LRES 110, Land Resources and Environmental Science                 

Analysis of Species Richness Data From "M" Field Trip,

Fall 1999    Assignment Answers

Null hypothesis (H_o):  There is no difference in plant species diversity between areas within leafy spurge patches and areas outside of patches. 

Download Excel data file with means and standard deviations calculated

Calculate:

1. Mean total species richness for sample frames inside and outside of leafy spurge patches.
2. Mean endemic species richness for sample frames inside and outside of leafy spurge patches.
3. Mean exotic species richness for sample frames inside and outside of leafy spurge patches.
4. Mean species evenness (Simpson Index, D) for sample frames inside and outside of leafy spurge patches.

Fill in the mean and standard deviation (in parentheses) values: [8 points]

 Create 4 bar charts: [4 points]

This chart not required, but you may find it an interesting alternative way to visualize the results.

Conclusions:   [3 points]

Our hypothesis (= the alternative hypothesis, H_a) was that leafy spurge, an exotic weed species would be associated with decreased species diversity (a measure of ecosystem function) in the plant communities below the "M".  

Thus, the null hypothesis (H₀) became that there is no association between leafy spurge and species diversity in the plant communities below the "M".  We try to provide enough evidence to reject the null hypothesis rather than providing evidence for our alternative hypothesis to avoid biasing our conclusions.

Species richness and evenness are measures of species diversity that we used as our dependent variables and inside leafy spurge patch vs outside leafy spurge patch (position relative to the occurrence of leafy spurge) was our independent variable. 

We then drew our conclusions by comparing the mean values for our dependent variables from samples of the plant community inside and outside 10 different leafy spurge patches. Rather than just accepting the differences that we see in the mean values, we consider the variation in our sample data and use the standard deviation as a measure of confidence in our mean. If the standard deviation lines on either bar in our charts include the top (mean value) of the other bar, we cannot be confident that there is a difference between the two mean values.

Thus, the conclusion that we must draw on all of our measures of species diversity was that there was no difference between inside and outside the leafy spurge patches. There was not enough evidence from our sample of the plant communities below the "M" to reject the null hypothesis.  This conclusion draws into question the issue statement that exotic weeds cause decreased ecosystem function.

As scientists, we would not stop here, but we would do further tests of our data to determine if we took enough samples to be confident in our assessment of the mean and variation in the data.  In addition, we would repeat our measures over several years and maybe at different times during the year to be sure that we didn't miss any species that may be present in the community.  We would also want to replicate our observations over the entire management area in order to draw meaningful conclusions for management.