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Population growth

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There are several predictable population growth phases, typically seen in lab-grown microorganisms in controlled environments.



The lag phase represents the beginning of their growth. Once they get adjusted to their new environment and start thriving, they are ready to divide. This takes place actively during the log phase when their growth is exponential (because 2 cells become 4, and 4 become 8, and 8 become 16). Once their expansion into the media has reached its maximum potential, and they begin to run out of space and nutrients, they reach the stationary phase where division halts.


If the medium is left the same, with excretory products ever increasing and nutrients running out, they begin to die. This is the death phase.


The carrying capacity of a flask of a particular volume, with nutrients for bacteria, kept at a specific temperature for a certain amount of time can be predicted. The carrying capacity describes the maximum limit on a population, kept there by environmental resistance.


Environmental resistance describes the factors in the environment of a species that limit population growth. In the case of lab bacteria, these are space, temperature, nutrient availability, cell density, oxygen, etc. In the wild, these factors could be light intensity, humidity, predators, disease and others.


Biotic potential, on the other hand, refers to the inherent ability of a population to increase under ideal conditions. Factors contributing to biotic potential are the rate of reproduction and number of offspring each time.


In r/K selection theory, the number of offspring a population can have in a habitat is used to contrast species that produce many offspring with those that produce few.



r refers to the rate of growth while K refers to the carrying capacity of the habitat. Species that invest less in their offspring but have many are on the r side of the equation, hence termed r-selected organisms.


Those that tend towards the K side have fewer offspring and invest more energy in each of them. These are K-selected organisms.


Several traits are associated with each reproductive strategy, such as organism size, the stability of the niche they occupy and the developmental speed of offspring. Large organisms have fewer offspring and take close care of them. The offspring reach maturity slower.


Smaller organisms have more offspring and don’t look after them as much. These reach sexual maturity quicker.


This model can accommodate a spectrum, and some species can’t be classified as r-selected or K-selected. For example, turtles are large and long-lived, but they have many offspring and release them without watching them closely.


Interactions between populations


Symbiosis describes various interactions between species. Some are mutually beneficial, while others are detrimental to one of the parties.


Competition is a situation where two species rely on the same resources. It is described as a -/- interaction because neither party has anything to gain. Both are under stress, and the outcome may be that one of them is outcompeted by the other. In this case, the fallen species is driven out or perishes.


Mutualism is a +/+ interaction as both parties benefit from each other. Green plants evolved their ability to photosynthesise by endosymbiosis with cyanoabcteria. This means plant cells joined up with photosynthesising bacterial cells. These became today’s chloroplasts which contain the photosynthesising chemicals in green plants.



This mutually beneficial arrangement can be seen in parallel with mitochondria which have now become eukaryotic cells’ powerhouses. This interdependent relationship sees both species relying on each other and benefiting in different ways. At the point in the very distant past when this relationship became established, early eukaryotic cells were much larger and more flexible than their prokaryotic counterparts, so engaged in engulfing other cells.


As it became more efficient to not destroy these cells for food, but rather allow them to reside within the host cell and produce lots of ATP the host cell could also use, the mutualistic relationship developed.


Predation, parasitism and grazing are examples of +/- interactions. Here, one species takes advantage of another one at its expense.


Tapeworms reside in the small intestine where they can conveniently tap into the host’s nutrients, and so interfere with the normal absorption of the host’s nutrients into its bloodstream, thereby depriving the host to potentially dangerous levels of malnutrition and other side effects such as anaemia and fatigue.



Lice live on the body, such as pubic lice or on eyelashes, or the scalp in the case of head lice, and feed on the host’s blood. Sensitivity to lice saliva causes itchiness, and some lice can be vectors (carriers) of dangerous infectious agents such as Epidemic typhus and Trench fever.


Suppose you start off with equal populations of wolves and rabbits, and all the wolves rely on the rabbits for food. As the wolves start predating the rabbits, the rabbit population will decrease, while the wolf population will be sustained. Now there are fewer rabbits, so some wolves won’t have any food left. These wolves will die, so the wolf population will decrease. What will happen to the rabbit population now? Well, there are fewer wolves so they are predated less. The rabbit population will increase, followed by an increase in the wolf population, and so on.



The predator-prey relationship is very intricate, so the two affect each other and hence their population sizes rise and fall accordingly.


Pesticides such as weedkillers (herbicides) and insecticides can be selective (or non-selective) and systemic (or contact). Selective plant protection chemicals only affect certain species, commonly certain weeds. Non-selective chemicals are useful in a large breakout, but risk contaminating wider areas, and weeds as well as other plants.


Systemic chemicals spread through the whole system of an organism, so if the leaves are sprayed, the chemical will reach the roots and other parts. Contact chemicals require application directly onto the target area in order to be effective.


Resulting issues with the use of these protective plant chemicals include leaching into the wider environment and potentially spreading through food chains, toxicity to certain animal species, and providing a strong selection force that results in resistance against the further use of pesticides, similar to the development of antibiotic resistance in pathogenic bacteria.



In order to mitigate these issues and provide the most efficient protection to crops, biological control strategies as well as integrated pest management strategies are employed.


Biological control involves the use of a natural predator of the pest being used to keep its spread in check, while the integrated management (IPM) involves the combination of both chemical control and biological control.


Human populations grow and shrink due the balance between births and deaths. The number of births per 1000 people is called the birth rate, and the number of deaths per 1000 people is the death rate.


The Population Growth Rate

Population growth rate = birth rate – death rate


It’s really not a complicated equation, you must admit. Say the birth rate of Switzerland was 23/1000 and the death rate 28/1000. The population growth rate works out as -5/1000 (23/1000 – 28/1000). As a percentage, this is -0.5% growth. This means that there is a net decrease of 5 people per thousand.


The Demographic Transition Model

This is a model used to classify different populations depending on their birth rate, death rate, overall population size, and the factors behind these numbers. There are 5 different stages:


Stage 1

Birth rate is high due to lack of contraception and low education.

Death rate is also high due to poor sanitation, high infant mortality and starvation.

Therefore, overall population size remains low.


Stage 2

In stage 2, the causes of high death rate ameliorate, but not those behind high birth rate, therefore:
Birth rate is high for all the reasons in stage 1.


Death rate decreases as more food becomes available and healthcare improves.

Overall population size increases steeply.


Stage 3

Birth rate decreases steeply due to contraception availability and family planning.

Death rate decreases more slowly.

Overall population size increases.


Stage 4

Birth rate is low due to more money being directed towards assets, and the lack of need for child labour.

Death rate is low.

Overall population size is high and stable.


Stage 5
Birth rate decreases as children are expensive to have.

Death rate is stable as many older people die, counteracting the effect of advancing healthcare.

Overall population size decreases for the first time.


Non-human populations have other factors affecting their population sizes, including competition. All individual actions between organisms form a web which impacts on all populations in an ecosystem, therefore determining their sizes.


Interspecific competition refers to competition between members of different species for the same resources (food, light, water. etc.). Often when a new species is introduced in a habitat, say the American ladybird to the UK, if the invader species is better adapted, then the host population decreases in size. This may lead to extinction in some cases of the host species.


[Can’t remember the difference between interspecific and intraspecific? Interspecific is like the internet – different things come together.]





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