Q.Study the figures given below that depict the comparative age distribution of human populations in Sweden and Rwanda. (International Data Base 2003) and answer the questions that follow : [Figure: Two age-pyramids plotted as Percent of population (Males on left, Females on right) vs Age (years) from 0-4 to 85+ — Sweden, roughly uniform width from base to apex; Rwanda, very broad base tapering to a narrow apex]

When organic matter — dead leaves, animal waste, plant litter — falls to the ground, it doesn't simply vanish. It is broken down through decomposition, a process driven by two groups of organisms working in concert: detritivores (earthworms, millipedes, woodlice) that physically fragment the material, and decomposers (bacteria and fungi) that chemically break down complex molecules into simpler nutrients the soil can absorb.

The speed of decomposition hinges on several factors. Temperature and moisture create the physical environment — warmth accelerates microbial activity, moisture keeps organisms hydrated and mobile. But given that all three soil samples in this question share identical temperature and moisture conditions, the deciding factor becomes the population of organisms doing the actual work — the contents 1-4 plotted in the source graph.

Concept and Intuition

Weeds are often defined as "plants out of place," but from an ecological perspective, they are typically pioneer species or ruderals. These are plants that are excellent at colonizing disturbed or open habitats. Abandoned fields represent a classic example of such a disturbed habitat.

When a field is abandoned, the regular human interventions like tilling, weeding, irrigation, and fertilization cease. This creates an environment where:

1. Competition changes: The dominant cultivated crops are no longer present, and the field is open for other plants to colonize.
2. Environmental conditions can be harsh: The soil might be depleted of nutrients, exposed to intense sunlight, and prone to drought, as there's no active management to maintain optimal growing conditions.
3. Herbivory pressure might increase: Without human protection, plants are more exposed to grazing animals.

Calotropis (commonly known as Aak or Madar) possesses specific biological traits that make it exceptionally well-suited to exploit these conditions, allowing it to thrive where many other plants would struggle.

Reasons for Flourishing

1. Reduced Competition and Absence of Agricultural Practices:

   In an abandoned field, the primary competitors—the cultivated crops—are no longer present. Farmers also cease practices like tilling, weeding, and herbicide application, which actively suppress weed growth. This creates an open niche with abundant resources (sunlight, space, water, and available nutrients) that Calotropis can readily exploit. As a fast-growing, opportunistic species, it can quickly establish itself and outcompete other early colonizers that might be less robust or slower growing. The lack of disturbance allows its root system to develop undisturbed and its aerial parts to grow without being cut back.

2. Tolerance to Harsh Environmental Conditions and Effective Anti-Herbivory Mechanisms: …