integrated lec 21 Flashcards
Symbiosis vs. Mutualism
Symbiosis: “Living together,” a close physical association between species.
Mutualism: A specific type of symbiosis where both species benefit.
Note: Not all symbioses are mutualistic.
Types of Mutualisms
Reciprocal Exchange of Goods or Services: Mutualisms often involve the trade of resources or services:
- Nutritional Mutualisms:
Legumes and Rhizobia: Rhizobia fix nitrogen for plants, which in turn provide carbon compounds.
Plants and Mycorrhizal Fungi: Fungi supply phosphorus in exchange for carbon. - Defensive Mutualisms:
Ants and Plants: Ants protect plants in return for food (extrafloral nectar) or housing.
Cleaner Fish and Client Fish: Cleaner fish remove parasites from clients, gaining food while clients benefit from parasite removal. - Dispersal Mutualisms:
Plants and Animal Pollinators: Animals disperse pollen in exchange for nectar.
Plants and Seed Dispersers: Seeds are spread in return for food.
Population Dynamics of Mutualism
Based on the Lotka-Volterra framework but with adjustments for positive interactions.
Problems in Models:
Unbounded exponential growth occurs in simple models due to mutual benefaction.
Limiting Factors:
-Intraspecific Competition: Overcrowding within a species limits growth.
-Third-Species Effects: Predators or competitors regulate population.
-Diminishing Returns: Benefits of mutualism decrease as population density increases.
How can strong intra-specific competition limit unbounded growth in mutualistic populations?
Strong intra-specific competition prevents unbounded growth in mutualistic populations by balancing the benefits of mutualism with density-dependent constraints.
Mechanisms:
- Resource Limitation:
As population size increases, individuals compete for limited resources (e.g., food, space), reducing growth rates. - Diminishing Returns of Mutualism:
At high population densities, the benefits of mutualism (e.g., resource sharing, protection) become less effective per individual. - Self-Regulation:
Overcrowding leads to competition for mutualistic services (e.g., pollinator access), slowing population growth. - Density-Dependent Feedback:
Increased competition at higher densities reduces individual fitness, preventing runaway growth.
Takeaway: Intra-specific competition acts as a natural check on population growth, ensuring mutualistic benefits are balanced by resource limitations and density-dependent feedback
Invasional Meltdown
Definition: Positive feedback between invasive mutualists accelerates their spread and impact.
Example:
Native ant (Aphaenogaster rudis) supports native plants, but invasive ant (Myrmica rubra) spreads invasive plants.
Experiment: With invasive ants, invasive plants dominate experimental plots.
Mutualism and community structure
Cleaner Fish Impact on Reefs:
Labroides dimidiatus maintains fish diversity by reducing parasite loads.
Evidence: Removing cleaner fish increases parasite abundance and decreases reef fish diversity.
Long-term studies demonstrate positive effects on reef ecosystems
Darwin’s Orchid and Reciprocal Adaptation
Angraecum sesquipedale: Orchid with a long nectar spur predicted by Darwin to be pollinated by a moth with an equally long proboscis.
Evidence of Coevolution:
Moth (Xanthopan morganii praedicta) discovered later confirmed Darwin’s prediction.
Reciprocal adaptation exemplifies the process of coevolution.
Mutualistic Networks
Generalists vs. Specialists:
Generalists: Many partners, widespread interactions.
Specialists: Few partners, often highly coevolved.
Pollination Networks: Most plants are visited by multiple pollinators, and pollinators visit multiple plants.
Key Insights:
Mutualistic networks are complex and not always one-to-one.
Understanding these networks is critical for conservation.
How do mutualistic networks differ between generalist and specialist species?
Generalist and specialist species play distinct roles in mutualistic networks, shaping their structure and stability.
Generalist Species:
Characteristics:
Interact with many partners.
Form the core of mutualistic networks.
Role in Networks:
Enhance network connectivity.
Increase network robustness to species loss.
Example: Bees pollinating a wide variety of plants.
Specialist Species:
Characteristics:
Interact with a few or one specific partner.
Often depend on generalists for survival.
Role in Networks:
Add diversity and unique interactions.
Are more vulnerable to partner loss.
Example: Fig trees and fig wasps.
Takeaway: Generalists increase network stability by connecting species, while specialists add unique, vulnerable links that enrich biodiversity.
Microbiomes
Definition: Microbiomes are communities of microorganisms living in or on hosts, influencing traits like metabolism and immunity.
Research Insights:
Gut microbiomes reflect diet, phylogeny, and morphology.
Advances in sequencing (e.g., 16S rRNA) allow microbiome analysis without culturing.
Humans rely on their microbiomes for health and disease resistance.
Endosymbiosis Example:
Aphids and Buchnera:
Aphids rely on Buchnera bacteria for essential amino acids.
Buchnera are vertically transmitted and have tiny genomes due to gene loss and outsourcing to the host.
What are nutritional mutualisms?
A: Mutualisms where species exchange nutrients, e.g., legumes and rhizobia (C for N).
What are dispersal mutualisms?
A: Mutualisms involving the movement of seeds or gametes, e.g., pollination by bees.
How does mutualism affect population dynamics?
A: It can lead to unbounded growth in simple models unless regulated by competition, predators, or diminishing returns
What prevents mutualist populations from growing indefinitely?
A: Factors like strong intra-specific competition, third-species effects, and diminishing benefits.
Why are simple Lotka-Volterra models for mutualism unrealistic?
A: They predict exponential growth without limitations, which is biologically implausible.
