Ecology Lecture 7 Ralph Kirby

Population (Ecology) –Group of individuals of the same species inhabiting the same area Interbreeding if sexual Limited in space Populations have –Density –Distribution in time –Distribution in space These are defined by –Rate of birth –Rate of growth –Rate of death Resulting in age & sex structure What is an individual –Easy for most large sexually reproducing animals –Not so easy for colonies of animals and many plants –Ants –Bees –Coral –Trees (Ramets) –Fungi

Distribution –Area over which a species occurs Abundance –Numbers of that species present in the area Both vary by available habitat –Temperature Red Maple –-40 o C Carolina wren –Northwards -7 o C –Westward>52mm –When all environmental factors within its range of tolerance, the organism can live in its habitat Horned lark –Avoids forests –Available territory

Density can be a key factor –Number of individuals per unit space –Affects growth rate ResourcesMatespredation Thus density controls in part –Birth rate –Growth rate –Death rate Density is difficult to define or measure –What area do you use –What distribution has the species within the area –How do you measure the density –Ecological density Some use a factor associated with the habitat Distribution –Clumped Normal –Uniform High density and competition –Random Social behavior Available nesting sites Metapopulation –Multi-Habitat based multiple populations

How do you count Direct Counts –Time consuming –Needs to be sure you can see all individuals in the area Quadrats –Organisms must be static –Needs laying of quadrats to be random –Statistical analysis essential Mark and recapture –Estimate Assumes equal chance of capture for all No deaths or births Marked animals random among unmarked No loss of marks No emigration or immigration Other factors include time of capture, stress of capture, sex, age, etc. Relative abundance –Using a factor such as tracks, bird song, etc –Get a relative measure

Disperal – Active and Passive Emigration – Escaping high density Immigration – Moving into empty habitat Migation – A round trip, perhaps involving mating

Age structure –Prereproductive –Reproductive –Postreproductive –Short life span Rapid increase –Long Life span Slow increase Measuring Age structure –Mark and recapture Accurate but hard to do –Special features Teeth Horn rings Tree rings Age Pyramids –Show status of population

Size/Age can affect the population structure Sex ratios in populations shifts with age –Humans More males Males have shorter life span –War –High risk activities Females need to survive giving birth to children Therefore sex ratios have change a lot over the last two hundred years –Parturition fever in 19 th Century –First World War in UK –Second World War in USSR

Plants are more difficult –Hard to detrmine age –Extremely high loss fo seedlings –Use of yield table Probability of survival is age specific –See death table –Probability of death in next year Life Table –n x Number in cohort –l x Probability of surviving from birth –q x Age specific mortality

Birthrate –Sex specific –Age specific Net reproductive rate depends on –Fecundity –Survivorship Given –Age specific mortality –Age specific birth rate –Project population growth/decline Exponential Growth –Depends on rate of reproduction, λ –If λ less than 1, there is cline Population growth is limited by the environment –Carrying Capacity –See reindeer

Nonconformity –Change in age structure –Immigration –Emigration –Changes in birth and death rates But stable once it reaches carrying capacity Populations should work via positive and negative feedbacks Does not work smoothly –Depends on environment such as overgrazing Population cycles in simpler systems –Food availability Can result in extinctions –Two types Change in environment. Start locally Mass extinctions. External event?

Three Models for regulation of population density K represents equilibrium –Partial density regulation A Birth Rate (B) independent of population density Mortality Rate (M) increases with density –Partial density regulation B Birth Rate (B) declines with population density Birth Rate (B) declines with population density Mortality Rate (M) independent of population density –Full density dependent regulation Birth Rate (B) increases with deceasing population density Mortality Rate (M) increases with increasing population density

Carrying capacity See effect of density for tadpoles and white clover –Maximum number of sustainable individuals –Results in intra-specific competition –Two types Scramble competition –Growth and reproduction depressed for all individuals Can result in local extinction or population crash Contest competition –Some individual deny resources to other individuals resulting in some growing and reproducing better than others Indirect via exploitation of resources Direct via interference

Population crash caused by Rindepest Note exponential increase from low base Grass limited in dry season Competition related to rainfall producing grass Once in equilibrium, rainfall most important

Similat effect for white clover

Effect of repotting Effects can differ –Bison is exponentia l curve –Grizzly bear is linear effect Competition can affect reproduction –harp seals Increas ed age of sexual maturity –maize Linear –marsh herb

Effects of high density on reproduction StressPheromonesDisease Increased mortality of young Results in dispersal –Sources Habitat –Sink Habitat Can be dangerous Lack protective cover High number of predators –Move no further than necessary –Move in forays

Social dominance can affect reproduction and dispersion Fighting and social position. Both male and female hierarchies are possible Results in limited mating –Alpha –Beta –Omega Home Range Can overlap Territory No overlap Fight to protect Dominant male gets best locations Males and female ranges can differ Male and female ranges overlap

Territory is protected –Food –Mating –Nesting site –Attraction of mates –Avoidance of suboptimal habitat –Needs energy May not be optimal strategy when resources are low

Other density independent factors can overrule density –Weather –Rainfall –Temperature