We explored the relationships between various socio-economic variables and the prevalence and density of invasive alien species
(IAS) on a global scale using country-level data sets. We did this by testing the hypothesis that the abundance and distribution
of populations of IAS are correlated with various socio-economic indicators, with the direction of causality being that the state
of IAS is determined by socio-economic conditions. We found a positive and statistically significant relationship between the
prevalence and density of IAS and the human development index (HDI), the satisfaction with life index and the gross domestic
product (GDP) among all the countries tested. Additionally, the density of IAS increased significantly with human-population
density, total geographic area, GDP and HDI. We also found a positive relationship between the density of IAS and the top 10
road networks of the world. This provides some insight into the development of renewed policies and management strategies for
invasive species across both continents and countries. We do caution, however, that the results are likely to be influenced by
the sampling factor, whereby affluent countries have more resources to measure and monitor IAS than poorer countries and hence
have better records of such, which then indicates a stronger relationship with the level of development.
The introduction and spread of invasive alien species (IAS) have been predicted from land-use and socio-economic factors. While links between
these factors have been made on certain scales (within-country, regional and limited-country comparisons), a geographically comprehensive
comparison has not yet been attempted. Therefore, we report on an evaluation of the hypothesis that the prevalence and density of IAS at
country level are correlated with the state and rate of change in the economic development of that country. We did this to determine
whether or not there is a link between the prevalence, density and distribution of IAS and the level of economic development.
Higher trade volumes and the increased movement of people are inextricably linked to increased levels of affluence and improved
transportation networks, both the result of economic growth. These levels of affluence and networks open the door to an increased
movement in both plant and animal species, which jump borders and continents in ways and at rates that would not have been possible
without this level and extent of trade and transport. It has been established that, for China, fast economic growth accelerates
invasion by exotic species.1 Furthermore, the relationships between the number of invasive alien plant species and
some socio-economic factors in China have been compared to those of the United States of America (USA), leading to the conclusion
that Chinese provinces are likely to become more invaded as economic development progresses, since international commerce will
bring new invaders.2 Economic trade has also been implicated in the spread of IAS on a global scale.3
Furthermore, it has been shown that, at a regional level, disturbed and anthropogenically induced transformed environments
are invaded more than pristine areas.4
Land-use and socio-economic factors apparently influence the introduction and spread of IAS directly.5 This
conclusion was reached after the exploration of the predictive power of land-use and socio-economic parameters on the density
of alien plants in European and North African countries. There is also a general notion that development and progress increase
the number of IAS.6 An analysis of the relative influence of several economic variables on the share of IAS in terms
of the density or intensity of invasion has suggested that disturbances associated with human activities and gross domestic
product (GDP) per capita are important determinants of a given country’s vulnerability to invasions.7
Economic development is associated with infrastructure expansion and it has been shown that road networks and their
development do indeed act as corridors and conduits for the spread of IAS.8,9 In developed countries, road
networks extend over large areas and are particularly dense in highly populated regions.8,9 In contrast,
road networks in developing countries are often less extensive in rural areas.10 Roads and activities
surrounding their development and maintenance also provide suitable conditions for the establishment, growth and
spread of IAS.11,12,13
While much work has been done on the relationship between the state of and change in country- and area-specific
economic activity and the prevalence and spread of IAS, to our knowledge, no global comparative analysis has been
done using country-level data. To bridge this knowledge gap, we assumed, firstly, that high human densities occur
in areas where there is high(er) economic productivity and, secondly, that areas with potentially high and positive
economic growth are measured as changes in the GDP. While GDP is an extremely weak indicator of human welfare and
economic development, it is a uniform measure of income and changes in income and of the income gap between and among
nations; levels of environmental transformation, including invasion, are linked to economic growth.
A variety of primary and secondary driving forces of invasion operates within and across national and international
boundaries. Primary driving forces (Table 1),14 such as the arrival of new propagules and disturbance
regimes, are inextricably linked and can operate individually or together to facilitate or hinder invasions.
Secondary driving forces, such as human-population growth and movement and global climate change, are likely
to have a direct effect on the primary driving forces of propagule arrival and changes in limiting factors,
respectively. From the perspective of first principles therefore, it is the secondary driving forces that
affect, influence and contribute to the primary driving forces and not the other way around. It is self-evident
that IAS do not cause economic growth or the movement of people or economic trade; they are a consequence thereof.
The causal relationship is therefore unidirectional.
The question, and the focus of this study, centres on the strength of this relationship and whether there is a
meaningful and statistically significant difference among countries with different levels of economic development.
Given that levels of environmental transformation are linked to economic growth and the need for resources
and given the established links between environmental transformation and the prevalence and spread of IAS,
one can accept that measures such as GDP per capita and human-population density can be considered as
conduits or even drivers for both propagule pressure and ecological disturbance, which facilitates the
establishment and spread of alien species.4,15 One can therefore anticipate that developed
countries are likely to have more IAS than developing countries.
We tested these assumptions against various indicators of the level and change in socio-economic well-being,
such as population density, GDP, the happy planet index (HPI), the satisfaction with life (SWL) index and the
human development index (HDI) (Table 2). We assumed that the indicators of social well-being, such as the HPI
and SWL indices, also provide insight into the ecological dimension of human well-being, in other words there
is more to happiness than economic prosperity alone. The latter can be achieved only when ecosystem services,
defined as the end products of nature that benefit humans,16 are optimal and without the hindrance
Table 1: Primary and secondary driving forces of invasion that operate within and across
national and international boundaries
Emerging correlations from this study may provide insight regarding the renewal of policies and management
strategies for invasive species across continents and countries.
We used the International Union for Conservation of Nature (IUCN, or World Conservation Union) definition of IAS as ‘an
alien species which becomes established in natural or semi-natural ecosystems or habitat, is an agent of change, and threatens
native biological diversity’.17 Ultimately, the degree to which IAS impact biodiversity is the most important
consideration; we thus did not take the broader approach of including all alien species in a country. Instead, we used the IAS
database of the Invasive Species Specialist Group of the IUCN, which is the most geographically comprehensive database on
invasive species worldwide.17 It includes 227 countries and profiles on 357 IAS across all taxa that are significant
threats to native biodiversity.17 We used the density of invasive aliens (i.e. the number of IAS divided by the
log10 area of the country concerned) and not the number of invasive aliens per se to avoid confounding effects
The HDI was retrieved from the Human development report 2007/2008 of the United Nations (UN) Development
Programme19 and GDP was retrieved from the World economic outlook database20 and the International
Monetary Fund.21 Human population was compiled from UN estimates and from the population clock for each country on 3
December 2007.22 Area for countries was retrieved from Wikipedia23, as was the SWL index24
and the HPI was obtained from the New Economics Foundation and Wikipedia25,26. A list of the top 10 countries
with the longest road networks, together with the extent of these networks, was compiled from maps of the world.27 The
FTSE index of the Financial Times and the London Stock Exchange groups countries into developed, advanced-emerging and
secondary-emerging (comprising largely developing or poorer) countries.28 We assumed that these groups provide an
indication of emerging international trade and therefore more opportunity for invasion, which is why we used the FTSE
classification and because it is the leading world index provider in terms of countries that are emerging financially
in economic investment and international trade. We used all the above-mentioned indicators of socio-economic
well-being and economic development as they are arguably the best predictors of the social and economic well-being
of countries (Table 2).
Correlations were performed among the various socio-economic variables and regression analysis was conducted for human
population, total geographic area, HDI and GDP for the 172 countries under investigation. Residual regressions were
performed between GDP and the density of IAS, with the successive removal of the top invaded countries using the SPSS
(SPSS Inc., Chicago, USA) statistical package.29 We used non-parametric correlations (Spearman’s
correlation) to investigate the relationships among various socio-economic factors and IAS.
Significant relationships between the density of IAS and the indicators of socio-economic well-being were obtained across the world
(Table 3). There was a positive and significant relationship between the density of IAS and the HDI (r2 =
0.509; p < 0.01), the SWL index (r2 = 0.298; p < 0.01) and the GDP
(r2 = 0.680; p < 0.01).
Table 2: Explanations of the socio-economic indicators used in the study
When all the countries (n = 172) were included in the regression analysis between GDP and IAS, we found that the
USA was the driving factor for the significance of the data. Residual-regression analysis revealed that significance
values decreased as the top invaded countries (and, correspondingly, those with the highest GDP) were removed from the
analysis (the outliers in the datasets) (Table 4).
More affluent countries have more potential for species cataloguing, which could explain the relationship between GDP
and IAS. There was a significant increase in invasive-species numbers with increasing GDP, total geographic area and
human population (Figure 1). GDP showed a significant positive correlation with the HDI and SWL index but was not
related to the HPI. Although the density of IAS was significantly positively related to the HDI, SWL index and GDP,
no significant relationship was found between the HPI and the density of IAS. Results clearly illustrated a positive
relationship between the number of IAS and the top 10 road networks of the world (r2 = 0.61;
p < 0.05) (Figure 2a). A change in pattern was observed when the density of alien species was related to the
density (km/km2) of the top 10 road networks, although the significance value increased (r2 =
0.90; p < 0.001) (Figure 2b).
A key point emerging from our study is that a country’s economic strength and socio-economic status, according to the most
commonly used indices, are excellent predictors of IAS density. This association is due mainly to three reasons. Firstly, more
funds are likely to be available for the research, surveying and cataloguing of IAS. Secondly, if GDP is high, there is greater
potential for more imports and therefore probably for more opportunities for the introduction of IAS. Thirdly, greater affluence
means improved road networks and, therefore, more opportunities for the introduction and spread of IAS. It is important to understand
these drivers and to interpret which plays a more important driving role.
Table 3: Correlation matrix among various indicators of socio-economic well-being and the
density of invasive alien species
Table 4: Countrywide residual regression relationship (Y = a + bX) between GDP and
density of invasive alien species
Research agendas are more directly influenced by economic priorities and practical limitations than by geographic and socio-political
barriers.30 The economic status of a region affects research efforts not only directly, by more resources being spent on the
problems of biological invasions in developed states than in developing ones, but also historically because developed states have more
advanced systems of science and education and therefore greater resources to survey and catalogue the presence of IAS. The surveying
and cataloguing of IAS in the USA, for example, are extensive due to the higher funding of research, thus revealing a potential
source of error in the IAS data set.
Due to the growing number of databases of alien plants and animals,31 reliable and balanced data for the unbiased
comparison of research effort with levels of invasion are likely to be difficult to obtain and the issue of circular reasoning
in exploring relationships with these data has been raised.32 Major global databases are selective because they are
not aimed at providing a complete overview of global invaders but rather at documenting those with a serious impact. Moreover,
the inclusion of species in these databases is based largely on published information, most of which comes from case studies;
the databases thus reflect what is recorded in scientific literature rather than the real state of affairs. The number of IAS
in most databases and check-lists is furthermore affected by sampling effort.33
The results presented in this study should, therefore, be interpreted with caution. Our study approach is statistically
exploratory but it does highlight the strong potential links between plant invasions and human economic activities. We argue
that, for a broader picture of the current scenario, such data sets can lead to some reliable conclusions with implications.
Our results suggest that human population, total geographic area and HDI have less of an influence on invasive alien spread
and that GDP plays a more prominent role. GDP could be a surrogate measure for the total imports and exports of a country.34
There is positive feedback between the degree of wealth and invasions because developed countries with high GDP and large trade
volumes are also those that receive the most alien species as an inescapable by-product of trade in commodities.
In a study from the continental USA, GDP was positively related to the establishment rates of alien species.35
The results suggested that the more developed countries (with a high GDP) are rich in IAS possibly due to the above reasons.
In most developing countries, however, the primary goal of government is GDP growth and not necessarily environmental
protection; invasive species are thus largely ignored.36
Economic practices and related incentives could, therefore, be a powerful policy tool to reduce future invasions.13
The concept of ‘the rich get richer’ has been subjected to theoretical debate for several
and here we demonstrate a situation in which economically richer
countries are becoming richer in IAS. We were unable, however, to detect a significant trend in the number of
IAS in the developed, advanced-emerging and secondary-emerging countries of the FTSE classification, due to the
high variability among the countries (Figure 3).
Road networks are the backbone of a country’s economic development. The significant positive relationship
between the countries with the top 10 road networks and road density and the number and density of IAS also indicates
that road development could facilitate the spread of IAS in these countries (Figures 2a and 2b). Turbulence created
by passing vehicles, for example, enhances plant dispersal.44 The recent construction of transport networks
(such as highways and railways) also enhances the immigration rates of new species and the spread of existing species.
44,45 A similar relationship has been found with road density and the density of invasive alien plants in provinces
of China and in states of the USA,2 suggesting that international cooperation and trans-boundary collaboration
could reduce the risk of new invasions into continents and into countries. The percentage of IAS has also been observed to
increase with an increasing number of visitors in protected areas.46 Positive relationships with IAS, land-use
variables (the length of traffic routes and protected land cover) and socio-economic variables (imports and HDI) have been
established for Europe and North African countries.5 The length of terrestrial traffic routes is also related to
the number of tourists who visit a country.5 Inevitable road development with the revival of an economy is therefore
also a major concern regarding the spread of invasive species. The message here is that precise policies and management practices
need to be in place during road construction and management to minimise the propagule pressure of invasives.
Figure 1: Regression analysis showing the relationship between socio-economic variables and invasive alien species numbers of the countries
Figure 2: Relationship between (a) the number of invasive alien species (IAS) and the top 10 road networks and (b) the density of invasive alien species and road density of the top 10 road
networks of the world (km/km2)
Figure 3: The number of invasive alien species in developed, advanced-emerging and
secondary-emerging countries, as per the FTSE index
We tested the relationship between changes in the population of invasive alien species and level of economic activity, finding
positive and statistically significant correlations. We further argued that, in terms of causality, it is the secondary
driving forces (economic growth, infrastructure development and population growth) that influence the primary driving
forces (such as the arrival of new propagules and disturbance regimes) of changes in IAS and not the other way around.
Causality is therefore unidirectional. While it may be possible for specific countries spending a considerable amount
of resources on the control of IAS to see a reverse trend (in other words, succeeding in controlling invasions, resulting
in the level of economic growth and IAS being negatively correlated), macro-statistics, at least in the short term, are
unlikely to pick up such trends.
We suggest that a new agenda of research in modelling and analysing the economic impacts of biological invasions should
be explored through interdisciplinary collaboration between ecologists and economists.47,48 We propose that
countries with emerging economies should dedicate funds for invasive species research and management to reduce future
costs associated with the management of these species. Resources should be used to support intercontinental cooperation
with properly designed research strategies, addressing issues of invasions where current biases can limit our understanding
of biological invasions.32 A comprehensive interdisciplinary study investigating the economic and ecological
variables of biological invasion would be a way forward in understanding this scenario in totality.
We acknowledge funding from, (1) the Department of Science and Technology and the National Research Foundation Centre of
Excellence for Invasion Biology at Stellenbosch University in South Africa, (2) the BIOTA project sponsored by the German
Federal Ministry of Education and Research and the NRF and (3) Pretoria vide Indo-South Africa funding, No. UID 67549.
We are grateful to the anonymous referees and to Ken Pringle and Brian van Wilgen for their valuable comments on previous
versions of this manuscript. Gyan P. Sharma, in particular, wishes to thank the CSIR, New Delhi, India, for its support.
We are also thankful for valuable suggestions from Prof. Dave Richardson at the Centre of Excellence for Invasion Biology.
At the time of proofreading this article, the authors became aware of the following paper that addresses similar issues: McGeoch
et al. Global indicators of biological invasion: species numbers, biodiversity impact and policy responses. Diversity Distrib.
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