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126 South African Journal of Science 100, January/February 2004 Working for Water
Recent developments in the science and
management of invasive alien plants:
connecting the dots of research
knowledge, and linking disciplinary boxes
D.M. Richardsona*, V.C. Moranb,D.C. Le Maitrec, M. Rougetd
and L.C. Foxcrofte
OVER 700 DELEGATES FROM 21 COUNTRIES
converged on Fort Lauderdale,
Florida, U.S.A., in November 2003
for an historic four-day summit on
‘Invasive plants in natural and managed
systems: Linking science and manage-
ment’. Participants were mainly research-
ers working in weed science and plant
invasion ecology, and managers and
policy-makers with wide-ranging inter-
ests in alien species and biological inva-
sions. The meeting, jointly sponsored by
the Ecological Society of America and the
Weed Science Society of America, was
held in conjunction with the 7th Interna-
tional Conference on the Ecology and
Management of Alien Plant Invasions.
Catalysts for the joint gathering were the
discovery that the two societies were
planning separate meetings in the same
area in the same year, and the realization
that escalating problems with invasive
alien species demand better integration
between weed scientists (working mainly
in agricultural settings) and ecologists
focusing on invasions in natural systems.
The conference was unique in bringing
together so many of the world’s top
researchers engaged in all aspects of alien
plant invasions. We provide a commen-
tary on this conference because:
•of its direct relevance to the subject
matter covered in this review issue of
the South African Journal of Science,
namely, the role of the Working for
Water programme in the management
of invasive alien plants in South Africa;
•in many respects, it represents cutting-
edge thinking in the ecology and
management of invasive plants; and
•it provides some measure of how South
African scientists and managers are
shaping up in this highly competitive
and burgeoning field.
In this report, we record our subjective
impressions of the conference. Given the
size of the meeting (more than 250 oral
presentations, mostly run in several con-
current sessions, and a display of over 200
posters) we make no claim to be compre-
hensive. Our summary comments are an
eclectic view of the conference, with
special reference to our own experiences,
current work, and our perceptions of the
relevance of the proceedings to South
Africa.
The extent and impact of plant
invasions
Alien plant invasions are getting more
widespread and serious at an alarm-
ing rate around the world. Many more
species are becoming invasive, more
processes are being disrupted, more eco-
systems damaged, and more costs are
being incurred. Many images of devastat-
ing invasions were presented, including
entire landscapes dominated by single
invasive alien species. Some of the most
damaging invaders discussed at the
meeting were virtually unknown even a
few decades ago. Notable in this category
is the Old World climbing fern (Lygodium
microphyllum), which has invaded southern
Florida at astonishing speed.
Many new or less well-known aspects
of plant invasions were discussed. For
example: (i) The complexity of real-life
systems was highlighted using quantita-
tive food-web models. These show that
changes in species composition caused
by plant invasions could have serious
consequences for higher trophic levels,
and may greatly affect organisms at levels
that have no direct connection with the
invasive plant species in question. (ii) Evi-
dence was presented of what was dubbed
‘invasional meltdown’, meaning syner-
gistic interactions between invasive
species that promote further invasions
and exacerbate their detrimental effects.
(iii) Particularly alarming was the revela-
tion that various elements of global
change (global warming, elevated atmo-
spheric CO2, nitrogen deposition, habitat
fragmentation) are already interacting to
worsen the impacts of plant invasions.
Some experimental results suggest that
elevated CO2levels have already had a
marked effect on traits of some key inva-
sive species in North America: increased
biomass production, expanded leaf area
and spininess, and enhanced pollen
loads.1All of these traits, alone or in com-
bination, affect how these plants influ-
ence native species, and the invasibility of
the ecosystems they occupy.
Advances in invasion ecology
Attributes of invaders
Considerable strides have been made in
the new domain of plant invasion ecol-
ogy. Each session at the conference pro-
vided examples of the bewildering array
of questions that are being asked and
methods applied to unravelling the
intricacies of alien plant invasions. The
three key questions that were explored
in the SCOPE (Scientific Committee
on Problems in the Environment) pro-
gramme on the ecology and management
of biological invasions in the 1980s2still
largely define the field: (i) what makes
some species more invasive than others?;
(ii) what makes ecosystems resistant or
susceptible to invasions?; and (iii) How
can we use insights from the previous
questions to manage invasions?
Scientistscontinue to relyheavily onthe
track record of various species as aliens
in order to predict their invasiveness (ex-
plaining why some species are more inva-
sive than others), rather than on mecha-
nistic understanding of invasions. Prob-
lems with this approach include the fact
thatonly a smallproportion of the world’s
flora has been widely planted as aliens,
and for long enough, to gauge their
invasiveness. Fewer than half of the cur-
rently invasive plant species have a
track record of being invasive elsewhere.
Species known to have invaded many
parts of the world are obviously more
likely to invade in other areas, but no
prediction can be made for species that
have no history of widespread introduc-
tion over long periods, or evidence of
their invasiveness. Nonetheless, it seems
that risk assessments based largely on the
experience of other invasive species are
still the most pragmatic approach in
predicting invasiblity worldwide. In any
event, continuing global changes will
complicate any predictions.
Several speakers reported on progress
towards a more mechanistic understand-
ing of invasiveness, contributing to the
debate on whether it is feasible or
desirable to aim for standardized criteria
for all weed lists, given that such lists
serve different purposes in different parts
of the world. Formal protocols for assess-
ing the risk of invasiveness are not widely
applied. Australia and New Zealand are
aInstitute for Plant Conservation, Botany Department, Uni-
versity of Cape Town, Rondebosch 7701, South Africa.
bZoology Department, University of Cape Town.
cCSIR, Division of Water, Environment and Forestry
Technology, PO Box 320, Stellenbosch 7599.
dKirstenbosch Research Centre, National Botanical Insti-
tute, Private Bag X7, Claremont 7735.
eSouth African National Parks, Kruger National Park,
Private Bag X402, Skukuza 1350.
*Author for correspondence.
E-mail: rich@botzoo.uct.ac.za
Working for Water South African Journal of Science 100, January/February 2004 127
the only countries using risk assessment
models in day-to-day decision-making
when evaluating applications for permis-
sion to introduce new species.
Attributes of ecosystems
Regarding invasibility (factors that de-
termine the susceptibility of an ecosystem
to invasion), evidence from many studies,
including palaeoecology, modern obser-
vational studies and experimental work
at different scales (mainly small plots)
shows that susceptibility/resistance of
systems to invasion fluctuates markedly
over time and space due to changing cli-
matic conditions, disturbance, nutrients,
availability of mutualists, and the avail-
ability and abundance of propagules to
initiate and sustain invasions. Invasibility
must thus be considered probabilistically
rather than deterministically. A strong
realization emerged at the meeting that
even areas currently thought to be
resistant to invasion will probably be
invaded as increased propagule pressure
and elements of global change alter key
processes in the system. A related debate,
discussed in several presentations, was
the link between native species richness
in a system and its capacity to accept new
invaders.
Below-ground biodiversity and its role
in maintaining ecosystem functioning
and integrity, is a new field of research.
There is growing evidence that invading
plant species alter this diversity and
may affect ecosystem functioning and
resource pools in ways that are not well
understood.
Lastly, two sessions were dedicated to
the use of mathematical models and
geographic information systems (GIS) in
understanding the effects of biological
invasions on ecosystems and as tools
to enhance the design of appropriate
management strategies.
What is being done to manage plant
invasions?
Although biological invasions have
rapidly become prominent environmen-
tal concerns in almost every part of the
world, only a handful of countries are
giving serious attention to the problem. It
was difficult to draw firm conclusions on
levels of awareness and commitment to
managing invasions in different regions
from the presentations at the meeting,
but clearly the most focused research,
planning, policy formation, and imple-
mentation of counter-measures is taking
place in Australia, Europe (with marked
variations between countries and re-
gions), New Zealand, South Africa and
the United States. At least there, perspec-
tives have changed radically in the last
few decades, from the consideration of
single-species problems to the incorpora-
tion of invasive species as a complex
component of global change requiring
substantial investment at multiple scales
of time and space. Yet, most developing
countries lack the capacity even to deal
with dramatic single-species problems,
let alone to respond holistically to the
escalating problems associated with
invasions that demand expensive, multi-
pronged, international efforts.
In this respect, several international
initiatives were mentioned, including the
Global Invasive Species Programme
(GISP), the International Plant Protection
Convention (IPPC), the Convention on
Biological Diversity (CBD), and the
South Pacific Regional Environment
Programme (SPREP). Although the list of
acronyms is long and daunting, we
cannot, unfortunately, report that there
has been substantial progress. On the
contrary, much effort is wasted through a
lack of cooperation, collaboration and
integration, and in many cases, open
rivalry exists between different agencies
and groups involved nationally and
internationally. There are at least some
notable exceptions to this gloomy picture,
which include the Nature Conservancy (a
U.S.-based non-governmental organiza-
tion), which leads in acquiring land, and
supporting multidisciplinary research
and control operations in conservation
areas, in the United States, and else-
where. South Africa’s Working for Water
programme, with its strong emphasis on
multi-departmental involvement and
international cooperation, and the Austra-
lian Cooperative Research Centre for
Weed Management are others
Many presentations by top-tier ecolo-
gists attested to the flurry of activity in
the wake of former U.S. president Bill
Clinton’s Executive Order (#13112 of
1997) which led to the establishment of
the National Invasive Species Council.
One of the main functions of this body
is to oversee the production, every two
years, of a National Invasive Species
Management Plan, and a massive effort is
now under way at every level in the
United States (including work on predic-
tion/prevention, early detection, eradica-
tion, containment, management, restora-
tion and education).
There was a particular emphasis in the
management of plant invasions on
prevention, early detection and/or eradi-
cation. In the area of prevention, that is,
the anticipation and interception of
invasions, Australia, New Zealand and
the U.S.A. are the only countries devoting
even remotely appropriate resources to
research, policy development and imple-
mentation.
Given the exponential increase of
invasive species, it makes sense to focus
effort on preventing the entry of known
invasive species and to maintain early
warning systems to detect nascent
invasions of species that do gain entry.
Much was made at the meeting of a few
cases where very small populations of
potentially invasive plants have been
eradicated (that is, totally eliminated).
While eradication may be a sound goal for
new and small invasions, it could be an
illusory and costly objective for well-
established mainland populations of
invasive plants.
Linking ecological and
socio-economic impacts
Linking the effects of invasive species to
ecosystem services is critical and remains
one of the least developed areas in inva-
sion ecology. Fewer than ten papers as-
sessed the economic consequences of
invading species, even superficially. This
may be due, in part, to the absence of key
researchers in the field from the meeting.
The work on costing impacts of alien
plant invasions on water resources in
South Africa3remains the most cited
example in the field. Also, the detailed
assessment of costs and benefits of the
commercially important but invasive tree
Acacia mearnsii in South Africa4is unique.
Several studies were presented on the
costs and benefits of biological control
and these yielded cost:benefit ratios
similar to those estimated for South
African species,5confirming the cost-
effectiveness of this type of control for
extensive invasions and for tackling
emerging species. Such information is
crucial in strategies to influence the
media, and hence politicians and decision-
makers.
Dealing with the human dimensions
of the problem
The human dimensions of invasions
were addressed in a number of sessions.
There were talks on education and out-
reach, and pre-conference workshops
looking specifically at materials for differ-
ent audiences such as school children.
Important contributions came from Aus-
tralia, New Zealand and the United
States. One of the plenary speakers ques-
tioned why, even in the United States,
there is little general public engagement
on the issue of invasives, despite almost
daily news coverage. He stressed the im-
portance of ‘connecting the dots’ — that
is, presenting research results appropri-
ately for different audiences — and ‘link-
ing the boxes‘ — meaning fostering of
cooperative and mutually beneficial in-
teractions between scientists and manag-
ers in different disciplines and in different
organizations. In another influential
128 South African Journal of Science 100, January/February 2004 Working for Water
plenary address the speaker emphasized
the need for management interventions
to be strategic, tackling invasions at key
points and devising incentives as well as
punitive measures to enhance results.
The constructive engagement between
the nursery trade in the U.S.A. and ecolo-
gists and authorities was encouraging.6
The nursery industry has adopted
voluntary codes of conduct to reduce the
dissemination of invasive species and is
funding research on, among other things,
breeding sterile varieties of popular but
invasive species, and improving our
ability to screen species. A similar initia-
tive in South Africa, the Working for
Water Nurseries Partnership Programme,
has also made good progress.
The notion that alien plants are accept-
able for short-term use in restoration
programmes where no suitable native
candidate species are available seems to
be gaining currency and was advocated
by several speakers. There was general
consensus that special measures were
needed to prevent the unwanted spread
of alien plants. The matter remains highly
controversial but there is recognition that
invasion and restoration ecologists need
to engage in dialogue and to seek
‘win-win’ medium- to long-term solu-
tions to this difficult problem, which will
yield the desired benefits while minimiz-
ing the risks.
Where does South Africa stand?
South Africa’s major investment and
faith in biological control of invasive
plants as an integral part of its overall
management strategy, and the many
recent success stories were widely
lauded. However, some countries with
serious invasive plant problems (such as
Portugal) have not yet initiated this prac-
tice or have doubts because of concerns
about the possible effects of biological
control agents on some non-target plant
species.7,8
South Africa is leading the field when it
comes to assessing the hydrological con-
sequences of invading species, espe-
cially in using a range of methods to com-
pile a comprehensive picture. Surpris-
ingly, Australia and New Zealand seem to
have paid little attention to hydrological
impacts.In much ofNorth America, water
shortages are not an important issue but
in the semi-arid regions of the southwest-
ern United States water shortages are
approaching critical levels. Yet the effects
of changes in plant-species composition,
for example when grasslands replace
shrublands, have not been quantified, or
the data are fragmentary and site specific.
The opportunities to paint the big picture
(‘connect the dots’) have not been taken
and the message that invaders are using
excessive amounts of water is, by and
large, not reaching the decision makers
and the wider public.
A lesson for South African research is
the emphasis on scientific rigour and
proper experimental design in studies of
the dynamics and impacts of invasions. A
recent review of the mechanisms of
invasion9highlighted the paucity of
studies aimed at identifying the various
factors and processes involved in invasions
and how ecosystems are affected. South
Africa is no exception to this problem.
Our ecosystems provide an excellent
natural laboratory for careful experimen-
tal studies, and we need to capitalize on
this.
There were numerous references in
many sessions to South Africa’s substan-
tial and innovative contributions in the
field. There is no doubt that the small
scientific community in South Africa has
made its mark.10 South Africa’s Working
for Water programme was hailed as a
model approach for addressing invasive
species while simultaneously satisfying
socio-political goals. The momentum
created by the serendipitous events lead-
ing to the establishment of the program-
me is encouraging, but the pillars upon
which it rests need to be strengthened.
Working for Water must be informed by
advances in invasion ecology in order to
ensure maximum efficiency in terms of
prediction and priority. The programme
must be supported by partner initiatives
addressing other facets of global change,
and much more attention should be
directed towards preventing invasions
and to the early detection and manage-
ment of emerging problems.
Because invasions are driven by the
needs of society, invasion pathways have
to be clearly defined, and innovative solu-
tions should address the root causes of
invasions— the societal needs that lead to
the demand for alien species. All role
players and stakeholders must be en-
gaged. Enhanced education at all levels is
pivotal. There is an urgent need for a
South African ‘National Strategy on Alien
Species’. This country has invested
heavily in research and management of
biological invasions in recent decades.
This investment will be eroded unless we
‘connect the dots’. We have the scientific
capacity, infrastructure and enlightened
political leadership to make rapid ad-
vances in this area. Such allocation of
effort is certain to yield significant divi-
dends ecologically, socially and economi-
cally.
Conclusions
The Fort Lauderdale conference was
timely, well conceived, impressively orga-
nized, and in all respects ran smoothly
and efficiently. The meeting presented a
feast of information and discussion for
anyone interested in the biology and
management of invasive alien plants. The
excitement generated as a consequence of
several hundred people talking together
about invasive plants, was palpable. The
conference was in many ways unique, a
first and, as such, represented a signifi-
cant step forward in achieving its objec-
tives, namely, forging links between
research scientists, managers and politi-
cians. There was a session entitled ‘Man-
agement and the Research Interface’.
Inevitably, researchers, managers and
politicians interacting for four days in a
single venue must have expanded all
their horizons. Whether the conference
will have succeeded in forging sufficient
mutual understanding and sustained
cooperation between busy people who
are all more or less entrenched in their
own disciplines, is moot. However, that is
what is needed, a concerted effort from
practitioners in all specialities — we need
to connect the dots in our research
endeavours, and to link the disciplinary
boxes in science and management — if
the global threat of invasive plants is to be
effectively countered.
The conference proceedings, compris-
ing most of the invited talks and some
contributed papers, will be published in
the journal Weed Technology in 2004.
1. Zizka L.H. (2003). Evaluation of the growth
response of six invasive species to past, present
and future atmospheric carbon dioxide. J. Exp.
Bot. 54, 395–404.
2. Drake J.A., Mooney H.A., Di Castri F., Groves R.,
Kruger F.J., Rejmánek M. and Williamson M.
(1989). Biological Invasions: A Global Perspective.
Wiley, Chichester.
3. van Wilgen B.W., Cowling R.M. and Burgers C.J.
(1996). Valuation of ecosystem services. A case
study from South African fynbos ecosystems. Bio-
Science 46, 184–189.
4. de Wit M.P., Crookes D.J. and van Wilgen B.W.
(2001). Conflicts of interest in environmental
management: estimating the costs and benefits of
a tree invasion. Biol. Invas. 3, 167–178.
5. van Wilgen B.W., de Wit M.P., Anderson H.J., Le
Maitre D.C., Kotze I.M., Ndala S., Brown B. and
Rapholo M.B. (2004). Costs and benefits of
biological control of invasive alien plants: case
studiesfrom South Africa. S.Afr.J. Sci.100, 113–122.
6. Baskin Y. (2002). The greening of horticulture:
new codes of conduct aim to curb plant invasions.
BioScience 52, 464–471.
7. Follett P.A. and Duan J.J. (1999). Nontarget effects of
biological control. Kluver Academic Publishers,
Boston.
8. Zimmermann H.G., Moran V.C. and Hoffmann
J.H. (2000). The renowned cactus moth, Cactoblas-
tis cactorum: its natural history and threatto native
Opuntia floras in Mexico and the United States of
America. Diversity Distrib. 6, 259–269.
9. Levine J.M., Vila M., D’Antonio C.M., Dukes J.S.,
Grigulis K. and Lavorel S. (2003). Mechanisms
underlying the impacts of exotic plant invasions.
Proc. R. Soc. Lond. B 270, 775–781.
10. Simberloff D. (2003). Confronting introduced
species: a form of xenophobia? Biol. Invas. 5,
179–192.
