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OCR for page 45
Colloquium
Human-causecl environmental change: impacts on
plant diversity and evolution
David Tilman* and Clarence Lehman
Department of Ecology, Evolution, and Behavior, 1987 Upper BuTord Circle, University of Minnesota, St. Paul, MN 55108
Human-caused environmental changes are creating regional com-
binations of environmental conditions that, within the next 50 to
100 years, may fall outside the envelope within which many of the
terrestrial plants of a region evolved. These environmental mod-
ifications might become a greater cause of global species extinc-
tion than direct habitat destruction. The environmental constraints
undergoing human modification include levels of soil nitrogen,
phosphorus, calcium and pH, atmospheric CO2, herbivore, patho-
gen, and predator densities, disturbance regimes, and climate.
Extinction would occur because the physiologies, morphologies,
and life histories of plants limit each species to being a superior
competitor for a particular combination of environmental con-
straints. Changes in these constraints would favor a few species
that would competitively displace many other species from a
region. In the long-term, the "weedy" taxa that became the
dominants of the novel conditions imposed by global change
should become the progenitors of a series of new species that are
progressively less weedy and better adapted to the new condi-
tions. The relative importance of evolutionary versus community
ecology responses to global environmental change would depend
on the extent of regional and local recruitment limitation, and on
whether the suite of human-imposed constraints were novel just
regionally or on continental or global scales.
The earth is undergoing rapid environmental changes because
of human actions (1-6~. Humans have greatly impacted the
rates of supply of the major nutrients that constrain the produc-
tivity, composition, and diversity of terrestrial ecosystems. Spe-
cifically, the natural rates of nitrogen addition and phosphorus
liberation to terrestrial ecosystems (1, 7, 8) have been doubled,
and atmospheric CO2 concentrations have been increased to
about 40% above preindustrial levels (9~. Soil calcium levels are
declining in some ecosystems because of increased rates of
leaching caused by acidic deposition (10~. Humans have relaxed
biogeographic barriers to dispersal by accidentally or deliber-
ately moving exotic species to new biogeographic realms (e.g.,
ref. 11~. Through both active fire suppression and increased use
of fire as a land clearing or management tool, humans have
regionally changed fire frequency (12, 13), which is a major force
structuring communities and ecosystems (14~. Humans now
appropriate more than a third of all terrestrial primary produc-
tion (15), and, in doing so, have simplified or destroyed large
portions of some types of ecosystems, leaving behind fragments
that often lack herbivores or predators that provided important
top-down constraints. Moreover, many human environmental
impacts are projected to be two to three times stronger within 50
years (16~. In total, humans may be imposing combinations of
constraints that already do, or may soon, fall outside the ranges
within which many species evolved.
Here we explore how and whether such changes could result
in the loss of local diversity and accelerated extinction (3), and
thus potentially decrease ecosystem functioning (e.g., refs. 17-
19~. The effects of environmental change on species composi-
tion, diversity, and ecosystem functioning are poorly understood.
www.pnas.org/cgi/doi/ 10.1 073/pnas.O9 1093198
As a tool to explore this issue, we use theories that potentially
can explain multispecies coexistence (20-29~. These models are
based on the interplay of environmental constraints and the
trade-offs organisms face in dealing with these constraints. They
can predict both the persistence of a large number of species
(24-29) and the conditions that could lead to extinctions.
Although mechanisms differ, all solutions to Hutchinson's (20)
paradox of diversity have a similar structure (26, 28, 29~. All
mechanisms assume that two or more factors constrain fitness,
and that intraspecific and interspecific trade-offs constrain each
individual or species to having optimal performance at a par-
ticular value of these constraints. These processes provide a basis
for interpreting the impacts of global human ecosystem domi-
nation on community composition, extinction, and speciation.
The physiology, morphology, and life history of a plant
necessarily constrains it to survival in only a range of environ-
mental conditions. In the classical literature, these conditions
were called its fundamental niche. Each species is, at best, a
superior competitor for a narrower range of conditions, classi-
cally called its realized niche (30-32~. The attributes of sites and
regions thus limit the types of species that can occur in them.
These classical concepts of fundamental and realized niches
underlie recent mechanistic approaches to competition, coex-
istence, and community structure (24, 25, 28, 33-36) and are a
useful way to summarize natural history (e.g., refs. 37-39~.
Moreover, they suggest that human-caused environmental
changes could create "vacant niches" (40) i.e., evolutionarily
novel suites of environmental conditions for which no species
in a region are well adapted. In this paper, we use recent
mechanistic theory to explore the potential impacts of human-
driven environmental change on the composition and diver-
sity of terrestrial plant communities, and on their patterns of
speciation.
Environmental Constraints in Plant Communities
What are the major environmental variables that limit the
abundance of terrestrial and aquatic plants, and which of these
variables are being impacted significantly by human actions? In
essence, plants may be limited by nutrients and other resources,
by pathogens and herbivores, by disturbances, by dispersal
abilities, and by the physical environment, including its climate.
These constraints are elaborated below.
Resource Limitation. Plants require N. P. K, Ca, Mg, S. trace
metals, CO2, water, light, and other resources. Depending on the
habitat and species, any one or several of these may be limiting.
The most commonly limiting resources of terrestrial habitats are
N. P. and water (24, 41-44~. N limitation is common because the
parent materials in which soils form contain almost no N.
This paper was presented at the National Academy of Sciences colloquium, "The Future of
Evoiution," held March ~ 6-20, 2000, atthe Arnold and Mabel Beckman Center in Irvine, CA.
*To whom reprint requests should be acldressed. E-maii: Tiiman~iter.umn.edu.
PNAS 1 May 8, 2001 1 vol. 98 1 no 10 1 5433-5440
OCR for page 46
Rather, the chemically stable form of nitrogen is atmospheric N2,
which is usable only by N-fixing plants via microbial symbionts.
Non-N-fixing plants obtain N as nitrate, ammonium, or organic
N. Some soils are either initially low in other mineral elements,
especially phosphorus and calcium, or become low in these after
millennia of leaching. The Park Grass plots of Rothamsted,
England have joint limitation by N. P. X, and early spring rainfall
(43, 44~. The greatest changes in plant community biomass,
composition, and diversity came from N addition in the grass-
lands of both Rothamsted and Cedar Creek, Minnesota (45-47~.
Water is a limiting factor in many terrestrial habitats, as can be
the atmospheric concentration of CO2. Light may also be
limiting, especially on productive soils in areas with low distur-
bance and low grazing rates.
Recruitment Limitation. All sessile plants have the potential to
have their abundance limited by dispersal (25, 48-51~. This
occurs because dispersal is a neighborhood process, and because
interspecific interactions also occur locally. Such "contact"
processes can cause plants to have spatially patchy distributions
(52), and thus to be missing from suitable habitat because of
recruitment limitation. A one-time addition of seed of plant
species that occurred in a savanna, but were absent from the local
sites, led to an 83~o increase in local plant species diversity and
to a 31% increase in total community plant abundance (53~.
Because the added species occurred nearby, but were absent
locally, their ability to germinate, grow, survive, and reproduce
after a one-time seed addition showed that their abundance was
limited by recruitment. Long-term observations in a Panama-
nian rainforest (51) also demonstrated strong recruitment lim-
itation, as have seed addition experiments in other habitats (54,
55~. Other evidence of dispersal limitation and of the rate of
movement of plant species comes from studies of secondary
succession. For instance, 10 to 15 years are required for Schiza-
chyrium scoparium, a prairie plant that is a strong nitrogen
competitor, to disperse from margins into abandoned fields, and
another 30 years are required for it to attain peak abundance
(46~. This 40-year time delay between creation of a site and
dominance is reduced to 3 years simply by adding seed of little
bluestem. Cornell and Lawton (56) found that local diversity was
limited less by local interspecific interactions than by recruitment
from regional pools. Davis (57, 58) followed the dynamics of
North American forests after glacial recession, and observed
time lags of thousands of years between a region having the
appropriate climate for a tree species and the arrival of that
species. Such time lags could greatly influence responses of plant
communities to human-caused environmental changes (58~.
Habitat fragmentation would lengthen such time delays.
Predators and Pathogens. Plant abundance in both terrestrial and
aquatic ecosystems is also limited by the densities and species
identities of pathogens and herbivores, which in turn can be
limited both by their predators and by dispersal. Thus, top-
down forces can greatly constrain both terrestrial and aquatic
ecosystems.
Disturbance. Physical disturbances also limit terrestrial plant
communities and sessile (benthic) freshwater and marine plant
communities. For many terrestrial ecosystems, fire frequency
has been a major constraint, as have been such physical distur-
bances as wind storms, landslides, mudslides, avalanches, clear-
ings caused by gophers or other fossorial animals, disturbances
caused by hooves, wallows, etc.
Temperature/Climate. The growth rates of terrestrial and aquatic
plants are temperature-dependent, with species (and genotypes)
having optimal growth and competitive ability at particular
temperatures, and thus in particular climates. This is likely the
5434 1 www.pnas.org/cgi/doi/10.1073/pnas.091093198
greatest cause of the geographic separation of species along
continental climatic gradients, such as north-south gradients
and elevational gradients. In addition, the geographic ranges and
abundance of many terrestrial plants are limited by temperature
extremes, especially by tissue damage associated with freezing or
subfreezing temperatures. In addition, within a region, differ-
ences in temperature-dependent growth could cause different
plant species to be specialized on different portions of the
growing season.
Temporal Variation. Plants respond not just to the mean levels of
limiting factors, but also to the extent and patterning of their
temporal variation. Some species may be limited or inhibited by
such temporal variation, whereas other species may have traits
that allow them to exploit such temporal variation (21, 22~. This
means that temporal variation, itself, can function as an addi-
tional limiting factor.
In total, there are a large number of factors and processes
that constrain abundance of plants in both terrestrial and
aquatic habitats. All of these limiting factors have been
implicated as potential determinants of the species composi-
tion and diversity of various plant communities. Various
combinations of two or, at times, three of these limiting factors
have been formally incorporated into theories that are poten-
tially capable of explaining the diversity and composition of
terrestrial and aquatic plant communities. Changes in any of
these constraints could thus change the abundance of species
and genotypes in a habitat.
Anthropogenic Global Change and Plant Constraints
Many of these constraints are undergoing large, rapid changes
because of human actions. Recent human activities have more
than doubled the preindustrial rate of supply of N to terrestrial
ecosystems (7~. Nitrogen had a preindustrial terrestrial cycle that
involved the annual fixation of about 90 to 140 Tg (teragrams)
of N/yr (1, 7), with an additional 10 Tg of N/yr provided by
atmospheric N fixation via lightening. Industrial N fixation for
fertilizer currently totals about 88 Tg/yr. About 20 Tg/yr of ~
is fixed during the combustion of fossil fuels, and about 40 Tg/yr
of N is fixed by legume crops. In addition, land clearing, biomass
burning, and other human activities mobilize and release about
an additional 70 Tg of N/yr. The projected expansion of global
population to about 9 billion people by year 2050 and shifts to
diets higher in animal protein suggest that, by 2050, global food
production will be double its current rate (19~. If so, anthropo-
genic terrestrial N inputs in 2050 would be about three to four
times the preindustrial rate (16, 19~. Much of this N would enter
rivers and be carried to near-shore marine ecosystems. N would
also be deposited atmospherically on nonagricultural terrestrial
ecosystems
Nitrate is readily leached from soil, carrying with it positively
charged ions such as Ca. Atmospheric N deposition may be
depleting Ca and other cations in hardwood forests of the eastern
United States (10~. This depletion of base cations could cause
elements that had not been limiting in a region to become
limiting. Plant species often have distributions constrained by
soil pH and Ca.
Phosphorus is a commonly applied agricultural fertilizer, and
current P application is a doubling of the natural global rate for
terrestrial ecosystems (8~. Projections to year 2050 are that
agricultural P fertilization will more than double. Much of this
P may enter aquatic ecosystems, which can be P-limited.
The accumulation of such greenhouse gases as CO2 and
methane may lead to global climate change, with the greatest
changes, especially warmer winter temperatures, forecast for
temperate and polar ecosystems (e.g., ref. 2~. Because climate
change and its potential impacts on terrestrial ecosystems are
widely studied, we will not review them here. Rather, we merely
Tiiman and Lehman
OCR for page 47
note that rainfall patterns, the frequency and severity of
droughts, and other aspects of climatic mean and variance, which
all constrain plant communities, are also forecast to change. In
addition, CO2 is a plant nutrient, and elevated levels of CO2
represent atmospheric eutrophication with a limiting plant
resource.
Fire frequency is a major variable controlling the species
composition and diversity of forests and grasslands (e.g., ref. 14~.
In the United States, active fire suppression, habitat fragmen-
tation, and other human activities have decreased by 10-fold the
area burned each year, from about 22 x 106 ha/yr in 1930 to
about 1.5 x 106 ha/yr since about 1960 (13~. In contrast, fire
frequency is greatly increasing in other habitats, especially
tropical habitats, where fire is used as a land-clearing or land-
management tool (59~.
Modern transportation and commerce have immensely in-
creased both accidental and deliberate introductions of species
to novel biogeographic realms (11~. About one quarter of the
vascular plant species of California, for instance, are exotics.
Exotic species are the second largest cause of native species of
the United States being listed as endangered (60~. Exotic species
can impact the abundance of native species in a large number of
ways, including via competitive suppression, via changes in
disease incidence or some other trophic interaction, via inducing
changes in the physical habitats, such as in fire frequency, and
changes in nutrient cycles (61, 62~. For instance, the invasion of
the N-fixing Myrica fava into the Hawaiian Islands greatly
increased local N fixation and thence soil N fertility. This
increased soil fertility allowed other exotic species to increase in
abundance once they were freed from N competition with native
plants that where efficient N users (63~.
Human actions have also fragmented habitats via conversion
of native ecosystems to agricultural lands, urban or suburban
lands, roads, power line rights-of-way, etc. Fragmentation is
likely to escalate as population and per capita incomes increase
globally. Habitat destruction can cause immediate extinction of
those species that lived only in areas destroyed, and delayed
extinction of poorly dispersing, perhaps competitively superior,
species of extant ecosystems (64~.
Finally, humans have decreased the geographic ranges and
abundance of top predators, especially large carnivores. De-
creased abundance of predators have had impacts in both
aquatic and terrestrial habitats that have cascaded down the
food chain (e.g., refs. 65 and 66), increasing abundance of some
herbivores, decreasing abundance of their preferred plant
species, and freeing herbivore-resistant species from compet-
itive pressure.
In total, human actions are modifying many environmental
constraints that, in combination with intraspecific and interspe-
cific trade-off, led to the evolution of extant plant species and
thus influenced the composition, diversity, and functioning of
terrestrial and aquatic plant communities. If current trends
continue, within 50 to 100 years the suites of factors constraining
the structure of many plant communities may fall outside the
envelope of values that existed both before the industrial revo-
lution and when many of the plant species evolved.
Ecological Responses to Environmental Change
How would such changes in environmental constraints impact
plant communities? Although there would be a continuum of
responses, it is instructive to consider two ends of this spectrum:
the more immediate, or "ecological" responses, and the more
long-term, or "evolutionary" responses, especially patterns of
speciation. Clearly, both ecological and evolutionary responses
happen simultaneously. We separate them because the evolu-
tionary response in which we are most interested is speciation,
which is much slower than changes in species abundance. Eco-
logical responses would depend on the constraints and trade-offs
Tilman and Lehman
that had structured a given community and on how these had
changed. Let us consider a case in which the composition and
diversity of a plant community are determined by competition
for nitrogen and light (e.g., ref. 28) and by dispersal limitation
(25, 49), and explore the impacts of elevated N deposition. The
qualitative changes that would occur in this plant community in
response to elevated N deposition are the same as those that
would occur in response to changes in any other environmental
constraint.
Concepts and Theory. Assuming similar underlying physiologies,
each plant species can be represented by the proportion of its
biomass that is in either roots (for uptake of nitrogen), stem
(which determines plant height and thus light capture), seed
(which determines dispersal ability), or leaves (light capture
via photosynthesis). For a given spatially homogeneous habi-
tat a site with a uniform soil of a given fertility (measured by
the annual in-site mineralization rate of nitrogen) and for a
given physiology, there would be one pattern of biomass in
root, stem, seed, and leaf that led to maximal competitive
ability (28~. On a low N soil, such as nutrient-maintained
(rather than grazing-maintained) grasslands, the best compet-
itor would have high root biomass, enough leaf biomass to
provide photosynthate to meet the needs of roots, little
biomass in stem (because light is not limiting), and little
biomass in seed or rhizome. It would, in essence, be a short
species that is an excellent N competitor but a poor disperser,
perhaps much like the bunchgrass S. scoparium (little
bluestem) of prairie grasslands on sandy soils in the United
States, which are ecosystems that have historically experienced
frequent burns. Plants with long-lived tissues, such as eracoids,
might fill this role in less frequently burned habitats, because
greater tissue longevity decreases plant N requirements (67~.
Even if soils were spatially homogeneous, theory predicts that
many other plant species could coexist with the best N compet-
itor if they had appropriate trade-off between their competitive
ability for N and their dispersal ability (23, 25, 27~. Although
there is an analytical limit to similarity for this mechanism of
coexistence (25), there is no simple limit to the number of species
that can stably coexist via this metacommunity process. This is
the predominant mechanism of coexistence illustrated in Fig.
L4. It allows numerous species, each represented by a dot, to
coexist with the major axis of differentiation being between root
biomass (i.e., competitive ability for soil N) and seed biomass
(i.e., dispersal ability). This defines the region of trait space in
which species can coexist (28), which has a highly elongated
shape (closed curve in Fig. 1A). This region of multispecies
coexistence spans species with seed biomass from a few percent
(the best competitor for N. which is more than 60% root) to more
than 40% (the poorest competitor, but the best disperser). The
region of coexistence includes species with different stem
biomasses because of assumed spatial heterogeneity in the N
content of soils. On more N rich soils, species with greater stem
biomass are favored over those with more root biomass, because
greater stem biomass allows better access to light. This, though,
is a minor axis of coexistence compared with the seed-root
trade-off for low N habitats.
A comparable pattern occurs for habitats with soils that have
high N content (Fig. 1B). The elongated region of coexistence
shown again represents coexistence mainly via a competition-
colonization trade-off, but in this case the trade-off is between
stem allocation (for light capture during competition for light)
and seed allocation (dispersal ability that depends on the number
and size of seed). Soils of intermediate fertility would favor
species intermediate between the extremes shown in Fig. 1 A
and B.
About a third of the globe has sandy soils with low N content.
What would happen if a region with such soils were to receive
PEAS I May 8, 2001 1 volt. 98 1 no. 10 1 5435
OCR for page 48
70
60
50
a'
40
30
2C
10
~ A. Species on
_ \ Low N Soils
~ L~
O ~ a:: .~
0 1 0 20 30 40 50 60 70
Percent Root
70
60
50
40
cot
30
20
. ~
10
Ot
70
6C
5C
4C
3C
2C
IC
C
~ B. Species on
- \ High N Soils
W~°o~
~ .~ ~ \
\ · \
\1 1 \~1
10 20 30
Percent Root
1\1 ~
46) 50 60 70
~ C. Empty Niches
- \ Because of High
\ Rates of N Deposition
0 1 0 20 30 40 50 60 70
Percent Root
Fig. 1. (A) Plant species can be represented by the proportion of biomass in
leaves, roots, stems, and seeds (28). In low nutrient habitats, superior com-
petitors have high biomass in root, low biomass in stem and seed, and
moderate biomass in leaves. Such superior competitors stably coexist with
species that are progressively poorer competitors, but better dispersers (25).
(B) In a fertile habitat, plant height and thus stem biomass is a determinant of
competitive ability for light. (C) A nutrient-poor region, experiencing high
rates of nutrient deposition. The region of coexistence includes only a few of
the species origina I Iy present in the nutrient-poor region. These species would
be competitively dominant and displace all of the other species, but be subject
to invasion by species in the vacant region enclosed by the solid curve. Because
Percent Root + Percent Stem + Percent Seed + Percent Leaf = 100%, Percent
Leaf is about 30% for all cases shown.
5436 1 www.pnas.org/cgi/doi/ 10.1 073/pnas.091093 198
projected increased rates of atmospheric N deposition? If all
possible species were present throughout the region (i.e., if the
whole triangular trait space of Fig. 1 were reasonably well
covered with species), there would be a transition, as N accu-
mulated, from a suite of species like those of Fig. 1A to a suite
like that of Fig. 1B. However, given that the region receiving
elevated N inputs started with low-N soil, the species of Fig. 1B,
which occur on N-rich soils, would not be present. Rather, the
responses observed would come from those species that hap-
pened to be present in the region those shown in Fig. 1A.
The long-term response of this low-N habitat to greatly
elevated N deposition should be dominance by superior light
competitors, which have greater stem biomass. However, only
two of the original species of the originally low N region would
fall within the new trait space favored by N addition (Fig. 1C).
These are both weedy species i.e., species with high seed
biomass compared with those that would be expected to be the
competitive dominants of the elevated-N habitat. These species
are favored initially because, of all of the species present in the
original low-N habitat, they have relatively high stem biomass.
Under conditions of elevated N. these two species would be
expected to increase greatly in abundance where present and to
rapidly spread to suitable sites because of their high seed
biomass. Some of the other original species of the low-N
community might coexist with them, if these additional species
had the appropriate trade-off between their competitive ability
for light and their dispersal ability. However, most species would
be competitively displaced. Thus, a striking feature of Fig. 1C is
that the vast majority of the species of the originally species-rich
flora of this originally low N region would be competitively
displaced by the new dominants. Thus, greatly elevated N
deposition should lead to great local extinction.
A second striking feature is the extent to which there are
"vacant niches" caused by environmental change—i.e., there are
almost no species present in the regional flora that have traits
that would normally be favored in such habitats. This is shown
by the large empty area within the solid closed curve of Fig. 1C.
Any species with traits that fell in this empty area should be able
to invade into the region. In total, because of N deposition, the
majority of the species that had been the dominants of a region
when it was a low N habitat would be competitively displaced by
a few formerly rare species, creating an ecosystem highly sus-
ceptible to invasion and species turnover until a community like
that of Fig. 1B had developed.
Results Qf Experimental N Additions. Just such changes in plant
diversity and composition are seen when one or a few such
factors have been experimentally manipulated for extended
periods of time. For instance, fertilization of the Park Grass plots
with 4.8 g.m-2 of N. as ammonium sulfate, led to dominance by
the grassAgrost~s (84% of community biomass compared with an
average abundance in unfertilized control plots of 12%) and to
the loss of 14 of the 19 plant species found, on average, in
unfertilized control plots (44, 68~. The addition of 14.4 g m-2 of
N as ammonium sulfate together with P. K, Mg, and other
nutrients led to extreme dominance by Holcus lanatus (Yorkshire
fog, a grass), which had an average abundance of 96% in the two
replicate high-N plots, compared with an average abundance in
the three unfertilized and unlimed control plots of 2%. Both of
the high-N plots contained only two plant species, whereas the
controls averaged 19 plant species. Experimental N addition in
a set of 207 grassland plots in Minnesota showed similarly strong
loss of grassland species diversity and similar shifts in species
composition at high rates of N addition (28, 69~. Moreover,
similar shifts in plant community diversity and composition have
been reported for ecosystems experiencing high rates of atmo-
spheric N deposition because of nearby intensive agriculture (70,
71~. For instance, the heathlands of The Netherlands are an
Tiiman and ~ehman
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Representative terms from entire chapter:
plant communities
Constraint 2
A ~ Human-caused
A/ Environ ~
recondition\ ~ Environmental
\
species niche is smaller than the entire niche space, then using
formulae for the volumes of e-dimensional hyperspheres and
hypercubes, that chance can be shown to be always less than
2 -n~z/2/(n/2~!,
where n is the number of environmental conditions changed, and
where the factorial is computed via the gamma function when n
is odd. Under these assumptions, if two environmental condi-
tions were changed (n = 2), at most about 80% of the species on
average would survive, but if eight conditions were changed
randomly at once, at most about 1% of the species on average
would survive. This multiplicative effect of changes in limiting
factors means that several small changes can have as great an
impact as one larger change, and that various combinations of
small and large environmental changes can, in combination, have
an immense impact. Thus, the ecological impacts of human-
caused environmental change should depend on the dimension-
ality of the suite of factors that constrain species abundance, and,
in a multiplicative manner, on the magnitudes of changes in all
these factors.
In the short-term, such shifts in environmental constraints
would eliminate many species and favor once-rare species. The
longer-term dynamics of these terrestrial plant communities
would depend on the dispersal rates of species both within a
region and from other regions, if any, that formerly had char-
acteristics similar to those that occur in the human-impacted
region. They also would depend on the evolutionary responses
of the species that remain in these habitats.
Evolutionary Responses to Global Change
What might the long-term outcome be of evolution under novel
environmental conditions? For one possibility, let us consider
again, but on an evolutionary time scale, the effects on a low-N
terrestrial plant community of a large increase in the regional
rate of N deposition. This could cause light and dispersal ability
to become major limiting factors, as illustrated in Fig. 1C. As
already discussed, the immediate effect of a high rate of N
deposition would be dominance by a few formerly rare, fast-
growing, rapidly dispersing plant species. These species would
rapidly spread and overtop low-N-adapted species and thus
out-compete them for light. However, a large portion of the
viable trait space of this community would be empty, as in Fig.
1C. Assuming that N deposition is occurring on a geographically
large region, or that habitat fragmentation or other dispersal
barriers prevent colonization by suitable superior light compet-
itors, or that the region has experienced other environmental
changes (e.g., Ca leaching, soil acidification, invasion by patho-
gens) that make it inhospitable for otherwise suitable superior
light competitors, its longer-term dynamics would be driven as
much, or more, by internal evolutionary processes than by
colonization.
The evolutionary dynamics of such systems have been ex-
plored for situations in which it is assumed that there is a strict
trade-off between competitive ability and dispersal ability (36,
73, 74~. Let us ask what might happen to a weedy plant species
that was the initial dominant of a formerly N-poor habitat that
experienced elevated N deposition, as shown in Fig. 1C. Nu-
merical solutions to a partial differential equation model (36)
show that, within the initially dominant weedy species (species 1
of Fig. 3A), those individuals that are better light competitors
have greater fitness than those that are better dispersers. This
causes the weedy species to evolve into a progressively better
light competitor (acquiring such traits as a larger proportion of
biomass in stem, greater height, and larger seed), but to produce
fewer seeds and/or allocate less to vegetative spread. Thus,
species 1 evolves to the right in Fig. 3A. As species 1 evolves into
a better local competitor (and thus a poorer disperser), it
5438 1 www.pnas.org/cgi/doi/10.1073/pnas.091093198
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`' 0.30
{
Q
0.20
0.10
0.00
0.10
0.05
0.00
A
time = 0
I ~
time = 50,000
time = 475.000
18
, ~~\
I ' 1 ' 1
0 20 40 60 80 100 120 140 160 180
T I T I T r
1
Low c Competitive Ability - ~ High
High C Dispersal Ability Low
Fig. 3. Numerical solutions of evolutionary change in a weedy species
growing in a spatially implicit habitat in which fitness is limited both by
dispersal ability and by competitive ability, based on a model of phenotypic
diffusion (36). (A) Given this trade-off, an initially weedy species, species 1,
undergoes evolutionary change, with its peak shown moving to the right. (B)
After 50,000 years, species 1 has evolved into a much better competitor, but
a much poorerdisperserthan itoriginallywas, and a newspecies, species2, has
appeared. Species 2 is a superior disperser, but an inferior competitor. It
survives in vacant sites in this spatial habitat. (C) Species 1 and 2 each evolve
toward being superior competitors. After some time a third species appears
that is a poor competitor, but excellent disperser. This third species evolves
into a superior competitor and a fourth species appears, etc. Shown here isthe
result after 475,000 years, at which time 21 peaks of abundance appear, each
peak representing a different phenotype, thus corresponding with different
species.
occupies fewer sites in the spatial habitat. After this has pro-
gressed sufficiently far, an interesting phenomenon occurs.
Individuals at the far end of the range of phenotypes, which are
good dispersers but poor light competitors, are also favored
(species 2 of Fig. 3B). These individuals are poor light compet-
itors, and thus do not competitively inhibit species 1. However,
they are good dispersers, which allows them to live in the sites
not occupied by species 1.
oilman and Lehman
In essence, there is a bimodal selective pressure created by
competition in a spatial habitat and by an analytical limit to
similarity for coexistence of organisms with traits at different
points on the trade-off curve (36~. This leads to two peaks on the
trade-off curve, each peak corresponding to an incipient species
(Fig. 3B). Such peaks appear even when all phenotypes are
initially rare, and result from the interplay of selection, muta-
tion/recombination, and the competitive limit to similarity.
Within each of these peaks, those individuals that are superior
light competitors but inferior dispersers are favored, causing the
peaks to move to the right in Fig. 3B. Once the second peak,
incipient species 2, moves sufficiently far to the right, a third peak
appears. It also evolves toward the right, and a fourth peak
appears, etc. In numerical solutions of the underlying reaction-
diffusion model, after a 475,000 year period, a single weedy
species had speciated into 21 species (Fig. 3C) that spanned the
empty niche space of Fig. 1C. Such speciation processes would
occur within each of the original weedy species, and eventually
would yield a local flora as species-rich as occurred before N
deposition.
In total, this process suggests that the imposition of novel
environmental constraints would lead to the eventual diversifi-
cation of the flora of a region, with the new flora filling in the
empty niches created by novel human-caused environmental
conditions. The process by which this is predicted to occur is one
in which the ancestral progenitors of this new flora are small,
fast-growing, weedy species. Interestingly, this is just what has
been suggested to have occurred during the evolution of the
angiosperms, during diversification in corals, and during the
diversification of terrestrial mammals.
Conclusions
Anthropogenic changes in environmental limiting factors are
likely to cause significant loss of plant diversity, leaving many
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