Population, Community, and Ecosystem Ecology: Key Concepts and Interactions

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Population, Community, and Ecosystem Ecology

Introduction

Population, community, and ecosystem ecology are branches of ecology that examine the distribution, abundance, and interactions of organisms within their environments. These topics are central to understanding how living things interact with each other and with their physical surroundings.

Species Distribution and Influencing Factors

Biotic and Abiotic Factors

The distribution of species is influenced by both biotic (living) and abiotic (non-living) factors.

Biotic Factors: Predation, herbivory, competition, mutualism, and parasitism can affect where organisms are found.
Abiotic Factors: Temperature, water, oxygen, salinity, sunlight, and soil are key non-living factors. These often vary in space and time.
Example: The distribution of saguaro cacti is affected by temperature, water availability, and interspecific interactions.

Dispersal and Distribution

Dispersal is the movement of individuals or gametes away from centers of high population density or from their area of origin. This process contributes to the global distribution of organisms.

Behavior and Habitat Selection

Some organisms do not occupy all of their potential range. Species distribution may be limited by habitat selection behavior, where individuals choose specific environments over others.

Population Ecology

Population Definition and Features

A population is a group of individuals of a single species living in the same general area. Populations are described by their boundaries and size.

Density: Number of individuals per unit area or volume.
Dispersion: Pattern of spacing among individuals within the boundaries of the population.

Human Population Growth

The human population is no longer growing exponentially but is still increasing rapidly. No population can grow indefinitely, and humans are no exception.

Current global population: Over 8.1 billion people.
Growth rate: Began to slow during the 1960s.

Global Carrying Capacity

Carrying capacity refers to the maximum population size that an environment can sustain.

Population ecologists predict a global population of 8.1–10.6 billion people in 2050.
Factors that may limit human carrying capacity include food, space, nonrenewable resources, and buildup of wastes.
Humans can regulate population growth through social changes.

Community Ecology

Definition and Features

A biological community is an assemblage of populations of various species living close enough for potential interaction.

Interspecific Interactions

Relationships between species in a community are called interspecific interactions. These include:

Competition (-/-): Species compete for resources in short supply.
Predation (+/-): One species (predator) kills and eats another (prey).
Herbivory (+/-): An herbivore eats parts of a plant or alga.
Symbiosis: Includes parasitism (+/-), mutualism (+/+), and commensalism (+/0).
Facilitation: Species have positive effects on the survival and reproduction of other species without direct contact.

Competitive Exclusion Principle

Strong competition can lead to competitive exclusion, the local elimination of a competing species. Two species competing for the same limiting resources cannot coexist in the same place.

G.F. Gause's experiment: Demonstrated competitive exclusion using Paramecium species.

Ecological Niche Concept

A niche is the total of all the ways an organism uses the resources of its environment, including space utilization, food consumption, temperature range, mating conditions, and moisture requirements.

Ecological niche: The sum of a species’ use of biotic and abiotic resources; its ecological role.
Resource partitioning: Differentiation of ecological niches enables similar species to coexist.

Fundamental vs. Realized Niche

The fundamental niche is the niche potentially occupied by a species, while the realized niche is the niche actually occupied. Competition can restrict a species to a smaller realized niche.

Example: Connell’s barnacle experiment showed that Chthamalus occupies a smaller realized niche due to competition with Balanus.

Character Displacement

Character displacement is the tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations. This reduces competition.

Example: Variation in beak size between populations of Galápagos finches.

Predation and Defensive Adaptations

Predation

Predation is a (+/-) interaction where one species kills and eats another. Both predators and prey have evolved strategies for survival.

Behavioral defenses: Hiding, fleeing, forming herds or schools, self-defense, alarm calls.
Morphological and physiological defenses: Physical traits and body functions that deter predators.
Mechanical and chemical defenses: Structures or chemicals that protect prey.
Aposematic coloration: Bright warning colors signal chemical defenses.
Cryptic coloration: Camouflage makes prey difficult to spot.
Mimicry: Batesian mimicry (harmless species mimics harmful one); Müllerian mimicry (two harmful species resemble each other).

Herbivory

Herbivory is a (+/-) interaction where an herbivore eats parts of a plant or alga. Plants have evolved mechanical and chemical defenses, and herbivores have adaptations to overcome these defenses.

Symbiosis

Types of Symbiotic Relationships

Parasitism (+/-): Parasite derives nourishment from host, harming it. Endoparasites live inside the host; ectoparasites live on the surface.
Mutualism (+/+): Both species benefit. Can be obligate (required for survival) or facultative (not required).
Commensalism (+/0): One species benefits, the other is unaffected. Often difficult to document.

Ecosystem Ecology and Energy Flow

Trophic Levels and Energy Transfer

Energy flows through ecosystems via trophic levels, which represent feeding relationships.

Autotrophs: "Self-feeders" that synthesize organic compounds from inorganic precursors. Includes photoautotrophs (use light) and chemoautotrophs (use inorganic oxidation reactions).
Heterotrophs: Cannot synthesize organic compounds from inorganic precursors; must consume other organisms.
Primary producers: Autotrophs at the base of the food chain.
Consumers: Heterotrophs that eat other organisms. Includes herbivores, primary carnivores, secondary carnivores, detritivores, and decomposers.

Productivity in Ecosystems

Gross Primary Productivity (GPP): The raw rate at which primary producers synthesize new organic matter.
Net Primary Productivity (NPP): GPP minus the respiration of primary producers.

Energy Transfer Efficiency

Energy transfer between trophic levels is inefficient. Only about 10% of the energy at one level is converted to organic matter at the next level.

Pyramid of Energy: Illustrates the decrease in available energy at higher trophic levels.
Example: 100 kg of plant material supports about 10 kg of herbivore biomass and 1 kg of carnivore biomass.

Trophic Level	Role	Energy Transfer Efficiency
Primary Producers	Autotrophs (plants, algae)	~1% of solar energy captured
Primary Consumers	Herbivores	~10% of producer energy
Secondary Consumers	Carnivores (eat herbivores)	~10% of herbivore energy
Tertiary Consumers	Top carnivores	~10% of secondary consumer energy

Food Chains and Food Webs

Food chains link trophic levels from producers to top carnivores. Food webs are more complex, showing branching feeding relationships and organisms that may occupy multiple trophic levels.

Limits on Food Chain Length

The energetic hypothesis suggests that food chain length is limited by inefficient energy transfer. Typically, only a few links exist in a food chain.

Community Structure and Species Impact

Species Diversity

Species richness: Number of different species in a community.
Relative abundance: Proportion each species represents of all individuals.
Communities with higher diversity are more productive, stable, and resistant to invasive species.

Dominant and Keystone Species

Dominant species: Most abundant or highest biomass; may be most competitive or best at avoiding predators.
Keystone species: Exert strong control by ecological roles, not necessarily by abundance. Example: sea stars in intertidal communities.
Invasive species: Introduced species that may become dominant due to lack of predators or disease.

Summary Table: Types of Interspecific Interactions

Interaction	Description	Effect on Species 1	Effect on Species 2
Competition	Two or more species compete for a resource in short supply	-	-
Predation	One species kills and eats the other	+	-
Herbivory	Herbivore eats parts of a plant or alga	+	-
Parasitism	Parasite derives nourishment from host	+	-
Mutualism	Both species benefit	+	+
Commensalism	One species benefits, other unaffected	+	0
Facilitation	Species have positive effects on others without direct contact	+	+

Key Equations

Net Primary Productivity:
Energy Transfer Efficiency:

Additional info:

Some content was expanded for clarity and completeness, including definitions and examples.
Tables were reconstructed to summarize key concepts and interactions.