Download 2002: the year of the `diversity–ecosystem function`

Research Update
2 Rees, M. et al. (2002) Snow tussocks, chaos and the
evolution of mast seeding. Am. Nat. 160, 44–59
3 Kelly, D. (1994) The evolutionary ecology of mast
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33 years by Dacrydium cupressinum Lamb. (rimu)
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benefits of mast seeding. Ecology 82, 117–126
8 Satake, A. and Iwasa, Y. (2000) Pollen coupling of
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weed density does not affect the long-term frequency
of weed control Crop Protection 16, 273-278
Rob P. Freckleton
Dept of Zoology, University of Oxford,
South Parks Road, Oxford, UK OX1 3PS.
e-mail: [email protected]
2002: the year of the ‘diversity–ecosystem function’
debate
Tom Cameron
The main conclusion of a detailed review on
biodiversity and ecosystem functioning in
2001 was to recommend continuing
research into general patterns between
species loss and ecosystem processes, in
relation to habitat management. Four key
papers have emerged from an explosion of
recent experimental tests addressing this
question. Studies by Pfisterer and Schmid,
and by Schaffers provide empirical evidence
for the lack of a unique relationship
between increased ecosystem diversity and
ecosystem function or stability. Paine and,
in a separate report, Willms et al. describe
the problems associated with ignoring
trophic links in the experimental testing of
the ‘diversity – production’ debate. These
findings highlight the importance of
rigorous testing of general ecological
theory before recommending it for use by
habitat or population managers.
Published online: 03 September 2002
There has never been so much attention
focused on the implications of biodiversity
loss for ecosystem functioning or primary
productivity. Two pairs of reports
published this year highlight the
importance of careful testing of ecological
paradigms and the thorough evaluation of
the focus of biodiversity research [1–4].
In a two-part report, Schaffers [1]
investigates the roles of soil, biomass and
management as factors that control plant
species diversity. The author identified
interesting hump-shaped species-richness
relationships with biomass and productivity;
a normal distribution, as identified in
Mediterranean-type ecosystems [5].
However, he points out that these
relationships fail if soil or management
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variables are considered. What Schaffers is
saying is that the biomass or productivity –
diversity relationships that are found in
plant systems can arise from covariation
of biomass and /or productivity with other
abiotic or management factors, which are
typically ignored in such studies. More
importantly he found that management
practices, such as mowing and hay removal
(i.e. grazing) promoted a positive relationship
with species evenness and the number of
rare or endangered species present. This
suggests that vulnerable species might be
threatened by competitive exclusion in
increased biomass environments, and that
a tradeoff between predation and
competition exists that promotes the
coexistence of rare and common species
and not simply a productivity relationship.
Complementing these findings, Pfisterer
and Schmid [2] report on an experimental
grassland system where increased
biodiversity increases biomass production.
However, they stress the importance of a
link between such systems and an inverse
relationship between the diversity and the
stability of ecosystem functioning [6]. The
authors found that biomass production in
species-poor ecosystems was reduced less
following perturbation, and returned to
pre-perturbation levels faster, than did
species-rich systems. Artificially increasing
spatial heterogeneity to accommodate
increased diversity and production might
therefore prove catastrophic if these
habitats contain rare or endangered
populations. This is in direct contrast to the
conclusions of a recent review by Loreau
et al. [7]. Instead, the indirect interactions
discovered by Pfisterer and Schmid could
promote increased extinctions in
increasingly variable environments.
These two reports [1,2] have questioned
the use of previous experimental results to
look for generalities in the ‘diversity begets
productivity’ debate because confounding
variables, such as soil microbe diversity,
soil quality and ecosystem resistance,
have been ignored [7,8]. Such problems
could be overcome with more complete
experimental null hypotheses.
Where have all the rare species gone?
Part of the burning interest in biodiversity
and ecosystem function relationships is
born from the concern over declining
species numbers in recent years and the
consequences of this decline to ecosystem
services and life on Earth [9]. This
interest has generated a large portfolio of
studies that describe the importance of
ecosystem function for diversity, and
vice versa [7,10,11]. But is it ecosystem
function that is important? Has this
generalization been experimentally tested
and, if so, how rigorously?
Recently, Paine [3] tested the hypothesis
that trophic links cloud the relationship
between species richness and ecosystem
output as measured by production (see also
[10,11]). Collecting data over seven years in
the low intertidal zone at Tatoosh Island,
WA, USA, he found that experimental plots
with restricted grazer access had higher
species richness and primary production.
However, this increase in production was
accounted for by the enemy release of only
one or two highly productive algal species.
Moreover, the increased richness (presence
of rare species) was accounted for by the
lack of grazers rather than the increased
production. These key results clearly
demonstrate a weakness in the ‘diversity
begets production’ debate. Paine’s report is
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496
Research Update
further evidence, and perhaps the most
convincing to date, that species-specific
properties in real systems (i.e. with trophic
links included) can prevent general
patterns emerging across ecosystems.
Complementing this is the finding by a
team from Canada [4] who, using a 70-year
grazer exclusion experiment in a mixed
prairie plant community, found a neutral
or positive effect on primary production of
disturbance exclusion (i.e. fire, grazing, etc.).
However, this was due to increased cover of
the most highly productive species, which
brought about a reduction in species
diversity. Willms et al. [4] conclude that the
potential effect that reduced diversity
might have on reducing production
stability is more than compensated for by
increased litter mass. This is similar to the
findings of Schaffers [1].
Both Paine [3] and Willms et al. [4] have
demonstrated the difficulty in finding a
general ecological relationship between
ecosystem function and diversity because of
species-specific effects and important
trophic links. Similar conclusions have been
drawn from laboratory experiments [12].
…but how do I get it?
I know what I want…
It might be reasonable to assume that
increased production will lead to increased
coexistence of species because competition
is relaxed (in the absence of adverse
indirect trophic links, which is unlikely).
It is also reasonable to accept that an
increased number of species could increase
ecosystem functioning through a more
complete utilization of resources [7] and
facilitation. However, it is not reasonable
for habitat managers to assume that
increasing the productivity of their
habitats would secure persistence or
increase diversity. The same caveat applies
to the advice that managing for increased
species richness, or preventing their loss,
will help increase or maintain productivity.
TRENDS in Ecology & Evolution Vol.17 No.11 November 2002
For the most part, these hypotheses have
not been accurately tested (but see [13]).
These four papers highlight
fundamental questions, particularly
regarding the conservation of biodiversity
and the methods recommended to habitat
managers and politicians. Can increased
production and its relationship with
ecosystem stability be exploited to promote
species conservation, more appropriate
habitat management strategies, and
sustainable biodiversity? I would suggest
not, because of the species-specific effects
and the complexities of indirect
interactions between biomass,
decomposition, competition and predation
illustrated by these studies. This is
especially the case in disturbed ecosystems,
which contain introduced species that can
monopolize ecosystems when released from
natural enemies and competition for
resources [14,15]. So, we know we want
conservation of vulnerable species in
ecosystems, but how do we achieve this?
Does increasing diversity or biomass
promote population viability or longevity
for rare species in ecosystems?
Given the universally accepted
importance of the ‘diversity–ecosystem
function’ debate, I believe two questions
now need to be addressed. First, is there
an unequivocal link between increased
ecosystem function and the protection of
rare or declining populations? Second,
does stable ecosystem function encourage
persistence of stable populations, rather
than the dominance of single highly
productive or invasive species? These are
both urgent issues in conservation ecology.
The four reports I have discussed [1–4]
stress that we need to change our thinking
towards such experimentation, because
too many applied projects and
conservation management plans are
based on untested theory.
Acknowledgements
I thank Steve Sait, Helen Wearing,
Pejman Rohani and two referees for
assistance with this article.
References
1 Schaffers, A.P. (2002) Soil, biomass, and management
of semi-natural vegetation. Part II. Factors
controlling species diversity. Plant Ecol. 158, 247–268
2 Pfisterer, A.B. and Schmid, B. (2002) Diversitydependent production can decrease the stability of
ecosystem functioning. Nature 416, 84–86
3 Paine, R.T. (2002) Trophic control of production in a
rocky intertidal community. Science 296, 736–739
4 Willms, W.D. et al. (2002) Response of the mixed
prairie to protection from grazing. J. Range
Manage. 55, 210–216
5 Bond, W. (1983) On Alpha diversity and the richness
of the cape flora: A study in southern Caper Fynbos.
In Mediterranean-type Ecosystems (Kruger, F.J.
et al., eds), pp. 337–356, Springer-Verlag
6 Sankaran, M. and McNaughton, S.J. (1999)
Determinants of biodiversity regulate compositional
stability of communities. Nature 401, 691–693
7 Loreau, M. et al. (2001) Biodiversity and
ecosystem functioning: current knowledge and
future challenges. Science 294, 804–808
8 Cavigelli, M.A. and Robertson, G.P. (2000) The
functional significance of denitrifier community
composition in a terrestrial ecosystem. Ecology
81, 1402–1414
9 Woodruff, D.S. (2001) Declines of biomes and
biotas and the future of evolution. Proc. Natl.
Acad. Sci. U. S. A. 98, 5471–5476
10 Tilman, D. (1999) Diversity and production in
European grasslands. Science 286, 1099–1100
11 Hector, A. et al. (1999) Plant diversity and
productivity experiments in European
grasslands. Science 286, 1123–1127
12 Bjornstad, O.N. et al. (2001) The impact of
specialized enemies on the dimensionality of host
dynamics. Nature 409, 1001–1006
13 Schmid, B. (2002) The species richness–productivity
controversy. Trends Ecol. Evol. 17, 113–114
14 Crawley, M.J. et al. (1999) Invasion-resistance in
experimental grassland communities: species
richness or species identity? Ecol. Lett. 2, 140–148
15 Byers, J.E. et al. (2002) Directing research to
reduce the impacts of nonindigenous species
Conserv. Biol. 16, 630–640
Tom Cameron
Centre for Biodiversity & Conservation,
School of Biology, University of Leeds, Leeds,
UK LS2 9JT.
e-mail: [email protected]
Meeting Report
Metersticks and microarrays
Jessica Ruvinsky
The annual meeting of The American
Society of Naturalists was held in Banff,
Canada, from 11 to 14 July 2002.
Published online: 11 September 2002
http://tree.trends.com
The American Society of Naturalists
(ASN) met independently of the
Society for the Study of Evolution for
the first time since 1987 this summer
in Banff, Canada. Among the
mountains, the posters and the few
concurrent sessions, members
discussed the role and direction of the
Society. Is the future in genomics or
in natural history?
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