An individual can be born or die, but a population has per capita birth and death rates. Similarly, an individual is male or female, but a population has a sex ratio. A population also has an age structure or age pyramid and a measurable size or density.
Populations possess birth rate, death rate, sex ratio, age distribution and population density; individuals do not possess these attributes as population-level properties.
For exponential growth, Nt = N0 e^(rt). If the population doubles in 3 years, 2N0 = N0 e^(3r). Therefore, 2 = e^(3r), so ln 2 = 3r. Hence r = ln 2 / 3 = 0.693 / 3 = 0.231 per year.
The intrinsic rate of increase is approximately 0.231 per year.
Plants cannot run from herbivores, so they use physical and chemical defences. Thorns in Acacia and cactus are morphological defences. Chemicals such as cardiac glycosides in Calotropis, nicotine, caffeine, quinine, strychnine and opium can make herbivores sick, inhibit feeding or digestion, disrupt reproduction or kill them.
Important plant defences include morphological defences such as thorns and chemical defences such as toxic, distasteful or digestion-inhibiting compounds.
The orchid benefits by getting support and better access to light as an epiphyte. The mango tree is neither harmed nor benefited. An interaction in which one species benefits and the other is unaffected is called commensalism.
This is commensalism.
Predators keep prey populations under control. Biological control uses this ecological relationship by introducing or encouraging natural enemies of pest insects, reducing pest density without relying only on chemical pesticides.
The principle is predation, where natural predators regulate prey or pest populations.
Population refers to members of one species, such as lotus plants in a pond or rats in a dwelling. A community contains many populations, such as plants, animals and microbes interacting with each other in a habitat.
A population is a group of individuals of the same species living in a defined geographical area and sharing or competing for similar resources. A community is an assemblage of populations of different species living and interacting in an area.
Commensalism is represented by +/0 interaction. Parasitism is +/-, with the parasite benefiting at the host's cost. Camouflage protects prey from detection by predators. Mutualism is +/+, where both partners gain, as in fungi and algae in lichens. Interspecific competition occurs between different species for resources or through interference, reducing fitness.
(a) Commensalism: one species benefits, the other is unaffected; orchid on mango tree. (b) Parasitism: parasite benefits and host is harmed; lice on humans. (c) Camouflage: concealment by matching surroundings; cryptically coloured insects or frogs. (d) Mutualism: both species benefit; lichen or fig-wasp relationship. (e) Interspecific competition: different species compete and one or both lose fitness; flamingoes and fishes competing for zooplankton.
When resources are limited, population growth cannot remain exponential. The population initially grows slowly, then rapidly, and later slows as resources become limiting. It approaches the carrying capacity K, the maximum number the habitat can support. The Verhulst-Pearl logistic growth equation is dN/dt = rN((K - N)/K). A correct diagram should show population density N on the y-axis, time t on the x-axis, and an S-shaped curve approaching K.
A logistic population growth curve is sigmoid or S-shaped. It begins with a lag phase, rises rapidly during acceleration, slows during deceleration and finally becomes nearly stable at the carrying capacity K.
- a. One organism is benefited.
- b. Both the organisms are benefited.
- c. One organism is benefited, other is not affected.
- d. One organism is benefited, other is affected.
In parasitism, the parasite benefits by obtaining food and shelter from the host, while the host is harmed. Therefore the interaction is represented as + for the parasite and - for the host.
(d) One organism is benefited, other is affected.
Population density is the size of a population in a habitat and may be measured by number, biomass or per cent cover. Birth rate is the per capita number of births added to the population in a given time. Age distribution is the proportion of individuals in different age groups; its age pyramid indicates whether the population is growing, stable or declining. Other valid characteristics include death rate and sex ratio.
Three important population characteristics are population density, birth rate and age distribution.