Large Size

Gentry plots have been widely used to describe the variation in diversity within and between geographical units of area. However, even within the same "type" of vegetation (e.g. upland forest), there is a high beta diversity in the vegetation at small scales (1-10 km) in tropical forests (Clark et al. 1999, Phillips et al. 2003, Ruokolainen & Tuomisto 1994, Vormisto et al. 2000, Tuomisto et al. 2003). Therefore, the area sampled for comparisons within the same site need to be large. For most taxonomic groups small plots (1-100 ha) capture a very small part of the diversity present at a particular site. This is the main reason why many plots of 0.1 ha are much more efficient than 1 ha plots with the same total area (Phillips et al. 2003). For comparisons between sites, we consider the grid system to be a single unit composed of many sub-plots. For analyzes within the same site, each plot sampling procedure or module should be considered to be independent.
The basic Long Term Ecological Research (LTER) survey unit used by PPBio is a system of trails in the form of a 5 km x 5 km grid over which permanent plots are systematically distributed. LTER sites are used to monitor changes in environmental and biological variables, and the site as a whole needs to be large enough to monitor phenomena that are of interest to professionals involved in forest management, reserve management, land use planning and so on. Smaller portions (1-100 ha) may be useful for specific issues, and even smaller plots can reveal undescribed species, but on their own are of little use in capturing the diversity of larger organisms, many of which are economically valuable or have a special conservation interest.
Ecosystem processes such as biomass accumulation, erosion, pollution and sedimentation are best evaluated over large areas, because then they are relevant for management and conservation. For example, within most stands of managed forest, it is possible to find 1ha. patches of virtually intact forest, 1ha. patches that have been cut, and extensive areas where roads and trails have compacted the soil and little or no vegetation exists. However, no one needs to point out these predictable effects to professionals in forest management. They are more interested in large-scale phenomena that may affect biodiversity and economic returns in the future. Only a sampling system covering tens of square kilometers can provide the information necessary for land use management.
In the tropics, alpha diversity (species richness) may not be much higher than at some temperate sites. However, between sites, the species composition (beta diversity) can lead to a much higher gamma diversity (Mendonca et al. 2005). Moreover, for a given plot size, alpha diversity indices are usually inversely related to the average size of the organisms being studied (Magurran 2004). For this reason, estimates of alpha diversity from small plots are usually of little use to applied or theoretical research (O'Hara 2005).
Distributing sampling plots over an area of about 25 km2 allows alpha and beta diversity evaluation over a range that is potentially useful for land use planning, and allows assessment of most, if not all, components of biodiversity i.e. alpha, beta and gamma.
In most regions and habitats small plots (1-100 ha) capture a very small sample of topographic and edaphic variation (see Compatibility with Existing Initiatives). This is important since, most species are small and and most small organisms are specialized to exist within in a particular range of topographic and soil gradients.
Small grids provide very little information about organisms strongly associated with characteristics related to drainage. This is an important omission since it is these which are often the first to suffer the impact of human activities. Large size is also important because space can act as a proxy for time in studies that integrate time-space and area (Adler et al. 2005). It is easier to get funding to short-term projects with quick turnaround than for long-term projucts whose results will take a long time to appear. PPBio grids form the single widely used survey system that includes all sizes of terrestrial non-flooded and aquatic environments and their constituent organisms at a reasonable cost and timely manner.
PPBio surveys are based on standard modules, each of which may be used for a wide variety of organisms. This means that data can be collected at much lower cost, and that they are useful to a great variety of users.