C4.1 Populations and communities

population

a group of organism of the same species living in a given area at a particular time

community

all of the organisms, of different species, living together and interacting in an ecosystem

interspecific competition

• competition between organisms of different species

• e.g. different plants competing for light

intraspecific competition

• competition between organisms of the same species

• e.g. kangaroos competing for mates

parasitism

• one organism (the parasite) lives in or on another organism (the host), obtaining nutrients from it

• the parasite benefits while the host is harmed

• e.g tapeworms

  • live in the digestive system and feed on food in the small intestine, host may suffer malnutrition

pathogenicity

• one species (pathogen) living inside another species (host) and causing disease

• can be bacteria, virus, fungi or protoctista

• e.g. rabies virus

  • virus infects the nervous system of mammals causing rabies (can be fatal)

mutualism

• two species living in close association with both benefitting from the relationship

• have usually co-evolved

zooxanthellae in hard corals (mutualism example)

• corals (polyps) are marine animals forming coral colonies

• zooxanthellae are photosynthetic algae

• corals provide zooxanthellae with:

  • protected environment by secreting CaCO₃ to form hard skeletons

  • compounds they need for photosynthesis

• zooxanthellae provide coral with:

  • oxygen from photosynthesis

  • help remove waste

  • supplies products of photosynthesis (e.g. glucose)

root nodules in legumes (mutualism example)

• legumes provide Rhizobium with:

  • sugar for respiration and low oxygen environment

  • root nodules for protection

• Rhizobium provide legumes with:

  • ammonium ions from fixation of atmospheric nitrogen - legumes use ammonium to produce amino acids

mycorrhizae in orchids (mutualism example)

• association begins at seed germination

• orchids provide mycorrhizae with:

  • sugars and other carbon-based compounds

• mycorrhizae provide orchids with:

  • mineral nutrients

  • water

  • carbon based compounds from decaying matter

herbivory

• primary consumers feeding on producers

• usually harmful but can be beneficial

• harmful example: colorado beetle with leaves

• beneficial example: lemurs with fruit (seed dispersal)

predation

• one consumer species killing and eating another consumer species

• e.g. foxes and hares

cyclical oscillations in predator-prey population

• ↑ predators → more prey killed = ↓ prey population

• ↓ prey → less food for predators = ↓ Predator population

• ↓ predators → fewer prey killed = ↑ prey population

• ↑ prey → more food for predators = ↑ predator population

predation case study

• red foxes (predators) and mountain hares (prey) in Sweden

• outbreak in sarcoptic mange reduced red fox population

• less foxes meant less predation, population of mountain hares increased

endemic species

a species which naturally occurs in a particular area

alien species

a species introduced into a non-native habitat

invasive species

an introduced species that causes ecological and economic damage

competitve exclusion priniciple

two species cannot occupy the same niche in a community as they will compete for the same limited resources, when one species has an advantage it will usually dominate

reasons for invasive species outcompeting endemic species

• higher reproductive rate

• more efficient use of resources

• no natural predators

invasive species case study

• grey squirrels introduced intro the uk from north america

• grey squirrels are larger and breed faster than red squirrels

• grey squirrels carry a disease which they are resistant too but kills red squirrels

• as a result population of red squirrels has decreased and are rare in the uk

how interspecific competition can be investigated

• laboratory experiments: grow species together and separately to compare their success

• field observations: use random sampling to investigate whether species occur together.

• field manipulation: remove one species and observe whether the other increases in population size or biomass.

evidence for interspecific competition

• one species grows or reproduces more successfully when another species is absent

• only implies, does not prove, could be due to other environmental factors

difference between experiments and observations

• experiments: researchers manipulate variables to investigate cause and effect

• observations: researchers collect data without manipulating variables

purpose of chi-squared test

to determine whether there is an association between the presence or absence of two species, which may indicate interspecific competition

intraspecific cooperation

• individuals of the same species working together to increase their chances of survival

• e.g. chimpanzees cooperate when hunting, mackerel swim in schools to reduce chance of predation, meerkats cooperate in caring for offspring

top-down and bottom-up control control

• top-down: organisms at higher trophic levels control populations at lower trophic levels (e.g predation)

• bottom-up: organisms at lower trophic levels controlling organisms at higher trophic levels

• both types of control can occur in a community but one is usually more dominant