Stage 2 Biology – Ecosystems Notes
E1. Populations are the units of the community.
·
Population = a group of individuals of one
species that interbreed together in a particular living space and produce
fertile offspring
o
Can be thought of as the unit of a community
o
Are not scattered at random – instead occur in
habitats that suit their particular requirements
·
Community = all of the living organisms found
living together in a particular area or habitat
o
Populations in a community inhabit a common
environment and interact with one another
o
Interactions shape the ecosystem, the
individuals in each population and the number of different species in the community
o
Unique set of properties and mix of organisms
o
Defining form of vegetation often used to name
the community
·
Ecosystem = sum of all of the organisms living
in a particular area, along with the habitat and the physical component of the environment
that affects organisms
·
Physical factors – oxygen levels, carbon dioxide
levels, water availability, sunlight and soil characteristics
Interactions
·
Competition
o
Organisms compete for the same resources –
light, space, oxygen, food
o
Most intense between members of the same species
or different species with similar requirements
o
No two populations have identical requirements,
but competition is seen where there is overlap
o
Vertical stratification – each level in a forest
provides its own set of physical conditions and consequently different organisms
feed and live in separate layers – therefore allowing them to co-exist in close
proximity but avoiding direct competition
·
Predation
o
One organism feeds on or eats another live
organism
o
Affects the number of organisms in a population,
biodiversity of the community and evolution of organisms
o
Strategies and physical characteristics of
predators are often easily observed
o
Mechanisms to avoid predators:
§
Plant defences:
·
Physical – thorns, spikes, hairs, waxy or silica
secretions
·
Chemical – chemicals that deter herbivores from
eating them, toxic chemicals
§
Animal defences:
·
Chemical – poisons to deter predators, defend
themselves or capture prey
·
Colouration – as a warning to others,
camouflage, mimicry
·
Symbiosis
o
A relationship between organisms of two
different species
§
Mutualism – both organisms from different
species benefit
§
Commensalism – one species benefits but the
other is unharmed
§
Parasitism – prey is harmed or eaten by an
organism that is smaller than it
Species definition
·
Species = a population or group of populations where
the members have similar physical characteristics and the capacity to
interbreed and produce fertile offspring
o
Does not apply to organisms that reproduce asexually
– in this case the definition is assigned by appearance and biochemical
characteristics
o
Main concept of a species is around genetic
isolation
·
Naming of species – binomial system with genus
and species (in italics, genus with capital, species with lowercase)
·
Members of the same genus have a number of
similar features, but cannot interbreed and produce fertile offspring
Reproductive
isolation
·
Mechanisms of reproductive isolation – prevent gene
flow between groups even though their habitats overlap
o
Barriers preventing fertilisation
§
Prevent mating between individuals – size/shape
of genitalia or flower
§
Interfere with fusion of ova/sperm or ova/pollen
·
Sperm destroyed in reproductive tract
·
Unable to penetrate ova
§
Breeding seasons / timing of flowering
o
Differing numbers of chromosomes
§
Development does not proceed
§
Horse + donkey = mule (it is infertile)
o
Attraction of mates
§
Pheromones are given off by one of the sexes
§
Mating calls
§
Courting rituals to attract only members of the
opposite sex of the species
o
Lack of development of fertile adults
§
Zygotes fail to develop
§
Fail to reach sexual maturity
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