Ecological Niches
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Ecological Niches
- A habitat is the place where a species lives; a niche is the role of a species within its habitat.
- A species' role includes what it eats, which other species depend on it for food, what time of day it is active, and exactly where in a habitat it lives and feeds.
- The niche also includes the abiotic factors acting on a species (e.g. oxygen and carbon dioxide exchange) and the biotic factors acting on it.
- No two species can fill the same niche within a habitat; if this happens, they compete directly for resources and one species will out-compete the other, causing it to die out in that habitat.
- Species may appear to share a niche, but subtle differences remain, e.g. feeding at different times of day or having different food sources.
- Feeding location is an example of a feature that may differ between niches.
Interactions in an ecosystem
Anaerobes & Aerobes
- All living organisms carry out some form of respiration; aerobic respiration requires oxygen, while anaerobic respiration does not.
- Obligate anaerobes are single-celled organisms that can only carry out anaerobic respiration and cannot tolerate oxygen.
- Early bacteria were obligate anaerobes, surviving in early Earth's oxygen-free atmosphere; today they are found only in oxygen-free environments such as lower soil layers, deep water, and inside other organisms.
- Facultative anaerobes mainly respire aerobically but can switch fully to anaerobic respiration in the absence of oxygen, with no negative effects.
- Examples of facultative anaerobes include brewer's yeast (×Saccharomyces cerevisiae×) and×Escherichia coli×.
- Obligate aerobes cannot survive without oxygen and rely on aerobic respiration; they may carry out anaerobic respiration for only a few seconds before damaging effects occur.
- Examples of obligate aerobes include most animals, most fungi (not yeast), and some bacteria such as×Mycobacterium tuberculosis×.
Methods of Nutrition
- Organisms need energy in the form of ATP, which is transferred during respiration from organic molecules such as carbohydrates.
- The way an organism gains organic molecules to fuel respiration is its method of nutrition; the two main modes are autotrophy and heterotrophy.
- An autotroph synthesises its own organic molecules from simple inorganic substances; photoautotrophs use light energy, while chemoautotrophs use energy from the oxidation of chemicals.
- A heterotroph gains organic molecules from the tissues of other organisms.
- Photosynthesis uses light energy to convert carbon dioxide into organic molecules such as carbohydrates; photosynthetic pigments such as chlorophyll absorb the light energy.
- Photosynthetic organisms are producers because they make their own organic molecules without relying on other organisms; they include plants, algae, and photosynthetic bacteria such as cyanobacteria.
- Photosynthesis transfers light energy into a chemical form usable by living organisms and releases oxygen into the atmosphere, enabling aerobic respiration.
Holozoic Nutrition, Mixotrophs & Saprotrophs
- Holozoic nutrition is used by heterotrophs that ingest, digest, absorb, and assimilate molecules from the tissues of other organisms; it involves internal digestion.
- Ingestion = eating; digesting = breaking down larger molecules into smaller ones; absorbing = transport of molecules from the digestive tract into cells; assimilation = using molecules to build cells and tissues.
- Animals such as house flies secrete enzymes onto their food before absorbing the products; they are heterotrophs but do not use holozoic nutrition because digestion is external.
- Mixotrophs use more than one method of nutrition, such as both auto- and heterotrophy; obligate mixotrophs must constantly have access to both methods, while facultative mixotrophs can survive using one method supplemented by the other.
- ×Euglena×is a single-celled freshwater mixotroph that can take in bacterial cells by endocytosis and digest them with lysosomal enzymes; it also has a light-sensitive spot to position itself for maximum light absorption by chloroplasts.
- Other mixotrophs include carnivorous plants, corals (gaining molecules from symbiotic algae and filter feeding), and marine plankton such as dinoflagellates.
- Saprotrophs are heterotrophs that secrete enzymes onto dead organisms and waste material, digesting externally before absorbing the products; examples include fungi and bacteria, which act as decomposers.
- Saprotrophs release mineral ions such as ammonium and phosphate; not all products are absorbed, leaving minerals in the soil for plants, so saprotrophs are essential to ecosystems and food webs.
Fungi as decomposers
Nutrition in Archaea
- Archaea are a diverse group of single-celled organisms forming one of the three domains; they vary metabolically, including phototrophic, chemotrophic, and heterotrophic groups.
- Phototrophic archaea use light energy to generate ATP; e.g.×Halobacteria×use the pigment bacteriorhodopsin to absorb light and pump H⁺ ions across a membrane, producing an ion gradient that drives ATP synthase.
- This is not oxygen-releasing photosynthesis, and×Halobacteria×are not autotrophic; they are photoheterotrophs because they gain carbon compounds from other organisms.
- Chemoautotrophic archaea produce their own carbon compounds using energy released from chemicals in the environment, such as hydrogen gas, ammonia, methane, and hydrogen sulfide.
- Chemosynthesis releases energy from chemicals which is transferred to carbon compounds, which can then be used for ATP production.
- Chemoheterotrophic archaea use chemicals to produce ATP but gain their carbon compounds from other organisms.
- Heterotrophic archaea gain carbon compounds from other organisms and use them to generate ATP, e.g. archaea that break down and absorb carbon compounds in dead plant material.
Nutrition in Hominidae: Skills
- Humans belong to the Hominidae family along with chimps, gorillas, orangutans, and gibbons; most existing hominids are omnivores.
- Chimps are mainly frugivores (fruit eaters) though they eat other plant matter and some small mammals; gorillas are mainly herbivores, feeding on leafy vegetation and sometimes insects.
- Jaw and dentition are specialised for diet: chimps have small jaw muscles, small incisors, and long canines for biting and tearing, while gorillas have strong jaw muscles and large molar and premolar teeth for grinding vegetation.
- Incisors are chisel-shaped for cutting and biting; canines are pointed for holding and tearing; premolars and molars are flat and ridged for grinding.
- The relationship between diet and dentition in existing hominids can be applied to extinct species, whose well-preserved skulls and jaws can be studied to infer diet and ecosystem structure.
- Dentition is not always a perfect indicator of diet: modern humans eat meat but have teeth more similar to plant-eaters; orangutans and gorillas have pointed canines but do not eat meat; male chimps have longer canines than females despite the same diet.
- Teeth may also function in defending territory or competing for mates, so other evidence such as microscopic patterns of tooth abrasion should be considered.
- Examples of extinct hominids include×Australopithecus africanus×,×Paranthropus robustus×,×Homo floresiensis×, and×Homo neanderthalensis×.
Nutrition: Adaptations of Herbivores & Plants
- Herbivores are heterotrophs that feed on plants; adaptations are characteristics that aid survival in an environment.
- Aphids have specialised mouthparts called stylets to pierce plant tissues and reach sugary sap in the phloem; caterpillars, grasshoppers, and beetles have mandibles to cut through leaves.
- Grazing animals such as sheep and horses have flat teeth for grinding plant matter; ruminants such as cattle and deer have multi-compartment stomachs and can regurgitate and re-chew food.
- Ruminants have specialised communities of bacteria in their digestive tracts that produce the enzymes needed to break down cellulose, which the herbivores themselves lack.
- Some mammals neutralise plant toxins: deer produce salivary proteins that bind tannins; proboscis monkeys have gut bacteria that neutralise leaf toxins; 'cautious sampling' avoids consuming large amounts of toxic chemicals.
- Plants cannot move away from herbivores, so they use mechanical deterrents such as cactus spines, nettle hairs containing toxins, thick bark, and tiny leaf hairs that hinder insects.
- Plants also produce toxic secondary compounds: foxgloves produce digitalis (affects heartbeat), deadly nightshade produces atropine (blocks neurotransmitters), tannins have a bitter taste and reduce digestive efficiency, and alkaloids such as caffeine and nicotine deter insect herbivory.
Adaptations of Predators & Prey
- Predators hunt and eat other animals or consume recently dead animals; prey are hunted and consumed by predators; their adaptations can be chemical, physical, or behavioural.
- Chemical predator adaptations include venoms: haemotoxic venoms (e.g. adders, rattlesnakes) damage the circulatory system, while neurotoxic venoms (e.g. mambas, cobras, scorpions) interfere with nerve impulses.
- Some predators use chemical mimicry (bolas spiders release moth pheromones) or chemical crypsis/scent camouflage (pirate perch fish) to attract or ambush prey.
- Physical predator adaptations include sense organs for detecting prey (excellent vision in birds of prey, forward-facing eyes for distance perception, Jacobson's organ in snakes, echolocation in bats) and body structures for catching prey (cheetah speed, streamlined swordfish, fast mantis shrimp limbs, large canine teeth).
- Behavioural predator adaptations include ambush strategies (puff adders motionless for weeks, mantis shrimps hiding in cracks, crocodiles bursting from water), pack hunting (orcas, wolves, lions), and pursuit hunting (cheetahs, wolves, painted dogs).
- Chemical prey adaptations include toxins (poison dart frogs, skunks, tiger moths) and scent camouflage (puff adders, harlequin filefish).
- Physical prey adaptations include eyes on the sides of the skull for wide vision, camouflage, mimicry (owl butterflies resembling owl eyes; king snakes mimicking coral snakes), aposematism (bright warning colours), and mechanical defences (exoskeletons, shells, spines).
- Behavioural prey adaptations include preference for dark sheltered places, fleeing, avoiding predator locations or times, grouping in large groups, mobbing predators, warning calls, and bluffing (playing dead, frill-necked lizards appearing larger).
Camouflage as an adaptation
Plant Adaptations for Harvesting Light
- Plants have whole-organism adaptations of form that maximise light absorption for photosynthesis, allowing them to compete with other plants.
- Trees form the canopy; emergent trees grow above the main canopy, and some trees form an understory layer; maximising height gives the tallest trees the most sunlight and allows high rates of photosynthesis for rapid growth.
- Lianas are woody vines that use tree trunks for support to reach the canopy; they germinate on the forest floor, grow toward tree trunks, then upwards, and their roots remain in the soil for nutrients and moisture.
- Epiphytes grow high in tree branches to absorb sunlight without beginning life on the forest floor; they often gain nutrients from the canopy, e.g. moss absorbs rainwater from bark, bromeliads collect rainwater in leaves, and some orchids have aerial roots that absorb moisture from air.
- Strangler epiphytes begin life in the canopy and grow roots downward to the forest floor, gaining nutrients and water from soil while using tree height for light; strangler figs can kill their host trees by taking all their resources.
- Shade-tolerant shrubs and herbaceous plants grow on the forest floor and absorb the limited wavelengths reaching the ground; they may have different photosynthetic pigments, large leaves for surface area, and brightly coloured or strongly scented flowers to attract pollinators in low light.
- Shrubs have woody stems but are not tall like trees; herbaceous plants lack woody stems and rely on soft tissues with turgid cells for support.
Competition Between Species
- A species' niche is its role within its habitat, taking into account biotic interactions (organisms it feeds on and that feed on it) and abiotic interactions (e.g. gas exchange with the atmosphere).
- The fundamental niche is the full range of conditions and resources in which a species could survive and reproduce, based on its adaptations and tolerance limits; it is the potential distribution with no limiting factors, competition, or predation, and is large in size.
- The realised niche is the actual conditions and resources in which a species exists due to biotic interactions; it is the actual distribution with competition and predation, and is smaller in size.
- Example: the barnacle×Chthamalus dalli×has a wide fundamental niche in rocky intertidal areas, but competition with×Balanus glandula×restricts its realised niche to higher up the shore where it is exposed to air for longer and avoids competition.
- A niche can only be occupied by one species; if two species try to occupy the same niche, they compete for the same resources.
- The result of this competition is that one species out-competes the other until the second species is forced into a new, slightly different niche or becomes locally extinct; both species may also be forced into a smaller part of their fundamental niches.
- The elimination of a competing species from its niche is known as competitive exclusion.
- A classic example is×Paramecium aurelia×and×Paramecium caudatum×: grown separately both thrive, but grown together×P. aurelia×outcompetes×P. caudatum×, leading to its elimination; such exact niche overlap is rare in nature.
Interspecific competition
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練習題
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1.Which of the following correctly describe aspects of an ecological niche? I. How a species gains its food II. Abiotic factors that influence species growth III. Biotic factors that influence species survival
Easy- AI only
- BI and II only
- CII and III only
- DI and III only
- EI, II and III
2.Saccharomyces cerevisiae is a species of yeast that can survive at a range of oxygen concentrations, including in the absence of oxygen. Which term describes Saccharomyces cerevisiae?
Easy- AObligate anaerobe
- BFacultative anaerobe
- CObligate aerobe
- DMixotroph
3.Which of the following are adaptations that aid herbivory? I. The presence of an enzyme that breaks down cellulose. II. Production of toxic secondary compounds. III. Proteins that bind to toxins.
Medium- AI only
- BI and II only
- CI and III only
- DII and III only
4.Which of the following statements about ecological niches is correct?
Medium- ACompetition reduces the size of an ecological niche.
- BAn ecological niche can be shared between two similar species.
- CCompetition for a niche always results in local extinction for one species.
- DA realised niche is larger than a fundamental niche.
5.The Ghost Orchid (Epipogium aphyllum) is a rare plant species which lacks leaves or chlorophyll. The plant has formed a symbiotic relationship with a fungus in order to obtain the nutrition it requires. The growth of the fungus is slowed as a result of the relationship. Which of the following rows correctly describes E. aphyllum?
Hard- AHeterotroph ✓, Autotroph ✗, Parasite ✗, Saprotroph ✓
- BHeterotroph ✗, Autotroph ✓, Parasite ✗, Saprotroph ✓
- CHeterotroph ✓, Autotroph ✗, Parasite ✓, Saprotroph ✓
- DHeterotroph ✓, Autotroph ✗, Parasite ✓, Saprotroph ✗
6.Which of the following are part of the role of a species that defines its ecological niche? (select all that apply)
Medium- AWhat it eats
- BWhat time of day it is active
- CThe colour of its fur or skin
- DWhich other species depend on it for food
- EThe abiotic factors acting on it
7.Which of the following statements about fundamental and realised niches are correct? (select all that apply)
Medium- AThe fundamental niche is the niche a species would occupy if there were no limiting factors.
- BThe realised niche is the niche a species actually occupies in the presence of competitors.
- CThe realised niche is larger than the fundamental niche.
- DThe fundamental niche represents the potential distribution of a species.
- EThe realised niche involves no competition for resources.
8.Which of the following are adaptations of plants that reduce herbivory? (select all that apply)
Medium- ACacti have sharp spines.
- BNettles have tiny hairs containing toxins.
- CGrazing animals have flat teeth for grinding plant matter.
- DFoxgloves produce the toxic compound digitalis.
- EAphids have stylets that pierce plant tissues.
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