Why trees need to stand together

Tropical forests are home to more species than nearly any other ecosystem on the planet, but increasingly this biodiversity is threatened. When forest is cleared, there is a reduction in forest area, which affects biodiversity.

There is also another highly damaging impact that is less obvious: fragmentation. This is when the remaining areas of forest become broken up into isolated patches or fragments. This fragmentation directly harms many species and alters the environmental conditions within the forest. Indirectly, fragmentation also disrupts important interactions between species, which can dramatically affect forest ecosystems.

A new review by Greenpeace, entitled Tropical Forest Fragmentation; Implications for Ecosystem Function, summarises the ecosystem impacts of tropical forest fragmentation including pollination, seed dispersal and, importantly, the implications of predator (tigers, jaguars etc.) loss on ecosystems.

Forest fragmentation can affect the pollination of trees and dispersal of their seeds, both of which are vital for tree reproduction. Long distance pollination is necessary to connect tree populations in separate forest fragments but many insect pollinators, such as orchid bees, will not even cross gaps as narrow as 100 metres.

Fragmentation can disrupt seed dispersal by animals. This is important since, in some tropical regions, up to 90% of tree species are dispersed by animals. Large-seeded trees are particularly vulnerable because fragmentation can cause the loss of nearly all animals large enough to dispersing large seeds (e.g. forest elephants). For example, forest fragments in Mexico had virtually no large-animal seed dispersers, causing the survival of seeds from large-seeded trees to drop by approximately 50%. Hence, fragmentation can have a large negative impact on tree reproduction.

Large predators, such as tigers threatened by expansion of palm oil plantations in Sumatra, are very susceptible to fragmentation and are often absent from a fragmented forest. The loss of top predators can trigger a “trophic cascade” (knock-on effects to the food web of an ecosystem) where populations of species lower down the food chain multiply out of control. This has been seen from grazing animals (herbivores) such as wild pigs, leaf-cutter ants and rodents, with densities becoming 10-100 times higher than in intact forest. Fragmentation-induced trophic cascades can have catastrophic impacts on tropical forests worldwide, showing the presence of top predators to be a fundamental component of a functional, healthy tropical forest ecosystem.

There is a growing recognition of the important role played by predators in regulating ecosystems, not only in tropical forests but also in other ecosystems. Scientists studying the ecological effects of the re-introduction of grey wolves in Yellowstone Park, USA in 1995/6 have seen a “trophic cascade” unfold. Although the evidence is not definitive, it seems that the wolves have kept elk populations in check, allowing trees such as aspen, cottonwoods and willow to regenerate. The greater abundance of willow trees along streams has encouraged beaver populations. Beavers build dams to create pools, which maintain stream flows in the drier months and provide cool, shaded waters for fish. The increase in trees provides more habitats for birds. Thus, the reintroduction of wolves appears to have caused a whole range of effects down the food web, a trophic cascade, creating a more diverse (and healthier?) ecosystem. This story is told in a short video narrated by the environment journalist George Monbiot.

It’s clear that fragmentation strongly alters many aspects of ecosystem function, which could reduce the stability and resilience of forest ecosystems. Whether higher biodiversity such as top predators leads to more robust ecosystem functioning has long been debated. Recent research is increasingly showing this to be the case, providing further evidence that fragmentation must be avoided to preserve healthy ecosystem function.

Finally, fragmentation increases the vulnerability of tropical forests to other threats, such as increasing the accessibility of previously remote areas of forest to hunters and agricultural settlers, while reducing the resilience of forest ecosystems to invasive species. This means once the fragmentation process has begun, it may be difficult to prevent it from continuing. Therefore, a precautionary approach is needed where intact areas of primary forests are protected from fragmentation.

Dr Janet Cotter is a Senior Scientist from Greenpeace International.