What is low biodiversity

Dramatic decline in biodiversity

There have been five phases of mass extinction of species in the course of the earth's history. After each of these, it took several million years for the biodiversity to recover. Today we are in the middle of a sixth period of mass extinction. In contrast to the previous ones, this mass extinction is not due to natural causes: it is caused by us humans.

BY DIETHART MATTHIES AND BERNHARD SCHMID

Many global environmental changes are in principle reversible. For example, the depletion of the ozone layer is of concern; one can hope, however, that it will recover when no more ozone-depleting substances are released. In contrast, the loss of biodiversity is irreversible. Species that are once extinct are irretrievably lost. The speed of the current extinction of species is enormous: According to various estimates, between 10 and 25 percent of all species will become extinct by the year 2000 alone.

Reasons for the extinction of species

The reasons for the current mass extinction lie in the constantly increasing demands on natural resources by humans. Today around 50 percent of the total biomass produced by plants worldwide is used directly or indirectly by humans. The main human activities leading to species extinction are habitat destruction or alteration, global climate change and environmental pollution, and the introduction of exotic species that threaten native species as predators, competitors or pathogens.

The most serious impact on global biodiversity is the destruction of tropical rainforests. Although these cover only about 7 percent of the earth's surface, they probably contain more than 50 percent of all species. The main causes of deforestation in tropical rainforests are timber production for export, the conversion of forests into pastures or plantations, and shifting cultivation.

Biodiversity is threatened with increasing dangers due to global environmental changes such as the rise in carbon dioxide concentrations in the atmosphere and the heavy fertilization of agricultural land. If the prognoses turn out to be true, according to which the increase in carbon dioxide will lead to a considerable warming of the earth (greenhouse effect), many organisms will suddenly find themselves in the “wrong” place, namely in a climate to which they are not adapted. It has also been found that an increased carbon dioxide content in the air can disturb the competitive equilibrium between different plant species and change the quality of plants as food for animals.

Biodiversity also threatened in Switzerland

Many species are threatened with extinction in Switzerland too. For example, 37 percent of mammals, 45 percent of breeding birds, 80 percent of reptiles and 95 percent of amphibians are endangered; 24 percent of the plants are also threatened. However, only a few of the animal and plant species found in Switzerland are restricted to this region in terms of their distribution. Therefore, the extinction of a species in Switzerland is generally not synonymous with its worldwide extinction. Nevertheless, one should also be concerned about the regional extinction of species, because this often means that genetic variants that were particularly well adapted to local conditions disappear. In addition, many species are endangered elsewhere and can possibly only be effectively protected here.

The cause of the threat to species in our region is the extreme change in the Central European cultural landscape over the past 50 years. During this period, for example, more than 90 percent of all dry grassland, wetlands and floodplain areas in Switzerland were destroyed. The intensification of agriculture, but also the development of industry and transport, are mainly responsible for these changes. Another problem is that human interventions such as the construction of roads and railway lines, but also the creation of settlements and intensification measures in agriculture increasingly fragment the remaining habitats of plants and animals. This habitat fragmentation leads to the fact that only individual "islands" remain of the originally contiguous habitats and originally large populations are split up into several small ones, between which there can often no longer be an exchange of individuals.

Small population problems

Various problems arise in small, isolated populations. Small populations are much more threatened by random environmental fluctuations than large ones. Inbreeding increases in small populations, which can have negative consequences for reproduction and survival of individuals. In addition, the interactions with other organisms can be disturbed. Ultimately, the isolation of habitats means that areas that would be suitable for certain species can no longer be reached and colonized by them. The increasing fragmentation of habitats thus has a number of negative consequences for animals and plants and contributes significantly to the decline and extinction of species at the regional level.

The investigation of processes in populations of rare or declining species is a focus of research at the Institute for Environmental Sciences (IfU) at the University of Zurich. Reproduction is a particularly critical phase in the life of plants. It is often disturbed in small populations. As part of a doctoral thesis, Markus Fischer examined the seed production of plants of the German gentian (Gentianella germanica) in different sized populations. Marc KŽry carried out similar investigations in his diploma thesis on the yellow gentian (Gentiana lutea) and the spring cowslip (Primula veris) by. These species grow on relatively nutrient-poor soils and are in decline in Switzerland. It was found that in all three species the individual plants in small populations produce far fewer seeds than in large populations (FIG. 1).

In fact, in some small populations of the yellow gentian and spring cowslip, the plants no longer seed at all. This shows how negative habitat fragmentation can affect reproduction.

The yellow gentian and spring cowslip are long-lived species. Low reproduction is therefore a warning sign, but it does not threaten these species in the short term. The situation of the German gentian is different, which generally dies after fruiting and whose populations therefore have to build up again and again from seeds. For this species it has to be stated that small populations are already threatened with extinction in the short term.

Species diversity and ecosystem processes

A controversial question is whether a reduced biodiversity can have an impact on the properties of ecosystems, for example whether their productivity changes or whether they can no longer provide certain “services” such as erosion prevention, climate regulation, air purification and soil formation to the same extent as before. Another working group at the IfU is investigating this question as part of an EU project in collaboration with eight other research groups from different European countries.

Meadows and pastures are of particular importance for Swiss agriculture and are well suited as model systems for ecosystem studies. Meadow plants are relatively low and have to grow back above ground every year after mowing or grazing. Possible changes are therefore quickly visible, unlike in a forest, for example.

There are numerous types of meadows in Switzerland that differ greatly in the number of species. However, soil and climatic conditions also vary greatly, and the possible effects of different species diversity on the properties of meadows are overlaid by other factors. In order to be able to make clear statements about the importance of species diversity for ecosystem processes, small patches of meadow with 1, 2, 4, 8 or 32 plant species were sown experimentally in one place and various properties of these mini ecosystems were investigated. The first results of the work are now available.

As Christof Binder and Angela Schreiber were able to show in their diploma theses, populations with few species (with 1, 2 or 4 species) had less cover and produced less biomass than species-rich meadows; in addition, they were more susceptible to the penetration of weeds (Fig. 2).

So it is by no means irrelevant how many species an ecosystem contains.

There may therefore be interdependencies between the decline in biodiversity and other global environmental changes: on the one hand, various global changes, such as the increase in carbon dioxide in the atmosphere, threaten biodiversity; on the other hand, a reduced biodiversity in ecosystems can change their properties and, for example, their absorption reduce carbon dioxide from the atmosphere.




University

press service press office of the University of Zurich
Felix Mäder ([email protected])
http://www.unizh.ch/upd/magazin/3-96/
Last update: October 3rd, 1996