Abstract
We derive statistical properties of standard methods for monitoring of habitat cover worldwide, and criticize them in the context of mandated seagrass monitoring programs, as exemplified by Posidonia oceanica in the Mediterranean Sea. We report the novel result that cartographic methods with non-trivial classification errors are generally incapable of reliably detecting habitat cover losses less than about 30 to 50%, and the field labor required to increase their precision can be orders of magnitude higher than that required to estimate habitat loss directly in a field campaign. We derive a universal utility threshold of classification error in habitat maps that represents the minimum habitat map accuracy above which direct methods are superior. Widespread government reliance on blind-sentinel methods for monitoring seafloor can obscure the gradual and currently ongoing losses of benthic resources until the time has long passed for meaningful management intervention. We find two classes of methods with very high statistical power for detecting small habitat cover losses: 1) fixed-plot direct methods, which are over 100 times as efficient as direct random-plot methods in a variable habitat mosaic; and 2) remote methods with very low classification error such as geospatial underwater videography, which is an emerging, low-cost, non-destructive method for documenting small changes at millimeter visual resolution. General adoption of these methods and their further development will require a fundamental cultural change in conservation and management bodies towards the recognition and promotion of requirements of minimal statistical power and precision in the development of international goals for monitoring these valuable resources and the ecological services they provide.
Highlights
Conservation monitoring, the regular observation of a valuable environmental resource, is the cornerstone of natural resource management programs worldwide [1]
We assume a monitoring decision tree in which there are three major statistical decision nodes, namely 1) direct versus indirect sampling methods, 2) fixed- versus random-plot methods, and 3) presence versus absence of habitat classification error. Prior to these nodes is a non-statistical decision between destructive and non-destructive methods; here we consider only non-destructive. Direct methods are those in which personnel carry out all observations in situ, which has the benefit of direct observation, but a high labor cost, especially for subtidal habitats that require trained SCUBA divers for all in situ work
Our results indicate that diachronic cartography of Posidonia with sidescan sonar is nearly always blind to habitat losses of about 20% or less (Fig 3), unless cover estimates are corrected with extensive ground truth studies using Eq 14, we are unaware of a single study that has employed such correction
Summary
Conservation monitoring, the regular observation of a valuable environmental resource, is the cornerstone of natural resource management programs worldwide [1]. Its purpose is to identify where and when that resource is in decline, so that prompt recovery and protection actions can be taken. It functions metaphorically as a sentinel, whose purpose is to provide the first alert to PLOS ONE | DOI:10.1371/journal.pone.0138378. If monitoring is not capable of reliably recognizing decline as it first occurs, it is a failure as a sentinel, and it fails the management program that depends on it [3]. To outside observers the program may, for a time, appear both responsible and successful, because it is built on a formally approved monitoring protocol, and at least initially, that protocol does not show any significant loss in the resource it is charged to watch
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