Tropical island model reveals how 'disconnected' young Estonian forests are

Gran Canaria is an ocean island with varied landscapes and distinctive wildlife.
Gran Canaria is an ocean island with varied landscapes and distinctive wildlife. Source: Martin Bek

Young Estonian forests created on former agricultural land are isolated habitats with distinct biodiversity that do not interact with, or support the older forest that surround them, Madli Jõks argues in her doctoral dissertation defended at the University of Tartu. She compared the bio-diversity potential of Estonia's young deciduous forests to that of oceanic islands and questioned whether they constitute an even more isolated habitat than that of islands.

According to the researcher, tropical oceanic islands and such "habitat fragments" that are produced in young forests in Estonia are both comparable ecological study objects. These ecosystems are distinguished by their sharp physical boundaries and the immigration and extinction bio-diversity dynamics.

However, unlike actual islands, the isolation of habitat in younger forests is caused not only by their location, as islands are isolated in the ocean, but also by the lack of "functional connectivity" between younger and older adjacent stands.

In her doctoral dissertation, Jõks argues that younger forests on former agricultural land are not "connecting" well to old-forest fragments in terms of species richness; older stands' ability to exploit younger forests is limited.

In her work, she studies simulations of habitat fragments as if they were oceanic islands. Jõks based her methodology on the initial assumption that old-growth forests support a high level of species richness. "The question was, how do the younger forests adjacent to the old forest support biodiversity in these older forests?" she explains. She was particularly interested to study Estonia's deciduous forests.

To investigate the topic, she developed computer simulations of biodiversity evolution in Hawaii and the Galápagos Islands, the Canary Islands, the Cape Verde Islands, and the Azores. The islands, according to Jõks, are an appropriate choice because they are model systems that perform very much as expected. "Oceanic islands of volcanic origin emerge from the seabed. Everything there had to have arrived by long-distance migration or evolved locally."

Today, old-growth forests have been separated from the rest of the environmental in the same way as ocean islands are. "We are increasingly talking about fragmented natural communities in the middle of a human landscape," she says. Simulations of the evolution of forests on the islands and in Estonia revealed that new woods do not support species richness as well as old forests. "The basic takeaway is that not all forests develop equal," Jõks explains.

Forest as an island

Despite the fact that the island was a data simulation or a metaphor in Jõks' doctoral thesis, she also examined genuine archipelagos. "The first two articles I wrote were about real oceanic islands, which have been considered ideal study systems for many years," she says. On the one hand, islands are fascinating in and of themselves. It has long been thought that the behavior of isolated colonies in one habitat, such as an island, will be transferable to other isolated colonies, such as forests, she explains.

What, in this situation, is the link between an island and an Estonian forest? "If we have an archipelago, and each island has a population of some kind, then these populations are connected to a certain extent through the distribution of individuals," Jõks says. The closer the two populations are, the better for the species as a whole: they are less likely to become extinct. "If something happens on the island, for example, if a small population dies out by chance, there are always more individuals coming in from elsewhere."

This is more true for fragmented communities, such as forests separated by fields, than for true islands, she says. On real islands, the populations are so isolated from each other that they start to grow apart. "In a forest or other natural community, the distances and timescales are smaller. There, it's more likely that sub-populations will save each other from extinction," she points out.

Jõks identified at least one similarity between the archipelagos she studied and Estonia's younger forests, i.e. forests that developed on former agricultural land. In both systems, the distribution limitation affected species diversity. "If there are no older forests nearby, there is simply nowhere for the species to come from; they cannot survive." Very young islands and young forests, she says, have the same problem: they simply do not have time necessary to develop rich biodiversity.

Over time, the habitat and environmental conditions there begin to have an effect on biodiversity. "In the case of both the island and the forest, we can ask what is limiting their biodiversity: is it arrival or survival in principle?" In her work, she was more concerned with the survival of species than their arrival.

What affects species richness?

Jõks used simulations to assess the influence of factors on the species diversity of isolated habitats. "Traditionally, the most important factor is the island's size: the larger it is, the more species it supports," she explains. A larger area accommodates more habitats.

that area influences the species diversity of an island in two ways: directly and indirectly. "Sometimes a small island can be very diverse in terms of habitats, allowing it to support a greater number of species than its size would suggest," she explains. On the other hand, based on its surface area, Fuerteventura, a large but ancient, shallow and eroded island in the Canary Islands, might be expected to support more species.

Although the island was very much a metaphor in Madli Jõks' doctoral thesis, she also studied real archipelagos. Source: Martin Bek

Jõks also looked at the impact of archipelago positioning. While Hawaii's islands are perfectly aligned, the Azores have three groupings of islands that are very far apart. "Hawaii is very linear: it's considered to be an absolutely ideal research system." The islands there are arranged in order of age, Jõks says, and species have also migrated from older to younger islands.

"There is even a separate theory regarding the evolution of species diversity on these islands, namely that it follows a curve," she continues. As is evident on Hawaii's youngest island, the species diversity of a young island initially increases. Middle-aged islands are the most species-diverse, as they have had ample time for new species to emerge and landscapes to evolve. Older islands, on the other hand, become shallower, smaller and loose species.

"However, when islands are clustered together, species divergence is less likely because there are more dispersal events between islands." On the one hand, this may result in fewer species. "On the other hand, islands that are close to each other are likely to have once covered a larger area, which again favours more species diversity," Jõks argues.

In another article, she also discussed the islands' history. Both the geological evolution of the islands and variations in sea level were simulated independently. This is the most intriguing aspect because it involves the least amount of speculation. How well do our present patterns of biodiversity correspond to our current conditions? Or how much of the past of the archipelago has been preserved?" Jõks asks.

Two of the three examined archipelagos exhibited the influence of geological evolution. In the Canary Islands, the researcher observed only sea-level effects: at lower sea levels, the two elder islands of Fuerteventura and Lanzarote were more connected. "In Hawaii, similarly, each island in the central island group is more biodiverse, so they have actually been one island with each other," she continues.

The apparent richness of forest diversity in Estonia

In simulations centered on Estonian forests, Jõks evaluated the species diversity on an imaginary island or old forest. "The observational data I used were always collected in old forests," she says. Jõks presumed, as said before, that old forests always support biodiversity successfully. The question was, however, about the younger forests surrounding the old forest.

"I modified the simulation parameters in the younger forests to determine which scenario and which parameters would produce results most similar to the actual data," she explains. Jõks hypothesized that a young forest does not support as much biodiversity as an older forest. However, the contrast was greater than she anticipated.

In a simulation in which she made the conditions of young forests as poor as possible, the results were most similar to the actual data. "From this, we can conclude that the surrounding young forests do not support the vitality of the old forests," she explains.

There is a conservation lesson to be learned from it: "We cannot assume that young forests that have been growing here since the 1970s will perform the same as old stands over 100 years old." Similarly, she asserts that ancient forest fragments wedged between young forests are ineffective because they are too small and isolated.

In the simulations focused on Estonian forests, Madli Jõks assessed species richness on a metaphorical island, namely an old forest. Source: Madli Jõks

For Jõks, the question is what would happen if she made the young forest figures in the simulation even worse. "If I were to say that young forests are like the ocean in my island forests, where everything dies, would the result become more realistic? I don't know," she says. According to her, the next stage in the research process would be to draw quantitative conclusions regarding where and how much of a forest can be felled while it continues to function normally.

The newly-minted Ph.D. says that the broader lesson is that biodiversity responses are durable. If a community in an environment was only recently fragmented, it may support more species than the area of the environment would imply due to its memory of the past. "Biodiversity has not yet responded to landscape changes, but it will in the future. In the case of islands, this process takes millions of years, whereas in the case of landscapes, it may take only a few decades or a century. In any event, there will be a response," Jõks says.

Madli Jõks defended her PhD thesis, "Biodiversity drivers in oceanic archipelagos and habitat fragments, explored by agent-based simulation models" on May 26 at the University of Tartu.


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Editor: Kristina Kersa

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