Topic 5 (ECOLOGY AND EVOLUTION) IB Biology SL

Light is the initial energy source for almost all communities, and is absorbed by producers and used in photosynthesis to produce chemical energy. This energy is transferred to primary consumers when they consume the producer. However only 10% of this energy is available at each trophic level. Therefore 90% of energy is lost at each trophic level as heat due to cell respiration & metabolism as well as through undigested/uneaten matter that is not consumed and defecation. For this reason, there are less consumers as there are producers because energy transfers between trophic levels are not 100% efficient.

• Pyramids of energy shows inflow of energy at each trophic level, in kJ m-2 per year.

• Pyramids of energy are pyramid-shaped because energy transformations between trophic levels is never 100 percent efficient.

• 90 percent of energy lost at each trophic level due to cell respiration and undigested matter of organisms.

• Pyramids of energy cannot be inverted because matter (organic compounds) cannot be created nor destroyed.

• Thus organic compounds would have to be created for there to be more energy available at higher trophic levels than lower trophic levels.

In an ecosystem, energy and nutrition are passed on from one trophic level to the next or between the biotic and abiotic components. Nutrition is recycled while energy is unidirectional and can be lost. Energy needs to enter and leave and leave an ecosystem while nutrients have to be recycled because the energy never goes back to the sun, so it doesn't come full circle.

Decomposers are organisms which gain nutrients through digesting dead organic matter. Decomposers are called saprotrophs because they obtain their food from digesting organic matter in their surroundings and absorbing them. They are very important in recycling nutrients.

• Increased usage of fossil fuels to generate electricity and to power cars has caused an increase in the concentrations of greenhouse gases, including water vapour, nitrogen oxides, carbon dioxide and methane, in the atmosphere. 

• During the greenhouse effect, shorter-wave radiation hits the earth from the sun and is reflected back into space mainly as heat. A proportion of the outgoing radiation is longer-wave radiation which is trapped by the atmosphere, increasing atmospheric temperatures to allow life to exist on earth. However, increased greenhouse gases cause what is known as the enhanced greenhouse effect. More longer-wave radiation is trapped by greenhouse gases and is reflected back to the earth, increasing ocean and atmospheric temperatures at a high rate than normal, which can disrupt climatic and ocean patterns, thereby threatening ecosystems.

• Conversely, some scientists are skeptical about whether humans are causing the enhanced greenhouse effect because throughout the Earth's history there have been many fluctuations in greenhouse gas levels and global temperatures.

Precautionary principle states that when a human-induced change could be potentially harmful, those responsible for the change must prove that it will not do harm before proceeding, instead of those against the change having to prove that it will do harm.

Evolution is the cumulative change in the heritable characteristics of a population

• When there is an abundance of resources, a population can achieve a J-curve maximum growth rate (biotic potential)

• However, with more offspring there will be less resources available to other members of the population (environmental resistance)

• This will lead to competition for available resources and a struggle for survival

• Intraspecific competition occurs when members of the same species compete for the same resources in an ecosystem (e.g. light, food, water)

• It is density dependent, as the available resources must be shared among members of the species

• Competition that occurs between different species for resources is interspecific

• The result of this competition will be an increase in the mortality rate, leading to an S-curve growth rate as the population approaches the carrying capacity (K)

Members of a species show variation, which can manifest itself in one of two forms:

• Discontinuous variation: A type of variation usually controlled by a single gene, which leads to distinct classes (e.g. ABO blood group in humans)

• Continuous variation: A type of variation controlled by many genes, which leads to a range of characteristics (e.g. skin pigmentation in humans)

There are three primary sources of variation within a given population

• Gene mutations (a permanent change to the genetic composition of an individual)

• Gene flow (the movement of genes from one population to another via immigration and emigration)

• Sexual reproduction (the combination of genetic materials from two parental sources)

Gametes are produced during meiosis, which is a special type of cell division where the chromosomes number is halved. Gametes show large variation because of crossing over during prophase 1 and independent assortment during metaphase 1. Both these process take place during meiosis 1.

As a result of the above processes, one human male can produce potentially 8 388 608 variations of sperms and a female can produce the same number of ova.

Fertilization is a random and can occur between any sperm and ovum. This means that a couple can potentially produce 7 x 10 to the power of 13 genetically different offspring.

1st name: Genus (always capitalized)

2nd name: species