CHARACTERISTICS OF INVASIVE ALIEN SPECIES - INVASIVENESS AND INVASIBILITY:

• High invasiveness of species means that they are more likely to invade certain habitats (to be invasive rather than non-invasive).

• Which species are most likely to become invasive?

• Which traits enable a species to invade a new habitat, and establish themselves successfully?

• Knowing this would allow one in principle to isolate species for restrictions on introduction or for early eradication.

Characteristics alien spp.

this implies flexible/adaptable responses to the following environmental variables:

INVASIVENESS AND INVASIBILITY

1. broad native range 2.rapid dispersal

traits that best explain variation among species in invasiveness (Dukes and Mooney, 1999)

difficult to identify traits because different traits favour invasiveness in different habitats

short generation time long fruiting period

large number of seeds small seed size

prolonged seed viability transport by wind or animals

3. greater phenotypic plasticity

disturbanceherbivory/grazing nutrient availability

waterlight

rapid dispersal is facilitated by the following traits:

Invasiveness & invasibility

This approach is useful as a checklist of potential warning signs, but is too broad to have much predictive value (Lake and Leishman, 2004).

INVASIVENESS AND INVASIBILITY

in 1974, Herbert Baker identified what he thought were the attributes for an ideal weed:

a plastic perennial germinates in a wide rangeof physical conditions

grows quicklyflowers early

is self-compatible produces many seeds which disperse widely

reproduces vegetatively is a good competitor

Herbert Baker

• This hypothesis dates back to Darwin (1859).

• It suggests that being unlike native species confers invasiveness. There is no evidence or theory to support this, with only a possibility that perhaps the native and invader, being different, have different resource needs and enemies i.e. they don’t compete with or attack one another, and its merely the physical environment with available resources that determines their success, rather than the presence of each other.

• In practice, the best predictor of invasiveness is whether or not the species has proven to be invasive elsewhere, especially under similar conditions (Reichard and Hamilton, 1997).

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THE UNLIKE INVADER HYPOTHESIS

The unlike invader hypothesis

• High invasibility of habitats means that they are more susceptible to invasion by invasive alien species. Low invasibility would make them more resistant to invasion.

• Which habitats are most susceptible to invasion?

• What determines the susceptibility of a community or habitat to the establishment and spread of new species?

• Knowing this might allow one to manage habitats with the emphasis on preventing the spread or establishment of invasive species.

INVASIBILITY OF HABITATS (Alpert et al, 2000)

Invasibility of habitats

• Types of habitats that are relatively invasible include islands and riverbanks.

• The combination of altered disturbance regimes with high resource availability may particularly promote invasibility (i.e. disturbed habitats are more prone to invasions).

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INVASIBILITY OF HABITATS cont. (Alpert et al, 2000)

Invasibility of habitats cont.

• Low spp. richness (aliens less likely to encounter close competitors).

• Island biotas have evolved in isolation (no indigenous predators & limited adaptability of indigenous spp.).

• Small size means that effects of human-induced disturbances are concentrated.

• Many islands are at the meeting point or cross-roads of inter-continental trade, so have been exposed to a large number of new species in a short time.

INVASIBILITY OF HABITATS cont.

Why are islands more susceptible to invasion than mainlands?

Islands

• Species-poor and temperate environments are easy to invade and therefore possess the greatest numbers of invasions. This is probably because of a lack of native predators, more space and available resources, therefore less competition.

• Habitats that tend to have relatively few alien species include dense or mature forests, arid but not necessarily semi-arid habitats, salt marshes, high montane habitats and larger fragments of fragmented habitats (Alpert et al 2000).

http://deserts.free.fr/ http://www.ewpnet.com/mkclimb.htm

BIOTIC RESISTANCE HYPOTHESIS

Biotic resistance hypothesis

• Propagule pressure has emerged as the most important factor for predicting whether or not a non-indigenous species will become established (Kolar & Lodge, 2001).

• Propagule pressure includes both the number of individuals introduced and the number of release events.

INVASIBILITY OF HABITATS cont.

five factors are thought to account for differences in invasibility between habitats:

evolutionary history

community structure

propagule pressure

disturbance

stress

Five factors

Links to other chapters

I hope that you found chapter 5 informative and that you will enjoy chapter six!

Chapter 1 Definitions

Chapter 2 History, globalisation and GMOs

Chapter 3 The human dimension

Chapter 4 Pathways of introduction

Chapter 6 The ecology of biological invasions

Chapter 5 Characteristics of invasive alien species

Chapter 7 Impacts of invasive alien species

Next

Chapter 8

Chapter 10

Chapter 9

Chapter 11

Chapter 12