species richness

Species richness

The number of species is an important biological variable that scientists try to quantify

Species diversity

The number of species weighted by their abundance.

For example, a forest with 10 tree species, each with ten individuals, is generally considered more diverse than a forest with 10 tree species, one of which is represented by 91 individuals with the other 9 species only represented by one individual each.

To estimate species richness one must first sample an area(s)

Sampling is measuring a small part of an entity assuming that this small part is representative of the larger entity

Ways to diminish biases in sampling

Use multiple sampling techniques

Sample at random locations (each location has an equal opportunity of being sampled--in other words, sampling should not be biased in favor of some locations and against other locations)

Sample repeatedly

In addition to sampling correctly, it is important to recognize that not all species are equally detectable

In addition, species may vary in their detectability across habitat types

Generally, the number of species detected (observed) through sampling, is not the best estimate of the species richness of an area

Ways to look at patterns and estimate diversity after sampling is completed

Diversity indices

Accumulation curves/Rarefaction curves

Using species abundance distribution data

Shannon Diversity Index

Also known as the Shannon-Wiener or Shannon-Weaver. This index uses species abundances to calculate an index of diversity

H = -Σ pi ln(pi)

pi= proportion of the ith species

Imagine the following are the abundances of individuals in a community of frogs detected by a biologist

Species 1 = 15Species 2 = 10Species 3 = 3Species 4 = 1

H = -Σ pi ln(pi)

15+10+3+1 = number of individuals in the community

p1 = 15/29, p2 = 10/29, p3=3/29, p4=1/29

H = -Σ (0.52(ln0.52)+(0.34(ln0.34)+(0.10(ln0.10) + 0.03(ln0.03) = 1.05

1.05 = diversity index, which we compare to other, similarly calculated diversity indices if we want to compare the diversities of different communities

Assumption of Shannon Index

All species are equally detectable

This assumption may be more true for some taxonomic groups than others. In general, for mobile organisms, the Shannon Index is not a strong way to estimate diversity

Accumulation curve—”records the total number of species revealed, during the process of data collection, as additional individuals (or sample units) are added to the pool of all previously observed or collected individuals or samples”

Gotelli, N.J. and R. K. Colwell. 2001. Ecology Letters 4:379-391.

Braulio Carillo

Tabanid fly

Tabanid fly species accumulation curves for La Selva and 1070 m in Braulio CarilloA Report on the Tabanidae of La Selva Biological Station and 1070m in Braulio Carrillo National ParkTaxonomic Collaborator: John F. Burger, Department of Zoology, Spaulding Hall University of New Hampshire, Durham, New Hampshire 03824 USA. Page author: John T. Longino, The Evergreen State College, Olympia WA 98505 USA

Accumulation curves show differences in species richness, given the number of individuals detected

When these curves reach an asymptote, sampling may be reasonably complete

Rarefaction curves are produced by “repeatedly re-sampling the pool of N individuals or N samples, at random, plotting the average number of species represented by 1, 2,…N individuals or samples. Sampling is generally done without replacement”

Rarefaction curves represent the statistical expectation for the corresponding accumulation curve

Black line is accumulation curve, red line is rarefaction curve, dashed lines are confidenceIntervals. Rarefaction curve is based on 100 samples. Data are for bats at site in Malaysia. http://www.wcsmalaysia.org/analysis/Biod_richness.htm

Williams et al. 2001. Variation in native bee faunas and its implications for detecting community change. Ecology and Society 5:7

If one bases rarefaction curves on numbers of samples versus individuals, one needs to be careful

If the density of individuals differs among samples, results will be misleading

From Gotelli and Colwell 2001.

Second growth forestis denser, greater no. of individuals, so rarefyingwith samples gives mis-leading result that secondgrowth forest is more species-rich. Rarefyingwith individuals controlsfor density differences.

Because each sample from second growth forest has more individuals than each sample from old growth forest, the samples for the two types of forest are not directly comparable.

Another technique for comparing species richness

Use program Specrich, available athttp://www.mbr-pwrc.usgs.gov/software/specrich.html

Patuxent Wildlife Research Center website

Community A Community B

No of spp. detected with one individual 60 30

No of spp. detected with two individuals 25 20

No of spp. detected with three individuals 10 20

No of spp. detected with four individuals 2 15

No of spp. detected with five individuals 3 15

Which community do you think has a greater species richness?

Species abundance distribution data (as presented in the previous slide) are used to estimate species richness in this method.

Estimated species richness (interpolated N)

Standard error of estimate

Range of estimate

Community A 174.3 14.1 160-188

Community B 130.0 7.7 122-138

Characteristics of particular taxonomic or ecological groups that are more likely to be more species-rich than others?

Characteristics of particular environments that are more likely to be more species-rich than others?

Control Island Plantation

Planting design--In islands or plantations?

San Gabriel, April 2007

San Gabriel March 2006

Loma Linda March 2006