Chapter 13 Part 1 Flashcards
how do populations fluctuate?
naturally, over time
Wild population numbers undergo significant
amounts of variation from year to year
variations
that occur in invertebrates as well as vertebrates ….
in plants as well as bacteria, fungi and even viruses
wild population numbers are dependent on
changing environmental conditions over time (weather/climatic conditions) - these factors can influence such things and competitive interactions, predation rates, herbivory, parasitism
variation in the density of a population depends on:
- the amoung of environmental fluctuation (both density dependent and density independent factors)
- the inherent stability in the population
all populations experience fluctuations due to factors such as
availability of resources, predation, competition, disease, parasites, and climate
Fluctuations include
random and cyclic changes through time
Some populations tend to remain
relatively stable over long periods
In contrast, some populations exhibit
much wider fluctuations
Small organisms (e.g., algae) tend to reproduce much faster than larger organisms (e.g., red deer), so their populations often respond
faster to favorable and unfavorable conditions (high surface areas)
Larger organisms have a lower surface-area-to-volume ratio, which allows them to
maintain homeostasis in the face of unfavorable environmental changes
some of the most pronounced population fluctuations in nature are the predictable cycles of certain
mammal and bird species at high latitudes
When an age group contains a high or low number of individuals, the population likely experienced
high birth or death rates in the past
Long-term fluctuations in age structure can be determined for a forest by
examining tree rings
Populations in nature rarely follow a smooth approach to their
carrying capacity
overshoot
when a population grows beyond its carrying capacity
when does overshoot occur
often when the carrying capacity of a habitat decreases from one year to the next
die-off
a substantial decline in density that typically goes well below the carrying capacity
when do die-offs often occur
when a population overshoots its carrying capacity
Populations With High Growth Rates Track
Environmental Fluctuations More Closely
Than
Those With Low Growth Rates
Environmental changes continually increase and
decrease the carrying capacity of the environment
for each population. How a population responds to such
changes through density-dependent effects, depends on …
the intrinsic capacity of the population to increase
in size … that is, its r value (which ultimately is
influenced by the extent to which the species is “r”
or “k” selected)..
The faster the potential rate of growth or decline
of the population – i.e., the higher the fecundity and
the shorter the life span of individuals
the greater is
its capacity to track change in its environment (e.g.
r-selected species).
Populations with r > 1 tend to
track environmental
changes
populations with r < 1 may
be
unresponsive to short-term environmental change
A population
with a higher growth rate (r = 0.5) tracks
a population with
a lower growth rate (r = 0.1)
Population Cycles May Result From
Intrinsic
Demographic Processes
Environmental changes generally tend to be
irregular rather than predictable
biological responses to environmental changes are also frequently
random and unpredictable
The sizes of many populations do, however,
change with
periodic frequency
a number of small mammal cycles have become part of the
lore of population ecology
Charles Elton, in 1924, showed that the lynx, and its
main prey, the snowshoe hare, display
large, regular fluctuations in population size - with each cycle lasting about 10 years
Fluctuations in population numbers may result in
a time delay
Fluctuations in population numbers may result in a
time delay in the response of
births and deaths to changes in the environment - just as the momentum imparted to a pendulum
The momentum provided to a population with
high birth rates at low densities or high death
rates at high densities …. results in
the population
“swinging” past its equilibrium point when
demographic responses are time-delayed
Times delays are inherent in models based on
discrete generations
Population cycles
regular oscillation of a population over a longer period of time
Some populations can exhibit
highly regular fluctuations in size
Cyclic populations can occur among
related species and across large geographic areas (e.g., the synchronous cycles of the capercaillie, black, and hazel grouses in Finland).
