Communities and Ecosystems: Interactions, Health, and Energy Flow

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Communities and Ecosystems

Introduction

Communities and ecosystems are fundamental units of ecological study, focusing on the interactions among organisms and their environment. Understanding these interactions is essential for grasping how biodiversity is maintained and how energy and nutrients flow through biological systems.

Community Structure and Interactions

Definition of a Community

A community consists of all populations of organisms living close enough for interaction. Each species within a community occupies a niche, which is its functional role, including its habitat, activity patterns, resources obtained, and interactions with other species.

Niche: The sum of a species' use of the biotic and abiotic resources in its environment.
Habitat: The physical environment where an organism lives.
Resource Partitioning: The differentiation of niches that enables similar species to coexist.

Cartoon of a community with various animals and plants

Types of Community Interactions

When niches overlap, populations interact in various ways. These interactions shape community structure and influence species survival and reproduction.

Competition: Both species are harmed by the interaction as they vie for the same resources.
Mutualism: Both species benefit from the interaction.
Predation: One species (predator) benefits and the other (prey) is harmed or killed.
Herbivory: An animal consumes plant parts, benefiting the herbivore and harming the plant.
Parasitism/Pathogens: The parasite/pathogen benefits while the host is harmed.
Commensalism: One species benefits while the other is neither helped nor harmed.

Interspecific Interaction	Effect on Species 1	Effect on Species 2	Example
Competition	–	–	Squirrels/black bears
Mutualism	+	+	Plants/mycorrhizae
Predation	+	–	Crocodiles/fish
Herbivory	+	–	Caterpillars/leaves
Parasites and pathogens	+	–	Heartworms/dogs; Salmonella/humans

Table of interspecific interactions and their effects

Competition and Resource Partitioning

Competition occurs when two species use the same limited resource. This can lead to competitive exclusion (one species outcompetes the other) or resource partitioning (species evolve to use different resources or habitats).

Diagram showing competitive exclusion and resource partitioning in Paramecium Finch beak size and resource partitioning

Mutualism

Mutualism is a relationship where both species benefit. Examples include pollinators and flowering plants, or mycorrhizal fungi and plant roots.

Bee pollinating a flower as an example of mutualism

Predation

Predation involves one organism (the predator) killing and consuming another (the prey). Predators have adaptations for capturing prey, while prey have evolved defenses to avoid predation.

Snake preying on a frog

Physical defenses: Thorns, shells, spines, etc.
Chemical defenses: Toxins, poisons.
Behavioral defenses: Fleeing, hiding, forming groups.

Examples of physical, chemical, warning, and camouflage defenses Examples of behavioral defenses: hiding or fighting back Horned lizard squirting blood as a defense

Herbivory

Herbivory is the consumption of plant parts by animals. Plants have evolved various defenses such as thorns, toxins, and tough leaves to reduce herbivory.

Caterpillars eating leaves as an example of herbivory

Parasitism and Pathogens

Parasites live in or on a host and derive nutrients at the host's expense. Pathogens are disease-causing organisms. Both have evolved strategies to maximize their success, sometimes manipulating host behavior.

Endoparasites: parasites that live inside their host Ectoparasites: parasites that live on their host Toxoplasma gondii and lancet fluke induce risky behavior in hosts Cholera bacterium as a pathogen

Commensalism

Commensalism is an interaction where one species benefits and the other is unaffected. An example is birds riding on large mammals to eat insects stirred up by movement.

Bird on buffalo as an example of commensalism

Community Health

Species Diversity

Species diversity, including species richness (number of species) and relative abundance, is crucial for community stability. Diverse communities are more resilient to disturbances and pathogens.

Comparison of diverse and monoculture plant communities

Keystone, Umbrella, Flagship, and Indicator Species

Certain species have a disproportionately large impact on community structure:

Keystone species: Their removal causes dramatic changes in community composition.
Umbrella species: Protecting them indirectly protects many other species.
Flagship species: Charismatic species used to rally conservation efforts.
Indicator species: Their presence, absence, or abundance reflects environmental conditions.

Diagram of keystone species concept Sea star as a keystone species Sea otters protect kelp forests from sea urchins Curlew as an umbrella species Indicator species illustration

Invasive Species

Invasive species are non-native organisms that cause ecological or economic harm in a new environment. They often outcompete native species, leading to reduced biodiversity and altered ecosystem processes.

Example: European rabbits in Australia multiplied rapidly, causing widespread ecological damage.

Ecosystem Determinants

Definition of an Ecosystem

An ecosystem includes all living organisms in a community plus the abiotic environment (chemical resources and physical conditions).

Biomes

Biomes are large ecosystems defined by climate, particularly temperature and precipitation. There are five major aquatic biomes (determined by salinity, depth, and water movement) and nine terrestrial biomes (determined by temperature and precipitation).

Diagram of aquatic biomes Diagram of terrestrial biomes

Climate Determinants

Temperature and precipitation are influenced by Earth's curvature, latitude, and ocean currents. These factors create distinct climate zones and influence biome distribution.

Sunlight and Earth's curvature affecting temperature Ocean currents affecting climate

Energy and Chemical Flow in Ecosystems

Energy Flow

Energy enters ecosystems as sunlight and is converted to chemical energy by photosynthesis (primary production). Energy flows through trophic levels (producers, consumers, decomposers), but only about 10% of energy is transferred from one level to the next; the rest is lost as heat.

Producers: Autotrophs (plants, algae, some bacteria) that convert solar energy to chemical energy.
Consumers: Heterotrophs that eat other organisms.
Decomposers: Organisms that break down dead material, recycling nutrients.

Energy flow from sun to producers Food web showing energy transfer

Chemical Cycling

Chemical elements cycle between abiotic reservoirs and living organisms. Major cycles include:

Water cycle: Involves evaporation, condensation, precipitation, and runoff.
Carbon cycle: Driven by photosynthesis and respiration; affected by fossil fuel burning and deforestation.
Nitrogen cycle: Nitrogen fixation by bacteria makes atmospheric nitrogen available to living things; fertilizers and runoff can disrupt the cycle.
Phosphorus cycle: Phosphorus moves through rocks, water, soil, and living organisms; mainly a local cycle.

Eutrophication

Eutrophication is caused by excess nitrogen and phosphorus, often from fertilizers, leading to algal blooms. When algae die, decomposers consume oxygen, creating "dead zones" where few organisms can survive.

Human Impacts on Communities and Ecosystems

Humans can have both positive and negative effects on ecosystems. Overconsumption, pollution, introduction of invasive species, and habitat destruction can destabilize communities and reduce biodiversity. Conservation efforts often focus on protecting keystone, umbrella, flagship, and indicator species to maintain ecosystem health.