IB Biology Sub-topic B4.2 Notes

Niches

You have now learned about organism evolution and speciation, biodiversity, and habitat. However, each habitat has a great number of species that each contribute differently. The functional role of an organism, in its environment, is its niche. There are two types of niches you need to be aware of:

1. Ecological niche - the species habitat and all its interactions. This includes what it consumes, its interdependence with other species, the time of day it's active, where it lives, and where it feeds.
2. Fundamental niche - the full range of conditions under which species can survive and successfully reproduce. 

Note that two species with the same niche cannot coexist as they would compete with one another, leading to the exclusion of one. This topic will focus on the distribution of organisms primarily on their:

1. Requirement of oxygen
2. Nutrition methods
3. Adaptations to nutrition
4. Competition

Oxygen requirements

In the discussion of oxygen requirements, you are required to be familiar with three types of organisms: obligate anaerobes, facultative anaerobes, and obligate anaerobes. The existence of each in a habitat is highly dependent on the presence of oxygen. 

While this is the case for many organisms today, Earth did not always host an atmosphere with free oxygen. The first billion years of our planet’s life did not have freely available oxygen, and it was not until 2.5 billion years ago that oxygen was produced by photosynthetic microorganisms splitting water. Thus, obligate anaerobes, facultative anaerobes, and obligate anaerobes evolved with different requirements:

1. Obligate anaerobes can be found in extreme environments found on earth, such as in deep oceans, deep mud, near active volcanoes or any where that limited oxygen exists.
2. Facultative anaerobes rely on sulphates, nitrates, and carbon monoxide as the final electron acceptor in respiration.
3. Obligate aerobes rely on oxygen, and are thus found where obligate anaerobes and facultative anaerobes are not.

Modes of nutrition

The main modes of nutrition exhibited by organisms include:

1. Autotrophic nutrition - nutrition reliant on chemical energy or light energy, such as photosynthesis. Green plants, bacteria, cyanobacteria, algae, and some archaea are all photoautotrophs.
2. Heterotrophic nutrition - nutrition reliant on the ingestion of other organisms or biological material. All animals are heterotrophs but can exhibit holozoic or saprotrophic nutrition.
   1. Holozoic nutrition - involves the ingestion, internal digestion, absorption, and assimilation of nutrients, exhibited by most animals. This is often broken down into carnivory, herbivory, and omnivory.
   2. Saprotrophic nutrition - involves the ingestion of dead organic material via external or internal digestion, exhibited by fungi and bacteria.
3. Mixotrophic nutrition - organisms with this nutrition mode can obtain nutrients autotrophically or heterotrophically. Euglena is a freshwater protist that exhibits this behavior.

You must also be aware that archaea bacteria shows a great variation of metabolic activity within their domain. They have:

1. Phototrophic archaea
2. Chemotrophic archaea
3. Heterotrophic archaea