The relationship between community diversity and exotic plants: cause or consequence of invasion? - Invasive Plants: Ecological and Agricultural Aspects - page 78

The relationship between community diversity and
exotic plants: cause or consequence of invasion?
Scott J. Meiners 1 and Mary L. Cadenasso 2
1 Department of Biological Sciences, Eastern Illinois University, 600 Lincoln Avenue, Charleston, IL
61920-3099, USA
2 Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545-0129, USA; and Yale School Of
Forestry and Environmental Studies, 205 Prospect St., New Haven, CT 06511, USA
Introduction
Biological diversity is the modern unifying metric by which the health and sta-
tus of communities and ecosystems are assessed throughout the world.
Diversity is used to identify areas of special conservation concern while threats
to biodiversity are the focus of remediation and legal action. Although the great-
est threats to biodiversity are typically thought of as being the direct or indirect
result of habitat loss and fragmentation, biological invasions follow closely
behind. Despite this high priority position, our knowledge of the relationship
between invasions and diversity remains relatively early in its development.
The invasion of terrestrial communities by exotic plant species is a major
concern for ecologists and natural resource managers. With the increased
speed of planned and unplanned movement of species throughout the world
and greater rates of disturbance, exotic plants are becoming a larger compo-
nent of regional floras [1-4]. Furthermore, exotic species already cover vast
areas of many community types and sometimes dominate local plant commu-
nities. Invasions affect both natural and agricultural systems and cause finan-
cial, as well as biological problems in areas of heavy invasion. While there has
been an increase in awareness of the problem of exotic invasive plant species,
their community-level impacts remain relatively unknown [5-8].
A frequently observed pattern within plant communities is a negative rela-
tionship between diversity, typically expressed as species richness, and the
cover of exotic plants (Fig. 1A). This simple pattern has been explained by two
community-level mechanisms that differ in the cause/consequence relationship
between diversity and invasion. In the first mechanism, diversity regulates the
invasibility of the local plant community, causing the diversity/invasion rela-
tionship. This view comes from theoretical and experimental community ecol-
ogists over the past several decades starting with Elton [9] and continues with
empirical work (e.g., [10-12]). In the second mechanism, plant invasion
results in the reduction of community diversity by driving other species from