Rarefaction (ecology)
In Ecology, rarefaction is a technique to compare species richness computed from samples of different sizes. Rarefaction allows the calculation of the species richness for a given number of sampled individuals and allows the construction of so called rarefaction curves. This curve is a plot of the number of species as a function of the number of individuals sampled. On the left, the steep slope indicates that a large fraction of the species diversity remains to be discovered. If the curve becomes flatter to the right, a reasonable number of individuals is sampled: more intensive sampling is likely to yield only few additional species. [1]
The issue that arrives when sampling various species in a community is that the larger the number of individuals sampled, the more species that will be found. Sampling curves generally ri very quickly at first and then level off towards an asymptote as fewer new species are found per unit of individuals collected. Rarefaction curves are created by randomly re-sampling the pool of N samples multiple times and then plotting the average n
酱油饭umber of species found in each sample (1,2, ... N). "Thus rarefaction generates the expected number of species in a small collection of n individuals (or n samples) drawn at random from the large pool of N samples." [1] Rarefaction curves generally grow rapidly at first, as the most common species are found, but the curves plateau as only the rarest species remain to be sampled.
History什么明星稀
The technique of rarefaction was developed in 1968 by Howard Sanders in a biodiversity assay of marine benthic ecosystems, as he sought a model for diversity that would allow him to compare species richness data among ts with different sample sizes; he developed rarefaction curves as a method to compare the shape of a curve rather than absolute numbers of species.[3]
Following initial development by Sanders, the technique of rarefaction has undergone a number of revisions. In a paper criticizing many methods of assaying biodiversity, Stuart
Hurlbert refined the problem that he saw with Sanders' rarefaction method, that it overestimated the number of species bad on sample size, and attempted to refine his methodology.[4] The issue of overestimation was also dealt with by Daniel Simberloff, while other improvements in rarefaction as a statistical technique were made by Ken Heck in 1975.[5]
Today, rarefaction has grown as a technique not just for measuring species diversity, but of understanding diversity at higher taxonomic levels as well. Most commonly, the number of species is sampled to predict the number of genera in a particular community; similar techniques had been ud to determine this level of diversity in studies veral years before Sanders quantified his individual to species determination of rarefaction. [6] Rarefaction techniques are ud to quantify species diversity of newly studied ecosystems, as well as in applied studies in community ecology, such as understanding pollution impacts on communities and other management applications.
Derivation
Deriving Rarefaction:
N = total number of items
K = total number of groups
Ni = the number of items in group i
(i= 1, ..., K).
Mj = number of Ni equal to j
From the definitions, it therefore follows that:
In a rarefied sample we have chon a random subsample n from the total N items. The relevance of a rarefied sample is that some groups may now be necessarily abnt from this subsample. We therefore let:
Xn = the number of groups still prent in the subsample of "n" items
It is true that Xn is less than or equal to K whenever at least one group is missing from this subsample.
Therefore the 现在流行什么眉毛rarefaction curve, fn李白的月下独酌 is defined as:
From this it follows that 0 ≤ f(n) ≤ K. Furthermore, 油画基础f(0) = 0,f(1) = 1,改性淀粉f(N) = K. Despite being defined at discrete values of n, the curves are most frequently displayed as continuous functions.[7]
Correct UsageWPS路由器
Rarefaction curves are necessary for estimating species richness. Raw species richness counts, which are ud to create accumulation curves, can only be compared when the species richness has reached a clear asymptote. Rarefaction curves produce smoother lines that facilitate point-to-point or full datat comparisons.
任职培训
One can plot the number of species as a function of either the number of individuals sampled or the number of samples taken. The sample-bad approach accounts for patchiness in the data that results from natural levels of sample heterogeneity. However, when sample-bad rarefaction curves are ud to compare taxon richness at comparable levels of sampling effort, the number of taxa should be plotted as a function of the accumulated number of individuals, not accumulated number of samples, becau datats may differ systematically in the mean number of individuals per sample.
One cannot simply divide the number of species found by the number of individuals sampled in order to correct for different sample sizes. Doing so would assume that the number of species increas linearly with the number of individuals prent, which is not always true.
