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Population Dynamics Population Dynamics • Populations of plants and animals change over time due to: • climate change, • natural disaster, • Changes in predator or prey abundance, • Anthropogenesis. Characteristics of Populations • • • • Size Density Dispersion Age Distribution Population Measurements of Population Characteristics • 1. Size – measure the number of individuals in a population at a given time. • Methods: Quadrant and transect. Population Measurements of Population Characteristics • 2. Population density – the number of individuals of a population in a certain space at a given time. • Terrestrial ecologists express density as the number of individuals per unit area. • Aquatic ecologists express density as the number of individuals per unit volume. Human Population Density Population Dispersion • 3. Population dispersion – the spatial pattern in which the members of a population are found in their habitat. a. Clumped Dispersal: (MOST COMMON) resources are usually patchy in nature. Additionally, some animal species form grazing herds, schools of fish, flocks of birds, and troops of primates to protect against predators, during migration, during mating season, or because they are social animals. b. Uniform Dispersal: (RARE) occurs mostly when individuals of the same species compete for resources that are scarce or when a species defends its territory by physical or chemical means. Example, Creosote bush in desert biome. It competes for water (limiting factor) by excreting toxic chemicals that prevent seedlings of other creosote bushes from growing near it. c. Random Dispersal (RARE) occurs when resources or conditions in the environment are fairly uniform and competition is limited. This condition is rare because environments are rarely uniform. Random dispersion is most common in weedy species that have a broad tolerance range for environmental conditions (‘generalists”) (a) Clumped (elephants) (b) Uniform (creosote bush) (c) Random (dandelions) Age Structure 4. Age Structure – the proportion of individuals in each age group in a population. a. Pre-reproductive b. Reproductive c. Post-reproductive A population with a large % of its individuals in the pre-reproductive category has a high potential for growth = “population momentum”. Human Age Structure Diagrams What Limits Population Growth? • • • • Births Deaths Immigration Emigration Population change = (b + i) – (d + e) Biotic Potential • Populations vary in their capacity for growth. When environmental factors are optimal, populations grow. Biotic Potential = Population Growth • The intrinsic rate of increase (r) is the rate at which a population could grow if it had unlimited resources. “r” is expressed as the # of individuals per existing individual per unit time. Example, # of new peppered moths per existing # of peppered moths/year. Biotic Potential • Where (dN/dt) is the rate of increase of the population and N is the population size, r is the intrinsic rate of increase. This is therefore the theoretical maximum rate of increase of a population per individual Populations with High Intrinsic Rates of Increase = “r” Strategists (Generalists) • • • • • • • • • • • Many small offspring Little or no parental care Early reproductive age Most offspring die before reaching reproductive age Small adults Adapt quickly to environmental changes High population growth rate Population size fluctuates greatly above/below carrying capacity Generalist niche Low ability to compete Early successional species (rabbit, quali, ringneck pheasant, dove, bobolink, gopher, pioneer plant species and weeds) Environmental Resistance • Environmental resistance limits the growth of a population when all of the limiting factors in the environment work synergistically. • This results in specialization of species (k-strategists): • Low reproductive rates • Specialized niche • Competition of resources • Loss of habitat which causes increased competition making organisms susceptible to disease and weakening them preventing them from reproducing. • Environmental Resistance = Population Decrease POPULATION SIZE Growth factors (biotic potential) Abiotic Favorable light Favorable temperature Favorable chemical environment (optimal level of critical nutrients) Biotic High reproductive rate Generalized niche Adequate food supply Suitable habitat Ability to compete for resources Ability to hide from or defend against predators Ability to resist diseases and parasites Ability to migrate and live in other habitats Ability to adapt to environmental change © 2004 Brooks/Cole – Thomson Learning Decrease factors (environmental resistance) Abiotic Too much or too little light Temperature too high or too low Unfavorable chemical environment (too much or too little of critical nutrients) Biotic Low reproductive rate Specialized niche Inadequate food supply Unsuitable or destroyed habitat Too many competitors Insufficient ability to hide from or defend against predators Inability to resist diseases and parasites Inability to migrate and live in other habitats Inability to adapt to environmental change Figure 9-4 Page 166 Environmental resistance Population size (N) Carrying capacity (K) Biotic potential Exponential growth Time (t) SYNERGY • Together, biotic potential AND environmental resistance determine the carrying capacity (K) which is the number of individuals of a given species that can be sustained indefinitely in a given area. Exponential Growth Curve • Exponential Growth – a population that does not have resource limitations grows exponentially. • Exponential growth starts out slowly and then proceeds faster and faster as the population grows. • “ J-shaped curve Logistic Growth Curve • Logistic growth involves exponential population growth when the population is small and a steady decrease in population growth with time as the population approaches “K”. • Logistic curves yield sigmoidal curves or “S” curves. What Happens When the Population Exceeds “K”? • 1. As populations use up resources they “overshoot” the carrying capacity (k) of the habitat. (+FBL) • The overshoot occurs BECAUSE of a “reproductive lag time” = the period required for the br to decrease and dr to increase in response to overconsumption of resources. (-FBL is delayed, therefore, recovery is delayed). Population size (thousands) 160 Hare 140 Lynx 120 100 80 60 40 20 0 1845 1855 1865 1875 1885 Year 1895 1905 1915 1925 1935 2.0 Overshoot Number of sheep (millions) Carrying capacity 1.5 1.0 .5 1800 1825 1850 1875 Year 1900 1925 Results • Population crashes! • The only way to avoid a crash in population is if individuals of that species emigrate out of the habitat and into a new one with more favorable conditions. • Once the existing “k” of the habitat has been exceeded, the environment becomes degraded and the new “k” is lowered. • Recall Easter Island! How Does Population Density Affect Population Growth? 1. Density-independent population controls – factors that will affect the size of a population regardless of it’s density. • Examples are: • Natural disasters (floods, hurricanes, earthquakes, landslides, severe drought, unseasonable weather, fire, destruction of habitat, tsunamis) • EX. Hurricane Katrina (2005), Indonesian Tsunami (2005) New Orleans After Hurricane Katrina Limiting Factors for Population Density 2. Density-dependent population controls – when limiting factors in the environment have a significant effect on the size of a population as it’s density per unit area increases. These include: • Competition for resources • Parasitism • Disease • This means that populations with a high density per unit area generally have lower birth rates and higher death rates = slow growth rates due to “k” of the environment. Density-Dependent Example in Animals • If there are a large number of prey (zebra) in the savanna biome where competition is high for limited resources THEN more zebra will become weakened, susceptible to disease, and die. • Most impacted = young, weak,and the elderly. Density-Dependent Example in Humans • Infectious Disease – bubonic plague • The bacterium that causes this disease lives in rodents and is transferred to humans via fleas (vector species). • Fleas feed on humans (become infected) and then bite humans transferring the disease. • The disease spread rapidly through crowded cities, where sanitary conditions were poor and rats were abundant. • Over 25 million people died. POPULATION CURVES IN NATURE Number of individuals 1. Stable – population fluctuates slightly © 2004rooks/Cole – Thomson Learning above/below “k”. (undisturbed tropical rain forest) (d) Irregular (a) Stable (c) Cyclic (b) Irruptive Time 2. Irruptive – normally have fairly stable population that may occassionally “explode” or “irrupt” to a high peak and then crash to a lower, more stable level. Can occur due to more predators/less food in habitat. (racoon) 3. Cyclic “boom-bust cycles” (Linx-rabbit, lion-zebra) 4. 4. Irregular unpredictable What Can We Learn From Nature? Living systems have six key features: • Interdependence • Diversity • Resilience • Adaptability • Unpredictability • limits Important Ecological Principles to Promote Sustainability • We are part of, not apart from, the earth’s dynamic web of life. • Our lives, lifestyles, and economies are totally dependent on the sun and the earth (solar capital and earth capital). • We can never do merely one thing without it affecting everything else. Solutions • Build societies based on conservation and not waste. • Preserve what we can’t replace. • Work with nature to help restore what we have degraded or destroyed. “Everything is connected to everything else; we are all in it together”