54 Populations in Space and Time The individuals of a species in a given area is a population. The distribution of the ages of individuals in a population and the way those individuals are distributed over the environment describe the population structure. The number of individuals of a species per unit of area (or volume) is its population density.

54 Populations in Space and Time Density of terrestrial organisms is measured as number of individuals per unit area. Density of aquatic organisms is measured as individuals per unit volume. For some species such as plants, the percentage of ground covered may be a more useful measure of density than the number of individuals.

54 Populations in Space and Time The structure of a population changes continually because of demographic events—births, deaths, and movment in and out of the population. Population dynamics is the change in population density through time and space. Demography is the study of birth, death, and movement rates that give rise to population dynamics.

54 Populations in Space and Time Population dynamics can be represented by: N 1 = N 0 + B – D + I – E  N 1 = number of individuals at time 1  N 0 = number of individuals at time 0  B = number of individuals born between time 0 and time 1  D = number of individuals that died between time 0 and time 1  I = number of individuals that immigrated  E = number of individuals that emigrated

54 Populations in Space and Time Life table information can be used to predict future trends in populations. A cohort is a group of individuals that were born at the same time. A life table can be constructed by determining the number of individuals in a cohort that are still alive at specific times, called survivorship.

Table 54.1 Life Table of the 1978 Cohort of the Cactus Finch on Isla Daphne (Part 1)

54 Populations in Space and Time In some populations (e.g., humans in the U.S.), most individuals survive for most of their potential life span and die at about the same age. In some (e.g., songbirds), the probability of surviving over the life span is the same once individuals are a few months old. In species that produce a large number of offspring and provide little parental care, high death rates for the young are followed by high survival rates during the middle of the life span.

54 Populations in Space and Time The age distribution of individuals in a population reveals much about the recent history of births and deaths. For example, in the U.S., population size increased during the “baby boom” of the 1950s and again during the “baby boom echo” of the 1980s.

Figure 54.2 Age Distributions Change over Time

54 Types of Ecological Interactions Species interactions fall into several categories.  mutualism (+/+ interaction).  Ex. Termites have protists in their gut that digest cellulose; they provide the protists, in turn, with nutrients.  commensalism (+/0 interaction).  Ex. Epiphytes living on other plants.  amensalism (0/– interaction).  Falling limbs damage smaller plants beneith them.

54 Types of Ecological Interactions  predator–prey and parasite–host interactions (+/– interactions).  Many examples.  competition (–/– interaction).  Countless examples.

54 Factors Influencing Population Densities Species that use abundant resources often reach higher population densities than species that use scarce resources. Species with small individuals generally reach higher population densities than species with large individuals.  Ex. Cockroaches, ants. Best example is bacteria!!!

54 Factors Influencing Population Densities Newly introduced species often reach high population densities. An example is species introduced into a region where their normal predators and diseases are absent. Zebra mussels whose larvae were carried from Europe in the ballast water of ships now occupy much of the Great Lakes and Mississippi River drainage. Complex social organizations (e.g., ants, termites, humans) may facilitate high densities.

Figure 54.5 Introduced Zebra Mussels Have Spread Rapidly

54 Fluctuations in Population Densities If a single bacterium were allowed to grow and reproduce in an unlimited environment, explosive population growth would result. Within a month, the bacterial colony would weigh as much as the visible universe and would be expanding outward at the speed of light. But while populations do fluctuate in density, even the most dramatic fluctuations are less than what is theoretically possible.

54 Fluctuations in Population Densities All populations have the potential for explosive growth because, as the number of individuals in the population increases, the number of new individuals added per unit of time accelerates. If births and deaths occur continuously and at constant rates, a graph of the population size over time forms a J-shaped curve that describes a form of explosive growth called exponential growth.

Figure 54.6 Exponential Population Growth (Part 1)

54 Fluctuations in Population Densities Exponential growth can be represented mathematically:  N/  t = (b – d)N  N = the change in number of individuals  t = the change in time b = the average per capita birth rate (includes immigrations) d = the average per capita death rate (includes emigrations)

54 Fluctuations in Population Densities The difference between per capita birth rate (b) and per capita death rate (d) is the net reproductive rate (r). When conditions are optimal, r is at its highest value (r max ), called the intrinsic rate of increase. r max is characteristic for a species. The equation for population growth can be written  /  t = r max N

54 RABBIT/LYNX ACTIVITY