Ecology 5th Edition by Manuel Molles

Exercise 3

The number of observations included in a sample, that is, sample size, has an important influence on the level of confidence we place on conclusions based on that sample. Let's examine a simple example of how sample size affects our estimate of some ecological feature. Consider an ecologist interested in how disturbance by flash flooding may affect the number of benthic insect species living in a stream. The stream is Tesuque Creek at about 3,000 m elevation in the mountains above Santa Fe, New Mexico. A flash flood, which completely disrupted one fork of Tesuque Creek, left a second, similar-sized fork undisturbed. Nine months after the flood, samples were taken to determine if there was a difference in the number of species of mayflies (Order Ephemeroptera), stoneflies (O. Plecoptera), caddisflies (O. Trichoptera), and beetles (O. Coleoptera) living in similar-sized reaches of the two forks. Samples of the benthic community were taken at 5 m intervals with a Surber sampler, which has a 0.1 m 2 metal frame, or quadrat, and an attached net. As a stream ecologist disturbs the bottom material within the quadrat of a Surber sampler, the net trailing in the current catches benthic organisms that are dislodged. In the study of Tesuque Creek, the number of benthic insect species captured in each 0.1 m 2 sample ranged from one to six in the disturbed fork and from two to eight in the undisturbed fork. However, our question concerns the total number of species in each fork and the number of benthic samples required to make a good estimate of that number of species.
Figure 1 plots the data in a way that provides an answer to both questions. The Surber samples are plotted in the exact order they were taken, beginning with the first that was taken at the downstream end of each study reach and ending with the twelfth sample taken 55 m upstream from the first. As shown in figure 1 , each of the first few samples adds to the cumulative number of species collected at each site, which rises steeply at first and then levels off at a maximum number of species in each study reach. The cumulative number of species stopped increasing at a sample size of seven quadrats in the undisturbed study reach and at five quadrats in the disturbed study reach.
How many samples should a researcher take? In the case of the benthic community just examined, seven replicate counts from 0.1 m 2 quadrats appears to be sufficient to estimate the number of benthic mayfly, stonefly, caddisfly, and beetle species living in a short reach of a small, high-elevation stream in the Rocky Mountains. (We will revisit this study in Investigating the Evidence 16, p. 359, which concerns estimating the number of species in a community.) In contrast, to make generalizations about global patterns of rooting among plants, Schenk and Jackson (2002) reported on 475 root profiles at 209 locations (see p. 134). The number of samples necessary depends on the amount of variability in the system under study and the spatial and temporal scope of the study. However, whether the scope of a project is large or small, sample size is one of the most important components of study design.
Judging from the data displayed in figure 1 , how did disturbance by flash flooding affect the number of mayfly, stonefly, caddisfly, and beetle species living in the disturbed reach?