Image: Encyclopedia of Life Flickr “On average, ants monopolize 15 – 20% of the terrestrial animal biomass.” – Schultz (2000) PNAS Image: Encyclopedia of Life Flickr Ants rule the world

What controls population size? Trophic efficiency Not everyone can be a human/eagle/tuna Age structure Density-dependent (bottom-up) Limiting resource: food, growing space Predation (top-down) Density-independent Fire, cold winter

Recall: ~10x the biomass of lower trophic level to support consumer Image: biologycorner.com

Energy is released

Age structure How many individuals fall into particular age categories (i.e. cottonwood data) Predict how fast a population can grow Reproductive age females In some species, egg production increases with body size/age

Density-independent Same effect on individual’s survival probability at any population size Cold winter kill birds or freeze ponds High temperatures kill coral or frogs mountain pine beetle

Density-dependent: limiting resource Bottom-up control Density-dependent: individual’s survival probability depends on size of population Recall carrying capacity: limit based on necessary resources. Resource in short supply is the limiting resource Limiting resource can be water, nutrient, food, space, light

Logistic growth

Food webs exert both top-down and bottom-up population controls Phytoplankton are marine autotrophs, use nutrients (like N, P, S) to make organic material Primary consumers are zooplankton Coastal food web, image: USGS

Simple ecosystem model http://coast.ocean.washington.edu/~neil/NPZvisualizer/ The size of the circle represents the amount of nitrogen through the plankton populations. Nitrogen is often a limiting resource. Arrows represent fluxes between the N reservoirs. Sliders let you control model parameters. Z decomposition Uptake N P

Human population 7 Billion and still climbing… but slower Kolkata street, National Geographic 7 Billion and still climbing… but slower

What is the percentage increase? United Nations Population Division Population size Annual increase / increment % increase = (new - old) / old

Human population National Population Immigration Emigration Birth Death Population explosion: accelerating rate of increase after Industrial Revolution Mortality rates fall as living conditions improve Growth rate probably topped out ~1970 at ~2% per year, perhaps 1000x faster than early human history Now at ~1% per year growth and falling