[19.5] investigating populations

the number of individuals of a species in a given space

• random sampling using quadrats • systematic sampling along a belt transect

• size of quadrat to use • number of sample quadrats to record in study area • position of each quadrat in study area

• 100 squares is easy for percentage calculation • only count a plant as present if it covers >50% of the square

• lay out 2 tape measures at right angles along 2 sides of the study area • obtain a series of coordinates by using random numbers generated by a computer • place a quadrat at the intersection of each pair of coordinates and record the species within it

• large sample size • use many quadrats • obtain mean of all samples the larger the sample, the more representative of the community as a whole the results will be

if you wanted to look at systematic sampling at a transition between one environment to another

• known number of motile animals are caught, marked and released back into the community • later, some individuals are randomly collected • number of marked individuals is recorded • size of population is calculated

(total number of individuals in 1st sample × total number of individuals in 2nd sample) / number of marked individuals recaptured

• proportion of M:UM idv in 2nd sample is same as proportion of M:UM idv in pop as a whole • MIs released from 1st sample distribute themselves evenly in rest of pop and have enough time to do so • pop has definite boundary so no immi / emigration • few, if any, deaths and births within population • method of marking isn’t toxic to idv • method of marking doesn’t make idv more conspicuous and therefore more liable to predation • mark isn’t lost or rubbed off during investigation

• sample death / birth • colour could rub off so estimate would be bigger • sample from same area • could have sample place, apparatus and method but confounding variables eg. weather, tide level • need to leave enough time between taking 2nd sample • sample could leave area entirely