Abstract. This report was conducted in order to look at the relationship between plant and arthropod communities on an urban college campus. We were interested in the overall biological significance of this experiment in order to see if the introduction of non-native species could affect trophic levels of native species like Johnson and his colleagues spoke of in the introduction of his paper (Johnson, Lajeunesse, Agrawal 2006). We tested these hypotheses by collectively setting up hundreds of bowls with water, dish detergent, and salt in order to trap arthropods in diverse plant communities around campus over 24 hour periods. Foundations of Biology II students surveyed the plants surrounding their bowls by using multiple diversity indices and then after collecting the trapped arthropods, students surveyed the arthropods using the same diversity indices. We found using a linear regression test that arthropod species diversity depends little on surrounding plant species diversity. We also found that overall, an increase in plant richness has little effect on arthropod richness and similarly an increase in plant Shannon diversity has little effect on arthropod Shannon diversity. An increase in plant richness however, seems to cause an almost equal, if not larger, increase in arthropod abundance. Using a more advanced test, we can reject the null hypothesis that there is no association between plant diversity and arthropod diversity on the Shannon Index (t=1.988, p<0.025). These results may be due to the fact that some arthropods only feed on a single species of plant (Siemann 2008), which would mean that as plant richness increases, arthropod species would also increase.
Introduction. For this experiment I was expecting to find increases in plant diversity would lead to increases in arthropod diversity as the arthropods feed and rely on surrounding plants for food. I predicted that as more plants were planted/grew in certain areas on campus, arthropods would flock to those areas and would be able to reproduce more due to the greater supply of food. Relating back to the overall biological significance of this experiment, if non-native species are planted on a college campus they may cause a “trophic cascade,” as seen in the Sustainable Human video in the Yellowstone National Forest. On our college campus, the effects may start from the bottom and cascade upwards, possibly leading to greater diversity in parasites and predators that would then threaten native plant species. In order to compare the diversities between plants and arthropods, we must first understand the different diversity indices. Richness is the most obvious way to measure diversity as it involves simply counting the number of species found. Although richness can be vastly different depending on sample sizes, this is factored in as richness is usually measured per unit area. Relative abundance is another diversity index as it is the total number of individuals of a species within the area of interest. Abundance and richness are not necessarily related as some communities may be rich in species, but certain species may be low in abundance. Relative abundance may be helpful in understanding dynamics of the communities we are studying as it can help us learn more about competition and predation. Shannon Diversity Index is another way of measuring diversity as it takes into account the number of species and the relative abundance of each species. We used all three of these indices to measure plant and arthropod species diversity. Most other field experiments of this nature focus on plant and arthropod diversities in rural forests (Crutsinger, Collins, Fordyce, Gompert, Nice, Sanders 2006), whereas our experiments were taken in a more urban setting meaning the results may be different than most, or different than what we would expect.
Methods. In order to collect data on the plants and arthropods, each pair of lab partners in our Foundations of Biology II lab placed 4 plastic bowls in 4 places around campus on a sunny day. Each bowl was filled with 8 oz. of water, 2 tsp. of salt, and 1-2 drops of dish detergent and then left in its specific location for 24 hours. The water trapped the arthropods while the dish detergent broke the surface tension and the salt prevented the animals from absorbing too much of the water. In order to count the surrounding plant species and their abundances, each pair of lab partners used a 1-meter length of string and walked in a circle around the bowl to measure a 3.14 m2 area in which they could record the number of plant species. Students collected plant species richness in order to compute the Shannon Diversity Index (H’) for each of the sites. The sites on campus were selected randomly by students. After 24 hours, the bowl was collected and the contents of each bowl were emptied into four labeled Ziploc bags and brought into lab for data collection on richness, abundance, and the Shannon Diversity Index. Students identified and counted the insects caught in each bowl based on the dichotomous key in the Foundations of Biology II Lab Manual (Fox JA, Patten MM). Data from all lab partners was compiled into an excel document in order to compare the relationship between plant diversity and arthropod diversity. Because these are continuous variables, we then used a linear regression to compute an r2 value in order to look at the relationship between diversities. We then followed those calculations by doing a t-test in order to see if the association of diversities is statistically significant. This test is necessary as it takes into account the size of the effect, the sample size, and the variability in data in order for us to either reject or accept the null hypothesis. **(Too much stats speak? How to reword this…?)
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