Final 5: Population Dynamics

Why to Study Population Dynamics

• better definition of life history parameters

  • survival for species and age classes within species

  • fecundity rates

  • age of sexual maturity

  • longevity

• becoming more sophisticated

  • more studies on extrinsic factors

    • environmental viability 

    • disease

    • natural toxins 

    • competition 

    • predation 

• importance of density dependence 

Population Growth of Longlived Animals 

• slow intrinsic rates of population increase

• consequence of life history characteristics 

Population Increase Dependence 

• how quickly a population can grow depends on

  • age at which females start reproducing

  • inter-birth intervals

  • lifespan and reproductive span

Marine Mammals Population Growth

• most marine mammals: 

  • take several years to reach maturity

  • have long gestation periods

  • can only have one calf/pup

  • do not reproduce every year

• marine mammal populations grow slowly vulnerable to overexploition

Measuring Population Growth

• can be measured in 2 ways: 

  1. abundance data collected over many years

  2. using life history data 

Abundance Estimation 

• more reliable

• abundance estimation

  • survey’s/counts over several years

• estimate the % change per year

• population growth is small and estimates can be imprecise

  • therefore long data sets of 10+ years is needed

• some species estimates are so imprecise you can’t determine growth

• several methods:

  • distance sampling

  • mark recapture

  • migration counts

  • colony counts

Distance Sampling 

• used with cetaceans which are easily detected and sparsely distributed

• line transect sampling is mostly used

• strip transect sampling is not efficient

• cue counting for baleen whales

• acoustic surveys

Mark Recapture

• populations aggregate locations each year

• estimate from tagged subset

  • mark a number of individuals from a population of unknown size, and release them. 

  • capture another subset of individuals and see how many are marked 

  • use the percentage of unmarked to calculate population size 

• using natural markings 

  • knowing % of unmarked individuals 

  • less reliable 

Life History Data

• less direct

• lesile matrixes or other models

  • estimates of age of sexual maturity, birth rate, juvenile and adult survival rate, and maximum age

  • estimate rate of increase from model

• useful to estimate the max potential for population growth

• rarely used for actual population estimates

  • not enough data on survival rates. only some species

Taxonomic Differences

• marine mammal population growth is small, but there’s a range of life-history strategies 

• maximum population growth rates per year

  • sea otters: 20%

  • otariids: 10%

  • phocids: 10%

    • steller sea lion (exception): 4%

  • mysticetes: 5%

    • humpbacks: 10%

  • manatees: 7%

  • dugongs: 5%

  • odontocetes: unknown, maybe 4%

Extrinsic Factors

• environmental variance 

• diseases and natural toxins 

• competition 

• predation 

• interactions 

Environmental Variance

• older age of sexual maturity + slow population growth = cant respond quickly to favourable conditions 

  • but, they also cant decline too often or too fast in unfavourable conditions (or else they would become extinct)

  • so they evolved life history strategies to buffer them from inter-year variability of environmental conditions 

• enough data to study environmental variance is rare 

  • pinnipeds are the best evidence effect of changing oceanographic conditions via their response to el nino years 

  • cetaceans reflect similar response to environmental variance, but at a lesser degree 

K-selected species 

• evolved to maintain populations close to carrying capacity (K)

  • K= number of living organisms that an area can support without environmental degradation 

Diseases and Natural Toxins

• probably natural events for marine mammals

  • though occur more frequently when populations are at or near carrying capacity 

• some of die-off may have been triggered by anthropogenic effects

  • hard to confirm 

• 3 natural toxins cause die offs

  • saxitoxin 

  • brevetoxin 

  • domoic acid 

    • all come from algal blooms, which are connected to increase nutrient load from human activities 

• some mortality from disease or toxins are bad enough to affect population dynamics 

Competition

• one organism has a negative effect upon another by consuming, or controlling access to, a resource that is limited in availability 

  • eating the same prey at the same location doesn’t mean there is competition 

  • resources might not be limiting (at the time and place)

  • species might exploit prey in different ways 

• competition with other species has provided little evidence of population control 

  • not important?

  • too hard to prove?

Predation

• in the past, was not considered a strong factor of population control

  • however, in the past 2 decades given more focus

• pinniped pups are vulnerable to predation

  • affects growth rates of rookeries, unsure of the effect for the whole population

• context of top down vs. bottom up influences

  • top down: removal (via predation and fishing)

  • bottom up: oceanographic productivity

    • usually plays a bigger role

Interactions

• interactions with multiple extrinsic factors:

  • sea otters with encephalitis are 4x more predated by sharks

    • maybe predation mortality is more focused on vulnerable individuals with lower survival rates already

  • correlated to influences from environmental stress or density dependence

Compensation 

• as populations become relatively larger, they tend to have lower population growth and eventually stop increasing 

• evidence found: 

  • life history parameters: when population is below K, females mature and start reproducing earlier than females from population close to K.

• hypothesis for how regulation for marine mammals works:

  1. affects rate immature survival

  2. age of sexual maturity lowers

  3. birth rate increases

  4. adult survival rate increases

Linear Density Dependence

• constant decline in growth and population increases

Non-linear Density Dependence

• no decline in population growth, until levels get close to carrying capacity, then rapid decline

Linear vs. Non-linear Density Dependence 

• both linear and non-linear happen with life history parameters of marine mammals

  • non-linear growth for a parameter doesnt necessarily translate to non-linear growth of the population, especially if another parameter has a linear growth

• we dont know for sure if marine mammal population growth is linear or non-linear

Density Dependence and Management 

• concepts incorporated into management and conservation 

  • many populations cetaceans and pinnipeds recovering from whaling times

  • observing over the next decades if they return to pre-whaling levels 

  • some populations have reached K

• we need to know when its density-depedence vs. human effect

  • populations close to K are more subject to environmental effects 

Allee Effect

• the other side of density dependence: 

  • populations at very low levels have very small growth

  • populations so small its hard to find a viable mate

  • can also happen due to inbreeding depression, or behavioural changes 

    • can’t forage as well in small numbers or be protected from predators 

• unsure how it affects marine mammal populations 

  • difficult to study, even more difficult to study small populations

• several whale species havent recovered, even after decades of no whaling 

  • is this the allee effect?

  • are they still strugling from other anthropogenic issues?

• pinnipeds were hunted, but are now showing recovery. 

  • how small were there smallest populations? small enough for Allee effect?