BIOEE1610 Mutualisms

Spatial and temporal variation in the strength and/or outcome of mutualism that can be attributed to the local environmental context; also referred to as conditionality.

Mutually beneficial interactions among individuals of the same species, often involving social interactions such as foraging or parental care.

Modification of some component of the abiotic or biotic environment by one organism that enhances colonization, recruitment, and establish- ment of another.

A mutualism that increases an organism’s success but that is not absolutely re- quired for its survival and/or reproduction.

A mutualism without which an organism will fail to survive and/or reproduce.

An interaction (positive, negative, or neu- tral) in which two species exist in intimate physical association for most or all of their lifetimes and are physiologically dependent on each other.

• Almost all species on earth partake in mutualism in one way or another, directly or indirectly.
• Influences of mutualism transcend levels of biological organization from cells to populations to communities to ecosystems.
• Mutualism is thought to have been key to the origin of eukaryotic cells.
• Mutualisms are crucial to the reproduction and survival of many organisms as well as nutrient cycles, leading to them being of increasing conservation priority.

Organisms offer their mutualist commodities that are relatively cheap to acquire or produce in exchange for commodities that would either be very difficult or impossible to acquire for them.

• Transportation of either the partner itself or of its gametes.
• Protection of the partner from the biotic or abiotic environment.
• The provision to the partner of limiting nutrients.

Rhizobium bacteria found in nodules on the roots of many legume (bean) species fix atmospheric nitrogen into NH3, a form that can be easily taken up by plants. In return, the bacteria receive carbon fixed by photosynthesis by their hosts. Conse- quently, legumes can thrive in nitrogen-poor environ- ments such as deserts where few other plant species can persist.

• Exchanged benefits
• Evolutionary origins (some seem to have been derived from predatory relations where the prey developed traits that benefit the predator so much that it was more advantageous to keep the prey rather than to fully consume it.)
• Indirectness (intermediate species) / Directness (aphid/ant interactions are indirect as the aphid only benefits from the interaction through the alteration of the behavior of aphid predators through ant defense)
• Degree of specificity (species specific where only one species derives benefits vs. generalized mutualisms where an array of species derive benefits)
• Level of dependency (facultative vs. obligate, this can differ between partners as well)
• Symbiotic (vast majority are nutritional) vs. not symbiotic

• They have costs: Mutualisms require a certain amount of investment which may either be fixed or varied according to the extent of partner presence. Natural selection thus operates on not only the benefits, but the costs as well. (to minimize the costs)
• In the act of conferring benefits on their partners, mutualists may cause collateral damage. (for instance, insects laying eggs on the plants they’re pollinating, wasps and fig trees) Somewhat more abstract costs include the species’ inability to survive without the presence of the mutualist (cold weather and fig trees for instance)
• Outcomes of mutualisms are context dependent and highly variable in space and time. In some cases, species interactions can differ all the way from commensalism to antagonism based on the context.
• Mutualisms are often beset with cheaters that take advantage of the rewards without conferring benefits in return. Can mutualisms evolve to minimize the effect of such exploitation? (Yes! Aci biber still gets equal seed dispersal from birds but doesn’t get eaten by rats)

• Abundance and prevalence of predators and competitors. (ants provide protection to multiple plants from herbivores. However, if herbivores aren’t present, this is more costly to the plant than beneficial.)
• The supply of resources such as nutrients.
• The density and distribution of mutualists.
• The size, stage or age classes of the interacting species.

• Without securing the well-being of the endangered species’ mutuliasts, conservation efforts may be in vain. Yet, at this point, remarkably little is known about either the processes that disrupt mutualisms or how easily they may be reassembled. The cascade of altered interactions as a result of the disturbance of one mutualistic interaction can scale up to entire communities.
• Example: Habitat fragmentation leading to population decrease of one species may decimate its mutualists. Loss of native bee populations due to forest fragmentation in Argentina has reduced seed production in ¾ of plant species.
• Example: Apart from the loss of mutualists, the addition of new species can be equally problematic. Invasive species can outcompete and replace native mutualists, often to the detriment of their partners. However, not all invasive species have detrimental effects. Additionally, invasive mutualists can replace native mutualists who have been driven to extinction to prevent a similar fate for their partners.

There is generally a direct correlation between traits.

• Improved fitness
• Improved population growth
• Persistence (across ecosystems)

• Facultative
• Obligate
• Note: there’s generally an asymmetry in the association levels between mutualistic organisms.

• Cleaning mutualisms: Oxpeckers: obligate for the bird but not the mammals. Typically eating ectoparasites.
• Defensive mutualisms: (mostly facultative) Species receive food or shelter from another species in return for defending partners against herbivores, predators or parasites. Example: aphids (produce sugar rich honeydew) and ants (eat honeydew and provide protection from predators and clean away microbial infections. Ants impact the abundance and distribution of aphids.)

• By-product: Not costly. A benefit that comes from the regular activities of the partner (aphid and ant, mixed species flocks)
• Investment: Costly product or service for the partner, typically not needed for self. (nectar produced by star orchid, nitrogen fixed by rhizobia)
• Purloin (costly product taken / stolen from partner: (plant pollen that is fed to bee larvae)

• Interaction is facultative
• Interaction is indirect (has multiple partners)
• Density / spatial distribution of partners matters
• Note: Conditions that flip the sign of an interaction often involve variation in the level of resources.