Populations Chapter 26

Population Science Studying growth rates of populations helps us understand: the effects of rapid overpopulation. how population growth is regulated. We can derive important lessons for humans from studies of populations in nature.

Why study populations? In the field, populations tend to be the unit of study. A population is a natural grouping, so studying populations reflects what is going on in nature. Even so – it’s not always easy to define a population!

General Principles A population consists of members of the same species living in the same ecosystem at the same time. Total population increases or decreases according to the number of births, deaths, immigration, and emigration that occurs.

Distribution Individuals distribute themselves in a population in three general patterns: Clumped Uniform Random

Distribution Clumped distribution is typical of organisms that move in groups (herds, flocks, etc.), or that cluster around resources, such as plants near a water source.

Distribution Uniform distribution is typical where resources are scarce. Individuals compete to claim enough territory to support them and keep a distance from others.

Distribution Random distribution is rare. Organisms may distribute randomly if resources are abundant and the organisms do not form social groups. Trees in a diverse forest may distribute randomly.

Growth Rate To determine the actual change in numbers of a population in a given unit of time, we look at the difference between losses (deaths and emigration) and additions (births and immigration) (births - deaths) + (immigrants - emigrants) = change in population size.

Growth Rate If we want to know the rate at which a population is increasing: r = population growth rate b = birth rate d = death rate Then: r = b - d

Growth Rate The number of individuals added to a population (G, or population growth) can be determined if we know the growth rate (r) and the population size (N): G = r x N

Exponential growth produces a J-shaped population graph.

Age of first reproduction affects the rate of population growth. Why? Exponential Growth Age of first reproduction affects the rate of population growth. Why?

Death rates and average lifespan also affects growth rate. Why? Exponential Growth Death rates and average lifespan also affects growth rate. Why?

Population Limits Species introduced to a new environment may experience exponential growth. Environmental resistance will eventually limit growth. Some populations experience “boom and bust” cycles. Others stabilize and show logistic growth.

Population Limits K = Carrying Capacity: # births = # deaths The upper limit for population growth is determined by the carrying capacity of the environment.

Available space limits barnacle populations. Population Limits Available space limits barnacle populations. Where there are many natural controls, populations tend to demonstrate logistic growth.

Population Limits If a population overshoots the carrying capacity of the environment, the result is a population crash.

Population Limits Where there are few natural controls, a population may rise rapidly, exceed carrying capacity, then crash as most of the population starves.

A growing population may become self-limiting. Population Limits A growing population may become self-limiting. In fruit flies, reproduction rate drops in response to crowded conditions. lifespan offspring per day days population density

Seasonal weather changes Population Limits Density-independent factors limit populations regardless of how large or small the population is. Examples: Seasonal weather changes Natural disasters Pollution

Population Limits Density-dependent factors affect a population more strongly the larger it grows. Examples: Predation Parasites Disease Resource competition

Survivorship Populations show different patterns in survival at different ages, which in turn can affect population growth. Early loss – many young die Constant loss – equal loss at all ages Late loss – high survivor of young, most deaths in old age

(in percentage of maximum life span) number of survivors late loss (human) constant loss (American robin) early loss (dandelion) age (in percentage of maximum life span)

(a) Developed countries 2006 2025 2050 75 and older male female 60 - 74 postreproductive (45–79 yr) 45 - 59 age 30 - 44 reproductive (15–44 yr) 15 - 29 0 - 14 prereproductive (0–14 yr) millions of people (b) Developing countries 75 and older male female 60 - 74 45 - 59 age 30 - 44 15 - 29 0 - 14 millions of people Different survivorship curves can have different consequences for populations, even of the same species.

Recap Population size changes through birth, death, immigration, and emigration. Population size is regulated by environmental restraints that increase deaths or decrease births. Populations are distributed in various patterns for social reasons or because of resource availability.