Download Lecture.6 - Cal State LA

Population Distribution and
Abundance
biosphere
region
landscape
ecosystem
community
interaction
population
individual
Evolutionary change driven by
ecological processes occurs at
the population level
Population
• a group of individual organisms of the same
species living in a specific area
Population
• Scale of area inhabited by a population varies
depending on the organisms in question – could
be a single mountain top or an entire continent
• Population may represent only a fraction of a
species, or a single population may include all
the individuals comprising a species.
Populations all share defining features:
• Distribution – physical location of individual
organisms in area that the population inhabits
• Abundance – number and density of individual
organisms within an area
Distribution Limits
• Physical environment imposes limits on the
distribution of populations
• the range of environments in which a particular species can
exist is limited
• at some point, the energy expended by an organism for
survival in an extreme environment exceeds the maximum
energy intake that is possible for the organism
Distribution Limits
Distribution Limits
• Physical environment is not always directly
responsible for limiting species distribution
• climate may act indirectly through its influence on other
factors such as food production, habitat, and incidence of
predators and parasites.
Distribution Limits
• Distribution of different species may overlap
geographically, yet may be divided on a
microclimatic scale
Distribution Limits
Distribution Limits
• Distribution of species can also be limited by
competitive interactions with other species
Distribution Limits – Interspecific
competition
Large – and small – scale distribution
of individuals within a population
• distinction between “large” and “small” scale
depends on the size of the organisms being studied
• small-scale = small distances over which there is
little environmental change significant to the
organism under study
• large-scale = areas over which there is considerable
environmental change from the perspective of the
study organisms
Small – scale distribution of individuals
in a population
• Random
• Regular
• Clumped
Small – scale distribution of individuals
in a population
• Random, regular or clumped distributions are
caused by the nature of interactions between
individual organisms in a population and / or the
structure of the physical environment
Small – scale distribution of individuals
in a population
• Random distribution – occurs when individuals
are indifferent to the presence of other
individuals and neutral with respect to
environmental conditions / resources
Small – scale distribution of individuals
in a population
• Regular distribution – occurs when individual
interactions are antagonistic and / or local
resources are depleted
Small – scale distribution of individuals
in a population
• Clumped distribution – occurs when individual
organisms attract each other and/or individuals
are attracted to a common resource
Dynamic small – scale distribution of
individuals in a population: creosote
Assessing small – scale distribution of
individuals in a population
• Count number of individuals of a species in a randomized
series of spatial plots
• transect
• quadrats
Assessing small – scale distribution of
individuals in a population
• for each plot, calculate the mean (x) number of
individuals of a species and determine the
variance (s2) around the mean
Assessing small – scale distribution of
individuals in a population
• if s2 is less than x, the distribution is regular
• if s2 is greater than x, the distribution is clumped
• if s2 is equal to x, the distribution is random
Large – scale patterns of population
distribution
• From a large-scale perspective, population
distributions always appear clumped
• individuals are concentrated around environmentally suitable,
resource-rich areas
Abundance
• smaller organisms tend to live at higher densities
(numbers of individuals per unit area) than
larger organisms
• Population density decreases as the size of
organisms increases
Abundance
Abundance
• Population density decreases as the size of
organisms increases – but…how much? Is the
relationship the same for all types of organisms?
• aquatic invertebrates tend to live at higher densities than
terrestrial invertebrates of similar size
• mammals tend to live at higher densities than birds of similar
size
Abundance
• Plant population density also decreases with
plant size, but population density is more
dynamic, changes as plants mature and grow
larger
Abundance
• Population density / organism size relationship
is important to quantify, as this generalization
can be used to calculate expected population
densities for a species of a given size and
taxonomic group.
• can determine whether a particular population is
unusually abundant or sparse
Extinction
• Patterns of population distribution and
abundance are a key factor determining a
species’ persistence or extinction
• rare species are more prone to extinction than
common species – but how are rarity and
commonness determined?
Rarity
• Classification of commonness and rarity based
on three factors:
1) geographic range (extensive vs. restricted)
2) habitat tolerance (broad vs. narrow)
3) local population size (large vs. small)
extensive geographic range
broad habitat tolerance
small local population size
Tiger
restricted geographic range
broad habitat tolerance
small local population size
Tasmanian Devil
Andean Flamingo
restricted geographic range
narrow habitat tolerance
large local population size
Pritchardia monroei
restricted geographic range
narrow habitat tolerance
small local population size
Artemesia californica (sagebrush)
Artemesia douglasiana (mugwort)
Encelia californica
Epilobium canum (CA fuchsia)
Eriodictyon crassifolium
(yerba santa)
Hereromeles arbutifolia (Toyon or
Hollywood)
Marah macrocarpus (wild cucumber or
Manroot)
Opuntia littoralis (coastal prickly pear)
Penstemon spectablilis
Salvia apiana (white sage)
Salvia mellifera (black sage)
Umbellularia californica (CA bay
laurel)
Population Dynamics and Growth
• Population distribution and abundance changes
through time – not a static feature, but one that
is in constant flux
Population Dynamics and Growth
• Patterns of population distribution and
abundance arise through the dynamic balance of
two types of processes:
• processes that ADD individuals to populations
and those that REMOVE individuals from
populations
Population Dynamics and Growth
• Immigration & births – add individuals to
populations
• Emigration & deaths – remove individuals from
populations
Dispersal
• movement of individuals to establish populations in
new areas (range expansion)
• movement of individuals within the bounds of an
established population range
• movement of individuals between spatially isolated
populations