Download - Invasion by e xotic species
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
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Invasions
Native bumblebee Exotic bumblebee
Native plants
Resources consumption
Disease transmission
-
-+“Steals” nectar
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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.
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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.
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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
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Predator hypothesis
•Could predators act as a mechanism promoting equilibrium?
•Two competitive preys one predator.
•Trade-off: competitive ability X susceptibility to predation?
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Study system• Estuarial plant communities in New England• Similar native and exotic plants• Herbivory by insects
Native species
Exotic species
(Heard &Sax, 2012)
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Dynamics – Model 1
Herbivores
Nativeplants
Exoticplants
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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
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First model
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No predators
Coexistence
No exotics
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Population size
Natives PredatorsExotics
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Dynamics – Model 2
Herbivores
Nativeseedlings
Exoticseedlings
Nativeadults
Exoticadults
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Second modelII Southern-Summer School on Mathematical Biology
No predators No exotics
No natives Coexistence
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Robustness of the modelsII Southern-Summer School on Mathematical Biology
Comparison between models
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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.
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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.
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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
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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
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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
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Experimental observations• Results (Heard & Sax, 2012):
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