The Biological Diversity of Forests

By Peder Agger

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Danida Forest Seed Centre's Seminar om skove og træer i miljø- og udviklingssamarbejde med fokus på bevaring og anvendelse af genetiske ressourcer. Mandag den 27. april 1998 på Skovskolen i Nødebo.

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Sustainable development is a claim for societal and thus political change in the way we live in our environment and share the benefits among nations and citizens. But we are far from sustainable development. Biodiversity is decreasing at an alarming rate and the need for action is met by reaction (Beder,1997; Lomborg 1998; Agger & Brendstrup,1998).

As an introduction I will confine myself to repeating the main stream opinion of the problem which is the basis for the Convention on Biological Diversity and thus for action and reaction.

There are four questions to consider: 1) is it a serious problem? 2) Are tropical forests species rich? 3) Are they decreasing at an alarming rate? And 4) Will this inevitably lead to mass extinction of species (and other forms of biodiversity)?

All four questions are recognized by the Convention and at the same time questioned by those that don't like it. I hope that some kind of an answer will be given in the following. And I will come back to the issue by the end of my contribution. But before we go on a few key concepts and the problems in dealing with them should be clarified.

Key concepts: Biodiversity, forests, habitats, and extinction

From a biological perspective biodiversity is a relative notion which will have to be specified by reference to a certain level of organization and by reference to a certain area. At the genetic level biodiversity may be defined in terms of variations in genotypes. Thus defined the notion of biodiversity may also be applied within a given species - a subspecific diversity. This is for example the case with domestic species where there is a concern to preserve genetic diversity within species as a resource for future breeds.

Other levels of organization where biodiversity may appear are: the population, the community, the landscape and the ecosystem level. However, until now, biodiversity at other than species level has not been easy to handle and therefore not very much in focus as regards to nature preservation. At the species level biodiversity may be defined in terms of number of species found within a certain area. And it is this type of biodiversity I will focus on. Biodiversity is thus a quality that can be expressed at different levels of organisation (gene, population, species etc.) within a specified spacial level (global, regional, local) and, when it comes to change, within a specific period of time.

It is not up to the biologists alone to define what the concept 'a forest' stands for, but for clarification I have to tell how I use the word in the following. According to Whitmore and Sayer (1992): Closed forests in the tropics can be of three types: rain forests, monsoon (or seasonal) forests and thorn forests. Together the two first are called tropical moist forests and these are what I am going to talk about in the following, unless anything else is stated.

Habitat is strictly speaking a species specific character that describes where an animal or plant species usually can be found. - its address as E. Odum (1971) called it. An overarching concept is the niche which Pianka (1978) defined as the sum total of all the various ways in which a given organismic unit conforms to its particular environment. Thus habitat is what some authors call a 'place niche'. But by all those other than ecologists, habitats are something that are defined not by the species, but as a quality of the environment.

Extinction means the absolute and final disappearance, disappearance at the global level, of species (or other types of organismic units). But very often disappearance at a lower spacial level, a 'local extinction', may be almost as negative as a global extinction and in the literature published in English any disappearance is called extinction. That is confusing.

We can conclude this section by saying that in the rest of this contribution we will focus on species diversity. We will emphasize its dependence on scale and the possible loss of habitats and consequent disappearance and eventual extinction.

The importance of forests as habitats

That the forest is rich in habitats means that many different species may find a living there. Tropical moist forest is a type of ecosystem which is specially rich. The forest is highly stratified. Trees generally form three layers. Climbing plants like liana and epiphytes abound. Often there are more tree species in a few acres than the entire flora of Europe (Odum, 1971) e.g. in a sample plot in the rainforest in the Andean foothills of Bolivia 403 bird species and 204 woody plants were found within a quarter of an acre (Heywood & Stuart, 1992) which is 10 to 20 times as much as we on average can be found in a Danish natural forests. But how many species can actually be found within a given forest depends, like in other ecosystems, on its size, age, and state.

Compared to other ecosystems virgin tropical moist forests are characterized by a very high gross primary production, but also by a high standing biomass and thus very little net community production. Almost all nutrient in the system is bound in the dead or living biomass. And the community is characterised by a relatively high number of different decomposers and fungal mycorrhiza forming species.

The tropical moist forest is, compared to other forests, characterised by having many fauna-flora interrelations. The trees are dependant either on pollinators or on animal species for dispersal of seeds e.g. many birds are herbivorous. Another decisive and disputed characteristic of the tropical moist forest is an extremely high frequency of endemic species i.e. species with a very confined area of distribution. And there is an equally important subspecific endemism as well. This is more true in Asia and Africa than in the neo-tropics. But even there it is much higher than what usually might be found outside the tropics. And generally many species in the tropical forest have a small geographical range. Simberloff (1992) mention that neotropical cloud forest plants can be endemic to isolated sites smaller than 10 km2. This makes them obviously more susceptible to threats.

This is the clue to understand why even a rather modest decrease of the overall area of tropical moist forests can be feared to have a proportional influence on the overall number of tropical species. It is also why a high number of endangered species are to be found in the tropics although there still exists much more virgin vegetation than there can be found at our latitudes.

Some areas are more species rich than others which means that nature protection and cautious management is more rewarding at some places than others. Such centres of biodiversity may be recognised on global, regional, and local scales. They are also called 'hot spots'. According to Gaston & Williams (1996) they have also been used to refer to areas where high biodiversity and threat coincided.

For example Hawksworth & Kalin-Arroyo (1995) refer to Myers (1988) for having identified ten areas, comprising 3.5% of the remaining primary forest, that harbour an estimated 34,400 endemic plant species and 27% of all tropical forests plants. But centres of diversity are not at all always congruent across taxonomic groups. And if not all areas are investigated equally well, the best investigated may be mistaken as centres of high diversity. They also refer to IUCN for having designated preliminary 234 centres for plant species diversity. These are species rich and have a high content of endemic species. Other criteria for designation have been high diversity of gene pool for potential useful species, diverse range of habitat types, significant proportion of species adapted to special edaphic conditions, and threats.

On a global scale centres of diversity are often found in areas with (geologically) rapid transition and therefore high ecological heterogeneity such as coastal zones, young mountains and islands. At a regional level endemism is often found on habitat islands e.g. mountain tops and real islands. A sixth of the world total of higher plants i.e. 40.000 species are endemic to oceanic islands (Heywood & Stuart,1992). At local scale diversity is strongly dependant of the heterogeneity of the landscape. And in a cultural landscape the degree of human disturbance is decisive.

Effects of degradation

Human disturbance is not by definition negative to biodiversity. At the global scale it is. But I will dare to postulate that this is less the case at a regional scale, and might be even less at the local scale, although it often not is the case. This is because mans activity in some areas and at some scales lead to higher heterogeneity of habitats and thus biodiversity.

Total deforestation, as we know it from many parts of Europe, is of course more damaging than partial deforestation, or we may say degradation of forests is better than removal of forests. And one clear cutting is less damaging than repeated clear cuttings, not least if it happens in areas with endemic species. Generally intensive use of the forest e.g. the number of trees removed, is more damaging to the biodiversity than more extensive uses, and uses affecting the whole forest are more damaging than uses in a more variate spacial pattern.

Spacial heterogeneity is thus the keyword. If the degradation only affects a part of the forest most of the original species may survive. Johns (1992) gives an example from Sabah where up to 73% of the trees in heavily logged forests were lost. Most of the animal species were able to persist either within the logged-over forest itself or within less damaged stands on slopes an along streams.

Rare species will be susceptible to depletion, particularly if they are highly valued timbers. As happened in the whaling industry we also see in the logging industry that the number of species used is increasing in many parts of the world along the depletion of the primary forests are emptied for their primary targets. Also, many animal species especially among insects such as larvae of butterflies and fungi which are specialists and bound to the occurrence of distinct plant species are threatened by any degradation of the forest. Genetic erosion becomes critically intense, where large blocks of land - especially in regions topography or vegetation - are treated in a more uniform fashion, such as in large Amazonian cattle ranches (Brown & Brown,1992).

One effect that only may be visible, long after the actual impact, is the consequences of fragmentation of the original continuous primary forest. The animal and plant populations on the remnant habitat islands will be subject to two different processes. The one is the edge effect from the adjacent logged or cultivated areas. It can be drying out of the forest edge, increased predation from predators in the surroundings, and other forms of disturbance.

The other is the island-effect where the rate of disappearance of species increases because of the populations now being smaller and more isolated and exposed to genetic drift, accidental fluctuation of abundance, and at the same time a decreased rate of immigration of new species because of the isolation of the habitat island. The net result being an average decrease in the overall species richness at the remnant habitat islands as well as in the logged areas of the forest.

At a regional level this island-effect may get another dimension. Today forests are more isolated from one another than they formerly were. If climate changes as a consequence of the greenhouse effect, the change in temperature and rainfall associated with even the most conservative predictions of climate change will have severe ecological impacts on protected areas (Whitmore and Sayer,1992).

The importance of biodiversity for maintenance of habitats

Many of the ecological services stemming from undisturbed tropical forests can be provided from degradated or planted forests as well. Carefully managed they can store carbon as well as the primary forest, moderate clima, and regulate water supply.

The major problem is the decrease in biodiversity. Biodiversity is specially important to provide resilience ('elasticity') to the ecosystems enabling them to adapt to changing conditions (Swansson,1997). Resilience measures if and when the system falls back into the previous realm of variation, or if it is reset to a new long-term dynamic.

Because of the before mentioned frequent and close interdependence between the species, the disappearance of one species often will consequently be followed by the disappearance of others in an ongoing 'cascade'. 'Each species lost from a community represents dozens of lost interactions, leaving a potentially gaping hole in the ecological web (Kunin and Lawton, 1996).

A well known example of such a dependence where the damage happened long before the threatening finale was found some years ago where a rare tree species from Mauritius, the fleshy fruited Calvaria trees, were found to be threatened by the extinction of the dodo (Raphus cucculatus) 200 years earlier. The bird was essential for the dispersal and germination of the seeds.

But how many species are needed to keep the ecosystems running? According to Kunin and Lawton (1996) there exists two contrasting hypothesis. The redundant-species hypothesis suggest that there is a minimum diversity necessary for proper ecosystem functioning, beyond which most species are redundant in their roles. And the all-species-make-a-contribution hypothesis that suggest that all species contribute to the ecosystem processes, so that the functioning of these declines progressively as species are lost.

It is not possible to come up with more precise predictions than the general observation that the adding of more species to an ecosystem follows generally deceleration functions, and vice versa that is subtracting of species will have an accelerating effect. An exception is, however, the keystone species. An example of this from temperate forests is the beaver that controls the water level, others are predator species that control the herbivores, and (the few) larger tree species which may dominate an ecosystem. The loss of keystone species may have large and dramatic consequences.

Methods of conservation

The best way of avoiding erosion of biodiversity is not necessarily to leave the forest as a reserve undisturbed by man. Most forest, almost all forest, even the tropical moist forest in the Amazonian Basin have been heavily influenced by man since long.

Durell Posey (1998), who has worked for more than a decade among the Kayapo indians in Matogrosso, Brazil, has demonstrated how, what was once thought to be the virgin tropical rain forest, is a cultural landscape. Seeds and saplings of beneficial plant species are transported by man over large distances. In the slash-and-burn agriculture he estimate that 86-98% of the needs among the indians are covered by non-domesticated resources i.e. post crop plants and plant species that just have been sawn or planted - not cultivated in the surrounding landscape by the indians.

The best method to avoid undue losses of biodiversity is in situ conservation i.e. to take care of it by careful planning. The use of forests should be seen as an integral part of the whole region in order to avoid accidental side effects for example access gained through logging roads that lead to intrusion of unwanted settling, poaching, forest fires, intrusion of exotic species etc.. The planning should also include designation of areas where forestry operations are banned combined with a design of a management scheme aiming at minimizing the impact of the logging.

One of the conventional and strongest means of conservation is thus establishing protected areas (PA's). Many PA's have gradually been established as the one large animal after the other has been threatened. Massive extinction of vertebrates can thus be considered as unlikely during the next few decades.

PAs are a necessary but not sufficient means. The long-term survival of many species which now only persists as isolated populations in relatively small PA's is in doubt unless the habitat available to them can be increased (Whitmore & Sayer, 1992). Therefore conservation should not only be seen as a separate activity. Concern about biodiversity should be an integrated element in all management of forestry operations as well.

In some cases an ex situ conservation is advisable or absolutely needed. Some red list species are nowadays classified as 'extinct in the wild', but for the large majority there is more hope. Here samples of the species concerned is kept in plantations, botanical gardens, seed banks, zoos etc. outside its natural habitat. Some times ex situ conservation is just for safety. Other times is it an integrated element and precondition for keeping a population alive. One extreme example is the maintenance of an artificial gene flow between small populations of adult female rhinos which are regularly cross-fertilized with males where the majority exists as sperm-samples in a deep freezer - 'a frozen zoo'. Habitat restoration, captive breeding, re-introduction, elimination of introduced predators or competitors are others but often expensive means.

But also the management i.e. the care taken during the planning and carrying through the logging operation, Low Impact Logging, is of high importance. Cautious planning of skidding lanes, clearing for log dumps, directional felling, saving of trees for food or nesting. Johns (1992) refers Therborg's estimate that as few as 1% of the trees in the Peruvian rain forest support the bulk of the frugivorous animal populations for several months each year. So it might be a good idea to save or totally spare such trees in the logging operation. Also post felling treatment and other such measures of good practice can make a large difference.

Complementarity of conserving ecosystems, species, and intra-specific variation

So far I have argued that a decrease in biodiversity can be almost totally avoided at the same time as the logging of trees and replanting goes on. However this is not possible. Any action will by definition have an effect. You can't drag out large trees from a forest without any disturbance. So when it comes down to earth, to the felling of the single tree, the effect at the locality will be dramatic and unavoidable. But at all other levels the negative impacts on biodiversity may be modified to some extent.

At the local level felling and roading may mitigate damages on the rest of the vegetation, some mother trees of valued species may be spared and eventually seed samples taken and stored, and saplings planted later on. Some keystone or threatened species may be saved as well. At a landscape- to the regional level a general pattern of delimitated 'hot spots' as core areas interconnected with dispersal corridors and surrounded by adjacent buffer zones should be designated. A systematic monitoring system should be set up enabling the management scheme to be revised currently.


Still these things are only studied very sporadically and our knowledge is very limited which is an argument for being precautious. Exact figures and precise rates must always be taken with caution. Heywood & Stuart (1992, p 99) express it by saying: 'It therefore appears that theory and reality do not converge on the matter of extinction rates. Either the theories are incorrectly formulated, the data are misleading, the time scale wrong, or, quite possibly, a combination of all three.' And they continue (on p.108): 'Because of these and other reasons outlined above, we feel that we cannot attach any great degree of confidence to any predictions of species extinction rates.'

Coming back to the initial questions: 1) Is it a serious problem? 2) Are tropical forests species rich? 3) Are they decreasing at an alarming rate? And 4) Will this inevitably lead to mass extinction of species (and other forms of biodiversity). My answer is however: 'Most probably'. But the focusing on extinction of large animal species caused by tropical afforestation is more for the headline-hungry media than for an understanding of the real threats.

The tropical moist forest is an extraordinary species rich system. The sum of species are only the top of an iceberg of genetic variation. Extinction is only the ultimate event of a often long period of foregoing genetic erosion (together they are called quasiextinction). Taking the relatively high number of endemic species (and genes), and the dense network of flora-fauna interrelations, into consideration it seems inevitable that the progressive exploitation of virgin tropical moist forest, points in one and only one direction. The rate of afforestation and degradation tend to increase and with the fragmentation, now and in the recent past, further losses can be predicted. The erosion of biodiversity continue.

Erosion of biodiversity should not necessarily be as strong as leading to extinction before it is to be considered a problem. Disappearance of biodiversity is a nuisance wherever it appears without being precisely planned and previously accepted. It is beyond doubt that deforestation has led, and continues to lead, to major losses of genetic variability within species populations, and there is field evidence to indicate that many species are reduced to populations that are below any of the suggested minimum viable levels for long-term survival, says Whitmore & Sayer (1992). Therefore any strategy for conserving biodiversity should not only aim at avoiding extinction but have a much broader objective of mitigating any not absolutely necessary loss of any form of biodiversity.

This leaves us with the question whether the rate of decrease of virgin tropical moist forests is considered as alarming or not. A one hundred year perspective is not unthinkable for foresters or ecologists. But it often is for market freaks and politicians. We should help them to think in a more far-sighted way.


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