Population Ecology: Key Concepts and Interactions
Ecology: Key Concepts and Interactions
Ecology
The study of the distribution and abundance of organisms and the flows of energy and materials between abiotic and biotic components of the ecosystem.
Demography
The study of rates of births and deaths, age and sex structure, and distribution.
Population
A group of potentially interbreeding individuals inhabiting a specific area.
Density
The number of individuals per unit area.
Ecological Density
The number of individuals per unit area of suitable habitat.
Crude Density
The number of individuals with no regard for variation in habitat.
Sampling Methods
Quadrat Sampling
Used when the area of the quadrat is known and the organisms are relatively immobile during sampling.
Mark-Recapture
Used when animals are mobile. Animals are marked and released, and then resampled to see what proportion of animals carry marks.
Lincoln-Peterson Index
One way to use mark-recapture data to estimate population size (M/N = m/n).
Statistical Concepts
Accuracy
Refers to how close an estimate is to the true value.
Precision
Refers to how close repeated estimates are to each other.
Confidence Interval
A range of values that contains the estimated parameter a high proportion of the time.
Calibration
When an index is related to the population size.
Morisita’s Index
Expresses how many times more likely it is that two individuals will occur in the same quadrat compared to a random distribution.
Population Dynamics
Rate of Increase
An indicator of the direction and pace of numerical change, but also a measure of population vigor.
Growth Increment
Refers to the difference in population size at two times.
Quadrat
An arbitrary sampling unit of known area.
Extent
In terms of scale, includes the size of a study area.
Resolution
In terms of scale, includes the area of a quadrat, grab, or net.
Sampling
Taking measurements of abundance such as density, frequency, biomass, or presence-absence.
Unbiased
Each unit has the same chance of being chosen.
Independent
The selection of one unit has no influence on the selection of other units.
Environments
- 1-dimensional environment: A transect, road, or trail.
- 2-dimensional environment: A forest or ocean floor.
- 3-dimensional environment: An entire forest, body of water, or air.
Wiegert’s Method
Determines the optimal quadrat size in estimating population density and considers two important factors: relative cost and relative variability.
Distribution Patterns
- Id > 1: Clumped distribution.
- Id < 1: Regular distribution.
- Id = 1: Random distribution.
Life Tables and Population Structure
Life Table
A concise summary of age-specific patterns of birth and death.
Age Pyramid
- Narrow base: Indicative that a population is declining.
- Broad base: Indicative that a population is growing.
Cohort Life Table
Follows a cohort through life (individuals born at the same time) until the last one dies.
Static Life Table
Takes a cross-section of the population one at a time.
Survivorship Curves
- Type I: A relatively high rate of survival among young and middle-aged individuals followed by a high rate of mortality among the aged.
- Type II: Constant rates of survival throughout life.
- Type III: A period of extremely high rates of mortality among the young followed by a relatively high rate of survival.
Sex Ratios
Primary Sex Ratio
The male:female ratio at fertilization.
Secondary Sex Ratio
The male:female ratio at birth.
Tertiary Sex Ratio
The male:female ratio at sexual maturity.
Quaternary Sex Ratio
The male:female ratio in the adult population.
Effective Sex Ratio
The sex ratio affected by the reproductive activities of organisms.
Reproductive Rates
Fecundity
Defined as the number of female births for each female of age x.
Net Reproductive Rate
The average number of female offspring produced per female during her lifetime.
Generation Time
Defined as the average time between the birth of a female and the birth of her female offspring.
Intrinsic Rate of Increase
The maximum per capita rate of increase when no resource is in short supply.
Population Growth Models
Geometric Growth Model
Occurs when there are abundant resources and discrete generations.
Exponential Growth Model
Occurs when there are abundant resources and overlapping generations.
Logistic Growth Model
Occurs when there are limited resources and overlapping generations, and the population growth slows as you approach carrying capacity.
Population Size Dynamics
- N > K: The population is declining.
- N < K: The population is growing.
- N = K: The population is stable.
Density Dependence
The negative relationship whereby the population growth rate declines as the population grows.
Limiting Factor
Any factor that affects the population rate of growth.
Regulation Factor
Any density-dependent factor that adequately depresses population growth as population density increases.
Life-Fecundity Table
The age-specific schedule of births and deaths.
Lotka Equation
Shows the link between the population rate of increase, schedules of survivorship, and fecundity.
Species Interactions
Mutualism
A positive interaction for both species involved.
Commensalism
A neutral interaction for one species and a positive interaction for the other species involved.
Neutralism
A neutral interaction for both species involved.
Predation
An interaction between organisms where the actions or presence of one organism reduces the fitness of another (a positive, negative interaction).
Amensalism
A negative interaction for one of the species involved and a neutral interaction for the other.
Competition
An interaction between organisms where the actions or presence of one organism reduces the fitness of another (negative interaction for both).
Intraspecific Competition
Competition with members of your own species.
Interspecific Competition
Competition with members of different species.
Exploitative (Scramble) Competition
Indirect competition that occurs through the depletion or occupation of a common limited resource.
Interference (Contest) Competition
Direct competition where aggressive interactions occur while exploiting a common resource.
Allelopathy
A common biological phenomenon by which one organism produces biochemicals that influence the growth, survival, development, and reproduction of other organisms.
Apparent Competition
Indirect competition where a common predator/parasite increases in abundance as a result of one species, thereby increasing harm to another.
Competition Coefficients
Variables that convert between the number of individuals of one species and the number of individuals of another species in terms of resource use.
Competitive Exclusion Principle
Complete competitors cannot coexist.
Ecological Niche
All of the environmental requirements for a species to persist.
Fundamental Niche
All the environmental requirements for an organism to survive and reproduce.
Realized Niche
The fundamental niche after considering interactions with other species.
Niche Partitioning
Species avoid competitive exclusion by”partitionin” resources in various ways by taking advantage of environmental heterogeneity.
Ghosts of Competition Past
Competing species may be less fit than a species which avoids competing because it occupies a fundamental niche that does not overlap theirs.
Ecological Character Displacement
Divergence of characteristics in two otherwise similar species due to competition.
Life History Strategies
Life History
Significant features of an organism’s life cycle, particularly relating to survival and reproduction.
Life History Strategy
A specific suite of traits shaped by natural selection to maximize reproductive output.
Trade-offs
The idea that organisms cannot maximize all traits simultaneously.
Parental Care
Any behavior that is directed towards the young to increase their likelihood of survival.
Semelparity
These species reproduce once per lifetime and have a large litter or clutch, small offspring, and provide little parental care.
Iteroparity
These species reproduce repeatedly and have small litters, large offspring, and provide adequate parental care.
R-Selection
Small body size, not strongly favored in competitive ability, high population growth rate, rapid development, single semelparity reproduction, and produce many, small offspring.
K-Selection
Large body size, highly favored in competitive ability, low population growth rate, slow development, multiple iteroparity reproduction, and produce few, large offspring.
Predator-Prey Dynamics
Predation Rate
The rate at which predators remove prey from the population. It is composed of the functional and numerical response.
Numerical Response
The change in predator density resulting from the change in prey density.
Demographic Response
Increases in predators result in higher birth rates or lower death rates.
Aggregational Response
Predators clustering in areas of high prey density.
Delayed-Density Dependence
The effects of prey density on predator populations are often delayed.
Functional Response
The relationship between prey density and an individual predator’s rate of prey consumption.
Attack Rate
The rate at which predators encounter prey items per unit of prey density.
Handling Time
Time spent attacking, subduing, consuming, and digesting.
Prey Switching
Some predators will switch to targeting a different prey type when the focal prey density is low, which can dramatically reduce predation at low prey densities.
Prey Refuges
Partial or complete invulnerability of prey to predation at low prey density.
Chemical Cues
Predator smells, alarm cues, diet cues, latent alarm cues, and learned associations.
Kairomone
A chemical emitted by an organism which mediates interspecific interactions in a way that benefits the receiver and harms the emitter.
Alarm Cues
A chemical passively released through injury that signals the presence of a predator.
Phenotypic Plasticity
The ability of one genotype to produce more than one phenotype when exposed to different environments.
Non-Consumptive Predator Effects
Effects of predators on prey that occur as a consequence of predators being present, even when prey are not being killed.
Maternal Effects
An organism’s phenotype is affected by the mother’s environment, often independent of genetic effects.
Compensatory Mortality
Predators eat individual prey that would have died anyway.
Life-Dinner Principle
Natural selection should act more strongly to protect prey than it does to help predators catch prey.
Community and Ecosystem Ecology
Community
Interactions among populations, species diversity, trophic dynamics, competition, and succession.
Ecosystem
Interactions between nutrient cycling, primary productivity, and material fluxes.
Biosphere
Global processes that include biotic and physical systems: oceans, atmosphere, and geology.
Ecological Succession
Progressive change in the composition of a biotic community following disturbance.
Climax Community
Stable, long-lasting community, primarily determined by”climat” (remains stable until disrupted).
Primary Succession
The development of biotic communities where none existed previously, i.e., on bare rock.
Secondary Succession
Occurs when an existing community is disturbed or destroyed, which tends to be more rapid than primary succession.
Facilitation
Modification by early colonizers paves the way for later species.
Tolerance
Modification by early colonizers does little to facilitate later species.
Inhibition
Modification by early colonizers makes a site less suitable for later species.
Gross Primary Production
Energy (carbon) fixed per unit time.
Net Primary Production
Energy (carbon) lost via respiration is equal to plant biomass.
Biogeochemistry
The science dealing with the relationship between the geochemistry of a given region and its flora and fauna, including the circulation of such elements as carbon and nitrogen between the environment and the cells of living organisms.
Biogeochemical Cycling
The cycling of nutrients through ecosystems via food chains and food webs, including the exchange of nutrients between the biosphere and hydrosphere, atmosphere, and geosphere (i.e., soils and sediments).
Nutrient Pool
A specific component or compartment where a nutrient resides.
Nutrient Flux
The rate of exchange (i.e., unit of material per unit time) of nutrients between pools.
Biological Nitrogen Fixers
Cyanobacteria (blue-green algae), free-living soil bacteria, and mycorrhizae (symbiotic bacteria living in root nodules).
Spiraling Length
The length of a stream required for a nutrient atom to complete a cycle.
Rate of Decomposition
The rate at which nutrients are made available to primary producers.
Keystone Species
Species that, despite low biomass, exert strong effects on the structure of the communities they inhabit.
Exotic Predators
Exotic species have dramatic impacts on communities because they were outside the evolutionary experience of local prey populations.
Biogeography and Global Ecology
Biogeography
The study of the geographical distribution of organisms, their habitats, and the historical and biological factors which produced them.
Species Richness
The number of species found in a given study area.
El Niño
When the sea surface in the western Pacific is cooler than average and barometric pressure is higher than average, which has global effects.
La Niña
Periods of lower sea surface temperatures and higher-than-average pressure in the eastern tropical Pacific.
Dead Zone
An area of oxygen-deprived bottom water.
Opportunistic Life History
Low juvenile survival, low fecundity, and early maturity.
Periodic Life History
Low juvenile survival, high fecundity, and late maturity.
Equilibrium Life History
High juvenile survival, low fecundity, and late maturity.
Capture Efficiency
The probability that an encounter will lead to a capture.
Co-Evolutionary Arms Race
The relationship between predator selection for improvements in foraging and prey selection for improvements in defenses.