Population Ecology
Population (def.): a local group of a species, useful as unit of study. Sometimes an interbreeding unit.
Pop Density = number of organisms per unit area (e.g. 10 lions per 1000 sq miles). Pop Size = just total number.
Changes in Pop Density over time: Population Dynamics:
(Births + Immigrants) - (Deaths - Emigrants). Often ignore I and E when studying dynamics, however a metapopulation stresses I and E connections among fragmented populations.
How to measure pop size? For plants, counting (but beware of clones). For animals, sign (droppings, scratch-posts, tracks, etc.); mark-recapture
Births: in general, size of an individual offspring is inversely related to number of offspring. The capability of a species to produces offspring under ideal conditions is called the: intrinsic rate of natural increase, or "irni", or "r".
Species common name r (day-1) Doubling Time
T phage virus 300 3.3 minutes
E. coli bacteria 58.7 17 minutes
Paramecium caudatum protistan 1.59 10.5 hours
Hydra hydra 0.34 2 days
Tribolium castaneum flour beetle 0.10 6.9 days
Rattus norvegicus Norway rat 0.0148 46.8 days
Bos taurus domestic cow 0.001 1.9 years
Nothofagus fusca Southern beech tree 0.000075 25.3 years
Deaths: 3 ideal types of Survivorship Curves, called I, II, III. These are obtained by following a cohort of organisms in a population from birth until death.
I. survival of juveniles is high, mortality mostly among the old
II. rate of mortality is fairly constant at all ages
III. juvenile mortality is high
Population Growth:
Exponential growth. No limits (food, space, disease, predators, etc.). Populations of all species have the tendency to grow in this way. Observe in nature on occaison (ex. cholera, pests,
weeds, introductions of species to habitats without normal checks and balances, etc.
Exponential growth is represented by a J-shaped curve.
Logistic growth. Limits to growth impose a Carrying Capacity, or upper limit to pop size. Limits are imposed in a density-dependent way---as density increases, limiting factors
increase in power. Exs. food, territory requirements, nutrients, etc. Limitation may also occur in density-independent fashion, e.g. freezing temperatures.
Logistical growth curves are S-shaped.
Ex. Mark a batch of nestling birds w/ numbered leg bands, and keep track of them until they all die.
Growth Curves: put births and survivorship together, and can track the Population Growth over time.
"r" and "K" selection--two general types of species
r-selected species maximize growth rate at the expense of stability in numbers: also known as "boom-and-bust" species, or "weeds", or "pests"
K-selected species maximize population stability at the expense of growth rate: these species have strong density-dependent population regulation mechanisms.
--Territories
--Social dominance hierarchies
Natural Selection and Reproduction: as early as possible or delay?? How many offspring at a time?
r- species maximize early reproduction; K-species balance learning and social structure
predation and reproduction: why do bamboos and century plants (agaves) wait so long to flower? Why are there 13-yr and 17-year periodic cicadas?
altricial versus precocial young. Example: ducks versus forest birds
Types of Natural Selection for Population Growth: r versus K selection
Pop Growth including the Age Structure of the population: Demography
Age pyramids
Exs. From humans, in different countries, r versus K type organisms
Population Ecology, cont.
Limits to population growth
Density-independent factors limit populations regardless of their density
Weather
Pesticides, pollutants, habitat destruction (anthropogenic factors)
Density-dependent factors become more effective as population density increases
Community interactions e.g parasitism and predation
Predators encounter more prey as prey populations increase
Parasites weaken their hosts but do not kill them outright. As populations increase, and crowding occurs, it is easier for parasites to move from host to host. Presence of other stressors may increase rate of parasitism or predation. E.g. harp seals & moribillivirus.
Predators and their prey and parasites and their hosts coevolve. Parasites and predators tend to destroy the less fit individuals, leaving the more fit to reproduce.
When organisms are introduced into areas where they have no natural predators or parasites, balance is lost. E.g. starlings, cheatgrass. New predators or parasites introduced into vulnerable populations… e.g. smallpox to Hawaii & N. America, brown tree snake into Guam, maybe humans into N.America.
Competition for resources - interaction between individuals for a limited resource
interspecific - individuals of different species
intraspecific - conspecifics - the more intense competition
scramble competition - a sort of free-for-all. Plants disperse hundreds of seeds, only a few germinate, large ones shade smaller
contest competition - social or chemical interactions used to limit access to important resources; limits direct competition e.g., territoriality
Emigration also reduce populations
Population control is complex interaction of density-dependent and density independent factors.
Population distribution
Ecologists recognize three kinds of population distribution
clumped - examples are family or social groupings such as elephants, wolf packs, flocks of some bird species, schools of fish. May be resource-based, as cottonwoods trees in groves along a river
uniform - territorial animals sometime seasonal, sometimes continuous. Some, like creosote bushes, use allopathic chemicals
random - rather rare, occurs when resources are not limiting, as trees in rain forests.
Community ecology - consists of all the interacting populations within an ecosystem
I. Community interactions
A. Ecological niche - 'occupation', as contrasted with habitat, which is 'address'. Ecological niche includes habitat, but also all the aspects of its way of life, trophic status, abiotic factors, TNZ, predator-prey interactions etc
B. Effects on balance of community of species
Competition - predation - symbiosis
C. Competition
1) no two species ever occupy the same niche - the competitive exclusion principle
Paramecium experiment of Gausse, chipmunks of Sierra
2) niches of different species may overlap - adaptations minimize overlap and competition - MacArthur's warblers, the nuthatches and brown creeper.
3) resource partitioning - develops over the course of evolution
MacArthur found that when two species with similar requirements coexist, they typically occupy a smaller niche than either would if it were by itself - resource partitioning
Darwin's finches are another good example
4) intraspecific competition helps to control population size
5) interspecific competition controls population distribution
D. Predation - Ecologists include herbivores among predators. Predators are usually larger than prey or hunt collectively. Nrg is lost at each trophic level
1) predator - prey interactions shape evolutionary adaptations - coevolution. AS prey become more difficult to catch, predators become more adept at hunting them
overcoming defenses - predators
Ex. some moths have evolved sound receptors for the frequencies bats emit - some bats have then evolved the ability to change frequencies! Some moths can produce their own high frequency clicks! So some bats can home in on the moth's own clicks!
2) camouflage - both predator and prey - many examples
3) warning coloration - Taricha tarosa, the CA newt - monarch butterfly
4) mimicry - prey species evolve characteristics to mimic dangerous poisonous species - viceroy butterfly, CA Mountain kingsnake looks like a coral snake. Gopher snake rattles, hisses
5) aggressive mimicry - predators resemble harmless species or objects - the frogfish blends into its background, prey see only the lure. Saber-tooth blenny looks like acleaner wrasse
Startle coloration - spots that look like big eyes on caterpillar
6) chemical warfare - plant alkaloids deter herbivores, most have evolved to deter insect herbivory not mammalian, as we haven't been around long enough.
But any herbivore that can evolve immunity to the toxin will have abundant food. Grasses have siliciious spike in leaves, only large herbivore teeth can withstand
bombardier beetle
plant chemicals, secondary metabolites
venoms, poisons
7) complex predator-prey relationships involving more than two species - clown fish hiding in the anemones's tentacles
plant - herbivore coevolution
species use other species to protect themselves
8) keystone predators and community stability - a keystone species plays a role that is out of proportion to its abundance in the community, and cannot be removed without radically altering community structure
E. Symbiosis - a close interaction between organism of different species for an extended time
Commensalism is a relationship in which one species benefits while the other is unaffected - neither harmed or benefitted. Barnacles attach themselves to whales
In mutualism, both species benefit - nitrogen-fixing bacteria on roots of plants, microorganisms in the digestive system of animals. Clown fish occasionally brings a acrap of food to its anemone partner
Parasitism - one organsim feeds on another, which is harmed. Unlike predators, parasites are usually smaller than their hosts. Many have complex life cycles involving 2 or more hosts
III. Succession - changes in a community and its non-living environment over time
A. Occurs in a predictable sequence
B. Primary succession - bare rock, barren environments
1. no previously established ecosystem (or a sterilized one, such as Mt St. Helens)
2) occurs over a longer period of time (thousands of years)
3) pioneer species - the ones that can exist in original inhospitable conditions, lichens, mosses first. These hold moisture and form the beginnings of soil in which seeds of small annuals can germinate. As these decay, woody shrubs like juniper can move in, then eventually trees.
willows on sand bars after a flood scouring
4) climax community - if left undisturbed, succession proceeds to a diverse and stable climax community.
environment determines the type of climax community that will occur in an area.
C. Secondary succession - occurs in a disturbed established ecosystem
Old farm fields are good example - "oldfield ecology" first annuals, then perennials, then shrubs, then pines and fast-growing deciduous trees and finally, after about a century, oaks and other hardwoods return.
Ponds also undergo succession, becoming marshy, eventually filling in. In forests, meadows are usually lakes undergoing succession bake to forest
What is the climax community for here? Modoc County? Eastern San Diego County?