Biological diversity, or biodiversity, is the variety and abundance of species in a defined area, and is one of the oldest and most basic descriptions of biological communities. Understanding how populations and communities are structured and change over space and time in response to internal and external forces is a management priority. Effective management practices and conservation strategies depend on our understanding of the relationship between changes in biodiversity and ecological drivers such as invasive species, land use and climate change. To demonstrate how changes in biodiversity may be monitored over a large (400 km2) tract of native forest habitat, we compared bird and plant community composition and structure in an upper montane region of Hawai‘i Island originally surveyed in 1977 as part of the Hawai‘i Forest Bird Survey (Scott et al. 1986) with a comprehensive sample of the same region in 2015.
Our findings suggest that across a region spanning an elevation range of 600 to 2,000 m considerable changes occurred in the plant and bird communities between 1977 and 2015. Endemic and indigenous plants species richness (i.e., total number of species) decreased dramatically in the low and middle elevations below an invasive weed front, whereas naturalized plant species richness did not change between the two periods at any elevation. Endemic bird abundance decreased and two species were lost in the lower elevations (< 1,100 m) between 1977 and 2015, while naturalized bird abundance and the numbers of species increased in the same area. In addition to changes in community composition, the structure of the forest showed evidence of changes in dominant and sub-dominant tree canopy cover, shrub and herbaceous cover, dominant tree canopy height, and matted fern cover. 
Biodiversity monitoring helps to define specific conservation targets and to measure progress towards reaching those targets. It is difficult to ascribe causative factors to a change in biodiversity without directly manipulating the environment. Forest habitat in a variety of settings (i.e., islands and regions with differing land-use histories and elevation ranges), however, can provide opportunities to evaluate the influence of ecological drivers. Declines in native bird biodiversity in low-elevation areas may be attributed to invasive species as land use and climate conditions have remained relatively similar over the 40-year period. Thus, the shift from an endemic-naturalized co-dominated community in 1977 to one dominated by naturalized, alien birds in 2015, and reduction in native bird abundance over that period, may reflect increasing dominance by naturalized plants within this forested area. Inferences drawn from analyses of region-wide surveys, especially with replicate datasets, will facilitate the identification of broad-scale changes in biodiversity, and provide a needed current datum in Hawaiian plant and bird biodiversity monitoring.