#### Calculation exercise - the importance of larvae´s survival and development time.

Imagine a population of 1 000 females that during a period of reproduction produce 1 000 000 eggs each. Suppose then that for every day the larvae spread, mortality is a constant ratio of those larvae that are alive. The number of larvae that survive the end of a given day (t) we call Nt. Nt is therefore a function of the number of larvae at the beginning of the reproductive period:

Nt = N0 xe-Rt

where e is the base in the natural logarithm system (about 2,718) and R is the momentary rate of death. In our example, at the outset number of offsprong is 109. Let the period that the larvae need to fully develope in the water be 20 days and let R be 0,69. If we include these values in the function above, the result would be exactly 1 000 offspring that survive and able to establish themselves at the end of a reproductive period. This means that at the end of a season, every female is replaced by one individual.

Calculate yourself and see if you can answer the following two questions:

(1) Mortality amongst larvae can vary from year to year, for example if the number of predators vary in number or they are more active certain years. Would it be natural to assume that a variation in mortality one year can have a large effect on the number of individuals that are able to establish themselves after the distribution period?
(2) The time it takes for larvae to develope can vary from year to year. Several factors effect growth, but food and tempeature are probably the most important. Can we assume that a change in the development time would have a noticable effect on the number of individuals that establish themselves after the distribution period?

Calculate what the effects of a 10% decrease and increase in mortality and development time would have.
You can compare your calculations with ours on the next page.
 Calculation exercise Page 1 of 2
Cliffs and rocks are fantastic!
Zoning and flecked occurrence
Animals that are attached
Modular construction
Variation and change
Variations in water level
Wave exposure
Both cliffs and rocks
Freshwater and saltwater
Geography, climate and history
Organisms life cycles
Organisms effect on each other
Energy and the flow of material

Home    Contents    Inspiration    Facts    Collaboration