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
• 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
• as nutrient concentrations increased, phytoplankton increased, water clarity
decreased, O2 concentrations dropped, and ﬁsh populations experienced massive
• 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
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
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.
• 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
• The population may grow exponentially until density-dependent factors regulate its
• Species such as the cattle egret colonize new regions by long-distance or jump
• 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
• 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.
• 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
• 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.
• Some populations have alternating periods of high and low abundance at regular
• 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