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from the USDA-ARS Monitoring Manual for Grassland, Shrubland and Savanna Ecosystems
A plot-level species inventory provides a rapid estimate of species richness. A thorough search of the plot can detect less-frequently occurring species that may not have been recorded in cover measurements (e.g., Line-point intercept). For a more intensive species richness measurement, see the modified Whitaker species richness approach.
The species inventory area is within at least a portion of the area covered by the Line-point intercept transects.
Figure 32. Three species inventory plot layout options to achieve an area of 1,641 m2: a) a single square species inventory plot with a side of 40.5 m (132.9 ft), b) three 10.9 (35.7 ft) x 50 m (164 ft) rectangular sub-plots, and c) a 22.9 m (75 ft) radius, circular plot. Dashed lines represent the path walked by the observer.
AF# = Annual forb (also includes biennials)
PF# = Perennial forb
PG# = Perennial graminoid
SH# = Shrub
TR# = Tree
Species inventories detect the presence of rare or invasive species which may not be detected by cover or density measurements along transects due to their infrequent occurrence, rarity, or recent establishment. This method can identify areas where additional plant surveys are needed. A plot-level species inventory also provides information on species richness, one indicator of biodiversity. Plot biodiversity indicators must be evaluated within the context of the ecological potential of the plot (e.g., as defined by an ecological site description). Consequently species richness, like bare ground and other indicators, cannot be directly compared among sites with different soils and climate.
Ecological heterogeneity can also affect richness: a plot that spans several soil types will likely have higher biodiversity than a plot located on a single soil. Similarly, a plot that includes several ecological states on the same or different soils is likely to have more species. Species richness may even be higher in a somewhat disturbed or degraded state than in an undegraded state as invasives colonize, but do not entirely replace spe¬cies native to the area. Within-plot comparisons over time must be carefully interpreted for the same reasons.
Consequently, caution should be used when comparing plots using species richness as an indicator of site biodiversity. Interpretation of species richness should always be made in an ecological context together with indicators derived from Line-point intercept, Gap intercept, and Soil stability.