Ch. 9 Flashcards
Mutualism
a reciprocally positive interaction between species (Bronstein, 2009).
Examples of facilitation
Nitrogen-fixing plants that release nitrogen into the soil that benefits non-nitrogen-fixing neighbors; salt marsh plants that provide shade and reduce evapotranspiration, thus reducing soil salinity and allowing less salt-tolerant species to survive.
Facilitation
s an interaction in which the presence of one species alters the environment in a way that enhances growth, survival, or reproduction of a second, neighboring species (Bronstein, 2009).
Foundational species
generally provide modified environments so that whole communities can develop within the environment.
provide structural habitat for other species
Ecosystem Engineers
modify the environment in a way that makes it more or less suitable for other species.
symbiosis
An intimate and persistent association between two species.
Could be positive or negative, and can exist on a continuum. Some debate about the definition in terms of whether or not it includes both positive and negative relationships.
For the purposes of this chapter we will only examine positive symbioses
Most mutualisms depend on
costs/benefits of the relationship.
Members of each species exchange resources or services, but
are vulnerable to exploitation.
How is cheating by one species or the other minimized?
Partner choice, partner fidelity
what is the modificed lotka-volterra model for two-species interactions
Used Lotka-Volterra model of two-species interactions; changed the sign of the interactions from negative to positive.
Species 1 equation: dN1/dt = r1N1 {K1 – N1 + α12 N2 / K1}
Species 2 equation: dN2/dt = r2N2 {K2 – N2 + α21 N1 / K2}
partner Choice - Example
Simms et al. 2006 found the yellow bush lupine preferentially allocates resources (makes larger nodules) to strains of Rhizobia bacterial strains (live in nodules on roots) that provided the greatest benefits to the plants.
When interaction coefficients are positive
both species together attain larger biomasses than separately, and a “stable knot” (equilibrium point) occurs.
As α and β increase
the equilibrium point continuously moves up and rightwards and finally passes into infinity.
Gause and Witt showed that
considering mutualisms within the L-V framework is unrealistic
For stable species coexistence to occur,
the positive effects of mutualism must be balanced by negative density dependence in some other area.
Examples of negative density dependence that could limit population growth:
Strong intraspecific competition (competition within species)
Feedbacks from other life stages (i.e. nurse plants benefit seedlings, but not adults)
Fluctuating environments (sometimes good for participating mutualistic species, sometimes bad)
Competition from other members within the community
Nurse plants
species whose canopies provide protective microclimate promoting the germination and growth of other species
more recently models have shown that mutualisms can evolve from
negative relationships like parasitism and competition.
This analysis (that the L-V elicits an unrealistic outcome), may be why beneficial interactions have been ignored in ecological theory.
??????
Mutualisms can be context dependent
One example: mycorrhizal fungi & land plants
In this relationship:
Plants deliver carbon compounds to their associated fungi
Fungi deliver mineral nutrients (commonly phosphorous, sometimes nitrogen)
In nutrient-poor soils plants may obtain up to 80% of their requirements for nitrogen and up to 90% of phosphorous from their fungi.
BUT, this is context dependent and may shift when in nutrient-rich soils. No benefit to mineral provisions to plants, and plant growth may be slightly inhibited due to carbon demands of the fungus.
A biotic example of context-dependent mutualisms
Ant-acacia mutualism
Acacia tree provides a nursery for ant broods. Ants will raise broods in the tree’s specialized thorns and are nourished by the tree’s extrafloral nectaries and Beltian bodies.
see slides
Mutualistic interactions often depend on
the inclusion of other species, which can influence positive/negative effects of the interaction.
Both biotic and abiotic factors can influence
the interaction between species to result in a beneficial or antagonistic relationship.
changes from positive to negative interactions can be altered across
life stages and environmental conditions
see slide
Mutualisms are extremely common in nature, but
much less work has been done to understand the complexities of their existence.
Mutualisms can involve complex, multi-species interactions creating
ecological networks
Gause & Witt (1935) utilized the Lotka-Volterra interspecific competition model and simply switched
the negative interaction to positive ones. But, the outcome seemed biologically unrealistic because it lead to an equilibrium point that would migrate upward.
Gause & Witt (1935) All likely because trade-offs (particularly negative density-dependent effects)
were not taken into account in the model.
Johnson (2010) developed a conceptual model to account for
trade-offs & the ant-acacia model is an example of a context-dependent mutualism.
Palmer (2008) found
the ant-acadia model is dependent upon herbivore presence.
minor differences in biotic or abiotic contexts can significantly alter
the mutualism context, leading to the evolution of antagonistic relationships.
