Nutrition: Adaptations of Organisms (HL IB Biology)

Adaptations for herbivory

• Herbivores are heterotrophs that feed on plants
• Different groups of organisms have different adaptations that allow them to survive on plant tissues
  • Adaptations are characteristics that aid an organism's survival in its environment
• Examples of adaptations for herbivory include:
  • Herbivory in insects
    • Aphids have specialised mouthparts known as stylets that are able to pierce plant tissues to reach the sugary sap inside the phloem
    • Insects such as caterpillars, grasshoppers and beetles have mouthparts called mandibles which allow them to cut through leaves
  • Herbivory in mammals
    • Grazing animals such as sheep and horses have flat teeth for grinding plant matter
    • Ruminant mammals such as cattle and deer have digestive systems adapted to improve their digestion of tough plant material; they have stomachs with several compartments from which they can regurgitate and re-chew their food, breaking down plant matter into smaller pieces to aid digestion
    • Ruminants have specialised communities of bacteria that live in their digestive tracts which aid the breakdown of cellulose
      • The bacteria have the enzymes needed to break down cellulose, while the herbivores do not
    • Some mammals have the ability to neutralise toxins produced by plants, e.g.
      • Some deer produce proteins in their saliva that bind to toxins called tannins
      • Proboscis monkeys have gut bacteria that can neutralise certain toxins found in leaves
    • Mammals may use 'cautious sampling' when they first encounter a new plant, meaning that any toxic chemicals will not be consumed in large enough quantities to be dangerous

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Aphids (left) feed by inserting their stylets into the phloem of plant stems, while caterpillars (right) cut through leaves with their sharp mandibles

Plant adaptations against herbivory

• Herbivory causes damage to plants, reducing their leaf surface area available for photosynthesis and their ability to transport substances
• Plants are unable to move away from herbivores, so they have other methods of deterring animals that might attempt to eat them:
  • Mechanical deterrents, e.g.
    • Cacti have sharp spines to deter herbivores that might attempt to eat their succulent stems
    • Nettles have tiny hairs that contain toxins which irritate the skin
    • Thick bark prevents insects such as aphids from piercing plant stems
    • Many tiny hairs on leaves may make it more difficult for insects to bite into/pierce plant tissues
  • Toxic secondary compounds, e.g.
    • Foxgloves produce a toxic compound known as digitalis which can affect the heartbeat of humans and animals
    • Deadly nightshade can produce a toxin known as atropine which can cause muscle paralysis by blocking the binding of neurotransmitters
    • Many plants produce chemicals called tannins which can deter herbivores by their bitter taste, as well as having a negative impact on the efficiency of digestive processes
    • Alkaloid chemicals, such as caffeine and nicotine, can deter insect herbivory, having toxic effects on growth and on nerve impulse transmission

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Nettles have tiny hairs (left) which contain skin irritant chemicals, and foxgloves (right) produce toxic secondary compounds

• Predators are animals that hunt and eat other animals, or that consume the tissues of recently dead animals
• Prey are animals that are hunted and consumed by predators
• The features of predators and prey are different, allowing them to adapt to their different roles
  • The adaptations of predators assist them in catching prey
  • The adaptations of prey assist them in avoiding predation
• The adaptations of predators and prey can be either
  • Chemical
    • Chemical compounds that assist in the catching of prey or the avoidance of predation
  • Physical
    • Physical features, such as specially adapted sense organs, that assist in the catching of prey or the avoidance of predation
  • Behavioural
    • Behaviours that aid the catching of prey or the avoidance of predation

Predator adaptations

Chemical adaptations of predators

• Some predators produce venom which can make prey easier to subdue and catch, e.g.
  • Snakes can produce venoms that act in different ways to kill prey, e.g.
    • Snakes such as adders and rattlesnakes produce haemotoxic venoms that damage the circulatory system, e.g. by interfering with blood clottihg
    • Snakes such as mambas and cobras produce neurotoxic venoms which interfere with the passage of nerve impulses
  • Scorpions can produce neurotoxic venom which they can use to subdue larger prey animals
  • Spider venom can contain various types of toxins, which they may also use to subdue their prey
• Some predators may use a strategy known as chemical mimicry to attract prey animals, e.g. bolas spiders release chemical pheromones normally used by female moths to attract mates, enabling them to catch male moths as prey
• A strategy known as 'chemical crypsis', or scent camouflage, allows ambush predators to lie in wait for prey without being detected, e.g. the prey of pirate perch fish seem to be unable to detect their presence; scientists think that this could be due to the production of a chemical which acts as camouflage

Physical adaptations of predators

• Predators have sense organs that help them to detect the presence of prey, e.g.
  • Birds of prey have excellent vision that allow them to detect small prey animal movement from a distance
  • The eyes of predators are often located in the fronts of their skulls, giving good distance perception
  • Snakes have an organ in the roof of the mouth known as the Jacobson's organ that allows them to use their tongues to detect chemicals that may be released by prey animals
  • Bats can detect and process information generated by sound waves bouncing off prey organisms, allowing them to find prey using echolocation
• Predators have body structures that allow them to catch and kill prey effectively, e.g.
  • Cheetahs can run at high speeds as a result of their long limbs and flexible spines
  • Swordfish can swim at 60 mph due to their streamlined body shape
  • Mantis shrimps can move their modified front limbs at 50 mph to catch their prey
  • Carnivorous mammals have large canine teeth which allow them to catch and hold onto prey

Behavioural adaptations of predators

• Ambush predators lie in wait without moving for extended periods, e.g.
  • Puff adders can remain motionless for weeks at a time while they wait for prey to come near
  • Mantis shrimps (also mentioned above) hide in cracks between rocks before they reach out and grab prey at high speeds
  • Crocodiles can approach their prey from underwater before bursting out of the water at high speed
• Pack predators cooperate with each other to increase their chance of success, e.g. orcas, wolves and lions
• Pursuit predators chase after their prey, either using a burst of speed, e.g. cheetahs, or persistence hunting over long distances, e.g. wolves and painted dogs

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Predators may use chemical, physical, and behavioural adaptations to assist them as they hunt and consume prey organisms. Adders (left) produce toxic venom, mantis shrimps (centre) have front limbs specially adapted for speed, and lions (right) co-operate with each other during pack hunting.

Prey adaptations

Chemical adaptations of prey

• Some prey animals produce toxins that deter predators by tasting bad, or by causing harm when consumed, e.g.
  • Poison dart frogs produce toxins in their skin that can kill predators
  • Skunks can produce chemicals that smell unpleasant to deter predators
  • Tiger moths contain chemicals that cause them to taste unpleasant to their bat predators
• The scent camouflage mechanism described above can also be used by prey animals, e.g.
  • Puff adders (described above as ambush predators) are also prey for animals such as mongooses; they produce chemicals which prevent their predators from detecting them while they lie in wait for prey
  • Harlequin filefish take on the scent of the corals on which they feed, meaning that predators are unable to detect their presence

Physical adaptations of prey

• As with predators, prey have sense organs that are adapted to detect predators, e.g. prey tend to have eyes positioned on the sides of their skulls, giving a wide field of vision
• Prey animals have body features which allow them to avoid or deter predators, e.g.
  • Prey animals may use camouflage; some insects have bodies that allow them to appear to be a leaf or a stick
  • Mimicry allows prey animals to look like predators; owl butterflies have wing patterns that resemble the eyes of owls, causing potential predators to avoid them
  • A strategy known as 'aposematism' involves the development of bright warning colours, sending predators a message about chemical defences, such as the brightly coloured skin of poison dart frogs
  • Certain types of mimicry allow prey animals to resemble species with chemical defences, without needing to invest in the production of toxins, e.g. king snakes mimic the striping colour and pattern of venomous coral snakes
  • Prey animals may use mechanical defences, such as tough exoskeletons in insects and crustaceans, tough shells in turtles, and spines in porcupines and hedgehogs

Behavioural adaptations of prey

• Prey animals will sometimes have innate preferences for dark, sheltered places, e.g. insects such as woodlice that will move around constantly until they encounter a dark hiding place
• Prey will often move away when they detect the presence of predators, e.g. rabbits will run into their burrows when they see birds with the wing shape of predators
• Prey may avoid locations or times of day where predators are present, e.g. desert rodents may spend the daytime in an underground burrow and only emerge at night
• Prey animals will often group together in large groups; this will reduce their chance of being caught, as well as potentially confusing predators, e.g.
  • Shoals of fish and large groups of birds move together in ways that make individual animals difficult to pick out
  • Some prey animals will 'mob' a predator, e.g. gulls may group together to attack a predator and drive it away
  • Some individuals may be able to warn others in a group of the presence of a predator, e.g. by using a warning call or by running away
• Bluffing techniques may allow prey animals to convince predators that they are not what they seem, e.g.
  • Opossums, some species of snake, and some species of shark may pretend to be dead; it is thought that this behaviour causes predators to lose interest
  • Frill-necked lizards may use their large neck frill to pretend to be larger than they really are

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Prey animals can use chemical, physical, or behavioural defences against predation. Poison dart frogs (left) produce toxins in their skin, owl butterflies (centre) physically resemble a predator, and grass snakes (right) can use apparent death behaviour.

• Plants rely on the process of photosynthesis to produce carbon-compounds, and their leaves are well adapted to carry out this process
• Plants also have adaptations at the level of the whole organism that maximise their ability to absorb light energy for photosynthesis
  • These whole organism adaptations can be described as adaptations of 'form'
• Form adaptations in a forest ecosystem allow plants to compete effectively with other plants for light

Trees

• Trees in a forest make up the uppermost layer of plants, known as the canopy
  • Some trees may grow above the main canopy; these trees are described as being emergent trees
  • Some trees form a layer beneath the main canopy, known as the understory
• The strategy of maximising height allows the tallest trees to gain the most sunlight, as there are no other plants between them and the sunlight
• Trees can carry out photosynthesis at a high rate, providing them with the molecules that they need to grow quickly and compete effectively with other plants

Lianas

• Lianas are woody vines that use the trunks of trees as their main supporting structure to gain height, allowing their leaves to reach the forest canopy where they can absorb light for photosynthesis
• Lianas germinate on the forest floor, growing toward the base of tree trunks before growing upwards
• The roots of lianas are in the soil, allowing them to gain their nutrients and moisture from the soil
• Lianas compete with trees for light, and for nutrients and moisture

Epiphytes

• Epiphytes use the height of trees to increase their absorption of sunlight by growing high up in tree branches, but they do not begin their lives on the forest floor, and often gain their nutrients from high in the canopy, e.g.
  • Moss gains water and nutrients from rainwater that runs across the tree bark on which it grows
  • Bromeliads collect rainwater in amongst their leaves
  • Some species of orchid have aerial roots which absorb moisture directly from the air
• Epiphytes have the advantage of height for gaining light energy, but do not need to expend their energy on upward growth

Strangler epiphytes

• Some epiphytes grow roots downward to the forest floor, allowing them to gain nutrients and water from the soil, while still taking advantage of height from trees to absorb sunlight
  • Note that this is different to lianas as strangler epiphytes begin their lives in the canopy and not on the forest floor
• An example is the strangler fig, which begins its life in the canopy, and is able to grow both upwards and downwards to maximise its access to resources
  • Strangler figs can kill their tree hosts by taking all of their resources

Shade tolerant shrubs & herbaceous plants

• Shade tolerant plants grow on the forest floor, and are adapted to absorb the limited range of light wavelengths that reach the ground through the leaves of the canopy and understory
  • Shade tolerant plants may contain different photosynthetic pigments, allowing them to absorb different wavelengths of light
• Plants that grow on the ground often have especially large leaves, maximising the surface area available for light absorption
• Flowers produced by these ground-living plants are often very brightly coloured or strongly scented to attract pollinators in low light levels
• Note that the terms 'shrub' and 'herbaceous plant' refers to the structure of the plant tissues
  • Shrubs are not tall like trees, but they do have woody stems
  • Herbaceous plants, or 'herbs', lack the woody stems of shrubs and trees, and rely on soft tissues with turgid cells for support

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Trees (top left) use height to maximise their light absorption, epiphytes (top right) grow in the branches of tall trees to gain light, strangler epiphytes (bottom left) grow in the branches of trees but grow their roots down into the soil to allow absorption of light and nutrients, and shade-tolerant herbs have large leaves to maximise surface area for light absorption