Scripta Varia

Plant conservation: Challenges and opportunities

Peter Raven[*]

As the papers presented in this collection confirm, plants present unique opportunities for conservation. They are rather easily grown in areas with an appropriate climate; moreover, the seeds of most species can be stored for decades, sometimes centuries, in seed banks or growth chambers. But what then? Their successful long-term conservation demands that suitable conditions for their growth continue to exist and eventually will be restored on our planet. In allegorical terms, the Ark is a vessel described in the Bible (Genesis, Chapter 6) as being 134 meters long, 13.4 meters wide, and 9 meters tall, thus with plenty of room for all domestic animals and a few others, but of very limited capacity considering all the species that live on Earth – very few of them could be accommodated. And after 150 days, the waters receded and the Ark came to rest on land.

I use the details of this Biblical tale because we all need to ask what are the limits of what living things we can save with our capacity, and also because we need to ask, having saved them, when and how will our “waters” finally recede?

In the roughly 11,000 years since our ancestors learned how to cultivate crops and domesticate animals for food and other purposes, our numbers have grown from approximately one million people, with only 100,000 of them in Europe, to the current 7.7 billion people, 500 million of us in Europe.[2] In other words, a multiplication by 7,700 has occurred in our population in less than 400 generations! And we are currently estimated to be consuming approximately 1.75 times the sustainable capacity available on Earth, so that last year, by August 1, we had used up all the Earth’s ecosystems could regenerate in the entire year, living on depletion for the rest of the year.[3] This year, the date was even earlier: on July 29. And our population is projected to grow by an additional 2.3 billion within just 30 years, that is, by mid-century.[4] At present, some 795 million people are estimated not to get enough food regularly to lead an active life, half of whom lack sufficient amounts of at least one essential Nutrient.[5] The inequality between nations and their rates of consumption is growing,[6] and some 40% of Earth’s ice-free land surface is already occupied by some form of agriculture (including grazing).[7] Much of the remaining 60% consists of cities or other forms of human encroachment. Within the remainder, the area occupied by natural vegetation is much smaller than it once was, and it is shrinking rapidly.

Over time, inequality within and between nations has also become a major problem. Thus, eight individuals have been estimated by British charity Oxfam to possess as much wealth as the world’s poorest 3.6 billion people, with nationalism and selfishness continuing to grow steadily when they clearly need to recede if we are ever going to be able to attain global sustainability. Although climate change is seen as a global concern, the efforts mounted so far have not reached a level at which the ongoing rise in temperature would ever be stabilized. In addition, the importance of biodiversity loss, and with it the disruption of conditions that alone make our lives on this planet possible, is a concern for relatively few people throughout the world.

Against this background, what are the future prospects for plants? Charismatic megafauna and a few butterflies, like the monarch butterfly (Danaus plexippus) charm us and capture most of the conservation headlines. Nevertheless, we are entirely dependent on plants for our food, many medicines, building materials, and a number of other products. We also depend on them collectively to maintain the composition of our atmosphere, to control erosion, precipitation patterns and other aspects of climate, and for the beauty and interest that they add to our lives, they have thus far not received the conservation attention they deserve. Overall, there are estimated to be 390,000 to 400,000 validly named species of vascular plants (lycopods, ferns, gymnosperms, and flowering plants; Chapman, 2009; Joppa, Roberts, & Pimm, 2010; Kew, 2017; Pimm & Raven, 2017), with perhaps as many as another 70,000 awaiting naming and in some cases even discovery. For eukaryotic organisms overall, estimates indicate that there are at least 10-12 million species, with fewer than 2 million of them having been recognized up to today and assigned scientific names. Considering this, vascular plants are relatively well known; but the situation becomes even more challenging when we consider how few of them, perhaps 50,000 or so, can be considered to be understood in any reasonable level of detail.

How many vascular plant species are estimated to be facing extinction over the next several decades? At least 600 species have become extinct over the past 260 years (Humphreys et al., 2019), and that estimate is very likely to be too low. During the next several decades, there seems to be justification for assuming that 20% of the total could disappear, with perhaps twice that fraction or even more in danger of being lost by the end of the century (Pimm et al., 2014; Kew, 2016). Ceballos et al. (2015) have demonstrated convincingly for the areas they have surveyed, such as Mexico, that some 60% of the populations of vertebrates have become extinct since 1950. The situation for plants seems to be similar, indicating that extinction may be proceeding far more rapidly than would be deduced from measuring species extinctions alone (Raven, in press). At any rate, the growth in human populations, the expansion and intensification of our agricultural activities, ongoing climate change, and the seemingly unrelenting demand for ever-higher levels of consumption, including the growing commercial exploitation of many plant species (Kew, 2016, 2017) will make it very difficult to preserve very much of the existing tropical forests until the end of this century. Specifically, conservation schemes that assume a forthcoming human acceptance of our common need for global stability strong enough for us to give up our nationalism and personal greed in the common interest are difficult to accept as realistic. We must beware of the distraction that they may bring us along with their valuable inspiration to do better. In a way, some such schemes might be said to be in essence “pipe dreams” that could be realized fully only within the parameters of a relatively stable and socially just world.

In a general sense, the recent publication of a comprehensive biodiversity conservation effort, Global Deal for Nature (GDN; Dinerstein et al., 2019), intended to match the existing Paris Climate Accord, represents an impressive accomplishment. On the other hand, the PCA not yet caused nations overall to come near what would be needed to achieve its targets, despite the fact that a much higher proportion of people, perhaps two-thirds of us in the U.S., are concerned with climate change than with biodiversity loss. Any such treaty can only become effective enough when the underlying causes of the problem being addressed are recognized as real barriers to its realization. These obstacles include continued population growth, ever-growing consumption levels, nationalism, and human greed, all formidable obstacles as we attempt to save our living planet.

The Global Strategy for Plant Conservation (GSPC) is dedicated to saving the world’s plant species and aligned with the U.N. Sustainable Development Goals, but even they must operate within the bounds set by human nature. It is certainly a good thing to set aspirational goals, but we shall clearly be able to achieve only limited success in attaining them until we fully address the moral underpinnings that alone can make their attainment possible. I hope this Vatican conference and the underlying values expressed so well in the Papal Encyclical Laudato si’ might help to open our eyes to the reality within the bounds of which we must operate.

What can we do to prepare our dispersed, contemporary “Noah’s Ark” for the human-driven deluge that I have just described? The world’s nearly 3,000 botanical gardens presently have in cultivation more than 115,000 species of vascular plants, more than a quarter of the known total. Botanic Gardens Conservation International (BGCI), together with its regional and national partners, is prioritizing the collection and conservation of rare and threatened plant species by employing a cost effective, rational sharing of responsibilities, skills and knowledge (Smith, 2016; this symposium). For example, there are very significant living collections held in particular places, such as the collections of palms and cycads at the Jardín Botánico de Quindío in Colombia, on which Alberto Gómez reports here, and the Montgomery Foundation Gardens in Coral Gables, Florida.

In general, botanical gardens are just starting to take into account the serious challenges they will face in maintaining these collections as the global climate continues to warm steadily, altering growing conditions in their gardens progressively and significantly. In this respect, seed banks and frozen tissue cultures may afford the better hope for preserving the rich treasure of species that still exists today. For many plant groups however, the palms and cycads just mentioned among them, protocols have not sufficiently been worked out for seed preservation; the absence of such protocols poses a real obstacle to the conservation of many plants. Coordinating centers such as the Center for Plant Conservation in the U.S., represented here by John Clark; the U.S. Department of Agriculture, represented by Chris Williams; the Australian National Botanical Garden and seed collection; and the Kunming Institute for Botany, Chinese Academy of Sciences, are doing good jobs in preserving the floras of their respective areas of focus.

On a regional basis, we can offer a few more detailed estimates. Australia is home to about 26,000 species, about 90% of them described scientifically. Damian Wrigley (pers. comm.) estimates that the Australian Seed Banks Partnership presently holds about 13,200 of the total species and about half of the estimated 4,000 threatened species of plants in seed banks (also see Australian Seed Bank Partnership, 2017), with a strong continuing effort to increase the sizes of many of the samples and add the missing species.

Roy Gereau (pers. comm.) has estimated that continental Africa (including Madagascar) is home to about 63,500 species of vascular plants. In North and South America, there are some 124,993 known species of vascular plants, as of December 2017 (Ulloa et al., 2017). The total restricted to the North American continent is about 42,941, with perhaps 12,000 found only north of Mexico. This indicates that a total of about 115,000 species were known from Mexico southward in the Western Hemisphere in the New World in 2017, but some 3,800 additional species have been described from or detected in this area (biogeographically roughly comparable to Africa and Madagascar) since the checklist was compiled.

Comparing these regions, the area from Mexico southward in the Americas, which has an area of about 18 million square kilometers, has about 120,000 species of vascular plants known at present, whereas Africa, with a total area of more than 30 million square kilometers, is home to only about 60,000 species. Compounding the difference between the two tropical areas, the number of recognized vascular plant species in Latin America is growing rapidly, while the number in Africa, where the floras are much better known, is growing much more slowly (R. Gereau estimates possibly an eventual 5% increase in the African total). Before such accurate comparisons were possible, we enumerated some of the likely reasons for this extraordinary difference (Raven and Axelrod, 1974), including the post-mid-Miocene (15 my) elevation of 2500 meters in eastern Africa; the Andes, rising in the past several million years, protecting much of America’s forested area from drying winds from the west while confining summer-dry (Mediterranean climate) vegetation of much more recent origin to a small area; and most of South America remaining at relatively low elevations throughout the Tertiary. About a third of Africa is desert, but less than 10% of the smaller area of the Americas from Mexico southward is also desert.

I estimate that Africa, Australia, Europe and Russia, North and South America have a known total of about 250,000 known vascular plant species, with China and Japan probably adding something like 20,000 species not found to their north or south, tropical Asia should be home to about 130,000 known species, with many more still to be discovered there.

Paul Smith (pers. comm.) has estimated that about 42% of the IUCN-denominated threatened species of plants may already be preserved in seed banks. This proportion would obviously be much lower if we had discovered all species already and knew the rare and tropical ones as well as we do those growing near most of the world’s botanical centers. Kew’s Millennium Seed Bank includes seed samples of nearly 40,000 species of vascular plants, a number that is increasing steadily. Overall, seed banks and tissue culture centers already include more than 60,000 species of vascular plants, but clearly not all of them are being held under optimum conditions (Paul Smith, pers. comm.). This still represents a relatively small proportion of the world’s species, and we much certainly do what we can to increase both the number of species represented and the dependability of their storage conditions. Unfortunately, we cannot really foresee when the “waters may fall” and there will be good opportunities to re-establish many of these in nature from material that we have preserved in our own modern “Noah’s arks”.

A great deal of experimentation is going on with re-establishing plant species in nature, where they should in principle be able to maintain themselves without further human interference. Both Volis (2019; this symposium) and Hitchcock (this symposium) provide good examples of how that can properly be done. As long as we continue to destroy large areas of natural vegetation for our growing needs; to allow native plants to be swamped by invasive exotics; the gathering of native species for commercial purposes; and the continuing warming of the climate, saving a large proportion of the existing vegetation will be impossible. As the decades pass, we may find global warming to be an irresistible force towards extinction, discussed exceptionally well in a recent book edited by Lovejoy and Hannah (2019). Climate change threatens many plant communities with extinction over the next few decades. For example, the rich arrays of endemic vascular plants along the southern edges of Africa and Australia; these plants and the animals that occur with them will literally have no place to go if their habitats warm to temperature levels at which they cannot survive. The same is obviously true of plants that grow at high elevations on mountains, or even in montane areas such as the biologically rich cloud forests of the tropics, where an entire climate regime is in the process of being lost (Janzen & Hallwachs, 2019; Helmer et al., 2019). Species can in principle be reestablished at higher latitudes and altitudes than those that prevail now in the areas where they occur naturally (e.g., Torreya floridana), but climate stability is ultimately required for them to persist even there.

The development of strategies for finding and if possible preserving plant species while they still exist would be extremely important if we are to do the best we can in this situation. As we have pointed out (Pimm et al., 2014), the species most likely to become extinct are by definition the rare ones, and most of the undescribed species are relatively rare. Probably a majority of plant species known to be of conservation concern grow in “hot spots”, originally defined by Myers et al. (2000) as areas that were 70% disturbed by human activities and having at least 2,000 endemic vascular plant species; so much of our conservation attention ought to be concentrated in these areas.

Hot spots of particular concern are those areas of the world with a Mediterranean (summer dry) climate, communities no more than a few million years of age, the summer drought having been formed as a result of the prevailing westerly winds passing over cold offshore currents (Ice age origin) on warmer lands in summer. If, as seems likely in the face on ongoing global warming, those currents warm, the pattern of summer drought will cease, and most of the thousands of plant species that are endemic in these areas and of relatively recent origin will then be driven to extinction. That would argue for a strong and urgent concentration of conservation efforts in and around California, central Chile, southwestern and southern Australia, the Cape Region of South Africa, and in the Mediterranean region itself, as well as along the southern edges of Australia and Africa for reasons already mentioned. Everywhere, though climates are changing, with the tropics expanding at about 30 miles per decade and the Globe’s wheat belts pushing poleward at up to 160 miles per decade (summary by Jones, 2019). Beach and other coastal plants may be in special danger of extinction, with a sea level rise of more than 2 meters projected in current scenarios (Bamber et al., 2019).

What about the tropics themselves? About a quarter of all tropical lowland forests have been destroyed since the Convention on Biological Diversity (CBD) came into effect in 1993 – within just 26 years! These forests are the most poorly known of all regions biologically, and the home of at least two thirds of the world’s species of vascular plants, probably 300,000 species overall. Specific areas of the tropics will be the world’s richest in species of most groups of organisms, certainly including flowering plants, and they are also the most poorly known and the most poorly represented in botanical gardens and seed banks. We provided some details for the individual countries in our collection of essays, “Voices from the Tropics” (Sodhi, Gibson, & Raven, eds., 2013), showing that the forests of some of the richest and most poorly known areas, such as New Guinea, are likely to be largely destroyed by mid-century. As if that were not bad enough news, the destruction of these forests will clearly increase the rate of warming everywhere on our planet, and is likely make the destruction of all ecosystems even more rapid. Relationships of this kind present us with a real sense of urgency, since there will be only so much we can learn during the time we have remaining, and suggest that those in a position to do something about the matter ought to develop some joint goals and pursue them vigorously. There will, unfortunately, be no second chances.

An especially telling demonstration of how little we actually know is demonstrated by the work of Tetsukazu Yahara of Kyushu University. Using molecular comparisons, Yahara and his colleagues have discovered many dozens of previously undetected species of Southeast Asian trees and shrubs, species which when detected usually exhibit clear morphological differences. Especially in families like Lauraceae, whose inconspicuous, usually yellowish-green flowers last only a very few weeks, the discoveries have been impressive in scope, indicating that many more species exist in the tropics that we have yet recorded scientifically.

To mention two additional non-woody families, André Schuiteman (pers. comm.) estimates that there are some 25,640 valid species of orchids (756 genera), with perhaps as many as 4,000 species remaining to be described. Thomas Croat (pers. comm.) estimates that there are 3,645 accepted species of Araceae (144 genera), but he has roughly 1,600 additional undescribed species in his collection. This total of about 4,250 species already in herbaria and the ongoing extraordinary discoveries that are still being made, especially in the Neotropics, indicates that our knowledge is very incomplete. Ultimately, as many as 10,000 species or even more may be found in this family if we can explore the relevant tropical areas while there is still time to do so.

The future of tropical forests is unfortunately unclear; although there are many plans and suggestions of how they might be conserved at least in part, they intrinsically all count on a degree of sharing between people within and between countries that is almost beyond imagining. In the absence of such a development, the prospect of many countries falling into what we have defined as an “Ecological Poverty Trap”, a point at which their already very low ability to consume is ever more limited by declining biocapacity per person, while also faced with too low income to afford imports from other places. One effect is the inevitable depletion of their natural assets. A rapidly growing number of countries are already in this situation or approaching it at a time when they have exhausted their resources and have no options but to sell what they can to other countries in order to survive (Wackernagel et al., 2019). Such countries will not be able to pursue schemes for the conservation of their biological riches by following plans and suggestions developed by the wealthy unless we can reverse our natural selfishness, recognize the worth of their people, and collaborate with them fully. Extinction and exhaustion of resources are proceeding rapidly, and one may reasonably wonder about the likelihood of those profound moral changes taking place while there is still time to accomplish something significant together.

This flood of changes has already begun, and extinction is already proceeding at something like 1,000 times the background rate. The pace of this destructive process has increased to such a degree that many biologists have concluded – notably Ceballos, Ehrlich, & Dirzo (2017) – that we have already entered the world’s Sixth Major Extinction Event, the first one since the end of the Cretaceous Period 66 million years ago – the time when the last dinosaurs became extinct and the character of life on Earth was permanently altered. In any case, we are very close! Carrying the analogy of this symposium further, it would be almost as if Noah and his sons were starting to build their Ark while the waters were already rising around them and the animals they wanted to save swimming or clustered on small islands. Everything about our situation now calls for collaboration, with the development and pursuit of appropriate collective goals, ultimately demanding recognition by our governments of the madness of maintaining their selfishness in the face of the disasters we are facing together. Anything less would not result is saving the biological richness with which our world has been endowed, and indeed would not be worth of us. As our colleague Dan Janzen put it recently when conversing on this issue, “If we don’t save it now, we can’t save it later”. It is time to get even busier, more focused, and more collective in our thoughts. As one of its organizers, I hope that this symposium will help us attain that necessary goal and look forward to the fine presentations that will now be provided by our botanical colleagues.


Australian Seed Bank Partnership. 2017. Safeguarding Australia’s Flora through a national network of native plant seed banks. Australian Seed Bank Partnership, Australian National Botanic Gardens, Canberra.
Bamber, J.L., M. Oppenheimer, R.E. Kopp, W.P. Aspinall, & R.M. Cooke. 2019. Ice sheet contributions to future sea-level rise from structured expert judgment. Proc. Natl. Acad. Sci. U.S. 116: 11195-11200.
Ceballos, G., P.R. Ehrlich, A.D. Baranosky, A. García, R.M. Pringle, & T.M. Palmer. 2015. Accelerated human-induced species loss: Entering the sixth mass extinction. Science Advances 1(5): e1400253.
Ceballos, G., P.R. Ehrlich, & R. Dirzo. 2017. Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines. Proc. Natl. Academy of Sciences U.S. 114: 6089-6096.
Chapman, A.D. 2009. Numbers of living species in Australia and the World. Australian Biological Resources Study, Canberra.
Dinerstein, E. et al. (19 authors total). 2019. A global deal for nature: Guiding principles, milestones, and targets. Science Advances 5(4), eaaw2869.
Helmer, E.H., E.A. Gerson, L.S. Baggett, B.J. Bird, T.S. Ruzycki, & S.M. Voggesser. 2019. Neotropical cloud forests and páramo to contract and dry from declines in cloud immersion and frost. PLoS ONE 14(4): e0213155.
Humphreys, A.M., R. Govaerts, S.Z. Ficinski, E.N. Lughadha, & M.S. Vorontsova. 2019. Global dataset shows geography and life form predict modern plant extinction and rediscovery. Nature Ecology & Evolution 3: 1043-1047.
Janzen, D.H. & W. Hallwachs. 2019. Perspective: Where might be many tropical insects? Biological Conservation 233: 102-108.
Jones, H. 2018. Redrawing the map: How the world’s climate zones are shifting. YaleEnvironment360, October 25, 2018, p. 1-6.
Joppa, L.N., D.L. Roberts & S.L. Pimm. 2010. How many species of flowering plants are there? Proc. R. Soc B. 278: 554-559. Published online 7 July 2010.
Kew. 2016. State of the world’s plants 2016. Royal Botanic Gardens, Kew, Richmond, U.K.
Kew. 2017. State of the world’s plants 2017. Royal Botanic Gardens, Kew, Richmond, U.K.
Lovejoy, T.E. & L. Hanah. 2019, Biodiversity and Climate Change. Yale University Press, New Haven.
Pimm, S.L., C.N. Jenkins, R. Abell, T.M. Brooks, J.L. Gittelman, L.N. Joppa, P.H. Raven, C.M. Roberts, & J.O. Sexton. 2014. The biodiversity of species and their rates of extinction, distribution, and protection. Science 344: 987.
Pimm, S.L. & Raven, P.H. 2017. The fate of the world’s plants. Trends in Ecology & Evolution 32: 317-320.
Raven, P.H. (in press). Biological extinction and climate change. In Al Delaimy, W.K., V. Ramanathan & M. Sánchez S. (eds.), 2019. Health of People, Health of Planet and Our Responsibility. Climate Change, Air Pollution and Health. Springer Verlag.
Raven, P.H. & Axelrod, D.I. 1974. Angiosperm biogeography and past continental movements. Ann. Missouri Bot. Gard. 61: 539-673. 10.2307/2395021
Smith, P.P. 2016. Building a Global System for the Conservation of all Plant Diversity: A Vision for Botanic Gardens and for Botanic Gardens Conservation International. Sibbaldia 14: 5-13.
Sodhi, N.S., L. Gibson & P.H. Raven (eds.). 2013. Conservation Biology. Voices from the Tropics. xxiv + 264 p. Wiley Blackwell, Oxford.
Ulloa, C. et al. 2017. An integrated assessment of the vascular plant species of the Americas. Science 358: 1614-1617.
Volis, S. 2019. Plant Conservation: The Role of Habitat Restoration. Cambridge University Press, Cambridge, p. xvi + 480.
Wackernagel, M., D. Lin, M. Evans, L. Hanscom, & P.H. Raven. 2019. Defying the Footprint Oracle: Implications of country resource trends. Sustainability, 11(7): 2164.


[*] PAS Academician.
[3] /




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