Chapter 12 Lecture 2 Flashcards
Populations have…
growth limits
what may regulate populations
the effects of density-independent factors or density dependent factors
density-independent factors
temp., precipitation, catastrophes; whose effects operate without regard to density
density-dependent factors
whose effects increase with increased density
what are density-independent factors sometimes referred to as
extrinsic factors… and most frequently affect “r” selected species
density-dependent factors are sometimes referred to as
intrinsic factors… and most frequently affect “K” selected species
Density independent
factors that limit population size regardless of the population’s density
common factors of density independent limitations
climactic events (tornadoes, floods, extreme temperatures, droughts)
Andrewarth and Birch
challenged density-dependent regulation of population size
the natural regulation of animal numbers by david lack
the importance of density-dependent factors in shaping population size English ornithologist
the distribution and abundance of animals by H.G. Andrewartha and L.C. Birch (Australian entomologists)
they argued that most populations, particularly those of insects and other small invertebrates, are influenced primarily by density independent factors - and that periods of favorable environmental conditions - ultimately control the size of populations
andrewartha and birch were largely influenced by
a study on populations of thrips (small insects that feed on plant parts)
brings ideal conditions for thrip population growth (good combination of moisture, warmth and plant flowering)
the spring in Australia - October through December
not only did the number of thrips follow seasonal changes in weather, but
the peak density of the population varied with climate from year to year. Food supply apparently had little to do with variations in the population
what did andrewartha and birch observe
- at no time were the numbers of thrips so dense to outstrip their food supply
- the variation in the population was accounted for satisfactorily by the physical conditions of the environment
what did andrewartha and birch claim
most of the evidence came from simplified and controlled lab populations, and had dubious application in natural populations
what did andrewartha and birch suggest
the demonstration of density dependence does not necessarily imply regulation of the population by density-dependent factors. one must show under natural conditions that such factors cause dense populations to the decrease and allow sparse populations to increase
who challenged Davidson and andrewartha’s data directly
frederick smith
what did smith argue
that davidson and andrewartha had analyzed their data in such a way that they couldn’t have detected density dependence
what did smith point out
arguments about density dependence do not address the absolute size of a population at a particular time of year; rather, they relate changes in population size to the initial size of the population
what did smith assume
the thrips had exponential growth, and that changes in the size of N may be represented by increments of the logarithm of N
how did smith poke additional holes in andrewartha’s argument
looking at the variances of the averages of his data
what did smith show
the variance in the log N declined between October and November, which suggest a lack of independence between the variables
density dependent
factors that affect population size in relation to the population’s density
negative density dependence
when the rate of population growth decreases as population density increases
the most common factors that cause negative density dependence are
limiting resources (food, nesting sites, physical space)
as a population’s size increases…
resources are divided among more individuals, and per capita resources decline to a level at which individuals find it difficult to grow and reproduce
what do crowded populations
generate stress, transmit disease, and attract predators
how do water fleas influence one another less directly
by eating the same resources (each water flea consumes millions of single-celled green algae and diatoms)
fecundity (progeny per female per day) of water fleas as a function of age at different densities
fecundity decreased as density increased
fecundity also decreased with increasing female age at each density level
the geometric rate of population growth (lambda) calculated from the water flea life tables decreased linearly with
increasing density and fell below 1.0 at a density of about 20 individuals per cm3
territorial behavior in many kinds of animals regulates population size in a…
density-dependent fashion
what does territorial behavior force
the young to socially subordinate individuals to leave the local population and seek space elsewhere (and possibly forego reproduction)
negative density-dependence in plants
when plants are grown at high densities, each plant has access to fewer resources such as sunlight, water, and soil nutrients
self-thinning curve
a graphical relationship that shows how decreases in population density over time lead to increases in the size of each individual in the population; often has a slope of -3/2
positive density dependence
when the rate of population growth increases as population density increases (aka inverse density dependence, or allee effect)
when does positive density dependence typically occur
when population densities are low, which may make it hard to find mates, particularly when sex ratios are uneven
what can low densities lead to
harmful effects of inbreeding and a higher predation risk
what are populations often regulated by
both positive and negative density dependence
what do increased densities provide
more individuals for breeding
above some density, resources become limiting and…
negative density dependence begins to play a role
