Semester 2 Flashcards
What are the 5 concepts of ecology?
- Ecological systems exist in a hierarchical organisation
- Ecological systems are governed by physical and biological principles
- Different roles organisms play in ecological systems
- Scientists use several approaches to studying ecology
- Humans influence ecological systems
What is ecology?
Study of how organisms interact with
Very broad topic
Various levels - individual to global
Helps us understand how the world works
What are ecological systems?
Biological entities that have their own internal processes and interact with their external surroundings
Population: population dynamics > ten unit of evolution
Individual: survival and reproduction > the unit of natural selection
Community: interactions among species
Ecosystem: flow of energy and matter
Biosphere - global processes
What is a species?
A group of organisms that can reproduce naturally with one another and create FERTILE offspring
Studying ecology at different levels
Individual approach: understands how adaptations or characteristics of an individuals behaviour, morphology and physiology enable it to survive in an environment
Population approach: examines variation in the number, density and composition of individuals over time and space
Community approach: understands the diversity and interactions of organisms living together in the same place
Ecosystem approach: describes the storage and transfer of energy and matter
Biosphere approach: examines the movements of energy and chemicals over the earths surface
What are the governing principles of ecology?
First law of thermodynamics - matter and energy cannot be created or destroyed but can change form (law of conservation of matter)
A dynamic steady state - occurs when gains and losses are in balance. Behaviour affects ecology
What is natural selection?
Differential survival and reproduction of individuals that possess certain phenotypes
What is evolution?
A change in the frequency of genes / characteristics in a population over generations
Individuals with better fitness will pass more copies of their genes to the next generation and that phenotype will come to dominate
Types of species interactions
Interactions that provide a benefit to a species are indicated by a ‘+’ symbol
Interactions that cause harm to a species are indicated by ‘-‘ symbol
Interactions that have NO effect on a species are indicated by a ‘0’ symbol
What is a habitat ?
The place or physical setting where an organism lives.
Distinguished by physical features such as dominant plant type
Habitat types overlap and absolute distinctions rarely exist
Examples:
Freshwater, marine, coastal, streams, forests, deserts, grasslands
Habitats and niches
Unique phenotypes: if not then extinction of a species!
Example: different insects like to feed on different crop species that may be growing in the same field
The scientific method
Hypotheses: ideas that potentially explain a repeated observation
Proximate hypotheses ‘how’: address the cause of immediate changes in individual phenotypes or interactions
Ultimate hypotheses ‘why’: address the fitness costs and benefits of a response. Behavioural ecology
Predictions: statements that arise logically from hypotheses
Manipulative experiments
Where a hypotheses is tested by altering factor hypothesised to be the cause of a phenomenon
Treatment: the factor that we want to manipulate in a study
Control: a treatment that includes all aspects of an experiment except the factor of interest
Example - researchers want to test if birds are an important factor in determining the number of insects on oak trees. They manipulate the presence of birds by placing cages around oak trees. Some trees were left uncaged as controls
Manipulative experiments
Experimental unit: the object to which we apply a manipulation
Replication: being able to produce a similar outcome multiple times (ie the number of experimental units per treatment)
Randomisation: a requirement for manipulation experiments, every experimental unit must have an equal chance of being assigned to a particular treatment
Experimental units may be natural (lakes) or artificial (microcosms) and may vary in size by several orders of magnitude
Alternative types of experiments
Natural experiments: an approach to hypothesis testing that relies on natural variation in the environment to test a hypothesis
Mathematical methods: representations of a system with a set of equations that correspond to hypothesised relationships among the systems components
Ecologists often test mathematic models using natural or manipulative experiments
What is the impact of humans as a species
8 billion - 15 November 2022 was predicted to be the day that the global population reaches 8 billion
Each year 78 million + added, greater than population of UK and 2x Ireland combined
How do humans influence everything
2% of remaining primary rain forest lost per year
50% of usable land used for agriculture
Semi arid subtropical regions turned to desert by overgrazing and firewood collection
Majority of fish stocks have collapsed
Climate change resulting from fossil fuel use
Humans use 20% more renewables than are actually renewed
6th great extinction
Passenger pigeon
Perhaps one of the greatest declines in population size
6 billion to none in 100 years, last died in 1914
The Allee effect - unpredicted effect of low densities
Terrestrial biomes
Are categorised by their major plant growth forms
Biomes > classified based on temperature and rainfall
Cold, wet are rare
There is often an association between the plant forms in a biome and the animal forms that live there
Boundaries between biomes can be unclear
Terrestrial biomes
There are 9 biomes within 3 temp ranges:
<5 degrees
5 degrees - 20 degrees
> 20 degrees
Climate diagrams
Graphs that plot the average monthly temperature and precipitation of a specific location on earth
Growing season > months that are warm enough to allow plant growth ie temps > 0 degrees > shaded regions in diagram
Plant growth is constrained by temperature
When precipitation line is ABOVE temp line, plant growth is limited by temp.
When line is BELOW temp line, plant growth is LIMITED by precipitation
Terrestrial biomes
There are 9 categories of terrestrial biomes
Tundras
The COLDEST biome, treeless expanse above permanently frozen soil (permafrost)
Upper soils thaw during brief summer growing season
Dry > precipitation is < 600mm
Extreme tolerators > soils are acidic and nutrient poor
Plants grow low to the ground to gain protection under snow and ice
Boreal forests
Dominated by evergreen needle leaves trees with a short growing season and severe winters
Temps are <5 degrees and low rainfall
Litter decomposes slowly and accumulated forming the LARGEST reservoir of organic carbon on earth
Soils are acidic and podsolised
Species diversity is LOW but the biome is a major source of timber and paper
Temperate rainforests
A biome known for mild temperatures and abundant precipitation and dominated by evergreen forests
Warmer conditions are due to nearby warm ocean currents
These forests typically support low species diversity
Temperate seasonal rainforests
A biome with moderate temp and precipitation conditions, dominated by deciduous trees eg maple, beech, oak
Conditions fluctuate because forests are NOT near warm ocean currents
Precipitation exceeds transpiration
Soils are podsolised, slightly acidic and support a layer of small plants beneath the dominant trees
Warmer and drier parts of the biome are dominated by pines
Woodlands / Shrub lands
A biome characterised by hot, dry summers and mild, wet winters
Combination that favours the growth of drought tolerant grasses and shrubs
There is a 12 month growing season but dry summers, cold winters and frequent fires limit plant growth
Dominated by Schlerophyllous vegetation which had small durable leaves that resist dessication
Temperate grasslands / cold deserts
A biome characterised by hot, dry summers and cold winters
Dominated by grasses, non woody flowering plants and drought adapted shrubs
Soils nutrient rich with lots of organic matter
Unproductive, cold deserts occur when precipitation <250mm
Tropical rainforests
A warm and Rainy (at least 2000mm annually) biome with multiple layers of lush vegetation
There is a canopy of 30-40 m trees with an understory containing smaller trees, shrubs, epiphytes and vines
Species diversity is higher than anywhere else in the world!
Organic matter decomposes quickly and vegetation rapidly takes up nutrients
Soils are devoid of humus and clay and retain nutrients very poorly
Tropical seasonal forests / Savannas
A biome with warm temps and pronounced wet and dry seasons > due to movement of the inter tropical convergence zone
Dominated by deciduous trees that shed leaves during the dry season
Savannas have long dry periods and contain grasses and occasional trees
Fire and grazing Maintain Savannas
Soils do NOT hold nutrients but the warm climate favours rapid decomposition and fast growth
Sub tropical deserts
A biome characterised by hot temps, scarce rainfall, long growing seasons and sparse vegetation
Soils are shallow and devoid of organic matter and neutral ph
Moister sites support succulent cacti, shrubs and small trees eg mesquite and paloverde
Global wind circulation
Inter tropical convergence zone
1> Hadley cell
2> Ferrel cell
3> polar cell
Aquatic biomes
Categorised by their flow, depth and salinity
Streams and rivers
Lotic > refers to flowing water systems
Stream support fewer species than other aquatic biomes
Small streams are limited in primary productivity > why?
Streams and rivers
Riparian zone > terrestrial vegetation alongside rivers and streams that is influenced by seasonal flooding and elevated water tables
Allochthonous > inputs of organic matter such as leaves that come from outside of an ecosystem (ie from a riparian zone)
Autochthonous > inputs of organic matter that are produced by algae and aquatic plants inside an ecosystem
Much of organic matter in streams is allochthonous (introduced) and rivers is autochthonous
Rivers typically accumulate sediments from land and high turbidity can block light and reduce primary production
Influence of dams
Dams are built to control flooding, produce water for irrigation or to generate electricity
Dams alter seasonal cycles of flooding and disrupt the natural movement of aquatic organisms upstream and down stream
Flooding also impacts terrestrial biodiversity
Ponds and lakes
Pond > aquatic biome that is smaller than a lake and is characterised by NON flowing fresh water with some area of water that is too deep for plants to rise above the waters surface
Lake > an aquatic biome that is LARGER than a pond and is characterised by NON flowing fresh water with some areas of water that is too deep for plants to rise above
Circulation in ponds and lakes > seasonal temps alter water density, water becomes more dense as it cools to 4 degrees and LESS dense as it cools below 4 degrees
As surface waters continue to warm during the summer, they gain heat faster than deeper waters and float on the surface
As surface waters cool during autumn they begin to sink
During the winter water less than 4 degrees floats beneath the ice
As surface waters warm during the spring, nutrients on bottom and oxygen on top are cycled
Freshwater wetlands
An aquatic biome containing standing fresh water or soils saturated with fresh water for at least part of the year, shallow enough for emergent vegetation throughout all depths
Wetlands provide > animal habitat > important natural purification systems
Swamps contain emergent trees
Marshes contain emergent NON woody vegetation
Bogs contain acidic water and plants adapted to these conditions
Salt marshes / estuaries
Salt marshes > a saltwater biome that contains NON woody emergent vegetation
Salt marshes are often found at continental coasts and in estuaries where the mouths of rivers mix with salt water from oceans
Estuaries contain abundant nutrients and sediments carried downstream by rivers
This supports extremely high biological productivity
Estuaries are often surrounded by tidal marshes which are some of the most productive habitats on earth
Mangrove swamps
A biome that occurs along tropical and sub tropical coasts and contains salt tolerant trees with roots submerged in water
Salt tolerance is key adaptation of trees in mangrove swamps
Mangrove trees prevent the erosion of shorelines from incoming waves
They provide habitat for many species of fish and shellfish
Inter tidal zones
A biome consisting of the narrow band of coastline between the levels of high tide and low tide
As the tide comes and goes, water exhibits widely fluctuating temps and salt concentrations
Can occur in a variety of areas from rocky coastlines to sloping mudflats
Coral reefs
A marine biome found in warm, shallow waters that are 20 degrees year round
Recent discovery > pristine coral reef 30m (twilight zone) off Tahiti > Nov 2021.
Corals are tiny animals in a mutualistic relationship with algae, corals produce co2 and algae produce sugars
They are hollow tubes with exo skeletons and tentacles that collect detritus and plankton
Corals live in colonies > their exo skeleton contributes to the structure of reefs
Corals reefs - reversing the damage
Rising temps is causing coral bleaching
Hong Kong > artificial reefs 2002
Using sound to repopulate reefs
Life history concepts
Life history traits represent the schedule of an organisms life
Life history traits are shaped by trade offs
Organisms differ in the number of times that they reproduce but they all eventually become senescent
Life histories are sensitive to environmental conditions
Life history
The schedule of an organisms growth, development, reproduction and survival > represents an allocation of limited time and resources to achieve maximum reproductive success
Slow to fast continuum
Variation in one life history trait is often correlated with variation in other life history traits eg. The number of offspring is negatively correlated with the size of offspring
Slow life history > long time to sexual maturity, low numbers of offspring, high parental investment
Fast life history > short time to sexual maturity, high numbers of offspring, little parental investment
Life history traits in plants
Conceptual model> J Philip grime proposed that plant life history depends on stress, competition and the frequency of disturbances
Plants functioning at the extremes of these environmental axes could be categorised as stress tolerators, competitors or ruderals
Life history traits in plants
Stress tolerators eg. Woody lousewort > typically small herbs with a long life span, slow growth and a long time to sexual maturity
Many stress tolerators rely on vegetative reproduction (reproducing from roots and stems) instead of producing costly seeds
Competitors eg. Goldenrod > when conditions are less stressful, grow fast, achieve early sexual maturity and devote little energy to seed production
Ruderals (eg. Weeds such as thistle) grow fast and devote a high proportion of their energy to seed reproduction
The principle of allocation
NO organism possesses the best of all life history traits
Principle of allocation > when resources are devoted to one body structure, physiological function or behaviour they cannot be allotted to another
Trade offs
Organisms face trade offs, when one life history trait is favoured and it prevents the adoption of other advantageous traits
Eg. Trade off between offspring number and offspring survival
Natural selection will favour individuals that allocate their resources in a way that achieves maximum fitness
Optimised life history resolves conflicts between competing demands of survival and reproduction to achieve maximum fitness
Offspring number vs size
Most organisms face a trade off between the number of offspring they can produce and the size of those offspring
The expected trade off is often not observed.
For many organisms the number of offspring can be variable but the size remains relatively constant why?
Offspring number vs parental care
As the number of offspring increases, the parental care per offspring decreases, reducing chances of offspring survival
Depends on environmental conditions eg. Number of daylight hours that parents have to find resources for their offspring
Test for this trade off > manipulate the number of offspring that a parent has
Example: removal of eggs from a magpies nest results in fewer total offspring
Parental care vs parental survival
Having more offspring can stimulate parents to hunt harder for food to feed their offspring
This additional effort can affect the parents fitness
Example: researchers added or removed 2 chicks or did NOT change (control) the number of kestrel eggs.
Removal and control nests > 98% of chicks survived. Chicks in enlarged broods > 81% survived.
Growth rate vs fitness
Allocation of energy to increased fecundity during one year occurs at the cost of further growth that year
Determinate growth > a growth pattern in which an individual does NOT grow any more once it initiates reproduction
Occurs in many species of birds and mammals ie. Should favour long life span organisms
Indeterminate growth - should favour short life span organisms
Growth rate vs fitness
Delaying sexual maturity allows an individual to grow large and produce more offspring per year once reproduction starts
Comparing across many species (within taxonomic groups):
The age of sexual maturity is positively associated with the number of years an animal will survive after reaching sexual maturity
Trade offs of Trinidadian guppies
The Trinidian guppy is common in the streams of Trinidad
In lower streams > guppies have short life expectancies, predation by pike cichlids and kill fish = high predation risk
In higher elevation streams > guppies have long life expectancies, predator free = low predation risk
Senescence
A gradual decrease in fecundity and body condition and an increase in the probability of mortality
Example:
Between the ages of 30 and 85, the rates of human metabolism, nerve conduction, blood circulation and breathing capacity decrease up to 65%. Over time, the function of the immune system also declines leading to higher death rates
Organisms differ in the number of times they reproduce before senescence
Semelparity
Arises when there is a massive amount of energy required for reproduction
Examples:
Bamboos, agaves, some octopus, cicadas
Semelparity and iteroparity
Examples:
Yuccas are mostly iteroparous (multiple) but some varieties are Semelparous (single)
Differences in breeding patterns lead to trade offs in flower and fruit numbers and in germination rates
Why does senescence exist?
Senescence is an inevitable consequence of natural wear and tear and may be the accumulation of molecular defects that fail to be repaired eg. From ultraviolet radiation
Long lived animals appear to have better mechanisms for reducing the production of reactive forms of oxygen and repairing damaged DNA and protein molecules
Stimuli for change
The right timing of life history events is critical so behaviour and physiology match changing environmental conditions
Organisms rely on various indirect, environmental cues
Photoperiod > the amount of lift that occurs each day, provides a cue for many events in the life histories of virtually all organisms
The effect of resources
Fluctuations in resource availability often determines the timing of life history events
Example:
Like many amphibians, the barking tree frog undergoes metamorphosis
The effect of predation
Predation can affect many life history traits (eg. Time to and size at hatching, metamorphosis and sexual maturity)
Example: hatching and sexual maturity
The effects of global warming
Small changes in temp can have substantial impacts on an organisms physiological processes
The increase in global temp has changed the breeding times of many animals and plants
Example: North American tree swallows
Changes in temp can alter initiation of flower production
Example: Thoreau and others observed the time of first flower for more than 500 species of flowering plants in Concord, Massachusetts
Consequences of altered breeding
Problems can arise when a species depends on the environment to provide necessary resources with an altered breeding season
Example: the pied flycatcher breeds in Europe each spring
Impact of humans
In addition to global warming, human activities can impose strong selection and have substantial impacts on organisms life histories
Example:
Commercial fisheries impose selection pressure on fish size by harvesting only the largest individuals.
Between the 1930s and 1970s the average age at maturity of north east artic cod decreased to 7-9yrs
This shift is likely associated with changes in fecundity and longevity
Concepts of ecology - population distribution
The distribution of populations is limited to ecologically suitable habitats
Population distributions have 5 important characteristics
The distribution properties of populations can be estimated
Population abundance and density are related to geographic range and adult body size
Dispersal is essential to colonising new areas
Many populations live in distinct patches of habitat
Distributions of populations
Spatial structure > the pattern of density and spacing of individuals in a population
Small scale variation in the environment creates geographic ranges that are composed of small patches of suitable habitat
Example: the geographic range of Fremonts leather flower is just 3 countries in Missouri
It is possible to test whether species are limited by unsuitable environmental conditions
Ecological niche modelling
General rule > populations can increase in more suitable habitats
Understanding the realised niche of a species aids in species conservation and can help to limit the spread of invasive species
Ecological niche modelling > the process of determining the suitable habitat conditions for species
Ecological envelope > the range of ecological conditions that are predicted to be suitable for a species (differs from the realises niche which describes conditions in which a species currently exists)
Can use historic distributions of species of few individuals or extinct
Modelling invasive species
Ecological niche modelling can predict the expansion of pest species
Example: the Chinese bushclover was taken to the US to control erosion, provide cattle feed and reclaim mined land
Effects of global warming
During the past century, the average temp of the earth has increased by 0.8 degrees
Temp change can cause a shift in the geographic range of species
Warmer northern temps in the North Sea has caused southern fish species to expand their ranges northward
Population characteristics
Geographic range > where is it found?
Abundance > the total number of individuals in a population within a defined area
Eg. Total number of lizards on a mountain
Population density
If population density is greater than what the habitat can support, some individuals must leave or the population will experience lower growth and survival
Largest density of individuals often near the centre of a populations geographic range
Population dispersion
Dispersion > the spacing of individuals with respect to one another within the geographic range of the population
Population dispersal
Dispersal > the movement of individuals from one area to another
Dispersal is distinct from migration, which is the seasonal movement of individuals back and forth between habitats
It is the mechanism by which individuals can move between suitable habitats
Dispersal allows species to colonise areas outside of their geographic ranges
Quantifying individuals
Area and volume based surveys > surveys that define the boundaries of an area or volume and then count all of the individuals in the space
The size of the defined space is related to the abundance and density of the population
By taking multiple samples, it is possible to determine how many individuals are in an average sample
Line transect surveys > surveys that count the number of individuals observed as one moves along a line. This data can be converted into area estimates of a population
Quantifying individuals
Many animals are sensitive to the presence of researchers and will leave the area when surveyed.
Other species are camouflaged and may be difficult to find.
Mark recapture survey > a method of population estimation in which researchers capture and mark a subset of a population from an area, return it to the area and capture a second sample of the population after time has passed
Population size is estimated by assuming that:
Initially captured individuals (M) divided by population size (N)
=
Marked recapture individuals (R) divided by total individuals captured in 2nd sample (C)
Quantifying dispersal
Dispersal can be quantified by measuring how far individuals travel from where they were marked
Eg. With ear tags, radio transmitters, leg bands
Lifetime dispersal distance > the average distance an individual moves from where it was born to where it reproduces
This provides an estimate for how fast a population can increase its geographic range
Quantifying dispersal
Dispersal can cause a geographic range to expand rapidly if a few individuals can disperse much farther than the average individual
Example: by marking different species of songbirds with rings
Population abundance and range
Populations with high abundance also have large geographic ranges
This pattern has been observed for many organisms (eg. Birds)
Population density and body size
The density of a population is negatively correlated to the body size of the species
Dispersal limitation
The absence of a population from suitable habitat because of barriers to dispersal
Habitat corridors
A strip of favourable habitat located between 2 large patches of habitat that facilitates dispersal eg. A narrow band of trees that connects forests
Conservation efforts have increasingly considered the preservation of corridor habitats
Example: biologists have pushed to protect riverside habitats along the Rio Grande that would allow species to move easily among large patches of protected land
The ideal free distribution
Whenever possible, individuals choose habitats that provide the most energy
As individuals move to high quality habitat, resource must be divided among more individuals ie. Reduced per capita benefit
Per capita benefit can fall so low that an individual would benefit by moving to low quality habitat!
Ideal free distribution > when individuals distribute themselves among different habitats in a way that allows them to have the same per capita benefit > assumes perfect knowledge of habitat variation
The ideal free distribution example
Milinski 1979
Stickleback fish were distributed proportionally throughout an aquarium
Researchers manipulated the abundance of prey (water fleas) on each side of the aquarium such that one side had 1/5th the abundance of water fleas as the other side
The ideal free distribution
Assumes perfect knowledge of habitat variation
Individuals may not be aware that other habitats exist
Individuals in nature rarely meet the expectations required by the ideal free distribution
Fitness is not solely determined by maximising resources, other factors may influence distribution such as the presence of predators or territory owners (despotic behaviour)
Models of spatial structure
Sub populations > when a large population is broken up into smaller groups that live in isolated patches
If individuals frequently disperse among sub populations, all sub populations increase and decrease in abundance synchronously
If dispersal is infrequent, each sub population fluctuates independently
Models of spatial structure
Basic meta population model > a model that describes a scenario in which there are:
Patches of suitable habitat embedded within a matrix of unsuitable habitat
All suitable patches are assumed to be of equal quality
Source sink meta population model > a population model that builds upon basic model and accounts for the fact that not all patches of suitable habitat are of equal quality
Source sub population > in high quality habitats they serve as a source of dispersers within a meta population
Sink sub population > in low quality habitats, they rely on outside dispersers to maintain the sub population within a meta population
Models of spatial structure
Landscape metapopulation model > a population model that considers both differences in the quality of the suitable patches and the quality of the surrounding matrix (eg. Habitat corridors)
Concepts of ecology - population growth
- Under ideal conditions, populations can grow rapidly
- Populations have growth limits
- Population growth is influenced by the proportions of individuals in different age, size and life history classes
Population demography
Demography > the study of populations
Incorporates > birth rates and death rates, movement (dispersal), age structure and sex / gender ratios
Can be used to predict growth of a population
Not all individuals contribute equally to population growth
Population dynamics
Births and immigration > adds individuals to the population
Deaths and emigration > removes individuals from a population
Change in population size = births + immigrants entering population - deaths - emigrants leaving population
The population growth rate (per capita rate of increase / intrinsic growth rate) can be expressed mathematically:
Triangle N divided by triangle t = B - D
B is the number of births
D is the number of deaths
Triangle N is the change in population size
Triangle t is the time interval
Growth rate = B - D
The per capita rate of increase (r) aka intrinsic growth rate is given by
Triangle N divided by triangle t = rN
Exponential growth model
Exponential growth > population increase under ideal conditions
A model of population growth in which the population increases continuously at an exponential rate and can be described by the equation:
Nt = N0e ^ rt
Nt = future population size
N0 = current population size
r = intrinsic growth rate
t = time over which population grows
e = natural exponential > approx 2.71828
J shaped curve > the shape of exponential growth when graphed
Exponential Growth model
Populations initially grow slowly because there is a small number of reproductive individuals, growth rate increases with the number of reproductive individuals
The rate of a populations growth at any point in time is the derivative of this equation:
Triangle N divided by triangle t = rN
Which means:
Change in population divided by change in time = intrinsic growth rate x population at a point
Exponential growth ( J shaped curve)
The rate of increase is constant but the population accumulated more new individuals per unit time when it is large then when it is small
Geometric growth model
A model of population growth that compares population sizes at regular time intervals (usually year)
It is expressed as a ratio of a populations size in one year to its size in the preceding year. It can NOT be less than 0
When it’s greater than 1, population size has increased
Population doubling time
Doubling time > the time required for a population to double in size, can be estimated by rearranging the exponential growth model:
t = loge 2 divided by r
For the geometric model, the equation is nearly the same. Recall that r = logeŷ so we can replace r with logeŷ
t2 = loge 2 divided by loge upside down y
Density independant
Factors that limit population size regardless of the populations density
Density dependant
Factors that affect population size in relation to the populations density
Negative density dependence > when the rate of population growth decreases as population density increases
Limiting resources > ad population increases, resources are divided among more individuals. Per capita resources decline to a level at which individuals find it difficult to grow and reproduce
BUT Allee effect > reverse may be true of below threshold number
The logistic growth model
Carrying capacity (k) > the maximum population size that can be supported by the environment
A growth model that describes slowing growth of populations at high densities
S shaped curve > the shape of the curve when a population is graphed over time using the logistic growth model
Inflection point > the point on a sigmoidal growth curve at which the population had its highest growth rate
The logistic growth model
As the population increases from a very small size, the rate of increase grows until reaching 1/2 of the carrying capacity (Corresponding to the inflection point)
Individuals in the population continually decline in their ability to contribute to population growth
Predicting human growth
The logistic growth model was formulated by Pierre Francois Verlhulst to describe human population growth in 1804
Survivorship Curves
Type 1 > depicts a population that experiences low mortality early in life and high later in life
Type 2 > a pop that experiences constant mortality throughout its life span
Type 3 > a pop with high mortality early in life and high survival later in life
Most populations exhibit a curve that combines features of type 2 and 3
Life tables
Age specific summary of the survival pattern of a population
They are typically based on the number of female offspring per female
Stable age distribution > when the age structure of a pop does NOT change over time, occurs when survival and fecundity of each age class stays constant over time
Survival rate > the probability of surviving from one age class to the next
Survivorship > probability of surviving from birth to any later age
Collecting data for life table
Cohort life table > follows a group of individuals born at the same time from birth to death of the last individual
Environmental changes can affect the survival and fecundity of a cohort, it is difficult to separate the effects of age and the environment
Time specific / static life table > quantifies the survival and fecundity of all individuals in a pop during a single time interval
Age is not confounded with time, all subjected to same environmental conditions, not as time consuming
Life tables
Net reproductive rate R0 > the total number of female offspring that we expect an average female to produce over the course of her life
Generation time T > the average time in years between the birth of an individual and the birth of its offspring
When upside down y or r is estimated from a life table, it is assumed that the life table has a STABLE age distribution
Age distributions fluctuate due to environmental conditions, so any approximation of upside down y or r is restricted to to the environmental conditions that the pop experiences at the time of measurement!
Concepts for ecology - population distribution 2
Populations fluctuate naturally over time
Density dependence with time delays can cause populations to be inherently cyclic
Chance events can cause small populations to go extinct
Metapopualtions are composed of sub populations that can experience independent population dynamics across space
Population fluctuations
All populations experience fluctuations due to factors such as availability of resources, predation, competition, disease, parasites and climate
Fluctuations include random and cyclic changes through time
Some populations tend to remain relatively stable over long periods
In contrast some populations exhibit much wider fluctuations:
Small organisms (eg. Algae) tend to reproduce fast and are not as buffered against starvation.
They have a high surface area to volume ratio so they maintain homeostasis
Age structure variations
When an age group contains a high or low number of individuals the population likely experienced high birth or death rates in the past
Long term fluctuations in age structure can be determined for a forest by examining tree rings
Overshoots and die offs
Populations in nature rarely follow a smooth approach to their carrying capacity
Overshoot > when a population grows beyond its carrying capacity
Die off > a substantial decline in density that typically goes well below the carrying capacity
Cyclic population fluctuations
Population cycles > regular oscillation of a population over a longer period of time
Some populations can exhibit highly regular fluctuations in size
Cyclic populations can occur among related species and across large geographic areas (eg. The synchronous cycles of grouse in Finland)
Cyclic behaviour of populations
Populations have inherent periodicity and tend to fluctuate up and down although the time required to complete a cycle differs among species
Populations are stable at their carrying capacity
If population size decreases, the population responds by growing and often overshooting carrying capacity
Overshoots can occur when there is a delay between the initiation of breeding and the time that offspring are added to the population
Delayed density dependence
When density dependance occurs based on a population density at some time in the past
Eg. Moose breed in autumn
As the time delay increases, density dependence is delayed and the population is more prone to both overshooting and undershooting k
Damped oscillations > a pattern of pop growth where pop initially oscillates but the magnitude of the oscillations declines over time
Stable limit cycle > a pattern of pop growth in which the pop continues to exhibit large oscillations over time
Cycles in laboratory populations
Delayed density dependence may occur because:
1) the organism can store energy and nutrient reserves
2) there is a time delay in development from one life stage to another
Extinctions in small populations
Small populations are more vulnerable to extinction than larger populations
Extinction due to growth rates
Data suggests that small populations are more likely to go extinct but growth models suggest that they should have more rapid growth and be resistant to extinction!
This contradiction can be resolved by incorporating random variation of growth rates into growth models
Deterministic model > designed to predict a result without accumulating for random variation in population growth rate
Stochastic model > incorporates random variation in pop growth rate and assumes that variation in birth and death rates is due to random chance
Extinction due to growth rates
Demographic stochasticity > variation in birth rates and death rates due to random differences among individuals
Environmental stochasticity > variation in birth rates and death rates due to random changes in the environmental conditions eg. Changes in weather
Low birth / high death for a number of years > extinction is more likely
Increased chance of having a string of bad years over time
Smaller populations are at more risk of extinction if they experience a string of bad years
Patchy habitats
Preferred habitat often occurs as patches of suitable habitat surrounded by a matrix or unsuitable habitat
Eg. A wetland
Habitat fragmentation
The process of breaking up large habitats into a number of smaller habitats
Some habitat fragments experience extinctions, other colonised by dispersers
Sources are: high quality patches that produce large number of individuals that disperse to other patches
Sinks are > low quality patches that produce few individuals and rely on dispersers to keep the sink population from going extinct
Patch connectivity influences the abundance of sub populations
Basic model of metapopualtions
Assumes that:
Habitat patches are of equal quality
Each occupied patch had the same sub population size
Each sub pop supplies the same number of dispersers to other patches
So species may be preserved by:
Protecting large fragments of habitat that reduce extinction risk
Or
Ensuring that individuals can disperse to and from patches
Patch size and isolation
Habitat patches are rarely equal in quality
Some patches are larger or contain a higher density of resources
Small patches are likely to experience higher rates of extinction and less likely to be occupied than large patches
Dispersal success is inversely related to the distance of dispersal, so more distant patches will have a lower probability of being occupied than closer patches
Unoccupied patches that are close to occupied patches are more likely to be colonised and reduced
Rescue effect > when dispersers supplement a declining sub population and thereby prevent it from going extinct
Successful use of metapopulation theory
Black footed ferret > numbers reduced by: habitat loss, poisons, disease
Thought extinct in 1979
Biologists conducted several reintroductions in locations throughout western North America
iPad
Competition
Occurs when individuals experience limited resources
Decrease in a population density causes an increase in growth rate of population
Leibigs law of the minimum
Not all resources limit consumer populations
The law states that a population increases until the supply of the most limiting resource prevents it from increasing further
Example > silica is a limiting resource for diatoms
Competition for a single limiting resource > the species that persists is the one that can drive down the abundance of that resource to the lowest levels
Leibig law assumes each resource has an independent effect on population growth so if you add more of another resource it will NOT improve the growth
Interaction among species
An increase in one resource can have a much larger effect on a population when there is also an increase in a second resource
Competitive exclusion principle
Two species cannot coexist indefinitely when they are both limited by the same resource
One species survives better when resources are scarce
Competition > related species
Darwin suggested that competition is most intense between related species because they have similar traits and consume similar resources
Related species that compete strongly - differences in habitat use should be favoured
Competition - non related species
Competition can also be intense among distantly related species that consume a common resource
Different types of direct and indirect competition
Indirect - through a shared resource
Apparent - two species have a negative effect through an enemy, including a predator, parasite or herbivore
Abiotic conditions
The ability to compete may well be overwhelmed by the ability to persist in harsh abiotic environments
Disturbances
Competitive interventions can also be altered by disturbances or lack of
Predation and herbivory
There are often trade offs between competitive ability and resistance to predators or herbivores
Predators can reverse the outcome of competition
Herbivores can also alter the outcome of competition
Predators and herbivores can limit the abundance of species
All organisms face attack by natural enemies - critical in communities as they lower the pop size
This can reduce the importance of competition in some natural populations
In stable population cycles, both species can coexist
Herbivores - effects can be seen by fencing areas ore removing herbivores to prevent grazing
Population cycles - pop of consumers and consumed populations fluctuate in retake cycles
The synchrony of population of population cycles between consumers and the populations they consume suggests that these oscillations are the result of interactions between them
Predator prey cycles
Stable predator prey population cycles can be achieved when the environment is complex so that predators cannot easily find prey
Lotka volterra model - predator prey interactions incorporates oscillations in predator / prey and shows predator numbers lagging prey
BUT model does not incorporate time delays, density dependence or realistic foraging behaviour of most predators
Functional response: the relationship between the density of prey and an individuals predators rate of food consumption
Prey density increases then a predator can consume a higher proportion of those prey
Predators can regulate the growth of prey population
Functional responses
Type 1: As prey density increases, predators consume a constant proportion of prey until satiation
Type 2: prey density increases and predators consumption of prey slows and plateaus.
Often happens because predators must spend more time handling more prey
Type 3: predator has low, rapid and slowing prey consumption
Low consumption at low prey densities may occur for 3 reasons
- Refugees for prey to hide
- Predators may have less practice at locating and catching prey but develop a search image at higher prey densities
- Predators may exhibit prey switching by changing their diet preferences to the more abundant prey
Herbivore population dynamics
Two types:
Interactive systems - herbivores affect plant populations traits
Non interactive - no relationship between herbivore densities and plant pop dynamics
One way: herbivores are affected, plants are not
Evolutionary consequences
All organisms can defend themselves against attack
Can predators select for prey defence and prey for increased predator success?
Predator hunting strategies
Active hunting - spend most of their time moving around looking for prey
Ambush - lie in wait for prey to pass
Hunting can be thought of as a series of events including:
Detecting, pursuing, catching, handling, consuming prey
Prey defences
Behaviour - call, move away
Crypsis
Aposematism and chemical
Mimicry
Physical - tough leaves, spines, hairs, scales
Communities can have distinct or gradual boundaries
Community > an assemblage of species living together in an area
Community zonation > species composition changes across a landscape, with changes in environmental conditions
Difference in: ranges for various environmental conditions and ability to compete with other species
Communities are often categorised by
Dominant organisms or physical conditions that affect the distribution of species
Ecotones
A boundary created by sharp changes in environmental conditions, over a relatively short distance and accompanied by a major change in the composition of species
Ecotones support a larger number of species from adjoining habitats, species specifically adapted to the Ecotone
Biodiversity of a community
Diversity = species richness (number of species) + relative abundance (proportion of individuals of each species)
Log normal distribution > a normal, bell shaped, logarithmic scale on x axis
Patterns of species abundance
Rank abundance curves > plots the relative abundance of each species in a community in rank order > from the most abundant to the least abundant
Species evenness > comparison of the relative abundance of each species in a community > if EVEN, all species = same abundance.
Calculating biodiversity
Shannon’s index
Both richness and evenness of each community
Weighted to richness
Higher value = more diversity
Simpsons diversity index
Both richness and evenness of each community
Gives more weight to dominant species eg. Abundance
Rare species will not affect index
Can use counts or percentage of cover
Simspons similarity index
Compares richness and abundance between 2 communities
Gives more weight to dominant species ie. Abundance
Effect of habitat diversity
Communities with higher diversity of habitats should offer more potential niches (eg. Places to feed and breed) and a higher diversity of species
Keystone species
A species that substantially affects the structure of communities
Species might not be numerous
Removal of keystone species can cause a community to collapse
Ecosystem engineers > keystone species that affect communities by influencing the structure of a habitat
Intermediate disturbance hypothesis
More species are present in a community that experience occasional disturbances compared to communities with frequent or rare disturbances
Food web
A complex and realistic representation
Arrows in a food web indicate consumption and the movement of energy and nutrients
Direct vs indirect effects
Direct > interaction between 2 species that does not involve other species
Indirect > intervention between 2 species that involves 1+ intermediate species, can occur between communities
Trophic cascade > indirect effects in a community that are initiated by a predator
Top down and bottom up effects
Abundances of trophic groups are determined
Bottom up control > by the amount of energy available from producers
Top down control > by the existence f predators at the top of the food web
How communities respond to disturbances
Community stability > ability of a community to maintain a particular structure
Community resilience > amount a community changes when acted upon by a disturbance eg. By addition or removal of a species
Community resilience > time taken for a community to return to its original state
Alternative stable state
When a community is disturbed so much that species composition and relative abundance change AND the new community structure is resistant to further change
Switching to alternative stable states typically requires a large disturbance eg. Removing a keystone species
Succession
Process by which the species composition of a community changes over time
Seral stage > each stage of community change during succession
Pioneer species
The earliest species to arrive at a site
Typically able to disperse long distances
Arrive quickly at disturbed sites
Often ruderals
Climax community
The final seral stage in the process of succession
Generally composed of organisms that dominate in a given biome
Often competitors
Observing succession
Direct observation of changes over time is the clearest way to record succession in a community
Indirect > examining pollen preserved in layers of lake and and pond sediments and chronosequence - a sequence of communities that exist over time at a given location
Primary succession
The development of communities in habitats that are initially devoid of plants and organic soil
Colonised by species - do not need soil, can live on rock surfaces
Secondary succession
The development of communities in disturbed habitats that contain no plants but still contain organic soil eg. Ploughed field, forests uprooted by a hurricane
Terrestrial succession
The sequence of seral stages that a site passes on its way to a climax community can differ depending on initial conditions
Chronosequence assumes sites pass through similar seral stages
Animal succession
Changes in the plant community can change the habitats that are available to animals, which causes changes in the animal community
Succession of intertidal communities
Succession in intertidal communities can occur rapidly after a disturbance due to the short generation time of dominant species
Traits of species through succession
Early and Kate succession species have differing trade offs:
Dispersal, growth, reproduction, competitive ability
Transient climax community
A climax community that is NOT persistent, occurs when a site is frequently disturbed so a climx community cannot persist
Gaps in a climax community
Small scale disturbances in an area with a climax community can allow growth of species that are not considered climax species
Extreme conditions
Fire maintained climax community > a successional stage that persists as the final seral stage due to periodic fires
Grazer maintained climax community > when a successional stage persists as the final seral stage due to intense grazing
