fluctuating resources in plants

1. ARTICLE:FLUCTUATING RESOURCES IN PLANTS;A
GENERAL THEORY OF INVASIBILITY
MARK A. DAVIS, J. PHILIP GRIME* and KEN
THOMPSON*
Department of Biology, Macalester College, Saint Paul,
Minnesota 55105, USA; and *Unit of Comparative Plant
Ecology, Department of Animal and Plant Sciences, The
University, Sheffi•eld S10 2TN, UK
EHSAN HAIDER
ROLL NO.6004 SEMESTER 7TH MORNING
SUBMIT TO PROF.ALI KMAL

2. INTRODUCTION
• The invasion of habitats by non-native species is a global
phenomenon with serious consequences
 The actual invasion of an environment by new species is
influenced by three factors
 The number of propagules entering the new environment
 The characteristics of the new species,
 The susceptibility of the environment to invasion by new
species (invasibility)

3. INTRODUCTION
 Invasibility depends upon
 region's climate
 the environment's disturbance
 The competitive abilities of the resident species
 Invasibility may also be aff€ected by the presence (or absence)
of herbivores and pathogens
 mutualists , and facilitative eff€ects of resident vegetation
 Although many hypotheses have been proposed to explain why
some communities are more invasible than others

4. THE THEORY
 A plant community becomes more susceptible to invasion
whenever there is an increase in the amount of unused
resources.
 Theory rests on the simple assumption that an invading species
must have access to available resources, e.g. light, nutrients,
and water
 That species will enjoy greater success in invading a
community if it does not encounter intense competition for these
resources from resident species
 increase the availability of limiting resources will increase the
vulnerability of a community to invasion

5. INCREASE IN RESOURCE AVAILABILITY
 An increase in resource availability can occur in two basic
ways
 use of resources by the resident vegetation can decline
 Resource supply can increase at a rate faster than the resident
vegetation can sequester it
 Resource use could decline due to a number of factors
 A disturbance could damage or destroy some of the resident
vegetation, reducing light, water, and nutrient uptake.

6.  Heavy herbivory due to grazing or a pest outbreak
 widespread disease among the resident vegetation, would also
reduce resource uptake
 removal of a tree canopy (increased light for the understorey
vegetation)
 Whether resource uptake goes down for a time, or gross supply
goes up, there are more resources available to invaders and this
is when the community is particularly vulnerable to invasion

7. RELATIONSHIP TO PUBLISHED DATA
 Nitrogen addition in a California serpentine grassland increased
the invasion success of several alien grass species
(Huenneke1999)
 Harrison (1999) argued that the low invasibility of serpentine
grasslands was due to low levels of soil nutrients
 Maron & Connors (1996) concluded that invasions by exotic
species in a California coastal prairie were facilitated by a native
nitrogen-fixing shrub
 An experimental study of plant invasions into a limestone
grassland in Great Britain showed that invasion was highest in
sites that were nutrient-enriched,

8.  Similar findings were found by Hobbs & Atkins (1988) who also
found that disturbance combined with eutrophication increased
a community's invasibility. The combination of disturbance and
eutrophication involves both a reduction in resource uptake by
resident vegetation and an increase in gross resource supply.
 So this theory intended to explain changes in investibility
 REFERENCE;WWW.journalofecology.org/view/0/toppapers