Density Dep/Indep Limiting Factors:
PPT & Readings:
PPT
https://docs.google.com/present/view?id=dfh23k67_2740dzvd74fr&pli=1
population overview incl. DD and DI factors
The reading on density dep and density indep factors is from the site below:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Populations2.html
Reading:
Density-Independent Checks on Population Growth
The vagaries of the physical environment, for example
drought
freezes
hurricane
floods
forest fires
often check population growth. Not only may they limit population growth but they often drive
existing populations well below their previous level. (And also make it unlikely that many
animals will survive long enough to show signs of aging — link to discussion.)
These factors are described as density-independent because they exert their effect irrespective of
the size of the population when the catastrophe struck.
This graph (from P. T. Boag and P.R. Grant in Science,
214:82, 1981) shows the decline in the population of one
of Darwin's finches (Geospiza fortis) on Daphne Major, a
tiny (100 acres = 40 hectares) member of the Galapagos
Islands. The decline (from 1400 to 200 individuals)
occurred because of a severe drought that reduced the
quantity of seeds on which this species feeds. The
drought ended in 1978, but even with ample food once
again available the finch population recovered only
slowly.
Catastrophic declines are particularly risky for populations living on islands. The smaller the
island, the smaller the population of each species on it, and the greater the risk that a catastrophe
will so decimate the population that it becomes extinct.
This appears to be one
reason for the clear
relationship between
size of island and the
number of different
species it contains.
The graph (redrawn
from R. H. MacArthur
and E. O. Wilson, The
Theory of Island
Biogeography,
Princeton University Press) shows the number of species of reptiles and amphibians on various
islands in the West Indies. In general, if one island has 10 times the area of another, it will
contain approximately twice the number of species.
The same principle applies to many habitats. In a sense, most habitats are islands. A series of
ponds, a range of mountain tops, scattered groves of citrus trees, even individual trees within a
grove, all are made up of patches of habitat separated by barriers to the free migration of their
inhabitants.
This has practical as well as theoretical importance. As the human population grows, jungles are
cleared for agriculture, farms are paved for shopping centers, rivers are dammed for
hydroelectric power and irrigation, etc. Although wildlife sanctuaries are being established, they
must be made large enough so that they can support populations large enough to survive densityindependent checks when they strike.
An example: Lake Guri
In 1986, the closing of a dam in Venezuela flooded over a thousand square miles (>2,500 km2)
turning hundreds of hilltops into islands. These ranged in size from less than 1 hectare (2.5 acres)
to more than 150 hectares (370 acres).
Within 8 years,
The tiniest islands (<1 hectare) lost 75% of the species that had lived there.
The larger the island, the fewer species it lost.
But all the islands — even the largest — lost their top predators; that is carnivores like
pumas, jaguars (image), and eagles at the ends of food chains.
Those animal species that did remain — mostly herbivores and small carnivores —
greatly increased their populations because of
o a reduction in competition for resources, and
o no longer being eaten by predators.
The intense grazing by the increased herbivore populations is degrading the variety of
plant life on the smaller islands.
Density-Dependent Checks on Population Growth
Intraspecific Competition
Intraspecific competition is competition between members of the same species.
In the summer of 1980, much of southern New England was struck by an infestation of the gypsy
moth (Porthetria dispar). As the summer wore on,
the larvae (caterpillars) pupated;
the hatched adults mated, and
the females laid masses of eggs (each mass containing several hundred eggs) on virtually
every tree in the region.
In early May of 1981, the young caterpillars that hatched from these eggs began feeding
and molting.
The results were dramatic:
o In 72 hours, a 50-ft beech tree or a 25-ft white pine tree would be completely
defoliated.
o Large patches of forest began to take on a winter appearance with their skeletons
of bare branches.
In fact the infestation was so heavy that many trees were completely defoliated before the
caterpillars could complete their larval development. [View!]
The result: a massive die-off of the animals; very few succeeded in completing
metamorphosis.
Here, then, was a dramatic example of how competition among members of one species for a
finite resource — in this case, food — caused a sharp drop in population.
The effect was clearly density-dependent. The lower
population densities of the previous summer had permitted
most of the animals to complete their life cycle.
The graph shows a similar population crash; in this case of
reindeer on two islands in the Bering Sea. Why the
population on St. Paul Island went through so much more
severe a boom-and-bust cycle than that on St. George Island is unknown.
Many rodent populations (e.g., lemmings in the Arctic) go through such boom-and-bust cycles.
DENSITY DEPENDENT/INDEPENDENT LIMITING FACTORS:
To clarify:
One Student told me that they found it easier to understand if they thought of it like
this:
DENSITY INDEPENDENT factors affect an entire environment; therefore it is affecting
MULTIPLE populations! (Ex. A hurricane or a tornado affects massive amounts of
different populations from insects to humans).
A DENSITY DEPENDENT factor affects only one population. (Overcrowding in a
Pond...the overcrowding is caused by the limited space and the fish have to then
compete for their lives)!
A comment was brought up in class about Humans and there affect on deer population
when they build a neighborhood...what is wrong with the statement is that the humans
building the neighborhood are affecting the entire ENVIRONMENT not just the deer...so
therefore that is a Density Independent factor…..but instead of a natural disaster the
cause is the human population, but that does not change the fact that the entire
environment is affected including Multiple Populations!