全 文 ：Plant diversity in a changing world: Status, trends, and conservation
needs
Richard T. Corlett*
Center for Integrative Conservation, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun, Yunnan, 666303, China
a r t i c l e i n f o
Article history:
Received 22 October 2015
Received in revised form
5 January 2016
Accepted 7 January 2016
Available online 24 May 2016
Keywords:
Land plants
Diversity patterns
Threats
In situ conservation
ex situ conservation
a b s t r a c t
The conservation of plants has not generated the sense of urgencydor the fundingdthat drives the
conservation of animals, although plants are far more important for us. There are an estimated 500,000
species of land plants (angiosperms, gymnosperms, ferns, lycophytes, and bryophytes), with diversity
strongly concentrated in the humid tropics. Many species are still unknown to science. Perhaps a third of
all land plants are at risk of extinction, including many that are undescribed, or are described but
otherwise data deficient. There have been few known global extinctions so far, but many additional
species have not been recorded recently and may be extinct. Although only a minority of plant species
have a specific human use, many more play important roles in natural ecosystems and the services they
provide, and rare species are more likely to have unusual traits that could be useful in the future. The
major threats to plant diversity include habitat loss, fragmentation, and degradation, overexploitation,
invasive species, pollution, and anthropogenic climate change. Conservation of plant diversity is a
massive task if viewed globally, but the combination of a well-designed and well-managed protected
area system and ex situ gap-filling and back-up should work anywhere. The most urgent needs are for the
completion of the global botanical inventory and an assessment of the conservation status of the 94% of
plant species not yet evaluated, so that both in and ex situ conservation can be targeted efficiently.
Globally, the biggest conservation gap is in the hyperdiverse lowland tropics and this is where attention
needs to be focused.
Copyright © 2016 Kunming Institute of Botany, Chinese Academy of Sciences. Publishing services by
Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-
NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
The conservation of plant diversity has received considerably
less attention than the conservation of animals, perhaps because
plants lack the popular appeal of many animal groups (Goettsch
et al., 2015). As a result, plant conservation is greatly under-
resourced in comparison with animal conservation (Havens et al.,
2014). Yet plants are much more important to us. Animals can
provide meat, leather, fur and other products, but none of these are
necessities for human survival and well-being, while many plant
products are essential. Plants provide food for us and our livestock,
as well as a huge diversity of other products and services, from
timber and fibers to cleanwater and erosion control. Althoughmost
commercial plant products come from a very narrow range of plant
species, a life based on only these species would be both unhealthy
and dull: even urban dwellers use a wide range of other plant
species for various purposes and rural people tend to use many
more. Wild plant foods contribute to nutrition and food security,
and numerous additional species have roles in traditional medicine.
Moreover plants are the basis for all terrestrial ecosystems,
providing the three-dimensional structure in which animals live
and move, and the food on which a majority feed.
This review focuses on the current status of global land plant
diversity, the major threats to its continued persistence, and the
priority actions for its conservation. It concentrates on the tropics,
where most plant species live but least is known about them.
2. How many plant species are there?
The updated Global Strategy for Plant Conservation (hereafter
GSPC) agreed at the CBD meeting in Nagoya in 2010 to include, as
its first target for 2020, ‘an online flora of all known plants’ (www.
* Tel.: þ86 18288059408 (mobile); fax: þ86 6918715070.
E-mail address: Corlett@xtbg.org.cn.
Peer review under responsibility of Editorial Office of Plant Diversity.
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Plant Diversity 38 (2016) 10e16
cbd.int/gspc/targets.shtml). This target is perhaps achievable, but it
explicitly omits unknown species, of which there are still many. A
recent paper estimated the total number of angiosperm species at
around 450,000, of which 10e20% are still unknown to science
(Pimm and Joppa, 2015). Recent estimates for gymnosperms (1000
species; Christenhusz et al., 2011), ferns (10,000 species; Ranker
and Sundue, 2015), lycophytes (1300 species), mosses (9000 spe-
cies; Magill, 2010), hornworts (200e250 species; Villarreal et al.,
2010), and liverworts (7500; Von Konrat et al., 2010) suggest that
the global total for all land plants is around 500,000 species. This
compares with around 10,000 bird species and 5400 mammals.
Indeed, the only taxonomic groups whose diversities are thought to
substantially exceed that of land plants are the largely plant-
dependent fungi (1.5e5.1 m; Hawksworth, 2012) and beetles (ca.
1.5 m; Stork et al., 2015).
3. Where are they?
Pimm and Joppa (2015) estimated that two-thirds of all angio-
sperm species are found within the tropics. Fern diversity is even
more highly concentrated in the tropics (Kreft et al., 2010) while,
among the bryophytes, liverwort diversity is highest in the tropics
but mosses show no clear latitudinal gradient (Geffert et al., 2013;
Chen et al., 2015). The distribution of plant species across the tro-
pics is far from uniform, with the highest diversities in the Neo-
tropics and the AsiaePacific region, and lower diversities in Africa
and on oceanic islands. For example, Slik et al. (2015) estimate that
there are 40,000e53,000 tree species in the tropicsd96% of all the
tree species on Earth (Poorter et al., 2015)dwith similar numbers
(19,000e25,000) in the Neotropics and the AsiaePacific, but far
fewer (4500e6000) in Africa. Although plant diversities are lower
on individual islands, endemism is high and around 50,000 species
of vascular plants are island endemics (Sharrock et al., 2014). Also,
not all concentrations of plant diversity are tropical: regional di-
versity is also very high in the Mediterranean region and in similar
climates elsewhere, as well as in the moist subtropical areas of Asia
(Barthlott et al., 2007; Joppa et al., 2013). At higher spatial resolu-
tions, the concentration of plant species is even more marked, with
67% of all plant species confined to, and 81% present in, only 17% of
the Earths land surface (Joppa et al., 2013).
For trees, there is enough data from plot inventories to look at
patterns of local diversity on a regional scale. For example, within
tropical East Asia (SE Asia plus S China and NE India), the highest
diversities (>210 tree species >10 cm diameter in 1 ha of forest) are
in lowland rainforests in Borneo and Sumatra, but high diversities
(>100 tree species) also occur in lowland rainforests from Sulawesi
to southern China (Corlett, 2014a). Plots at higher altitudes
(>1200m), on extremesoil types, and in areaswith a longdry season
have lower tree diversities, as do all sites north of the tropics. The
contrast between tropical and temperate zone tree diversity is
highlightedby the fact that just 52haof lowland rainforest at Lambir
in Borneo supports as many tree species (1175) as all the temperate
forests in thenorthernhemisphere together: Asia, Europe andNorth
America (Wright, 2002). Similar diversity patterns occur in other
tropical regions, with land plant diversity best predicted by the
number of wet days per year (Kreft and Jetz, 2007).
4. Do we need them all?
The conservation of all plant species can be justified on a range
of aesthetic, scientific and ethical grounds－it is simply good
stewardship－but these arguments seem to have been used more
effectively in support of animal conservation. Unlike butterflies or
frogs, plants are expected to be useful. In low-diversity ecosystems,
most plant species do have specific human uses that justify their
protection, but this is not true in the hyperdiverse tropical forests,
where local people appear to know only a subset of the flora
(personal observations in Papua New Guinea and SE Asia). The use
of relatives of species with specific human uses in plant breeding
programs considerably extends the list of ‘useful’ plant species. For
example, a recent study in China identified 871 species of wild
relatives of major crops (Kell et al., 2015), although this is still a
relatively small proportion of Chinas total angiosperm flora of
around 30,000 species (Wang et al., 2015). All wild plant species,
however, are parts of natural ecosystems which, in turn, provide
services for human populations. Are they all necessary for ecosys-
tems to function?
High local plant diversities in tropical forests have been
explained in multiple ways, with most of these depending on dif-
ferences between species, in their resource use (water, nutrients,
light) and/or in their pests and diseases (Wright, 2002; Corlett,
2014a). Neutral theory, in contrast, suggests that coexistence de-
pends on the ecological equivalence of species rather than their
differences (Rosindell et al., 2012). The available evidence strongly
supports the idea that coexistence depends on differences (Corlett,
2014a), but this does not necessarily imply that these differences
are important to the maintenance of ecosystem functioning. In the
most species-rich forests, most species are rare and the common
species are likely to dominate ecosystem functions. For example, a
remarkably few, common, large tree species (1.5% of the total tree
flora in both the Amazon Basin and Central Africa) contribute
disproportionately to carbon storage and fluxes in tropical forests
(Fauset et al., 2015; Bastin et al., 2015). However, a recent study
showed that the rare species in high diversity ecosystems support
themost distinctive and vulnerable functions, and that these species
make a disproportionate contribution to the potential range of
functions that can be provided by the ecosystem (Mouillot et al.,
2013). In an era of rapid global change, this functional redundancy
is likely to be a useful insurance policyagainst unpredictable threats.
5. How many species are threatened?
Target 2 of the GSPC is assessing the conservation status of all
known plants by 2020, but we are still a long way from achieving
this. Fewer than 20,000 plant species have been formally assessed
so far at the global level using the IUCN Red List criteria, so the
proportion of land plants that are threatened is not accurately
known. Pimm and Joppa (2015) suggest that a third of all angio-
sperms are at risk of extinction, including most of those that have
not yet been described, since these are likely to have small ranges
and be locally rare. Brummitt et al. (2015) assessed the status of a
random sample of 7000 plant species against the Red List criteria,
including bryophytes, ferns, gymnosperms and angiosperms (rep-
resented by monocots and the well-studied legumes) and
concluded that 22% were threatened (IUCN categories Vulnerable,
Endangered, or Critically Endangered) and 30% threatened or near-
threatened. For the major groups assessed, the percentage threat-
ened ranged from 11% for legumes to 40% for gymnosperms.
Compared with other groups assessed in the same way, plants are
more threatened than birds, similar to mammals, and less threat-
ened than amphibians.
Note, however, that this sampling approach necessarily excludes
the species still unknown to science and thus almost certainly
underestimates the overall threat levels. Moreover, Data Deficient
species were assumed to be threatened in the same proportions as
those with enough information, while it is more likely that data
deficiency most often reflects rarity and thus higher vulnerability.
The habitat with most threatened species was overwhelmingly
tropical rainforest in both the above studies. A recent model-based
assessment of the conservation status of 15,200 Amazonian tree
R.T. Corlett / Plant Diversity 38 (2016) 10e16 11
species estimated that 36%e57% would likely qualify as threatened
under IUCN Red List criteria and the authors go on to suggest that
the majority of the worlds >40,000 tropical tree species may be
threatened (ter Steege et al., 2015). If these estimates are confirmed,
it suggests that tropical trees are among the most threatened taxa
on Earth.
In contrast to threat assessments, known global extinctions
among plant species are still few. The current IUCN Red List of
Threatened Species (Version 2015-3. www.iucnredlist.org Down-
loaded on 26 December 2015) includes only 139, of which 37 still
survive in cultivation. This compares with 140 bird and 78mammal
species, out of much smaller total numbers. The IUCNwarns that all
these numbers are ‘likely to be significant underestimates’ and lists
another 113 species as ‘Possibly Extinct’. This informal list, which is
dominated by plants, was developed to highlight Critically En-
dangered species that cannot be listed as Extinct without additional
surveys, and it shows the difficulty in determining if a rare plant
species is extinct or not. Plants dont sing or come to baits, and dont
walk past camera traps, which makes it much more difficult and
time-consuming to assess their conservation status than it is for
vertebrates and some invertebrates. There are an estimated 1.4
trillion individual trees >10 cm in diameter in the tropical and
subtropical forests that harbor most of the worlds plant diversity
(Crowther et al., 2015), and trillions more small trees, shrubs and
herbs, so it is easy to overlook the last few individuals of a
threatened species, particularly when the key characters for iden-
tification are high up in the canopy, as is true of most large trees, or
only visible for a brief period each year, as is true for many smaller
plants.
6. What are the major threats?
6.1. Habitat loss, fragmentation, and degradation
Habitat loss and associated fragmentation is the biggest single
threat to plant diversity, particularly in the tropics (ter Steege et al.,
2015), where conversion of tropical forests to pastures and com-
mercial crop monocultures (oil palm, rubber, soy etc.) has replaced
small-scale cultivation by poor farmers as the major driver of forest
loss. Few forest-adapted plant species survive complete defores-
tation, and even if a substantial fraction of the original forest cover
remains, fragmentation drives changes that tend to reduce plant
diversity (Kettle and Koh, 2014). Large areas of the remaining forest,
whether fragmented or not, are degraded by logging, fire, and other
impacts, including fuelwood harvesting in densely populated areas
(Specht et al., 2015). Non-forest habitats, from savannas and
grasslands to deserts, are similarly threatened by agricultural
development. Many plant species are confined to specialized hab-
itats, such as limestone or ultramafic outcrops, which are unsuit-
able for commercial agriculture, but such habitats often have
different, highly specific threats, such as mining of limestone to
make cement (Clements et al., 2006) and of ultramafic rocks for
nickel (Losfeld et al., 2015), which are particularly damaging
because of the small areas involved.
6.2. Overexploitation
Overexploitationdof the whole plant or enough of it to reduce
the chance of survivaldis the second most important threat to
plant species. It is usually more or less species-specific, although
some species can be lumped together for specific uses, for example
dipterocarps with similar wood in the timber trade or plants with
similar properties in the medicine trade. People collecting plants
for home use or for sale face the same difficulties in locating them
as botanists do, so exploitation to the point of extinction is likely
only in species with a restricted range or where the value increases
with rarity, as will often be the case for luxury products. Collection
for the horticultural trade and for private collections is the biggest
single threat to the cacti (Goettsch et al., 2015) and, in many areas,
orchids (Phelps andWebb, 2015), as well as cycads and ornamental
species in many other families (Sharrock et al., 2014). Extinction is
also likely to be slow where only the largest individuals are har-
vested, as happens with timber trees, since seedlings, saplings and
undersize adults survive each round of logging. Logging affects
more than half of all remaining tropical forests (Sharrock et al.,
2014), but over-logging threatens the timber supply long before it
threatens individual tree species (Shearman et al., 2012). Damage
from the harvesting of non-timber forest products (NTFPs) varies
widely, depending on whether whole plants are removed or killed,
and, if only parts of each plant are removed, how this affects
growth, reproduction and survival. Note that overexploitation of
animals may also threaten plant species in the long term, by
restricting seed dispersal (e.g. Harrison et al., 2013) or, in some
cases, pollination.
6.3. Invasive species
Invasive alien species are another potential threat to native plant
diversity. A recent study showed that more than 13,000 spe-
ciesd3.9% of the worlds vascular plant florad have become natu-
ralized somewhere outside their native range as a result of human
activity (van Kleunen et al., 2015). Tropical regions generally have
fewer naturalized species than temperate regions, but these
numbers are increasing as direct trade between tropical countries
overcomes the geographical barriers that have isolated the major
tropical regions during the period when most modern species
evolved (Corlett and Primack, 2011). Although invasive plant species
can havemassive local impacts, reducing native plant diversity, and
changing fire regimes and nutrient cycling (Py
sek et al., 2012), we
actually knowvery little about their longer-termimpactson regional
and global plant diversity. Even on oceanic islands, where local
impacts tend to be much greater than on the mainland, non-native
plants generally add to the total plant diversity, rather than replac-
ing native species, and in continental areas there is little evidence
that invasive plant species currently threaten any native species
with extinction (Ellis et al., 2012; Thomas and Palmer, 2015). It is
possible that competitive exclusion of native species is simply very
slow (Gilbert and Levine, 2013), but current evidence suggests that,
despite often large local impacts, the extinction risk from invasive
plants is low. Invasive animals may be more of a threat, particularly
generalist herbivores, such as goats, on islands that lack native
vertebrate grazers and browsers (Chynoweth et al., 2013).
6.4. Air pollution and nitrogen deposition
Every plant on Earth today is exposed to an atmosphere that
differs significantly in composition from any that its ancestors
would have experienced. Changes in the concentration of the major
greenhouse gases (CO2, CH4, N2O) are considered separately below,
but other air-borne pollutants can also impact plant diversity
(Corlett, 2014b). The major source of air pollution is the burning of
fossil fuels and the most important primary pollutants are sulphur
dioxide and nitrogen oxides. Ozone, which is produced from hy-
drocarbons and nitrogen oxides in the presence of sunlight, is the
most important secondary pollutant. Particulates (or aerosols: solid
and liquid particles suspended in the air) are derived from a variety
of primary and secondary sources. Air pollution is declining in
Europe and other developed regions, but increasing in much of
Asia. Wet and dry deposition of nitrogen compounds not only
acidifies the soils but can also dramatically change nutrient cycles,
R.T. Corlett / Plant Diversity 38 (2016) 10e1612
as has happened over much of southern China, with a largely un-
known impact on plant diversity (Zhu et al., 2015). Indeed, for
tropical forests in particular, our current understanding of the im-
pacts of air pollution and nitrogen deposition on plant diversity is
very limited.
6.5. Climate change
The impacts of anthropogenic climate change are also complex
and unpredictable, and even more pervasive. After around 1 C of
global warming so far, many temperate zone plants are leafing and
flowering earlier in spring anddless consistentlyddelaying leaf fall
in autumn (Ge et al., 2015). Some species have extended their
ranges towards the poles and/or to higher altitudes, although other
species have not done so (Hijioka et al., 2014). Growth rates have
generally increased where temperature is limiting and decreased
where water is. Although no global plant extinctions have yet been
attributed to anthropogenic climate change, there is evidence that
local extinctions have occurred at the climatic margins of species
ranges (Buse et al., 2015).
The recent Paris Agreement (UNFCCC, 2015), signed (but not yet
ratified) by 195 countries, set a target of keeping “the increase in
the global average temperature to well below 2 C above pre-
industrial levels and to pursue efforts to limit the temperature in-
crease to 1.5 C above pre-industrial levels”, but the pathway to
these ambitious targets is still unclear. Without rapid cuts in
emissions, 3e4 C is more likely. Even a 2 C rise in global tem-
perature means generally greater warming of land surfaces,
particularly at high northern latitudes, and will be associated with
less predictable changes in rainfall and other climatic parameters
(IPCC, 2013). Climate change will also interact with other impacts:
both negatively, as with fires and fragmentation, but also perhaps
positively with rising carbon dioxide levels (Corlett, 2014b). When
changes in the local climate exceed the range of natural variation,
plant populations can either acclimate (i.e. adjust physiologically
within the lifetime of an individual), adapt (by evolutionary
changes over multiple generations), move to somewhere with a
more suitable climate, or die. There is very little information
available on either acclimation capacity or evolutionary potential
for all but a fewmodel plant species, but the capacity for movement
is better e although still incompletely e understood (Corlett and
Westcott, 2013; Corlett, 2015). These studies suggest that most
plant species will find it difficult or impossible to track the expected
rate of climate change, except in steep topography where climatic
gradients are equally steep. Moreover, some current climatic con-
ditions cannot be tracked, since they will completely disappear,
while large areas of tropical and subtropical lowlands will have
climates by the mid to late 21st century that do not currently exist
anywhere on Earth (IPCC, 2013).
7. How can they be conserved?
Viewed globally, conserving plants is a huge job: 500,000 spe-
cies spread over the Earths land surface, of which
100,000e160,000, many currently unknown to science, may be
threatened. The fact that most species and most threatened species
are in tropical rainforests has been a problem for their conserva-
tion, since most conservation expenditure and the most ambitious
plant conservation projects are in high-income countries outside
the tropics. However, rapid economic development in recent years
has lifted most tropical rainforest countries in Asia and the Neo-
tropics, and some in Africa, into the middle-income bracket, so they
have more financial and other resources potentially available for
conservation, even if they are not currently using them for this.
International donors are still an important source of funding for
conservation in some of these countries, but national governments
are in the best position to provide the continued baseline support
that is likely to be most effect in the long term.
7.1. Completing the inventory
The most urgent task is to complete the inventory of all land
plant species. We need a complete global list of species, including
the estimated 50,000e100,000 species that not yet been collected
and described (i.e., overcome the Linnean shortfall), and determine
their distributions (the Wallacean shortfall), as well as understand
their phylogenetic relationships (the Darwinian shortfall) (Diniz-
Filho et al., 2013). We also need to make this information easily
accessible on-line (Meyer et al., 2015). GSPC target 1, ‘to produce an
online flora of all known plants’, covers much of this, but it does not
include a strategy for ensuring that as many as possible of the
currently unknown plants become known by 2020. Moreover, the
short time-frame means that, in practice, the first global flora will
largely be a compilation of existing information. Recent studies
suggest that spatial bias in collection activity in botany is idiosyn-
cratic, with large under-collected areasdparticularly in the
tropicsdas well as pockets of intense activity (Vale and Jenkins,
2012; Yang et al., 2014).
7.2. Conservation status assessment
Only slightly less urgent is a global assessment of the status of
the 94% of land plant species not yet evaluated globally under the
IUCN Red List criteria, so that both in and ex situ conservation can be
targeted efficiently. A recent assessment of all but two of the large
family Cactaceae (1480 species), in which only 11% of species had
been evaluated before 2013, suggest that this goal is no less
achievable for at least some plant groups than it is for animal
groups, such as the amphibians, which have received much more
conservation attention (Goettsch et al., 2015). Clearly botanists
need to make this a higher priority than it is at present. A pre-
liminary assessment of the global status of 15,200 Amazonian tree
species, using spatially explicit models of tree species abundance
and deforestation, and interpreting the results using the IUCN Red
List criteria, demonstrates the potential for scaling up the assess-
ment process, although the authors point out that species-by-
species assessments are still needed (ter Steege et al., 2015).
Regional and national assessments using IUCN Red List criteria
include many species that have no global assessment yet, and can
provide a basis for targeting conservation work in these areas
(Havens et al., 2014; Sharrock et al., 2014).
7.3. Improving the protected area system
When we know what is threatened we can assess current pro-
tected area systems and add to them where they provide inade-
quate in situ coverage, as will be true in most areas. However,
assessing the adequacy of existing coverage of threatened plants is
difficult in most of the world, because inventories are lacking
(Sharrock et al., 2014). In response to this information gap, the
Royal Botanic Gardens Kew is proposing to identify and map
‘Tropical Important Plant Areas’ (TIPAs) which support high con-
centrations of threatened species (www.kew.org/science-
conservation/kews-science-strategy/2020-strategic-outputs/
tropical-important-plant-areas). Currently an estimated 15% of the
Earths land surface is legally protected for conservation, but
coverage varies widely among ecosystems, as does the effective-
ness of the protection. General reviews of the effectiveness of
protected areas in conservation show that they retain more biodi-
versity than alternative land uses (Coetzee et al., 2014) and that
R.T. Corlett / Plant Diversity 38 (2016) 10e16 13
management effectiveness is generally increasing (Geldmann et al.,
2015), but there have been no long-term studies of their ability to
maintain viable populations of threatened plant species. A review
of tropical forest reserves found mixed results and concluded that
what happens inside reserves is strongly linked to what happens in
the surrounding area (Laurance et al., 2012). Experts reported de-
clines in large-seeded old-growth trees and epiphytes, and in-
creases in pioneers and generalists, lianas, and invasive species.
Many protected areas fail to prevent overexploitation of valuable
plants and/or the animals on which they depend, and many are
subject to encroachment by farmers or their fires. Invasive species
and air pollution are also not excluded by legally designated
boundaries. Moreover, the fragmentation of the protected area
system in many countries means that even the more mobile plant
species will not be able to track climate change over future decades
unless their dispersal agents can cross large intervening areas of
agriculture or urban development (Corlett and Westcott, 2013).
The best answer we have currently is to try to preserve large
areas of forest (and other habitats, such as natural grasslands) over
broad altitudinal gradients in steep topography, which should
maximize the ability of plants to respond to climate change by
movement. Where this is no longer possible, as will often be the
case, it may be possible to use ecological restoration to recreate the
missing links between fragments, and/or to make the intervening
agricultural areas more wildlife friendly. Where none of these are
possible, or in areas where there is no steep topography, vulnerable
plant species may need to bemoved artificially to cooler (or drier or
wetter) areas that they do not currently inhabit (Corlett and
Westcott, 2013; Corlett, 2015). Such managed translocation (or
assisted migration) is, rightly, controversial, but alternative options
are limited.
To be effective, in situ conservation requires species-level
monitoring to ensure that viable plant populations of threatened
species persist within protected areas. If declines are detected,
appropriate interventions, such as habitat management, invasive
species control, prevention of overexploitation, and/or managed
translocation to a new site, can be considered. Ideally, each
threatened species would have a separate species management
plan (Heywood, 2015). Monitoring the many unknown species in
tropical forests is clearly impossible, but changes in status of known
species may flag general problems in protected area management
that can be reduced, to the benefit of both known and unknown
species.
7.4. Controlling overexploitation
Overexploitation, within and outside protected areas, will need
additional action. Controlling subsistence use runs into practical
and moral issues, but most damage is caused by collection for
markets, often by professional collectors. Many countries have laws
against this which are inadequately enforced, but often commercial
collection makes use of legal loopholes which need to be closed.
The CITES convention (Convention on International Trade in En-
dangered Species of Wild Flora and Fauna) can be effective in
limiting transboundary trade in plant species (Sharrock et al., 2014),
but for many species the major markets are internal and in this case
enforcement depends on overworked police forces without rele-
vant training and with many higher priorities.
7.5. Ex situ conservation
Target 8 of the GSPC is the ex situ conservation (i.e. outside their
natural habitat) of at least 75% of threatened plant species, with at
least 20% available for recovery and restoration programs. Most
angiosperms (75%e80%; Walters et al., 2013) have ‘orthodox’ seeds
that can be dried and then stored at low temperatures for a varying
length of time, so this is a potentially attainable target, although we
are still far from achieving it. The Millennium Seed Bank in the UK,
the worlds largest, has a target of storing 25% of the worlds plant
species by 2020 (www.kew.org/science-conservation/collections/
millennium-seed-bank) and several other large seed banks have
ambitious targets. Low storage costs mean that it is possible to
maintain many genetic individuals of each species and also, where
necessary, to keep separate collections of multiple wild populations
(Hoban and Schlarbaum, 2014). In practice, however, the quality of
collections in seed banks varies, with some suffering from low
viability and many failing to represent the genetic diversity of the
wild populations.
The situation for the other 20%e25% of species, with ‘recalci-
trant’ seeds that cannot be stored under standard conditions is
currently far worse. Cryopreservation, usually in or over liquid ni-
trogen, is currently the only practical means of long-term storage.
Storage of whole recalcitrant seeds is rarely possible, but excised
embryos, embryonic axes, or dormant buds of many non-tropical
and some tropical species have been stored successfully, although
protocols often need adjusting for each species (Berjak and
Pammenter, 2013). As a result, these techniques are currently
used mostly with crop plants and their wild relatives. The majority
of wild species with recalcitrant seeds are in the lowland humid
tropics, however, where the numbers of threatened species are
highest and the resources for conservation least. More research is
urgently needed, but as ultra-low temperature freezers become
cheaper, avoiding the need to replace liquid nitrogen as it evapo-
rates, cryopreservation is likely to become easier and, hopefully,
more widespread.
Currently the simplest ex situ conservation strategy for these
species is to grow them in ‘living collections’ in botanical gardens,
arboreta, and similar facilities. However, growing enough trees to
represent the full genetic diversity of a species may require an
inordinate amount of space, while living collections of shorter lived
species need to be carefully managed to avoid inbreeding, hybrid-
ization, or selection for the garden conditions (Ensslin et al., 2015).
The use of new molecular techniques for genetic optimization of
living collections can help (Wee et al., 2015), but these are not yet
widely available where they are most needed. In many botanical
gardens and other institutions in the tropics the living collections
include many threatened species, but the majority of these are
represented by only one or a few genetic individuals, so these
collections are of little use for conservation. Economically impor-
tant timber treesdwhich are rarely threateneddare usually the
only tree species with adequate living collections in the tropics
(FAO, 2014). There is an urgent need for botanical gardens to move
beyond the ‘stamp-collecting’ mentality to collecting for conser-
vation purposes (Cavender et al., 2015). Networking between gar-
dens in the same climate zone can help with space constraints, as
can the ‘safe sites’ approach used by the International Conifer
Conservation Programme, based at the Royal Botanical Gardens
Edinburgh. These sites include public parks, golf courses, hospitals,
and private land owners that grow threatened conifers as part of a
coordinated, out-sourced, ex situ conservation programme
(Cavender et al., 2015). Overall, the ex situ conservation of tropical
plants continues to be a major gap in global plant conservation and
needs to be a major focus of both research and action.
8. Priorities
Despite the daunting global scale of plant conservation needs, a
target of ‘zero extinction’ is not implausible at the local level, if ex
situ facilities are available to fill the gaps in a well-designed pro-
tected area system. However, prioritization of threatened species
R.T. Corlett / Plant Diversity 38 (2016) 10e1614
for protection is necessary at a global scale because of the large
variation in local conservation capabilities, particularly in the tro-
pics.While a variety of criteria have been suggested for this process,
the most obvious ones are: potential economic value (e.g., wild
relatives of crop and medicinal plants; Sharrock et al., 2014),
ecological importance (i.e., species with key roles in ecosystem
functioning), and phylogenetic uniqueness (i.e. species with no
close relatives) (Corlett, 2014a). The first two criteria are likely to be
most useful at the local level, while phylogenetic distinctiveness
makes most sense when setting global priorities. The EDGE
(Evolutionarily Distinct and Globally Endangered) program at the
Zoological Society of London has drawn conservation attention to
many previously neglected animal species and could do the same
for plants (www.edgeofexistence.org). EDGE plants are an irre-
placeable part of the plant kingdom, each representing millions of
years of evolutionary history, and are also likely to have unique
ecological roles.
9. Conclusions
There do not need to be any more plant extinctions. The com-
bination of a well-designed, well-monitored, and well-managed
system of protected areas, with ex situ conservation in seed banks
and, where necessary, living collections and cryogenic storage,
should be enough to protect all land plant species through the next
few decades of rapid global change. The major barriers to this goal
of zero global plant extinctions are: the many undescribed plant
taxa, which cannot receive targeted protection; the low percentage
of known taxa whose status has been assessed, so we cannot effi-
ciently assign protection; the uneven global coverage of protected
areas, particularly in the hyperdiverse humid tropics, and the lack
of plant inventories within them; the massive underrepresentation
of tropical taxa in ex situ collections; and the apparent absence of
any sense of urgency among everyone from plant biologists to
government officials, conservation NGOs, and the general public.
None of these problems are inherently intractable and all the gaps
could be filled. We know how to do this, but we are not currently
doing enough.
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