Assessing the effectiveness of global protected areas in conserving plant community completeness

The establishment of terrestrial protected areas (PAs) is an effective
strategy for conserving plant species diversity across multiple spatial
scales—from local to global. This effectiveness can be assessed using the
concept of plant community completeness, which quantifies the degree to
which the local biodiversity potential (i.e., the site-specific species
pool) is realized. Although community completeness offers valuable
insights for nature conservation, few studies have applied this metric to
evaluate PAs. We present the first global assessment of plant community
completeness for PAs. We analyzed 41,980 plots located inside PAs and
28,599 plots outside PAs, comparing species richness, species pool, and
community completeness across various ecoregions and biomes. Using the
Getis-Ord Gi* statistic, we identified areas of high community
completeness and evaluated the effectiveness of PAs in conserving this
metric. Furthermore, we determined ecoregions with the potential to
increase community completeness by the expansion of conservation areas
outside PAs. Significant variations in species richness, species pool, and
community completeness were observed both inside and outside PAs across
biomes. At the global scale, the threshold for high community completeness
was estimated at 0.208. The lowest 10% quantile values of high community
completeness were approximately 0.1 for different biomes in the PA
assessment. Biome-specific thresholds revealed that PAs effectively
conserve community completeness in 57.2% of the study ecoregions but
exhibit limited effectiveness in maintaining community completeness in 71
ecoregions. Community completeness outside PAs showed considerable
potential for further enhancement. Given the nearly fixed nature of the
species pool and its relationship with community completeness, the high
completeness observed outside PAs underscores the need to expand
conservation areas in 40.7% of the study ecoregions, spanning tropical,
temperate, and desert biomes. Additionally, 105 ecoregions show potential
for improving community completeness through restoration efforts outside
existing PAs. Synthesis and applications: Community completeness is a
powerful tool for evaluating the effectiveness of PAs in conserving plant
diversity. It is critical to test whether PAs are in high completeness
areas, i.e., in appropriate sites, and suggest which areas are in theory
better suited for restoration and transformation into PAs.