Ecology
Niches and Distributions: Chapters 4 and 6
KEY CONCEPTS
All species have a limited geographical range, and the goal is to discover what causes these limits
Transplant experiments can help to identify the potential range of species
Shelford’s law of tolerance can be used to define the critical environmental limits to survival and reproduction and this the potential of geographical range for a species
The tolerance ranges of species can change via natural selection
Temperature and moisture are the main limiting factors for both plants and animals on a global scale
Light, fire, pH, and other factors can limit distributions
Species may evolve adaptations that overcome the limitations set by physical and chemical factors
Some of these adaptations may allow a species to extend its geographical range
Climatic warming in this century will have major impacts on the geographical ranges of species that are currently limited by temperature and moisture
KEY TERMS
Control: a treatment or plot in which nothing is changed so that it serves as a baseline for comparison with the experimental treatments
Dispersal: The movement of individuals away from their place of birth or hatching or seed production into a new habitat or area to survive and reproduce
Habitat selection: the behavioral actions of organisms in choosing the areas in which they live and breed
Liebig’s law of the minimum: the generalization first states by Liebig that the rate of any biological process is limited by the factor in least amount relative to requirements, so there is a single limiting factor
Shelford’s law of tolerance: the ecological rule first described by shelford that the geographical distribution of a species will be controlled by that environmental factor for which the organism has the narrowest range of tolerance
LECTURE
Distributions can be set by abiotic physical factors (temp moisture, salinity, light) and biotic factors such as competition
How to measure distributions:
Niche: range of conditions and resources within which a species can persist indefinitely (excludes factors such as temp, salinity, resources, etc)
Grinnell defined a niche as the habitat a species lives in
Charles Elton (1927) described a niche as a place in the biotic environment
Fundamental Niche: the ecological space occupied by a species in the absence of competition and other biotic interactions from other species
Realized Niche: the observed resource use of a species in the presence of competition and other biotic interactions; contrast with fundamental niche
species distributions can be graphed along environmental gradients
examples of gradients:
Each gradient represents a “niche axis”
how many niche axes are there?
Dimensions of the Niche:
Look at how one factor affects a populations growth rate (lambda)
niche = range of conditions where lambda is greater than one
two dimensional niche: more than one factor being looked at simultaneously (temp and salinity)
Competition and Niches:
The amount of niche overlap determines the degree and intensity of competition
not too much overlap limits to population growth set by intraspecific competition
much overlap limits to the population growth set by interspecific competition, competitive exclusion is possible in ecological time, over long period of time niches can shift via natural selection to reduce competition
Direct Influences/Physical factors:
In general, youn organisms are often more sensitive to environmental extremes than adults, adults may be able to survive but not reproduce if beyond their natural range
Northern rock barnacles: all live where the winter temp is 8 degrees C or lower, reproduction depends on temp, barnacle gonads only develop with 20 days of cold temps at minimum
Eastern Phoebe: winter range is set by minimum January temp (4 degrees C), max metabolic rate is 2.5 x the base rate, and at 4 degrees the bird runs at 2.5 just to keep warm
Rattlesnakes in Pa and NJ, northern limit set by the number of warm days in the summer, young will fail to develop properly and die is the mom is not warm enough for long enough
Indirect Influences:
temperature affects precipitation and evaporation, which affects soil moisture, which affects tree distribution, which affects outcome of chipmunk competition, which affects chipmunk distributions
Chipmunks:
Cliff chipmunk lives below 2100m and Uinta chipmunk lives above 2100-2000m
there is a 20 deg C temp gradient
On the pilot mountains:
No uinta chipmunks
cliff chipmunks cover the entire mountains up to 3000m
On the ruby mountains:
No cliff chipmunks
Uinta chipmunks cover the entire range
High mountains have dense trees, low has sparse shrubs
cliff chipmunk is aggressive but stays on ground, chases uinta away
high up, the uintas can go into the trees
the cliff chipmunk spends so much time chasing uintas, it doesn’t have enough time to gather enough food
Physiology and Ecology:
When limits to distribution are set by direct physical factors, organisms are at the limits set by their physiology
Examples: temp tolerance, energy requirements, photosynthetic rate, drought tolerance
Adaptations:
Are genetically influenced traits that enhance the fitness of an individual organism under a particular set of limiting factors
can expand the distribution of a species by allowing it to survive in more marginal conditions
arise by natural selection
can be physiological: freezing tolerance, hibernation, shade tolerance
can be physical: lungs, a thick waxy cuticle, spines, aerobic respiration
can be behavioral: genetic memory (instinct)
Acclimation:
Certain roadside weeds (agrostis tennis) had evolved a high resistance to lead pollution
This genotype was only adaptive within 2 meters of the highway
Transplants:
Useful in determining the limits of potential distributions
Beech bark disease ex:
Fungal disease, nectria spp
native to Europe and asia
arrived in Halifax Nova Scotia in the 1890s on a shipment of ornamental European beech trees
now parasitizes and kills American beech (fagus grandifolia)
dispersal is via the beech scale insect cryptococcus fagi which feeds on the bark of the trees
moved 30km per year on average
north woods of Maine, adirondacks of NY, west of Michigan, south of Tennessee
effects: 37% beech nut production loss, ripple effect on animals that feed on beech nuts
Continental Patterns:
Latitude: temperature, wind patterns, seasonality
Mountains: rain shadow effect, temperature, precipitation, and elevation
coasts and large lakes: moderate temp extremes, precipitation events
angle of the sun importance:
Causes differences in temperature, which creates wind and ocean currents
wind over the oceans picks up moisture, which becomes precipitation over land
salinity gradients in the ocean also drive ocean currents, solubility of salt in water changes with temp
greenhouse effect (CO2, CH4)
Evapo-Transpiration:
Combines evaporation from the soil and other abiotic surfaces with transpiration from plant leaves
potential (PET): depends on temperature
actual (AET): depends on both temp and precipitation, tied strongly with plant productivity
C3 vs C4 plants:
Success depends on climate
C3 plants
“Normal pathway for photosynthesis
evolved first
uses enzyme RUBP Carboxylase (RUBISCO) to fix CO2 from the atmosphere
RUBISCO is inefficient at high temperatures
Common C3s include trees, shrubs, wildflowers, some grasses
C4 plants
More efficient method of fixing CO2 in hot dry areas
uses enzyme PEP catboxylase instead of RUBISCO to fix CO2 out of the air
common C4s include many grasses, corn, pineapple
Local Microclimates
Local microclimates may favor different species with different tolerances and adaptations
Hills:
S & W: warmer, sunnier
N & E: colder, shadier
windward side: colder, more wind
peaks: colder in the day, warmer at night
valleys: warmer in the day, colder at night
Open fields:
Warmer days, colder nights in summer
low humidity
higher snow cover in winter
faster wind velocities
high direct sunlight
Closed Forests:
Moderate temps, warmer in winter
high humidity
low snow cover
low wind velocities
diffuse light and sun flecks
Climate Change:
Upper limits of species/community distributions are set by climate
conern because many species have evolved for specific climates and microclimates
climates have changed in the past but slowly
change is occurring faster than many plant species can evolve or adapt, their pollinators and seed dispersal animals must also be able to adapt or move with them
EX: forest have moved 70m up since 1945, beech is being squeezed from below by holm oak
Historical Effects:
Also set species distributions
species pool in any given location is a function of the species that have evolved in a certain place, minus the species that have gone extinct, plus the species that have dispersed in (S = Ev - Ex + Im - Em)
LIFE HISTORY AND COMPETITION: CHAPTERS 5 AND 10:
KEY CONCEPTS:
Some species do not ibhabit an area because they have not yet been able to disperse there. Dispersal limitation can be tested by transplant experiments
Global distributions are often limited by barriers that block dispersal. On a local scale adaptations for dispersal are common and few species are limited in distribution by a failure to disperse
Animal species may be limited in their geographic distribution by selecting a range of habitats that is more restricted than the range they could occupy successfully
The presence of other organisms—predators, parasites, pathogens, or competitors—may limit the geographic distributions of many species
Predator limitations on prey distributions often operate on a local scale. Diseases and parasites may affect geographical distributions
Competition between species can result from exploitation of resources that are in short supply or from interference in gaining access to needed resources
competition between species can be analyzed with simple mathematical models based on the logistic growth equation
competition is common in natural populations of plants and animals, and is particularly stron among herbivores
in natural populations, competition over evolutionary time leads to niche differentiation, observed as character displacement, which acts to minimize competition between species
to understand the effects of competition we need to study the mechanisms by which it operates and the resources that are being utilized
KEY TERMS:
Allelopathy: organisms that alter the surrounding chemical environment in such a way as to prevent other species from using it
fitness: ability of a particular genotype or phenotype to leave descendants in future generations, relative to other organisms
ideal despotic distribution: a theoretical spatial spread of members of a population in which the competitive dominant individuals take up the best resources or territories and the less competitive individuals take up areas or resources in direct relationship to their dominance status
ideal free distribution: a theoretical spatial spread of members of a population in which individuals take up areas with equal amounts of resources in relation to their needs, so all individuals do equally well
reids paradox: the observed large discrepancy between the rapid rate of movement of trees recolonizing areas at the end of the ice age and the observed slow dispersal rate of tree sets spreading by diffusion
tens rule: the rule of thumb that 1 species in 10 alien species imported into a country becomes introduced, 1 in 10 of the introduced species becomes established, and 1 in 10 of the established species becomes a pest
character displacement: the divergence in morphology between similar species in the region where the species both occur, but this divergence is reduced or lost in regions where the species’ distributions do not overlap; presumed to be caused by predation
Gause’s hypothesis: complete competitors cannot coexist, also called the competitive exclusion principle
Lotka-Volterra equations: set of equations that describe competition between organisms for food or space; another set of equations described predator-prey interactions
r-selection: the type of natural selection experienced by populations that are undergoing rapid population increase in a relatively empty environment
LECTURE
Life History:
Life history traits:
Describes the species’ own characteristics
a set of co-adapted traits designed by natural selection to solve particular ecological problems (stearns)
idea of trade-offs
energy allocation
evolution is evolved
most important ecological problem: reproduction
a species’ life history traits should maximize reproduction
R - K theory:
Pianka (1970)
describes a continuum of life histories
idea of trade-offs
selected for by the environment
r selection:
Climate: variable/uncertain
mortality: catastrophic density-independent
survivorship: type 3
pop size: variable over time and below k
competition: weak
selection favors: rapid development, high rmax, early reproduction, small body size, many offspring
lifespan: short
leads to: productivity
stage in succession: early
K selection:
Climate: constant/predictable
mortality: density-dependent
survivorship: type 1 or type 2
pop size: stable over time, near k
competition: strong
selection favors: slow development, competitive ability, delayed reproduction, large body size, few offspring
lifespan: long
leads to: efficiency
stage in succession: late climax
C-S-R Theory for plants:
Three way trade off:
competitive ability
stress tolerance
reproduction
Competition:
As a resource is reduced, it limits reproduction of both species A and species B
Species A and/or species B actively block each others access to the resource
one species is almost always better at either gathering resources when resources are scarce or being more efficient with their resources when scarce
competitive exclusion is possible if resources are drawn down to low levels. The better competitor may cause competitive exclusion because…
2 mechanisms of competition:
Exploitative: 2 fish feeding on zooplankton, diurnal hawks and nocturnal owls feeding on mice
interference: chemical competition (allelopathy), overgrowth competition, territoriality, encounter competition
Escape negative effects of competition:
Select a different habitat
select a different food
Reciprocal Interaction:
Competition is a reciprocal interaction
if two individuals use the same resource, both suffer negative effects because of its depletion
intraspecific competition:
same species
individuals deplete resource abundance
this causes feedback, reducing the population growth rate (lambda)
birth and death rates are affected
Characteristics favored by natural selection when intraspecific competition is strong:
Evolution of a lower R*
delayed reproduction, lower allocation to current reproduction
interference competition methods
territoriality
interspecific competition:
different species
influences distribution and abundance of species
Cattails:
Grace and Wetzel studied competition of two species of cattails around a point in Michigan, both were capable of living across the gradient of water depth at the edge of the pond, when grown together they restrict each others niche breadth and biomass production
Density Dependence:
Birth rate is negatively density-dependent
death rate is positively density-dependent
most relationships between N and population growth rates are not linear
real life examples: birth rates decline precipitously at a very high N (young per female in cattle populations, young per female in breeding sparrow populations)
R *
Is the level of resources that results in: zero population growth
birth rate = death rate
resources may start high, decline to R*
population will start low, increase to K
a species with a lower R* needs less of a resource to grow the same amount
Ecological interaction:
If species A and species B live in the same area and use the same resources:
The R* of each will determine which species will persist and which will be competitively excluded
if species A has the higher R*, it will grow fastest initially but if species B drops its R* then species A will decline and go extinct
David Tilman:
University of minnesota
studied diatoms that have a shell made of silica
2 species of diatoms
Asterionella - R* =1.0
Synedra - R* - 0.4
synedra wins out
Main lessons:
Competition can lead to one species winning and the second species going extinct
some competitive interactions can lead to coexistence
We can understand competitive interactions only by knowing the resources involved and the mechanisms by which species compete
Resource competition model:
Simultaneously considers 2 resources needed by a species to survive and grow, the two can be anything such as nitrogen and water
both resource levels need to be above the R* for the population to be able to grow
a species uses the two resources up at different rates: resource use is represented by a vector
The outcome of competition depends on:
The R*s of both species for each of the resources (determines the species’ ZNGI’s)
SNGI: zero net growth isocline
the consumption vectors of both species for each of the resources
the resource supply vector of the environment
Outcomes by starting zone:
1 - both species die, as neither can exist here
2 - species B can live here but A can’t
6 - species A can live here but B can’t
start in zone 3: the species use up resources, pushing the environment into zone 2, species goes extinct
start in zone 5: the species use up resources, pushing the environment into zone 6, species B goes extinct
zone 4: resource use creates a stable oscillation, so that the environment “circles” and stays in the coexistence zone, both species can survive indefinitely
What’s happening in zone four?
Both species use water, so water declines (drought)
species b is better than A at low water so B increases and A declines
B uses up nitrogen faster than A; so nitrogen declines (soil infertility due to many B) as water recovers (without many A around to use it)
species A is better than B at low nitrogen, so A increases and B declines
water delines (used up fast by A) as nitrogen increases (without many of B around to use it)
this goes on and on with nobody driving the other extinct
Gause:
Russian ecologist
studied parameciums
three species:
Aurelia
bursaria
caudatum
A & C — A wins out even though C reproduces faster
C & B — coexist, but at much lower levels than when grown alone
Why did B & C coexist?
Top of test tube: O2 levels support bacteria for parameciums to eat
Bottom of test tube: low O2 levels supported yeast
resources and conditions in the environment were heterogenous
Gause’s Principle/Rule of competitive exclusion:
2 species can’t coexist indefinitely if they depend on the same resources
2 species could coexist if they depend on different resources
2 species could coexist if they live at different conditions
Joe Connell:
Scottish ecologist, worked with barnacles
2 species in the intertidal zone; Cthalamus, semibalanus
S adults survive between mean high neap tide and mean low neap tide, larvae settle everywhere, is better at exploiting prey than C, S can pry C off the rock
C adults can survive between mean high neap tide and mean high spring tide, larvae do not occur below the mean low neap tide
When S is removed, C can increase its range down into the ocean therefore it must have been competitively excluded from lower areas
competition is asymmetrical; the effect on C is greater than the effect on S, S must pay for its greater competitive ability with energy allocation
S is not as good at surviving warming and drying at low tide like C is
How often does competition really occur in nature?
Two theories:
Competition is ubiquitous and powerful, affecting and organizing almost every natural system (i.e., Dave tilman)
competition exists, but is only important in the absence of other natural forces that normally overwhelm it, such as disturbances
evidence supports both
PREDATOR AND PREY: CHAPTERS 11 AND 12
KEY CONCEPTS:
Predator-prey interactions can be analyzed with simple models for one predator-one prey systems
simple models of predation often lead to predator-prey cycles rather than a stable equilibrium
laboratory systems rarely lead to stable interactions between predators and prey, but they show the importance of prey refuges and spatial heterogeneity
predation can be broken down into components-numerical, functional, developmental, and aggregation responses of predators to prey-to aid our understanding of the predation process
multiple predator-multiple prey systems lead to more complex dynamics, and show the importance of predation to the evolution of escape behavior and warning coloration of animals
the world being green implies that herbivores are prevented from completely destroying their food sources, either by their own behavior, by their enemies, or by plant defense strategies
the resource availability hypothesis predicts that plants growing slowly in poor habitats should invest most in plant defense because they have the most to lose from herbivory
herbivores may not achieve a stable interaction with their food plants, and many ungulates undergo irruptions with subsequent oscillations in numbers
models of predator-prey dynamics can be applied to grazing systems to determine the kinds of plant-herbivore interactions that might lead to stability
not all plant-herbivore interactions are detrimental to plants. Mycorrhizal fungi grow on most plant roots to the advantage of both the plants and the fungi. Many mutualistic interactions have evolved in which both plants and the herbivores gain from their association with
KEY TERMS:
Aposematic: warning coloration, indicating to a predator that this prey is poisonous or highly defended against attack
coevolution: the mutual evolutionary influence between two species; each party in a coevolutionary relationship exerts selective pressures on the other, thereby affecting each others’ evolution, back and forth
environmental heterogeneity: variation in space in any environmental parameter such as soil pH or tree cover
functional response: the change in the intake rate of a predator in relation to the density of its prey species
generalist predators: predators that eat a great variety of prey species
handling time: the time utilized by a predator to consume an individual prey item
numerical response: the change in numbers or density of a predator in relation to changes in the density or its prey species
optimal foraging theory: a detailed model of how animals should forage to maximize their fitness
prey isocline: the contour line of densities of predator and prey at which the prey are in equilibrium; the impact of a predator exactly balances the preys rate of population growth, so the prey population growth rate is zero
safe sites: for animals, sites where prey individuals are able to avoid predation; for plants, sites where seeds can germinate and plants can grow
grazing facilitation: the process of one herbivore creating attractive feeding conditions for another herbivore so there is a benefit provided to the second herbivore
inducible defenses: plant defense methods that are called into action once herbivore attack occurs and are nearly absent during period of no herbivory
mutualism: a relationship between two organisms of different species that benefits both and harms neither
mycorrhizae: a mutually beneficial association of a fungus and the roots of a plant in which the plants mineral absorption is enhanced and the fungus obtains nutrients from the plant
optimal defense hypothesis: the idea that plants allocate defenses against herbivores in a manner that maximizes individual plant fitness, and that defenses are costly to produce
overcompensation hypothesis: the idea that a small amount of grazing will increase plant growth and fitness rather than cause harm to the plant
plant stress hypothesis: the idea that the herbivores prefer to attack stressed plants, which produce leaves that are higher in nitrogen
plant vigor hypothesis: the idea that herbivores prefer to attack fast-growing, vigorous plants rather than slow-growing, stressed plants
resource availability hypothesis: a theory of plant defense that predicts higher plant growth rates will result in less investment in defensive chemicals and structures
secondary plant substances: chemicals produced by plants that are not directly involved in the primary metabolic pathways and whose main function is to repel herbivores
LECTURE:
Disturbance:
Density-dependent mortality
likelihood of death is set by the event, not by the density of the population
can keep a top competitor (lowest R*) from winning out by lowering its population size
examples: windfall or fire gaps in a forest, storm surges in rocky intertidal shore communities
Predation:
Can be a density-independent or dependent process
if density-dependent, the more prey, more more predators, and the better they keep the prey in check
predator N affects prey death rate
prey N affects predator birth rate
classification of predators:
True predator
grazer (herbivores)
parasites
parasitoid
factors controlling the density of predators
Prey switching: when one prey gets rate, the predator will eat something else
territoriality: predators limit themselves from becoming too numerous
predators may not even control prey densities at all
conditions allowing for coexistence:
Non-interactive predator-prey relationship
one or both species is limited by something else
presence of refuges
predators can't eat all of the prey (predator satiation)
predator kill efficiency is low
cant kill all the prey as prey become more abundant
cant find prey well at low prey densities
functional responses:
Type 1: constant catching efficiency
type 2: predator satiation at high prey densities
type 3: combination of one and two
Type one is not that common in nature, two and three are more common
evolution and predation:
Natural selection favors traits in prey that lower predation efficiency
long legs in ungulates
taking care of eggs
breakaway tail in salamanders
good camouflage
be obnoxious or deadly and advertise it
predator evolution:
Natural selection increases predation efficiency
no “prudent predators” that voluntarily reduce their predation pressure at low prey densities
cryptic coloration
better prey catching mechanisms
arms race:
Prey must win or else they would be extinct
natural selection is more effective on prey- if prey loses, it dies. If a predator loses, it just skips a meal
”life-dinner principle”
modeling predator-prey systems:
You can overlap the growth rate of the population of prey with the functional response of the predator
if predation rate is greater than prey population growth rate, then the prey decline
if prey population growth rate is greater than predation rate, then the prey increase
at places where the lines cross, we have equilibrium, where both populations stay the same
optimal foraging theory:
The choice of what to eat is determined by the abundance of preferred prey
profitability = energy value/handling time
handling time becomes larger at low prey density
REGULATION OF POPULATION SIZE: CHAPTER 14
KEY CONCEPTS:
Two questions are central to population dynamics: what stops population growth and what determines average abundance
to stop population growth, natality, mortality, or movement rates much change with population density. Population regulation requires density dependence
biotic agents such as predators and diseases can limit or regulate populations, as can climatic and physical factors
individual differences in physiology, genetics, or behavior can limit or regulate populations through intraspecific competition for resources
KEY TERMS:
Allee effects: population growth rates that decrease below replacement level at low population density, potentially leading to extinction
balance of nature: the belief that natural populations and communities exist in a stable equilibrium and maintain that equilibrium in the absence of human interference
density-dependent rate: as population density rises, births or immigration decrease or deaths or emigration increase, and consequently a graph of population density versus the rate will have a positive or negative slope
density-independent rate: as population density rises, the rate does not change in any systematic manner, so that a graph of population density versus the rate will have a slope of zero
limiting factor: a factor is defined as limiting if a change in the factor produces a change in average or equilibrium density
metapopulations: local populations in patches that are linked together by dispersal among the patches, driven by colonization and extinction dynamics
regulating factor: a factor is defined as potentially regulating if the percentage of mortality caused by the factor increases with population density or if per capita reproductive rate decreases with population density
self-thinning rule: the prediction that the regression of organism size versus population density has a slope of -1.5 for plants and animals that have plastic growth rates and variable adult size
sink populations: local populations in which the rates of production is below replacement level so that extinction is inevitable without a source of immigrants
source populations: local populations in which the rate of production exceeds replacement so that individuals emigrate to surrounding populations
LECTURE:
What controls populations?
Carrying capacity of the environment
competition for limited resources
predators, parasites, disease
disturbance and environmental stress/fluctuation
Fundamental observations:
Abundance varies from place to place
some habitats are “good” in supporting many individuals and some are “poor” in supporting fewer individuals
no population goes on increasing without a limit
Closed system:
A population in a closed system will increase until the per capita birth rate = the per capita death rate
b = d
b + d = 0
lambda = 1
changing the slope of d changes the equilibrium population density, same would be true of b
No closed population stops increasing unless:
Birth rate is density dependent
death rate is density dependent
both are Krebs first principle of population regulation
Comparing Populations:
Differences between two populations in equilibrium density can be caused by variation in either:
Density-dependent per capita birth and death rates
density-independent per capita births and death rates
these are krebs 2nd principle of population regulation
Limiting factor:
Change in the factor changes equilibrium density
ex: deer population is higher when disease is absent
potential regulating factor:
Percentage mortality increases with population density (or reproduction decreases)
ex:more losses to disease as deer density increases
Compensation:
Change in one factor produces the opposite change of identical magnitude in another factor
with their combined effects the population remains unchanged
intrinsic factors:
Internal to a population
affect population by interactions between individuals in it
extrinsic factors:
Affect a population by the action of other species, outside factors such as climate, nutrients, etc
(In)stability: (in order of decreasing stability)
Stable equilibrium:
Biotic coupling
compensation
resource limitation
density dependence
tight patterns
few stochastic effects
Biotic instability:
Overconnectedness
strong interactions
competitive exclusion
overexploitation
limit cycles, chaos
Stochastic domination:
Biotic decoupling
species dependence
abiotic limitation
density independence
large stochastic effects
loose patterns
Allee effects:
Small populations can suffer reduced population growth rates
complicates the simple density-dependent model of population growth
ex: shearwaters in New Zealand, passenger pigeons in US