Ecosystem Energy Flow, Trophic Structure, and Community Interactions

Ecosystem Energy Flow

Net Primary Production (NPP)

Energy enters ecosystems primarily through net primary production (NPP), which is the amount of energy captured by autotrophs (such as plants and algae) via photosynthesis, minus the energy they use for respiration.

• NPP provides the energy base for all other organisms in the ecosystem.

• Energy flows through ecosystems via trophic interactions (who eats whom).

Energy Transfer and Trophic Efficiency

Energy is transferred from one trophic level to the next, but only a fraction is passed on due to losses from respiration, waste, and inefficiency.

• Consumption efficiency: Proportion of available biomass ingested by consumers.

• Assimilation efficiency: Proportion of ingested biomass assimilated by digestion.

• Production efficiency: Proportion of assimilated biomass used to produce new consumer biomass.

Most energy is lost as heat or waste, and only a small percentage is transferred to higher trophic levels.

Key Equations

• Trophic Efficiency: $\text{Trophic Efficiency} = \frac{\text{Production at one trophic level}}{\text{Production at the previous trophic level}}$

Trophic Structure

Trophic Pyramids and Biomass

Trophic pyramids illustrate the distribution of biomass and energy among different trophic levels in an ecosystem.

• Primary producers (plants, algae) form the base of the pyramid.

• Primary consumers (herbivores) eat producers.

• Secondary consumers (carnivores) eat herbivores.

• Tertiary consumers (top predators) eat other carnivores.

• Decomposers break down dead organic matter, recycling nutrients.

Example Table: Trophic Levels

Trophic Level	Example Organisms
Producers	Green plants, algae
Primary Consumers	Rabbits, insects
Secondary Consumers	Snakes, fish
Tertiary Consumers	Eagles, lions
Decomposers	Bacteria, fungi

Food Webs

Food webs represent complex networks of feeding relationships, showing how energy and nutrients flow through communities.

• Include multiple predator-prey and other interactions.

• Help illustrate the interconnectedness of species within an ecosystem.

Wider Implications of Trophic Structure

Conservation Biology and Human Health

Trophic structure has important implications for conservation and human health.

• Disruption of trophic levels can lead to loss of biodiversity and ecosystem services.

• Humans at higher trophic levels are more vulnerable to toxins due to biomagnification.

Biomagnification and Mercury Pollution

Biomagnification is the process by which toxins become more concentrated at higher trophic levels.

• Mercury pollution in oceans leads to high mercury levels in top predators (e.g., tuna, humans).

• Toxic effects include kidney, lung, cardiovascular, and neurological damage.

Example Table: Mercury Concentration Across Trophic Levels

Trophic Level	Mercury Concentration
Water	Low
Algae	Higher
Insects	Higher
Fish	High
Humans	Highest

Community Structure and Interspecific Interactions

Predator-Prey and Competition

Organisms interact in various ways, including predation and competition for resources.

• Interspecific competition: Occurs when different species compete for the same limited resource.

• Competitive Exclusion Principle: Two species with identical resource requirements cannot coexist indefinitely in the same community.

• Resource partitioning: Coexisting species divide resources to minimize competition.

Example Table: Resource Partitioning

Species	Resource Use
Species 1	Uses resource at one time/space
Species 2	Uses resource at different time/space

Ecological Niche and Niche Partitioning

An ecological niche is the sum total of an organism's use of biotic and abiotic resources in its environment.

• Niche partitioning allows similar species to coexist by utilizing different resources or habitats.

• Example: Different warbler species forage at different heights in the same tree.

Consequences of Interspecific Competition

Community Effects

Competition can shape community structure in several ways:

• Exclusion of species from a community.

• Alteration of species' habitat or diet.

• Complex effects on community composition and stability.

Kelp Forests: A Case Study

Diverse Community Structure

Kelp forests are highly diverse marine ecosystems, providing habitat and nursery grounds for many species.

• Common inhabitants: polychaetes, amphipods, decapods, gastropods, sponges, tunicates, anemones, urchins, abalone, starfish, rockfish, seals, sea otters, birds, and others.

Food Web Dynamics

Kelp forests have complex food webs, with interactions among producers, consumers, and decomposers.

• Loss of key species (e.g., sea otters) can disrupt the food web, leading to population explosions of other species (e.g., urchins) and decline of kelp.

Example Table: Population Density in Kelp Forests

Location	Sea Otter Density	Sea Urchin Density	Kelp Density
A	High	Low	High
B	Medium	Medium	Medium
C	Low	High	Low
D	Low	High	Low

Keystone Species and Ecosystem Engineers

Keystone Species

A keystone species is one that has a disproportionately large effect on its ecosystem relative to its abundance.

• Helps maintain species richness by controlling populations of dominant competitors or predators.

• Examples: Sea otters in kelp forests, wolves in forests.

Ecosystem Engineers

Ecosystem engineers are organisms that directly or indirectly modulate the availability of resources to other species by causing physical changes in biotic or abiotic materials.

• Examples: Beavers (build dams), elephants (modify vegetation).

Human Impacts and Conservation

Marine Mammal Protection Act (1972)

Legislation aimed at protecting marine mammals, such as sea otters, which are slow to recover due to various threats.

• Urchin culling operations may be necessary to restore kelp forests.

• Kelp is threatened by rising ocean temperatures.

Energy Sources in Ecosystems

Dependence on Sunlight

Most ecosystems rely on sunlight as the primary energy source, but some deep-sea ecosystems exist without light.

• Photic zone: Sunlit upper layer of water where photosynthesis occurs.

• Aphotic zone: Deep water layer with no sunlight; some ecosystems rely on chemical energy (chemosynthesis).

Example Table: Ocean Zones

Zone	Depth (m)	Light Availability
Photic	0-200	High
Mid-Water	200-1000	Low
Aphotic	1000+	None

Additional info: Chemosynthetic bacteria can support entire communities in aphotic zones, such as hydrothermal vent ecosystems.