Population density is the number of individuals of a particular species per unit area or volume.
Examples:
35 alligators per square km of a swamp
1,000,456 bacteria per cm2 of an agar plate
120 earthworms m2 of soil
Population density problems
On rare occasions you can count all the individuals in
a population, such as the number of beech trees in a
forest measuring 50 square kilometers (km2).
Population density = Individuals = 1000 trees = 20 trees
Unit area 50 km2 km2
Population density is a helpful measurement for comparing populations in different locations.
35.2 There are limits to a population’s growth
Exponential growth: growth of a population that multiplies by constant factor
Limiting factor:condition that restricts a population’s growth, such as space, disease and food availability.
Carrying capacity: number of organisms in a population that an environment can maintain.
Exponential Growth
Figure 35-5 This table shows how many bacteria are in a population that doubles every 20 minutes. The graph is another way to show the same data.
CARRYING CAPACITY
Figure 35-6 Before the early 1900s, hunting kept this population of fur seals below the carrying capacity of the environment. Then, after hunting was reduced, the population grew almost exponentially for two decades. The population began to level off as it reached the carrying capacity.
Growth factors (increase in pop.)
Growth factors (increase in pop.)
Immigration: individuals moving into a population
Births
Shrinking factors (decrease in pop.)
Emigration: individuals moving out of a population
Competitive exclusion: One species succeeding over another, when the growth of both species is limited by the same resource.
Figure 35-14 Two similar species may each thrive in separate locations, but one may exclude the other when they are placed together. The results of an experiment with two Paramecium species demonstrate this principle of competitive exclusion.
NICHE
Niche: a unique living arrangement of an organism defined by its living place (habitat), its food sources, the time of day it is most active, and other factors
The local loss of a species is likely to occur if 2 species have niches that are very similar
niches are rarely identical.
Example: one lizard in a tropical forest feeds on insects in low shrubs, while a similar lizard may eat insects high in the trees.
Predation
Predation: an interaction in which one organism eats another.
The lion attacks and eats an injured zebra or an egret catches and eats a fish.
Predator: the organism that kills/ eats the prey.
Prey: the organism that gets eaten.
eating and avoiding being eaten are important to survival,
many effective adaptations have evolved in both predators and prey.
Surrounding towns and villages had to be immediately, permanently abandoned.
Secondary succession: trees are colonizing uncultivated fields and meadows
Human Activities and Species Diversity
humans have had the greatest impact on communities worldwide.
60 % Earth's land is used by humans, mostly as cropland or rangeland.
Human disturbances have a negative effect on species diversity
Clearing the Land
for lumber
land for farming
Land for building.
paved over or eventually recolonized by weeds and shrubs, as in abandoned city lots.
Introduced Species
Introduced species are organisms that humans move from the species' native locations to new geographic areas, either intentionally or accidentally (exotic species).
Kudzu, a Japanese plant planted widely in the American South (1930s) to help control erosion. especially along irrigation canals. But kudzu soon grew out of control, taking over vast expanses of landscape.
Some introduced species gain a foothold and may disrupt their new community.
Some introduced species prey on native species or outcompete native species
Invasive species
Kudzu vines
Zebra Mussels clogging pipes
35.5 DISTURBANCES
Bioaccumulation: is the process by which substances not readily broken down or excreted can build up and be stored in living tissue (usually in fatty tissue.)
Bioaccumulation: is the process by which substances not readily broken down or excreted can build up and be stored in living tissue (usually in fatty tissue.)
Biomagnification: is the process by which substances become more concentrated in the bodies of consumers as one moves up the food chain (trophic levels).
PCBs, or polychlorinated biphenyls, are a group of man-made chemicals.
PCBs, or polychlorinated biphenyls, are a group of man-made chemicals.
Introduced in 1929 and widely used in electrical transformers, cosmetics, varnishes, inks, carbonless copy paper, pesticides and for general weatherproofing and fire-resistant coatings to wood and plastic.
The federal government banned the production of PCBs in 1976
PCBs can effect the immune system, fertility, child development and possibly increase the risk of certain cancers
DDT is a pesticide that was widely used until being banned in the U.S. in 1972
DDT is a pesticide that was widely used until being banned in the U.S. in 1972
DDT accumulates in living tissue, particularly in fat tissue
High concentrations in some bird species caused failure of eggs by thinning the shells