Community Ecology Study Guide
Warm-Up Calculations & Foundational Terminology
Quantitative Population Growth Calculation:
Given parameters:
Per capita birth rate:
Per capita death rate:
Initial population size:
Per capita growth rate formula:
Annual change in population size calculation:
Result: 60 individuals are added to the population in one year.
Overview of Interaction Symbols:
Predation:
Parasitism:
Mutualism:
Commensalism:
Key Chapter Definitions:
Fundamental Niche: The full range of environmental conditions (abiotic and biotic) under which a species can survive and reproduce, representing the niche potentially occupied by that species in the absence of interspecific competition.
Realized Niche: The actual set of environmental conditions and resources occupied and utilized by a species when restricted by interspecific interactions, competition, and predation.
Symbiosis: An intimate, close physical relationship between individuals of two or more different species living in direct contact over extended periods.
Parasitism: A symbiotic interaction in which one organism, the parasite, derives its nourishment directly from another organism, the host, which is harmed in the process.
Mutualism: A symbiotic interaction that yields direct fitness benefits to both participating species.
Commensalism: An interaction between species that benefits one organism while leaving the other unaffected.
Keystone Species: A species that exerts strong control over community structure and species diversity by virtue of its ecological role or niche, rather than its relative abundance or total biomass.
Invasive Species: An organism introduced outside its native distribution range that establishes self-sustaining populations and causes ecological or economic disruption.
Ecological Succession: The predictable, sequential process of community change and ecosystem development over ecological time following a disturbance or creation of new land.
Primary Succession: Ecological succession occurring in virtually lifeless environments lacking formed soil.
Secondary Succession: Ecological succession occurring in an area where an existing disturbance has cleared the biological community but left the soil layer intact.
Community & Interspecific Interactions
Ecological Community Definition:
A group of populations consisting of different species living close enough together in a defined habitat to interact directly or indirectly.
Overview of Interspecific Interactions:
Interspecific interactions are relationships between populations of different species within a community.
These relationships can be classified based on their net effects on individual survival and reproduction: positive , negative , or neutral .
Comprehensive Interaction Types:
Competition : Occurs when two or more species compete for a limited shared resource.
Predation : One species (the predator) kills and consumes another species (the prey).
Herbivory : An animal herbivore consumes portions of a plant or macroalga.
Symbiosis:
Parasitism : Parasite derives nourishment from host, harming host fitness.
Mutualism : Both interacting species experience increased survival or reproductive success.
Commensalism : One species benefits while the host species is unaffected.
Facilitation : Species positively influence the survival and reproduction of other species without requiring direct physical, symbiotic contact.
Interspecific Competition & Ecological Niches
Mechanics of Interspecific Competition:
Occurs when individuals of different species compete for a resource that limits their growth and survival (e.g., food, water, light, nesting sites).
Because competition imposes costs on both species, the interaction is designated as .
Competitive Exclusion Principle:
Two species competing for the exact same limiting resources cannot coexist indefinitely in the same ecological niche.
If two species have identical ecological niches, the species with even a slight reproductive advantage will outcompete and localise the complete exclusion or elimination of the other.
Ecological Niche Concept:
The sum total of a species' use of the biotic and abiotic resources in its environment.
Described as an organism's ecological role or "profession" within the ecosystem.
Fundamental Niche vs. Realized Niche:
Fundamental Niche: The niche potentially occupied by a species under ideal conditions without interspecific competition.
Realized Niche: The portion of the fundamental niche that a species actually occupies in nature as a result of competitive pressure and biotic interactions.
Classic Field Experiment (Joseph Connell's Barnacle Study):
Target Species: Two intertidal barnacle species along the Scottish coast—Chthamalus stellatus and Balanus balanoides.
Observations:
Chthamalus occupies the higher intertidal zone, while Balanus occupies the lower intertidal zone.
Experimental Removal:
When Balanus was experimentally removed from the lower intertidal zone, Chthamalus spread downward and successfully colonized the lower zone.
Conclusion:
The fundamental niche of Chthamalus encompasses both the high and low intertidal zones.
Interspecific competition from Balanus excludes Chthamalus from the lower zone, restricting Chthamalus to a smaller realized niche high on the rocks.
Resource Partitioning:
The differentiation of ecological niches that enables similar species to coexist in a community.
Through natural selection, competing species evolve distinct microhabitats, feeding behaviors, or active time periods, reducing direct competition.
Predation Adaptations & Defensive Strategies
Predation Dynamics , Adaptation, and Coevolution:
Predation drives natural selection, resulting in specialized predator weapons (e.g., claws, fangs, acute senses, venom) and specialized prey defensive strategies.
Prey Defensive Strategies:
Cryptic Coloration (Camouflage):
Morphological adaptations that make organisms difficult to spot against their natural background environments.
Examples: Eastern screech owls blending into tree bark, Satanic leaf-tailed geckos mimicking dead leaves, octopuses altering skin texture and color to match coral reefs, canyon tree frogs matching granite rock surfaces.
Aposematic Coloration (Warning Coloration):
Bright, high-contrast coloration displayed by animals with chemical defenses (toxins, poison, venom).
Signals danger to visual predators, which learn to avoid attacking organisms with those distinct markings.
Examples: Poison dart frogs (vivid red/black patterns), ringneck snakes (displaying bright orange-red ventral undersides), ladybug beetles (bright red with black spots).
Batesian Mimicry:
A harmless or palatable species evolves to mimic the warning signals or appearance of an unpalatable, harmful model species.
Examples:
Non-venomous scarlet king snakes mimicking the red, yellow, and black banded patterns of highly venomous coral snakes.
Hawkmoth larva expanding its body when threatened to visually resemble the head of a venomous green parrot snake.
Hornet clearwing moth featuring transparent wings and yellow-and-black striped abdomen that mimics a European hornet.
Müllerian Mimicry:
Two or more unpalatable, toxic, or dangerous species resemble each other.
Predator avoidance learning is accelerated because predators encounter a unified warning image across multiple distinct species.
Examples:
Monarch butterfly and Viceroy butterfly (both toxic and unpalatable to bird predators).
Cuckoo bee and Yellow jacket wasp (both capable of delivering painful stings).
Herbivory Defense Mechanisms:
Plants avoid being consumed using physical defenses (spines, thorns, tough trichomes) and chemical defenses (secondary metabolites, toxins, irritants).
Herbivores evolve adaptations such as specialized digestive enzymes and specialized teeth to consume plant tissue safely.
Symbiotic & Facilitative Relationships
Detailed Symbiotic Categories:
Parasitism .
Parasite derives nutrients at the direct expense of the host.
Examples: Parasitic wasps laying eggs inside caterpillars; endoparasites like Cordyceps fungi infecting ants and manipulating ant host behavior prior to host death.
Mutualism .
An interaction where both participants gain measurable fitness advantages.
Examples:
Ants (Pseudomyrmex) and Bull's-horn acacia trees: The acacia provides shelter in enlarged hollow thorns and food via nectar and protein-rich Beltian bodies. In exchange, ants attack herbivores and trim encroaching vegetation.
Ants and aphids: Aphids provide ants with nutrient-rich excreted honeydew; ants farm and protect aphids from predators.
Commensalism .
One organism gains a benefit while the host organism is unaffected.
Example: Cattle egrets foraging alongside African buffaloes. As buffaloes walk, they flush out ground insects, which egrets capture without harming or helping the buffaloes.
Facilitation .
Species exert positive impacts on the performance, survival, or reproduction of other co-occurring species without maintaining direct symbiotic contact (e.g., black rush stabilizing salt marsh soil and shading soil surface, reducing salt accumulation).
Community Structure, Diversity & Invasive Species
Quantifying Species Diversity:
Species Richness: The total number of distinct species present in a community.
Relative Abundance: The proportion each species represents of all individuals in the community.
Simpson Diversity Index: A mathematical formula used to calculate community biodiversity by integrating both species richness and relative abundance.
Community Diversity Comparison Case Study:
Community 1: 90 Species A, 10 Species B, 0 Species C, 0 Species D.
Low diversity; high dominance by Species A.
Community 2: 25 Species A, 25 Species B, 25 Species C, 25 Species D.
Highest overall diversity; equal relative abundance (high species evenness).
Community 3: 80 Species A, 5 Species B, 5 Species C, 10 Species D.
Moderate diversity; skewed abundance.
Ecological Value of High Biodiversity:
High diversity communities are more productive, exhibit lower year-to-year biomass fluctuations, and are significantly more resistant to invasion by non-native species.
Invasive Species Case Studies:
Kudzu (Pueraria montana):
An Asian vine introduced to the United States.
Grows rapidly over existing trees, plants, and structures, smothering vegetation and blocking sunlight.
Dutch Elm Disease (Ophiostoma ulmi):
A fungal pathogen vectored by elm bark beetles, imported to North America from Europe on Dutch timber logs.
Caused widespread mortality of native elm tree populations across North America, Europe, and Canada.
Management efforts focus on breeding and cultivating disease-resistant elm varieties.
Potato Blight (Phytophthora infestans):
A fungus-like oomycete pathogen imported to Ireland via ships from the United States in the 1840s.
Destroyed potato crops across Ireland, precipitating the Great Famine and causing over 1 million human deaths.
Highlights the vulnerability associated with crop monoculture and lack of genetic diversity in agricultural ecosystems.
Trophic Structures & Energy Flow
Trophic Levels and Food Chains:
The trophic structure of a community describes the feeding relationships among its member organisms.
Food Chain: The linear sequence of energy transfer from primary producers up through consumers and decomposers.
Terrestrial Hierarchy:
Primary Producers: Plants
Primary Consumers: Herbivores (e.g., Grasshopper)
Secondary Consumers: Carnivores (e.g., Mouse)
Tertiary Consumers: Carnivores (e.g., Snake)
Quaternary Consumers: Carnivores (e.g., Hawk)
Marine Hierarchy:
Primary Producers: Phytoplankton
Primary Consumers: Zooplankton
Secondary Consumers: Carnivores (e.g., Small Fish)
Tertiary Consumers: Carnivores (e.g., Large Fish)
Quaternary Consumers: Carnivores (e.g., Orca / Killer Whale)
Constraints on Food Chain Length:
Energetic Hypothesis: Inefficiency of energy transfer along the chain limits chain length (typically only ~10% of energy stored in organic matter at one trophic level is converted to organic matter at the next level).
Stability Hypothesis: Longer food chains are less stable because top predators are sensitive to fluctuations at lower levels.
Food Webs:
Complex networks of interconnected, branching food chains within an ecosystem.
Species often feed at multiple trophic levels simultaneously (e.g., humans or smaller toothed whales eating multiple types of marine organisms).
Community Dynamics: Species Roles & Ecological Succession
Dominant vs. Keystone Species:
Dominant Species: Species in a community that have the highest total biomass or greatest numerical abundance.
Keystone Species: Species that exert strong control over community structure through pivotal ecological niches, despite low relative abundance or biomass.
Sea Otters: Feed on sea urchins in coastal ecosystems. In the absence of sea otters, sea urchin populations expand rapidly, overgrazing and destroying kelp forest ecosystems.
Grizzly Bears: Act as nutrient vectors by capturing salmon in aquatic habitats and depositing salmon carcasses and nutrient-rich excrement across terrestrial forest floors.
Prairie Dogs: Construct extensive burrow networks, aerate soil, and trim grass, creating microhabitats that support hundreds of co-occurring animal and plant species.
Disturbances and Succession:
A disturbance is an event (e.g., fire, flood, drought, storm, human disruption) that changes a community by removing organisms or altering resource availability.
Ecological Succession: The sequence of community changes following a disturbance.
Primary Succession:
Occurs in virtually lifeless areas where soil has not yet formed (e.g., retreating glaciers, newly formed volcanic islands, bare lava flows).
Pioneer species (e.g., lichens, mosses, fireweed) colonize bare substrate, slowly breaking down rock and creating organic soil over time.
Secondary Succession:
Occurs when an existing biological community is cleared by a disturbance, but the underlying soil layer remains intact (e.g., post-forest fire recovery, abandoned agricultural land).
Pioneer species recolonize quickly, followed by shrubs, softwoods, and hardwood trees.
Yellowstone Post-Fire Example: Immediately after severe forest fires, patchy landscapes remain. Within one year, abundant herbaceous plants cover the ground and initiate community recovery.
Biogeographic Factors Influencing Diversity
Latitudinal Gradients:
Species richness is highest in tropical regions near the equator and declines toward the poles.
Influenced by evolutionary history (longer growing seasons without major glacial disruptions) and climate (high solar input and water availability).
Area Effects (Species-Area Curve):
All other factors being equal, larger geographic areas contain more species because they offer a greater diversity of microhabitats.
Island Biogeography Model:
Biogeographic islands (including terrestrial habitat fragments) act as natural laboratories for studying species diversity.
Key Variables: Island size and distance from the mainland source population.
Effect of Island Size: Larger islands experience higher immigration rates and lower extinction rates due to larger target size and resource abundance.
Effect of Distance: Islands located farther from the mainland experience lower immigration rates and higher extinction rates due to dispersal limitations.