However, even in the United States, several of these indicators currently cannot be monitored due to insufficient data. Status of Biodiversity Generally, biodiversity tends to be higher in southern areas and to decrease gradually toward the north. This pattern is especially true for flowering plants but it also emerges with the diversity of vertebrates. A similar longitudinal gradient is observed in global biodiversity (Gaston 2000). However, many basic questions related to the current status of biodiversity remain unanswered. For instance, the total number of species in the world is unknown. Estimates vary from a few million species to more than 100 million species, with the current consensus around 14 million species (Table 1). The species counts and their precision vary considerably across different taxonomic groups. Plant species are among the most completed inventories. It generally is accepted that approximately 300,000 to 500,000 plant species exist (Hammond 1995). For many groups of organisms, however, the precision of the estimated species counts is considered poor or moderate. The number of actually recognized and described species is fewer than 2 million.
The taxonomic group with the largest number of known species—about 1.3 million—is animals. Out of all known animal species, the vast majority, almost 1.1 million are insects and other arthropods. In addition, animal species include about 45,000 known chordates and about 70,000 thousand mollusks. Sponges, jellyfish, corals, hydras, and other aquatic animals comprise approximately 20,000 known species. The rest of known animal species mostly are worms of different kinds, such as flatworms, roundworms, or segmented worms (Hammond 1995). Vertebrates and plants have been catalogued quite comprehensively, and their estimated numbers are not expected to change dramatically as more information is gathered over time. Viruses, bacteria, and fungi are the major groups of organisms with the largest estimated number of nondescribed species. New species are identified in all taxonomic groups; every year, more than 10,000 completely new species are identified. This rate of
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