populations grow until stopped by some external or internal force
internal force might be inter-individual stresses
external force:
physical
reduction in resource such as water, air, food, or shelter
biological:
interspecies competition
predation
when a population is introduced to a new and favorable environment
initial growth is slow="lag phase"
growth then becomes very rapid=exponential growth or log phase
continues until restricted by environmental force(s)
typically, the exponential growth levels off and stabilizes at the carrying capacity of the habitat for the species
population growth with a lag phase, an exponential phase, and a stable phase is called sigmoidal or S-curve growth
alternatively, the population can overshoot its carrying capacity and die off rapidly (perhaps permanently)
this is called J-curve growth
population growth rate is the result of the struggle between biotic potential and environmental resistance (note the electrical metaphor)
the ability of a population to grow in an unrestricted environment is called its biotic potential
houseflies and roaches are famous for high biotic potential (one male roach and 19 fertile female roaches can reach the mass of an aircraft carrier in a year)
large slow-gestating animals have much slower biotic potentials
the set of factors which tend to reduce population size or slow reproduction is called environmental resistance
the size of a population is set by ability of the individuals to utilize the resources and by the level of those resources (ie, the carrying capacity of the habitat)
the more efficiently individuals can utilize resources, the greater their population can be
Population interactions (see below)
these can be important factors for population dynamics
competition
symbiosis
predation and parasitism
Population cycles
driven by two types of factors:
density dependent factors cause higher death rates as the population increases (eg a limited food supply)
density independent factors cause the same die off regardless of population
ecologists favor control by one type or the other
Habitat and Niche
Habitat
habitat = the place or places where a population can find the resources it needs and where we might expect to find members of the population
a population may not exist in a particular area if it has not reached that area, but if introduced the species should do well if the area is a suitable habitat
people may introduce species accidentally or intentionally with mixed results
the physical limits we discussed earlier (last week) determine the suitability of a habitat for a particular species
the range of a parameter (eg temperature) that a species can survive is called the zone of tolerance for that parameter
within that range is/are zones in which the organisms do not do well, though they may survive; these are called zones of stress
animals can occupy their space in different ways
activity patterns
at specific times of day, individuals of a species are likely to be doing something in some part of the environment
different diurnal patterns can be observed at different times of the year
spatial distribution
animals may select and defend a specific nesting location
grazing animals or large predators may travel circuits or other paths throughout a larger area (the home range)
a community may migrate to different areas on a seasonal basis to exploit variable food supplies or breeding grounds (monarch butterflies travel thousands of miles)
Niche
the functional role a species employs in its habitat is called the niche of that species
food supply and means of obtaining
protective mechanisms
interspecific competition occurs when two or more species have overlapping niches; usually, species do not compete head to head
instead there is some resource partitioning: the optimum ranges for all biotic and abiotic factors line up next to each other rather than on top of each other
two species that have similar niches in different places are called ecological equivalents
these are the product of convergent evolution
they are not necessarily very close relatives; but since they are functionally similar, they often look alike
the slow evolution of habitats often permits populations to modify their niche or find a new one
the rapid alteration (as by human activities or natural disasters) can bring about the destruction or even extinction of a population, but even slow change can bring about population change in a given area (as occurs for example in ecosystem succession)
Population interactions
actually occur between individuals and groups of individuals
can be beneficial or detrimental to either population or both
competition
intraspecific
between members of the same species for food, mates, nesting materials, etc
interspecific
between members of two or more species for resources (sunlight, food, habitat, etc)
results can be:
extinction of one
coexistence
often, coexistence can be achieved (this makes sense since species can evolve to exploit a unique ecological niche)
humans have such voracious appetites for space and other resources that they can "compete" species into extinction
competition may be between a species we favor (cattle) and one we do not (bighorn sheep)
symbiosis
as we've discussed, this is different organisms living together
often to the mutual benefit or mutual interdependence of each
beans and nitrogen fixing bacteria
organisms may be simply compatible rather than actually beneficial to each other
symbiosis is one aspect of the intricate balance of all parts of an ecosystem
if one part of a symbiotic relationship is disturbed, other parts of the ecosystem are also disturbed
predation
one individual (the predator) feeding on others (the prey) usually of another species
an essential part of energy flow in ecosystems
the relationship between predator and prey ("predator-prey interactions") are slowly evolving mechanisms that maintain stable population levels
increase in prey population allows increase in predator population which acts as negative feedback, decreasing prey population
BUT, when humans are the predators:
some prey species are hunted to or close to extinction
some prey species are modified and monocultured
in this process, some other predators may be outcompeted
by removing predators, humans tend to disturb the stable population of prey species
people remove predators
to protect favored prey (eg cattle)
by disturbing and destroying habitat
by introducing a prey species into an environment without natural predators (eg the gypsy moth)
when predatory feedback is disrupted, prey populations can grow out of control
alternatively, people can introduce a new predator can wreak havoc on the unsuspecting prey
parasitism
predation by an organism (parasite) smaller than its prey (host)
the parasite does not kill its host, at least initially, but rather feeds on the living tissues or food of the host
parasite may kill the host provided it has some mechanism for transmitting its offspring to a new host before or after the host's death
disease causing organisms are human parasites
human environmental manipulations can cause population disturbances that increase or decrease the incidence of various parasites
others terms:
mutualism = an interaction which benefits both interacting species
commensalism = an interaction which benefits one species without affecting the other
Ecosystem succession
over time, the structure of an ecosystem changes; the process of change is predictable in an undisturbed system and is known as "succession"
each community eventually renders its habitat unsuitable for that community
primary succession:
soil is formed out of rock by plants, or deposited by wind or water
the presence of plants produces more and richer soil
as soil accumulates, additional plant species can move in....
secondary succession:
occurs when an ecosystem has been forced back to an earlier stage
a cleared field is a good example (most of Connecticut a century or so ago):
annual grasses the first year
perennial grasses, in a few years
shrubs, ten years
trees, ten to fifty years
first are shade intolerant
later are shade tolerant
forest in a century (like CT today)
in western Oregon:
oaks eventually shade out oak seedlings
firs move in, then make the soil inhospitable to fir-lings
other trees move in
eventually ecological conditions in a stable physical environment can reach a steady state embodied by a climax community
stable and balanced birth and death rates
low net productivity (most energy is used by the inhabitants of the community)
earlier communities (eg perennial grasses) produce a lot of extra energy