water X predator — 1 X time — 1), or in terms of numbers or biomass of prey captured (predator —1 X time —1) and are often combined with estimates of predator and prey densities in order to estimate the effects of predation on prey populations. One characteristic of gelatinous predators is that clearance rates tend to be constant even at extremely high prey densities. In species with few tentacles (e.g., Pleurobrachia spp.), however, ingestion is limited by prey handling. Saturation feeding at typical prey densities rarely occurs in situ. Gelatinous predators feeding on small prey seldom fill themselves at natural prey densities. They may not reduce populations, but they do keep them in check. Predators that consume ichthyoplankton often appear in areas of intense spawning activity and are major causes of fish egg and larva mortality. In some areas large schy-phomedusae consume high numbers of commercially important fish larvae, and they also compete with fishes for food. Swarms of jellyfish may be so dense that they clog and damage fishing nets. Many hydromedusae species, including Por-pita spp., Velella spp., and Physalia spp., also occur in huge concentrations, particularly in tropical seas.
Some species have such a significant ecological effect that they are considered "keystone species." If these species disappear or appear in an environment, the entire habitat can shift dramatically. For example, active predation by sea stars can significantly affect prey populations. The crown-of-thorns sea stars (Acanthaster spp.) feed on coral polyps in tropical reef habitats worldwide. Occasionally, population explosions of A. planci occur that can have devastating effects on coral reefs. Thousands of square miles (kilometers) of bare coral skeletons can result. Species composition and diversity of other inhabitants are affected secondarily as algae and other organisms colonize the reef. Organisms that depend on live coral for survival either leave the area or die.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.