Community Ecology: Mutualism, Trophic Levels, and Keystone Species
The Coral-Zooxanthellae Mutualism
Photosynthetic Pigments: Corals possess pigments that allow for photosynthesis to occur, providing the majority of the energy required by the corals.
Benefits to the Zooxanthellae (The Algae):
Protection: By living inside the coral, which is a much larger multicellular organism, the zooxanthellae gain a physical refuge.
Defense: Corals are related to jellies and possess stinging cells on their tentacles, providing a well-protected environment against potential attackers.
Access to Nutrients: Corals capture and consume other organisms to obtain essential nutrients such as Nitrogen (), Phosphorus (), and Calcium (). These nutrients are shared with the zooxanthellae.
Photosynthetic Requirements: To perform photosynthesis, zooxanthellae require water (readily available in the ocean) and Carbon Dioxide ().
Carbon Dioxide Supply: The coral generates as a waste product of cellular respiration. As the coral animal breaks down sugars to obtain energy molecules, it releases , providing a critical raw material back to the zooxanthellae for photosynthesis.
Costs and Risks in Coral Mutualism
Dependency Risks: Corals are highly dependent on zooxanthellae as a primary energy source.
Energy Contribution: Depending on the species, corals may receive up to of their energy from zooxanthellae. Losing these algae is equivalent to a cut in a human's caloric intake, which drastically reduces the chances of survival and reproduction.
Environmental Stress and Reactive Oxygen Species:
As oceans become warmer and more acidic, the thylakoids or photosystems in the chloroplasts of the zooxanthellae overwork.
This stress causes the zooxanthellae to produce Reactive Oxygen Species, which are stressed molecules that are chemically damaging to coral tissues.
The Bleaching Mechanism: When the coral senses the production of these toxic reactive oxygen species, it literally ejects or spits out the zooxanthellae from its cells to protect its own tissue. This is a major cost of the association; while the algae provide benefits, their stress response is toxic to the host.
Costs to Zooxanthellae: If ejected, the zooxanthellae lose the protection of the coral body, lowering their chances of survival and reproduction.
Opportunity Costs: Zooxanthellae "pay" for the mutualism by giving up a massive portion of the sugars they produce. If they were living independently, they could use all that energy for their own growth and reproduction.
Classifications of Mutualism
Obligate Mutualism:
Derived from the root word "obligation."
In this type of relationship, both players depend on each other for survival and reproduction.
Corals and zooxanthellae are often considered an obligate mutualism because the large-scale loss of the partner (bleaching) typically leads to the death of the coral and significantly lower survival rates for the algae.
Facultative Mutualism:
A less dependent interaction where both species benefit when together, but both can exist independently if the partnership breaks down.
Example: Ants and Aphids:
Aphids drink plant fluids and excrete excess sugar water (honeydew).
Ants consume this sugar water as a major energy source.
In return, ants "tend" to the aphids like livestock or pets, defending them from predators.
If the ants leave, the aphids can still survive (though at higher risk of predation), and if the aphids leave, the ants can find other food sources.
Mixed Mutualism:
Occurs when the level of dependence differs between the two species.
Example: Hummingbirds and Flowers:
For the Flower (Obligate): If a specific flower species can only be pollinated by a specific hummingbird, it cannot reproduce without that bird. The interaction is obligate for the flower.
For the Hummingbird (Facultative): The hummingbird gets nectar from the flower but can easily fly to other flower species or consume insects to survive. The interaction is facultative for the bird.
Other Species Interactions: Commensalism and Amensalism
Commensalism (): An interaction where one species benefits while the other experiences no meaningful effect.
Example: Gila Woodpeckers and Elf Owls:
The Gila woodpecker pecks holes in cacti to build nests and raise young. Once finished, they move on and do not return to the same nest.
The Elf owl cannot make its own holes but moves into the abandoned woodpecker nests to raise its own young.
The owl benefits (), but the woodpecker is unaffected () because it doesn't use the nest anymore and the owl doesn't compete for resources or provide services.
Amensalism (): An interaction where one species is harmed while the other experiences no effect.
Example: Wild Boar and Grasses:
Wild boars forage in grassy fields for grubs and insect larvae.
As they move, they rip up and kill patches of grass.
The grass is harmed (), but because the boar is not eating the grass (it is looking for insects), the grass has no direct effect on the boar ().
Amensalism is most common when there is a significant size difference between species; a larger organism harms a smaller one simply by moving through the environment.
Community Ecology and Trophic Levels
Trophic Levels: Defined as the "levels of feeding" in a community.
Producers (): Organisms that make their own food via photosynthesis (e.g., plants, algae, cyanobacteria). They create biomass without eating other organisms.
Primary Consumers ( Consumers): Also known as herbivores; these organisms consume producers.
Secondary Consumers ( Consumers): Predators that feed on primary consumers.
Tertiary Consumers ( Consumers): Predators that feed on other predators.
Decomposers: Organisms (bacteria, fungi, beetles) that feed on dead organic material from all trophic levels, returning biomass to the soil or sediment. They are difficult to slot into a single level because they consume everything.
Challenges in Classification: Parasites (like ticks) and viruses also feed across multiple trophic levels, making simple "chains" more complex.
Trophic Cascades and Indirect Interactions
Trophic Cascade: An indirect positive effect of a predator on a producer that occurs when the predator limits the population of an herbivore.
Direct vs. Indirect: A direct interaction involves two species physically interacting (e.g., predator eating prey). An indirect interaction (represented by dashed lines in diagrams) is the effect of one species on another mediated by a third species.
Example: Yellowstone National Park:
Trophic Chain: Wolves (Predator) Elk (Herbivore) Cottonwoods/Aspen/Willow (Producers).
The Cascade: When wolves were reintroduced in the mid-, they consumed elk. The decline in elk populations reduced the amount of "browsing" (herbivory).
Result: The height and number of trees (Aspen, Cottonwood, Willow) increased dramatically. The wolves had an indirect benefit () on the trees by removing their "enemy" (the elk).
Keystone Species
Definition: A species that has a disproportionately large impact on its community relative to its low abundance. It is central to the functioning of the entire ecosystem.
Example: Wolves: In Yellowstone, just a few dozen wolves transformed the landscape of thousands of acres.
Example: Sea Stars (Pisaster):
In rocky intertidal communities, the sea star Pisaster acts as a keystone species.
When Pisaster is present, community diversity is high (over species).
When Pisaster is experimentally removed, the number of species drops dramatically because mussels (their prey) are allowed to overgrow and competitively exclude almost all other species.
Questions & Discussion
Question: Regarding the coral video, what was the negative effect the zooxanthellae had on the corals?
Response: Because oceans are warmer and more acidic, the chloroplasts in the zooxanthellae overwork and produce reactive oxygen species. These are toxic chemicals that damage coral tissue, leading the coral to eject the algae (bleaching).
Question: Is the number of trophic levels limited?
Response: Yes, there is a limit to how many levels can be stacked, which will be discussed later in the week.
Question: Is the trophic level basically based on the food chain?
Response: Yes, it is based on how energy passes from one level to the next. While a "food chain" is a simplification, it is a useful way to track energy movement through a community.