Final answer:
Isolation typically leads to speciation, but for fern-like plants, their spore-based reproductive strategy enables them to disperse widely and colonize available niches despite being isolated. This reduces the impact of isolation on speciation, which is why fern numbers may increase without a corresponding increase in species diversity.
Step-by-step explanation:
The reason isolation does not affect the number of fern-like plant species is linked to the reproductive strategy of ferns. Isolation often leads to speciation in animals and some plants, due to the lack of gene flow between separated populations. However, with ferns, reproduction occurs via spores that can travel long distances, potentially reaching many different environments and reducing the effects of isolation.
Ferns are seedless vascular plants with a diverse presence in moist and shaded environments, and they have existed since the Devonian period. The vast majority of fern species are not reliant on sexual reproduction involving pollinators. Instead, each spore can develop into a new individual without fertilization. Given that many fern spores survived both below and above the boundary where extinctions occurred, they had numerous vacated niches to occupy. This increased the abundance of ferns, rather than diversifying them into many new species.
Therefore, even when conditions such as isolation occur, which would normally result in speciation due to genetic drift or natural selection, the ferns' ability to spread their spores can counteract the potential for new species to develop. This highlights why tree ferns and filmy ferns might look very plant-like without the diversity expected from isolated plant species.