Key Points

Ecology studies the interactions among organisms and between the organism and its physical environment. It is concerned with four main levels: organisms, populations, communities, and biomes.

A population is a group of individuals of the same species that live in a defined geographical area, share or compete for similar resources, and can potentially interbreed.

A population has attributes that an individual does not, such as birth rates, death rates, sex ratio, and age distribution. These are measured per capita for the entire group.

An age pyramid is a graphical representation of the age distribution of a population. Its shape indicates whether the population is expanding (pyramidal), stable (bell-shaped), or declining (urn-shaped).

Population size is technically called population density (N). It can be measured by total number, biomass, or percent cover, depending on the species and habitat.

Population density fluctuates due to four processes: Natality (births) and Immigration (individuals arriving) increase density, while Mortality (deaths) and Emigration (individuals leaving) decrease it.

The population density at time $t+1$ is given by the formula: $N_{t+1} = N_{t} + [(B+I) - (D+E)]$, where B is births, I is immigration, D is deaths, and E is emigration.

When resources are unlimited, a population shows exponential growth, resulting in a J-shaped curve. The rate of increase is described by the equation $dN/dt = rN$, where 'r' is the intrinsic rate of natural increase.

The parameter 'r' represents the 'intrinsic rate of natural increase' and is calculated as the per capita birth rate (b) minus the per capita death rate (d), i.e., $r = b - d$.

In nature, with limited resources, populations exhibit logistic growth, forming a sigmoid or S-shaped curve. This is described by the Verhulst-Pearl equation: $dN/dt = rN(\frac{K-N}{K})$.

Carrying capacity (K) is the maximum population size that a given environment can sustain indefinitely. In the logistic growth model, population growth slows as it approaches K.

Interspecific interactions arise from the interaction of populations of two different species. These can be beneficial (+), detrimental (-), or neutral (0) for the species involved.

In both predation and parasitism, one species benefits while the other is harmed. A predator kills and eats its prey, while a parasite derives nutrition from a living host.

Competition is an interaction where both species are negatively affected. Gause's Competitive Exclusion Principle states that two closely related species competing for the same limited resources cannot coexist indefinitely.

Mutualism is an interaction where both interacting species benefit. Examples include lichens (fungus and algae) and the relationship between figs and their pollinator wasps.

Commensalism is an interaction where one species benefits, and the other is neither harmed nor benefited. A classic example is an orchid growing as an epiphyte on a mango tree.

Plants have evolved various defense mechanisms against herbivores. These include morphological defenses like thorns (Acacia, Cactus) and chemical defenses like producing poisonous compounds (Calotropis).