Unit 2: The Living World: Biodiversity

Ecosystem Services and Why Biodiversity Matters

An ecosystem is a community of living organisms (plants, animals, microbes) interacting with each other and with the nonliving environment (water, air, soil, climate). Biodiversity can sound like a “nature appreciation” idea, but in AP Environmental Science it is tightly linked to how ecosystems function and to the concrete benefits humans receive.

What ecosystem services are

Ecosystem services are the benefits people obtain from ecosystems. A core APES idea is that healthy ecosystems do work “for free,” and when ecosystems are damaged, society often has to replace those services with technology or infrastructure, which can be expensive and sometimes less effective.

Ecosystem services are commonly grouped into four categories:

1. Provisioning services are tangible goods harvested or used from ecosystems. These include food (fish, crops), fresh water, timber, medicinal resources, and raw materials from livestock such as fiber (wool), meat, and milk.
2. Regulating services are natural processes that regulate environmental conditions. Examples include water purification, flood control, climate regulation through carbon storage, erosion control, and disease regulation. A particularly testable regulating service is natural pest control: predators and parasites help keep pest populations in balance, which can keep food prices lower and reduce the need for pesticides.
3. Supporting services are underlying processes that make the other services possible, such as nutrient cycling, soil formation, and primary production (photosynthesis creating biomass). Supporting services also include forming new soil and renewing soil fertility, which can increase crop yields and reduce the need for fertilizers.
4. Cultural services are nonmaterial benefits such as recreation, aesthetic value, and spiritual or educational value. For example, sustainable fisheries and aquaculture can support recreation; recreational fishing is directly linked to healthy aquatic ecosystems.

A common misconception is that supporting services are “less important” because they feel indirect. In reality, supporting services are the foundation; if they collapse, provisioning and regulating services often collapse too.

Why biodiversity supports ecosystem services

Biodiversity is the variety of life at multiple levels (genes, species, ecosystems). It matters because different organisms often play different ecological roles. Ecosystems with greater functional variety are often better able to maintain productivity under changing conditions, resist disturbances (drought, pests, storms), and recover after disturbances through regrowth and recolonization. A useful way to explain this on FRQs is to treat biodiversity like a diversified “team” for ecosystem tasks: if roles are shared across many species, the system tends to be less fragile.

Pollination as a concrete example

Pollination is a classic ecosystem service with a direct link to food production. Many flowering plants depend on animal pollinators (insects, birds, bats). Pollinator diversity can stabilize pollination: if one pollinator declines due to disease or pesticide exposure, other pollinators may still provide the service.

A common student mistake is assuming pollination is “just bees.” In reality, many organisms pollinate, and ecosystems with multiple pollinator species can be more resilient.

Water purification and wetlands

Wetlands (marshes, swamps, bogs) often act like natural water filters. Water slows down in wetlands, sediments settle out, and plants and microbes can absorb or transform nutrients and some pollutants. If wetlands are drained or filled, communities often need engineered water-treatment solutions and can become more vulnerable to flooding.

Trade-offs and unintended consequences

Environmental decisions often involve trade-offs between short-term provisioning services and long-term regulating/supporting services. Clearing a forest might increase short-term provisioning (timber, farmland) but can reduce regulating services (carbon storage, flood control) and supporting services (soil formation, nutrient cycling). A key APES skill is explaining these cause-and-effect chains clearly.

Exam Focus

• Typical question patterns:
  • Explain how a specific ecosystem (wetland, forest, mangrove, coral reef) provides at least two ecosystem services.
  • Describe how biodiversity loss can reduce ecosystem stability or resilience.
  • Evaluate a land-use decision by identifying ecosystem services gained and lost.
• Common mistakes:
  • Confusing supporting vs. regulating services (nutrient cycling is supporting; water purification is regulating).
  • Listing services without explaining the mechanism (you must explain how wetlands filter water, how forests reduce erosion, etc.).
  • Treating biodiversity as “just number of species,” ignoring that roles and interactions matter.

Biodiversity: Levels, Patterns, and How We Measure It

Biodiversity is the variability among species, between species, and of ecosystems. In APES, treat biodiversity as multi-layered rather than a single number. Biodiversity is important because it helps keep the environment in a natural balance and helps ecosystems remain stable when conditions change.

The three levels of biodiversity

1. Genetic diversity is variation in DNA within a species. It includes the range of all genetic traits (both expressed and recessive) that make up the gene pool of that species. Genetic variation increases the chance that some individuals can survive changes such as disease outbreaks or climate shifts.

   Population bottleneck: a large reduction in the size of a single population due to a catastrophic environmental event. After a bottleneck, the remaining gene pool is smaller, so future generations often have less genetic diversity.

   Minimum viable population size (MVP): the number of individuals remaining after a bottleneck and how that compares to the smallest possible size at which a population can exist without facing extinction from a natural disaster. MVP connects to genetic diversity, inbreeding risk, and the chance that random events wipe out the population.

2. Species diversity is the variety of species in an area. A basic way to describe it is the number of different species that inhabit a specific area, but in APES you also separate this into:

   • Species richness: the number of different species represented in a community or region.
   • Species evenness: how evenly individuals are distributed among species.

3. Ecosystem diversity is the range of habitats/ecosystems found in a specific area. More ecosystem types can support more species and provide a broader set of ecosystem services.

A frequent student error is mixing up these categories (for example, calling the number of habitats “species diversity”). Keep the levels distinct.

What high biodiversity ecosystems tend to look like

Ecosystems with high biodiversity are often characterized by abundant natural resources, large genetic diversity, complex food webs involving many ecological niches, large numbers of organisms of different species, and large numbers of different species.

Species richness vs. evenness (why both matter)

Two communities can have the same richness but very different diversity depending on evenness. If one species dominates and the others are rare, the community may be more vulnerable if that dominant species is harmed. If individuals are distributed more evenly, ecological roles may be shared more broadly.

Concrete illustration:

• Community A: 100 individuals, 10 species, but 91 individuals are from one species.
• Community B: 100 individuals, 10 species, and each species has about 10 individuals.

Both have the same richness, but Community B is typically described as having higher species diversity because it has greater evenness.

Diversity increasers vs. diversity decreasers

On exams, you’re often asked to predict whether biodiversity will rise or fall under certain conditions. The patterns below are commonly used as general tendencies.

Anthropogenic activities that can reduce biodiversity

Human activities can reduce biodiversity directly (habitat loss, pollution, harvesting) and indirectly (changing environmental conditions that exceed tolerance levels). You should be able to explain a mechanism and propose a realistic remediation.

Loss of habitat and the role of specialists vs. generalists

Habitat loss often eliminates specialist species first.

• Generalist species live in many types of environments and often have varied diets.
  • Example: raccoons are omnivores and can survive on a wide variety of foods.
• Specialist species require unique resources, often have limited diets, and may need a specific habitat.
  • Example: the giant panda survives almost entirely on bamboo and lives in remote bamboo forests in China.

Keystone species: small changes, big effects

A keystone species has an impact on ecosystem structure that is disproportionately large relative to its abundance. Removing one can cause major ecosystem shifts and can even trigger extinctions of other species (often through trophic cascades, habitat modification, or loss of a key mutualism). Keystone species are not always predators; they can also be ecosystem engineers or critical mutualists.

Examples:

• Certain bat species pollinate critical rainforest trees and disperse their seeds.
• Grizzly bears transfer nutrients from oceanic to forest ecosystems.
• Prairie dogs aerate soil through burrowing and provide burrows used by other animals for shelter and hibernation.
• Sea stars prey on sea urchins, mussels, and other shellfish with few natural predators, keeping those populations in check.

Indicator species (bioindicators)

An indicator species is an organism whose presence, absence, or abundance reflects a specific environmental condition and can indicate ecosystem health.

Examples:

• Caddisflies, mayflies, and stoneflies indicate high dissolved oxygen in water.
• Lichens (some species) indicate air pollution.
• Mollusks can indicate water pollution.
• Mosses indicate acidic soil.
• Sludge worms indicate stagnant, oxygen-poor water.

Additional related resource: Chapter 3: Populations

Exam Focus

• Typical question patterns:
  • Distinguish genetic, species, and ecosystem diversity and explain why each matters.
  • Use richness vs. evenness to compare communities.
  • Explain how a population bottleneck reduces genetic diversity and why MVP matters.
  • Identify or explain the role of a keystone species or indicator species and predict consequences of changes.
  • Connect specific human activities to biodiversity loss and propose a remediation.
• Common mistakes:
  • Treating “biodiversity” as only species richness and ignoring evenness and genetic diversity.
  • Mixing up biodiversity levels (genes vs. species vs. ecosystems).
  • Treating keystone species as “always predators,” or listing an indicator species without stating the condition it indicates.

Island Biogeography, Habitat Fragmentation, and Edge Effects

In APES, “islands” are not only literal islands. An island can be any suitable habitat for a specific ecosystem that is surrounded by a large area of unsuitable habitat. A forest fragment surrounded by farmland can function like an island for forest species.

Island biogeography theory

Island biogeography examines factors that affect the richness and diversity of species living in isolated natural communities. The theory of island biogeography proposes that the number of species on an island is determined by a balance between:

• immigration (new species arriving)
• extinction (species disappearing)

Key drivers:

• Island size: larger islands typically have more biodiversity because they are bigger “targets” (easier for migrants to find), contain more habitats, and can support larger populations with lower extinction rates.
• Degree of isolation (distance to mainland/nearest island): closer islands tend to have more immigration and thus more biodiversity.

Other influences that may be relevant in scenarios include habitat suitability, climate, initial plant and animal composition, current species composition, human activity and disruption, and location relative to ocean currents.

Island biogeography is used to predict biodiversity patterns and extinction rates in fragmented habitats on continents.

Habitat fragmentation and edge effects

Habitat fragmentation occurs when a habitat is broken into pieces by development, industry, logging, roads, and similar disturbances. It is often described as a main threat to terrestrial biodiversity.

Fragmentation can reduce biodiversity by:

• reducing total habitat area
• increasing isolation between patches (lower immigration, higher local extinction)
• increasing the proportion of edge habitat

Edge effects are changes in abiotic and biotic conditions near boundaries (often more light, wind, temperature fluctuation, and easier access for predators and invasive species). Some species thrive at edges, but many interior specialists decline.

Exam Focus

• Typical question patterns:
  • Predict how changing island size or isolation changes immigration, extinction, and species richness.
  • Apply island biogeography to habitat fragments (forests, parks, reserves) and predict extinction risk.
  • Explain how fragmentation increases edge effects and how edge effects change species composition.
• Common mistakes:
  • Explaining fragmentation only as “less space,” without addressing isolation and edge effects.
  • Forgetting that “islands” can be habitat fragments, not just land surrounded by water.

Ecological Tolerance and Adaptations

Species distributions are shaped by both biotic and abiotic factors, and a major organizing idea is that organisms can only survive and reproduce within certain environmental ranges.

Law of Tolerance

Earth’s ecosystems are regulated by the Law of Tolerance, which states that the existence, abundance, and distribution of species depend on each species’ tolerance level to physical and chemical factors in its environment. Each organism’s success depends on a complex set of conditions, including minimum, maximum, and optimum ranges. Biological, climatic, and topographic factors affect abundance and distribution; if conditions exceed tolerance, species numbers will decline.

Adaptations

An adaptation is a biological mechanism by which organisms adjust to new environments or changes in their current environment.

• Behavioral adaptations include instincts, mating behavior, or vocalizations.
• Physiological adaptations include internal methods of temperature control or how food is digested.
• Structural adaptations involve physical features such as body coverings.

Adaptations can also be described by timescale:

• Short-term adaptations develop in response to temporary environmental changes. They involve temporary changes, are not inherited, do not change DNA, and play no role in evolutionary processes.
• Long-term adaptations can involve DNA changes over long time periods in response to natural selection and evolutionary processes.

Exam Focus

• Typical question patterns:
  • Use the Law of Tolerance to explain why a species is present/absent along an environmental gradient (temperature, salinity, pH, dissolved oxygen).
  • Identify whether an example is behavioral, physiological, or structural adaptation.
  • Distinguish short-term responses from long-term evolutionary adaptation.
• Common mistakes:
  • Treating tolerance limits as identical for all species.
  • Calling any short-term adjustment an “evolutionary adaptation.”

Terrestrial Biomes: Climate as the Organizing Principle

A biome is a large ecological region characterized by climate and typical plant and animal communities adapted to it. In APES, terrestrial biomes are organized primarily by temperature and precipitation, including their seasonal patterns. Climate shapes vegetation, and vegetation shapes habitat structure, so plants often define the look and function of a biome.

How climate patterns create biomes

Two major geographic patterns help explain why biomes occur where they do:

1. Latitude: farther from the equator, sunlight is generally less direct and average temperatures decrease.
2. Elevation: higher altitudes are typically cooler and can shift precipitation patterns.

Seasonality matters: two locations can have the same annual precipitation but very different ecosystems if one has steady rain and the other has a short wet season followed by a long dry season.

How to “read” biome descriptions and climate graphs

Focus on:

• Water availability (are plants water-limited?)
• Growing season length (how long can plants photosynthesize?)
• Disturbance patterns (fire, storms, frozen soil)

These factors drive plant adaptations, which then influence animals.

Major terrestrial biomes (core APES set)

Tundra

Cold with low precipitation and a short growing season. Permafrost is common.

Permafrost limits root growth and slows decomposition, affecting nutrient cycling. Warming can change soil stability and ecosystem processes. Adaptations include low-growing plants, insulation in animals, and seasonal migration.

Boreal forest (taiga)

Dominated by coniferous evergreens.

Long, cold winters and short summers. It stores large amounts of carbon in biomass and soils, and decomposition is slow. Needle-like leaves reduce water loss, and conical shapes shed snow.

Temperate rainforest

Cooler than tropical rainforest but receives high precipitation.

High biomass and significant carbon storage. Dense vegetation and epiphytes are common.

Temperate seasonal forest (deciduous forest)

Warm summers, cold winters, moderate precipitation.

Trees drop leaves to reduce water loss and damage during cold/dry seasons. Often heavily populated by humans, so land-use change is a major issue.

Woodland/shrubland (chaparral)

Mild, wet winters and hot, dry summers.

Fire is a common natural disturbance. Many plants resprout after fires or have seeds that germinate after heat/smoke cues.

Temperate grassland

Moderate precipitation but not enough for dense forests.

Soils are often very fertile due to deep root systems and organic matter accumulation. Fire and grazing maintain grass dominance. Many grasslands have been converted to agriculture.

Tropical rainforest

Warm year-round with high precipitation.

Very high species richness. Soils can be nutrient-poor because nutrients are rapidly taken up by plants and heavy rains can leach nutrients. A key misconception to avoid is “rainforests have rich soil”; productivity is high, but nutrients are often stored in biomass rather than soil.

Tropical seasonal forest / savanna

Warm with distinct wet and dry seasons.

Savanna is grass with scattered trees, shaped by seasonal rainfall, fire, and grazing.

Subtropical (hot) desert

Defined by low precipitation, not temperature.

Adaptations include water storage (succulents), nocturnal animals, and reduced leaf surface area.

Biome boundaries are not sharp

Biomes blend through ecotones (transition zones). Ecotones can have high biodiversity because they contain species from both neighboring regions plus species specialized for the transition area.

Exam Focus

• Typical question patterns:
  • Identify a biome from a climate graph and justify using temperature/precipitation patterns and seasonality.
  • Explain an adaptation to limiting factors (water, temperature, fire).
  • Predict how climate change could shift biome boundaries.
• Common mistakes:
  • Defining deserts by heat rather than precipitation.
  • Assuming high plant growth always means nutrient-rich soil (tropical rainforest is the classic counterexample).
  • Ignoring seasonality when comparing places with similar annual precipitation.

Aquatic Biomes: Water, Salinity, and Light Control Life

Aquatic biomes are organized less by temperature and precipitation and more by salinity, water depth, light availability, nutrient levels, and water movement. Organisms must solve physical challenges (oxygen availability, currents, salinity) while competing for resources (light and nutrients).

Freshwater vs. marine: the role of salinity

• Freshwater systems have low salt concentrations.
• Marine systems have high salt concentrations.

Salinity matters because it affects osmosis (movement of water across membranes). Many organisms tolerate only a narrow salinity range, so changes such as saltwater intrusion can reshape communities.

Light and photosynthesis in water

Light decreases with depth, creating:

• the photic zone (enough light for photosynthesis)
• the aphotic zone (too little light for photosynthesis)

Because aquatic food webs often start with photosynthetic producers (phytoplankton, algae, aquatic plants), light strongly shapes productivity.

Freshwater biomes

Streams and rivers

Flowing freshwater systems.

Upstream water is often colder, clearer, faster-moving, and has higher dissolved oxygen. Downstream water is often warmer, slower, and has more sediments and nutrients. Moving water transports nutrients, organisms, and pollution, which is why upstream land use can affect ecosystems and communities far downstream.

Lakes and ponds

Standing freshwater systems.

Common zones:

• Littoral (near shore): shallow, well-lit, often high plant growth.
• Limnetic (open water): dominated by plankton.
• Profundal (deep water): low light; decomposition can dominate.

Lakes can show thermal stratification (temperature layering), which affects oxygen distribution. Low oxygen in deep water can stress fish and change nutrient cycling.

Freshwater wetlands

Include marshes and swamps.

They store water, mitigate flooding, provide habitat, and filter water. Many wetlands have high plant productivity and support diverse food webs.

Marine and coastal biomes

Estuaries

Mixing zones where freshwater meets seawater (brackish and variable).

Highly productive due to nutrient inputs from rivers and shallow waters with good light. Estuaries are nurseries for many fish and shellfish and buffer storms.

Intertidal zone

Coastal area exposed at low tide and underwater at high tide.

Organisms must tolerate drying out, wave action, and changing temperature and salinity.

Coral reefs

Warm, shallow, clear marine waters.

Extremely high biodiversity. Clear water matters because light penetration is high; reefs often thrive in relatively low-nutrient waters. Corals are sensitive to temperature change, pollution, and changes in ocean chemistry. A key misconception: reefs do not “need” nutrient-rich water; excess nutrients can fuel algal growth that harms reefs.

Open ocean

Covers most of Earth’s surface and is often nutrient-limited, so productivity is concentrated where nutrients are supplied (upwelling or coastal delivery).

Eutrophication (recurring aquatic concept)

Excess nutrients (often nitrogen and phosphorus) can trigger algal blooms. When algae die, decomposition can consume dissolved oxygen, creating low-oxygen conditions that can cause fish kills and reduce biodiversity. Being able to explain this mechanism clearly is a frequent exam requirement.

Exam Focus

• Typical question patterns:
  • Compare productivity and biodiversity across estuaries, coral reefs, wetlands, and the open ocean.
  • Explain why light and nutrient availability affect aquatic primary productivity.
  • Predict ecological impacts of damming rivers, nutrient runoff, or wetland loss.
• Common mistakes:
  • Assuming the open ocean is highly productive everywhere.
  • Confusing photic vs. aphotic zones or assuming photosynthesis occurs at any depth.
  • Describing estuaries as stable environments rather than highly variable brackish systems.

Natural Disruptions, Earth-System Process Scales, Sea Level Change, and Wildlife Migrations

Ecosystems are not static. Natural disturbances can dramatically affect which species thrive, which decline, and how communities change through time.

Disturbance: what it is and why it isn’t always “bad”

A disturbance is an event that disrupts ecosystem structure and changes resources or the physical environment. Examples include wildfire, hurricanes, insect outbreaks, floods, droughts, and volcanic eruptions. Disturbance can reset ecological conditions, open space and resources, and change which species can survive.

Many ecosystems evolved with certain disturbances and may depend on them (for example, periodic fire that prevents trees from taking over a grassland).

Flooding

Flooding can kill wildlife and their food sources and can remove the stabilizing role of plant roots, increasing erosion. Water-saturated soils can “drown” plant roots because roots require oxygen. Flooding can also increase runoff of water and nutrients across land surfaces. Burrows, dens, and nests can be destroyed by rushing water, forcing animals to move.

At the same time, floodplain species are often adapted to occasional flooding, and floods can deposit nutrient-rich sediment along stream banks.

Volcanic eruptions

Volcanic eruptions can kill wildlife and their food sources and can remove vegetation that holds soil in place. Over time, volcanic materials weather into some of Earth’s richest soils, which have supported human civilizations.

Across geologic time, volcanoes and cooling magma contributed to Earth’s water and to a large portion of the early atmosphere. Volcanic sulfur gases can also form microscopic droplets in the atmosphere that persist for years, cooling the troposphere by roughly 2–3 degrees.

Wildfires

Wildfires can kill wildlife and their food sources and can leave soil more vulnerable to erosion because roots no longer hold it in place. However, fire can also benefit ecosystems by clearing dead and dying vegetation, increasing light for surviving plants. Ash and charcoal can add nutrients to depleted soils, creating rich conditions for regrowth. Several plants require fire in their life cycles.

A key real-world pattern is that fire suppression can allow fuels to build up, leading to later fires that burn hotter and more destructively.

Earth system processes operate on a range of scales

• Episodic processes occur occasionally and at irregular intervals (example: El Niño and La Niña).
• Periodic processes occur at repeated intervals (example: tides).
• Random processes lack a regular pattern (example: meteorite impacts).

Sea levels

Global sea level has changed significantly over Earth’s history. Sea level is affected by the amount and distribution of water and by the shape and volume of ocean basins. Factors include ocean temperature, water stored in aquifers, glaciers, lakes, polar ice caps, rivers, and sea ice, along with changing ocean basin shape, tectonic uplift, and land subsidence.

Today’s global mean sea level rise is primarily driven by warming oceans (thermal expansion) and melting of land-based ice (glaciers and ice sheets). Coastal land subsidence does not increase global ocean volume, but it can strongly increase relative sea-level rise measured in many coastal locations, worsening flooding and erosion risk.

Wildlife migrations

Wildlife migrations often occur to:

• escape harsh weather (for example, seeking warmer water for breeding and raising young while returning to colder water for feeding where food is more available)
• escape natural disasters and their aftermaths (wildfires, floods, storms)
• find natural resources for food

Exam Focus

• Typical question patterns:
  • Explain how a disturbance (fire, hurricane, flood, volcanic eruption) can both reduce and support biodiversity.
  • Describe how changes in disturbance frequency/intensity/seasonality affect ecosystems.
  • Classify processes as episodic, periodic, or random and connect the scale/pattern to ecological impacts.
  • Explain drivers of sea level change and distinguish global sea-level change from local relative sea-level change due to subsidence.
  • Predict how disturbance or resource changes can trigger wildlife migration.
• Common mistakes:
  • Treating all disturbances as purely negative without noting disturbance-adapted systems.
  • Ignoring how topography, season, intensity, and frequency change disturbance outcomes.
  • Confusing global sea level drivers with local relative sea-level change.

Ecological Succession

Ecological succession is the gradual and orderly process of ecosystem development brought about by changes in community composition through time, often after disturbance. Succession is commonly described as directional, non-seasonal, and cumulative, involving colonization, establishment, and local extinction.

Species interactions that shape succession

Succession isn’t only about climate and soil; it’s also about how species change the environment for one another.

• Facilitation occurs when one species modifies the environment so that it better meets the needs of another species.
• Inhibition occurs when one species modifies the environment in a way that is not suitable for another species.
• Tolerance occurs when species are not affected by the presence of other species.

Pioneer species and life-history tendencies

Pioneer species are early successional organisms and are often generalists. Pioneer plants often have short reproductive times. Pioneer animals often have low biomass and fast reproductive rates.

A common large-scale pattern is that early succession is often dominated by r-strategists (rapid maturity, short-lived, high population size, niche generalists, often lower biodiversity), while later succession tends to include more K-strategists (slow maturity, long-lived, lower population size, niche specialists, often greater biodiversity).

Primary vs. secondary succession

A high-frequency exam skill is identifying the type of succession by whether soil is present.

Primary succession occurs when life colonizes an area where no soil is present initially (barren, lifeless habitat). Examples include new volcanic rock and areas exposed by retreating glaciers. Pioneer species such as lichens and mosses help break down rock and add organic matter, slowly forming soil. As soil develops, grasses and small plants establish, followed by shrubs and trees if climate allows.

Secondary succession occurs after a disturbance that removes organisms but leaves soil intact. Examples include fires, storms, and abandoned farmland. Fast-growing plants (grasses and weeds) colonize, followed by shrubs, young trees, and then later-successional trees.

Typical timelines (highly variable by ecosystem): primary succession can take 1000+ years; secondary succession is often faster (for example, 50–200 years) because soil, seeds, roots, and nutrients may remain.

How productivity, biomass, and diversity tend to change

The impact of a disturbance and the resulting successional pathway depend on intensity and frequency, season, size and spatial pattern, and topography.

As succession proceeds, species richness generally increases and often peaks in mid-to-late succession. In some systems, diversity may level off or even decline slightly as a late-stage community becomes dominated by strong competitors.

In early succession, gross productivity is low because conditions are harsh and producer populations are small. In later succession near a climax-like community, gross productivity may be high, but respiration also rises, so net productivity can approach zero and the GP:R ratio approaches 1:1.

Common directional trends discussed in APES include:

• energy flow becomes more complex
• soil depth, humus, water-holding capacity, mineral content, and cycling increase
• organism size tends to increase
• NPP and GPP can rise and later fall as respiration increases
• biomass-to-production patterns shift through time

Early vs. late successional plant community characteristics

Resilience and resistance

Two useful disturbance-response terms:

• Resistance: how much an ecosystem changes when disturbed.
• Resilience: how quickly it returns to its prior state.

Biodiversity can support resilience when multiple species can perform similar ecological functions; if one declines, others can partially compensate.

Exam Focus

• Typical question patterns:
  • Identify primary vs. secondary succession and justify using soil presence.
  • Explain how facilitation, inhibition, or tolerance could shape a successional sequence.
  • Predict changes in biodiversity, productivity, biomass, and stability across successional stages.
  • Use resistance vs. resilience correctly in disturbance-and-recovery scenarios.
• Common mistakes:
  • Confusing primary vs. secondary succession (primary starts without soil).
  • Saying succession always ends in a single “final” community; disturbance can keep systems in earlier stages.
  • Confusing resilience (recovery rate) with resistance (change magnitude).

Invasive Species: Mechanisms, Impacts, and Management

Invasive species are a major driver of biodiversity loss in many ecosystems, especially when combined with habitat fragmentation and global trade. You’re expected to know what makes a species invasive, why invasions can spread rapidly, and the trade-offs of control strategies.

What makes a species “invasive”

A species is typically considered invasive when it:

1. is non-native (introduced intentionally or accidentally),
2. spreads rapidly, and
3. causes harm (ecological, economic, or human health).

Not all introduced species become invasive; “invasive” implies harmful spread.

Why invasive species can outcompete natives

Invasives often succeed due to:

• enemy release (predators/parasites/pathogens from the native range are absent)
• high reproductive rate
• generalist traits (broad diet or habitat tolerance)
• strong competitive ability

A key mechanism to explain is that native species may lack defenses or competitive strategies because they did not coevolve with the invader.

Pathways of introduction

Common human-linked pathways:

• shipping (ballast water, hull attachments)
• transport of firewood, plants, soil
• pet releases or escape from captivity
• intentional introductions for pest control or landscaping

Prevention often targets these pathways and is typically the most cost-effective approach.

Ecological impacts: how invasives reduce biodiversity

Invasive species can reduce biodiversity through:

• predation (prey lack defenses)
• competition for light, nutrients, or space
• habitat alteration (fire regimes, hydrology, soil chemistry, physical structure)
• disease introduction

Invasions often simplify ecosystems, reduce food-web complexity, and can degrade ecosystem services.

Management strategies (and realistic trade-offs)

1. Prevention: inspections, quarantines, transport restrictions. Often most effective because eradication after establishment is difficult.
2. Mechanical control: physical removal (pulling plants, trapping animals). Best for small invasions; labor-intensive.
3. Chemical control: herbicides/pesticides. Effective but can harm non-target species and create pollution concerns.
4. Biological control: introducing a natural predator/parasite/pathogen. Must be evaluated carefully because biocontrol agents can become problems themselves or harm non-target species.

A common misconception is that “biological control is always environmentally friendly.” It can be useful but carries risks and requires careful testing and monitoring.

Connecting invasives to fragmentation and edge effects

Fragmentation increases edge habitat and human contact, which can increase opportunities for invasives to enter, create disturbed high-light conditions where invasives thrive, and reduce native populations, making invasion easier.

Example-style reasoning you should be able to do

If an invasive plant spreads along roadsides, explain:

• roads create disturbed, high-light edge conditions
• vehicles and soil movement spread seeds
• disturbed areas reduce competition from established native plants

This pattern (pathway → ecological advantage → impact) is a strong FRQ structure.

Exam Focus

• Typical question patterns:
  • Explain why a non-native becomes invasive using enemy release, rapid reproduction, or generalism.
  • Evaluate control methods with benefits and drawbacks.
  • Predict impacts on native biodiversity and ecosystem services, especially in fragmented habitats.
• Common mistakes:
  • Calling any introduced species “invasive” without evidence of harm.
  • Proposing biocontrol as a simple fix without noting risks to non-target species.
  • Describing impacts vaguely instead of using mechanisms (competition, predation, disease, habitat change).