Population Ecology - Weber State University

A population subjected to onlydensity-independent factors cannot persist over a long period oftime – eventually go to extinction

K is constant over timedoes not vary year to year etc. dN / Ndt declines linearly with Nalternative nonlinear decline Effectof density N on dN / Ndt is instantaneous no delays alternative density now affects dN / Ndt some timein the future (time lag) Continuousoverlapping generations

Laboratorypopulations ofParamecium aurelia&ParameciumcaudatumLaboratory population ofDrosophila melanogaster

Cycles most common in higher latitudesTundra3-4 year cyclesPredators – Arctic fox, snowy owl, Rough-legged HawkPrey – lemmingsTaiga8-10 year cyclesPredators – LynxPrey – Snowshoe Hare, Ruffed GrouseTemperate Grasslands3-4 yearPredators - mustelids?Prey - voles, Prairie Chickens?TropicsProbably not ?

2 major cycles identifiedvoles, lemmings, and grouse –3-4 year periodSnowshoe Hares – 8-10 year

Current research has been concentrated on lowphase -Voles and hares as small mammals have atremendous capacity for growth during increasephaseMicrotines 8-22 fold increase in 6months

What prevents population growthduring low phase of cycle?2 broad categories of hypotheses1. Classical ModelSomething is wrong with the environment Food supplies damaged by overgrazing atpopulation peak Predators sufficiently abundant to preventrecovery Disease or parasites Or interaction

Food HypothesisSeems unlikely, numerous food supplement experiments have failedto prevent low cycle

PredatorsDifficult to tease apart cause and effect

2 Possibilitesa. predators simply follow changes in preyabundance and thus have little consequence onprey demographyb. predators drive cycles and are cause of cyclesLows would be caused by predators that keep prey# depressed for extended period following thedecline

Snowshoe hares – decline directly related to intensepredationLow and increase phase --- 23% population disappearedoverwinter (50% due to predation)Decline phase ---50-77% disappeared overwinter (70% dueto predation).Decline phase in Yukon -- 1% survived entire year (83%predation)Thus predation is a major factor in the decline phase butseems not to have an effect during the low phase.Same for microtines

Indirect effects HypothesisResponse of prey to risk of predation may affectreproductive fitness and ultimately cause the low phase.Decline - Snowshoe hares minimize risk to predation byaltering foraging to areas of dense cover.The correlate of good cover is poor food qualityTrade-off reproduction for survivalShift to dense spruce stands which results in poorernutrition, lower mass of female, reduced fecundity.This causes chronic stress with long term consequences.

All physiol measurements associated with stressresponse (immune system, mass loss, reprodhormone response, hypothalamic-pituitaryadrenal response) all were severely affectedduring the hare decline phase. They did improveremarkably after predators had declined.Low phase is thus a lag caused by indirect effectsof predation

2. Self-regulating modelSomething is wrong with the individualEnvironment is ok but something isworking on individuals to prevent theirincrease.Physiol & Behavioral characteristicspotential source of limitation

Population Cycles – Continued2. Self-regulating modelAnecdotal evidence supports hypothesis

Population Cycles – Continued2. Self-regulating modelMaternal Effects HypothesisChange in maternal quality occursduring peak phase and carries over todecline and low phase

Maternal Effects HypothesisProduced through d-d social inhibitionof maturation in peak years

The use of a resource by two or more differentindividuals, or the interference by one individualwith the resource use by others

Exploitation– competition in which 2 or moreindividuals consume the same limitedresource Interference – competition in which oneorganism prevents the other from havingaccess to a limiting resource. Activeinhibition is used to deny others the resource

Ultimateeffect is decreased contribution oforganism to next generation. To be apparentcompetition must lead to a reduction insurvivorship or fecundity. Resource must be limited. The effect of competition on any individualis greater, the greater the number ofcompetitors there are (density dependent)

11 young/year 27% adults survive 6% of youngDoes intraspecific competitionlimit population size of GreatTits?Over 4 years removed 60% of young (4 young/year)

Do results suggest survivalrates are density-dependent?TreatmentAdult SurvivalYoung ReturnControl27%6%Removal56%22%Increases were sufficient to compensate forthe loss of young and breeding numberscontinued to fluctuate around same numbersas before.

Competition in plant hoppersDenno and Roderick 1992. Ecology 73:1323Lives onsalt marshgrass Spartina

Density controlled by enclosinginsects with Spartina seedlings3, 11, 40

Results-survivorship-development-Body length- % Macropterousfemales

Colonial SpeciesSuggested that density may have a positive effect- Fecundity increases with increased density

Common Guillemot Breed in subcoloniesof different densities Females lay 1 egg Subcolonies withhigher densities hadhigher reproductivesuccess

Common Guillemot Inverse densitydependence

Common Guillemot Driving factor –deterrence ofpredators (GreatBlack-backed Gullsand Herring Gulls)

Tendency for destabilizing effect – as birthrates rise with increased density, death ratesremain the sameLarge populations get largerSmall populations get smaller

Other Mechanisms of Allee Effects predator dilution/swamping antipredator vigilance social thermoregulation reduction of inbreeding genetic drift antipredator aggression social facilitation of reproduction

Too simplistic to think environmentalconditions are equally favorable everywherean organism exists

Population SourceProductivityExcess young producedPopulation SinkProductivity not sufficient tobalance mortality

What level of nesting success isneeded to sustain a “healthypopulation”?

SNPLMayfield Nesting Success (%)Study Site2005 2004 4.734.512.911.338.1SHORE1993 1992 1991(Paton 1995)1990

CALCULATION OF LAMBDA(finite population growth rate)SNPL PA PJ where,PA annual adult survival (set to 74% or 50% based on Paton 1994and Nur et al. 1999);PJ juvenile survival from fledging to the following breeding season(set to 50% based on Nur et al. 1999) mean annual production (calculated from this study, Paton 1995,Nur et al. 1999)

λ PA PJ βλ 1 Population Sourceλ 1 Population Sink

1.8SNPL1.6OptimisticEstimate 0.5Nest Success0.60.7

SNPLMayfield Nesting Success (%)Study Site2005 2004 4.734.512.911.338.1SHORE1993 1992 1991(Paton 1995)1990

A ‘metapopulation’ is a “population ofpopulations” (Levins 1969,1970); in whichdistinct subpopulations (local populations)occupy spatially separated patches ofhabitat.

A metapopulation exists when dispersal rates are “lowmoderate”, meaning that an individual will move from one patchto another at a rate high enough to maintain some interactionamong subpopulations but low enough that those subpopulationsremain distinct.

High rates of dispersal lead to the unification of patches into asingle large, patchy population. All “patchy” populations are notnecessarily metapopulations.

Metapopulation theory is particularly useful to wildlife biologistsbecause most wildlife habitats are fragmented or maintain somedegree of patchiness.Animal dispersal among patches is an obvious concern forpopulations existing in heterogeneous landscapes. Rate of animaldispersal is affected by aspects of life history traits and populationdynamics, but animal movement is also affected by aspects oflandscape heterogeneity, including patch size, patch isolation,edge characteristics, and matrix characteristics.

Life History TheoryA. DefinedDefinition – Set of evolved strategies includingbehavioral, physiological, and anatomicaladaptations that influence survival andreproductive success directlyEverything we know about NS indicates thatthose individuals with fecundity andsurvivorship schedules most suited tomaximize fitness will be favored.

Teleology is the idea that purposeexists in evolution in the same sensethat it does for human intention. Evolution has no predesigned or intentional goal

“Strategy”does not imply a conscious choiceby the organism. Adaptive strategy does not meanthe best possible. Like all adaptations, life history strategy issubject to constraints and needs only to be goodenough.Optimal life history strategy means the best ofthose existing in a certain population undercertain environmental conditions.

Crucial aspects of these schedules representLH Strategies1. Age and size reproduction begins2. Relative effort devoted to growth,reproduction, and survival3. The apportionment of reproductive effortbetween many small or few large offspring4. Distribution of reproductive effort overlifetime

Investmentin eachlife history trait hasa benefit and a costto the organism. Investment beyondthat optimumreduces fitness bylimiting energyavailable for otherimportant functions.

B. Trade-offsHypothetical Organism-Reproduces immediately after birth- Large number of Large offspring- Lavishes parental Care- Reproduces repeatedly throughout a longlifeTE G M R(c f)For real organisms, its LH Strategy must be acompromise or a trade-off

C. Cost of ReproductionIndividuals devoting considerable energy toone aspect of its strategy must pay for itby reducing investments in another.Is there a cost to reproduction?Nur Journal of Animal Ecology 53:479:496

Surgical removalSurgical addition Removal of an egg from the female Anole lizards (Anolis sagrei) leads tohigher survival to the next reproductive event due to the increase infemale’s stamina, sprint speed, and growth.

Thecollaredlemming(Dicrostonyxgroenlandicus):the weaningweight ofindividuals in largelitters is less thanthat in smalllitters.

Average clutch size in birds increases with the latitude

A study of nest predation in Venezuelan cloudforest. Hypothesis: High predation rates in the tropicsselect for smaller nests and thus smaller clutchsizes. Prediction 1: Nest predation rates increase withnest size. Prediction 2: Clutch size increases with nestsize. Prediction 3: Nests in the tropics are smallerthan nests at higher latitudes.

Test 1: Nest-swapexperiments intropical andtemperate sites. Two nests of differentsizes placed near anatural nest in theenvironment. Nests baited withquail eggs andchecked daily forpredation. Prediction 1supported. Ecology: Evolution, Application, IntegrationDavid T. KrohneCopyright 2015 Oxford University Press

Whenthe nest size wasadjusted for body size, therewas no increase in clutch sizein large nests. Prediction 2 not supported. Tropical nests were notsmaller relative to body sizein the tropics than at higherlatitude. Prediction 3 not supported.Ecology: Evolution, Application, IntegrationDavid T. KrohneCopyright 2015 Oxford University Press

K is constant over time does not vary year to year etc. dN / Ndt declines linearly with N alternative nonlinear decline Effect of density N on dN / Ndt is instantaneous no delays alternative density now affects dN / Ndt some time in the future (time lag) Continuous overlapping generations