Chapter 52 ~ Population Ecology

Scope of Ecology Ecology The study of the interactions of organisms with Other organisms and the physical environment Habitat - Place where an organism lives Population - All the individuals of a species within a particular space Community – Various populations of multiple species interacting with each other Ecosystem - Community interacting with the environment Biosphere - All the communities on Earth whose members exist in air and water and on land

Demographics of Populations Population characteristics Density~ # of individuals per unit of area counts sample size estimate indirect indicators mark-recapture Dispersion~ pattern of spacing random~ unpredictable, patternless spacing clumped~ patchy aggregation uniform~ even spacing

Estimating Population Size The Mark-and-Recapture Technique   Emphasize that this method comes with a whopping assumption! Namely, that marked individuals have the same probability of being captured as unmarked individuals.

Let’s Try an Example! Twenty individuals are captured at random and marked with a dye or tag and then are released back into the environment. Therefore s = # of animals marked = 20 At a later time a second group of animals is captured at random from the population Emphasize that this is a simple proportionality and they should “expect easy math”! In other words, the numbers won’t be too scary or abstract.

Let’s Try an Example!  

Population Growth Models Demography: factors that affect growth & decline of populations Birthrate (natality, fecundity)~ # of offspring produced Death rate (mortality) Age structure~ relative number of individuals of each age Survivorship curve~ plot of numbers still alive at each age

Survivorship curves What do these graphs indicate regarding species survival rate & strategy? 25 1000 100 Human (type I) Hydra (type II) Oyster (type III) 10 1 50 Percent of maximum life span 75 Survival per thousand I. High death rate in post-reproductive years II. Constant mortality rate throughout life span Type I curve is flat at the start, reflecting low death rates during early and middle life, then drops steeply as death rates increase among older age groups. Humans and many other large mammals that produce few offspring but provide them with good care often exhibit this kind of curve. Type II curves are intermediate, with a constant death rate over the organism’s life span. This kind of survivorship occurs in Belding’s ground squirrels and some other rodents, various invertebrates, some lizards, and some annual plants. Type III curve drops sharply at the start, reflecting very high death rates for the young, but then flattens out as death rates decline for those few individuals that have survived to a certain critical age. This type of curve is usually associated with organisms that produce very large numbers of offspring but provide little or no care, such as long–lived plants, many fishes, and marine invertebrates. An oyster, for example, may release millions of eggs, but most offspring die as larvae from predation or other causes. Those few that survive long enough to attach to a suitable substrate and begin growing a hard shell will probably survive for a relatively long time. III. Very high early mortality but the few survivors then live long (stay reproductive)

Ideal Survivorship Curves 1,000 100 II Number of survivors (log scale) 10 Ask some graphical analysis questions: Which type of curve would best depict the survivorship curve of jellyfish? Type III Which type of curve would best depict the survivorship curve of a lizard? Type II III 1 50 100 Percentage of maximum life span 9

Demographics of Populations Survivorship Curves Type I Characteristic of a population in which most individuals survive past the midpoint of the life span and death does not come until the end of the life span Type II Death is linear over time (unrelated to age) Type III Typical of a population in which most individuals die very young

Population Growth Models Exponential model (blue) • idealized population in an unlimited environment (J-curve); r-selected species (r=per capita growth rate) Logistic model (red) •carrying capacity (K): maximum population size that a particular environment can support (S-curve); K-selected species

Population life history “strategies” r-selected (opportunistic) Short maturation & lifespan Many (small) offspring; usually 1 (early) reproduction; no parental care High death rate K-selected (equilibrial) Long maturation & lifespan Few (large) offspring; usually several (late) reproductions; extensive parental care Low death rate

Regulation of Population Size Density-independent Factors The population density does not influence the intensity of the factor’s effect Natural disasters Weather/Climate Density-dependent Factors The percentage of the population affected increases as the population density increases Competition Predation Parasitism Stress/Overcrowding Waste Accumulation