BIOL 286: Lecture #9 Review (Communities: Competition, Niches, & Symbioses)

Community ecology examines interactions between . . .

co-occurring populations

Major types of species interactions: (5)

(1) parasitism; (2) commensalism; (3) mutualism; (4) predation/herbivory; (5) competition

competition

Interactions between individuals over a limiting resource that may lead to decreased fitness of competing species

Types of competition: (2)

(1) interspecific competition; (2) intraspecific competition

Types of competition: (1) Interspecific competition

Competition between individuals of different species

Types of competition: (2) Intraspecific competition

Competition between individuals of the same species

Overlap in niches can lead to . . .

competition

niche

The role and position a species has within its habitat, encompassing how it interacts with both biotic and abiotic factors to survive and reproduce (i.e. includes interactions with other species and the environment)

fundamental niche

The full range of environmental conditions and resources a species can theoretically use and tolerate in the absence of limiting factors like competition, predation, or disease

The fundamental niche represents . . . based solely on . . . and . . ., without . . . or . . .

the potential ecological space a species could occupy; its physiological tolerances; resource requirements; interference from other species; environmental constraints

realized niche

The range of abiotic and biotic conditions under which a species actually lives; the part of a species fundamental niche that it actually uses, limited by abiotic and biotic factors (including competition)

Factors affecting the intensity of competition: ()

(1) niche overlap

Describe how niche overlap impacts the intensity of competition among different species.

Greater niche overlap intensifies interspecific competition because species that use the same resources experience stronger reductions in fitness when those resources become limited. As the overlap increases, individuals interact more frequently and compete more directly for shared requirements such as food, space, or microhabitat conditions. Under high overlap, the realized niche of each species becomes a contracted subset of its fundamental niche, since competition excludes species from portions of the environmental space they could occupy in the absence of competitors. When niche overlap is extensive, these competitive constraints may lead to competitive exclusion or evolutionary shifts such as character displacement that reduce overlap and alleviate competitive intensity. By contrast, minimal niche overlap weakens competitive interactions because resource use is sufficiently partitioned to limit direct ecological interference; consequently, the realized niche of each species more closely approximates their respective fundamental niches.

To demonstrate competition, you must demonstrate that either: (2)

(1) An increase in the density of species A decreases the fitness of individuals of species A (evidence of intraspecific competition); (2) An increase in the density of species A decreases the success of species B (interspecific competition)

Evidence of interspecific competition: . . .

An increase in the density of species A decreases the fitness of individuals of species A

Evidence of intraspecific competition: . . .

An increase in the density of species A decreases the success of species B

Discuss the impact of niche overlap on interspecific competition between Chthalamus stellatus and Semibalanus balanoides, with particular attention to how the fundamental and realized niches of each species is impacted.

In Chthamalus stellatus and Semibalanus balanoides, the intensity of interspecific competition is shaped by the extent of niche overlap across the intertidal zone, and this overlap determines how each species' fundamental niche is expressed as a realized niche. Chthamalus is physiologically capable of occupying both shallow tide‑pool habitats and deeper mid‑intertidal zones, whereas Semibalanus is restricted to deeper waters because it is comparatively more sensitive to desiccation. This physiological limitation constrains the fundamental niche of Semibalanus to lower, consistently submerged regions, while Chthamalus—being more desiccation‑tolerant—has a broader fundamental niche extending into higher, periodically exposed zones.

Where their distributions overlap, Semibalanus exerts strong competitive pressure: it overgrows, crushes, or physically displaces Chthamalus from shared substrate. As a result, Chthamalus' realized niche contracts upward into the shallow intertidal zone, despite its ability to occupy deeper waters. Experimental removals demonstrate this asymmetry: when Semibalanus is removed, Chthamalus expands downward into deeper regions, revealing its full fundamental niche; when Chthamalus is removed, Semibalanus remains confined to deeper waters because its physiological intolerance of desiccation prevents upward expansion. Thus, niche overlap produces asymmetric competition, compressing the realized niche of Chthamalus while leaving Semibalanus limited primarily by abiotic stress rather than by biotic exclusion.

asymmetric competition

Ecological competition between two species in which one species suffers a much greater fitness decline than the other

de Wit replacement series

An experimental method used to study interspecific and intraspecific competition by growing two or more species together at varying proportions while keeping total density constant

The de Wit replacement series is designed to analyze how . . ., particularly in terms of . . .

species interact when grown together; competition for resources (e.g. light, nutrients)

The de Wit replacement series allows researchers to quantify both . . . and . . ., comparing . . .

interspecific competition; intraspecific competition; the performance of species in monocultures vs. mixed cultures

Mechanisms of competition: (3)

(1) exploitation competition; (2) interference competition; (3) preemptive competition

Mechanisms of competition: (1) Exploitation competition

A class of competition in which one species reduces the amount or availability of the limiting resource

Mechanisms of competition: (2) Interference competition

A class of competition in which one species actively inhibits another from obtaining the resource (e.g. allelopathy, interspecific territoriality)

Mechanisms of competition: (3) Preemptive competition

A class of competition in which a mechanism deployed by a species ensures access to the limiting resource (e.g. early establishment in a resource-rich area)

Potential outcomes of competition: (4)

(1) coexistence; (2) character displacement; (3) competitive exclusion; (4) resource partitioning

competitive exclusion

A potential outcome of competition, where, if there is too much overlap in the niche space, one of the competing species may go extinct

Competitive exclusion occurs when . . .

there is competition for a limiting resource

character displacement

A potential outcome of competition whereby competition may cause the traits and, consequently, the niches of two species to diverge

Discuss character displacement as an outcome of interspecific competition with reference to G. fortis and G. fulginosa, where G. fortis occupies Daphne Major in isolation, G. fulginosa occupies Los Hermanos, and the species co-occur on Santa Cruz.

Character displacement arises when interspecific competition in sympatry favors the divergence of traits that reduce niche overlap. The classic pattern in G. fortis and G. fuliginosa illustrates this process clearly. When each species occurs allopatrically—G. fortis on Daphne Major and G. fuliginosa on Los Hermanos—they exhibit substantial overlap in beak size and exploit similar seed resources, reflecting the breadth of their fundamental niches in the absence of competitors. However, on Santa Cruz, where the species co‑occur, competition for shared seed resources favors divergence in beak morphology. G. fuliginosa tends to exhibit an intermediate beak size, whereas G. fortis evolves a smaller beak, reducing direct resource overlap. As a result, their realized niches in sympatry become more distinct than their fundamental niches in isolation. This shift in trait distributions—driven by competitive interactions—constitutes character displacement and allows the two species to coexist with reduced ecological interference.

Character displacement has contributed to many . . ., such as . . .

adaptive radiations; the adaptive radiation of the Honeycreepers of the Hawaiian Islands

Describe how character displacement can contribute to adaptive radiations such as that of the Honeycreepers on the Hawaiian Islands.

Character displacement can facilitate adaptive radiations by reducing niche overlap among closely related species and thereby enabling the diversification of ecological roles. In the Hawaiian honeycreepers, early lineages arriving on newly formed islands encountered abundant, unoccupied resources, but as diversification proceeded, sympatric species increasingly competed for similar food types and foraging substrates. Under these conditions, selection favored divergence in traits such as bill morphology, allowing species to specialize on distinct dietary niches—nectar, seeds, arthropods, or even wood‑boring prey. This trait divergence reduced interspecific competition and stabilized coexistence, while simultaneously opening new ecological opportunities. Over evolutionary time, repeated rounds of character displacement across islands and habitats contributed to the honeycreepers' extensive adaptive radiation, expanding their realized niches into a wide array of feeding strategies that collectively reflect the breadth of their ancestral fundamental niche.

resource partitioning

The division of environmental resources by coexisting species such that the niche of each species differs by one or more significant factors from the niches of all coexisting species

Describe how resource partitioning as a potential outcome of competition may lead to the reordering of a community's structure, with reference to McArthur's warblers.

Resource partitioning can reorder a community's structure by reducing niche overlap among species that would otherwise compete intensely for the same resources. MacArthur's warblers provide a classic example: although several Dendroica species forage in the same trees, competition has led each species to specialize on distinct microhabitats within the canopy. By dividing the tree into vertically and horizontally differentiated foraging zones, the warblers minimize direct interference and stabilize coexistence. This partitioning effectively restructures the community, transforming what would be a set of strongly competing, ecologically similar species into a spatially organized assemblage in which each species occupies a more narrowly defined realized niche. In this way, resource partitioning acts as a community‑level outcome of competition, reshaping species interactions and the spatial organization of the habitat itself.

Coexistence is possible if . . . and . . .

resources are abundant (i.e. not overly limiting or limiting whatsoever); niche overlap is small

coexistence

A potential outcome of competition whereby two or more species may share the same fundamental niche but yet live sympatrically without either going extinct

Coexistence may be facilitated by . . ., . . . or . . .

(1) toleration of niche overlap; (2) selective pressure for specialization (e.g. sympatric Galapagos finches); (3) selective pressure to increase the range of resources used

How can competitors coexist? (5)

(1) if there is limited niche overlap; (2) character displacement; (3) if resources are abundant; (4) if niche breadths are small (i.e. species specialize); (5) if species can exploit resources other species cannot

When niches are projected into 2-dimensional niche space, each species is represented by . . .

an ellipse or circle

Explain the conceptualization of the ecological niche as an n-dimensional hypervolume.

The ecological niche as an n‑dimensional hypervolume is conceptualized as a multidimensional space that captures all environmental conditions and resources necessary for a species' survival and reproduction. Each ecological factor—such as temperature, humidity, prey size, salinity, or microhabitat position—constitutes a separate axis in this space, and the full set of axes together defines the species' fundamental niche. The "hypervolume" represents the region within this multidimensional space where the species' population growth rate is positive, meaning the conditions collectively permit persistence. This framework emphasizes that niches are not single traits or habitats but integrated combinations of environmental variables, and it provides a quantitative way to compare niche overlap, realized niches, and the effects of biotic interactions on niche expression.

symbioses

A type of ecological interaction in which species interact in a permanent or prolonged close relationship

In symbioses, . . . (3)

(1) one species can benefit, and the other is negatively affected (e.g. predation, herbivory, parasitism); (2) both species can benefit (e.g. mutualism); (3) one species can benefit, and the other is not affected (e.g. commensalism)

Types of symbioses: (3)

(1) mutualism; (2) commensalism; (3) parasitism

Types of symbioses: (1) Mutualism

A type of symbiosis in which both interacting species benefit from the relationship

Elucidate mutualism as a type of symbiosis with reference to aphids and ants.

Mutualism is a form of symbiosis in which both interacting species experience a net fitness benefit, and the association between aphids and ants is a classic example. Aphids feed on plant phloem and, after extracting a portion of the sugars they require, excrete the remainder as honeydew, a carbohydrate‑rich resource that ants readily consume. In return, ants provide protective services: they guard aphids from predators and parasitoids, reducing aphid mortality and increasing their overall reproductive success. This reciprocal exchange of nutrition for defense illustrates the defining feature of mutualism—each partner gains a benefit that enhances its ecological performance and contributes to the stability of the interaction.

Types of symbioses: (2) Commensalism

A type of symbiosis in which one of the interacting species benefits while the other is unaffected

Elucidate commensalism as a type of symbiosis with reference bromeliads and trees.

Commensalism is a form of symbiosis in which one species benefits while the other is neither harmed nor helped, and the association between bromeliads and trees exemplifies this relationship. Bromeliads use tree trunks and branches as elevated substrates that provide access to light and moisture, thereby expanding their realized niche without extracting nutrients from or damaging the host tree. This commensal relationship, however, is contingent on the bromeliad remaining below a size threshold at which its biomass would begin to impede the tree's photosynthetic capacity. As long as the bromeliad's growth does not shade or mechanically burden the tree, the tree remains ecologically unaffected, preserving the asymmetry that defines commensalism.

Types of symbioses: (3) Parasitism

A type of symbiosis in which one of the interacting species benefits while the other is harmed

Elucidate parasitism as a type of symbiosis with reference mistletoe and trees.

Parasitism is a form of symbiosis in which one species gains a fitness benefit at the expense of another, and the interaction between mistletoe and trees exemplifies this asymmetry. Mistletoe attaches to tree branches and inserts specialized structures into the host's vascular tissue, extracting water and nutrients rather than producing them independently. This resource withdrawal imposes a physiological cost on the tree, reducing growth and potentially impairing reproductive output, while the mistletoe gains a reliable supply of nutrients that supports its own survival and reproduction. Unlike commensalism or mutualism, the host receives no benefit and experiences measurable harm, making mistletoe a clear illustration of parasitism as a symbiotic relationship defined by unidirectional exploitation.

Types of antagonistic interactions in symbioses: (3)

(1) herbivory; (2) predation; (3) parasitism

keystone predator

A predator species that reduces the density of the strongest competitors in a community, thereby helping maintain species diversity

Factors influencing predation rates (# prey/unit time): (6)

(1) search time; (2) identifying prey; (3) pursuit time; (4) capture time; (5) ingestion time; (6) processing time

functional response

Predator individuals change their ingestion rate in response to prey number

numerical response

An increase in the predator population through reproduction and/or immigration in response to prey density

In predation, the relative strength of the functional response and the numerical response is dependent on: (3)

(1) how difficult it is to find prey (search time); (2) how long it takes to pursue and capture prey; (3) how long it takes to handle prey (manipulate, process, consume)

Types of functional responses: (3)

(1) Type I; (2) Type II; (3) Type III

Types of functional responses: (1) Type I

A functional response in which a predator's rate of prey consumption increases in a linear fashion with an increase in prey density until satiation occurs

In predation, a type I functional response may be observed when . . .; that is, . . . are . . . and . . .

consuming the food does not interfere with searching for food; prey items; small; easy to find and handle

Provide an example of a type I functional response in predation/herbivory: . . .

Herbivores grazing on abundant food items can continue to eat as they search for more food, yielding a linear relationship between the density of the prey population and the number of prey consumed.

Types of functional responses: (2) Type II

A functional response in which a predator's rate of prey consumption begins to slow down as prey density increases and then plateaus when satiation occurs

In predation, a type II functional response is observed when prey . . ., meaning . . .

are difficult to find at lower prey densities; search time will slow consumption

In a type II functional response, if the density of prey increases, initially . . ., but, at higher densities . . .

predators can keep up and eat more prey in proportion to density; handling time and/or digestive constraints will cause consumption rates to plateau

Provide an example of a type II functional response in predation/herbivory: . . .

A type II functional response describes a predator's intake rate that rises rapidly with increasing prey density but then asymptotically levels off as handling time becomes the limiting factor. Wolves hunting caribou exemplify this pattern: at low caribou densities, wolves can increase their kill rate as encounters become more frequent, but as prey density continues to rise, each kill still requires substantial handling time—pursuit, subduing, and consumption—which prevents the predation rate from increasing linearly. The result is a saturating curve in which wolves consume more caribou as they become more abundant, but only up to a maximum rate constrained by the time and effort required to process each prey item.

Types of functional responses: (3) Type III

A functional response in which a predator exhibits low prey consumption under low prey densities, rapid consumption under moderate prey densities, and slowing prey consumption under high prey densities

In predation, a type III functional response is observed when prey . . ., meaning . . .

are difficult to find at low densities; search time will slow consumption

In a type III functional response, as prey density increases, . . ., but, at higher densities . . .

predator efficiency at detecting and handling prey increases; handling time or digestive constraints cause consumption rates to plateau

Provide an example of a type III functional response in predation/herbivory: . . .

A type III functional response is characterized by low consumption at very low prey densities, followed by a rapid increase once prey become more abundant, and finally a plateau as handling time becomes limiting. Deer mice feeding on sawfly cocoons provide a clear example. When cocoons are rare, deer mice encounter them infrequently and may rely on alternative food sources, resulting in low predation rates. As cocoon density increases, mice learn to locate them more efficiently and switch their foraging effort toward this profitable prey, causing predation rates to rise disproportionately quickly. At high densities, however, consumption again becomes constrained by handling time, producing the characteristic sigmoidal curve of a type III functional response.

A type III functional response curve might be observed if . . . for . . .

there exists a learning curve; a predator hunting a particular prey item

Distinguish between type II and type III functional responses in predation/herbivory.

A type II functional response and a type III functional response differ in both shape and ecological interpretation, especially in how predators respond to changing prey densities.

A type II response shows a hyperbolic, saturating curve: consumption rate increases quickly at low prey densities but then levels off as handling time becomes the limiting factor. Predators with type II responses—such as wolves hunting caribou—continue to encounter prey easily even at low densities, so intake rises immediately and then plateaus.

A type III response shows a sigmoidal (S‑shaped) curve: consumption is initially low at very low prey densities because predators may have difficulty locating prey, may not recognize them as profitable, or may switch to alternative prey. As prey become more abundant, predators increase consumption rapidly, before eventually reaching the same handling‑time‑limited plateau seen in type II. This pattern reflects prey switching, learning, or refuge effects.

Type II responses therefore emphasize handling constraints, whereas type III responses incorporate density‑dependent changes in predator efficiency.