Getting to the Point: Accuracy of Point Count in Monitoring Ecosystem Change

Eric Pante,Phillip Dustan

doi:10.1155/2012/802875

Eric Pante, Phillip Dustan

Open Access

https://doi.org/10.1155/2012/802875

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Abstract

Ecological monitoring programs depend on the robust estimation of descriptive parameters. Percent cover, gleaned from transects sampled with video imagery, is a popular benthic ecology descriptor often estimated using point counting, an image-based method for identifying substrate types beneath random points. We tested the hypothesis that the number of points needed to robustly estimate benthic cover in video imagery transects depends on cover itself, predicting that lower cover will require more points/frame to be accurately estimated. While this point may seem obvious to the statistically inclined, the justification of point density has been largely ignored in the literature. We examined the statistical behavior of point count estimates using computer-simulated 20 m-long transects patterned after data from a Bahamian reef. The minimum number of points necessary to insure accurate percent cover estimation, the Optimal Point Count (OPC), is a function of mean percent cover and spatial heterogeneity of the benthic community. More points are required to characterize reefs with lower cover and more homogeneously distributed coral colonies. These results show that careful consideration must be given to sampling design and data analysis prior to attempting to estimate benthic cover, especially in the context of long-term monitoring of degrading coral reef ecosystems.

Highlights

A common problem while working in ecological characterization and monitoring programs is how to effectively test and optimize methods and experimental designs
The number of points to be used per unit area is crucial to obtaining a robust estimate of percent cover, and initial statistical tests should be performed to establish the point density that will provide adequate precision and accuracy while maximizing efficiency
Coral reef ecologists adopted some of the general survey techniques from plant ecology owing to the structural similarities of forests and reefs

Summary

Introduction

A common problem while working in ecological characterization and monitoring programs is how to effectively test and optimize methods and experimental designs. The number of points to be used per unit area (point density and the unit area being an image frame from a video transect) is crucial to obtaining a robust estimate of percent cover, and initial statistical tests should be performed to establish the point density that will provide adequate precision and accuracy while maximizing efficiency (the time spent identifying substrate types underlying points). Such tests include power analysis [1, 4, 10], performance curves (graphical examination of standard deviation inflation relative to point density [4, 11,12,13]), bootstrap estimate of bias [13], Pearson’s product moment correlations [14], and examination of the interaction term in a two-way ANOVA for transects sampled twice using different point densities [4]

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Results

Conclusion