We are down to the final six lectures of the semester -- began about 12 weeks ago with the simplest levels of biological organization (e.g. cells) -- we are now ready to consider the most complex levels of biological organization: populations, communities, and ecosystems.
So in the time that remains, we are going to consider the relationships of organisms to their environments -- this is the branch of biology known as ecology.
Ecology: scientific study of the interactions that determine the distribution and abundance of organisms -- more simply, the study of how organisms interact with their environment.
Ecology -- an often misused word -- is sometimes used incorrectly as a synonym for environment -- e.g. "I 'm concerned about the 'ecology' so I recycle newspapers.
Environment: "where the organism lives" -- also called the organism's habitat.
An organism's habitat consists of two general kinds of environmental factors:
1. biotic factors -- living, biological factors that the organism interacts with -- e.g. predators, parasites and diseases -- competitors (both intra- and interspecific).
2. abiotic (physical) factors -- nonliving factors in the environment such as temperature, moisture, sunlight, nutrient concentrations in the soil or water, pH, etc.
Ecologists study interactions at several levels:
1. individual -- some ecologists are interested in how organisms respond physiologically to their environments -- physiological ecology -- e.g. how desert organisms physiologically deal with high temperatures and low water availability.
2. populations -- groups of organisms of the same species present at a particular place at a particular time-- e.g. kangaroo rat population on a 2 acre plot in Anza-Borrego Park -- what interactions influence population size, growth rate, age structure etc.?
3. communities -- collection of populations at a given locality -- e.g. rodent community on the 2 acre plot -- how do community members interact?.
4. ecosystem -- the community and the abiotic (physical) environment considered together -- e.g. desert ecosystem; freshwater lake ecosystem --
We can think of the earth as one large ecosystem -- sometimes referred to as the biosphere -- obviously a very complex biological system.
Studying populations, communities and ecosystems presents some special problems for biologists:
1. Experimental manipulations of these systems are expensive and labor intensive.
2. Because of the complexity of these systems and the large number of variables involved, it is usually difficult to predict the results of experiments or disturbances -- e.g. global warming.
Let's talk about the ecology of populations.
Populations have a number of characteristics that are of interest to ecologists.
Two most important are distribution and density (population size).
Distribution: where organisms are found on earth -- determined by biotic and/or abiotic factors -- e.g. large trees (oaks, hickories, pines) are not found in dry habitats like grasslands and deserts because there is not enough rainfall to sustain their growth -- evolution has adapted trees to moist habitats -- organisms may be absent from a habitat due to the presence of predator species or competing species.
Where a particular species of organism does occur, the spatial relationships of individual organisms to one another may take several different forms -- this is called dispersion.
Dispersion: spatial distribution of individuals of a population -- three general kinds of patterns:
1. clumped (aggregated) dispersion -- individuals clumped together -- resources and suitable habitat may be patchy in distribution which causes organisms to form "clumps" -- e.g. pillbugs under rocks -- some organisms may come together in clumps for breeding purposes-- e.g. seals,
2. regular dispersion -- individuals all about the same distance from one another -- often caused by competition among individuals in the same population -- e.g. desert shrubs compete for water and often display a regular dispersion pattern.
3. random dispersion -- habitat is relatively uniform so individuals are neither repelled or attracted to one another.
Population density -- number of individuals per unit area or unit volume -- density is a measure of population size.
Like distribution, density is influenced by biotic and abiotic factors in the environment.
There are four primary factors that determine population size.
1. Natality -- addition of organisms to population through reproduction -- increases population density.
2. Mortality -- lose of organisms from the population due to death -- decreases population density.
3. Immigration -- addition of organisms that move into a population -- same effect as natality (+).
4. Emigration -- loss of individuals that move out of the population -- same effect as mortality (-).
We will ignore immigration and emigration and concentrate on natality and mortality.
Contributions of these processes can be expressed as rates:
e.g. birth rate (b) = number born/individual in pop./unit time. -- e.g. if a population of 500 mice produces 100 young by reproduction in a month, then b = 100/500 per month = 0.20/individual/month.
death rate (d) = number dying/individual in pop./unit time -- e.g. if during one month 80 individuals in our population of 500 die, then d = 80/500 per month = 0.16/individual/month
The difference between b and d determines whether the population increases, decreases or remains constant in size.
Clearly, if b > d, the population will increase size through time; if b = d, population is not changing size; if b < d, population is getting smaller.
rate of population increase = r = b - d -- can be positive, negative, or zero.
Note: rate of population increase can be made larger by increasing b, decreasing d or both!
Change in population size over a time interval (DN) can be predicted if we know r and the size of the population at some time (Nt): DN = rNt
For you business majors: this is just like an interest calculation, where Nt is the principle, r is the interest rate, and DN is the amount of interest accrued.
This equation describes a form of population growth called exponential growth -- population will grow by ever increasing increments as N gets larger -- if plotted against time (t), a J-shaped curve is generated.
Figure 39.4.
Under optimum conditions, the rate of increase for a particular species population will reach a maximum value called the biotic potential (rm).
In the real world, populations never realize their biotic potential, nor do they display exponential growth for very long -- to have exponential growth, factors such as space, food, etc. must be unlimited.
In the real world, resources required for population growth are limited -- populations face environmental resistance to further growth as resources become less and less available -- this environmental resistence prevents populations from growing exponentially or achieving their biotic potential.
Carrying Capacity (K) -- a given environment only has enough resources to sustain a given population size for a particular organism -- if the population size is less than carrying capacity, the population increases in size; at carrying capacity population size doesn't change.
We can modify our equation for population growth to include this concept of carrying capacity -- magnitude of change in population size should be a function of how close population is to its carrying capacity:
DN = r [(K-N)/K] N
Notice that if N is small, (K-N)/K is nearly 1 and pop. will approximate exponential growth -- if N = K, population won't change in size -- if N is greater than K, population will decrease in size.
This is called the equation for logistic population growth and plotting N vs. time produces an S-shaped or sigmoidal curve (See Diagram).
Figure 39.6.
Recall that mortality and natality underlie a population's ability to change in size.
Mortality and natality rates are influenced by another important characteristic of populations and this is age structure -- this is because both mortality and natality are age-specific processes -- individuals of different ages have different probabilities of dying and different reproductive contributions.
Age structure often depicted graphically -- percentage of population in each age class and half of pyramid for each sex -- shape of graph tells us something about population dynamics -- e.g. more-developed countries (MDCs) and less-developed countries (LDCs).
1. pyramid shape -- characteristic of LDCs -- means high proportion of population in young age classes -- usually indicates high reproduction and high population growth.
Figure 39.11.
2. bell or urn shape -- characteristic of MDCs -- greater proportions of older individuals means slow or even negative growth.
Survivorship curves: based on following a cohort of individuals from birth until all have died -- age at death recorded.
1. Type I -- good early survival and die of old age -- human populations show this type of survivorship -- usually have low birth rates -- provide lots of parental care.
2. Type II -- organisms die at a relatively constant rate regardless of their age -- e.g. some birds, lizards, small mammals.
3. Type III -- poor early survival -- survival improves with age -- many invertebrates, fish, plants -- produce lots of offspring and low parental care.
Figure 39.8.
Species have reproductive rates and survivorship curves that have been shaped by natural selection -- determined by the kinds of environment the species has evolved in -- the patterns of reproduction and survivorship displayed by species are called life history patterns.
Ecologists have used the terms of the logistic growth equation to name two general kinds of life history patterns.
1. r-selected species -- species
inhabiting fluctuating or unpredictable environments -- environments
keeps populations well-below carrying capacity -- selection favors
traits for increased reproductive rates (r) -- selection favors the
following life history traits:
a. early reproduction
b. many offspring per reproductive effort
c. little or no parental care.
r-selected species are often found in disturbed habitats -- e.g. weeds -- good dispersers and good colonizers.
2. K-selected species -- species inhabiting relatively stable, predictable environments -- population size often near caring capacity and resources are scarce -- selection favors traits that make organisms good competitors -- selection favors a different set of life history traits:
a. delayed maturity
b. few offspring per reproductive effort
c. much parental care.
K-strategists are good competitors and often specialists -- become established and exclude invaders/colonizers.
A couple of pertinent points about r and K selection:
1. These strategies are actually two ends of a continuum -- some species may be intermediate.
2. These are relative terms -- e.g. Species A is an r-strategist compared to Species B.
3. Some populations of a single species may be more r-selected than other populations of the same species due do differences in the environments where they live.
Next time: Population Interactions