What is a population? A population is all the individual organisms found in a given habitat, of one species. So you could talk about a population of wolves in the woods. If you want to talk about the wolves and rabbits in the woods, then you’d be referring to a community. A community is made up of the various populations in a habitat. So the summation of all the living things in a given area is called a community. What then is an ecosystem?
An ecosystem comprises the community of living organisms in a habitat, together with all the non-living components such as water, soil, temperature, etc. called abiotic factors.
Why are different organisms of different species able to coexist in the same habitat? How come they don’t directly compete with one another and drive others out? Have a watch…
So that’s the last and loveliest new term: niche. It rhymes with quiche. A niche is the interaction, or way of life, of a species, population or individual in relation to all others within an ecosystem. It’s how it behaves, what it eats, how it reproduces, where it sleeps, etc.; a species’ niche is determined by both biotic factors (such as competition and predation) and abiotic factors.
Different things may determine the population sizes within an ecosystem.
Climatic and edaphic factors (abiotic factors)
Non-living factors such as light intensity, temperature and humidity determine the number of organisms that a habitat can sustain. All species have a varying degree of ability to withstand harsh or fluctuating conditions, called resilience. If an abiotic factor changes dramatically in favour of a population – for example, plenty more light in a field – then the population will increase provided no other factors are limiting. The opposite is true if an abiotic factor changes against the resilience limit of a population – it will decrease.
Climatic factors refer to those abiotic factors that pertain to the air, temperature, light and water. These are key to photosynthesis, metabolic rate and other fundamental processes.
Edaphic factors specifically refer to soil properties such as its pH, availability of macronutrients and micronutrients, and its aeration (air inside it). Edaphic factors are key to organisms which rely directly on the earth to live e.g. land plants and decomposer microorganisms.
“Living factors” refer to all interactions between organisms, be it a bunny rabbit being predated, or two shrubs competing for sunlight. 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.]
Intraspecific competition refers to competition between members of the same species. If a population of apple trees all compete for a source of light, then each apple tree is taking up some light that has now become unavailable to a different apple tree. There are only so many apple trees which that habitat can sustain. The maximum population size sustainable indefinitely in a habitat is called the carrying capacity.
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.The diversity of life is built on the same biochemical basis. All life operates with carbohydrates, lipids, nucleic acids and proteins.
These building blocks have different properties and serve to function as structural and functional components such as cell walls, enzymes, genetic material, metabolites, energy stores, etc. Just as 4 DNA variables (adenine, guanine, cytosine and thymine) can serve to encode the entire genetic diversity of life, all these basic classes of chemicals together can create so many different configurations of life that it generates diversity of organisms at the individual and species level.
Species diversity is the diversity of species in a community. Put simply, how many different species are there in a community? 5 or 5,000? W
hich has the higher diversity? Not rocket science I hope.
^That’s some rocket science, I don’t really know what it is, but I don’t wish to find out, and neither do you. Just a little motivator to not complain about biology.
Species richness is defined by the number of different species in a habitat. However, in order to have biodiversity, the relative abundance of each species is also key. The more species, the higher the diversity. What if there are two separate communities like this:
Community #1 has 150 individuals per each of 20 different species (3000 individuals in total)
Community #2 has 10 individuals per each of 19 species, and 2990 individuals of the last species (3000 individuals in total)
It doesn’t take a complex formula to figure out that community #1 is far more diverse compared to community #2, despite them having the same number of species and individuals. The distribution of individuals to species is important in determining a community’s diversity.
Now for a little talk about deforestation and agriculture. Deforestation is the removal of trees in forests. and agriculture is the cultivation of useful plants to people which are often carefully selected for, and occupy a large area by themselves (like corn).
It’s not hard to figure out the impact both have on species diversity. Deforestation practically removes many, whole trees, and with them goes the shelter and food source of many other organisms. A great reduction in species diversity can be expected as a result.
Agriculture by humans results in a single dominant species which occupies vast land at the expense of others. Humans actively remove other species by the use of pesticides, insecticides and (indirectly) fertilisers. This, too, will lead to a great decrease in species diversity.
Other factors affecting species diversity include the degree of isolation, for example as seen on islands. The ability of individuals of certain species to move between different habitats can affect the biodiversity of different areas. When an area that is smaller harbours niches that can only cater to animals that can fly, or those that can eat fruit, the resulting community of populations of different species would not be as diverse as a larger, better connected area with more niches.
Alongside species diversity, other levels of diversity can be measured including genetic diversity at the level of different alleles, and ecosystem diversity at the level of different ecosystems in an area.
In the wild, each species may exist as one population or multiple populations. Different populations correspond to defined areas – habitats.
The sum of all present alleles for a given gene in a given population is known as the gene pool.
This is essentially a way of thinking about all the individuals in a population contributing their alleles towards the overall allele frequency. The extent of different alleles present gives the genetic diversity of a population.
The allele frequency in a population’s gene pool can change as a result of selection. The effectors of selection can be varied, yet the outcome is similar: advantageous or preferred alleles and the traits associated with them increase in frequency, while detrimental or disfavoured alleles and the traits associated with them decrease in frequency.
On the Earth as a whole, ecological diversity is overarching and includes within it both species diversity and genetic diversity.
It can be assessed in different ways, including geographically by ecosystem features e.g. deserts, oceans, forests, as well as biologically e.g. through the number of trophic levels in an ecosystem.