population dynamic Flashcards
Definition of population
A group of individuals of a single species living in the same general area, so that members
• rely on the same resources
• are influenced by similar environmental factors
• are likely to interact and breed with one another
Properties of populations
Boundaries
Size/Density (Not a static property)
Distribution (clumped, uniformed or random)
Structure (age structure, sex ratio)
Boundaries
- Before studying a population, we need to define its boundaries
- May be natural ones (e.g. island, lake)
- Or arbitrarily defined (e.g. a specific area of a national park)
- Need to be appropriate to the organism under study and to the questions asked
What is population ecology
- The scientific study of of populations in relation to their environment
- i.e. how biotic and abiotic factors influence the abundance, dispersion and composition of populations
Why is population ecology important
- Sometimes pure scientific interest, but often applied focus
- Understand, but also predict and manage
- Guide how to:
- Conserve threatened species
- Control pests
- Manage harvested species
Managing populations
• Can act on different processes (births, deaths, immigration, emigration)
Step in Quantifying aspects of populations
- This may include:
- Estimating the size of the population
- Monitoring fecundity and survival
- Monitoring movement
estimating population size
Counting
• In some rare cases, may count all individuals in the population…
Sampling
…. but in most cases we need to use sampling techniques to estimate the size of a population
Sampling method
Set plots randomly
• Count within plots
• Calculate average density
• Extend estimate to whole area
Precision depends on:
• Number of plots
• Variability in counts
Mark-recapture technique
First sampling session (e.g. �”=12 individuals captured)
Marked (in some appropriate way), then released
WAIT FOR INDIVIDUALS TO MIX BACK WITH REST OF POPULATION
Capture a second time
estimated population= firstcapture number x second capture number divided by number of recapture
Some key assumptions in mark-recapture
• Marks are not lost between sampling sessions
• Marking does not alter the behaviour of individuals (trap-shy; trap-happy)
• No births, deaths, immigration, emigration between sessions (= closed population)
à If assumptions not well met, estimates will be biased (population size underestimated/overestimated)
à Model extensions exist to deal with some of these issues, e.g. suite of methods for open populations
markings
-NATURAL MARKS
-ARTIFICIAL MARKS (tag, paint, band, PIT tags)
• Batch marking vs. individual marking
• Invasiveness of marks (ethics; assumptions method)
NON-INVASIVE GENETIC METHODS ( DNA analysis of animal product that was left behind)
A typical life style Life cycle
- after birth is a juvenile phase mostly for growth
- growth stoped and move into reproductive phase
- after reproductive phase enter post reproductive phase and death by senesceane
Life cycles of all (unitary) organisms are variations of this basic outline
Difference in life cycle
• Length of generations
- Several generations per year
- One generation per year (annual)
- One generation over several years (perennial)
• Repeated reproduction?
- Iteroparous species: individuals breed multiple times; resources during breeding dedicated to
future survival
- Semelparous species: single reproductive event; no resources dedicated to future survival;
reproduction followed quickly by death
- We can find iteroparous and semelparous species among both annual and perennial species
Longer life cycles breeding
- Seasonal breeding, triggered by photoperiod, to match to availability of abundant resources
- Continuous breeding (e.g. in equatorial areas with little variability in temperature, rain, photoperiod; many primates)
- Semelparous species; most life in pre-reproductive stage, then breed and die
Demography and life tables
• To assess how the patterns of birth and death affect how a population may grow or decline in specific scenarios,
we need to monitor them in a quantitative way
• Demography: the study of the vital statistics of populations and how they change over time
• A useful way to summarise demographic information for a population is to make a life table
• To build a life table:
• We usually follow the fate of a cohort (a group of individuals of same age) from birth to death
• We determine the proportion of the cohort that survives from one age group to the next…
• … and the number of offspring produced in each age group
Survivorship curves
- A graphical representation of the survival rate data in a life table
- Plot of the proportion of a cohort still alive at each age (column 3 in previous life table)
- Usually plot log() values
Survivorship curves type
- type 1 Most individuals die late in life
- type 2 Individuals die at a uniform rate
- type 3 Most individuals die at a young age
Population dynamics
• Size is not a static property of populations. Populations can grow or decline in size
Change include birth, death, immigrate, emigrate
Problem with growth
When resources are abundant, populations have the potential to grow greatly…… but populations cannot grow infinitely!
Exponential growth = growth proportional to the population size
Types of mechanisms that limit population growth
Density independant
-any force that affects the growth of a population regardless of the density of the population. often abiotic
Density dependant
-factors that affect the growth of a population differently depending on the density of the pop. à often biotic
Negative density-dependence population growth rate decreases because the population is too crowded
Positive density-dependence population growth rate decreases because the population is too small
Density-dependent population regulation
- Negative density-dependence introduces a negative feedback in the system
- Population ↑ à Growth rate ↓
- “Stable” system, maximum population size
Carrying capacity = the maximum population size of a species that an environment can sustain Carrying capacity can vary substantially in space and time!
Density-dependence mechanisms
- INTRASPECIFIC COMPETITION FOR RESOURCES (FOOD, SPACE)
- DISEASE
- PREDATION
- TOXIC WASTES
Controversy: importance of density-dependent regulation vs. density independent factors?
some argue that density-dependant regulator and mainly competition is the main reason for limit in pop
some suggest fluctuation in the enviroment
-both is right, case specific
Mathematical models
- equation, or set of equations, used to describe a phenomenon
- quantitative representation of the truth
- a simplified one, but hopefully a useful one!
- balance between complexity and utility
- include model parameters that take different values depending on the scenario that is being represented
Exponential growth model (discrete)
Let’s consider:
- Discrete time (e.g. t refers to year t)
- Births and deaths only (we’ll talk about movement in the next lecture)
- Females (more convenient for modelling, common practice)
pop size at time t+1 = pop size at time � + births − deaths
Nt+1=Nt+Ntbt-Ntdt =Nt(1+bt-dt) =Nt(1+rt)= Ntrt
Nt=N0(1+r)t
Logistic growth model
Logistic growth model (discrete)
Logistic growth model (continuous)
Model assumptions and departures of logistical growth model
- Like all models, the logistic model makes assumptions
- If not met it lead to fluctuations in the population growth trajectory compared to the real model
Some assumptions:
- No variability in the environment (constant intrinsic growth rate and carrying capacity)
- No effects of chance (particularly relevant for small pops!)
- No consideration of population structure
- No delays (i.e. populations adjust growth instantaneously approaching carrying capacity smoothly)
- If there are delays, a population may overshoot carrying capacity.
- And during an overshoot period, the carrying capacity may be lowered by resource destruction
What is stochasticity?
- Stochasticity = randomness
- A process is stochastic if it cannot be predicted accurately
- E.g. roll of a dice (we can predict the frequency of events, but not their order)
deterministic
no randomness involved
Environmental stochasticity
- Unpredictable fluctuation in environmental conditions in space and time
- E.g. variability in rainfall, temperature, etc… and through this also variability in resources, e.g. food
resources and conditions that individuals need to survive and reproduce are not constant -> birth and death rates (and therefore growth rate) are not constant
Demographic stochasticity
- Arises because the processes of birth and death of individuals are probabilistic
- Even if the birth and death rates are constant, from year to year, there will be variability in the actual proportion of individuals that are born or die
An example of demographic stochasticity
- Population of individuals, all equal, with a survival probability of 0.6
- If there are 10 individuals… 6 individuals will survive to the next time step? Could be… but maybe not!
- In proportion, more variability the smaller the population
- Increases the risk of extinction in small populations
Accounting for stochasticity
- Both environmental and demographic stochasticity can be important for the dynamics of populations
- Not considered in the deterministic exponential and logistic population growth models
- But there are more advanced modelling approaches that allow accounting for these sources of variability (and other aspects, e.g. population structure)
Definitions: dispersal
- Most often used to describe the spreading of individuals away from others
- Can be divided in 3 phases: emigration/transfer/immigration
Definitions: migration
Mass directional movement of a large number of individuals of a species from one location to another
Dispersal
- Organisms need resources and good conditions for reproduction and survival
- There may be forces favouring aggregation… (e.g. individuals gather around a resource or for protection)
- … and forces favouring separation (e.g. individuals moving away to avoid competition)
à Dispersal is often age and sex biased (young, males)
- Immigrants/emigrants not only influence population size, but can also affect its composition _> Dispersal particularly relevant in the context of invasions
- What brings individuals to the new area in the first place à Dispersal also key for metapopulations (“a population of populations”)
Emigration dispersal is commonly density-dependent
- Commonly triggered by the more intense competition in crowded areas
- Can also go the other way: individuals leave low density patches (e.g. to avoid inbreeding)
- Regulatory effect on populations (similar to density-dependent mortality)
- Possibly the main effect of dispersal for the dynamics of single populations!
Measuring movement
- We focus on animals but movement also relevant for other organisms (e.g. seed dispersal in plants!)
- Broadly, three types of approaches:
-Individual marking
+ observation Using tracking technology
=Analyzing “intrinsic markers”
