Despite decades of biological inventories worldwide, we still do not know how many species exist and how they are distributed. Although global patterns of estimated vascular plant species richness and distribution have become more clear, no previous study has focused on trees as a distinct growth form. As a consequence, our estimation of the number of tree species in tropical forests still depends on untested expert opinions rather than on an appropriate methodological framework and data set.
Although we can say with much confidently 'the tropics are diverse,' the answer to 'how diverse' still remains open to speculation. Tropical tree identification is notoriously difficult - hampered by hard-to-access terrain and the sheer number of rare species.
A new study published in PNAS tries to give an answer to the question 'how diverse' by estimating the number of tropical tree species worldwide. This work is a collaborative effort of over 170 scientists from 126 institutions. They analysed a dataset composed of 207 forested locations across tropical America, Africa and the Indo-Pacific. Much of the data came from CTFS-ForestGEO study sites, where standardized pan-tropical survey methods create opportunities to much more accurately gauge tropical diversity. Each forest plot contains at least 250 individual trees identified to species, ensuring comprehensive coverage of the total species diversity in each geographical area.
The global analysis raised the minimum estimated number tree species to at least 40,000 to 53,000 in the tropics, in contrast to only 124 across temperate Europe. Among their findings, the researchers note that, contrary to previous assumptions, the Indo-Pacific tropics contain as much species diversity as tropical America - at least 19,000 species. Both tropical America and the Indo-Pacific are about five times as species-rich as Africa (4,500–6,000 tree species), whose forests are hypothesized to have experienced extensive extinction events during the Pleistocene era of glaciation and climate change. All three regions contain distinct tree lineages reflecting unique evolutionary histories.
The colleagues note that their calculations excluded some 10 percent of unidentifiable trees in the dataset which comprised 657,630 individuals. As these trees could reasonably represent rare or previously unknown species, there's a high likelihood that the world's estimates of total tree species diversity will keep increasing as more of the tropics are surveyed and studied.
By raising the estimated minimum number of tree species in the world, estimates for the number of arthropod and microbe species associated with tropical trees also increases, placing an even higher premium on the protection of these forest ecosystems.
Meanwhile, as deforestation and development increase the extinction risk for many unique species, lessons may be learned from Africa's reduced tropical diversity. When forest areas shrink, rare species are usually the first to disappear. Consequently, even if the extinction pressure is eventually lifted, a much more limited palette of species remains to repopulate the region. While the tropics are vast and diverse, their individual components are irreplaceable.
The stunningly high tree diversity of the tropics is represented by thousands of rare species, whose sparse populations may not be sustained in the long term by isolated protected areas. This study once again validates a strategy of making forest reserves as big as possible, and also trying to prevent their isolation from adjoining areas of forest.