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BIOTIC INTERACTIONS Three Categories of interactions

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1 BIOTIC INTERACTIONS Three Categories of interactions
Predation Competition Symbioses As a result organisms evolve (change): develop to maximise the benefit of their interaction (minimise the disadvantage) Intraspecific Between individuals of the same species Interspecific Between individuals of different species

2 Density Dependence/ Independence
Density dependent – the severity of the effect increases as the population size increases. Density independent – the severity of the effect is the same irrespective of the population density.

3 Density Dependent Density Independent Predation Any abiotic factor e.g. Food Temperature Water Light Disease pH Space


5 PREDATION Predation is a force for natural selection
Selection pressure Causes co-evolution of predator/prey Predator gets faster, stronger, hunt co-operatively etc. Prey gets faster, form herds, modify behaviour/ physical characteristics Grazing is classified as a predation?! Grazing promotes biodiversity by selectively reducing dominant (more frequently encountered) species VIDEO

6 Predation Predators are adapted to enhance their success:
have highly evolved senses sight – eagles/ cats smell – anteaters, pigs Infra red (rattlesnake), hearing – owls echo location – bats, electrical – sharks, platypus Predators can cooperate (lions, army ants, chimpanzees) Allow exploitation of resources beyond the capability of a single individual of the species Predators can also use: mimicry - angler fish camouflage - lion, preying mantis Prey evolve to avoid predation

7 Avoiding Predation Prey can use: behavioural adaptations
hiding (fish on coral reefs) running away (antelope from lion, seal from killer whale) mobbing (kittiwake on gulls) herding (musk ox) distraction displays (plover broken wing, butterfly eyes on tail) Active defence fight back (water buffalo/ gnu mothers) Camouflage (crypsis) animal is coloured to merge into background e.g. stonefish, chameleon, stick insect

8 Avoiding Predation Prey can use: Aposematic (warning) colouration
Animals advertise their toxicity wasps & bees yellow & black Mimicry – one organism resembles another Batesian mimicry a harmless species mimics a toxic one Hoverfly looks like a wasp need more wasps than hoverflies otherwise predators learn yellow & black is not toxic. (but see below) Mullerian mimicry two or more aposematically coloured species develop similar warning colouration e.g. bees & wasps warning signal is greatly reinforced by such large numbers showing the same warning

9 Avoiding Predation Mechanical & chemical defences
plants contain toxins, grow spines/ thorns Animals secrete toxins/ bitter taste/ slimes (slugs, frogs), grow armour (pangolin), spines (hedgehog)

10 Predator Prey Interactions

11 Predator prey Interactions
Cyclical oscillations in predator population reflects cyclical oscillations in prey population Carrying capacity = population which can be supported by the ecosystem As snowshoe hare population increases carrying capacity (lynx) of ecosystem increases - more food available Lynx population increases hare population eventually exceeds carrying capacity of the ecosystem (food, space run out) population (hare) crashes lynx population no longer has sufficient food resource consequently lynx population crashes Grass (hare food) population would peak BEFORE the hare population FIRST in food chain peaks FIRST in cycle NB the predator DOES NOT usually control prey population, it is a species’ food supply which controls its population size

12 Competition Resources are limited (e.g space, food, water)
The ability of organisms to gain resources will determine their success. As the density of population increases competition becomes more severe Some organisms are more effective at securing resources Those are successful, survive and reproduce Less successful organisms perish Competition causes natural selection Species change (evolve) to reduce competition

13 Competition Intraspecific competition is more severe than interspecific because the same species compete directly for exactly the same resource.

14 Galapagos Finch- fortis
                                                   Fortis eats seeds. During drought big tough seeds are all that are available to eat Big beaks make this easier Prior to drought average beak was 10,68mm long and 9.42 mm deep After a drought period, average beak length 11.07mm long and 9.96mm deep Competition for food caused nearly 6% change in beak shape in one year.

15 Two types of Competition
Exploitation competition - occurs indirectly through a common, limiting resource, which acts as an intermediate. For example the use of the resource(s) depletes the amount available to others, or they compete for space. e.g grey/ red squirrel food; e.g. Interference competition - occurs directly between individuals via aggression etc. when the individuals interfere with foraging, survival, reproduction of others, or by directly preventing their physical establishment in a portion of the habitat. e.g ant & Rattan & herbivores e.g ant, acacia & giraffe Private life of Plants- Living Together 8 min in Ant & Rattan 9.5 min in Ant Acacia & Giraffe

16 Niche A niche is an organism’s position within an ecosystem described in terms of abiotic and biotic interactions: abiotic interactions (i.e. mineral needs/ pH tolerance, moisture/temperature range) The larger the range of physical conditions tolerated, the wider the niche and more widespread the organism is biotic interactions (i.e. position in the food chain, diversity of food sources exploited, diversity of species which exploit it as a food source) The greater diversity of these interactions the more widespread the organism Within an ECOSYSTEM no two organisms can occupy the same niche

17 Competitive Exclusion Principle
Two organisms cannot coexist sharing the same niche in an ecosystem. They will compete, one will be more successful and the second will become extinct. Experimentally demonstrated using Paramecia species (Gause,)

18 B competes more strongly
A competes more strongly

19 B competes more strongly
A competes more strongly

20 Fundamental & Realised Niche
The fundamental niche is the entire range of abiotic & biotic parameters an organism can survive within. Fundamental niches can overlap Realised niche is the actual range of parameters within which the species occurs. Realised niche can be smaller than the fundamental niche Realised niches cannot overlap Species cannot share exactly the same resources Competition would lead to the exclusion of one of the two species occupying the same niche Adaptations of species is such that they are best suited to a subset of their fundamental niche parameters e.g. barnacle zonation on the shore

21 Resource Partitioning
To reduce competition between organisms with overlapping niches, species adapt and diverge to become specialised for a smaller realised niche




25 Resource Partitioning
e.g. Cormorant/ Shag Cormorant nests high on cliffs or broad ledges Shag –nests on shallow ledges, low on cliffs Cormorant – feeds on mixed diet no sand eels/ sprats Shag – Eats mostly sand eels/ sprats

26 Importance of Niche overlap
Within a population some individuals are adapted to living at the extremes of the niche i.e. they are adapted to conditions slightly different to those currently found in the ecosystem such organisms will survive, albeit less successfully, in the overlap of niches This variability within a population is vital for allowing a species to survive change These weaker individuals may have traits ideally suited to the new conditions

27 Alien Species Realised niches cannot overlap
Indigenous species are adapted to exploit niches within their home ecosystem, and resist competition from other indigenous species A new species (alien, exotic or introduced) may: Prey on other species in the ecosystem, not adapted for defence against their predation Compete more effectively for resources, ousting an indigenous species from a niche Be immune from natural biological control mechanisms so grow unchecked Introducing species, particularly to islands can cause grave harm to the established species (extinction)

28 Alien species prey on defenceless animals
Examples Hawaiian Islands Hawaii’s endemic moths destroyed by introduced parasitic wasps Hawaii’s plants threatened by seed & fruit predation by rats Hawaiian native snails threatened by introduced snails Hebrides Hedgehogs eat eggs of ground nesting birds

29 Alien species grow unchecked
Australia Cacti are not native to Australia Prickly pear (S. America) grows unchecked, not natural predator Native plant species are ousted (no space) 1925, Cactoblastis (moth), lays its eggs specifically on the cactus and larvae burrow in causing bacterial infection Good biological control

30 Alien species grow unchecked
Australia Rabbit rapid reproduction & poor control by predators Population explosions occur Eat grass Myxomatosis introduced as biological control in 1950’s Similar explosions seen with mice

31 Alien species steal Niches
Example Hawaiian Islands Ants are not native to Hawaii Their introduction has led to loss of endemic flightless fly which previously filled the niche now occupied by ant

32 Resource Partitioning
e.g. Shore birds (beak length) e.g herbivores on African plains (Giraffe, Elephant & Antelope)

33 Symbiosis Symbiosis = living together
Two species form a close relationship They co-evolve to maximise the benefits from their interactions (parasitism only one species benefits) Three types of symbioses: Parasitism Commensalism Mutualism

34 Parasitism The symbiont (the parasite) benefits, the host (parasitised) loses Two forms of parasitism: Ectoparasite – live externally on the host e.g. ticks & fleas, leeches, Endoparasite – live inside the host e.g. malaria, tapeworm, hookworm, most gut bacteria are not parasites

35 Parasite transmission
Transmission is: vertical (mother to baby – HIV, rubella) horizontal (amongst members of species) direct close contact – cold, measles sexual contact – HIV, syphilis indirect contact – polio, cholera (through water) vector contact – malaria, sleeping sickness Parasites develop ingenuous strategies to transfer between host Often complex multistage , multihost life cycles involved

36 Pinworm Human gut parasite
Eggs transferred into mouth (oro-faecal transmission) Develop and grow in small intestine Warm, moist, good food supply Once mature females fill with eggs Migrate to anal region In evening/sleep, migrate out of anus, lay eggs perianally (around anus) Secretion causes irritation/ redness of perianal region (pruritus ani) Host scratches irritation Poor hygiene allows transfer of egg into mouth

37 Important aspects of host- parasite interactions
Parasites adapt to improve effectiveness of parasitism Obligate parasites – must live as a parasite Facultative parasites – can live as parasites when host is alive, but switch to saprophytes once host dies Hosts adapt to counter parasitism immune system preening behaviour plants produce defensive chemicals, galls develop to seal off parasite from rest of host Escalation of “war” leads to specificity in host/ parasite relationships e.g. smallpox virus, fleas

38 Commensalism A biotic interaction between two species
one species benefits, the other is UNAFFECTED Difficult to find clear examples Lichen on a tree is possibly one case Where carriage is provided e.g. hermit crab & anemone, energy is expended in transporting the anemone, But hermit crab appears to benefit because it actively replaces the anemone when removed – likely mutualism In the nitrogen cycle, Nitrobacter depends on Nitrosomonas for its nitrite The two species otherwise live entirely independently in the soil

39 Mutualism A biotic interaction in which both species gain benefit
Ant & acacia Ant – gains secure home, food supply Acacia – gains protection from predation Coral & Algae Coral – gains carbohydrate from photosynthesis Algae – protection and mineral nutrients Mycorrhizae & plants Mycorrhiza – gains photosynthetic product Plant – improved mineral and water absorption Ruminant herbivore & bacteria Ruminant - gets its food digested Bacteria – gains protection, warmth, moisture & food Lichen Fungus – photosynthetic products Algae – gains water, minerals and structural support Rhizobium and legumes Rhizobium – gains photosynthetic product Plant – gains nitrate for protein synthesis

40 More on Rhizobium Rhizobium responsible for N fixation in nodules on roots of legumes Nodules form as a result of interaction between bacteria and root hair cells 90% of fixed nitrogen passes to plant plant gives carbohydrate to bacteroids Enzyme involved is NITROGENASE Rhizobium produces NITROGENASE However nitrogenase is poisoned by OXYGEN The PLANT produces a protein which binds the oxygen and prevents NITROGENASE being poisoned leghaemoglobin traps oxygen

41 Cost, Benefits & Consequences
INTERACTION Effect on Population Density Predation Parasitism Commensalism Mutualism Competition Predator increases, prey decreases Parasite increases, host decreases Commensal increases, host density is unaffected Both species in mutualism increase Both species in competition decrease

42 Effect of External factors
Quantitatively, the outcome of a species interaction is determined by: Biotic factors e.g. disease, food availability Abiotic factors e.g. temperature, water availability If there is a pre-existing stress, negative interactions are more damaging. Humans further complicate the interaction by using medicines, fertilisers, pesticides & herbicides to alter the consequences of species interaction between ourselves and our crops

43 Coral Bleaching Coral is dying in a number of areas around the world
bleaching – when coral dies it turns white death is due to loss of algal mutualism this due to increase in sea temperatures (1ºC)

44 Competitive Exclusion
In closed conditions Competition between two species will lead to the exclusion of one of the species The triumphant species will ultimately depend on the conditions within the system In real ecosystems, competition may lead to the exclusion of a species through most of its range Local conditions may allow pockets of reduced density to survive, because they are better suited to these local conditions Should conditions change to favour the outcompeted species these pockets are sources from which the species can migrate and colonise its former range

45 Essay Compare parasitic, commensalistic and mutualistic interactions, using neamed examples. (15) Parasitism: Definition (1) Ecto/endo (1) +1 Obligate/ facultative (1) +1 Effect on host/ parasite (energy) (1) Evolutionary pressures (1) Life cycle vs host (1) +1 Max. 7 Commenalism: Definition (1) Examples (1) Diffiulty relating clear examples (1) Benefits analysis (energy) (1) Max. 3

46 Keystone species A keystone species is one whose removal will have an extremely detrimental effect on the community e.g. The removal of sea otter from californian kelp forest

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