Ecology – Biology 3500
POPULATION ECOLOGY UNIT
Chapter 9: Population Distribution and Abundance
Population Ecology -- Important characteristics of populations include:
1. distribution (on Earth) -- why organisms occur where they do (and why NOT elsewhere)
2. density -- within the population, how many individuals occur within a unit area
3. abundance -- overall numbers within the population as a whole
Distribution Limits: the environment limits the distribution of
Each species has a niche (what's a niche?)
Fundamental vs. Realized niche
Animal example: a Tiger Beetle of cold climates (Cicindela longilabrus)
higher metabolic rates than most other species, but similar across entire range
Plant example: the genus Encelia and pubescence of leaves
Encelia californica -- along coast of CA; cool environment and relatively moist.
This species has the least pubescent leaves (Why?)
Encelia actoni -- inland; temps somewhat warmer but conditions also much drier.
Leaves more pubescent than E. californica; gives leaves higher albedo (reflectance)
In the hot Sonoran desert farther east, E. frutescens, with sparsely pubescent leaves,
and E. farinosa, with strongly pubescent leaves, grow side by side. Yet, both species
leaves are similar in temperature during the hot day. HOW? E. frutescens has much
higher transpiration rates. So, how does E. frutescens avoid too much water loss? It
grows in a different microhabitat, in the much deeper soils of seasonally wet washes
with much deeper roots. E. farinosa grows mostly along banks with shallow soils.
Intertidal barnacles (Balanus and Chthamalus) and competitive exclusion (more in Chap. 13)
Distributions (Dispersion) Patterns on a Small Scale:
Tied with resource availability, distribution, abundance; age structure can change distribution
Distributions can be:
1. random -- likely when? Resources incredibly abundant and competitive interactions
reduced (individuals "ignore" one another)
2. regular (uniform) -- sparse resources, individuals "claim" exclusive use of a patch of
landscape (in essence, individuals compete with each other to claim "territory")
3. clumped -- when resources are clumped; food, water, mates, etc. Creates a situation
where individuals are "attracted" to one another. Obviously happens with social groups.
Examples: Creosote bush (adults) in the desert -- regular (uniform) distribution. WHY?
(see discussion of root systems page 208)
However, if you look at young individuals, they tend to be clumped. WHY?
Distribution Patterns on a Large Scale: always clumped (WHY?)
Resources are NEVER uniformly distributed over large areas. Many organisms clumped
around water sources, for instance.
Different species with different tolerances of various conditions may have abundance gradients
that follow resource gradients: moisture, temperature, salinity, etc. Can result in "hot spots".
This may also mean that in a given area, different species with different needs/tolerances are
found at different places along the gradient (see bird and plant examples in text).
Organism Size and Population Density
It should surprise no one that pop density necessarily decreases with increasing organismal size,
all other factors being equal (though rarely are other factors equal). These trends can be
seen across comparisons of major taxonomic groupings (see page 213), and even within
species. Simply put, larger size requires more resources, and therefore fewer individuals
can be supported by a given amount of resources.
Abundance/rarity -- for reasons I will discuss in class, I do not like to use the term "rare" except
under very specific circumstances (circumstance "8" on page 214)
Based on three factors: 1. geographic range (extensive vs. restrictive), 2. habitat tolerance
(broad vs. narrow), and 3. local population size (large vs. small)
Chapter 10: Population Dynamics
Chapter 9 discusses populations from a static (snapshot) point of view; however, populations are,
of course, dynamic.
Population size at some point: Nt = Nt-1 + B + I - D - E
Dispersal: WHY do organisms disperse? Understand that migration is directed dispersal.
Expand ranges (why is this important?), move away from competition/dwindling resources, follow
food resources as they move/experience pop. fluctuations, with changing weather/climate, etc.
Mechanisms for dispersal: many, many organisms have very specialized dispersal mechanisms for
some part of the life history (animals -- immatures, adults; plants -- seeds, runners, fruits; fungi
-- spores). You should be able to provide EXAMPLES.
Dispersal of Expanding Populations -- Birds
provide many good examples (after all, have wings)
Example: Eurasian Collared Doves. Odd in the sense that, unlike introduction of starlings or
English House Sparrows into new countries, the Eurasian Collared Doves, once restricted to
Turkey in Europe, began around 1900 spreading into the rest of Europe, so that by 1980, every
country in Europe had been "invaded" (see Fig. 10.3), all without human introduction or
apparently much other influence.
The dispersal, although apparently reasonably steady, took place in small "jumps". Only
young doves are active dispersers; adults are highly sedentary. Each of these bursts of juvenile
dispersers averaged about 45 km. of expansion yearly. Compared with the 300 to 500 km.
yearly expansion by Africanized Honeybees in the Americas (see Fig. 10.1), 45 km is pretty
modest, but still impressive compared to most other animals.
The RATE of expansion clearly differs for different species (see Fig. 10.5), and is based upon
MANY different factors.
Also, UNDERSTAND that dispersal is likely taking place in all directions, but the only direction in
which it will be apparent will be away from the already established populations.
Example: North American Trees -- Maple and Hemlock
Both spread northward after the last ice age, reaching present northern limits at different times,
based on different rates of dispersal. Maple disperses faster than Hemlock (anyone know why?).
QUESTION: are we likely to see these ranges begin to expand northward farther yet in the near
future, and, if so, WHY?
Dispersal in Response to Changing Food supply
Example: Small mammal populations (eg., lemmings, voles) experience large population fluctuations
from year to year; these fluctuations are thought to be rapid reproductive responses to changes in
their food. Yet we see rapid response in predator populations as well, and these are almost
exclusively due to dispersal and NOT production of more offspring (preds. reproduce slower)
Owl and Kestrel densities match vole pop. fluctuations in Finland almost perfectly -- if owls/
kestrels were exhibiting reproductive responses to prey densities, there would be lag time. This
suggests owls/kestrels nomadic. Not all preds can follow prey as well as these, because, after
all, wings can make a huge difference!!
Dispersal in Water
Current is a natural disperser, but true aquatic organisms are, of course, quite RESTRICTED as
to where they can disperse. As far as dispersal is concerned, however, perhaps the most
important question here is "How do organisms PREVENT dispersal in water?"
As there are a huge number of dispersal mechanisms, there are a large number of characteristics
that allow organisms to maintain position within streams (be sure you know some). Even so,
certain flooding/weather events still move these organisms downstream -- drift.
Therefore, organisms that permanently reside in a stream must offset drift with upstream
colonization cycles. Sometimes these upstream dispersal movements are coordinated in
spectacularly large groups (see migrating snails in the Rio Claro, Costa Rica, page 224).
Metapopulations -- subpopulations connected by moving individuals. Example? Remember
the Glanville Fritillary populations in Finland (Chapter 4).
North American Example: Parnassius smintheus (an alpine butterfly)
The butterflies tend to leave small subpopulations more than large, and move into larger pops., so
that the larger meadows have an even proportionally larger density of individuals. With the
long standing policy of fire suppression, and in the face of global climate change, alpine meadows
are shrinking in many places, leaving alpine species with nowhere to go.
Patterns of Survival (already approached to an extent in lab)
Survivorship Curves/Life Tables/Age Distribution pyramids
Follows cohorts through life
Types of Survivorship Curves:
"I" represents high survivorship of young; typically occurs with parental care of few young
"II" represents equal chances of surviving/dying throughout life
"III" represents high young mortality; typically occurs with tremendous numerical output of
young, but no parental care.
You should, of course, be able to recognize examples of all of these, of which copious examples
are presented in the book (and I imagine you could think of more on your own).
These graphs give an indication not only of age structure, but in turn whether or not a pop.
is likely to continue growth, remain level, or decline.
White Oaks (Illinois pop.) and Rio Grande Cottonwood (Belen, New Mexico pop.) example
A Truly dynamic example: Geospiza conirostris on Genovesa (Galapagos Islands)
Snapshots in 1983 and 1987 indicate droughts in 1977, 1984 and 1985 caused a lack of
young production during those years, and also killed some of the older birds. Shows that
survivorship is typically MUCH MORE COMPLEX than a simple survivorship curve.
Life Tables: Rates of Population Change -- requires knowledge of a fecundity schedule
Net Reproductive rate (per individual), geometric rate of increase, generation time, per capita rate
You should be able to determine various questions asked about life tables and to fill in life tables
when given nx and mx.