ORGANISMS AND POPULATIONS · Comparison Tables

1. Organism vs population

Levels relevant to population ecology
Feature	Organism	Population
Meaning	One individual living entity.	Individuals of one species in a defined area, sharing/competing for resources and potentially interbreeding.
Attributes	Birth and death occur to individuals.	Natality, mortality, sex ratio, age distribution and density are population attributes.
Evolution	Individual copes with changed environment.	Natural selection acts at this level and changes populations over time.

2. Measures of population density

Density is not always a head count
Measure	When useful	NCERT example
Number	When individuals can be counted and are comparable in size.	Lotus plants in a pond.
Biomass / percent cover	When size or ecological role differs greatly.	One huge banyan versus many carrot-grass plants.
Relative density	When absolute count is impractical.	Fish caught per trap; tiger census by pug marks and faecal pellets.

3. Natality and immigration vs mortality and emigration

Changes in population size
Process	Effect on N	Meaning
Natality (B)	Increases	Births added during a given period.
Immigration (I)	Increases	Individuals entering from elsewhere.
Mortality (D)	Decreases	Deaths during a given period.
Emigration (E)	Decreases	Individuals leaving the habitat.
Equation	N_t+1 = N_t + [(B + I) - (D + E)].

4. Exponential vs logistic growth

Population growth models
Feature	Exponential growth	Logistic growth
Resources	Unlimited.	Finite and progressively limiting.
Equation	dN/dt = rN	dN/dt = rN ((K-N)/K)
Curve	J-shaped / geometric.	S-shaped / sigmoid.
Key term	r = intrinsic rate of natural increase.	K = carrying capacity of habitat.
Realism	Applies when checks are absent.	Generally more realistic in nature.

5. Age pyramids and population status

Age distribution of males and females
Shape	Population status	Age-group pattern
Triangular	Growing	Large pre-reproductive group.
Bell-shaped	Stable / stationary	Comparable pre-reproductive and reproductive groups.
Urn-shaped	Declining	Pre-reproductive group is relatively small.

6. Major population interactions

Effect on two interacting species
Interaction	Effect	Example
Mutualism	+/+	Lichen; fig and wasp; mycorrhiza.
Competition	-/-	Balanus excluding Chthamalus.
Predation	+/-	Predator and prey; herbivore and plant.
Parasitism	+/-	Lice on humans; Cuscuta on hedge plants.
Commensalism	+/0	Orchid on mango; cattle egret with cattle.
Amensalism	-/0	One species harmed, other unaffected.

7. Predation vs parasitism

One benefits while the other is harmed
Feature	Predation	Parasitism
Relationship	Predator feeds on prey, usually killing/consuming it.	Parasite derives food/shelter from host and reduces host fitness.
Ecological role	Transfers energy, regulates prey and can maintain diversity.	Can reduce host survival, growth and reproduction.
Adaptations	Prey show camouflage, chemical defence and plant thorns/toxins.	Parasites may lose sense organs/digestive system, have suckers and high reproduction.
Examples	Pisaster starfish; cactus moth controlling prickly pear.	Human liver fluke; malarial parasite; brood parasite cuckoo.

8. Ectoparasite vs endoparasite

Where parasites live
Feature	Ectoparasite	Endoparasite
Location	External surface of host.	Inside host body.
Examples	Lice on humans, ticks on dogs, copepods on marine fish.	Human liver fluke; malarial parasite.
Special point	Cuscuta is an external parasitic plant that has lost chlorophyll and leaves.	Often has complex life cycle with intermediate hosts or vectors.

9. Competition vs competitive exclusion

When species use similar resources
Feature	Competition	Competitive exclusion
Meaning	Fitness of one species (r) is lower in presence of another.	Competitively superior species eliminates the other under given conditions.
Resources	May be limited, or interference can occur even when resources are abundant.	Two closely related species competing for the same resources cannot coexist indefinitely (Gause).
Evidence	Competitive release after removing competitor.	Balanus excludes Chthamalus from intertidal zone.

10. Commensalism vs mutualism

Positive interactions
Feature	Commensalism	Mutualism
Outcome	One benefits, other unaffected (+/0).	Both species benefit (+/+).
Examples	Barnacles on whale, clown fish and sea anemone, cattle egret and cattle.	Lichen, mycorrhiza, fig tree and wasp.
Co-evolution	Not necessarily tightly linked.	Often tightly linked, as with a fig species and its pollinating wasp.