5.1 Population Potential


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UBC Bio 111 - Intro to Biology

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5.1 Population Potential

  1. 1. 5.1 Population Potential Learning Outcomes: 1. Identify factors that contribute to population growth and explain how they contribute to population size. 2. Compare real population growth curves to models 3. Explain what factors determine carrying capacity. Readings: Chapter 13: 343-355; selections from Plan B 3.) by Lester Brown (follow link in Wk 5 Learning Module) Reminders: Reading Quiz 5 due before the start of today’s class Assignment #2 on Food Webs due before start of today’s class Midterm Friday Oct 9th from 12:00 to 12:50 in your normal lecture room What is a population?: Individuals of the same species living in the same geographic area at the same time The rate at which populations grow is determined by 4 key factors: Factor Effect on population size Birth Adds new individuals increase Death Removes individuals decrease Immigration New individuals enter population from elsewhere increase Emigration Individuals leave population to go somewhere else decrease Other influences Generation time or The younger the age of the parents when they reproduce… Faster increase Parental age The older the parents when they reproduce… Slower increase Amount of Healthier individuals living longer….lowers death rate… Increase available resources AND/OR (don’t think about Individuals with more resources for growth (and reproduction)….can Homo sapiens) delay reproduction and spend more time growing eg long-lived trees Slower increase Individuals with few resources are less healthy….increases death Decrease or slower rate….and they have less energy for birthing healthy offspring increase AND/OR If resources are limiting….organisms may put available resources into reproduction and then die shortly afterward (big-bang reproduction eg weedy plants) Faster increase How can we reconcile the seemingly contradictory effects that factors like the amount of available resources can have on population growth? How can the existence of plentiful resources potentially both increase or decrease population growth rates? We should consider the typical pattern of growth and reproduction for an individual species first (its life history) and then consider the available resources second.
  2. 2. Life history: how an individual within a species allocates resources to growth and reproduction as well as activities and adaptations it has related to its survival Life history traits form a continuum: Some species put most of their energy into reproduction and little energy into actively insuring the survival of those offspring eg. A tree frog lays a large mass of eggs but does not protect the eggs and offspring nor does it nurture the offspring. A fireweed plant is shown at the far right releasing seeds for wind dispersal. Some species give birth to few offspring but put relatively more energy into insuring the survival of those few offspring. eg: An elephant usually only gives birth to one offspring at at time after gestating 22 months, the mother feeds her young offspring milk and keeps it with her protecting and teaching the young for ~4 years. eg2 A coconut tree only produces a relatively small number of seeds (coconuts) each season but each seed has a large energy store for germination and early growth and the seed itself is well protected against predation. For their life history all species fall somewhere in between these two extremes of high reproduction vs. high survivorship. The life history strategy employed by a species will affect how a population responds to environmental changes such as resource limitation.
  3. 3. What order would you place these species on a continuum of reproduction (aka fecundity above left shown by crabs) vs. survivorship far right shown by coconuts)? QUESTION 1. Number the species from 1 to 5 with 1 being the species most focused on reproducing high numbers of organisms and 5 putting the most emphasis on survivorship of offspring. American coot puffball lizards Japlura swinhonts Paramecium caudatum chimpanzees Fulica americana Lycoperdon (amoeboid protist) Pan troglodytes nigrescens #3rd #4 #1 #5 #2 (Highest (Chimpanzees take longer to Fecundity) teach offspring stuffs) QUESTION: 2. If you had any trouble deciding the order of some of these species what questions would help you better decide on the order? - How often do they reproduce? - What is the generation time? - How many offspring in total does an individual have? - How much energy is invested per offspring? How do populations grow? Whooping cranes (Grus americana) were near extinction mainly due to habitat loss. Intensive recovery efforts have been ongoing since the 1970s including protecting remaining habitat, introducing whooping crane eggs into sandhill crane nests, creating 1 non-migratory population in Florida, and creating a second more easterly migratory population. QUESTIONS: This migratory crane population appears to still be increasing (the last data point is from 2007). 3. What is permitting the population to continue to grow? - Few competitors (less competition) - Available food (because they move around) - Few predators - Migration may favour a larger population - Available nesting sites 4. What would limit its growth? (Hint: remember your lectures on Biotic factors)
  4. 4. - Predators - lack of habitat - competitors - lack of food - ( - unfavourable abiotic conditions ) This graph shows increases in the number of wolves in the Rocky Mts of the United States after recovery efforts occurred. NWMT..= wolves in Montana, GYA.. = wolves in the greater Yellowstone area and CID.. = Idaho. Approximately the last 5 years on the graph indicate population growth from the wolves is now only coming from wolves in Idaho, the other two areas have reached a stable or maximal size. QUESTION: 5.Why is the Idaho population likely still growing while the other two are not? - Bigger capacity - Available food - More available habitat - Fewer predators - Less contact w/ humans (who feel threatened and kill them :X ) QUESTION 6: For this population of yeast cells growing in a culture in a test tube draw a simple plot below of this data and predict the population size at 120 and 135 minutes. Time in Number of cells minutes 15 27 30 932 45 850,000 60 980,000 75 823,000 90 788,000 105 500,000 120 135 QUESTION: 7.What has begun to happen beyond 60 minutes? - Resources (eg: food, gases) have become limiting (this happens at the beginning of stationary phase) - Carrying capacity: The maximal population size a habitat can support without any habitat degradation. The population growth curves for whooping cranes and wolves were shown for populations in recovery because most natural populations we observe are not starting from scratch moving into a new habitat and beginning to grow. They have already reached some particular level and are either stable or showing some type of fluctuations depending on abiotic and biotic conditions. Estimating what population level a habitat can support is a key question for population biologists. For example here is a graph generated by wildlife biologists in Wisconsin concerned with their wolf population:
  5. 5. Fig 13.6 The growth shown by many populations in initially exponential as shown by the first part of the above S-shaped curve. Once a population gets close to carrying capacity (where the graph flattens out) population growth slows. This overall trend is called logistic growth as shown by Fig 13.6 in your text. When a population is dense and close to its carrying capacity a number of things can occur: -wastes build up and can harm the health of the species itself as well as other species in the same habitat -diseases may increase QUESTION 8. What else? - You begin to see the effect of competition between organisms within the population - Food supplies can become limiting - These factors which affect a population that is close or at carrying capacity are called density-dependent factors. ***QUESTION: 9. Considering that a population at carrying capacity is at the maximum number of individuals that can be supported without habitat degradation, has the human population surpassed its carrying capacity? Populations can also be affected by density-independent factors that can affect a population no matter what size it is. These factors tend to be abiotic.
  6. 6. QUESTION: 10. What could be influencing the population size of the song sparrow population from Mandarte Island (BC) shown left? - Extreme weather events -