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Quadrat or circular plot sampling requires that target species are reliably identified and not missed, if they are present within a sampling unit, which can be square, rectangular or circular.
Quadrat or circular plot sampling assumes that target species are detected with certainty and can be reliably identified. It is a flexible survey method that measures a wide range of indicators, in a wide range of settings. Sampling units can be permanently demarcated to monitor trends over time or can be newly marked out during every survey.
This method is used to monitor vegetation attributes or small, relatively sessile terrestrial organisms, such as insects and other invertebrates. It can also be used specifically to detect the presence and abundance of invasive species.
In terrestrial environments, a sampling unit is marked out randomly, using either pegs and tape along with a measuring device, or else a flexible lightweight frame of the desired size and shape. Once the sampling unit is marked out, the organisms of interest are surveyed. Depending on the goal of the study, individuals can be identified to species if possible, or to functional groups such as pollinators. Ideally, a number of sampling units should be sampled along a transect to gain a complete picture of the community being surveyed. If needed, specimens can be collected and removed (e.g., for museum collections or to aid identification), or else the sampling can be done with minimal disturbance by leaving everything in situ. When performing surveys for invasive species, in general non-target species can be ignored, though there may be interest in monitoring native species simultaneously to track the effects the invasive species may be having on them.
Biomass
Vegetation health
Vegetation species richness
Vegetation species abundance
Marine habitat sampling, e.g. coral cover, algae cover, substrate types
Marine invertebrate species richness
Marine invertebrate abundance
Sampling units can be created relatively inexpensively, using materials such as PVC, or items designed for other purposes such as hula hoops. They can also be marked out using pegs or poles, along with rope and something to make precise measurements and angles. Permanent sampling units can be created using the latter methods, and with heavier materials being preferred to resist weather, wildlife, and other disturbances.
If sampling units are intentionally being sampled in a non-invasive way by doing all identification and measurement in the field, a comprehensive identification guide to the organisms of interest will be needed. A camera would also be highly recommended to capture images, if field identification is not possible. If samples are being collected, then suitable equipment will be needed based on the organism types involved, along with bags or other containers to transport the specimens. If only presence-absence is recorded when quadrat or circular plot sampling for invasive species, then this is easier than surveying for abundance, as there is no need to quantify the number of individuals present.
An older, but still applicable, manual on conducting quadrat surveys, provided by the United States government’s Natural Resources Conservation Service (NRCS), can be found at this link. A guide to specifically collecting and preserving arthropod specimens, from the United States Department of Agriculture, can be found here.
Traditional Knowledge holders may be able to contribute important information on organism identification and use. Including Traditional Knowledge holders in quadrat or circular plot sampling surveys can increase the efficacy of this method.
To understand how to integrate Traditional Knowledge into your project's monitoring protocol, please refer to COMET's Practitioners Guide to Engaging with Indigenous Peoples and Local Communities in Conservation Monitoring.
