Population Ecology
 either examine populations of a single species or interactions between species
 investigation on a single species: population numbers and distribution
 why are populations of pandas decreasing? what would it take to increase them
 how do you control aphids on a crop?

what factors increase or decrease population?

temp? competition? predation? Plant quality?
Counting populations
 numbers per unit area: density
 births or recruitment (establishment in population)
 deaths
 immigration (input) and emigration (output)
 seems straitforward—but.…conceptually simple, practically difficult
 animals and plants pose different problems

animals: mobility results in recounting, hiding, problems estimating density-mark-recapture

plants: standing still helps with density, etc. but have another problem:
What is an individual?
 counting unitary vs modular organisms:

unitary--growth form is determinate--four legs, one mouth... a genetic
individual is easy to identify

modular--growth form is indeterminate--module may be determinate, but the
whole body of the organism consists of repeated modules giving a branched
structure, of indeterminate size and number of components

plants are modular, and some animals such as coral,bryozoans, hydroids, and
many fungi and protists

module--in plants it is usually a bud, its associated stem, leaves and a
reproductive part--but definition not absolute

plants grow by accumlating modules--the final size of a plant is not
effected as much by the size of the modules, but by the number-contrast young vs mature oak trees

modules can accumulate vertically--trees--modules can be slightly
independent but linked together

modules can accumulate horizontially--strawberry or grass--modules
can be completely independent or share resources

those that grow horizontially can break to pieces, and counting genetic
individuals becomes a problem-genet
vs
ramet

a plant is a population of modules--significant implications for their
ecology
How do populations grow?
 all organisms produce more offspring than needed to replace themselves—
how many on average do they produce?
 depends on the environment they live in and the type of organism
 survivorship curves Fig 10.6
 example: each individual produces 2 offspring (or 4 per female)

start with 10 individuals 10 x 2 = 20; 20 x 2 = 40; 40 x 2 = 80; 80
x 2 = 160; 160 x 2 = 320; 320 x 2 = 640; 640 x 2 = 1280; 1280 x 2
= 2560

plot it on a graph over time

model it mathematically
 r the intrinsic rate of increase determines the rate of population growth
 r represents the potential growth rate of a species under ideal conditions—

depends on number of offspring (fecundity) and generation time

small organisms have short generation times and high r

large organisms have longer generation times and lower r

holding size constant still get variation in r –discuss later
 lack of resources caused by competition will slow population growth
 population growth involves intra specific competition
 define competition:

interaction between individuals, brought about by shared requirement for a
resource in limited supply and leading to reduced survivorship, growth,
and/or reproduction
 population growth is density dependent
 competition can be intraspecific or interspecific
 mortality and fecundity are Density-dependent
 fecundity--eggs or offspring produced--says nothing about their survival-fitness
 data to support this statement

planthoppers (fig 13.6 Molles); Isopods (fig 13.7 Molles)
 intraspecific competition regulates population size
 because mortality and birth rate depend on density--they can control population
size

where births = deaths you get stable numbers

where births greater than deaths, numbers increase

where deaths greater than births, numbers decrease

stable point is called the carrying capacity
 If you follow increases in numbers over time from a starting point of very low
density you get an S-shaped curve or sigmoid curve

Fig 11.7, 11.8, 11.9, 11.10, 11.11 Molles
 the situations this occurs in

experiments where a few individuals put into a cage with abundant resources

recovery of animals after population crashes due to disease or predation
 there is no set carrying capacity for a population--it varies from year to year
depending on resource supply, conditions
 competition impacts growth
 competition impacts numbers in a population, AND the size of individuals (ie.
Fig 13.3, 13.4, 13.5 Molles)
 the final biomass of a population is often very similar--even when starting from
different densities--ie. plant example

law of constant final yield

density x mean weight = constant
10 plants x 10 gm each= 100 gm
5 plants x 20 gm each= 100 gm
competition's impacts on growth are asymmetric
 the rich get richer; the poor get poorer
 individual plant size becomes skewed to left as density increases

the few, large individuals may be little affected by the smaller ones--but the
reverse is not true--asymmetric competition
 territoriality is asymmetric competition among animals
 size differences may be present, but not as extreme as in plants--outcome is the
same: few winners and many that don't reproduce at all
INTERSPECIFIC COMPETITION
Examples of strong competition:
 Gause's paramecium experiments- sigmoid growth curves for P. aurelia and P. caudatum by themselves (Fig
13.15), but together P. aurelia out competed P. caudatum.
 P. caudatum and P. bursaria coexisted together, but close examination revealed
that P.caudatum feed on bacteria suspended in medium, and P. bursaria lived on
yeast that fell to bottom of the tubes.
 These results led to Gause's principle or the competitive exclusion principle:

two species cannot occupy the same niche

if two species coexist in the same habitat, it is because of niche
differentiation, (realized niche)
 At the same time Gause was working with Paramecium, mathematician
produced models of competition based on population growth--these solidified
the exclusion principle as they predicted the outcome: unless competitive
abilities exactly equal for the two species, the weaker one was always
eliminated over time
Resources and conditions define the niche of a species.
 Niche: description of all the conditions and resources necessary for a species
to survive and reproduce.
 for example, for saguaro cactus, temperature is very important part of niche, but
must add others: water--too little and it will die--too much and it may not be
able to compete with something else.
 Most accepted definition is Hutchinson (1957) n-dimensional hypervolume
(defined on p. 307)
 think of temperatures influence on a plant--one dimension
 add water--two dimensions
 add pH of soil—three dimensions
 Fundamental Niche is determined by physical conditions and resources

Realized Niche is influenced by biotic interactions
 competition-interspecific
 predation
Examples of interspecific competition:
 Barnacles
 Connell (1961) studied two species of barnacles: Chthamalus and Balanus
(Cham-a-lus and Ball-a-nus)
 ECO--sp2003_files\balanus.ppt (and Fig 13.21)
 live on same rocky shores, but
 Chthamalus occurs higher up on shore than Balanus.
 Chthamalus larvae settle in both zones, but survival is low in the presence of
Balanus
 if Chthamalus larvae are protected from Balanus, their survival is high, ie., it
was not the physical conditions of lower shore that was killing Chthamalus
larvae
 Balanus was limited to lower shore due to its sensitivity to desiccation--not
competition
 You can see niche here. The fundamental niche (defined by physical
environment) of Chthamalus included both the higher and lower intertidal area,
however the reaized niche (due to interactions with other organisms-
competition and predations) was only the upper tidal zone. For Balanus the
fundamental niche and realized niche were similar: the lower zone.
 bedstraws
 two species: one on acidic soils, the other on basic soils
 in pots, alone, they could survive either soil type
 competitive exclusion must be occurring
 small granivores
 Dipodomys removal plots and control plots
 smaller granivores increase in number on Dipodomys removal plots (Fig 13.24
and 13.25)
Summary of what happens when strong interspecific competition occurs:
 evolutionary time--niche differentiation
 selection against strongly competing individuals within a population (fig 13.26)
 for niche differentiation to occur, competition and the competitive winner must
be consistant over time
 ecological time--competitive exclusion from part of niche that overlaps with
stronger competitor
 restriction of fundamental niche to realized niche
Do not expect natural selection under these conditions:
 patchy environments--species may be competitively excluded from some
environmental patches in the habitat, but still exist in other patches, i.e., dung
piles

environmental disturbance that does not allow competitive exclusion to run to
equilibrium, i.e., prairie fires
 Examples of strong competition in nature not muted thru niche
differentiation:
 patchy in space or time: patches of good resources are unpredictable in space
and time. A poorer competitor may be a better colonizer. A patch appears, is
colonized by fast disperser (sp1), then by slower disperser. Sp.1 is a poor
competitor, and is competitively excluded: competitive exclusion occurs, but
only in a local gap
 pre-emption of space--the species to arrive first in a gap gets a head start and
can outcompete other species, even if it would not be the winner if started at the
same time. Different patches have different species as the first to arrive due to
chance colonization.
 fluctuating environments: paradox of the plankton
Exploitation-
.
predators
kill & consume
whole prey many prey?
example
yes
yes
lion, tiger, bear, ladybug, seed-eating mice
grazers
no
yes
cows, mosquitoes, cookie-cutter shark
parasitoids yes
no
flies, wasps, monster in ALIEN
parasitesand
no
tapeworm, tick, insect galls, bacteria
no
fungi that cause disease,
dodder,
 Impact of predation on prey individuals.
 predators and parasitoids kill individuals
 grazers and parasites reduce growth or reproduction


Parasites—

impact depends on the number of parasites

parasites can change behavior
Herbivores-
impact depends on ability of plant to compensate--when parts are eaten,
other parts remain to continue growth
 grazers and parasites can make a prey more susceptible to other negative factors

competition

predation concentrated on parasitized individuals

parasitized individuals arrive at breeding ground late
 Impact of predation on prey population
 impact on population is not always negative

individuals not chosen by predator randomly--attack weak, old, young....
minks prey on muskrats that do not have established territoriesthese individuals already stressed, and had little chance of
reproductive contribution

individuals that escape may compensate--Net recruitment curve vs density
if intraspecific competition holding population numbers down (i.e.,
K) then predator removal of individuals will be compensated for
by increased food to survivers which can increase fecundity—

compensation by prey population not complete
 exploiters can regulate population size of prey.

impact of a caddisfly on algae in stream

many examples when we accidently indroduce plants or animals without
their predators

prickly pear cactus in Australia

ECO--sp2003_files\prickly pear.ppt
 Effect of consumption on predator
 too little food: death
 too much: full predators and many escaping prey
 prey population can exploit this

masting in nut trees—pecans every 4 yrs; oaks;

cicadas in Arkansas—13 yrs between emergences
 variability of plants as resources for predators—it is not how many you eat, but
their quality that counts.
 dynamics of predator/prey populations
 Math models and lab models back in 30’s, but we are not looking at them; they
predict oscilations
 Gause did test tube experiments on paramecium by adding predators

Examine Fig 14.18 in Molles,
 Refuges are key to a prey’s ability to survive under exploitation

Huffacker’s oranges

predator satiation, i.e., masting and cicada examples
Mutalisms
 examples of mutalisms
 plants and animals

pollinators and seed dispersers

defense with food and shelter payoff
 plants and fungi

mycorrhizae-fungi and plant roots
 animals and fungi

termites with fungi in gut
 animals and animals

oxpeckers, honeyguides
 important in interactions within a community
 important in natural selection