0610 · 19.4Populations

Population, community and ecosystem

0:00Imagine a pond containing a small population of water fleas, tiny animals that feed on microscopic organisms. If they have plenty of food and keep reproducing, why does the pond not eventually become completely packed with them? The answer is that reproduction increases population size, but the environment places limits on that increase. Understanding those limits explains why populations grow, level off, and sometimes decline. By the end of this video, you will understand the difference between a population, a community and an ecosystem.

0:28You will also be able to explain how food supply, competition, predation and disease affect population growth. We will use those ideas to explain each phase of a population growth curve, then work through how to interpret one rather than simply memorising its shape. First, we need one basic idea about reproduction. Reproduction produces new individuals, while death removes individuals from a population. If more individuals are added than are lost over the same period, population size increases.

0:56For our main example, imagine a population with no individuals entering or leaving, so its changing size depends on reproduction and death. A population is a group of organisms of one species, living in the same area, at the same time. All the water fleas of one species in our pond today form a population. Water fleas of that species in a different pond are not part of this particular population.

1:17The species, area and time all matter because they define exactly which organisms we are counting. A community is all of the populations of different species in an ecosystem. Our pond community includes the populations of water fleas, fish, algae, reeds and other organisms living there. A community therefore contains different species, whereas a population contains only one species. The water and mud are not themselves members of the community because they are not organisms.

1:44An ecosystem is a unit containing the community of organisms and their environment, interacting together. The pond ecosystem includes its living community and environmental features such as water, light and temperature. The interaction is important, because organisms depend on their surroundings, and their activities can change those surroundings. Think of population, community and ecosystem as increasingly complete views of the same place.

Factors limiting population growth

2:08Now return to the question of growth. Population size tells us how many organisms there are, but the rate of population growth tells us how quickly that number changes. A population of a thousand organisms could be increasing, remaining approximately constant, or decreasing. Knowing its size alone does not tell us which of those things is happening. The first factor affecting the rate of population growth is food supply. Organisms need food to provide energy and materials for growth and reproduction.

2:35When food is plentiful, more individuals may survive and reproduce successfully. If food becomes scarce, fewer offspring are produced and more individuals die. Population growth then slows, or the population starts to fall. The second factor is competition, which happens when organisms need the same resource and there is not enough available for all of them. Water fleas may compete for food, while plants may compete for light, water or mineral ions.

3:01Competition can occur between members of the same species or between different species. When a population becomes crowded, competition gets stronger, because more individuals share the same resources. The third factor is predation, where one animal kills and eats another animal. If fish eat more water fleas, more individuals are removed from the water-flea population. This can reduce its rate of growth or cause its size to decrease. Predation does not automatically mean a population falls, because reproduction might still replace more individuals than predators remove.

3:31The fourth factor is disease, which can reduce survival or successful reproduction. An infectious disease may spread more easily when individuals are crowded close together, although its spread depends on how it is transmitted. If disease increases deaths or reduces reproduction, population growth slows. In each case, ask how the factor changes the number of births compared with the number of deaths. When a factor restricts population growth, we call it a limiting factor.

3:57For example, food becomes limiting when its availability prevents the population from growing as quickly as it otherwise could. These factors often interact rather than acting separately. A reduced food supply can increase competition, leaving some individuals less able to survive and reproduce.

Lag and exponential phases

4:12Let us put these ideas onto a graph. Time goes on the horizontal axis, and population size goes on the vertical axis. In an environment with limited resources, population growth often follows an S-shaped pattern called a sigmoid curve. We will identify the lag, exponential, stationary and death phases, remembering that the downward death phase can follow the S-shaped growth section if conditions deteriorate. The lag phase is the initial period of slow population growth.

4:39A small starting population contains relatively few reproducing individuals, so only a small number of offspring may be added at first. Organisms may also need time to become established in their new environment before reproducing rapidly. In a microorganism culture, for example, cells may need time to adjust to the available food. Notice that slow growth during the lag phase does not necessarily mean food is scarce. There may be plenty of food, but the population has not yet reached the stage of rapid reproduction.

5:08The graph stays relatively shallow because population size changes only a little over time. Some growth can occur during lag, so this phase does not have to be perfectly horizontal. Next comes the exponential phase, also called the log phase. Resources are plentiful relative to the number of organisms, and competition has relatively little effect. Reproduction adds individuals faster than death removes them, and the growing population contains more individuals capable of reproducing.

5:33That is why the increase becomes progressively faster and the graph rises steeply. Imagine an idealised population that doubles over equal time intervals. Ten individuals become twenty, then forty, then eighty, provided conditions allow that pattern to continue. The same proportional increase produces a larger numerical increase each time because there are more organisms reproducing. These numbers illustrate exponential growth, but they are not a claim that every real population always doubles at a fixed interval.

6:02However, the environment does not gain unlimited resources simply because the population grows. More organisms consume food, and competition becomes stronger as resources become less available per individual. Disease or predation may also increasingly restrict growth, depending on the population and its surroundings. The curve therefore becomes less steep as limiting factors reduce the net increase in population size.

Stationary and death phases

6:25The stationary phase is the plateau, where population size remains approximately constant. In our population with no movement into or out of the area, reproduction adds individuals at approximately the same rate as death removes them. Limited food and increased competition help prevent continued growth. The population has reached a size that the available conditions can support at that time. Stationary does not mean that every organism stops reproducing or that nothing happens.

6:50Individuals can still be born and die while the total remains approximately unchanged. Imagine adding ten individuals while losing ten during the same interval, and the population size stays the same. A flat graph shows no net change, not an absence of biological activity. Finally, the death phase occurs when deaths exceed the addition of new individuals through reproduction. Food supplies may become severely depleted, competition may be intense, or disease may increase deaths.

7:17In a closed culture, harmful waste products can also accumulate and worsen conditions. The population size falls, giving a downward section of the graph, although a death phase does not necessarily mean every individual dies.

Interpreting population growth graphs

7:29Now let us interpret an example step by step. These are illustrative numbers for practice, not results from a real experiment. The population contains forty organisms on day four, one hundred and twenty on day six, and one hundred and eighty on day eight. It remains at one hundred and eighty on day ten, then falls to one hundred and twenty on day twelve. Suppose the question asks us to calculate the average rate of population growth between day four and day six.

7:55First, find the change in population size by subtracting forty from one hundred and twenty, giving eighty organisms. Next, find the time interval, which is two days. Dividing eighty organisms by two days gives an average increase of forty organisms per day. Why divide by time? Because a growth rate tells us how much the population changes during each unit of time, not just its total change. Between day six and day eight, the population increases by sixty organisms over two days, giving thirty organisms per day.

8:25Its size is still increasing, but its growth rate has slowed from forty to thirty organisms per day. Between day eight and day ten, population size remains at one hundred and eighty, so the net growth rate is zero. This interval represents the stationary phase, not necessarily the end of reproduction. Between day ten and day twelve, the population loses sixty organisms over two days. That means an average decrease of thirty organisms per day, consistent with the death phase.

8:52A useful graph-reading rule is to distinguish height from steepness. The height tells you population size, while the slope tells you how quickly population size is changing. A high, flat section represents a large population with no net growth. A lower but steeply rising section can represent a smaller population growing rapidly, so the highest point is not automatically where growth is fastest. Here is an exam-style question using Cambridge command words.

9:17Identify the phase at section A, and explain why population size remains approximately constant at section B. For identify, give the phase name directly. For explain, link the environmental conditions to their effect on reproduction and death, rather than simply repeating that the line is flat. Section A is the exponential phase. At section B, food supply is limited. Competition for food reduces successful reproduction and survival. The rate at which new individuals are produced is approximately equal to the death rate.

9:46Therefore, population size remains approximately constant. Watch out for three common mistakes. A community is not simply a large population, because it includes populations of different species. Exponential growth cannot continue indefinitely in an environment with limited resources, because limiting factors eventually restrict the increase. And a downward curve means the population is getting smaller, not merely growing more slowly. Slower positive growth still produces an upward curve. Let us bring everything together.

10:13A population contains one species in the same area at the same time, while a community contains all the populations of different species in an ecosystem. An ecosystem includes that community and its environment interacting together. Food supply, competition, predation and disease affect growth by changing how successfully organisms reproduce and survive. The lag phase has slow growth as the population becomes established, and the exponential phase has rapid growth while conditions are favourable.

10:42Limiting factors slow the increase until the stationary phase, when additions approximately balance deaths. If deaths exceed additions, the population enters a death phase. Once you connect the graph to that changing balance, its shape becomes an explanation of what is happening, not just a pattern to memorise.