Size matters
Forest friends are having a hard time these days: Their field of activity with its animals and plants is rapidly disappearing worldwide. Some protected areas are intended to preserve at least parts of it for the future. But if they are too small, they are too weak for their inhabitants - and maybe also the people.
The year 2006 saw some positive news for the largest rainforest on earth: shortly before the end of the year, Brazil's government designated the world's largest rainforest protection area in the state of Pará, as a kind of Christmas present for ecologists, which will in future permanently protect 15 million hectares of untouched nature should be inaccessible to humans. Falling soybean prices and a moratorium on the part of large soybean buyers to forego goods from freshly cleared areas also halved the destruction rate compared to previous years.
However, 13,000 square kilometers of destroyed rainforests still mean total losses, which almost correspond to the area of Schleswig-Holstein. But much larger areas are affected by the clear-cutting and fire, because every bare farmland or every newly laid road opens the interior of the forest to outside influences: wind and sun dry out the undergrowth, light-loving plants crowd out shade plants, and animal populations are separated from each other - and possibly endangering species. It is therefore no wonder that isolated forest plots lose a large part of their species over time or do not have them at all because the habitat is deteriorating, the necessary ecological niche does not exist there or new colonization by individual species is ruled out because the distances between the suitable habitats are too large.
Up to now, it was primarily unclear whether and how occurrence, extinction and resettlement actually depend quantitatively on area size and location and how these processes all interact. For example, some bird species do not live in a natural area simply because they would not have colonized this area in the previous primary forest due to lack of suitability? And then isolation amplifies this niche problem or does it not affect the appropriately specialized species?
To clarify this, ecologists led by Gonçalo Ferraz from the Instituto Nacional de Pesquisas da Amazônia observed the fate of 55 bird species over 13 years on 23 primary forest areas [1]. Originally all of these areas, ranging in size from one to 600 hectares, were embedded in a continuous carpet of trees, but over time eleven of these have been isolated by newly established cattle pastures. Among the birds selected were species that roam in mixed flocks or are closely linked to army ants and the range of prey they scare. Overall, there was a colorful mixture of undergrowth and canopy species, of very sedentary and highly mobile.
Not surprisingly, smaller plots had lower diversity than larger ones. Because, according to Ferraz, many bird species are so rare that they would not even occur in undisturbed areas because their niche is missing. The rare black-throated ant shrike (Frederickena viridis) is such an example, since it tends to live in clearings or clear cuts, which already have very dense bushes, which only occurs in small areas in extensive forests. The smaller the remaining forest areas were, the faster it disappeared from them - as did similarly strongly connected species. Conversely, the risk of extinction fell the more habitat was preserved: a clear confirmation of previous conservation theories and studies.
The loss of species in areas that are too small does not have to be the result of isolation, as the researchers determined. In 36 out of 55 species, they established a connection with the remaining area available: if they got too cramped or uncomfortable, they disappeared and later stopped immigrating. However, the biologists were only able to pinpoint the isolation of the wooded areas in the case of twelve representatives of this group, so the causes for the remaining two thirds must lie elsewhere. This large remnant would not even colonize these small spots in uninterrupted primary forests, because they would simply lack the basis for life there - an important finding for conservationists, because in future the size of a reserve will count even more than before.
Which and how many reasons are now responsible for the increased losses in small residual natural areas has also not yet been clarified. In addition to the aforementioned climatic changes, increased enemy pressure or increased inbreeding, the consequential damage of which can permanently weaken the population, parasites also play a major role. At least that's what Jason Tylianakis from the University of Göttingen and his colleagues have found out. They compared the food webs of 33 bee and wasp species and nine parasitic insects from five different Ecuadorian ecosystems - including forest, coffee plantations, pastures and fields [2].
The landscape change only changed the composition of the species communities to a small extent, but very clearly the infestation of the host species with parasites: It was relatively lowest in forests and in the relatively natural shade coffee plantations, but it increased in the non-natural habitats sharply - with negative consequences for the services of the host insects such as pollination or pest control. In addition, in the open country, the parasites were concentrated on a few common hosts, while in the forest they were nourished by a broader range of feeders. However, this in turn could weaken some exploited species and thus endanger their long-term survival in these regions.
Especially in the case of key species such as pollen-carrying bees, this in turn triggers chain reactions that sometimes ruin other species: A lack of pollinators is now also considered a crisis factor for biodiversity. And ultimately it also affects people, because bees and the like increase the production capacity of almost ninety crops and a third of the world's food production - perhaps one more argument when the next nature reserve is to be set up.
