- network of fluid filled canals derived from the body cavity (coelom) that functions in gas exchange, feeding, sensory reception and locomotion. 

- water enters the system through the madreporite plate, which is above a small sac and connected to a duct called the stone canal. 

- The stone canal runts to a circular ring canal, which radial canals run outwards along the ambulacral grooves. Each side of the groove have radial canals which give rise to ampullae. The ampullae are connected to sucker like podia, which all fills with water and forms the tube feet. 

uses its tube feet to handle its prey. If the prey is too large to be swallowed whole, then it can use its tube feet to open shells. It can evert its stomach through its mouth and engulf its prey, liquify it with digestive enzymes and ingest the processed food. Pisaster ochraceus can insert part of its everted stomach, or some digestive juices, through the narrow gap that exists where the byssal threads emerge from the shell.

• Pisaster sexes can only be separated by the presence of eggs or sperm in the gonads. They reproduce by broadcast spawning and there is no parental investment beyond spawning.

• Many sea stars live to a minimal age of four years. The Pisaster ochraceus could live as long as twenty years

The cardiac stomach can be extended out through the mouth to digest food outside the body. Suspension-feeding starfish use their tube feet to pass food to the mouth. The cardiac stomach is connected to a pyloric stomach (located above it), which in turn is connected to both the anus and to the pyloric ducts and pyloric cecum which extend out into each arm.

• Competition = competing for resources, 3 types: Interference competition: occurs when an individual of one species directly interferes with an individual of another species. Examples include a lion chasing a hyena from a kill, Exploitative competition: This occurs via the consumption of resources.Apparent competition: occurs when two species share a predator. The populations of both species can be depressed by predation without direct exploitative competition.

also referred to as closed or unitary) is one where the species within the community are interdependent on each other for keeping balance and stability of the system. These communities are oftentimes described as working like a superorganism, meaning that every species plays an important part in the overall well being of the ecosystem which the community resides. Holistic communities have diffused boundaries, and an independent species range. Co-evolution is likely to be found in communities structured after this model, as a result of the interdependence and high rates of interaction found among the different populations. Developed by Frederic Clements. Populations in a community grew at the same rate, and each population was affected by the same things in the same way. 

interaction that benefits both, Rhizobium bacteria and the roots of legumes

one is harmed, another is helped, this includes predation and parasitism 

one is harmed, one is neutral (cows trampling grass) Spanish ibex and weevils eat the same bush, the weevil does not affect the ibex, the ibex negatively affects the weevil because it eats more of the bush and accidentally eats the weevils themselves. 

H. A. Gleason (1939), gained credence which held that communities were largely a coincidence of individualistic species characteristics, continuously varying environments and different probabilities of a species arriving on a given site. Essentially, a population could change, but its change was not because and does not affect another population. Populations grew and changed at a different rate than others in their community 

Dr. Steven Hubbell species are functionally equivalent, and the abundance of a population of a species changes by stochastic demographic processes (i.e., random births and deaths). Each population would have the same adaptive value (competitive and dispersal abilities), and local and regional composition would represent a balance between speciation or dispersal (which increase diversity), and random extinctions (which decrease diversity)]

species at higher latitudes have a wider geographical range than species at lower latitudes, expands on the idea that biodiversity has an inverse relationship with latitude is an ecogeographical rule that states that latitudinal ranges of plants and animals are generally smaller at lower latitudes than at higher latitudes.

says that predation opens up niches and reduces prey. Fewer prey lead to liberated resources which, if used by other species, leads to an increase in species richness. The concept of a keystone species is important to this theory. The keystone species is the species that a community is dependent upon and is often a predator. The keystone species maintains stability of the community which will increase species richness.

introduces the concept of r and k selection. r-selected organisms are described as rapid growing organisms that have broad resource requirements and k-selected organisms are described as slow growing organisms that have narrow resource requirements. r have many offspring and die young, k have a few offspring and long lives

suggests that diversity is directly related to the length of time that an area of land has been around. Specifically, land that has been around for a long time, undisturbed, tends to have more diversity.

states that temperature has an impact on the metabolic rates of organisms. 

that as species are added to an ecosystem, each one representing a potential disturbance, the native system is perturbed in such a way that the system reaches a threshold, at which point it cannot resist any further and invasions occur exponentially. Every individual invader is a threat, but it is their collective impact that can cause the greatest damage to a habitat

species-rich communities are more resistant to invasion because these types of communities are able to use the resources/nutrients more effectively/efficiently than communities with low species-richness.

hat local species diversity is maximized when ecological disturbance is neither too rare nor too frequent.

he more diverse a community is, the more stable it is and the more likely it will be able to bounce back from disturbance.. An experiment by Nelson Hairston using bacteria and Paramecium in a two tiered trophic system showed that the extinction rate would decrease with an increase of bacteria species, this result had an increased stability and supported the hypothesis. Dr. Tilman did a field experiment carried out over eleven years and found that variation in community biomass decreases with increased species richness. But when it came to year to year variation the biomass of a particular species increased with increasing species richness. These results concluded that diversity increases community stability but not population stability.

species that dominates an ecosystem in abundance and influence, plays a role in structuring the community, exert physical changes 

Type 1- Juveniles survive, most deaths occur at old age

Type 2- death occurs equally among ages

Type 3- death occurs mainly in juveniles, the old survive more