Introduction to Ecology
and the concepts of
Populations, Species, Communities, Biomes, and the Biosphere
 Reading: Freeman , Chapters 48-50.

Ecology is  the scientific study of the distribution and abundance of organisms, and their interactions with the environment.
The word Ecology comes from the Greek words "oikos", meaning house, and "logos",  to study.

What do we mean by "scientific study"?
There are hundreds of different sciences, each employs different methods.
In one form or another, the "scientific study" of any subject uses the hypothetico-deductive approach.
This approach is rather new historically, tracing its roots back to the European enlightenment of the sixteenth and seventeenth centuries
Essentially, it goes something like this:
A scientist, or group of scientists, becomes interested in explaining one aspect of   how the natural world works.  (This comes easily for scientists, as well as most people, people are curious...)
Both intentionally and unintentionally, scientists conduct observations.  (Some observations may be commonsense or centuries old (i.e., the tides), others may require specialized experience (variation in scutellar hair number on different strains of Drosophila)
    A hypothesis is constructed based on all the pertinent observations.

A hypothesis is a working model of the process in question
Good hypotheses generate predictions that can be tested.
In experimental sciences, experiments are set up to test the hypothesis ;     In the historical sciences, additional evidence is gathered in the place of experiments (although not nearly as incisive, some areas do not easily lend themselves to experiments (i.e., stellar astronomy))

If the predictions of the hypothesis are not borne out, the hypothesis is falsified.
Interestingly, a hypothesis cannot be proven true by an experiment, even when it correctly  predicts the outcome.
This is because an infinite number of hypotheses can usually be generated which predict the same outcome.
    The strongest hypotheses are usually accepted as true if multiple experiments fail to falsify them.
    These hypotheses, and their underlying models, form scientific paradigms which may last for centuries or longer.  Scientific paradigms are explanations for how the world works, they inspire future research, but also limit its direction to certain avenues.

• Observation-Zonosemata flies have dark bands on their wings. They wave them up and down when disturbed.
• Based on the prevailing scientific paradigm in biology (Darwinism)-researchers ask "How could the behavior evolve by natural selection?"

• Hypothesis 1-Wing patterns and displays evolved by sexual selection. (courtship displays such as this are common in flies)
• Hypothesis 2-Wing patterns and displays evolved to scare predators by "mimicking" a jumping spider. (mimicry is a common defense in the animal kingdom)
• Hypothesis 3-Displays evolved to deter predation by jumping spiders themselves.

• Create five experimental groups
• 1-untreated controls
• 2-Zonosemata with own wings re-glued
• 3-Zonosemata with housefly wings
• 4-Houseflies with Zonosemata wings
• 5-Untreated houselfies
• Subject flies from each treatment to starved jumping spiders and other predators. Conduct 20 trials. Measure number of times spider or other predator attacks v.s. retreats.
• Questions-Why group 2? Why not just 1 trial?

• Hypothesis 1-All flies should be attacked.
• Hypothesis 2-Jumping spiders should attack all groups. Other predators should attack groups 3-5, but leave 1 and 2 alone.
• Hypothesis 3-Other predators will attack all groups. Jumping spiders will avoid groups 1 and 2 but attack everything else.

• # of kills out of 20 trials
• Jumping spider Anole
• Group 1 4 19
• Group 2 4 20
• Group 3 20 19
• Group 4 20 20
• Group 5 20 19

Ecology has a long history as a historical science, and is becoming an experimental science.
    At the moment, ecology lacks a scientific paradigm because many of its underlying ideas have not been sufficiently tested or have crumbled under the weight of new experiments.
    This makes ecology an exciting field, because new discoveries can have an enormous impact

What do we mean by "environment"?
The environment includes both biotic and abiotic components.
Biotic components are living aspects of the environment.  Biotic factors include other members of the same species and members of different species.

Important biotic factors in our own environment include influenza viruses, members of the opposite sex (who might become possible mates), other members of the same species that compete for resources (that other person in line for our job).

    Other species deal with different biotic factors.

...Desert mice must be constantly vigilant to avoid predation by owls and snakes.
...Males and females maintain separate systems of territories to prevent other members of the same species from tapping into their food supply.
...During mating season, males actively seek females as mates, by seeking to build a territory that includes as many females as possible

Abiotic components are nonliving aspects of the environment.
Abiotic components of the environment include rainfall and temperature, fire and disturbance.
Organisms tend to seek environments most suitable for themselves.
...Even in the same place, different organisms face different biotic and abiotic factors.

EXAMPLE:
In an office, a human being may experience a constant temperature and low humidity (central air conditioning), while the bacteria in our mouths experience a very different, much more constant, temperature and humidity.
...Bacteria in our mouths must confront periodic disturbance every time we brush our teeth, and respond to the sudden influx of food every time residue is left on our teeth after eating.

Individuals are single, discrete organisms.
Species: groups of actually or potentially interbreeding organisms,or organisms which share certain morphological or biochemical traits in common
Populations are groups of organisms of the same species living in the same place
Communities are assemblages of populations of different species living in the same place.
Biomes are broad assemblages of plant and animal communities generally defined by the dominant vegetation or by important abiotic factors.
Individuals are single, discrete organisms.
...In some cases, it is easy to define an individual organism, i.e., mice and people.
In other cases, the distinction between individuals is arbitrary or nonexistent.  A stand of aspen trees looks like many individuals above the ground, but below ground they are all interconnected.  A single fungus mycellium may occupy tens of square kilometers beneath the surface of a forest.

What is a Species?
The concept of a species is fundamental to biology, yet biologists disagree on exactly what constitutes a species.
There are several different, working definitions of a "species"
Different branches of biology use different working definitions because they have access to different information or because of fundamental differences in the biology of the organism in question.
"Biological Species Concept"
"A species is a group of actually or potentially interbreeding organisms that can mate and produce fertile offspring" –Ernst Mayer
This definition is very interesting from an evolutionary perspective.  It defines species as "real", objective entities, defined by the limits of gene exchange
Morphological Species Concept
"Species are Groups of Organisms that Share Certain Morphological or Biochemical Traits"
Some species do not reproduce sexually, some are known only from fossils.  This definition is the working definition used by biologists that cannot, or should not, use the "Biological Species Concept"
It is more subjective

Phylogenetic Species Concept
"A species is a discrete lineage"

Populations are groups of organisms of the same species living in the same place.
Individuals of the same population interact with one another: most interaction among members of a given species occurs within a population
They may compete for resources and mates, they may combine alleles via sexual reproduction.
Examples of populations include:
...a herd of wildabeast
...all the bullhead catfish living in a midwestern lake
the Sarcophaga bullata fly population on a single, isolated carcass
the E. coli population in a single person’s gut
Communities are assemblages of populations of different species living in the same place
Some species within the community may interact to a great extent, while interaction between other species may be virtually nonexistent

Imagine a herd of wildabeast:
Now imagine the two species of grass on which it feeds.  Now imagine the ten species of forbs and sedges it tramples in the processes.  Now imagine the species of grasshopper that also eats the same grasses.  Now imagine the soil microorganisms, the ticks that live on the wildabeast, and the birds that eat the ticks that live on the wildabeast, as well as the dung beetles that harvest wildabeast droppings.  This is a simple list of species that may make up a grassland community.
 
 
 

Biomes are broad assemblages of plant and animal communities generally defined by the dominant vegetation or by important abiotic factors.
Terrestrial Biomes (some scientists count them differently)
Chaparral
Grasslands
Temperate Deciduous Forests
Coniferous Evergreen Forests
Tropical Rain Forests
Tropical Dry Forests
Tundra
Deserts

Terrestrial biomes have characteristic vegetation determined by the amount and timing of water, the amount of available sunlight, and by temperature.
Vegetation provides a spatial structure to terrestrial biomes.
Spatial structure creates micro-environments important to various different organisms
Biomes differ greatly in their productivity and their biodiversity

Tundra
Tundra is characterized by the lack of trees, the dominant vegetation is lichens, annual grasses, and, in some places, very specially adapted shrubs and woody plants.
Tundra occurs in polar climates and at high elevations, where the growing season is very short.  Nearly all precipitation falls as snow.
Plants and insects flourish during the short growing season.

Taiga or Coniferous Forest
Cone-bearing trees such as pine, spruce, and fir dominate.  most are dominated by one or a few species
This biome is very common, covering huge areas at high latitudes or high elevations
the cool to warm summers are the  growing season
Plants are dormant during the winter, when temperatures drop far too low for photosynthesis.
Often, much of the precipitation falls as snow.  Snow melt releases a great deal of water into the communities that make up the taiga

Desert
Low rainfall, generally less than 30cm per year, characterizes a desert
Many deserts are very hot (at least during the day), but some, such as the Great Basin and the Gobi Desert, are cold most of the year.
The dry environment often causes a dramatic difference in temperature between day and night
Grasses are usually found when water is available (they are dormant underground, or as seeds, when there is no water).
Succulents, such as cacti and euphorbs, and shrubs with specialized water-saving adaptations, are present.
Trees are uncommon or totally absent.

Chaparral:
Cool, rainy winters and dry summers
Dense, spiny, evergreen shrubs dominate
Seasonal fires
Most of the plants are fire-adapted.  Many species can only reproduce via periodic fires
This biome has high biodiversity, with a great deal of endemism.  Endemism is when species are found in one place only
This biome is rather uncommon, being confined to relatively few locations with a Mediterranean climate, California, Costal South America, South Africa, the Mediterranean, and Western Australia

Temperate Deciduous Forests
Moderate rainfall and mild to warm summers with cool to cold winters.
Occur at mid-latitudes, where moisture is sufficient to support the growth of large trees
Winters are cold enough to prohibit photosynthesis, and most organisms go dormant or hibernate over the winter
Dense stands of deciduous trees predominate
Trees tend to have distinct vertical layers; including one or two strata of trees and an understory of shrubs

Grasslands
Grasses and forbs dominate.  Fire and grazing prevent the establishment of shrubs and trees
Moderate to low rainfall and a wide range of temperatures permit grasslands
Seasonal drought, occasional fires, and grazing by herbivores prevent the establishment of trees as the dominant vegetation
Grasslands are a widespread biome; examples include the Prairies of North America, the Steppes of Asia, the Pampas of Argentina, the Veldts of South Africa, and the Puszta of Hungary.

Savannah
Tropical Rain Forests
The most productive biome on the planet, also harbors the most biodiversity.
Most are located near the equator, where temperatures are warm and relatively constant year round
Plants are broad-leafed evergreen trees, shrubs, woody vines, and epiphytes.
epiphytes-plants that live on other plants, usually trees.  Examples include orchids and mistletoe
High rainfall-usually with a pronounced rainy season and dry season.
Competition for light is intense.  Pronounced stratification with several layers of trees.

Tropical Dry Forests
tend to occur in lowland areas, where the distinction between the wet season and the dry season can be very pronounced
deciduous trees, thorny shrubs, and succulents are very common

Major Physical Factors influencing terrestrial biomes
 Rainfall, and its timing
 Sunlight
 Disturbance

Aquatic Biomes are determined by the availability of sunlight and nutrients
The water column itself provides spatial structure.  Within a given biome, communities are stratified spatially.
Some Aquatic Biomes
Inter-tidal
Continental Shelf
Open Ocean
Freshwater Rivers
Freshwater Lakes

Continental Shelf
    Neritic Zone-near shore
    Intertidal: vertically stratified environment harboring many specially-adapted species
    Oceanic Zone-offshore
    Photic zone-light penetrates
    Benthic Zone-the bottom of the body of water

Coral Reefs-conspicuous and distinctive communities harboring many plants and animals, reefs are constructed of the skeletons of coral, and to a lesser extent, algae and other organisms.  Algae, and coral harboring photosynthetic sybionts act as producers

Open Ocean
    Pelagic Zone:  Light penetrates the top few meters
Except in areas of upwelling, nutrient concentrations are generally low, because the remains of dead organisms sink to the bottom of the ocean.
All organisms are floating or free-swimming "nekton".  The major producers are photosynthetic algae.
    Abyssal Zone:  Nutrients reach this zone by falling from above.
Light is absent.  This zone supports a wide variety of specially-adapted organisms

Wetlands
Wetlands are areas covered in water that support aquatic plants
They range from periodically flooded regions, to soil that is saturated with water during the growing season, to permanently flooded areas
Typical Wetlands
Estuaries-occur at the mouths of rivers
Swamps-flooded areas dominated by trees
Marshes-flooded areas dominated by sedges and grasses
Bogs and Fens-have distinctive vegetation because their soil is either very alkali (fens), or very acidic (bogs).

Rivers and Lakes
Lakes-Can be zoned like the ocean.
   The littoral zone shallow and near the shore, the limnetic zone is on the surface farther from shore.  Lakes also have a benthic zone.
Rivers and Streams-support different biological communities than lakes, partially because the nutrient content of a lake is determined by the terrain and vegetation through which it flows.  Streams tend to be well oxygenated via the action of moving water
 
 

Reading: Freeman, Chapter 48
and Chapter 2 in Murray and Nyberg

Most species have a geographic range
A geographic range describes where a species may be found.
In the United states, most species have a range of 4-24 states.
Cosmopolitan species are an exception, they are worldwide in distribution.
Endemic species are found in only a small, restricted area.

No species are truly ubiquitous, in  the sense that all species are restricted to a particular habitat.
Suitable habitats tend to be clustered within the geographic range of a population, therefore, most species are composed of discontinuous groups called populations.
Clearly, the distinction between populations can be somewhat subjective.
What constitutes a population depends upon the species in question

Populations are groups of individuals of the same species living in the same place
Individuals within a population occupy the same general area, rely on the same resources, and are influenced by the same general environmental conditions.
Most of the interaction, including sexual reproduction, between individuals of a species is among members of the same population.

Example
A grassland in North Dakota may support a single population of Buffalo, many separate populations of prairie dogs, and hundreds of populations of frogs.  Each population of frogs would be localized in a patch of wetland called a prairie sinkhole.  Likewise, each prairie dog town would support its own population of fleas.  There might be several streams bisecting the grassland, each would support an independent population of fish, such as the bullhead catfish, and several islolated populations of water loving (hydric) plants.
Some populations move
A single population of salmon may spawn upstream in a river of the pacific northwest, and return to the ocean to breed.   Most of the interaction between individuals and exchange of alleles, occurs among individuals nesting in the same stream.  Likewise, a population of monarch butterflies may migrate, en masse, to over-wintering grounds in Mexico for the winter, returning to the Midwest to reproduce during the summer.

Populations have certain emergent properties
These properties are consequences of the way an organism interacts with the environment, and with other organisms, and influence its evolution.
Size
Density
Patterns of Dispersion
Age Structure
Spatial Structure
Sex Ratio
Variability

Size
Simply the number of individuals in the population at any given time.  Sometimes called abundance.

Density
The number of individuals in the population per unit area or unit volume.

Example Problem
There are 10,400 mice living in a 1000m x 1000m field.  What is the density of this population?

Patterns of Dispersion
Populations follow into three different patterns of dispersion, generally.

Clumped
Regular
Random

Clumped
This is the most common pattern of distribution,  it occurs when individuals aggregate into patches.
Sometimes clumping occurs because some areas of habitat are more suitable than others
i.e., Plethodon sp. salamanders are found clumped under fallen logs in the forest
the night lizard Xantusia sp. is found clumped within fallen Joshua trees in the Mojave desert
Sometimes species clump for other reasons.
Plants often clump because their seeds fall close to the parent plant or because their seeds only germinate in certain environments.  Impatiens capensis seeds are heavy and usually fall close to the parent plant-this species grows in dense stands.
Species may clump for safety, or social reasons.  Ground nesting bees Halactus sp. prefer to nest in the presence of other bees, forming aggregations of solitary nests

Random distribution
This pattern occurs in the absence of strong attraction or repulsion among individuals.
 It is uncommon.
The trees of some forest species are randomly distributed within areas of suitable habitat.
For example, fig trees in the amazon rain forest.  This random distribution might be due to seed dispersal by bats.
Regular Distribution
This generally happens because of interactions between individuals in the population.
Competition:  Creosote bushes in the Mojave desert are uniformly distributed because competition for water among the root systems of different plants prohibits the establishment of individuals that are too close to others.
Territoriality:  The desert lizard Uta sp. maintains somewhat regular distribution via fighting and territorial behavior
Human Intervention:  I.e., the spacing of crops.

Spatial Structure
The scale matters a great deal in describing the spatial distribution of a species.
A species may be clumped on the large scale, but evenly distributed on a finer scale.
Example:  Ground nesting wasps,  Sphex sp. are clustered in areas of suitable nesting substrate (packed sand).  Within these areas, their nests are evenly distributed because of aggressive interactions.
.  Alternately, a species may be clumped at both spatial scales, or regularly distributed on a large scale, but clumped on a fine scale.
I.e., Pipe-organ mud-daubers cluster their nests in areas of suitable habitat (under bridges and culverts).  Within these areas, their nests are often clustered, because individuals prefer to build nests where other individuals have successfully built nests
I.e., Ant colonies, Pogomyrmex sp., are evenly distributed in the Arizona desert.  Each colony contains thousands of individuals.

Age structure
This is the relative number of individuals at different ages.

Sex ratio
Sex ratio is the proportion of individuals of each sex.  The number of females is more important in the overall growth rate of populations
Examples: elk; fewer males of reproductive age than females; males breed with more than one female.
Wasps: Melittobia sp. may have as many as a hundred females per male.  These males never leave the nest and mate with their sisters.  Population growth is essentially independent of the number of males.
Scale is important in determining how organisms respond to their environment.
A habitat is called course grained if the scale of variation in the environment is large compared to the size of individuals.
A habitat is called fine grained if the scale of environmental variation is small compared to the movements of individuals.
I.e.,  to a foraging butterfly, a field of wildflowers would be course grained      .. to a zebra, the same field would be fine grained.

Variability is differences among individuals in the population
.Most populations show differences among individuals.
Some variation has a genetic basis.
Other variation is largely environmental.
In many cases, variability is caused by both genes and the environment.
Sexual Dimorphism is when the two sexes differ greatly in appearance.
Metamorphosis is when individuals differ in appearance as they age.

Short Discussion
Is the human species composed of a single population?, or many smaller populations?

How are we distributed?

Are we variable?  What biological purpose might this variation serve?
It is a fundamental characteristic of living things that all organisms reproduce.

Every species is capable of population growth under some set of possible conditions

Arithmetic Growth
Imagine a species where all the births occur at once (natality).
All the deaths occur in the interval before the births (mortality).
 In the same interval, individuals can leave the population by immigration, and enter by emmigration.
This is called arithmetic growth.
Some species exhibit this kind of growth, i.e., annual grasses and grasshoppers.

Demographic Processes
Birth (Natality)
Death (Mortality)
Immigration
Emmigration

Where:
N(t+1)=populalation at time (t+1)
N(t)=population at time t
Where B=Number of Births (natality)
            D=Number of Deaths (mortality)
            E=Number of Emmigrants
            I=Number of Immigrants
 

Example Problem:
A population of field mice, Peromyscus sp.  Consists of 371 indiviuduals at the start of 1999.
That year, 115 individuals die, 201 are born, 37 immigrate and 75 emmigrate.

What is the population at the start of 2000?

 l is an arithmetic growth parameter, it describes the amount of population growth per generation

N(t+1)/N(t)=l

if l =1 the population is constant, if l is<1 if the population is decreasing,if l > 1 if the population is increasing

N(t)=N(0)lt

t is the elapsed time in generations
proof:
N(t)=lN(t-1)
N(t)=l * l N(t-2)
N(t)=l * l* lN(t-3) etc.

Exponential growth
In exponential growth models, births deaths, emmigration and immigration take place continuously
this is a better approximation for the growth of most biological populations
I.e., human populations grow exponentially when resources are plentiful
 

DN/DT=bN-dN
Where:
b is the per capita birth rate
d is the per capita death rate
ignoring immigration and emmigration.
dN/dT=rN (define r as the instantaneous population growth rate; r=b-d)
you can integrate this to get the exponential growth formula that follows...

Exponential growth formula

where r is the exponential growth parameter
N₀ is the starting population
t is the time elapsed
r=0 if the population is constant, r>0 if population is increasing, r<0 if the population is decreasing.

Example problem
The human population of the earth is growing at approximately 1.8%per year.
The population at the start of 2001 was approximately 6 billion.
If nothing were to slow the rate of population growth, what would the population be in the year 2101?

Limits to Growth
No population can continue growing forever.
Even organism that reproduce very slowly, such as elephants, rhinos, whales, and humans, would outstrip their resources if they reproduced indefinitely.

Carrying Capacity
Populations grow until one or several limiting resources become rare enough to inhibit reproduction so that the population no longer grows.
The limiting resource can be light, water, nesting sites, prey, nutrients or other factors.
Eventually, every population reaches its carrying capacity, this is the maximum number of individuals a given environment can sustain.

Logistic Growth Model
The logistic growth model accounts for carrying capacity.
K= Carrying Capacity, this is the maximum number of individuals that the population can sustain.
N=The Number of individuals in the population at a given time
rmaxis the maximum population growth rate

Question:  What is dN/dT when N=K?
Answer
When N=K, dN/dT=0
Likewise, when N is small,
dN/dT =approximately r_max
When NK the population declines.

How Well Does the Logistic Model Fit Actual Populations?
For laboratory populations of paramecia, crustaceans, etc.., the logistic provides a pretty good fit.
For actual populations, the logistic does not provide such a good fit.
There are usually other factors involved.
One factor: lag time is the time it takes between reaching carrying capacity and the slowdown in reproduction.