Eighth Grade Science: Predation, Competition, and Symbiosis
Within any biological community, no species lives in complete isolation. Organisms are bound together by a complex web of ecological interactions that dictate survival, reproduction, and population stability. These ecological relationships fall into three primary categories: predatory interactions, competitive struggles, and intimate symbiotic partnerships. By analyzing predation, niche differentiation, mutualism, commensalism, and parasitism, you can understand how species interactions structure natural ecosystems.
Predation: The Dynamics of Hunter and Hunted
Predation is an ecological interaction in which one organism—the predator—captures, kills, and consumes another living organism—the prey. Predation exerts strong selective pressure on both species, driving coevolution: predators evolve sharper teeth, enhanced sensory vision, and silent locomotion, while prey evolve camouflage, warning coloration, defensive spines, and speed.
Predator-Prey Population Oscillations
Population (N)
^ /\ /\ /\
| / \ PREY / \ / \
| / \ (Hare) / \ / \
| / \ / \ / \
| / \ / /\ \ / /\ \
| / /\ \ / / \ \ / / \ \
| / / \ \ / / \ \ / / \ \
+---+---+----+-----+--+---+------+----+---+------+----+----> Time
\ / \ / \ /
\ / PREDATOR \ / \ /
\/ (Lynx) \__/ \__/
(Predator population peaks SLIGHTLY AFTER prey population peaks!)
The Lotka-Volterra Oscillation Cycle
The population densities of predators and prey follow cyclic oscillations, famously documented in historical Canadian fur-trapping records of the snowshoe hare and the Canada lynx:
1. Prey Abundance: When prey populations expand, predators have abundant food; predator survival and birth rates climb.
2. Overpredation: As predator numbers swell, they overhunt the prey; prey mortality exceeds natality, causing a prey population crash.
3. Predator Decline: With prey scarce, predators starve; predator populations decline sharply.
4. Prey Recovery: With predatory pressure relieved, surviving prey reproduce rapidly, restarting the cycle.
Competition and the Ecological Niche
Competition occurs when two or more individual organisms or species attempt to utilize the same limited resource in an ecosystem, such as food, water, sunlight, nesting sites, or mates. Competition can occur between members of the same species (intraspecific competition) or between different species (interspecific competition).
+-----------------------------------+-----------------------------------+
| Fundamental Niche | Realized Niche |
+-----------------------------------+-----------------------------------+
| The full theoretical range of | The actual, restricted niche |
| environmental conditions and | space an organism occupies in the |
| resources a species COULD occupy | presence of interspecific |
| in the ABSENCE of competition. | competitors and predators. |
+-----------------------------------+-----------------------------------+
Gause's Competitive Exclusion Principle
In the 1930s, Russian biologist G.F. Gause formulated the Competitive Exclusion Principle: two distinct species competing for the exact same limiting resources cannot coexist indefinitely in the same ecological niche. The species with even a slight competitive advantage will outcompete and displace the other, driving it to local extinction or forcing it into a different niche.
Resource Partitioning in Warblers
Top of Conifer Tree Canopy: Cape May Warbler (Eats top canopy insects)
Middle Interior Branches: Black-Throated Green Warbler
Lower Base and Trunk Branches: Yellow-Rumped Warbler
Three species coexist peacefully by dividing the physical tree niche!
Resolving Competition: Resource Partitioning
To avoid competitive exclusion, competing species undergo resource partitioning—evolutionary adaptations that divide shared resources spatially, temporally, or anatomically. Robert MacArthur famously documented five species of warbler birds coexisting in the same spruce forest: each species fed on insects in a different vertical zone of the trees, allowing all five species to thrive without driving one another to extinction.
Symbiosis: Close and Prolonged Biological Partnerships
Symbiosis (meaning "living together") refers to an intimate, long-term biological interaction between two different species. Symbiotic relationships are classified into three primary forms based on whether each participating species benefits, is harmed, or remains unaffected.
+-----------------------+-----------------------+-----------------------+
| Symbiotic Form | Species 1 Effect | Species 2 Effect |
+-----------------------+-----------------------+-----------------------+
| Mutualism (+ / +) | BENEFITS (+) | BENEFITS (+) |
| Commensalism (+ / 0) | BENEFITS (+) | UNAFFECTED (0) |
| Parasitism (+ / -) | BENEFITS (+) | HARMED / DAMAGED (-) |
+-----------------------+-----------------------+-----------------------+
The Three Symbiotic Archetypes
MUTUALISM (+ / +): Clownfish and Sea Anemone
Clownfish gets protective stinging defense;
Anemone gets food scraps and aeration!
COMMENSALISM (+ / 0): Barnacles on a Humpback Whale
Barnacles get mobile access to plankton-rich waters;
Whale is unharmed and unaffected!
PARASITISM (+ / -): Tick on a White-Tailed Deer
Tick feeds on blood and completes lifecycle;
Deer suffers blood loss and disease transmission!
Mutualism: Reciprocal Benefits ($+/+$)
In a mutualistic partnership, both interacting species derive survival or reproductive benefits. A classic terrestrial example is the relationship between flowering plants and pollinators (bees, hummingbirds). The plant rewards the pollinator with nectar (carbohydrate energy), while the pollinator transfers pollen grains to other flowers, facilitating plant cross-fertilization. Another vital mutualism is mycorrhizal fungi living on plant roots: the fungal hyphae absorb water and soil phosphorus for the plant, while the plant supplies the fungus with sugars produced via photosynthesis.
Commensalism: One-Sided Benefit Without Harm ($+/0$)
In commensalism, one organism benefits while the other organism is neither helped nor harmed. Epiphytic orchids growing high on the bark of tall rainforest trees benefit from accessing sunlight above the dim forest floor, while the massive tree is unaffected by the lightweight presence of the orchid roots.
Parasitism: Exploitation and Harm ($+/-$)
In parasitism, one organism (the parasite) lives on or inside another organism (the host), deriving nutrients and metabolic energy at the host's direct expense. Unlike predators, parasites typically do not kill their host immediately, as doing so would destroy their own food source and habitat before they can reproduce. Examples include endoparasites (tapeworms, malaria-causing Plasmodium) and ectoparasites (fleas, ticks, leeches).
Chapter Exercises
Exercise 1: Classify each of the following four ecological relationships as mutualism, commensalism, parasitism, or predation: first, a remora fish attaching to a shark to eat leftover food scraps without affecting the shark; second, nitrogen-fixing rhizobia bacteria living inside legume root nodules, trading usable nitrates for plant sugars; third, a tapeworm living in the intestines of a mammal; fourth, an osprey diving into a river to catch and consume a trout.
Exercise 2: Explain Gause's Competitive Exclusion Principle. Describe how resource partitioning allows two species of predatory owls to hunt in the same forest without driving either species to extinction.
Exercise 3: Explain why it is evolutionary disadvantageous for a parasite to kill its host rapidly, comparing the ecological survival strategy of a parasite with that of a carnivore predator.
Solutions and Explanations
Solution 1: The remora fish attaching to the shark is commensalism ($+/0$): the remora benefits by gaining effortless transportation and feeding on discarded food scraps, while the shark is unharmed and receives no measurable benefit. The nitrogen-fixing bacteria living in legume roots is mutualism ($+/+$): both organisms benefit, as the bacteria convert atmospheric nitrogen into usable nitrates for the plant, and the plant supplies the bacteria with carbohydrates manufactured through photosynthesis. The tapeworm inside the mammal's intestine is parasitism ($+/-$): the tapeworm benefits by absorbing digested nutrients, while the host mammal is harmed through malnutrition, abdominal pain, and weight loss. The osprey catching a trout is predation: the osprey acts as a predator that captures, kills, and consumes the prey fish for food.
Solution 2: Gause's Competitive Exclusion Principle states that two distinct species competing for the exact same limiting resources cannot coexist indefinitely in identical ecological niches; the species with greater competitive fitness will outcompete and displace the other. Resource partitioning resolves this competitive pressure by dividing the niche so that the two species do not compete for the exact same resource at the same time or place. For example, two owl species in the same forest can practice temporal resource partitioning: one species (the Great Horned Owl) hunts at dusk and during the first hours of darkness for large rodents, while a second species (the Barn Owl) hunts during midnight hours for small mice. By hunting at different times or targeting different prey sizes, both owl species occupy distinct realized niches and coexist stably.
Solution 3: It is evolutionary disadvantageous for a parasite to kill its host rapidly because the host represents the parasite's continuous source of nourishment, shelter, and reproductive habitat. If a parasite kills its host prematurely, the parasite loses its food supply and dies before its eggs or larvae can mature and transmit to a new host. Natural selection favors parasites that exploit host resources moderately, keeping the host alive long enough to maximize parasite reproduction. In contrast, a predatory carnivore kills its prey immediately upon capture because the prey serves solely as a direct nutritional meal to be consumed, with no ongoing dependency on the prey's continued living survival.