Biological Communities and Interspecific Interactions: Exhaustive Study Guide

Biological Communities and Interspecific Interactions

• Biological Community Definition: A biological community is defined as an assemblage of populations consisting of various species that live in close enough proximity to one another for potential interaction.
• Interactions and Benefits: Some interactions within a community are beneficial to both species involved.     * Named Example: The bluestreak cleaner wrasse and the moray eel. The wrasse swims inside the mouth of the moray eel and consumes tiny parasites located there, benefiting from a food source while the eel benefits from parasite removal.
• Classification of Community Interactions: Ecologists classify relationships between species in a community as interspecific interactions (comparatively, intraspecific interactions occur between members of the same species).     * Functional Categories: These interactions include competition, predation, herbivory, parasitism, mutualism, and commensalism.     * Effects on Fitness: Interspecific interactions can significantly affect the survival and reproduction of each species involved. These effects are summarized using symbols:         * Positive (+): The species benefits.         * Negative (–): The species is harmed.         * No Effect (0): The species is neither helped nor harmed.

Competition

• General Definition: A competition (–/– interaction) occurs when individuals of different species compete for a resource that limits their survival and reproduction.
• Necessary Conditions: For competition to occur, resources must be in short supply.
• Documented Examples:     * Weeds growing in a garden competing with crop plants for soil nutrients and water.     * Lynx and foxes competing for the same prey source, such as snowshoe hares.
• Competitive Exclusion: Strong competition can lead to competitive exclusion, which is the local elimination of a competing species.     * G.F. Gause’s Principle: The Russian ecologist G.F. Gause concluded that two species competing for the exact same limiting resources cannot coexist permanently in the same place.     * Paramecium Study: When two species of paramecium are cultured together, they compete for resources. Ultimately, only one species will survive—the one with the competitive edge.

Ecological Niches and Resource Partitioning

• Ecological Niche Definition: An ecological niche is the sum of an organism's use of biotic and abiotic resources in its environment. It is often described as the organism's "ecological role."
• Niche Differentiation for Coexistence: Ecologically similar species can coexist in a community if there are one or more significant differences in their respective niches.
• Resource Partitioning: This refers to the differentiation of ecological niches that enables similar species to coexist.     * Spatial Partitioning: Utilizing different physical spaces in the environment.     * Temporal Partitioning: Utilizing the same resources at different times.         * Temporal Example: The golden spiny mouse and the common spiny mouse. Both are naturally nocturnal when not competing. However, the golden spiny mouse becomes diurnal (active during the day) when competing with the common spiny mouse.     * Spatial Example (Anolis Lizards): Various species of the genus Anolis partition their habitats to coexist.         * A. distichus perches on fence posts and other sunny surfaces.         * A. insolitus usually perches on shady branches.         * Other related species include A. ricordii, A. aliniger, A. christophei, A. cybotes, and A. etheridgei.
• Fundamental vs. Realized Niches:     * Fundamental Niche: The niche potentially occupied by a species in the absence of competition.     * Realized Niche: The niche actually occupied by a species in its natural environment.     * Relationship: As a result of competition, a species' fundamental niche may differ significantly from its realized niche.     * Barnacle Experiment (Connell):         * Species: Chthamalus (A) and Balanus (B).         * Observation: Chthamalus is found in the high tide zone, while Balanus is in the low tide zone.         * Manipulation: When Balanus was removed, Chthamalus spread into the lower tidewater area.         * Conclusion: Balanus limits the realized niche of Chthamalus. The fundamental niche of Chthamalus includes both high and low tide zones.

Character Displacement

• Definition: Character displacement is the tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species.     * Sympatric: Geographically overlapping species.     * Allopatric: Geographically separate populations.
• Example (Galápagos Finches): Variations in beak size between G. fuliginosa and G. fortis.     * In allopatric populations (e.g., Los Hermanos for G. fuliginosa and Daphne for G. fortis), beak depths are nearly identical, around $10\,mm$.     * In sympatric populations (e.g., Floreana and San Cristóbal), beak depths diverge: G. fuliginosa evolves a shallower beak (average $8\,mm$) and G. fortis evolves a deeper beak (average $12\,mm$ to $14\,mm$) to utilize different food sources.

Exploitation

• General Definition: Exploitation refers to any (+/– interaction) in which one species benefits by feeding on the other species. It encompasses predation, herbivory, and parasitism.

Predation

• Interaction Definition: Predation (+/– interaction) is an interaction in which one species, the predator, kills and eats another, the prey.
• Predator Adaptations: Adaptations for finding, identifying, catching, and subduing prey include fangs, claws, help from venom/poison, and acute senses.
• Prey Adaptations and Defenses:     * Behavioral Defenses: Hiding, fleeing, and forming herds or schools.     * Mechanical Defenses: Physical structures such as the quills of a porcupine.     * Chemical Defenses: Secretions such as those produced by a skunk.
• Coloration Strategies:     * Aposematic Coloration: Bright warning coloration exhibited by animals with effective chemical defenses. Predators are particularly cautious around such prey. Example: Poison dart frog.     * Cryptic Coloration: Also known as camouflage, this makes prey difficult to spot in their environment. Example: Canyon tree frog.
• Mimicry:     * Batesian Mimicry: A palatable or harmless species mimics an unpalatable or harmful model. Example: The nonvenomous hawkmoth larva mimics the appearance of the venomous green parrot snake.     * Müllerian Mimicry: Two or more unpalatable species resemble each other, providing a collective defense. Example: The cuckoo bee and the yellow jacket. Another example is the Viceroy butterfly and the Monarch butterfly, both of which are toxic and resemble each other.     * Predator Mimicry: Mimicry evolved in predators to approach prey undetected. Example: The mimic octopus can assume the appearance and movement of over a dozen marine animals, including a sea snake, a flounder, and a stingray.

Herbivory

• Interaction Definition: Herbivory (+/– interaction) involves an herbivore eating parts of a plant or alga.
• Diversity of Herbivores: While large mammals are familiar herbivores, the majority of herbivores are actually invertebrates.
• Adaptations:     * Herbivore Adaptations: Specialized teeth for grinding vegetation or complex digestive systems for processing plant matter.     * Plant Defenses: Plants produce mechanical defenses (spines, thorns) or chemical defenses (toxic or distasteful chemicals).

Parasitism

• Interaction Definition: In parasitism (+/– interaction), the parasite derives nourishment from the host, which is harmed but usually not killed immediately.
• Types of Parasites:     * Endoparasites: Parasites that live within the body of their host.     * Ectoparasites: Parasites that live on the external surface of their host.
• Characteristics and Impacts:     * Many parasites have complex life cycles involving multiple intermediate hosts.     * Some parasites can manipulate host behavior to increase the likelihood of transmission to the next host.     * Parasites significantly affect the survival, reproduction, and population density of their hosts across the community.

Positive Interactions

• General Definition: Interactions where at least one species benefits and neither is harmed. These interactions include mutualism and commensalism.
• Mutualism (+/+):     * An interspecific interaction that benefits both species.     * In mutualism, both species incur costs, but the benefits to each partner must exceed those costs.     * Degree of Dependency: Some species are obligate mutualists (depend on each other for survival/reproduction), while others are facultative (can survive alone).     * Example (Ants and Acacia): Stinging ants of the genus Pseudomyrmex live in the hollow thorns of acacia trees. The ants clear the area around the base of the tree and protect it from herbivores; in return, the tree provides shelter and food for the ants.
• Commensalism (+/0):     * An interaction where one species benefits and the other is unaffected.     * Example (Wildflowers): Shade-tolerant wildflowers depend on the shade provided by forest trees, but the trees are unaffected by the presence of the flowers.     * Example (Cattle Egrets): Cattle egrets benefit by eating insects flushed out of the grass by grazing bison. Typically, the bison is unaffected.     * Shift in interaction: Commensalism can become mutualistic if the egrets begin removing and eating ectoparasites from the bison's skin.

Community Structure and Foundation Species

• Facilitation and Diversity: Positive interactions can heavily influence the structure of ecological communities.
• Example (Black Rush - Juncus): The black rush affects community diversity by making soil more hospitable for other plant species.
• Statistics/Quantitative Data:     * In salt marshes, the presence of Juncus significantly increases plant diversity.     * Data Comparison: Areas with Juncus sustain approximately $8$ plant species, whereas areas without Juncus sustain only approximately $2$ plant species.