Population Sampling and Enumeration PDF

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Population Sampling and Enumeration...

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Population Sampling and Enumeration Sampling and Counting a Population - If we are studying a population and wish to know something about it, then we ask such questions as, “How many individuals are there? Where are they? How many samples?” - Before sampling a population, it helps to know something about the plant or animal of interest, such as their biology and ecology o Then you’ll know what time of day, time of year, season, etc to look for them - May want to determine population sizes, or understand why a population is declining, find out specific habitat requirements, or monitor populations over a long period of time to quantify its population dynamics - Abundance (how many) is related to prevalence (where in space) x intensity (how many/ unit area) - Plants, invertebrates, fish, reptiles and amphibians, birds, and mammals all have their own unique sampling methods Sampling Techniques - There are many ways to count individuals in a population: o Direct counts o Mark-recapture o Trapping o Point and line transect o Sampling o Others - Needs balance between precision of samples and the cost of taking more samples, if time permits (more samples will narrow the sampling error, but it ‘costs’ more in terms of time and resources) - Beware collecting more samples than you have time to process - Plants: o Frame quadrats o Transects o Soil cores o Mapping - Invertebrates: o Aquatic and terrestrial o Pond nets o Sweep netting o Blacklighting o Emergence traps o Pitfall traps o Malaise traps o Bait traps - Fish: o Bankside counts o Underwater observations

o Electrofishing o Trawling o Hook and lining o Trapping o Hydroacoustics o Egg counts - Reptiles: o Trapping o Hand capture o Marking - Amphibians: o Recognizing individuals o Drift fencing o Scan searching o Netting o Trapping - Birds: o Counting nests, leks, roosts, flocks, migrants o Map territories o Point counts o Line transects o Mist netting o Timed species counts - Mammals: o Breeding sites o Bat roosts o Strip and line transects (ground and aerial) o Trapping o Call counting o Count dung o Locate signs of feeding o Footprints o Cameras Direct Counting - All plants in an area can be directly counted, but only if you can positively identify each species - Harder to do for animals that would move around a lot o Will you find them all or miss too many? o Population estimates depend on accuracy of counts - Detectability can be reduced if counting is done during inclement weather Mark-Recapture

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If you capture a sample of animals from a population, mark them, and release them to mix with the overall population, then if another sample is taken, the proportion of marked/unmarked individuals in the second sample can be used to calculate the total population size - Marking is done with paint, dyes, fluorescent dust, toe-clipping - Assumptions are that marks are retained, all animals in the population have equal trapability - Methods include Petersen (closed population) and Jolly-Seber (open population) methods - Closed population = no gains (by birth/immigration or losses (by death/emigration) during study period - Open population = has gains/losses - The Petersen method (aka Lincoln index): o One capture and mark, and one recapture session  Intended to quantify individuals that actively move in a population o A population contains N individuals (unknow, estimated from MRR) o A sample of M individuals is taken from this population, marked, and released back into the population o Later, another sample of n individuals is taken from the population, containing R recaptured individuals (those that were captured and marked in first sample) o Population size (N) can be estimated as follows:  N = (Mn)/R  M = marked, n = sample, R = recaptures o Example: a sample of 200 butterflies is taken from population on day 1, marked with a dot and then released  On day 2, 250 butterflied collected. 50 have been marked  M = 200, n = 250, R = 50  N = (200x250)/50 = 1000 Trapping - Partial trapping out: the removal method - Individuals in a population can be trapped and removed if this is done repeatedly over time, the numbers of individuals trapped will be fewer and fewer - The numbers caught in each trapping session can be plotted on a graph...