23.2 Ecosystems, Food Webs, Populations & Biomes

Overview & Exam Relevance

Competency 014 of the TExES Core Subjects EC-6 Science exam evaluates candidate expertise in ecological concepts, energy and matter flow through ecosystems, food chains and complex trophic food webs, ecological energy pyramids, symbiotic species interactions, global biome characteristics, and the dynamics of ecological succession. In the Texas Essential Knowledge and Skills (TEKS), ecology forms a core conceptual strand across every elementary grade level. Texas students trace how organisms depend on each other and their environment in Kindergarten and Grade 1, construct basic food chains demonstrating energy flow originating from the Sun in Grades 2 and 3, and analyze food webs, energy transfer, and environmental changes in Grades 4 and 5.

On the TExES 391 examination, you will be assessed on your ability to correctly interpret food web diagrams (especially understanding the mandatory directionality of energy arrows), calculate energy loss across trophic levels using the 10% Rule, differentiate between bioaccumulation and biomagnification, classify symbiotic relationships (mutualism, commensalism, parasitism), analyze the climatic and ecological characteristics of major biomes, and contrast primary versus secondary succession. Furthermore, questions frequently test your diagnostic ability to resolve pervasive student misconceptions, such as confusing energy flow (which is open and unidirectional) with nutrient cycling (which is closed and circular), or misinterpreting the role of decomposers in food webs.

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The Ecological Hierarchy & Environmental Factors

Ecology is the scientific discipline investigating the interactions between living organisms and their physical and biological environment. Ecologists organize these complex interactions into a nested ecological hierarchy of increasing spatial scale and complexity:

THE ECOLOGICAL HIERARCHY
│
├── 1. ORGANISM ──────► An individual living entity (e.g., a single White-tailed Deer)
├── 2. POPULATION ────► Group of conspecific individuals interbreeding in the same area at the same time
│                       (e.g., all White-tailed Deer in a Texas Hill Country state park)
├── 3. COMMUNITY ─────► All interacting populations of diverse species coexisting in a shared habitat
│                       (e.g., deer, live oaks, grasses, ticks, coyotes, and soil microbes)
├── 4. ECOSYSTEM ─────► The biological community PLUS all abiotic physical/chemical environmental factors
│                       (e.g., the living organisms + limestone rock, stream water, sunlight, soil pH)
├── 5. BIOME ─────────► Broad regional/global ecological zone defined by climate and dominant vegetation
│                       (e.g., Temperate Deciduous Forest, Tropical Rainforest, Desert, Tundra)
└── 6. BIOSPHERE ─────► The global ecological sum of all ecosystems on Earth supporting life

Biotic versus Abiotic Environmental Factors

Every ecosystem is structured by dynamic interactions between two foundational classes of environmental variables:

• Biotic Factors: All living, formerly living, or organically derived components of an ecological system. These include primary producers (plants, algae), consumers (herbivores, carnivores, omnivores), detritivores (earthworms, vultures), decomposers (bacteria, fungi), leaf litter, and fallen logs. Biotic interactions encompass predation, competition, parasitism, mutualism, and disease transmission.
• Abiotic Factors: The non-living, physical and chemical parameters of the environment that establish the physical boundary conditions within which living organisms must survive. These include solar irradiance, annual temperature range, total precipitation, humidity, wind patterns, atmospheric gas concentrations ($O_2, CO_2$), soil composition (mineral content, grain size, porosity, nitrogen/phosphorus availability), aquatic salinity, water depth, and dissolved oxygen.

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Trophic Levels, Energy Flow & Food Web Dynamics

Energy and nutrients sustain all living communities, but their operational pathways through an ecosystem are fundamentally distinct: energy flows unidirectionally through an ecosystem (entering as solar radiation and dissipating into space as metabolic heat), whereas matter cycles indefinitely through biogeochemical cycles (carbon, nitrogen, water, phosphorus).

Trophic Classifications: Autotrophs, Heterotrophs, and Decomposers

Organisms occupy specific nutritional positions called trophic levels based on how they obtain biochemical energy:

• Producers (Autotrophs): Organisms capable of synthesizing organic macromolecules (glucose) from inorganic raw materials. The vast majority are photoautotrophs (plants, phytoplankton, cyanobacteria) that capture radiant sunlight via photosynthesis. In extreme environments lacking sunlight (such as deep-sea hydrothermal vents), chemoautotrophs (sulfur-oxidizing bacteria) harness chemical energy from inorganic molecules like hydrogen sulfide ($H_2S$) to produce carbohydrates.
• Consumers (Heterotrophs): Organisms that cannot produce their own organic nutrients and must ingest other living or once-living organisms:
  • Primary Consumers (Herbivores): Feed directly on autotrophic producers (e.g., zooplankton, caterpillars, cottontail rabbits, deer).
  • Secondary Consumers (Carnivores / Omnivores): Feed on primary consumers (e.g., frogs consuming insects, bluebirds consuming caterpillars).
  • Tertiary and Quaternary Consumers (Apex Predators): High-level carnivores that prey on secondary consumers. Apex predators occupy the uppermost trophic position in an ecosystem and have no natural predators (e.g., red-tailed hawks, orcas, wolves, lions).
  • Omnivores: Organisms whose diet routinely spans multiple trophic levels, consuming both producers and consumers (e.g., humans, black bears, raccoons).
• Decomposers and Detritivores: Essential ecological recyclers that break down moribund organic matter, fallen vegetation, and metabolic feces, returning vital inorganic mineral nutrients (nitrates, phosphates, potassium) to the abiotic soil and water:
  • Detritivores: Physically consume non-living organic detritus internally (e.g., earthworms, dung beetles, millipedes, sea cucumbers).
  • Decomposers (Saprotrophs): Heterotrophs (predominantly fungi and bacteria) that secrete extracellular hydrolytic digestive enzymes directly onto dead matter, breaking down complex polymers (cellulose, lignin, protein) externally and absorbing decomposed nutrients through cellular membranes.

Food Chains versus Complex Food Webs

• Food Chain: A simplified, linear diagram illustrating a single, direct sequence of feeding relationships through which energy and nutrients pass (e.g., Sun $\longrightarrow$ Grass $\longrightarrow$ Grasshopper $\longrightarrow$ Toad $\longrightarrow$ Snake $\longrightarrow$ Hawk).
• Food Web: A complex, highly realistic, interconnected network of multiple overlapping food chains reflecting the diverse, multi-trophic dietary relationships within an ecological community. Food webs provide ecological resilience: if disease decimates one prey species, predators can shift foraging effort toward alternate prey.
• The Golden Rule of Ecological Arrows: On the TExES 391 exam, you will encounter diagrammatic questions featuring food chains and food webs. The arrows ALWAYS point in the direction of energy and matter transfer—from the organism being eaten to the organism doing the eating (from prey to predator; from food into the belly of the consumer). Arrows do NOT point to what an organism hunts!

TERRESTRIAL FOOD WEB & ENERGY DIRECTIONALITY

            [ Hawk ] (Apex Predator / Tertiary Consumer)
               ▲                    ▲
               │                    │
        [ Rat Snake ]          [ Bullfrog ] (Secondary Consumers)
               ▲                    ▲
               │                    │
         [ Field Mouse ]      [ Grasshopper ] (Primary Consumers)
               ▲                    ▲
               │                    │
         [ Wild Clover ]      [ Prairie Grass ] (Primary Producers)
               ▲                    ▲
               └──────────┬─────────┘
                          │
                    [ THE SUN ] (Initial Radiant Energy Source)

* Arrows represent direction of energy flow (from organism consumed to consumer)

The 10% Rule of Ecological Energy Transfer (Lindeman's Efficiency)

In 1942, ecologist Raymond Lindeman formulated the 10% Rule of Ecological Efficiency, governing how energy is partitioned across successive trophic levels:

• On average, only approximately 10% of the net chemical energy stored as biomass at any given trophic level is successfully transferred and incorporated into the cellular biomass of the next higher trophic level.
• Approximately 90% of the energy is lost at each trophic transition:
  • The overwhelming majority is dissipated into the environment as low-grade metabolic heat during cellular respiration.
  • Significant energy is expended in daily life processes (locomotion, foraging, thermoregulation, cellular repair).
  • Portions of organisms are unconsumed (woody trunks, bones, teeth, claws, feathers) or undigested (feces, cellulose excreted as waste).

THE TROPHIC ENERGY PYRAMID (10% RULE)

          ▲
         / \
        / 10 J \      ◄── Tertiary Consumers (Hawk)
       /─────────\
      /   100 J   \    ◄── Secondary Consumers (Snakes)
     /─────────────\
    /   1,000 J     \  ◄── Primary Consumers (Herbivorous Mice)
   /─────────────────\
  /     10,000 J      \◄── Primary Producers (Grasses & Plants)
 /─────────────────────\

Consequences of the 10% Rule:

1. Energetic Limits on Food Chain Length: Because available energy declines exponentially at each successive trophic tier, ecosystems rarely support more than 4 or 5 trophic levels. Beyond that, the energy pool is insufficient to sustain viable breeding populations of predators.
2. Biomass Pyramid Configuration: In terrestrial ecosystems, the total standing biomass of producers must vastly exceed the biomass of primary consumers, which in turn exceeds secondary consumers. Apex predators require vast territorial foraging ranges to meet their metabolic requirements.

Bioaccumulation versus Biomagnification

Elementary candidates must clearly distinguish between two interrelated ecotoxicological phenomena involving persistent environmental pollutants (such as heavy metal methylmercury, polychlorinated biphenyls [PCBs], and synthetic organochlorine pesticides like DDT):

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Symbiotic Relationships & Interspecific Interactions

Symbiosis is defined as an intimate, prolonged physical and biological association between two distinct biological species. Symbiotic relationships are classified according to the net fitness effect (positive $+$, negative $-$, or neutral $0$) experienced by each participant:

INTERSPECIFIC ECOLOGICAL INTERACTIONS
│
├── MUTUALISM (+/+) ────────► Both participating species derive net biological survival/reproductive benefits
│   ├── Legumes & Rhizobium bacteria (root nodules fix atmospheric nitrogen for plant in exchange for glucose)
│   ├── Mycorrhizal fungi & plant roots (fungi extend water/phosphorus uptake; plant provides photosynthates)
│   ├── Clownfish & sea anemones (clownfish gains protection; anemone gains defense and nitrogen waste)
│   └── Flowering plants & insect pollinators (bees obtain nectar/pollen; plants achieve cross-pollination)
│
├── COMMENSALISM (+/0) ─────► One species benefits while the other species is neither helped nor harmed
│   ├── Epiphytic orchids anchoring on high rainforest canopy branches for light without harming the host tree
│   ├── Barnacles cementing to the skin of baleen whales, gaining mobility and filter-feeding currents
│   └── Cattle egrets foraging in pastures alongside grazing cattle, feeding on insects flushed from grass
│
├── PARASITISM (+/-) ───────► Parasite derives nutrition at the direct physical expense of the host
│   ├── Ticks, fleas, and lice feeding on mammalian blood, transmitting microbial pathogens
│   ├── Intestinal tapeworms absorbing pre-digested nutrients across their cuticles, causing host malnutrition
│   └── Mistletoe (*Phoradendron*) penetrating tree xylem via haustoria to steal water and mineral nutrients
│
├── PREDATION (+/-) ────────► Non-symbiotic: Free-living predator actively hunts, captures, kills, and consumes prey
│
└── COMPETITION (-/-) ──────► Interspecific: Two or more species compete for identical limiting resources
    ├── Gause's Competitive Exclusion Principle: Two species cannot permanently occupy the identical niche
    └── Resource (Niche) Partitioning: Species evolve specialized foraging zones to minimize competition

The Competitive Exclusion Principle versus Resource Partitioning

• Gause's Principle of Competitive Exclusion: Russian ecologist G.F. Gause demonstrated that two distinct species competing for the exact same limiting environmental resource (food, light, nesting cavities) cannot coexist indefinitely at stable population levels if their ecological niches are identical. The species with even a fractional competitive advantage will outcompete and drive the inferior competitor to local demographic extinction.
• Resource (Niche) Partitioning: In diverse natural ecosystems, potentially competing sympatric species coexist through the evolutionary divergence of their ecological niches. Classic empirical research by ecologist Robert MacArthur demonstrated that five distinct species of North American warblers (Setophaga) coexist within the exact same spruce trees by partitioning the canopy into distinct vertical foraging zones (some foraging exclusively on high outer tips, others in mid-canopy interior needles, and others on low dead branches), eliminating direct competitive conflict.

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How Organisms Respond to Stimuli

A stimulus is a change that causes a response. Internal stimuli include hunger, thirst, and changes in body temperature; external stimuli include light, temperature, sound, touch, and the presence of predators.

• Plant responses (tropisms): Phototropism is growth toward light, as when seedlings on a windowsill bend toward the window. Gravitropism (geotropism) sends roots down and shoots up. Thigmotropism is a response to touch, as when vines coil around a fence. Some plants respond quickly: a Venus flytrap snaps shut, and a sensitive plant (Mimosa pudica) folds its leaves when touched.
• Animal behaviors: Instinctive (inherited) behaviors include a spider spinning a web and a newborn mammal nursing. Learned behaviors include a dog learning to sit on command. Seasonal behaviors include migration (monarch butterflies migrate through Texas each fall to overwinter in the mountains of central Mexico), hibernation (winter dormancy), and estivation (summer dormancy during heat and drought).

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Populations: Competition, Limiting Factors, and Carrying Capacity

• Competition: Organisms with similar needs compete for limited resources such as food, water, light, and space. Competition occurs within a species and between species. Each species' adaptations give it a unique niche, its role in the ecosystem.
• Limiting factors keep populations from growing forever: food and water supply, space, shelter, disease, predators, and weather.
• Carrying capacity is the largest population an environment can support over time. With plentiful resources, a population may grow rapidly (a J-shaped curve). As it nears carrying capacity, growth slows and levels off (an S-shaped curve).
• Predator-prey cycles: When prey populations rise, predators have more food and increase. Predators then reduce the prey population, which later causes the predator population to fall.

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Organisms Change Ecosystems

Populations and species modify the ecosystems they live in. Beavers build dams that turn streams into ponds and wetlands, creating habitats for other species. Earthworms aerate and enrich soil. Invasive species, such as zebra mussels in Texas lakes and feral hogs across the state, outcompete native species and damage habitats. Humans change ecosystems through farming, building, and pollution, as well as through conservation and restoration.

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Terrestrial & Aquatic Biomes

A biome is a broad geographical regional complex of ecosystems characterized by a distinctive climate regime (specifically mean annual temperature and precipitation), adapted soil types, and dominant climax plant and animal communities.

Major Terrestrial Biomes

• Tropical Rainforest: Equatorial regions; characterized by warm temperatures year-round ($20\text{–}30^\circ\text{C}$) and extraordinarily high precipitation ($>200\text{ cm/year}$). Displays multi-tiered canopy stratification (emergent layer, canopy, understory, forest floor) and the highest terrestrial biodiversity on Earth. Despite luxurious vegetation, soils (oxisols) are heavily leached, acidic, and nutrient-poor because high heat and humidity accelerate decomposition, causing nutrients to be instantaneously reabsorbed by plant root systems rather than stored in topsoil.
• Temperate Deciduous Forest: Mid-latitude regions (eastern North America, western Europe); characterized by four distinct seasons (warm summers, freezing winters) and moderate precipitation ($75\text{–}150\text{ cm/year}$). Dominated by broadleaf deciduous hardwood trees (oaks, maples, beeches, hickories) that undergo leaf abscission in autumn to conserve water during freezing winter conditions. Produces rich, fertile topsoil (mollisols/alfisols) rich in organic humus from decaying leaf litter.
• Desert: Arid regions situated along $30^\circ$ North and South latitudes (Sahara, Sonoran, Chihuahuan); characterized by extreme aridity ($<25\text{ cm/year}$ of precipitation) and violent diurnal temperature fluctuations (scorching daylight, freezing nights). Dominated by specialized xerophytic flora (succulent cacti, deep-rooted mesquite) and nocturnal fauna.
• Grasslands (Temperate Prairie and Tropical Savanna):
  • Temperate Prairies (North American Great Plains): Characterized by moderate rainfall, freezing winters, hot summers, and periodic seasonal droughts and wildfires that prevent tree establishment. Boasts the deepest, most fertile topsoils on Earth, supporting vast herds of grazing ungulates (bison).
  • Tropical Savannas (African veld): Characterized by high temperatures year-round, prolonged dry seasons alternating with monsoon wet seasons, open grasslands interspersed with scattered drought-resistant acacia trees, and large migratory herbivore herds.
• Taiga (Boreal Forest): High northern latitudes immediately south of the tundra (Canada, Scandinavia, Russia); characterized by long, bitterly cold, snowy winters and short, moist, mild summers. Dominated by cold-tolerant, cone-bearing evergreen coniferous gymnosperms (spruce, fir, pine, larch) possessing needle-shaped leaves with thick waxy cuticles that resist freezing and shed heavy snow loads without limb breakage. Acidic, nutrient-poor podzol soils.
• Tundra: High Arctic latitudes and alpine mountain summits; characterized by extreme cold, ferocious desiccating winds, an exceptionally short growing season ($6\text{–}8\text{ weeks}$), and low precipitation ($<25\text{ cm/year}$, creating a "cold desert"). Underlain by permafrost—a permanently frozen subterranean layer of soil and ice that acts as an impermeable barrier preventing deep root penetration and causing summer surface pooling. Completely treeless landscape dominated by low-lying lichens, mosses, grasses, and dwarf perennial shrubs.

Aquatic Biomes

• Freshwater Ecosystems: Aquatic environments with low dissolved mineral salinity ($<0.5\text{ parts per thousand}$). Divided into lentic (standing water: lakes, ponds, wetlands/marshes) and lotic (flowing water: springs, streams, rivers). Characterized by vertical light zones (photic zone where photosynthesis exceeds respiration, and aphotic profundal zone).
• Marine Ecosystems: Ocean environments with high salinity (averaging $\approx 35\text{ parts per thousand}$). Encompasses intertidal zones, open pelagic oceans, biodiverse benthic coral reefs (constructed by symbiotic reef-building cnidarians and photosynthetic zooxanthellae), and aphotic abyssal zones powered by marine snow and hydrothermal vents.
• Estuaries: Semi-enclosed coastal transition environments where freshwater rivers merge with oceanic saltwater, creating dynamic brackish water gradients. Estuaries are subject to tidal fluctuations and trap massive sediment and organic nutrient loads from terrestrial runoff. They rank among the most biologically productive ecosystems on Earth, functioning as critical ecological nurseries that provide shelter, food, and breeding grounds for juvenile fish, crabs, shrimp, and migratory birds.

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Ecological Succession: Primary versus Secondary

Ecological succession is the predictable, chronological process through which the structure and species composition of an ecological community evolve and transform over time following a physical disturbance.

THE TWO PATHWAYS OF ECOLOGICAL SUCCESSION
│
├── PRIMARY SUCCESSION (Begins on lifeless, bare substrate DEVOID of soil)
│   ├── Substrates: Cooled volcanic basalt lava, retreating glacial moraines, sand dunes
│   ├── Pioneer Species: Lichens (fungus + alga symbiosis) and mosses
│   ├── Mechanism: Lichens secrete weak organic acids to weather bare rock into mineral dust;
│   │              accumulating dead pioneer organic matter generates the FIRST topsoil layer
│   └── Timeline: Exceptionally slow, requiring hundreds to thousands of years to reach climax
│
└── SECONDARY SUCCESSION (Begins following disturbance where SOIL REMAINS INTACT)
    ├── Disturbances: Wildfires, hurricanes, floods, abandoned agricultural farmland, clear-cutting
    ├── Pioneer Species: Fast-growing, opportunistic ruderal annual weeds, grasses, and wildflowers
    ├── Mechanism: Viable soil seed banks, fungal spores, and root networks already exist;
    │              succession proceeds rapidly through weed, shrub, and softwood forest stages
    └── Timeline: Relatively rapid, reaching mature climax community in decades to centuries

• Primary Succession: Unfolds in barren, lifeless terrestrial environments that completely lack pre-existing soil and biological organisms (such as newly cooled basaltic lava flows from a volcano, pulverized rock exposed by retreating glaciers, or shifting coastal sand dunes).
  • Pioneer Species: The initial hardy colonizing organisms must be capable of surviving on bare mineral rock under harsh environmental extremes. The classic pioneer species are lichens (a mutualistic symbiotic partnership between a fungus and a photosynthetic green alga or cyanobacterium) and specialized bryophyte mosses.
  • Soil Genesis: Lichens physically anchor to bare rock and secrete weak organic acids that chemically weather the rock surface into microscopic mineral particles. When lichens die, their decomposing organic biomass mixes with weathered rock particles, slowly forming the first thin layer of primitive soil. This thin soil allows windblown seeds of small annual weeds and grasses to germinate; their deeper roots accelerate rock fragmentation, producing thicker soil capable of supporting perennial herbs, shrubs, softwood trees, and ultimately a stable climax community (such as an oak-hickory forest).
• Secondary Succession: Occurs in an ecological area that previously supported a thriving biological community, but was disturbed or destroyed by a catastrophic event (such as a wildfire, hurricane, agricultural plowing, or clear-cut logging) that removed the vegetation while leaving the underlying fertile soil intact.
  • Ecological Rate: Secondary succession proceeds substantially faster than primary succession because nutrient-rich soil, subterranean dormant seed banks, fungal mycorrhizae, and underground root systems are already present.
  • Successional Trajectory: The pioneer species in secondary succession are opportunistic, fast-growing annual "weeds" (such as crabgrass and ragweed) whose wind-dispersed seeds capitalize on direct sunlight. Over decades, these are sequentially replaced by perennial grasses, woody shrubs (brambles), shade-intolerant sun-loving softwood trees (such as loblolly pines), and eventually shade-tolerant hardwood canopy trees (such as oaks and maples).

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Comparison Table: Symbiotic Interactions & Global Biomes

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Classroom Scenario Application

Classroom Context: Mrs. Kalu is facilitating a 5th-grade science investigation exploring food webs and ecological energy transfer. Students dissect sterilized commercial barn owl pellets, carefully separating fragile micro-mammalian bones (skulls, jaws, femurs, and ribs of voles, shrews, and field mice) from matted rodent fur.

Inquiry Task & Data Analysis: Students use bone identification dichotomous keys to count the number of prey organisms consumed by a single owl, construct an ecological food chain representing the barn owl's trophic pathway (Sun $\longrightarrow$ Prairie Grass $\longrightarrow$ Meadow Vole $\longrightarrow$ Barn Owl), and calculate the biomass required to sustain the owl using the 10% Rule.

Student Misconception: While sketching their food chains, a student draws an arrow pointing from the Barn Owl toward the Meadow Vole, explaining: "The arrow points to the vole because the owl flies down and attacks the mouse." Another student argues: "Decomposers like mushrooms shouldn't be in the food web at all because they don't hunt anything and they aren't part of the animal food chain."

Teacher's Guided Pedagogical Intervention:

1. Correcting Arrow Directionality: Mrs. Kalu redirects students back to the fundamental physics of ecological modeling: arrows in food chains and food webs always represent the transfer of chemical energy and biomass. She prompts: "When the owl eats the vole, does the vole's body energy go into the owl, or does the owl's energy go into the vole?" The students immediately recognize that the energy flows from the vole into the owl, requiring the arrow to point toward the consumer: $\text{Meadow Vole} \longrightarrow \text{Barn Owl}$.
2. Clarifying the Indispensable Role of Decomposers: She presents a closed ecological terrarium and asks students what would occur if decomposers (bacteria and fungi) were completely eliminated. Students realize that without decomposers, dead organisms, fur, and bones would accumulate indefinitely, and vital nitrogen, carbon, and mineral nutrients would remain permanently locked away, starving producers and causing the entire food web to collapse. She guides them to understand that decomposers connect every single trophic level back to abiotic nutrient reservoirs.