Evolution & Ecology

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Population ecology

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explores how biotic and abiotic factors influence density, distribution, size, and age structure of populations

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Population

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A group of individuals of a single speciesliving in the same general area at the same time.

Most often described by their boundaries and size

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Density

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Number of individuals per unit area or volume

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Dispersion

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Pattern of spacing among individuals withinthe boundaries of a population.

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Density

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Although density can be calculated by counting all individuals in a population, in most cases it is impractical or impossible.- Calculate population density by extrapolating countsfrom a sub-sampled area- Estimate population size based on indicator such asnests, burrows, or fecal droppings- Mark-recapture methods

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Density

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Density is not a static property, constantly changing based on individuals being added or removed from the population.

Birth: Individualsadded topopulation (all forms ofreproduction)- Death: Individualsremoved from population.

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Patterns of Dispersion

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Territoriality, Clumped Distribution, Random Distribution, and Uniform Distribution.

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Clumped Distribution

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Individuals are found in groups or patches within their particular habitat.

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Random Distribution

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An absence of strong attractions or repulsions from among individuals. Individuals are spread out in the environment irregularly; the position of one individual is independent of another.

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Uniform Distribution

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Distribution results from direct negative interaction between individuals. Rare in nature, individuals who are spaced evenly. Presence of one hinders another. Distance between individuals maximized.

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Territoriality

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Distribution results from direct negative interaction between individuals. Defense of a bounded physical space against encroachment by other individuals in the population.

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Demography

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study of the vital statistics of a population and howthey change over time.- Most often done using life tables

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A life table

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is an age-specific summary of the survival pattern of a population- Initially developed in 1950s for insurance companies

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Cohort

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a group of individuals of the same age

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A life table is an:

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age-specific summary of the survival pattern of a population (3 main types):(1) Cohort Table: Follows group of same aged individuals from birth(2) Static Life Table: Made from data collected at all ages at oneparticular time(3) Death Table: Measures mortality data from generation togeneration

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Semelparity

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Have only one reproductive event in their lifetime

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Iteroparity

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Capable of multiple reproductive events

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Demographics

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A cohort life table can be constructed from counts/estimates of all individuals in a population as it progresses through time.- The first column (x) specifies the age class while the secondcolumn (nx) is the number of individuals at start of each age class.

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Demographics

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From this data, we can calculate several life history features- Proportion surviving to each age class (lx); divide each n by n0- Portion of individuals dying (dx); lx – (lx+1)

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Demographics

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From this data, we can calculate several life history features- Stage specific mortality rate (qx); dx/lx. Next three columns used to determine the population’s reproductive output.- Fx: number of offspring produced at each age- mx: Individual fecundity, offspring produced per survivingindividual (Fx/nx)- lxmx: number of offspring produced per original individual ateach age

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By summing up lxmx across all ages, you can calculate the net reproductive rate (R0).

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One can think of R0 as the population’s replacement rate. If R0 = 1.0, than population is just replacing itself. If R0 > 1.0, populationis growingwhile R0 < 1.0meanspopulation isdeclining.

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T (generation time) or the time between the birth of one cohort and the birth of their offspring.

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T = Sxlxmx

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r, per capita rate of increase

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r = ln (R0)TPer capita rate of increase = ln (1.043)/7 = .006

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Life tables may be static (also called vertical), they provide a “snapshot” of a population at all life stages at same time.

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However, they can be dynamic (horizontal). These follow one cohort, say the progeny of a single breeding season, throughout their lives.- Much harder to do- Take more time and moneyThe two types of tables are theoretically identical assuming(A) the environment is not changing(B) population is at equilibrium (B=D; I=E)

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Survivorship Curves

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A survivorship curve is a graphical way of representing the data presented in a life table.- usually survivorship curve extrapolated to begin with cohort ofconvenient size (1000 individuals)

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Survivorship Curves

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A survivorship curve is a graphical way of representing the data presented in a life table.- Essentially the graph is a plot of nx vs x (on log scale)

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Survivorship Curves

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Survivorship curves can be classified into three general types (idealized)Type I: Low death rates during early and middle life and an increase in death rates among older age groupsType II: A constant death rate over the organism’s life spanType III: High death rates for the young and a lower death rate for survivors

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Reproductive Rates

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Demographers studying sexually reproducing species generally ignore males.- Usually sustainability is function of females ability to give riseto more femalesReproductive table, or fertility schedule, is an age-specific summary of the reproductive rates in a population.- Calculated by measuringthe reproductive outputof a cohort from birthuntil death.

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Per Capita Rate of Increase

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a population’s growth rate (per capita increase) equals birth rate minus death rateMore appropriately, births and deaths can be expressed as the average number of births and deaths per individual over a specified time frame.B  bN = where b annual per capita birth rateD  mN = where m annual per capita death rate

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Exponential Growth

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When all members of a population have access to unlimited resources and are free to reproduce at their physiological capacity, that population will experience exponential population growth.- This is maximum per capita rate of increase

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Exponential Growth

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The size of a population that is growing exponentially increases at a constant rate.- species introduced to newenvironment- rebounding species fromcatastrophic numbers loss

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Logistical Model and Reality

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Growth of several species in laboratory (beetles, crustaceans, Paramecium, etc…) fit logistical model well. However, some species’ populations appear to grow well beyond their carrying capacity.- Model assumes populations adjust instantaneously to growthand increasing lack of a limiting resource- In many cases though, there is a delay. Continued reproductiondespite the burden of a limited resource(s) cause a population toovershoot its carrying capacity for a short time.

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Allee Effect:

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Individuals may have a more difficult time surviving or reproducing if the population size is smallEcological mechanisms include mate limitation, cooperative defense, cooperative feeding, and environmental conditioning.

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Life history

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The traits that affect an organism’s schedule of reproduction and survival. (3 main variables)- The age at which reproduction begins- How often the organism reproduces- How many offspring are produce per reproductive episode

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Population Dynamics

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The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size.

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Population Cycles

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Predator and prey populations also respond to each other. Predator populations increase as their prey population increases, but this naturally leads to more predation which begins to decrease prey population. This, in turn, limits food availability and the predator population begins to decline. (Boom-and-Bust Cycles)

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Regional Patterns of Population Change

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Ultimate goal is to achieve zero population growth (ZPG), when the number or people being born is equal to the number dying—replacement fertility rate.

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Climate

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Long-term prevailing weather conditions in an area including temperature, precipitation, sunlight, and wind.- Macroclimate consists of patterns on the global, regional,and landscape level- Microclimate consists of very fine patters (local level)Global climate patterns are largely determined by solar energy and Earth’s movement in space.- Sun’s warming effects drive evaporation, air, and watercirculation- Latitudinal variations in climate

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Seasonality:

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Caused by Earth’s titled axis rotation and annual passagearound the sun (mainly mid to high latitudes)- Changes in day length, solar radiation, and temperature- Wet and dry seasons in tropics caused bymovements of wet and dry air on either sideof the equator (20°N or S). Varied dailyhours ofsunlight at Polesafter solstices.

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Biomes

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are major life zones characterized by vegetation type (terrestrial biomes) or physical environment (aquatic biomes)

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aquatic biomes

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are characterized by their physical environment (mainly salinity, temperature, and light environment)

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Thermocline

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Thermal energy from the sun warms surface waters to whatever depth the sunlight penetrates. In most aquatic/marine systems, there is a narrow layer of abrupt temperature change corresponding to light penetration.

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Aquatic Biomes

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can be characterized by their physical environment, chemical environment, geological features, photosynthetic organisms, and heterotrophs

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Dispersal

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is the movement of individuals or gametes away from centers of high population density to areas of lower population density.- Can also be away from their area of origin

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adaptive radiation

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In rare cases, long-distance dispersal can lead to the rapid evolution of an ancestral species into new species

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Behavior and Habitat Selection

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If organisms are not occupying their potential range. Capable of feeding on wide variety of plants, though in wild found almost exclusively on corn- Females attracted to odorsproduced by maizeBehavior restricts the geographic distribution of this insect. Ostrinia nubilalis

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What influences species distribution?

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Both biotic and abiotic factors.

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Abiotic Factors

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Sunlight, rock and soil, temperature, salinity and oxygen.

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Biotic Factors

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Predation, Herbivory, Mutualism, and Parasitism.