Biotic Interactions: Competition, Keystone Species, and Symbiosis – Study Notes
Competition and Community Structure
Biotic factors are organisms' interactions that influence what different organisms live in which areas and the niches they occupy.
Competition acts as an organizing force determining where organisms can live (who gets resources first, who can persist).
Two kinds of competition:
Intraspecific competition: individuals within the same species compete for limited resources.
Interspecific competition: individuals from different species compete for the same resources.
Link to natural selection and evolution:
Not necessarily the absolute “strongest” survive; rather, those better at competing for resources pass on advantageous genes.
Over time, gene changes can spread in populations, helping explain species differences.
Intraspecific Competition
Territoriality as a classic form of intraspecific competition.
Example: Parrotfish (coral reef fish) with distinct male and female coloration.
Males typically defend territories that hold groups of females.
They chase away rivals to protect mating opportunities.
Sneaker males in wrasses (parrotfish family):
Sneaker males quietly enter a territory and mate with a female while the resident male is distracted.
Their genes can still be passed on, despite not holding the territory).
Sex change dynamics in wrasses:
If the dominant male is removed, the largest female often changes color within a couple of hours to signal the shift.
Within about 2\,\text{weeks}, she develops testes and functions as a male.
This sex-change ability is widespread among wrasses and related species (collectively called wrasses).
The dominance of the largest individual (male or female-to-male) determines access to territory and reproductive success.
Implication: territoriality and sex-change strategies illustrate how intraspecific competition shapes social structure and reproductive roles.
Interspecific Competition
Competition between different species can also drive interactions for space and resources.
Examples of competitive interactions for space:
Anemones: even though relatively sessile, they compete for space on rocks and for access to light and prey.
Corals and sponges: compete for space and sunlight; corals rely on symbiotic algae for energy, so shading and space are critical.
Competitive outcomes can be observed as “carved out” space where one species dominates due to competitive advantages (competitive exclusion).
Competitive exclusion (end result): when one species outcompetes another for a limiting resource, leading to the latter's decline or local extinction.
Resource partitioning (a complementary outcome):
Multiple species with similar resource needs divide resources to minimize direct competition.
Example: damselfish (small reef fish) occupy different microhabitats and dietary niches (e.g., hovering at different depths or feeding on different patches of algae).
Result: species coexist by using slightly different resources or habitats rather than directly competing for the exact same resource.
Keystone Species and Community Balance
Keystone species are not merely predators; they are one species whose presence maintains the structure of the community.
Analogy: a keystone in an arch—the arch would collapse without that single stone.
Important example: Pacific starfish (a starfish) – historically observed population declines (wasting disease) led to a dramatic shift in rocky intertidal communities.
After starfish decline, mussels aggressively dominated, overgrowing rocks and shading other organisms.
Recovery of starfish allowed patches of bare rock and a more diverse community to reappear (limiting mussel overgrowth).
Another keystone example: sea otter – helps regulate urchin populations and maintain kelp forest ecosystems.
Takeaway: removing a keystone species can destabilize communities and reduce biodiversity; protecting keystone species supports overall ecosystem health.
Symbiosis: Interactions that Bind Species
Definition: symbiosis is a relationship where two species live in close association, and at least one cannot live without the other.
Not all symbioses are equal; outcomes can be mutualistic (both benefit), commensal (one benefits, the other is largely unaffected), or parasitic (one benefits at the expense of the other).
Mutualism (plus-plus): both species benefit.
Commensalism: one species benefits, the other is not significantly affected.
Parasitism: one benefits (parasite) at the expense of the other (host); the host is not typically killed outright in the short term.
Mutualism
Classic example in marine systems: coral and their symbiotic algae (zooxanthellae).
Zooxanthellae: photosynthetic dinoflagellates living inside coral tissues.
Benefits to zooxanthellae: protection from predators and access to light by living in coral tissue.
Benefits to coral: receives sugars produced by the algae via photosynthesis, providing energy for growth and reef building.
Coral bleaching mechanism:
When water temperatures rise, zooxanthellae can die or be expelled from coral tissues.
Loss of algae reduces energy supply, leading to coral bleaching and potential coral mortality if stress persists.
Significance: mutualism is a major driver of reef productivity and reef-building capacity; temperature stress threatens this mutualism and reef health.
Commensalism
Clownfish and sea anemones: clownfish gain protection from predators due to the anemone's stings, while anemones are largely unaffected.
Clownfish have a mucus coating that reduces stinging; eggs may be protected by clownfish behavior.
Barnacles on large marine animals (e.g., turtles, whales): barnacles gain transport and access to feeding currents; hosts are generally not significantly harmed unless barnacle load is extreme.
Note: in some cases, heavy colonization can slow the host or increase drag, turning a commensal interaction into a more detrimental one, but classic examples emphasize the neutral effect on the host.
Parasitism
Parasitism is a symbiotic relationship where the parasite benefits and the host is harmed, but the host is typically not killed immediately; the parasite aims to keep the host alive for continued feeding.
Differences from predation: parasites do not usually kill the host outright; they exploit tissues, fluids, or nutrients over time.
Types of parasites:
External parasites: e.g., marine isopods that attach to fish and suck blood.
Internal parasites: e.g., worms living in the gut or tissues; absorb nutrients as they pass through.
Practical note: parasite loads influence host health and can affect behavior, growth, and survival; cooking and food safety considerations arise from parasite risks in seafood.
Activity Context: Predictions about Relationships
The lesson includes an activity where you predict the relationship between two organisms based on a picture before watching a short video clip.
Purpose: practice forming hypotheses, testing them with evidence, and understanding that correct answers are not the point of the exercise; the process matters.
Instructions reminders:
Do not skip ahead to see answers; start and stop at designated points to test understanding.
This is a scientific method exercise: generate predictions, then evaluate against observed footage.
Real-World Relevance and Connections
Foundational principles: competition and niche theory explain species distribution and community structure; natural selection drives adaptations that reduce or exploit competition.
Energy and trophic considerations: mutualisms like coral-zooxanthellae increase energy efficiency and reef-building capacity; these relationships influence ecosystem productivity and resilience.
Environmental stressors: temperature rise disrupts symbioses (e.g., coral-zooxanthellae), with cascading effects on reef biodiversity and coastal protection.
Conservation implications: protecting keystone species and maintaining habitat heterogeneity through resource partitioning can sustain resilient communities.
Practical applications: understanding symbiotic relationships informs fisheries management, reef restoration, and bioindicator programs.
Connections to Foundational Principles and Real-World Relevance
The material links to core ideas in ecology and evolution:
Niche concept and competition shape species distributions.
Natural selection favors traits that improve competitive success, including behavioral strategies (e.g., territoriality, sneaker mating) and physiological changes (e.g., sex change in wrasses).
Symbiosis illustrates co-evolution and energy transfer across species boundaries, with broad implications for ecosystem function.
The content also highlights critical ecosystem services and risks:
Keystone species maintain community balance and biodiversity.
Coral-algae mutualism underpins reef structure and productivity; bleaching links climate change to ecosystem health.
Human activities that affect temperature, pollution, or overfishing can disrupt these relationships with far-reaching consequences.
Quick Reference: Key Terms and Concepts
Competition: interactions over resources that influence where organisms live and how they reproduce.
Intraspecific competition: within-species competition for resources.
Interspecific competition: between-species competition for resources.
Territoriality: defense of a spatial area to secure mating opportunities and resources.
Sneaker males: alternative mating tactic where non-territorial males achieve paternity without holding a territory.
Sex change (reversal) in wrasses: largest female changes to male in response to social cues; coloration shifts within hours; gonads develop to produce sperm within about 2\,\text{weeks}.
Competitive exclusion: one species outcompetes another, leading to the latter’s decline.
Resource partitioning: coexisting species divide resources to reduce competition.
Keystone species: a single species whose removal destabilizes the ecosystem (e.g., Pacific starfish, sea otter).
Symbiosis: close, often long-term interaction between two species, with at least one unable to live without the other.
Mutualism: both species benefit (e.g., coral and zooxanthellae).
Zooxanthellae: symbiotic algae inside coral tissues; provide sugars to coral via photosynthesis.
Coral bleaching: loss of zooxanthellae due to heat stress, leading to pale or white corals and potential mortality.
Commensalism: one benefits, the other is largely unaffected (e.g., clownfish and anemones; barnacles on turtles/whales).
Parasitism: parasite benefits, host is harmed but not usually killed immediately (external and internal parasites).
Zooxanthellae and corals have a tightly linked energy dynamic critical for reef growth and resilience.