Population Equilibrium Births Immigration Deaths Emigration
Exponential growth: population not limited by external factors Time population size N1N1 t1t1 slow increase at first t2t2 N2N2 pop goes to infinity
“The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval and surviving till one hundred years old; if this be so, after a period of from 740-750 years there would be nearly nineteen million elephants alive descended from the first pair.” Charles Darwin But……. the planet isn’t covered with elephants. What limits exponential growth?
Carrying capacity (K) = upper limit of population size due to limits of available resources (space, nutrients, food, water….)
Density dependent regulation: factors connected to the population density slow the rate of growth as population size increases. Time population size rapid (exponential) growth resources become limiting K
Time population size idealized logistic growth what a real population might do, with oscillations and time lags Called “s-curve” in book
Density independent regulation: factors external to the population can limit numbers. ex. If a pond dries up, all fish will probably die, regardless of how many are there.
Another scenario, exponential growth and crash (J curve)
Biotic potential: ability of populations to increase Reproductive rate Migration and dispersal Defenses Coping strategies Environmental resistance: combination of biotic and abiotic factors that may limit population increase Predators, competitors, disease Adverse weather, limited food/nutrients
Time population size Set by biotic potential K Set by environmental resistance
Time population size Critical number After a decline……… rebound extinction
Human population estimates http://www.prb.org/Content/NavigationMenu/PRB/Educators/Human_Population/Population_Growth/Population_Growth.htm
Biotic potential: ability of populations to increase Reproductive rate Migration and dispersal Defenses Coping strategies
Reproductive strategies: Many offspring with low parental care Few offspring with high parental care
lakes ~ islands (also segregated habitats) rivers & other corridors Mechanisms flying: adult insects, birds swimming:fish, invertebrates Walking: wildebeest, most mammals ducks feet & other animals Humans often cited, scorned by many ecologists, should follow migration pattern I hate it when I get zooplankton stuck in my toes Migration & Dispersal
Defenses of prey: examples from freshwater Coloration: reduce visibility, in water be clear Don’t make waves plays dead (playing possum) trade off between feeding and avoiding detection http://www.cnas.smsu.edu/zooplankton/bosmina.htm
be hard to eat: works best against small predators gelatinous sheath protuberances D. ambigua D. retrocurva D. lumholtzi
The cost of defense constitutive: always turned on snail shell, porcupine needles constant or unpredictable predation no too costly to make induced: only turn on when threat of predation detected change in zooplankton body size (mostly) periodic or predictable predation don’t incur the cost till you need it
examples of costs Energy spent constructing case, shell, body protrusion not devoted to reproduction & may take extra energy to swim Time spent hiding not spent foraging, avoid death, but lower growth & reproduction There’s almost always a trade off (no free lunch) Stream invertebrates must deal with fish (visual) and predatory invertebrates (tactile), good defense for fish may make you vulnerable to inverts.
Current Effects complex: can sweep you to a bad habitat or bring you a snack Adaptations to stay in place: be flat, have suction disc Adaptations to catch food Caddis fly uses nets used to trap food
Fish-buoyancy saves energy sharks & a few bony fishes have lipid filled livers deep sea fishes (>1000m) have reduced skeletal & muscle tissue http://www.pelagic.org/ image _lib/ http://www.pbs.org/wgbh/nova/abyss/ these methods limit activity & hard to regulate w/ change in depth Most fish have a gas bladder that maintains buoyancy