SNHS Biology Blog 5: Populations

Andrew Park

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Describing the Population

One of the characteristics of a population is its geographic range. This is the place that the population lives at. The population range may vary greatly in size. Another characteristic is the growth rate. The number of individuals in a population may change over time or it may stay the same. Another characteristic is the population density and the population distribution. The population density is how many individuals are living in a given amount of reason. If there are lot of individuals in that region, then the population density would be higher. The population distribution describes how the population is spread out in the given location. The population may be uniform, random, or clumped. This information would help identify if there are competitions for food in the area or not as uniform occurs when there are competitions and random occurs when there are no competitions. Lastly, age structure, the spread of ages in the population, may be important. This structure can help identify if the population will increase, stay the same, or decrease as some animals have a range of ages where they can reproduce.

Population Growth

One change that the population would go through is population growth. This can be caused by multiple factors. The abundance of resources, high birth rate, immigration, etc. There are two main types of population growth. One of them is known as an exponential growth. This type of growth occurs when the species is introduced into a new region with plentiful resources and few predators. This growth will result in a population to grow fast exponentially. One of them is known as a logistic growth. There are three phases in logistic growth. First, the population is introduced in an environment with plentiful resources. In this phase, the population will grow exponentially. The second phrase is when the resources become limited. The population will still increase, but the rate will be decreased due to the competition. The third phase is where the carrying capacity is reached, due to the limiting factor, a factor that controls the growth of a population. This means that the population has reached the maximum number of individuals that the environment can hold. The population will remain the same in this phase.

Density-Dependent Limiting Factors/Density-Independent Limiting Factors

Density-dependent limiting factors are factors that don’t strongly affect a small, scattered population. These factors include competition, stress from overcrowding, parasitism, disease, predation, and herbivory. Competition occurs when the population becomes crowded. The individuals begin to compete for food, water, space, sunlight, and other resources. This competition allow some to get enough resources to survive, while the others may die from the lack of resources available to them. Parasites and disease causing organisms can weaken their host and result in death. This is a density-dependent limiting factor because they can spread very easily when the population is dense. Some organisms may fight each other in an overcrowded environment. The stress from this overcrowding can cause the organisms to neglect and kill their offsprings, affecting the population size. Population of prey and predators may rise and fall. When there are many prey, then the amount of predators will increase as there are more resources available to them. When this happens the number of prey will decrease. This decrease limits the resources for the predators, resulting in their decrease. After this happens the prey’s population increases again, and the process repeats. Density independent limiting factors affect the population regardless of their size and density. These include environmental factors like hurricanes, droughts, floods, etc. It can also include natural disasters like wildfires. These factors can cause the population to crash or reach very low numbers. The effect from these factors will last for a long time before the population can recover from the damage.