Populations Wednesday, August 24th, 2016

Populations = Individuals of a species that live in the same place at the same time Are dynamic – constantly changing Evolution occurs at this level

Population Dynamics Population dynamics refers to changes in a population over time Population dynamics includes four variables: Age Distribution Size Density Dispersion

1. Age structure The age structure of a population is usually shown graphically The population is usually divided up into prereproductives, reproductives and postreproductives The age structure of a population dictates whether is will grow, shrink, or stay the same size

Age structure diagrams Pyramid = + growth Vertical edges = 0 growth Inverted Pyramid = - growth

2. Population growth Exponential Logistic Populations show two types of growth Exponential Logistic

Exponential growth J-shaped curve As early as Darwin, scientists have realized that populations have the ability to grow exponentially All populations have this ability, although not all populations realized this type of growth J-shaped curve

The graph at right shows what exponential growth looks like Exponential growth is growth that is independent of population density

Doubling time/ Rule of 70 Doubling time is the amount of time that is takes for a population to double in size when growing exponentially It is calculated as Doubling Time = 70 / percent increase Ex. A rabbit population has an r value of 1.5, so the percent increase = 150% D.T. = 70/150 = .46, or 5.5 months

Do all species enjoy exponential growth? NO! The exponential growth of most populations ends at some point Two general outcomes can be observed….

1. Populations increase so rapidly that they over shoot the pop size that the environment can support, and the pop size crashes ex. reindeer

2. Populations increase to some level, and then maintain that stable level ex. sheep

What limits population growth? Density-independent factors: affect populations randomly (without respect to density) ex. Hurricanes, tornadoes, fire, drought, floods poor regulators of populations Density-dependent factors: affect populations when densities are high ex. Disease, competition, predation, parasitism good regulators of populations

Population Regulation/Logistic Growth Most populations grow exponentially until the effect of density- dependent factors increases and limits population growth S-shaped growth curve (logistic growth)

1. The population experiences exponential growth. 2. Population size (and density) increases, the growth rate decreases. 3. The population approaches the carrying capacity, K, the number of individuals that the environment can support

3. Population Density Population density (or ecological population density) is the amount of individuals in a population per unit habitat area Some species exist in high densities ex. mice Some species exist in low densities ex. Mountain lions Density depends upon social/population structure mating relationships time of year

4. Population Dispersion Population dispersion is the spatial pattern of distribution There are three main classifications clumped: individuals are lumped into groups ex. Flocking birds or herbivore herds due to resources that are clumped or social interactions most common

Uniform: Individuals are regularly spaced in the environment ex. Creosote bush due to antagonism between individuals, or do to regular spacing of resources rare because resources are rarely evenly spaced Random: Individuals are randomly dispersed in the environment ex. Dandelions due to random distribution of resources in the environment, and neither positive nor negative interaction between individuals rare because these conditions are rarely met

Species Distribution Related to THREE things: Fundamental Niche (abiotic conditions that it can tolerate) Ability to disperse to a new area Interactions with other species (symbiosis)

Interactions Among Organisms Symbiosis An intimate relationship between members of 2 or more species Participants may be benefited, harmed or unaffected by the relationship Results of co-evolution Types: Competition, Predation, Mutualism, Commensalism

Competition Interaction among organisms that vie for the same resource in an ecosystem Intraspecific Competition between individuals in a population Interspecific Competition between individuals in 2 different species

Limiting Resources Any environmental resource that, because it is scarce or at unfavorable levels, restricts the ecological niche of an organism

Predation Predation- the use of one species as a resource by another species. True predators: the consumption of one species by another Prey is killed by predator Herbivores Parasitism: relationship in which one species is benefited and the other is harmed Parasites rarely kill their hosts

Mutualism A relationship in which both organisms benefit Example: Honey guide birds alert and direct badgers to bee hives. The badgers then expose the hives and feed on the honey first. Next the honey birds eat. Both species benefit. Example: Oxpeckers feed on the ticks found on a rhinocerous. The oxpeckers get a meal and the rhinocerous is helped by the removal of ticks.

Commensalism A relationship in which one organism benefits and the other is not harmed nor helped. Example: Barnacles attach to whales. Barnacles get a free ride, but it doesn’t affect the whales. Example: A sparrow will build its nest under the next of the osprey. The smaller bird gets protection. The osprey is neither helped nor harmed.

Interactions between species and their effects Type of Interaction Species 1 Species 2 Competition - Predation + Mutualism Commensalism