Stages of Invasion
These are the basic stages of an invasion. Most plants (95%) and ~half of all introduced
animals never get through the establishment phase. After the establishment phase only
about 5‐50% of introduced species actually spread outside their point of introduction. Out
of all species that are introduced only about 0.3% of plants and 25% of animals cause
significant negative effects. It is important to remember that even though these stages are
presented as sequential much of this is happening at the same time and there are feedback
loops between the stages.
Questions that Define Invasion
What characteristics make an introduced species likely to be established and what makes the new community/environment vulnerable for invasion?
An introduced species...
must be able to tolerate the abiotic and biotic conditions it encounters.
Animals introduced to similar enviroments
usually have success becoming established.
Having a broad tolerance to many temperatures, moisture levels. Makes individuals more likely to survive the abiotic features of a new environment.
Highly competitive organisms, which are dominant in aggression, resource economy, and fast development, survive better against native species. Greater chance for establishment.
Behavioral Flexibility (intelligence)
Willingness to try new things and methods. This is one of the traits that make humans an incredibly great invasive species.
Piranhas in the Midwest and Great Lakes
Sometimes fishermen catches these dangerous fish in the lakes because people release these pets into the wild. However, these species cannot tolerate the winters and do not become established.
Piranhas and Texas
Piranhas are banned in Texas, however, because they could become established here.
Intelligence and Innovation: Brain Size
Birds that had larger brain size, relative to their body size, have more invasion potential. Innovation Frequency was also positively related to invasion potential. Thus higher intelligence and willingness to try new foraging methods lead to becoming invasive.
A measure of behavioral flexibility.
Red Foxes and Australia
Red Foxes were introduced to Australia for hunting purposes. However, authorities did not account for its high behavioral flexibility, among other things like high competitiveness and prey naivety.
Red Foxes and Prey
The red fox ended up being an extremely effective predator and drove many species to extinction who were vulnerable never having seen foxes before. The Bettong is one such animal that was once abundant, but now only exists on nearby islands that are free of foxes.
One of the Most Important Invasion Aspects
Number of individuals that get released in a single event or across multiple events. The more individuals introduced, the higher the propagule pressure > Higher chance of establishment.
Human Assisted Invasions....
Have high propagule pressure because many individuals are usually introduced in a continuous fashion.
Why does Propagule Pressure Increase Success?
1. Increased numbers can overcome extinction by random processes.
2. More likely an adapted Genotype will be introduced.
3. More likely individuals can find mates.
4. Increase Genetic Diversity
Prop. Pressure: Increased Numbers
Having a larger population creates a resistance to random events. Also, a high population means organisms will be able to locate each other for mating easier. High pop means more genetic diversity and high chances a genotype might thrive.
One would assume a small population should thrive, since fewer individuals means that there would be less competition for resources. However, a small population can get smaller and go extinct if it is too hard for organisms to find mates.
Acclimation Societies and Bird Intros
Records for Acclimation societies in New Zealand show that the number of release attempts correlates positively with establishment success, chances of success increased with more releases. The average was 8 releases.
Marketing Time Predicts Plant Invasiveness
These researchers used nursery records in Florida for ornamental plants to determine how
long the plants were sold and whether that impacted the chance they became invasive -
the answer was yes - the longer a plant was sold the more likely it would become
established and the more likely it would become invasive.
Paradox of Invasive Species
How do the small introduced populations with low genetic variability become established while fighting inbreeding depression and local adaptation?
Lag Times Invasion
There is often a lag period before an invasion takes off, due to demographic growth, undergoing selection in the new range, awaiting infusion of new genetic material.
Size of Human Assisted Invasions
Large, very large.
Do multiple introductions facilitate invasion?
Invasion of Florida by Cuban Anole
Cuban Anoles were brought to the Florida keys as pets and stayed there and in Southern Florida for a long time. Eventually they spread northward. Genotyping revealed that as you travel north, the genotypes of the lizards are more diverse, thus new genetic material was introduced from Cuba while the species moved up the peninsula. The new invasions probably brought genetic material that allowed the anoles to adapt and move north.
How open a community is to invasion. New species will likely become established in communities with high invasibility.
The hypothesis states that communities with high species richness should resist invasions and so there should be a negative correlation between the number of non‐native species and the species diversity of a community. Rich environments will have few to none open niches, thus less place for new species to survive.
Support of the Invasibility-Diveristy Hypothesis
Islands have poor species diversity, due to being cut off, and have high numbers of invasive species. Peninsulas have a similar setup, they are mostly secluded and have only one connection to mainland.
Communities are Rarely Saturated!
A large number of studies have now revealed that communities may never be saturated - you can always add more species. In this series of figures the number of non‐native species are plotted against year for six different islands - notice they are always increasing and there is never a leveling off suggesting the communities are not saturated or even beginning to be saturated. Similar patterns are seen for aquatic and mainland systems as well.
Non-Native Species are More Common in Species Rich Areas
This makes sense, since the abiotic conditions that would favor an area to have high species richness would also be appropriate for many invasive species.
Why Non-native species are common in Rich areas...
High spatial and temporal variation in resource availability may be the main drivers of species diversity, not competition. Resource variation keeps one or a few species from becoming so dominant they use all resources and keep others from coexisting. (Starfish and foundation species)
Resource Availability and Species Richness
The relationship between resource variability and species richness is expected to peak at
intermediate levels of resource availability or disturbance as shown in this figure.
Why do Islands have more introduced species?
They have higher propagule pressure: more imports means more events at which individuals are introduced. Increased chance for establishment.
Invasive Species Facilitate Eachother
American Bullfrogs have expanded beyond it natural range. Some ponds, however, had bullfrogs and some didn't Hypothesis was that bluegill, which were in ponds with frogs, ate dragonflies that preyed upon tadpoles (dragonflies appeared alone in ponds).
Experimental tanks were set up with and without dragonfly larvae and with and without
bluegill (bullfrog tadpoles were put into all the tanks).
Bluegill reduced the number of dragonfly larvae by eating them, thus allowing Bullfrogs to survive better. When bluegill were absent, dragonflies ate all the tadpoles. Alone, the tadpoles lived with no ill effects similar to tadpoles with bluegill. Thus bluegill facilitated bullfrogs.
In summary: The traits of an organism, the propagule pressure, and the invasibility of the
community all determine whether a species will become established. Propagule pressure
is still the main determinant of establishment - simple persistence - if you add enough of
them they will eventually stick! After establishment, adaptation and facilitation can then
help the spread of the new species.