invasion by exotic species a possible mechanism that allows competitive coexistence between native...

Invasion by exotic speciesA possible mechanism that allows competitive coexistence between native and exotic plants.

•Augustina di Virgilio•Ewaldo L. de O. Júnior•João Pinheiro Neto•Luiz H. de Almeida•Melina O. Melito•Pedro G. A. Alcântara

II Southern-Summer School on Mathematical Biology

Invasions

Native bumblebee Exotic bumblebee

Native plants

Resources consumption

Disease transmission

-

-+“Steals” nectar


Relevance

• Worldwide phenomena.

• Invasion can have strong effects on the environment.

• Diversity of species could be at risk.

• Conservation polices have to take this into account.


Competition

•A basic competition dynamics should eventually force the

elimination of the weaker competitor (Competitive

Exclusion Principle).

•However, there is no evidence to suggest that this is a

common occurrence. (Lonsdale 1999; Stohlgren et al. 1999)

•The species forge a kind of coexistence.


So how can there be coexistence?

• There must be mechanisms regulating the interactions.

• What could they be?

• Predator, niche, space, delay for predators to attack (enemy release), or many other possibilities.

• It could even be that the timescale in which the elimination happens is just too large for we to observe


Predator hypothesis

•Could predators act as a mechanism promoting equilibrium?

•Two competitive preys one predator.

•Trade-off: competitive ability X susceptibility to predation?


Study system• Estuarial plant communities in New England• Similar native and exotic plants• Herbivory by insects

Native species

Exotic species

(Heard &Sax, 2012)


Dynamics – Model 1

Herbivores

Nativeplants

Exoticplants


Assumptions

• Natives and Exotics - different growth rates

• Herbivore rates are different for exotics and natives

• Competitive strength is not symmetrical

• Capture rate (), conversion rate (), and the parameter D are the same for both species


First model


No predators

Coexistence

No exotics


Population size

Natives PredatorsExotics


Dynamics – Model 2

Herbivores

Nativeseedlings

Exoticseedlings

Nativeadults

Exoticadults


Second modelII Southern-Summer School on Mathematical Biology

No predators No exotics

No natives Coexistence


Robustness of the modelsII Southern-Summer School on Mathematical Biology

Comparison between models


General conclusions

• Both models fit the observations.

• Predator dynamics could act as a mechanism to promote coexistence between competitors.

• A basic trade off in adaptability and susceptibility to predators could explain coexistence without loss of biodiversity.

• We must remember they may not be the only mechanism at work.


References:

[1] Heard, M.J. and Sax, D.F. , Coexistence between native and exotic species is facilitated by

asymmetries in competitive ability and susceptibility to herbivores. Ecology Letters 16 (2013) 206.

[2] Adler,P.B. et alli, Coexistence of perennial plants: an embarrassment of niches. Ecology Letters 13 (2010) 1019.

[3]Keane, R.M. and Crawley, M.J. Exotic plant invasions and the enemy release hypothesis. Trends in Ecology and Evolution 17 (2002) 164.

[4] Davis, M.A. et alli. Don't judge species on their origins. Nature 474 (2011) 153.

[5] Stromberg, J.C. et alli. Changing Perceptions of Change: The Role of Scientists in Tamarix and River Management. Restoration Ecology 17 (2009) 177

Images from:

•http://ian.umces.edu/

•Augustina di Virgilio - Argentina

Special thanks to group 6 for their

work on preferences, it was very

useful.


Parameters – Model 1

Natives growth rate

Exotics growth rate

Natives carrying capacity

Exotics carrying capacity

Competition coefficient

Feeding efficiency

Natives herbivore rate

Exotics herbivore rate

Saciation coefficient

Predators mortality rate

Conversion coefficient


Parameters range – Model 1

Model parameter Values Range References

Natives growth rate (rn) 0.1Wilson et al. 1991, Marañon and Grubb 1993, Hoffmann and poortman 2002

Exotics growth rate (re) 0.2 r2 < 0.3 Marañon and Grubb 1993, Hoffmann and poortman 2002Natives carrying capacity (Kn) 100.0

Exotics carrying capacity (Ke) 80.0 K2<100Competition coefficient (beta) 0.013 beta < 0.016 Levins and culver 1971, Hulbert 1978

Feeding efficiency (theta) 0.09 theta < 7 Wilson et al. 1991

Saciation coefficient (D) 20.0 6 < D < 45 Ben-Shahar and robinson 2001

Natives herbivory rate alfa n 0.30 Pacala y Tilman 1994

Exotics herbivory rate alfa e 0.7 alfa 2 > 0.33 Pacala y Tilman 1994

Predators mortality rate (mu) 0.03 0.013<mu<0.044

Anderson and ray 1980, Wilson et al. 1991, Chapman et al. 1998

Conversion coefficient (gamma) 0.06 Wilson et al. 1991


Parameters range – Model 2

Model parameter Values Range References

Natives growth rate (rn) 0.1Wilson et al. 1991, Marañon and Grubb 1993, Hoffmann and poortman 2002

Exotics growth rate (re) 0.2 Marañon and Grubb 1993, Hoffmann and poortman 2002

Carrying capacity (Kn) 100.0

Prop. seedling to adults (G) 0.01 G < 0.3

Natives herbivore rate alpha n 0.9 0.01 < alfa 1< 5 Pacala y Tilman 1994

Competition coefficient (beta) 0.01 beta < 0.3 Levins and culver 1971, Hulbert 1978

Natives mortality rate (mu) 0.03

Exotics mortality rate (mu) 0.07 0.05 <mu< 0.15

Seedlings to adults ratio for support 1.1 < 1.1

Predators mortality rate (mu) 0.028

Conversion coefficient (gamma) 0.2 0.15 <gamma < 0.5 Wilson et al. 1991

Feeding efficiency (theta) 0.09 0.01<theta<7 Wilson et al. 1991

Saciation coefficient (D) 100.0 Ben-Shahar and robinson 2001


Experimental observations• Results (Heard & Sax, 2012):


invasion by exotic species a possible mechanism that allows competitive coexistence between native...

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