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A downside of diversity?

DOI:10.1016/j.tree.2015.02.005
Authors:
James T. Stroud at Washington University in St. Louis
James T. Stroud
Kenneth J. Feeley at University of Miami
Kenneth J. Feeley
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Abstract

Gallagher et al. [1] propose that the niche breadth of a species is a potentially reliable predictor of extinction vulnerability. Species with narrow niches (specialists) generally have lower ecological resistances (i.e., are more sensitive to environmental disturbances) than similar species with broader niches (generalists). Gallagher et al. demonstrate this relationship between niche breadth and extinction vulnerability by highlighting the elevated extinction probabilities for specialist versus generalist species across a broad range of taxonomic groups.
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A
downside
of
diversity?
A
response
to
Gallagher
et
al.
James
T.
Stroud
1,2*
and
Kenneth
J.
Feeley
1,2
1
Department
of
Biological
Sciences,
Florida
International
University,
Miami,
FL
33199,
USA
2
Fairchild
Tropical
Botanical
Gardens,
Coral
Gables,
FL
33146,
USA
Niche
theory
as
a
predictor
of
extinction
probability
Gallagher
et
al.
[1]
propose
that
the
niche
breadth
of
a
species
is
a
potentially
reliable
predictor
of
extinction
vulnerability.
Species
with
narrow
niches
(specialists)
gen-
erally
have
lower
ecological
resistances
(i.e.,
are
more
sensitive
to
environmental
disturbances)
than
similar
spe-
cies
with
broader
niches
(generalists).
Gallagher
et
al.
demonstrate
this
relationship
between
niche
breadth
and
extinction
vulnerability
by
highlighting
the
elevated
extinction
probabilities
for
specialist
versus
generalist
species
across
a
broad
range
of
taxonomic
groups.
We
suggest
that
the
incorporation
of
‘niche
packing’
theory
allows
us
to
predict
which
communities
should
have
con-
stituent
species
at
greatest
risk
of
extinction.
Specialists
can
be
highly
prone
to
local
extinction
fol-
lowing
habitat
loss
because
the
resource(s)
required
by
the
species
are
often
missing
from
any
remaining
habitats
[2].
In
the
case
of
climate
change,
changes
in
temperature
and
precipitation
regimes
can
quickly
shift
habitat
condi-
tions
beyond
the
narrow
requirements
of
specialized
spe-
cies
[3].
Therefore,
in
the
absence
of
rapid
adaptation
and
‘evolutionary
rescue’,
relatively
minor
changes
in
climate
can
force
specialized
species
to
shift
their
geographic
dis-
tributions
[4].
Even
with
migrations,
specialized
species
will
be
at
an
inherent
disadvantage
because
areas
that
offer
both
suitable
climates
and
the
required
environmen-
tal
conditions
will
be
relatively
sparse.
By
contrast,
gen-
eralists
will
be
better
able
to
tolerate
environmental
changes
because
these
species
are
by
definition
capable
of
persisting
across
a
wider
range
of
conditions.
Considering
the
strong
connection
between
the
degree
of
specialization
of
a
species
and
its
sensitivity
to
distur-
bance,
we
argue
that
extinction
probability
must
then
be
predicted
to
be
highest
in
those
areas
that
support
the
largest
numbers
or
proportions
of
narrow-niched
species.
Theory
provides
us
with
one
tool
for
predicting
where
these
specialist
species
are
most
likely
to
occur.
Specifi-
cally,
the
theory
of
‘niche
packing’
states
that,
because
of
heightened
interspecific
competition,
the
species
that
occur
in
biologically-diverse
communities
will
tend
to
have
narrower
niches
(i.e.,
will
be
more
specialized)
than
will
similar
species
in
less-diverse
communities
[5].
Taken
together,
these
two
theories
–
the
increased
sensitivity
of
specialized
species
and
greater
niche-packing
in
more
diverse
communities
–
dictate
that
the
intrinsic
extinction
vulnerabilities
of
species
should
generally
increase
with
diversity.
In
other
words,
we
hypothesize
that
there
is
likely
to
be
a
‘downside
to
diversity’,
such
that
the
species
comprising
more-diverse
communities
are
in-
herently
at
greater
risk
of
extinction
than
are
species
of
depauperate
communities.
The
downside
of
diversity:
a
tropical
problem?
The
most
biodiverse
communities
in
the
world
are
located
in
the
tropics
[6].
Tropical
species
are
widely
believed
to
be
more
sensitive
to
climate
change
than
their
temperate
counterparts
because
of
(i)
the
absence
of
a
marked
latitu-
dinal
gradient
of
temperature
within
the
tropics,
which
results
in
greater
distances
between
current
and
future
climate
analogs,
and
hence
faster
climate-change
veloci-
ties,
necessitating
faster
rates
of
species
migration
[7,8];
(ii)
rapid
rates
of
habitat
loss
which
decrease
habitat
availability
and
increase
the
distances
that
species
will
be
required
to
migrate
to
keep
pace
with
changing
climates
[9];
and
(iii)
the
prevalence
of
species
with
narrow
climatic
niches
due
to
the
short-
and
long-term
climatic
stability
of
tropical
environments
[10].
As
discussed
above,
the
diverse
communities
of
the
tropics
will
also
generally
exhibit
intense
interspecific
competition
and
niche
packing.
There-
fore,
tropical
species
can
be
predicted
to
have
narrower
niches,
even
regarding
non-climatic
factors
such
as
diet
preference
and
habitat
use,
than
their
temperate
counter-
parts
[11].
According
to
our
proposed
‘downside
of
diversity’
hypothesis,
extinction
probabilities
may
therefore
be
even
higher
in
the
biologically-diverse
communities
of
the
tro-
pics
than
was
previously
anticipated.
With
the
massive
number
of
extinctions
that
are
fore-
cast
as
we
enter
the
‘Anthropocene’
[12],
it
is
crucial
that
we
identify
the
systems
and
communities
under
greatest
risk
of
species
loss
–
we
cannot
afford
to
wait
to
construct
models
post
hoc
based
on
observed
extinctions.
Combining
the
theories
synthesized
by
Gallagher
et
al.
with
the
classic
theory
of
niche
packing,
we
can
predict
that
highly-speciose
communities
and
their
constituent
species
are
at
high
risk
of
extinction
from
environmental
disturbances
such
as
climate
change
and
habitat
loss.
Given
this
potential
downside
to
diversity,
we
argue
that
there
is
additional
motivation
to
prioritize
the
conservation
of
high-diversity
communities
in
the
tropics.
References
1
Gallagher,
A.J.
et
al.
(2015)
Evolutionary
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Diamond,
J.M.
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(1987)
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Schwartz,
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0169-5347/
ß
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Elsevier
Ltd.
All
rights
reserved.
http://dx.doi.org/10.1016/j.tree.2015.02.005
Corresponding
author:
Feeley,
K.J.
(kjfeeley@gmail.com).
Keywords:
conservation;
specialization;
ecology;
extinction;
biodiversity;
niche.
*
Twitter:
@jtstroud
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2
... In their response to our recent article on using evolutionary theory to predict extinction risk [1], Stroud and Feeley [2] suggest that extinction probabilities are highest in regions where there is a higher density of narrow-niched species. More specifically, Stroud and Feeley [2] suggest that incorporating theory of 'niche-packing' in our framework [1] might also be useful for predicting where extinctions may occur, due to the fact that competition between species will result in higher degrees of specialization. ...
... In their response to our recent article on using evolutionary theory to predict extinction risk [1], Stroud and Feeley [2] suggest that extinction probabilities are highest in regions where there is a higher density of narrow-niched species. More specifically, Stroud and Feeley [2] suggest that incorporating theory of 'niche-packing' in our framework [1] might also be useful for predicting where extinctions may occur, due to the fact that competition between species will result in higher degrees of specialization. We commend Stroud and Feely [2] for highlighting these issues, but the framework we presented in [1] already integrated the theory of niche packing as it relates to extinction risk, although the term 'niche packing' was not explicitly used. ...
... More specifically, Stroud and Feeley [2] suggest that incorporating theory of 'niche-packing' in our framework [1] might also be useful for predicting where extinctions may occur, due to the fact that competition between species will result in higher degrees of specialization. We commend Stroud and Feely [2] for highlighting these issues, but the framework we presented in [1] already integrated the theory of niche packing as it relates to extinction risk, although the term 'niche packing' was not explicitly used. In fact, in our framework, we included geographic range and population density, the two main points of Stroud and Feeley [2], as two of several parameters for estimating resilience. ...
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The Future of Tropical Species on a Warmer Planet
Article
Modern global temperature and land cover and projected future temperatures suggest that tropical forest species will be particularly sensitive to global warming. Given a moderate greenhouse gas emissions scenario, fully 75% of the tropical forests present in 2000 will experience mean annual temperatures in 2100 that are greater than the highest mean annual temperature that supports closed-canopy forest today. Temperature-sensitive species might extend their ranges to cool refuges, defined here as areas where temperatures projected for 2100 match 1960s temperatures in the modern range. Distances to such cool refuges are greatest for equatorial species and are particularly large for key tropical forest areas including the Amazon and Congo River Basins, West Africa, and the upper elevations of many tropical mountains. In sum, tropical species are likely to be particularly sensitive to global warming because they are adapted to limited geographic and seasonal variation in temperature, already lived at or near the highest temperatures on Earth before global warming began, and are often isolated from cool refuges. To illustrate these three points, we examined the distributions and habitat associations of all extant mammal species. The distance to the nearest cool refuge exceeded 1000 km for more than 20% of the tropical and less than 4% of the extratropical species with small ranges. The biological impact of global warming is likely to be as severe in the tropics as at temperate and boreal latitudes.
The velocity of climate change
Article
The ranges of plants and animals are moving in response to recent changes in climate. As temperatures rise, ecosystems with 'nowhere to go', such as mountains, are considered to be more threatened. However, species survival may depend as much on keeping pace with moving climates as the climate's ultimate persistence. Here we present a new index of the velocity of temperature change (km yr(-1)), derived from spatial gradients ( degrees C km(-1)) and multimodel ensemble forecasts of rates of temperature increase ( degrees C yr(-1)) in the twenty-first century. This index represents the instantaneous local velocity along Earth's surface needed to maintain constant temperatures, and has a global mean of 0.42 km yr(-1) (A1B emission scenario). Owing to topographic effects, the velocity of temperature change is lowest in mountainous biomes such as tropical and subtropical coniferous forests (0.08 km yr(-1)), temperate coniferous forest, and montane grasslands. Velocities are highest in flooded grasslands (1.26 km yr(-1)), mangroves and deserts. High velocities suggest that the climates of only 8% of global protected areas have residence times exceeding 100 years. Small protected areas exacerbate the problem in Mediterranean-type and temperate coniferous forest biomes. Large protected areas may mitigate the problem in desert biomes. These results indicate management strategies for minimizing biodiversity loss from climate change. Montane landscapes may effectively shelter many species into the next century. Elsewhere, reduced emissions, a much expanded network of protected areas, or efforts to increase species movement may be necessary.
Evolutionary rescue can prevent extinction following environmental change
Article
The ubiquity of global change and its impacts on biodiversity poses a clear and urgent challenge for evolutionary biologists. In many cases, environmental change is so widespread and rapid that individuals can neither accommodate to them physiologically nor migrate to a more favourable site. Extinction will ensue unless the population adapts fast enough to counter the rate of decline. According to theory, whether populations can be rescued by evolution depends upon several crucial variables: population size, the supply of genetic variation, and the degree of maladaptation to the new environment. Using techniques in experimental evolution we tested the conditions for evolutionary rescue (ER). Hundreds of yeast populations were exposed to normally lethal concentrations of salt in conditions, where the frequency of rescue mutations was estimated and population size was manipulated. In a striking match with theory, we show that ER is possible, and that the recovery of the population may occur within 25 generations. We observed a clear threshold in population size for ER whereby the ancestral population size must be sufficiently large to counter stochastic extinction and contain resistant individuals. These results demonstrate that rapid evolution is an important component of the response of small populations to environmental change.
Niche packing and coevolution in competition communities. Proc Natl Acad Sci USA 78: 5021-5025
Article
  • Sep 1981
A coevolutionary model of species packing is developed that allows evolutionary adjustment in both niche position and within-phenotype niche width of one-three competing species. The environment is specified as a single resource dimension x and availability of resources along x is given by a gaussian curve that has parameters x[unk](R) and sigma(R). The model predicts that, for S species, the ratio of optimal niche width w[unk] to sigma(R) is roughly independent of sigma(R) and can be approximated by 1/S when the competitors are completely resource limited. Niche separation (d[unk]/w[unk]) increases only moderately with increases in resource diversity sigma(R) and is greater for two than for three competing species. To the extent that the competitors are not completely resource limited, both coevolutionary niche separation and niche width decrease. Many of the general trends in niche width and niche separation predicted by this coevolutionary model parallel those from optimal foraging theory and limiting similarity models of community structure. The coevolutionary model stands out, however, in the singularly high values predicted for niche separation, making coevolved communities highly invasible. Hence, the theory suggests, as some empirical evidence indicates, that coevolved competition communities can only eixst as such on remote islands or in other habitats that might be free from invasion by outside species.