Introduction A population is a group of individuals of a single species that simultaneously occupy the same general area. The characteristics of populations are shaped by the interactions between individuals and their environment. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

1. Two important characteristics of any population are density and the spacing of individuals. Populations have size and geographical boundaries. The density of a population is measured as the number of individuals per unit area. The dispersion of a population is the pattern of spacing among individuals within the geographic boundaries. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Measuring density of populations is a difficult task. We can count individuals; we can estimate population numbers. Fig. 52.1 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

This information allows estimates of population changes to be made. Unfortunately, it is usually impractical to attempt to count individuals in a population. One sampling technique that researchers use is known as the mark-recapture method. Individuals are trapped in an area and captured, marked with a tag, recorded, and then released. After a period of time has elapsed, traps are set again, and individuals are captured and identified. This information allows estimates of population changes to be made. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Mark & Recapture Practice Problem R / C = M / N R (marked recaptures) / C (total in second sample)  =  M (marked initially) / N (total pop. size)

Mark & Recapture Practice Problem Problem: Suppose that you capture 10 individuals of a rare subspecies of brook trout from an impounded watershed. You place a tag in the body cavity of each individual and then release these fish. You come back a month later and capture 20 fish and find that four of these are individuals that you had previously captured and released. What is your best estimate of the population size, N?

Clumped dispersion is when individuals aggregate in patches. Fig. 52.2a Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

By contrast, uniform dispersion is when individuals are evenly spaced. Fig. 52.2b Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Overall, dispersion depends on resource distribution. In random dispersion, the position of each individual is independent of the others. Overall, dispersion depends on resource distribution. Fig. 52.2c Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Chi Square Break! An area has previously had a fairly uniform dispersal of a certain type of plant. An industry moved in and wants to check to see if their factory is adversely affecting species dispersal. They do not believe that they are, but they want to appease nearby neighbors who are expressing concerns.

Chi Square Break! They select 10 sample plots and count the species in the plots. The results are on the next slide. What is the null? Degrees of freedom? Would you recommend that they reject or fail to reject the null?

Plot Number Number of Plants

1. Life histories are highly diverse, but they exhibit patterns in their variability Life histories-the traits that affect an organism’s schedule of reproduction and survival Some organisms, such as the agave plant, exhibit what is known as big-bang reproduction, where large numbers of offspring are produced in each reproduction, after which the individual often dies. This is also known as semelparity. Fig. 52.4 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

By contrast, some organisms produce only a few eggs during repeated reproductive episodes. This is also known as iteroparity. What factors contribute to the evolution of semelparity and iteroparity? Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Variations also occur in seed crop size in plants. The number of offspring produced at each reproductive episode exhibits a trade-off between number and quality of offspring. Fig. 52.7 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Populations have the potential for tremendous growth If N represents population size, and t represents time, then N is the change is population size and t represents the change in time, then: N/t = B-D Where B is the number of births and D is the number of deaths Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Expressing births and deaths as an average number of births and deaths per capita during the specified time interval B = bN D = dN So if 34 births per year in a population of 1,000 individuals, per capita birth rate is .034. Now we can calculate expected number of births per year in a population of any size! Try with a population of 500.

Population ecologists are most interested in the difference between the per capita birth rate and per capita death rate. Per capita rate of increase, r r = b-d r>0 it is growing r<0 it is declining

If r = 0 then there is zero population growth (ZPG). We can simplify the equation and use r to represent the difference in per capita birth and death rates. N/t = rN OR dN/dt = rN If r = 0 then there is zero population growth (ZPG). Under ideal conditions, a population grows rapidly. Exponential population growth is said to be happening Under these conditions, we may assume the maximum growth rate for the population (rmax) to give us the following exponential growth dN/dt = rmaxN Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Figure 52.8 Population growth predicted by the exponential model

The J-curve – exponential growth Characteristic of some populations that are introduced into a new environment Can be a populations whose numbers have been reduced by a catastrophic event and are rebounding

2. The logistic model of population growth incorporates the concept of carrying capacity Typically, unlimited resources are rare. Population growth is therefore regulated by carrying capacity (K), which is the maximum stable population size a particular environment can support. Energy, shelter, refuge from preds, nutrient availability, water, and suitable nesting sites can all be limiting factors Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The logistic growth equation We can modify our model of population growth to incorporate changes in growth rate as population size reaches a carrying capacity. The logistic population growth model incorporates the effect of population density on the rate of increase. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

dN/dt = rmaxN((K-N)/K) The graph of this equation shows an S-shaped curve. Fig. 52.11 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The s - curve When N is small compared to K, the term (K-N)/K is large Per capita rate of increase, rmax (K-N)/K is close to the maximum rate of increase

Table 52.3 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

How well does the logistic model fit the growth of real populations? The growth of laboratory populations of some animals fits the S-shaped curves fairly well. Fig. 52.12 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The logistic population growth model and life histories. This model predicts different growth rates for different populations, relative to carrying capacity. The life history traits that natural selection favors may vary with population density and environmental conditions. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Natural Selection Favors Certain Life History Traits Depending on the Circumstances at the Time K-selection: selection for life history traits that are sensitive to population density r-selection: selection for traits that maximize reproductive success in uncrowded environments

K-selection characteristics: Maximizes population size (moves towards “K”) Operates in populations living at or near carrying capacity In other words, they stay near the maximum level supported by the available resources Species that can survive and reproduce with few resources are favored Competitive ability and maximum efficiency of resource utilization are favored Also called density dependent selection

r-selection characteristics: Maximizes “r”—the intrinsic rate of increase Favored in an “empty” environment (like an area recently cleared by fire) Very little competition for resources Adaptations that favor rapid reproduction are favored Increased fecundity (being fruitful and multiplying!) and rapid maturation are favored Also called density-independent selection

What environmental factors keep populations from growing indefinitely?

This is a type of negative feedback. Density-dependent factors increase their affect on a population as population density increases. This is a type of negative feedback. Density-independent factors are unrelated to population density, and there is no feedback to slow population growth. Fig. 52.13 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Reproductive Competition ex: fruit flies living in crowded conditions will lay fewer eggs; honeybee queen regulates laying based on availability of food; lab rats will eat their young when space is restricted even if food and water is plentiful. These are limits on the number of offspring Territoriality, defense of a space, may set a limit on density. This limits the number of parents. Interspecific (between species) competition for food and resources Migration: member of the population emigrate as numbers grow. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Predation: as the numbers of prey increase, predators can harvest it more easily. Ex: vole population goes through 3 year boom and bust due to predator regulation Parasitism and Disease: as the numbers increase, disease and parasites spread more easily. Ex: “black death” in Europe killed ¼ of population between 1348 and 1350.