Organisms and Populations -2 Flashcards
Growth Models: exponential growth model
Resource (food and space) availability is
obviously essential for the unimpeded growth of a population.
Ideally, when resources in the habitat are unlimited, each species has the ability to realise fully its innate potential to grow in number,
as Darwin observed while developing his theory of natural selection.
Then the population grows in an exponential or geometric fashion
Any species growing exponentially under unlimited resource conditions can reach enormous population densities in a short time, even slow growing animals such as elephants.
formula for exponential growth rate
If in a population of size N, the birth rates (not total number but per capita births) are represented as b and death rates (again, per capita death rates) as d, then the increase or decrease in N during a unit
time period t (dN/dt) will be
dN/dt = (b – d) × N
Let (b–d) = r, then
dN/dt = rN
The r in this equation is called the ‘intrinsic rate of natural increase’ and is a very important parameter chosen for assessing impacts of any biotic or abiotic factor on population growth.
To give you some idea about the magnitude of r
values, for the Norway rat the r is 0.015, and for
the flour beetle it is 0.12. In 1981, the r value for
human population in India was 0.0205.
curve of exponential growth model
The above equation describes the exponential or
geometric growth pattern of a population (Figure
11.3) and results in a J-shaped curve when we plot N in relation to time.
Nt= N0 e^rt
Nt = Population density after time t
N0 = Population density at time zero
r = intrinsic rate of natural increase
e = the base of natural logarithms
logistic growth model
- No population of any species in nature has at its
disposal unlimited resources to permit exponential growth.
-This leads to competition between individuals for limited resources. Eventually, the ‘fittest’ individual will survive and reproduce.
what is meant by carrying capacity of environment
. In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. Let us call this limit as nature’s carrying capacity (K) for
that species in that habitat.
graph of logistic growth curve
A population growing in a habitat with limited resources show initially a lag phase, followed by phases of acceleration and deceleration and finally an asymptote, when the population density reaches the carrying capacity.
A plot of N in relation to time (t)
results in a sigmoid curve. This type of population growth is called
Verhulst-Pearl Logistic Growth
equations of logistic growth
dN/dt = rN( K-N/K)
Where N = Population density at time t
r = Intrinsic rate of natural increase
K = Carrying capacity
Since resources for growth for most animal populations are finite
and become limiting sooner or later, the logistic growth model is
considered a more realistic one.
Life History Variation
Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (high r value), in the habitat in which they live.
Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy.
Some organisms breed only once in their lifetime (Pacific salmon fish, bamboo) while others breed many times during their lifetime (most birds and mammals).
Some produce a large number of small-sized offspring (Oysters, pelagic fishes) while others produce a small number of large-sized offspring (birds, mammals).
Ecologists suggest that life history traits of organisms have evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat in which they live.
Evolution of life history traits in different species is currently an important area of research being conducted by ecologists.
define mutualism and its benefits
This interaction confers benefits on both the interacting species.
Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria.
Mycorrhizae are associations between fungi and the roots of higher plants.
The fungi help the plant in the absorption of essential nutrients from the soil.
The plant in turn provides the fungi with energy-yielding carbohydrates.
Plant-Animal Mutualism
The most spectacular and evolutionarily fascinating examples of mutualism are found in plant-animal relationships.
Plants need the help of animals for pollinating their flowers and dispersing their seeds.
Animals have to be paid ‘fees’ for the services that plants expect from them.
Plants offer rewards or fees in the form of pollen and nectar for pollinators and juicy and nutritious fruits for seed dispersers.
The mutually beneficial system should also be safeguarded against ‘cheaters’ (e.g., animals that try to steal nectar without aiding in pollination).
what is meant by coevolution
Plant-animal interactions often involve co-evolution of the mutualists.
The evolution of the flower and its pollinator species are tightly linked with one another.
mutualism in fig tree
A given fig species can be pollinated only by its ‘partner’ wasp species and no other species.
The female wasp uses the fruit as an oviposition (egg-laying) site and uses the developing seeds within the fruit to nourish its larvae.
The wasp pollinates the fig inflorescence while searching for suitable egg-laying sites.
In return for pollination, the fig offers the wasp some of its developing seeds as food for the developing wasp larvae.
Orchid and Pollinator Co-evolution
- Orchids show a bewildering diversity of floral patterns, many of which have evolved to attract the right pollinator insect (bees and bumblebees) and ensure guaranteed pollination.
- Not all orchids offer rewards.
The Mediterranean orchid Ophrys employs ‘sexual deceit’ to get pollination done by a species of bee.
One petal of its flower bears an uncanny resemblance to the female bee in size, colour, and markings.
The male bee is attracted to what it perceives as a female, ‘pseudocopulates’ with the flower, and during this process is dusted with pollen from the flower.
When the same bee ‘pseudocopulates’ with another flower, it transfers pollen and thus pollinates the flower.
- Co-evolution operates in this interaction:
If the female bee’s colour patterns change even slightly during evolution, pollination success will be reduced unless the orchid flower co-evolves to maintain the resemblance of its petal to the female bee.
Commensalism
This is the interaction in which one species benefits
and the other is neither harmed nor benefited. An orchid growing
as an epiphyte on a mango branch, and barnacles growing on the
back of a whale benefit while neither the mango tree nor the whale
derives any apparent benefit
cattle egret and cattle relationship
The cattle egret and grazing cattle in
close association, a sight you are most likely to catch if you live in farmed rural areas, is a classic example of commensalism.
The egrets always forage close to where the cattle are grazing because the cattle, as they move, stir up and flush out insects from the
vegetation that otherwise might be difficult for the egrets to find and catch.
