Key Terms
lag phase: The stage at the start of a population growth curve where the population remains small and grows only very slowly.
log/exponential phase: The stage in a population growth curve where the population grows at its maximum rate.
stationary phase: The stage in a population growth curve where the population remains roughly constant.
death phase: The final stage in a population growth graph where the population falls.
sigmoid growth curve: an S-shaped curve showing the change in size of a population through all the phrases in population growth.
age pyramid: A diagram showing the relative numbers of individuals of different ages in a population.
Sigmoid growth curve
Changes in the size of a population can be summarized into four stages, and these four segments will form a Sigmoid curve. 一个种群的数量变化可以被总结成四个阶段，这四个这段会形成一条Sigmoid曲线。
In the first stage, the lag phase, the base of the population is small, so it does not become large and the population size grows slowly.在第一个阶段，也就是滞后期，种群的基数很小，所以不成规模，人口数量增长缓慢。
In the second stage, the log phase, the population grows at the fastest rate. 在第二个阶段，也就是对数期，人口数量以最快的速度增长。
In the third stage, the stationary phase, the population peaks and no longer fluctuates significantly. 在第三个阶段，也就是固定期，种群的数量达到顶峰，不再有大幅度波动。
In the last stage, the death phase, the population eventually begins to decline. 在最后一个阶段，也就是死亡阶段，种群的数量最终开始下降。
| phase 阶段 | population 种群数量 | birth 出生率 |
|---|---|---|
| lag | Increasing but not significant | birth rate > death rate |
| log | Explosive growth | birth rate >> death rate |
| stationary | Remains roughly constant | birth rate = death rate |
| death | Constantly getting smaller | death rate > birth rate |
The size of a population can be limited by many factors that are called limiting factors. There are different limiting factors in different populations, for example for yeast the limiting factor is the food supply. 一个种群的数量可以被很多因素限制住，这些因素被称为限制因素。在不同的种群中有着不同的限制因素，例如对酵母菌来说限制它们是食物的数量。
Possible examples of limiting factors:
- number of predators
- amount of food available
- amount of space
- amount of infection
In an ecosystem, the numbers of predators and prey interact with each other. When there are too many predators, the number of prey decreases, but when the number of prey decreases to a certain number, the number of predators also decreases due to the decrease in food, resulting in a decrease in population size. 在一个生态系统当中，捕食者和猎物之间的数量会相互影响。当捕食者过多时猎物的数量会减少，但是当猎物的数量减少到一定数量时捕食者的数量也会因为食物的减少导致种群数量下降。
The age pyramid shows the weight of different ages within the same event, with different regions having different numbers of people. 年龄金字塔展示了同一事件内不同年龄的比重，不同的地区有着不同数量的人。
The human population has grown rapidly in recent years, mainly due to preventive measures against many diseases and to the increasing food yield resulting from modernization and industrialization. 人类的人口数量在近几年飞速增长，主要原因是人们对于许多疾病的防范措施以及现代化和工业化所带来的粮食产出。
Questions
- Name three diagrams in this topic, describe their uses and their axises.
- Explain how the population of prey and predator changes.
- Name three practical limiting factors to human population, and state the current stage in Sigmoid growth curve.