Population dynamics

Populations are all a game of maths really. After all, the number of individuals in a population is just an number and the growth of a population can be simulated using graphs and equations etc. So why are populations so important in biology? In fact, it is often the size of a population which determines whether a group of organisms can survive. You need to have a population size within a set of defined limits in order to live. For example, like the pandas, if a population is too small it might be hard for individuals to find mates to reproduce sexually, hence the population cannot be sustained and it will go into decline and this will only exacerbate the problem. On the other hand, populations which are too large (some say like us humans) result in a lot of interspecific competition. Competition can even be expressed as war, and not just in humans, in ants, crows and elephants too! As a matter of interest, it is only humanity's ability to innovate and adapt using technology which has allowed us to produce enough resources to keep up with our population. But anyway, let us delve into the dark recesses of populations and find out exactly why, how and who they are.

To begin, it is important to know exactly what a population means. A population is (according to AQA): "all the organisms of one species in a habitat". Within a macrohabitat (e.g. a sand dune ecosystem) there are many different populations of different species, and together these make up a community. So what is an ecosystem then? If you imagine all of these organisms interacting with each other and with the other biotic and abiotic factors in a habitat, you have an ecosystem. Speaking about all of these interactions has brought me very nicely onto my next point: niches.

The idea of a niche is a very important concept in ecology. Think of a niche as a counterpart to a habitat. Whilst a habitat is where an organism lives, a niche is exactly what the organism does there. Habitat = workplace, niche = job. Of course, the organism will not really understand what job it is doing, or why it is doing it (unless it is a particularly altruistic sea cucumber) because like all things in biology which seem to be guided by some omnipotent hand, the reason an organism falls into a niche is nothing more or less spectacular than natural selection, leading to evolution where the best adaptations survive to pass on their genes to the next generation. Now that's a big concept - it took Darwin most of his life and a letter from Wallace to realise it - so let's look at an example.

Rabbits, they're everywhere because they breed like... well, rabbits. They seem to be everywhere in the British countryside and for a good reason! Because they have found and become very well adapted to their niche and exploit it to the full. Let's look at a few adaptations of rabbits. They're brown, which camouflages them against lots of woody/muddy/leafy backdrops which means they can survive well in woodland and grassland. What do they do? They eat grass all day long. They also have highly adapted incisors for eating grass which means that they are good at it. What's more, they are fast which means they can run away from fantastic Mr. Fox. Now you don't see many other animals doing that in the woods do you? Precisely, because this is the niche of the rabbit.

But what if another species with similar adaptations were to encroach on this niche? Perhaps that evil and nefarious creature: the hare! Two species cannot occupy the same niche, because not only is it socially awkward but it means you get interspecific competition. When two species occupy the same niche, they want the same resources: food, space, water, coal, belgian chocolate etc. But of course, there isn't enough to go around so scramble competition occurs. Both species want that damn chocolate, the belgian hare of course being the finest connoisseur of chocolate, so they both gather it. But because one species will have better adaptations to gather the resource, it will outcompete the other and reclaim its niche for itself and all other members of its species (huzzah). But hang on, members of the same species also compete for the same resources, which means that intraspecific competition will also occur. Therefore only the best adapted will get their precious resources (like Golem) and this leads us back to survival, breeding and more natural selection.


Now we know this about populations, any good scientist will ask how do we know this? Well here comes the #applianceofscience (use it, you have my permission) with sampling methodology.

One way scientists can sample populations is using random quadrat placement. Very good for plants, but not so much for animals as they have a habit of moving which would produce invalid results as they could be counted twice or not at all. So here's my method:

1. Get a map of the study area and break it up into a grid using a computer/pen/sonic screwdriver.
2. Decide how many quadrats to use with the method explained here.
3. Generate random numbers using a random number simulator e.g. random.org which is cool because it uses electromagnetic noise from space to generate the numbers.
4. Turn these numbers into coordinates.
5. Place quadrats on these coordinates.
6. Go crazy counting plants and cataloguing their species.
7. You can then use these data to estimate % cover or the abundance of a species or species diversity/richness.

Never ever throw the quadrat. For one thing, nobody wants a quadrat in their eye. Also it biases the results which is exactly what random sampling aims to eliminate.

The other important quadrat method is using transects:

1. Put a long line of string in whatever direction you want to measure (e.g. distance from a cliff edge).
2. Place quadrats along the length of the string and count the species in them like in random sampling.

But what if the transect is really long and I'm really lazy? Easy, just place a quadrat at every x metre interval along the transect or every y metre height interval. This is called an interrupted transect.

But alas, we have limitations. Like I said ages back, this is rubbish for counting most animals as they like to run away. So we have capture, mark, release, recapture which is much more fun, but science doesn't care about that. But here's the method:

1. Introduce some kind of trap into the habitat e.g. a pitfall trap.
2. Have patience whilst all sorts of animals drop into it.
3. Sort out the one(s) you want and mark them using a method which wont affect them or the rate at which they are predated and note down how many were marked.
4. Release them and wait for them to mix back in with the rest of the population (a really long time if you're investigating snails/slugs/tortoises).
5. Replace the traps (under similar conditions so as not to introduce a confounding variable).
6. Wait until more animals are collected.
7. Count how many you have in total and how many of these are marked.
8. Multiply the number of animals collected on both traps together and then divide by the number which were marked on the second trap.

Wasn't that a joy? What's even more fun is carrying out risk assessments on fieldwork. Basically, don't do anything stupid and mention some hazards and appropriate steps to avoid them and you'll be fine (I don't like risk assessments - can you tell?).

Ah, but won't all this poking around in delicate ecosystems harm the environment? Yes, yes it will. So we have to take appropriate steps to mitigate our impact. For example: if we have an ultra-rare shrew and we want to know its dry biomass to construct a pyramid of biomass, heating it in an oven until dry won't do the population much good. So, maybe making an estimate from already known values of similar species would be more ethical. There are many many many ethical considerations which should be made when carrying out fieldwork, but it's really just a case of common sense.

Oh, and do you want to hear the punchline of all this investigative work? Conclusions drawn from correlations in the data have to be done so very cautiously and tentatively because as we all know correlation does not necessarily prove a causal relationship. So you might say, all of that work just means we can now say that what we thought may or may not be true, but we are a little bit more sure of it being true now than we were last week. Score: science 1-0 ignorance.

This blog is called 'population dynamics' and of course 'dynamic' means movement, so what moves in populations? The size of populations of course! They vary!

Populations can vary because of biotic and abiotic factors. Remember that, it's important, I put it in bold and everything. Let's make some more examples, shall we?

Okay, so we have our lovely squid (Sid) and his niche dictates that he can only survive in waters from 15-17˚C. Because of global warming and all that jazz, his waters are getting warmer, and his bad luck doesn't even end there. He happens to live in a closed lake, so the whole squid population is stuck there. The temperature in the water is now 17.5˚C. Some squid can survive this, but most can't. Sid is dead and the population has decreased as squid die due to warming waters.

But of course, biotic factors are also important. Let's forget Sid's unfortunate demise in the warming lake and imagine that some careless person has introduced a sperm whale into the lake (I know whales are big, but it's a big lake, go with it). Sperm whales like to eat squid - they predate it - because this large predator has been introduced, the squid population decreases and then increases again because of the classic lynx-snowshoe hare model. Sid is dead, and inside a whale. Poor Sid.

It is also worth remembering that because of interspecific and intraspecific competition, the populations will vary too. Go back to the whole hare-rabbit fiasco earlier if you want to read it again. Both of these types of competition are biotic factors.

Finally, we come to the geographical concept of human populations (woot).

Human populations grow depending on four major factors:

• Birth rate
• Death rate
• Emigration
• Immigration

Each of these factors is usually measured as a rate per 1000 of the population. For example, if per 1000 people this year, 25 die, the death rate is 25 deaths per 1000 people. Easy, right? Good, because you can also express it as a percentage. 25 per 1000 can be expressed as (25÷1000)% = 2.5%. Again, nice and easy. The total growth rate of the population (which can be negative) = (birth rate + immigration)-(death rate + emigration).

There is a very famous graph called the demographic transition model (DTM) and here it is:

It's a jolly-looking thing, but really is quite important. Because I'm feeling particularly lazy tonight, I found an annotated model which describes what is going on. Here are my explanations:

1. Only really seen in remote tribes and apocalypse survivors. There is a lack of proper healthcare and sanitation so the death rate is high and occasionally spikes due to disease or famine. The birth rate is high because there is a lack of family planning and contraception. Despite the high birth rate, the death rate keeps the population low.
2. Improved healthcare means people live for longer, so the death rate drops sharply, but the birth rate remains high due to the continued lack of family planning and contraceptives.
3. The death rate continues to fall, but more slowly now as the initial breakthroughs reach their limit of effectiveness. Birth rate begins to fall as people become more educated on sex and families and jobs begin to dictate family life.
4. Jobs dominate family life, so the birth rate levels off, occasionally spiking due to affluent periods in time. The birth rate means that the population stops growing. This is where the UK is now.
5. I know it's not on the graph, but birth rates may start to drop even further as more elderly dependents emerge so people cannot afford to have families. Only a few countries exhibit this behaviour, such as Germany and Japan.

There are many other ways to represent human populations, such as population growth curves, survival curves and age-population pyramids. Each of which can be used to find out which stage of the DTM a country is in. Each one has its own purpose and I strongly recommend that you look up the differences between them, but interpreting them is pretty straightforward. But this is starting to look more and more mathsy, and I now regret saying that populations are pretty much maths at the beginning, because this maths isn't fun maths. Nevertheless it is important, so don't disregard it so quickly as I just did!

Well, that's population dynamics. I thank anyone who managed to stick with it to the end, but it is important, regardless of how dull it can seem compared to 'proper biology' #controversy. Good night.