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



