Ecosystems

Ecosystem Components

Definitions

An ecosystem is a dynamic system comprising a community of living organisms (biotic components) interacting with their non-living environment (abiotic components) through energy flows and nutrient cycles.

Biotic components are classified by their role in energy transfer:

• Producers (autotrophs): organisms that convert solar energy (or, rarely, chemical energy) into organic matter through photosynthesis or chemosynthesis. Includes green plants, algae, and some bacteria.
• Consumers (heterotrophs): organisms that obtain energy by consuming other organisms. Classified as primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), and tertiary consumers (top carnivores).
• Decomposers (detritivores and saprotrophs): organisms that break down dead organic matter and waste products, releasing nutrients back into the environment. Includes fungi, bacteria, earthworms, and some insects.

Abiotic components include climate (temperature, precipitation, sunlight, wind), edaphic factors (soil type, pH, texture, mineral content), topography (altitude, aspect, slope), and hydrology (water availability, water chemistry).

Trophic Levels and Food Chains

A trophic level represents the position of an organism in a food chain. Energy flows from lower to higher trophic levels:

$\mathrm{Producer} \to \mathrm{Primary consumer} \to \mathrm{Secondary consumer} \to \mathrm{Tertiary consumer}$

A food web is a network of interconnected food chains, reflecting the complexity of real ecosystems where most organisms occupy multiple trophic levels and have varied diets.

Ecological Niches

An organism's niche describes its role within the ecosystem: its habitat, feeding relationships, interactions with other species, and environmental tolerances. The competitive exclusion principle states that two species cannot occupy exactly the same niche in the same habitat; one will outcompete the other.

Energy Flow

Laws of Thermodynamics in Ecosystems

The First Law: energy cannot be created or destroyed, only transformed. Solar energy entering the ecosystem is converted to chemical energy in producers and then transferred through the food chain.

The Second Law: energy transfers are inefficient; at each trophic level, energy is lost as heat through respiration, excretion, and incomplete consumption. This means energy flow through an ecosystem is unidirectional and diminishes at each step.

Ecological Efficiency and Pyramids

Gross Primary Productivity (GPP) is the total amount of solar energy converted to chemical energy by producers per unit area per unit time:

$\mathrm{GPP} = \mathrm{Total photosynthesis}$

Net Primary Productivity (NPP) is the energy remaining after plant respiration:

$\mathrm{NPP} = \mathrm{GPP} - R_p$

Where $R_p$ is plant respiration. NPP represents the energy available to herbivores.

Net Secondary Productivity (NSP) is the energy stored in consumer biomass after accounting for respiration and excretion:

$\mathrm{NSP} = \mathrm{Energy consumed} - \mathrm{Energy lost (respiration, excretion)}$

Ecological efficiency (trophic efficiency) is typically $10$--$20\%$, meaning that only $10$--$20\%$ of the energy at one trophic level is transferred to the next. This is why food chains rarely exceed four or five trophic levels.

Pyramids of number, biomass, and energy:

• Pyramid of numbers: the number of organisms at each trophic level. Usually pyramid-shaped but can be inverted (e.g., one tree supporting many insects).
• Pyramid of biomass: the total dry mass of organisms at each trophic level. Usually pyramid-shaped but can be inverted in aquatic systems where phytoplankton have rapid turnover and low standing biomass but high productivity.
• Pyramid of energy: always pyramid-shaped (cannot be inverted) because energy is lost at each trophic level.

Nutrient Cycles

Unlike energy, nutrients cycle continuously within ecosystems. The major cycles involve carbon, nitrogen, and phosphorus.

The Carbon Cycle

Carbon exists in four main reservoirs: the atmosphere ($\mathrm{CO_2}$), the biosphere (organic carbon in living and dead organisms), the oceans (dissolved $\mathrm{CO_2}$, bicarbonate, marine organisms), and the lithosphere (fossil fuels, carbonate rocks).

Key processes:

• Photosynthesis: $\mathrm{6CO_2 + 6H_2O \to C_6H_{12}O_6 + 6O_2}$
• Respiration: $\mathrm{C_6H_{12}O_6 + 6O_2 \to 6CO_2 + 6H_2O + \mathrm{energy}}$
• Decomposition: organic matter broken down by decomposers, releasing $\mathrm{CO_2}$
• Combustion: burning of fossil fuels and biomass releases stored carbon as $\mathrm{CO_2}$
• Ocean exchange: $\mathrm{CO_2}$ dissolves in ocean water; marine organisms fix carbon through photosynthesis; carbonate shells accumulate on the ocean floor

The Nitrogen Cycle

Nitrogen is essential for amino acids, proteins, and nucleic acids. Despite comprising $78\%$ of the atmosphere, atmospheric $\mathrm{N_2}$ is inert and unavailable to most organisms.

Key processes:

• Nitrogen fixation: conversion of atmospheric $\mathrm{N_2}$ to ammonia ($\mathrm{NH_3}$) by nitrogen-fixing bacteria (e.g., Rhizobium in legume root nodules) or lightning
• Nitrification: conversion of $\mathrm{NH_3}$ to nitrite ($\mathrm{NO_2^-}$) by Nitrosomonas bacteria, then to nitrate ($\mathrm{NO_3^-}$) by Nitrobacter bacteria. Nitrate is the form most readily absorbed by plants.
• Assimilation: plants absorb nitrates through their roots and incorporate nitrogen into organic compounds; animals obtain nitrogen by consuming plants or other animals
• Ammonification: decomposers break down organic nitrogen (in dead organisms and waste) into $\mathrm{NH_3}$
• Denitrification: conversion of nitrates back to atmospheric $\mathrm{N_2}$ by denitrifying bacteria (e.g., Pseudomonas) in anaerobic conditions (waterlogged soils)

The Phosphorus Cycle

Phosphorus is a key component of DNA, RNA, ATP, and cell membranes. Unlike carbon and nitrogen, phosphorus has no significant gaseous phase; it cycles primarily through the lithosphere and hydrosphere.

Key processes:

• Weathering: phosphate rocks are slowly weathered, releasing phosphate ions ($\mathrm{PO_4^{3-}}$) into soil and water
• Uptake by plants: plants absorb phosphate from the soil
• Consumption and excretion: animals obtain phosphorus by eating plants; phosphorus is returned to the soil through waste and decomposition
• Sedimentation: phosphate is transported to oceans by rivers, accumulates in marine sediments, and is eventually returned to land over geological timescales through uplift and weathering
• Human inputs: phosphate mining for fertiliser has significantly accelerated the phosphorus cycle, causing eutrophication

Biomes

A biome is a large-scale ecosystem characterised by its climate (particularly temperature and precipitation), vegetation, and fauna. The major terrestrial biomes are:

Tropical Rainforest

Location: within $5^{\circ}$--$10^{\circ}$ of the equator (Amazon Basin, Congo Basin, Southeast Asia).

Climate: hot ($25$--$28^{\circ}\mathrm{C}$ year-round), wet ($> 2000\;\mathrm{mm/year}$), no dry season. High humidity ($> 80\%$).

Structure: multiple vertical layers:

• Emergent layer: tallest trees ($40$--$60\;\mathrm{m}$), exposed to high wind and light
• Canopy: continuous layer ($25$--$40\;\mathrm{m}$), intercepts most sunlight and rainfall, highest biodiversity
• Understorey: shrubs and saplings adapted to low light ($3$--$15\;\mathrm{m}$)
• Forest floor: dark, humid, rapid decomposition due to warmth and moisture; thin nutrient-poor soil (latosol)

Characteristics: extremely high biodiversity (over $50\%$ of terrestrial species), dense vegetation, buttress roots, epiphytes, lianas. Nutrients are locked in the biomass, not the soil; deforestation rapidly depletes the nutrient store.

Threats: deforestation for agriculture (cattle ranching, soybean cultivation), logging, mining, road construction, climate change.

Tropical Grassland (Savanna)

Location: between tropical rainforest and desert belts ($5^{\circ}$--$20^{\circ}$ north and south).

Climate: hot, distinct wet and dry seasons. Annual rainfall $500$--$1500\;\mathrm{mm}$. Dry season can last $5$--$8$ months.

Characteristics: tall grasses with scattered trees (baobab, acacia). Frequent fires maintain the grassland. Large herbivore populations (wildebeest, zebras, elephants) and their predators.

Threats: overgrazing, conversion to cropland, desertification.

Temperate Grassland (Prairie/Pampas/Steppe)

Location: interior of continents in the mid-latitudes (North America, South America, Eurasia).

Climate: continental climate with hot summers and cold winters. Annual rainfall $250$--$750\;\mathrm{mm}$, insufficient to support forests.

Characteristics: deep, fertile soils (chernozem/mollisol), now extensively cultivated for cereal production. Natural vegetation is drought- and fire-adapted grasses.

Desert

Location: approximately $20^{\circ}$--$30^{\circ}$ north and south (Hadley cell subsidence), continental interiors, rain shadow zones.

Climate: arid (rainfall $< 250\;\mathrm{mm/year}$). Extreme diurnal temperature range. High evapotranspiration.

Characteristics: sparse vegetation adapted to water scarcity (succulents, deep-rooted shrubs, ephemeral annuals). Soils are thin, saline, and low in organic matter.

Temperate Deciduous Forest

Location: western and central Europe, eastern North America, eastern Asia.

Climate: moderate temperatures ($4$--$18^{\circ}\mathrm{C}$), distinct seasons, annual rainfall $600$--$1500\;\mathrm{mm}$ distributed throughout the year.

Characteristics: trees that shed leaves in autumn (oak, beech, maple) to reduce water loss during winter. Rich, fertile soils (brown earths). Moderate biodiversity.

Taiga (Boreal Forest)

Location: $50^{\circ}$--$70^{\circ}$ north (Canada, Scandinavia, Russia — the world's largest terrestrial biome).

Climate: long, harsh winters ($-30^{\circ}\mathrm{C}$ or lower), short, cool summers ($10$--$15^{\circ}\mathrm{C}$). Annual rainfall $300$--$800\;\mathrm{mm}$, mostly in summer. Low evapotranspiration.

Characteristics: dominated by coniferous trees (pine, spruce, fir) adapted to cold and low light. Needle-shaped leaves reduce water loss and snow accumulation. Evergreen canopy allows year-round photosynthesis during the short growing season. Thin, acidic, nutrient-poor soils (podzol) with a thick layer of undecomposed litter (mor humus) due to slow decomposition in cold conditions.

Adaptations:

• Trees: conical shape sheds snow; needle leaves reduce water loss and frost damage; shallow root systems suit the thin active layer above permafrost; evergreen habit maximises the short growing season
• Animals: migration (birds), hibernation (bears), thick insulating fur (lynx, wolves), seasonal coat colour change (arctic fox, snowshoe hare), fat storage

Threats: commercial logging, forest fires (increasing in frequency and severity due to climate change), permafrost thaw destabilising root systems, oil and gas extraction, climate-driven northward shift of the biome boundary.

Tundra

Location: north of the taiga ($> 70^{\circ}$), also at high altitudes (alpine tundra).

Climate: extremely cold ($-40^{\circ}\mathrm{C}$ in winter), short growing season ($6$--$10$ weeks), low precipitation ($< 250\;\mathrm{mm}$), permafrost.

Characteristics: no trees; low shrubs, mosses, lichens, and grasses. Permafrost restricts root growth and drainage. Low biodiversity but high endemism.

Common Pitfalls

• Confusing GPP with NPP. NPP is the energy available to consumers; GPP includes the energy used by plants in respiration.
• Stating that the pyramid of biomass is "always pyramid-shaped." Inverted pyramids of biomass occur in aquatic ecosystems where phytoplankton have rapid turnover but low standing biomass.
• Confusing the carbon cycle with the nitrogen cycle. Carbon cycles through the atmosphere, biosphere, oceans, and lithosphere; nitrogen is fixed from the atmosphere by bacteria and has no significant sedimentary reservoir.
• Describing the taiga as having "poor soil" without specifying why: low temperatures slow decomposition, producing acidic, nutrient-poor podzols with a thick mor humus layer.
• Confusing weathering (release of phosphorus from rocks) with nitrogen fixation (conversion of atmospheric $\mathrm{N_2}$). These are entirely different processes in different nutrient cycles.

Practice Problems

Problem 1: Energy Flow Calculation

A field of grass has an NPP of $20000\;\mathrm{kJ/m^2/year}$. Cattle grazing on the field consume $8000\;\mathrm{kJ/m^2/year}$. The cattle lose $6000\;\mathrm{kJ/m^2/year}$ through respiration and excretion. Calculate the ecological efficiency of energy transfer from the grass to the cattle.

$\mathrm{Energy consumed by cattle} = 8000\;\mathrm{kJ/m^2/year}$

$\mathrm{Energy stored in cattle biomass (NSP)} = 8000 - 6000 = 2000\;\mathrm{kJ/m^2/year}$

$\mathrm{Ecological efficiency} = \frac◆LB◆\mathrm{NSP}◆RB◆◆LB◆\mathrm{NPP of producer}◆RB◆ \times 100 = \frac{2000}{20000} \times 100 = 10\%$

The ecological efficiency is $10\%$, consistent with the typical range of $10$--$20\%$.

Problem 2: Biome Identification

A biome experiences average annual temperatures of $-15^{\circ}\mathrm{C}$, receives $350\;\mathrm{mm}$ of precipitation annually (mostly in summer), has permafrost beneath the surface, and is dominated by coniferous trees. Identify the biome and explain the adaptations of the vegetation.

This is the taiga (boreal forest).

Vegetation adaptations:

• Conical shape: reduces snow accumulation on branches, preventing breakage
• Needle leaves: small surface area reduces water loss through transpiration; thick waxy cuticle further reduces water loss; needles shed snow easily
• Evergreen habit: allows photosynthesis to begin immediately during the short growing season without needing to grow new leaves
• Shallow root systems: the permafrost creates an impermeable layer; shallow roots exploit the thin active layer above the permafrost
• Dark green colour: maximises absorption of the limited solar radiation
• Slow growth rate: conserves energy in the nutrient-poor environment

Problem 3: Nutrient Cycle Evaluation

A tropical rainforest is cleared for cattle ranching. Explain the impact on the local nutrient cycle and the long-term consequences for soil fertility.

In an undisturbed tropical rainforest, nutrients are rapidly taken up by vegetation and recycled through decomposition. The nutrient store is primarily in the biomass, not the soil (latosols are thin, acidic, and nutrient-poor).

When the forest is cleared:

• The biomass store is removed (burning or harvesting), releasing a pulse of nutrients into the soil
• The protective canopy is removed, exposing the soil to heavy tropical rainfall, which leaches nutrients rapidly
• High temperatures accelerate decomposition of remaining organic matter, further releasing nutrients
• Without vegetation to absorb them, nutrients are leached beyond the root zone and lost
• Within $2$--$3$ years, the soil becomes severely depleted of nutrients
• Cattle ranching productivity declines rapidly; farmers often abandon the land after a few years and clear more forest

Long-term consequence: soil degradation (laterisation and hardening of the soil surface), loss of soil structure, reduced water infiltration, and increased surface runoff and erosion. Recovery of the original rainforest is extremely slow once the nutrient cycle is disrupted.

Problem 4: Inverted Pyramid of Biomass

Explain how an inverted pyramid of biomass can exist in a marine ecosystem despite the second law of thermodynamics.

In a marine ecosystem, the pyramid of biomass can be inverted because phytoplankton (producers) have a very high turnover rate: they reproduce rapidly and are consumed quickly by zooplankton (primary consumers). At any given moment, the standing biomass of phytoplankton is low, but their total productivity (the amount of biomass produced per unit time) far exceeds that of the zooplankton.

The pyramid of energy, however, is never inverted. The total energy flow through the phytoplankton trophic level exceeds the energy flow through the zooplankton level, in accordance with the second law of thermodynamics. The inversion of the biomass pyramid reflects the timing of measurement (a snapshot of standing biomass) rather than a violation of energy laws.

Problem 5: Climate Change Impact on Biomes

Discuss how climate change is likely to affect the taiga biome over the next 50 years.

• Northward migration: as temperatures rise, the taiga's southern boundary will shift northward, and the tundra to the north will be replaced by taiga vegetation
• Permafrost thaw: warming causes the active layer (soil above permafrost that thaws in summer) to deepen, destabilising tree root systems and causing "drunken forests" (trees tilting as the ground subsides)
• Increased wildfire frequency and severity: warmer, drier summers increase fire risk; larger and more frequent fires release stored carbon, reduce forest area, and alter species composition
• Pest outbreaks: warmer winters allow insect pests (e.g., spruce bark beetle) to survive and expand their range, causing large-scale tree mortality
• Species composition shifts: broadleaf deciduous species (e.g., birch, aspen) may outcompete conifers at the southern margin of the taiga
• Carbon cycle feedback: permafrost contains vast quantities of frozen organic carbon; thawing releases $\mathrm{CO_2}$ and methane, accelerating warming (positive feedback)
• Hydrological changes: permafrost thaw alters drainage patterns, creating thermokarst lakes and wetlands while drying previously waterlogged areas