Organisms live as members of populations Population size Population density Population dispersal How do populations grow?
Exponential growth growth without limits at a maximal rate (biotic potential) Actual growth rate: difference between birth and death rates, corrected for migration
Some species show a very high rate of population growth (exponential) reproduce at early age produce a lot of offspring life span is short “r-selected adaptations” for organisms with that type of life history (and which don’t consume a lot of resources and where resources are not limiting)
Populations have innate capacity for exponential growth Usually this is temporary, until carrying capacity is reached. What sorts of things limit population growth?
Most populations are affected by limitations in their environment “logistic growth model”: growth is rapid (exponential) for awhile then tapers off
Many species are adapted for life at or near carrying capacity (K-selected) Compete for resources Reproduce late in life Have few offspring-and care for the offspring they have Tend to live long lives and have low mortality rate (low birth rate balanced by low death rate)
At carrying capacity, birth rate is equal to the death rate What sorts of factors affect population growth? Density-dependent factors: increasing density limits availability of resources for all Limited food supply Waste accumulation (toxicity)
Density-independent factors independent of population size but do affect its growth Weather (can contribute to population cycles) Natural disasters
Seen in populations of insects, birds and mammals (e.g., lemmings and other rodents) Predation? Food supply? Overcrowdingstress hormone imbalance and infertility?
How predators control prey population As prey population increases, more opportunities for predators. As predators switch to alternatives, original prey can recover. Predators remove weaker prey. Parasites weaken prey
Competition within a community Interspecies Intraspecies Competition for what? Food, water, space, breeding sites, shelter, light…
Introduced species can upset the whole balance of activity Snails Starlings Kudzu Zebra mussels Prickly pear cacti in Australia- a predatory cactus moth was introduced to control the cacti!
Structure of populations Density Dispersion- how organisms are grouped in their geographical range
Clumped pattern Most common in nature Unequal distribution of resources Social behavior mating safety in numbers
Uniform distribution Often results because of interactions between individuals in the population (to deal with scarce resources) Creosote bushes in the desert- roots compete for water and nutrients Nesting penguin pairs (on islands)
Random- no pattern no factors contribute to attraction or repulsion between individuals Rare, but could occur when there are no pressures due to environment Exhibited mainly by plants (why?) What kind of dispersion is shown by humans?
Demographics- statistical methods for analyzing populations Cohorts- age groups- birth and death rates populations with many young members tend to grow faster Sex ratio- proportion of males to females effect depends on mating practices
Community interactions Community- all the interacting populations within an ecosystem Interactions tend to maintain balance between resources and consumers Populations act as agents of natural selection (e.g. predator-prey) Over time- coevolution
Major types of community interactions Competition (harms both) Predation (benefits predator, harms prey) Symbiosis parasitism (helps one, harms the other) mutualism (helps both) commensalism (helps one, no effect on the other)
Competition: how much do ecological niches overlap? In plain English: ecological niche is everything an organism needs to survive No two species occupy exactly the same niche, although some aspects of a niche may be shared
Competitive exclusion One survives, the other dies
Resource partitioning Minimizes niche overlap
Remember Darwin’s finches? Finches had evolved different beak shapes and feeding behaviors that minimized competition among them
Members of species, or closely related species, compete for limited resources Two species of barnacles that live in intertidal zone One species (A) dominates the upper zone The other (B) predominates in the lower (wetter) zone) When the lower species is removed, the upper species spreads into the area
Conclusion: B outcompetes A (A can’t grow in the lower region unless B is removed) However, A has an advantage on the upper shore because it can tolerate low tide (studies of Connell et al.)
Predator-prey interactions Prey usually outnumber predators Herbivores “prey” on plants Predators and prey exert a lot of pressure on each other: Prey evolve to become more difficult to catch Predators evolve to become better hunters
Behaviors evolved by predators and prey I. Camouflage (by both)
Mimicry Coral snake (poisonous) King snake (harmless)
Both of these butterflies taste bad monarch viceroy
Monarch butterfly has adapted to milkweed (a toxic plant) Lays eggs on milkweed Larvae feed on milkweed Caterpillars store toxin in their own tissues as defense against predators!
Even grasses and their consumers have coevolved Grasses have tougher consistency Herbivores evolved teeth for grinding