IB Biology SL 2025 Exam - Unit 8 Study Guide

Covering topics from Unit 8 that could be on the 2025 test!

What is a habitat?

The place where a species, population, or organism lives.

How are Mangrove Trees Adapted to Mangrove Swamps?

Secretion of excess salt from salt glands in the leaf

Root epidermis coated in suberin to reduce permeability to salt and prevent excessive absorption

Cable roots grow close to surface to take in most oxygen

Vertical root branches called pneumatophores grow into the air to absorb more oxygen

Stilt roots that grow in a downward arch from the central trunk to buttress the tree

Large buoyant seeds

Accumulation of mineral ions and carbon compounds such as mannitol, which increases the osmotic potential of root and leaf cells

How is Lyme Grass Adapted to Sand Dunes?

Thick waxy cuticle to reduce transpiration

Stomata in indentations where humid air can remain

Leaves that roll up during droughts to create a humid chamber

Tough sclerenchyma to prevent wilting

Underground stems called rhizomes that grow upward and extend deep into the dune to obtain water

Accumulation of carbohydrates known as fructans in root and leaf cells to increase osmotic potential

Abiotic Variables Affecting Species Distribution

Plants - temperature, water availability, light intensity, soil pH and salinity, and availability of mineral nutrients

Animals - water availability and temperature

What is Range of Tolerance?

The range where an organism can reasonably survive and reproduce. An organism cannot live if it’s outside of its range of tolerance

What are coral reefs?

Biodiverse marine ecosystems that contain hard corals, which have mutualistic zooxanthellae that undergo photosynthesis to provide energy

What are the proper conditions for coral reefs to form?

Less than 50 m depth of water

7.8 pH or higher

32-42 parts per thousand of dissolved ions

Clarity

23-29 degrees Celsius

What are the two factors that determine biome distribution?

Temperature and Rainfall

What are biomes?

Groups of ecosystems that resemble each other

Characteristics of the Tropical Rainforest

High temperature, high precipitation, high light intensity, minimal season variation

Characteristics of the Temperate Forest

Medium temperature, high/medium precipitation, medium light intensity, warm summers, cold winters

Characteristics of the Taiga

Low temperature, high/medium precipitation, medium/low light intensity, short summers, long, cold winters

Characteristics of the Tundra

Very low temperature, medium/low precipitation, low light intensity, very short summer, very cold winter

Characteristics of the Desert

High temperature, very low precipitation, high light intensity, minimal season variation

How is the Saguaro Cactus Adapted to Deserts?

Widespread root system to collect water up to 30m from the stem

Deep tap roots to collect water 1m under the soil

Fat stems with storage tissue to conserve water

Pleated stems that allow shrinkage in droughts and swelling after rains

Vertical orientation of stems to reduce interception of sunlight at midday and maximize at night

Thick waxy cuticle on the stem epidermis to reduce transpiration

Leaves reduced to spines to reduce transpiration area and prevent herbivores from eating the cactus

Stomata opening at night and closing during day thanks to CAM metabolism

How is the Fennec Fox Adapted to Deserts?

Nocturnal

Lives in an underground den

Long thick hair to provide insulation for cold nights and hot days

Hairs covering pads of feet to provide insulation when walking on sand

Pale-colored coat to reflect sunlight

Large ears that radiate heat and keep body temperature down

Variable ventilation rate to cause heat loss by evaporation

How are the Meranti (Shorea faguetiana) trees Adapted to Tropical Rainforests?

Grow over 100m high to outcompete other trees

Trunk of hard dense wood that’s buttressed to provide support

Smooth trunk to shed rainwater rapidly

Broad oval leaves with pointed tips to shed rainwater rapidly

Evergreen leaves that take advantage of ideal conditions for photosynthesis

Enzymes of photosynthesis adapted to tolerate temperatures as high as 35C

Flowers and seed produced in large quantities every five years

How is the spider monkey (Ateles geoffroyi) Adapted to Tropical Rainforests?

Long arms and legs for climbing and reaching fruit

Flexible shoulders for swinging

Large hook-like hands without thumbs that can grasp branches and lianas to pick fruit

Feet that can act like extra hands

Long tail that can grip onto branches and act as a 5th hand

Highly developed larynx allowing a wide range of sounds to be communicated

Sleeping at night and active in daytime when vision is most acute

Breeding at any time of the year

What are ecological niches?

Every species in an ecosystem fulfilling an unique role

Zones of tolerance determine the habitat of a species

Species must specialize and create adaptations for the mode of nutrition

Obligate Anaerobes

Inhibited or killed by oxygen so only live in anoxic environments

Clostridium tetani, methanogenic archaea

Facultative Anaerobes

Use oxygen if available so live in oxic or anoxic environments

Baker’s yeast

Obligate Aerobes

Require a continuous oxygen supply so only live in oxic environments

All animals and plants

Photosynthetic Prokaryotes

Several groups of bacteria including cyanobacteria and purple bacteria that participate in photosynthesis

What is Holozoic Nutrition?

Type of digestion commonly used by animals when whole pieces of food are swallowed before being fully digested

What is Mixotrophic Nutrition?

Organisms (certain protists) that are neither exclusively autotrophs nor heterotrophs

What is Saprotrophic Nutrition?

When organisms secrete digestive enzymes into the dead organic matter and digest it externally. They then absorb the products of digestion

Many types of bacteria and fungi are satrotrophic

Also known as decomposers

Why is Archaea so varied in nutrition?

Archaea are adapted to extreme environments such as hot springs, salt lakes and soda lakes, and therefore need to take in energy through a variety of ways

What is the relationship between dentition and the diet of omnivorous and herbivorous representative members of the family Hominidae?

Physical collections of skulls reveal that humans have a mix of both teeth: flat molars to crush and grind food like herbivores, but also have sharper canines and incisors to tear tougher food

Adaptations of herbivores for feeding on plants and of plants for resisting herbivory

Beetles and other insects have jaw-like mouthparts for biting off, chewing and ingesting pieces of leaf

Aphids and other insects with tubular mouthparts for piercing leaves or stems to reach phloem sieve tubes and eat the sap

Plants resist with some having sharp-pointed spines, stings, or synthesize toxic substances (called secondary metabolites and stored typically in seeds)

Adaptations of predators for finding, catching, and killing prey and of prey animals for resisting predation

Vampire bats (Desmodus rotundus) have large, sharp incisors to pierce prey and feed on the blood

Blue-striped snappers (Lutjanus kasmira) swim in a tight group, resembling a “schooling” behavior and making it hard for predators to catch any one individual

Adaptations of plant form for harvesting light

Trees have a dominant leading shoot that grows rapidly and tops others

Lianas climb through trees, using them for support, and in turn not needing to produce as much xylem tissue

Epiphytes grow on trunks and branches, so they receive higher light intensity

Strangler epiphytes climb up the trunks of trees and outgrow the branches to shade out the leaves

Shade-tolerant shrubs and herbs absorb light reaching the forest floor

Fundamental Niche

Full range of tolerance of a species

Realized Niche

The actual extent of the potential range that a species can occupy

Competitive Exclusion Principle

When two species in an ecosystem have overlapping fundamental niches and one species outcompetes the other in all parts of the fundamental niche

Population

A group of individual organisms of the same species living in a given area

Why must one choose random sampling for population size?

To avoid unconscious bias and provide the most accurate result

How can non-motile organisms be counted?

Through quadrats and transects

How can motile organisms be counted?

Mark-Capture-Release-Recapture

What is Mark-Capture-Release-Recapture?

When a group of organisms are captured and marked, released back into the wild, and then another group of organisms caught

Lincoln Index Formula

(M * N / R)

Where M is the number of individuals caught in the first round

Where N is the total number of individuals caught in the second round

Where R is the number of marked individuals caught in the second round

Carrying Capacity

The maximum size of a population that an environment can support

Factors that limit Carrying Capacity

Water, light, soil nutrition, space, food

How does negative feedback control population size?

If a population starts to increase over the carrying capacity, negative feedback will bring the population numbers back down, and vice versa

Density-dependent Factors

Have an increasing effect as the population becomes larger

Density-Independent Factors

Have the same effect as the population becomes larger

What are population growth curves?

Graphs that record and show the trend of populations in certain areas

Sigmoid Population Growth Curve

Typical population growth that is shaped like an S

First phase is exponential growth, then comes the transitional phase, and finally the plateau phase

Competition vs Cooperation

Two types of intraspecific relationships within ecosystems

Competition helps with natural selection, while cooperation helps all organisms benefit

Community

All the interacting populations of different species living in the same area

Herbivory

Primary consumers feeding on producers

Bison grazing on grasses

Predation

One consumer species killing and eating another consumer species

Anteaters feeding on ants or termites

Interspecific Competition

Two or more species using the same resource, with the amount taken by one species reducing the amount available to the other species

Barnacles competing for space and food on rocky shores

Mutualism

Two species living in a close association, with both species benefiting from the association

Rhizobium bacteria living in root nodules of plants in the Fabaceae family and exchanging materials with the plant

Parasitism

One species living inside, or on the outer surface of, another species and obtaining food from them

Ticks living on the skin of deer and feeding by sucking blood

Pathogenicity

One species living inside another species and causing a disease

Myxomatosis virus infecting rabbits

Root Nodules in Fabaceae Mutualism

Root nodules in Fabaceae

Plant grows root nodules in which the bacteria can live with protection, plant maintains low oxygen conditions and supplies sugars made by photosynthesis, providing energy

Rhizobium absorbs nitrogen and fixes it to produce and supply ammonium, and prevents nitrogen deficiency in the plant

Resource Competition between Endemic and Invasive Species

Alien species occupy similar niches to the endemic species, causing the endemic species to have a smaller realized niche or die out

Chi-Squared Test

Used for testing independence or association, as well as goodness or fit

Row total times column total, all over grand total

Predator-Prey Relationship Study Example

Normally, prey numbers remain relatively the same despite being constantly eaten, as the reproduction rate is relatively the same

However, there are instances where there are spikes of prey numbers that cause greater predator numbers, only for negative feedback to bring these numbers back down

Top-Down Control

Acts from a higher trophic level to a lower one

Bottom-Up Control

Acts from a lower trophic level to a higher one

Allelopathy

A biological interaction where one plant negatively affects the growth and development of another through the release of chemical compounds

Open System

Where resources can enter or exit, including both chemical substances and energy

Closed System

Where energy can enter or exit, but chemical resources cannot be removed or replaced

What is the principal source of energy for ecosystems?

Sunlight

What type of energy flows through a food chain?

Light energy transfers to chemical energy

Autotrophs

Organisms that make carbon compounds for energy. Require sunlight and chemical reactions to build carbon compounds

Heterotrophs

Organisms that feed on others for carbon compounds, and energy

Photoautotrophs

Autotrophs that use light to build carbon compounds

Chemoautotrophs

Autotrophs that use exothermic inorganic chemical reactions to build carbon compounds

Trophic Levels

Organisms that are classified into groups based on how they obtain energy and carbon compounds

Energy Pyramids

Another way of showing how energy flows through organisms

How is energy lost going up the energy pyramid?

Incomplete consumption, incomplete digestion, and cell respiration

Primary Productivity

The amount of energy that is synthesized by autotrophs from carbon compounds and other simple substances to increase biomass

Secondary Productivity

The accumulation of carbon compounds in biomass by animals and other heterotrophs

Carbon Cycle Diagrams

A way of showing how carbon flows through an ecosystem

Carbon Sinks

When photosynthesis exceeds respiration, meaning there is a net uptake of carbon dioxide

Carbon Sources

If respiration exceeds photosynthesis, meaning there is a net release of carbon dioxide

4 Places where CO₂ can be released

Natural gas and oil

Coal

Peat

Biomass

Keeling Curve

Shows the flux in how much CO₂ is produced throughout the year

Dependence of aerobic respiration on atmospheric oxygen produced by photosynthesis, and of photosynthesis on atmospheric carbon dioxide produced by respiration

All respiring organisms require oxygen, and that only comes from photosynthesis. This cyclical exchange of gases maintains the balance of O2 and CO2 in the atmosphere, supporting life on Earth

Recycling of all chemical elements required by living organisms in ecosystems

Since there are limited quantities of each chemical element on Earth, there are certain organisms that recycle these elements for other organisms to use

Ingestion

First stage of holozoic nutrition when the organism takes food into the gut

Digestion

Second stage of holozoic nutrition when the organism breaks down large food molecules into smaller molecules

Absorption

Third stage of holozoic nutrition when digested food is transported across the plasma membrane of epidermis cells and into the blood and tissues of the body

Assimilation

Fourth stage of holozoic nutrition when digested food is used to synthesize proteins and other macromolecules to make them part of the body’s tissues

Egestion

Final stage of holozoic nutrition when undigested material is voided from the end of the gut

Facultative Mixotrophs

Organisms that can be entirely autotrophic, entirely heterotrophic or use both modes

Euglena gracilis has chloroplasts and carries out photosynthesis, but also feeds on detritus or smaller organisms by endocytosis

Obligate Mixotrophs

Organisms that cannot grow unless they utilize both autotrophic and heterotrophic modes of nutrition

Phototrophic

Organisms that absorb light energy by pigments, but pigments other than chlorophyll are used

Chemotrophic

When organisms oxidize inorganic chemicals for energy. For example, Fe2+ ions to Fe3+

Mycorrhizae in Orchidaceae

Roots that form close associations with the fungi

Orchid supplies carbon compounds made by photosynthesis, including sugars

Fungus absorbs nitrogen and phosphorus from the soil and supplies them to the orchid; supplies fixed carbon in organic compounds, obtained from the soil by digesting dead organic matter or parasitizing other plants; supplies water absorbed from the soil

Zooxanthellae in Hard Corals

Coral provides a safe and protected environment; grows close to the surface so algae have a reliable source of light; supplies carbon dioxide from cell respiration

Zooxanthellae provides carbon compounds such as glucose and amino acids produced by photosynthesis; supplies oxygen as well

Geographical Location

Specific area or region of where a species is found (such as continents or islands)

Physical Location

Focuses on the immediate environment where the species lives (types of terrains, types of vegetation)

Characteristics that can describe a habitat

Latitude, climate, soil type, water availability, flora and fauna, altitude, human impact, sunlight exposure

Adaptation

Features of organisms that aid their survival by allowing them to be better suited to their environment

Structural Adaptations

Physical differences in biological structure

Teeth of a lion, neck of a giraffe

Behavioral Adaptations

Differences in patterns of behavioral activity

Lions typically kill by crushing the windpipe in the neck of its prey

Physiological Adaptations

Variations in detection and response by vital organs

Color perception or homeothermy