Source: Exam 1 Review Guide 2, Prof. Jensen
Difficulty: Intermediate | Prerequisites: Population ecology, predation concepts, basic understanding of species interactions.
Community ecology zooms out from individual species interactions to ask: what determines which species live together, and in what proportions? This topic brings together everything from competition and predation into a bigger-picture framework. It matters because community structure directly affects biodiversity, ecosystem stability, and conservation decisions. You should already be comfortable with predation, competition, and basic population ecology before tackling this.
Community structure depends on both biotic interactions (competition, predation, mutualism) and abiotic factors (temperature, rainfall, disturbance). Ecologists have debated for decades whether communities are tightly integrated units or loose assemblages of species that happen to share the same habitat. Disturbance, succession, and latitudinal gradients in diversity are central themes.
Community
All the species that live and interact in a particular area. Think of it as the full cast of organisms sharing a habitat.
Community structure
The composition and relative abundance of species in a community, including which species are present and how many individuals of each exist.
Species richness
The total number of different species in a community. More species = higher richness. This is a simple count, nothing more.
Evenness
How equally individuals are distributed among species. A community where every species has roughly the same number of individuals has high evenness. One dominated by a single species has low evenness.
Species packing
The number of species that can coexist in a given area. Relates to how finely niches are divided.
Biotic factors
Living influences on community structure, such as competition, predation, mutualism, and disease.
Abiotic factors
Non-living influences on community structure, such as temperature, rainfall, soil type, and disturbance.
Succession (primary vs secondary)
The process by which community composition changes over time after a disturbance. Primary succession starts from bare substrate with no soil (e.g. after a lava flow). Secondary succession starts from existing soil where a community has been disrupted but not completely erased (e.g. after a fire).
Climax community
The relatively stable community that develops at the end of a successional sequence. In the Clementsian view, this is a predictable endpoint.
Keystone species
A species whose impact on community structure is disproportionately large relative to its abundance. Remove a keystone species, and the whole community shifts dramatically.
Intermediate disturbance hypothesis
The idea that species diversity is highest at moderate levels of disturbance. Too little disturbance lets dominant competitors exclude other species. Too much disturbance eliminates all but the hardiest colonisers.
r-selected species
Species that reproduce quickly and in large numbers, investing little in each offspring. They thrive in disturbed or unpredictable environments. Think of it as the "quantity over quality" strategy.
K-selected species
Species that reproduce slowly, invest heavily in fewer offspring, and compete well in stable environments near carrying capacity. Think of it as the "quality over quantity" strategy.
Interactive hypothesis (Clements)
The view that communities are tightly integrated units where species are bound together by strong interactions. Communities are predictable, repeatable, and behave almost like superorganisms.
Individualistic hypothesis (Gleason)
The view that communities are loose collections of species that happen to share similar environmental requirements. Species are distributed independently along environmental gradients, and community composition is less predictable.
Latitudinal gradients in diversity
The well-documented pattern that species richness tends to increase from the poles toward the tropics. The causes are debated but likely involve a combination of habitat stability, energy availability, and evolutionary history.
Community structure is shaped by both biotic factors (species interactions) and abiotic factors (physical environment).
Components of community structure include species richness (how many species), evenness (how evenly individuals are distributed), and the types of interactions among species.
Understanding community structure matters for conservation, land management, and predicting how ecosystems respond to change.
Clements (Interactive hypothesis): communities are tightly knit, predictable associations of species. Succession leads to a specific climax community. Species are interdependent, and communities behave almost as superorganisms.
Gleason (Individualistic hypothesis): species respond independently to environmental gradients. Communities are coincidental assemblages. There is no single predictable climax; composition depends on which species happen to arrive and tolerate the local conditions.
This is a foundational debate in ecology. Modern evidence supports elements of both views, but leans toward Gleason for most systems.
Primary succession: begins on bare substrate with no soil (volcanic rock, glacial till). Pioneer species (lichens, mosses) gradually build soil, enabling later species to establish.
Secondary succession: begins after a disturbance that removes the community but leaves the soil intact (fire, logging, farming). Recovery is faster because soil nutrients and seed banks remain.
r-selected species tend to dominate early succession (fast reproduction, good dispersal). K-selected species tend to dominate later stages (strong competitors in stable environments).
A disturbance is any event that removes organisms or disrupts a community (fire, flood, storm, volcanic eruption, human activity).
Low disturbance favours K-selected competitive dominants, which exclude weaker competitors and reduce diversity.
High disturbance eliminates most species except r-selected colonisers, also reducing diversity.
Intermediate disturbance keeps both types in play, maximising diversity.
Species richness increases from poles to tropics. This pattern is one of the most consistent in ecology.
Likely causes: greater energy availability (more sunlight), higher habitat stability (fewer ice ages, less seasonal variation), and more evolutionary time for speciation in tropical regions.
The exact mechanism remains debated.
A keystone species has an outsized effect on community structure relative to its abundance.
Classic example: sea otters. When otters are removed, sea urchin populations explode, which destroys kelp forests and collapses the entire community.
Identifying keystone species is critical for conservation because losing one can trigger cascading effects.
An n-dimensional hypervolume describes the full range of abiotic and biotic conditions under which a species can survive and reproduce. Each dimension is a different environmental variable (temperature, humidity, food size, etc.).
The "ghost of competition past" concept: a species may retain a restricted niche (its realised niche) long after the competitor that confined it has disappeared. Over evolutionary time, the species loses the traits needed to exploit the broader niche, so the restriction becomes permanent.
Students often assume high species richness automatically means high diversity. It does not. Diversity also depends on evenness. A community with 10 species but 95% of individuals belonging to one species has low diversity despite decent richness.
Students confuse Clements and Gleason. Remember: Clements = communities as integrated, predictable units (interactive). Gleason = species as independent agents along gradients (individualistic).
Students think succession always reaches a fixed climax community. In many systems, disturbances prevent climax from ever being reached, and the Gleasonian view questions whether a single predictable climax exists at all.
Students sometimes think the intermediate disturbance hypothesis means disturbance is always good for diversity. It is not. Only moderate levels promote diversity; too much or too little reduces it.
⚠️ Be ready to compare Clements (interactive) vs Gleason (individualistic) views and explain which one evidence tends to support.
⚠️ Know the difference between species richness and evenness, and how both contribute to diversity.
⚠️ Understand the intermediate disturbance hypothesis well enough to explain why diversity peaks at moderate disturbance levels.
⚠️ Be able to distinguish primary from secondary succession and connect r-selected vs K-selected species to early vs late successional stages.
⚠️ The n-dimensional hypervolume and "ghost of competition past" are likely exam material. Be ready to apply them to scenarios (like the leopard frog/bullfrog pond question from the review guide).
True or False: Species richness and evenness are the same thing. (False: richness is the number of species; evenness is how equally individuals are distributed among them.)
Fill in the blank: Clements viewed communities as tightly integrated units, while Gleason viewed them as ________ assemblages of independent species. (Individualistic / loose.)
True or False: Primary succession begins on existing soil after a disturbance. (False: that is secondary succession. Primary succession starts on bare substrate with no soil.)
Fill in the blank: According to the intermediate disturbance hypothesis, diversity is highest at ________ levels of disturbance. (Moderate / intermediate.)
True or False: A keystone species is always the most abundant species in a community. (False: keystone species have disproportionately large effects relative to their abundance, which is often low.)
Q: What is meant by the term "n-dimensional hypervolume," and how does it relate to the effects of habitat destruction on a species?
A: The n-dimensional hypervolume represents the full range of environmental conditions (each a separate dimension) under which a species can survive and reproduce. If development removes shallow water from a pond, the habitat no longer provides the conditions that fall within the leopard frog's hypervolume, effectively eliminating one or more critical dimensions of its niche.
Q: Leopard frogs evolved alongside bullfrogs for 40,000 years, then were moved to a pond without bullfrogs. They still live only along the edges. Explain this using the "ghost of competition past."
A: Over thousands of generations of competition with bullfrogs, leopard frogs that thrived in open water were outcompeted and eliminated. Natural selection favoured individuals specialised for the edges. Eventually, the genes for exploiting the full pond were lost. Even when the competitor was removed, the leopard frogs could no longer use the broader niche because the traits needed for it had been selected against.
Q: What is a disturbance? Give one example and describe its impact on r-selected vs K-selected species.
A: A disturbance is any event that removes organisms or disrupts community structure. Example: wildfire. After a fire, r-selected species (fast reproduction, good dispersal) colonise quickly and dominate early. Over time, K-selected species (strong competitors, slower reproduction) establish and gradually replace the early colonisers.
Q: Briefly state Clements's view on community dynamics and explain why it is called the "interactive" view.
A: Clements argued that communities are tightly integrated, predictable units that develop through succession toward a specific climax community. It is called the interactive view because species are seen as deeply interdependent, with strong biotic interactions binding the community together, almost like a superorganism.
Q: Four communities each have 6 species but different numbers of individuals per species. Which has the highest species richness?
A: They all have the same species richness (6 species each). Richness is just the count of species present. The communities differ in evenness, not richness.
This connects to predation because top-down control by predators (including keystone species) is one of the major forces shaping community structure. It connects to trophic levels and productivity because energy flow through ecosystems determines how many species and how much biomass a community can support. The niche concept and competitive exclusion tie back to population ecology and interspecific competition.
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