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What are populations?
a group of individuals of a single species living in the same general area (individuals in a species in a specific area)
- the number of individuals per area
- Number increases with births and immigration and decreases by deaths and emigration
How individuals are arrayed in an area (clumped, uniform, and random)
clumped dispersion patterns?
- beneficial for mating or protection from predation (most common pattern in nature and example is a starfish colony)
- better together
Uniform dispersion patterns
may result from territoriality (need own space/nutrients from soil)
Random dispersion patters
Few random patterns exist because the abiotic and biotic environment is not random (dandelions that grow in lawns is an example) (not really an attainable goal?)
The study of vital statistics (use a plot called a survivorship curve/plot, the plot uses a logarithmic scale)
What is a cohort study?
done by starting with a group of organisms of the same age and recording their death/demise (females outlast males in example shown, because males show off)
What are survivorship curves called in medicine? how are they different?
- Kaplan-meier plots, they are not logarithmic
- in these studies cohorts are subjects receiving the same therapy. For study to be valid, there must be a control group (vital)
What do different curves on survivorship curves indicate?
- different species. Differences reflect evolutionary strategies to insure species survival.
- ex- humans have low birth rate but nurture (provision) all young, but oyster disperse millions of eggs into the sea and only few survive
What is reproduction rate important? what should be noted about studies including reproduction rate?
- important for population dynamics
- only females are used in these studies because they are the only ones that can change the population size
- Change in population size
- change in population size=births+immigrants entering population-deaths-emigrants leaving population
- N- change in the population number
- t- change in time
- B- the number of births during the time interval
- D- the number of deaths during the time interval
- (immigration and emigration usually ignored because they usually cancel. We are interested in population change per time (ex per year))
- b=birth rate
- d=death rate
- N=number in population
- (we usually use rate, i.e. births per person per time)
- Number of births (or deaths) proportional to time and population size
- if growing b increases and death decreases, and vice versa
What are the units of the constant r (rate)?
1/individual organism/time. in physical science we call this the rate constant, not just rate
How do populations increase in size?
- exponentially (if curving upwards it is a constant rate, but population is increasing, no rate! if plotted with logarithmic scale, it'd be a strait line for rate, not increasing)
- If there is unlimited resources and no predation, populations expand exponentially
- (elephants are an example)
Most populations can reach a limit (carrying capacity), why?
- that's what can be sustained by the environment. (defined as K for carrying capacity)
- Called the logistic population growth model- says growth will be 0, when carrying capacity is reached
Why don't natural populations follow growth curves exactly?
because it assume that a mechanism to limit reproductive success as resources become scarce.
When reproduction begins, how reproduction occurs, how many offspring are produced per event
- reproducing only once (salmon)
- (a strategy for survival)
- reproducing repeatedly (birds)
- (survival strategy)
- (life history, semelparity and iteroparity are the ends of a spectrum of patterns)
When is semelparity favored?
What organisms tend to be semelparous?
What's an example?
- When the environment is unpredictable
- organisms that require buildup of nutrients over time
- Salmon that feed in the ocean for a variable number of years before spawning just once
Are reproductive strategies sensitive to population density?
depends, some are and some are not.
What are K-selection and r-selection?
- reproductive strategies that can be sensitive to population density
- Ex- a tree in an old growth forest would respond to K, the limiting resource, whereas dandelions invading a lawn disregard K and respond to r.
Feedback mechanisms that contribute to density dependent limitation
- Competition for resources (soil nutrients for wheat)
- Toxic buildup (yeast that can tolerate ethanol)
- Intrinsic factors (field mice dress leads to hormone change and limit reproduction)
- Territoriality (gannets peck neighbors- distance goes as far as necks reach)
- Predation and disease (predation can be density dependent)
What is an example of dynamic populations?
- The moose and wolves thought to be classic example of predator and pray fluctuation where more moose=more wolves followed by collapse of both.
- now we understand that weather, limited resources, and disease contribute to fluctuation
What's an example of a population that does not respond to food supply? and is dynamic.
snowshoe hare fluctuation responds to predation levels and sunspots, but not food supply
What is happening to the human growth rate?
beginning to decline
How do you reach zero population growth?
achieved by either a high birth rate and high death rate, or low birth rate and low death rate.
What is societal transformation between death rate and birth rate called?
What is low death rate likely a function of?
better sanitation and immunization
Low birth rate is likely the function of what?
the security that children will reach maturity, negating the need to produce many children to help sustain a family
What's happening in Mexico?
birth rate declining. Decrease of immigrants coming in to US, could be one reason for the decrease in birth rate?)
How many births per woman does a population require to support it's constant size?
if growth/population graph bottom heavy, what does it mean?
heavily skewed to younger portion of society
What if population graph is top heavy?
that people are scared to have children, reducing population overall, will cause problems later, want uniformity like US
What do longer longevity and low infant mortality signify in a population?
demographic transition (usually developed countries)
What did malthus recognize
- exponential growth of populations
- argued that ever time humans were able to increase their wealth, they would reproduce to squander wealth accumulation.
- Darwin used malthusian concept that organisms tended to produce more offspring than their environment could support.