**Succession in a heavily overgrazed pasture**

Succession refers to the process of gradual and predictable changes in the composition of a community over time. It occurs in response to changes in environmental conditions, such as bare ground, disturbances, or the introduction of new species. In the given options, succession is most likely to occur in a heavily overgrazed pasture, which is option B. Let's explore why.

**Explanation**

1. **Definition of succession:** Succession is the process by which a community of organisms changes over time following a disturbance or changes in environmental conditions. It involves the colonization of new species, their growth, and interactions, leading to a more stable and complex community.

2. **Heavily overgrazed pasture:** When a pasture is heavily overgrazed, it means that the vegetation has been extensively consumed by grazing animals, leaving the ground bare or with only a few remaining plants. This excessive grazing can disturb the ecological balance and significantly impact the plant community.

3. **Disturbance and bare ground:** Overgrazing leads to bare ground, exposing the soil. Bare ground provides an opportunity for new plant species to colonize the area. The bare soil is often rich in nutrients and has reduced competition from existing plants.

4. **Pioneer species:** In the absence of vegetation, pioneer species such as annual grasses or herbaceous plants are often the first to colonize the bare ground. These species have adaptations that allow them to quickly establish themselves in harsh environments, such as rapid growth, efficient resource utilization, and high reproductive rates.

5. **Facilitation and competition:** As the pioneer species establish themselves, they modify the environment, making it more suitable for other plant species. They improve soil fertility, moisture retention, and provide shade, creating microhabitats for other species to thrive. This process is known as facilitation.

6. **Succession stages:** Over time, the pioneer species are replaced by other plant species that are more competitive and better adapted to the changing conditions. This leads to a sequence of different plant communities, each influencing the environment and preparing the site for the next stage of succession.

7. **Timeframe of succession:** In a heavily overgrazed pasture, where the vegetation has been significantly impacted, the process of succession can occur relatively quickly compared to other options given. The availability of bare ground and the rapid colonization by pioneer species can initiate a series of changes in the plant community within a short period.

Therefore, the correct answer is option B, a heavily overgrazed pasture, where succession is likely to occur in a short time due to the availability of bare ground and the rapid colonization by pioneer species.