Ecology

Ecology is the study of the interrelationsips

• between organisms; and
• between organisms and the physical environment

Levels of organisation

1. Species
2. Population: group of organisms of the same spcies in the same habitat
3. Community: groupd of organisms of different species in the same habitat
4. Ecosystem: living components + non-living components, self-supporting, stable, dynamic
5. Biome: complex of communities that occupy a large geographical area, dominant type of plant
6. Bioshpere: entire space on earth surface where oranisms exist

Basic features of an ecosystem

1. interactions of organisms with one another and the physical environment
2. Flow of energgy
3. Cycling of materials

Types of ecosystems in Hong Kong

1. Freshwater stream
2. Mangrove
3. Rocky shores
4. Woodland
5. Grassland

Freshwater stream

• small, fast-flowing water
• Fast-flowing: fish
• Slow-flowing: insects
• Under rocks: larvae
• Amphibians: attached to plants along stream banks.

Mangrove

• intertidal mudflats: freshwater streams run into the sea
• water: slow flow
• fine soild particles >> soft mud: low O₂ content & high salt content

Rocky shore

• marine coastal areas with large rocks
• High tide: covered with seawater
• Low tide: shores become exposed to air and small rock pools may form.
• Organisms: hard shells, attach firmly onto rocks

Woodland

• Evergreen broad-leaved trees
• exotic species vs native species.

Grassland

• Abandoned farmlands/ regular burning --> prevent growth of shrubs and trees
• relatively low biodiversity
  • Grass and herbs

Abiotic factors of an ecosystem

1. Temperature

Animals:

• Homiotherms: can maintain constant body temperature --> maintain active in wide temperature range
• Poikilotherms: Cannot maintain constant body temperature --> restricted to areas with narrow range
• Animals in hot areas: develop organs with large surface area to promote heat loss
• Animals in cold areas:
  • Thick layer of subcutanoeus fat
  • Hibernate/ migrate

Plants:

• Temperaure affects flowering

2. Light

Animals:

• Affect behaviour
  • Dinural: active in day
  • Nocturnal: active at night --> develop night vision

Plants:

Duration of light in a day afffects the flowering of the plants

3. Rainfall and humidity

Animals:

• Stay in underground holes during the day to minimize the evaporation of water by sweating.

Plants:

• leaves with thick cuticle
• leaves modified to form spines
• store water in their stems
• long roots the absorb water

4. Wind speed

Animals:

• Sucker-like fins to attach themselces firmly on rocks
• flattened body to protct them from being hamred by strong waves

Plants:

• Wind: medium of pollination and dispersal of seeds
• Rate of transpiration increases with wind speed

5. Water current

• Organisms living in streams or along coastlines may hace flattened body/ hide under rocks/ sucker-like fins for holding themselves onto the rocks

6. Salinity

• Def: Conc. of dissolved saltes or ions --> water potential of water --> availability of water and minerals to organisms.
• e.g.: mangroves have salt glands in leaves to remoce excess salts

7. Oxygen concentration

• Factors: temp, water flow, presence of organic matter
• Soil with smalle particles: air spaces are small --> [O₂]⁺ is lower --> soil easily flooded with water and becomes waterlogged

Animals:

Aquatic insects have specialized breathing structure that sticks out of water to obtain O₂ from air

Plants:

Mangrove plants: to obtain O₂ from air

8. Soil Properties

e.g. soil PH, air content, nutrient content --> types of animals and plants

Biotic community of an ecosystem

Niche and habitat

Habitat = the living place of organisms
Each species occupies a specific niche
Niche: is determined by: temperature range, types of food they eat, space they occupy

Species diversity

• Species richness = number of species in the community
• Relative abundance of different species = ratio of species within the community
• Species diversity = Species richness + Relative abundance of different species

Dominant species

• most common type of plant

Interaction of organims in a community

1. Predation (+,-)
   Predator eats prey
2. Competition (-,-)
   Intraspecific ~ : same species
   Interspecific ~ : different species
   Intraspecific: more intense because individuals of the same species have the same needs
3. Symbiosis
   1. Commensalism (+,0)
      One gains benefits while the other does not
   2. Mutualism (+,+)
      Both organisms gain benefits
   3. Parasitism (+,-)
      Some organisms live on or inside organisms, obtaining nutrients for them and harming the host

Ecological succession

• Def: series of changes in the composition of a community over a period of time
• Climax community = max. no. of organisms that can be supported by the habitat
• Primary succession
  1. Lichens
  2. Mosses, ferns, grass
  3. Shrubs
  4. Trees
• Secondary succession

Energy flow in an ecosystem

• Ultimate origin: the sun

graph LR;
Sun-->Plant;
Plant-->Herbivore;
Herbivore-->Carnivore/Omnivore;
Carnivore/Omnivore--> .......

• Energy loss:
  • excretion
  • egestion
  • uneaten body parts when organisms die
  • heat during respiration

Food chain

e.g.:

graph LR;
grass-->rabbit;
rabbit-->fox;

1. Producers

• produce their own food
• most are photosynthetic
• light energy --> chemical energy stored in organic compounds

2. Consumers

• "heterotrophs"
• transfer energy along food chains in the form of food
  • Primary consumers: feed on producers
  • Secondary consumers: feed on primary consumers, etc.

3. Decomposers

• saprophytes
• feed on dead bodies
• break down organic matter into inorganic nutrients
• important in material cycling

Food web
Connect food chains together

Trophic levels and energy flow

Less energy at higher trophic level

1. Uneaten body materials

• Chemical energy stored in the uneaten body material is not passed to higher trophic levels and is lost from the food chain

2. Egested material

• eaten at higher trophic level, large proportion is indigestible and unabsorbed (e.g. celluose, hair and bones), passed out as faeces --> not transferred

3. Excretory products

• CO₂, urea --> lost

4. Respiration

• chemical energy in food relesased as heat or used to support various body functions

Energy "lost"

refers to energy not transferred to the next trophic level
1,2,3 --> broken down by decomposers
Energy loss in food chain --> sledom more than fice trophic levels

Ecological pyramids

1. Pyramid of numbers
   Predator: larger in size than prey --> require more energy to maintain life
2. Pyramid of biomass
   energy proportional to amount of living material

limitations of the ecological pyramids

• trophic level of an organism may vary with food eaten/ change diet as they grow
• energy flow to decomposers is not considered