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Chapter 10

2483: Ecology Chapter 10.pdf

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Biology 2483A
Hugh Henry

Chapter 10: Population Dynamics A Sea in Trouble: A Case Study • in the 1980’s the comb jelly was introduced into the Black Sea accidently • the timing was horrible because the black Seas ecosystem was already in decline due to increased inputs of nutrients from sewage, fertilizers, and industrial waste, and also overfishing • the increased supply of nutrients were devastating as the shallow waters of the Black Sea are prone to problems that stem from eutrophication (addition of nutrients to water) • as nutrient concentrations increased, phytoplankton increased, water clarity decreased, O2 concentrations dropped, and fish populations experienced massive die-offs • then the comb jelly fish was introduced • the jelly eats zooplankton, fish eggs, and young fish and it continues to eat even when it is full • this causes it to regurgitate large quantities of prey stuck in balls of mucus which can lead to death • the jelly’s population exploded jelly’s eat zooplankton, which eat phytoplankton... so the phytoplankton population • increased even more • when jellys and phytoplankton die they provide food for bacterial decomposers which use O2 to decompose the dead--> leading to a decrease in O2 concentrations--> decreasing fish populations • because comb jellys eat fish eggs, the fish population couldn’t repopulate • By 1999, the Black Sea showed signs of recovery. Nutrient inputs had been reduced and phytoplankton abundance decreased. Another comb jelly had arrived, Beroe, which feeds almost exclusively on Mnemiopsis. • Within 2 years, Mnemiopsis numbers plummeted. • The Mnemiopsis decline led to a rebound in zooplankton abundance and increase in native jellyfish species. • There was also an increase in anchovy catches for commercial fishermen. Events in the Black Sea ecosystem illustrate two types of causation in ecological communities: Bottom-up control—increased nutrient inputs caused eutrophication and increased phytoplankton biomass, decreased oxygen, fish die-offs, etc. Top-down control—top predators control the abundance of populations. • Overfishing was also a factor in the Black Sea: Decline of top predator fish leads to increase in plankton-eating fish, which decreases zooplankton populations, and then phytoplankton abundance increases. Introduction Population dynamics: The ways in which populations change in abundance over time. • Population size changes as a result of four processes: Birth, death, immigration, and emigration. Nt+1 = Nt + B - D + I - E • Nt = Population size at time t • B = Number of births • D = Number of deaths • I = Number of immigrants • E = Number of emigrants Patterns of Population Growth Populations exhibit a wide range of growth patterns, including exponential growth, logistic growth, fluctuations, and regular cycles. • These four patterns are not mutually exclusive. A single population can experience each of them at different times. Exponential Growth • Population increases by a constant proportion at each point in time. • When conditions are favorable, a population can increase exponentially for a limited time. When a species reaches a new area, exponential growth can occur if conditions are favorable. • The population may grow exponentially until density-dependent factors regulate its numbers. • • Species such as the cattle egret colonize new regions by long-distance or jump dispersal events. • Local populations then expand by short-distance dispersal events. In Logistic Growth, The Population Approaches an Equilibrium • These populations first increase, then fluctuate by a small amount around the carrying capacity. • Plots of real populations rarely match the logistic curve exactly. • “Logistic growth” is used broadly to indicate any population that increases initially, then levels off at the carrying capacity. ex. • For K (carrying capacity) to be constant, birth rates and death rates must be constant over time at any given density. • This rarely happens in nature. Birth and death rates do vary over time, thus we expect carrying capacity to fluctuate. Population Fluctuations • In all populations, numbers rise and fall over time. • Fluctuations can be deviations from a growth pattern, e.g., the Tasmanian sheep population • or erratic - In Lake Erie phytoplankton populations, fluctuating abundance could reflect changes in environmental factors such as nutrient supplies, temperature, or predator abundance. population outbreak - # of individuals increases rapidly • ex. An ongoing outbreak of the mountain pine beetle has killed hundreds of millions of trees across British Columbia. This has altered forest composition, and CO2 is released as the trees decay—17.6 megatons every year. Population Cycles • Some populations have alternating periods of high and low abundance at regular intervals. • Populations of small rodents, such as lemmings and voles, typically reach a peak every 3–5 years. • Different factors may drive population cycles in rodents. • For collared lemmings in Greenland, field studies and modeling indicated that the 4- year cycle is driven by predators, such as the stoat. • In other studies, predator removal had no effect on population cycles. Factors driving population cycles may vary by place/species. • Some population cycles may stop if certain environmental factors change. • Warmer winter temperatures affect snow conditions at high latitudes, making it more difficult for lemmings to feed and avoid predators. Their populations have stopped cycling every 3 to 4 years. Delayed density dependence: Delays in the effect that density has on population size. • Commonly, the number of individuals born in a given time period is influenced by population densities that were present several time periods ago. Delayed density dependence can cause populations to fluctuate in size. • Example: A predator reproduces more slowly than its prey. • If predator population is small, prey population may increase, then the predators will increase, but with a time lag. • Many predato
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