Until recently, efforts at assessing and monitoring forests have focused on the amount (area) of forest remaining and/or its standing timber volume. Increasingly the multiple benefits and functions of forests, including provision of non-wood forest products, hydrological functions, carbon sequestration and biodiversity protection have been recognised, and their consideration in forest assessment has been recommended. There is now a perception that not all forests are equal, whether it be for timber production or for conservation of biodiversity. The international community has placed a high priority on assessing and monitoring the capacity of forests to provide any given range of services and conveying this information to policy and decision-makers and the general public (Nyyssonen & Ahti 1996).
In this paper we discuss:
• ways in which the capacity of forests to maintain their original biodiversity can be assessed at broad geographical scales;
• the problems inherent in such assessments;
• the most useful ways of presenting the results, and
• steps that need to be taken to ensure effective monitoring in the future.
Biological diversity, or biodiversity, is an imprecise term that is used to refer to the diversity of life at the levels of genes, species and ecosystems, in a single locality or over broad geographic areas, including the Earth as a whole. Biodiversity is used to refer to not only the numbers but the types of genes, species and ecosystems existing in the area in question. There is usually an implicit assumption that naturally occurring or original components of biodiversity are of more value than articficially introduced or degraded ones, and therefore biodiversity preservation or protection is frequently understood to refer to the preservation of these qualities and components.
Forests play a significant role in maintaining the world’s biodiversity. They provide habitat for more than half of the world’s species (Groombridge & Jenkins 2000). A forest area contributes to global forest biodiversity by the number of species present and shared with other areas and the number of species it contains that are found nowhere else (endemics). The biodiversity complement of individual forests varies with forest type, and the climatic and soil factors that relate to it, as well as the biogeographic position and isolation of the forest. In general forests at low latitudes and low altitudes, with warmer and wetter climates, have higher species richness than those at high latitude or altitude, or with cooler and drier climates. Natural perturbations, such as storms and wildfires can also be important influences on forest biodiversity.
Natural global patterns in biodiversity are also altered by human action (Groombridge & Jenkins 2000). In the contemporary world, human activities may be the most important influence on forests’ capacity to maintain their original biodiversity. Such activities as commercial and artisanal logging, large scale land conversion, fuelwood and charcoal production, slash and burn agriculture, harvesting of non-timber forest products, hunting and mining all affect forest biodiversity. Climate change resulting from modification of the atmosphere by anthropogenic emissions of carbon dioxide is also affecting the distribution and status of forest biodiversity.
Each of these types of human influence affects forests differently, and the magnitude of the effects will depend strongly on the methods employed locally, the forest type, and on other factors within and around the ecosystem. For example, commercial logging in temperate forests is often by clear cutting, which entirely removes forest cover in some areas and fragments remaining forest cover. In contrast, commercial logging in tropical forests is usually by selective felling which disrupts canopies and forest structure and alters species composition, but may not alter total forest cover or its spatial configuration. Secondary effects of logging such as increased access and resultant hunting are also important determinants of the status of forest biodiversity and the prospects for its preservation. Small-scale timber extraction differs yet again in its effects on forest condition. The effects of climate change are less localised, and are only beginning to be recognised. Thus, many factors influencing forest biodiversity are affected in varying and complex fashions by human activity.
In general human activities tend to affect any of three major aspects of forests:
1) The total area of forest remaining – many of man’s activities remove forest cover either temporarily or permanently. Some forest types may disappear locally, and reduction in the total amount of habitat is a significant pressure on some forest species that can lead to local extinction.
2) The configuration of remaining forest cover – reduction in forest area is often accompanied by division of remaining forest cover into fragments, rather than continuous blocks. Forest biodiversity is affected by the consequent local reduction in habitat area, by the exposure of forest edges to new environmental and biotic influences and by isolation from other forest areas (more detailed discussion below).
3) The structure and composition of remaining forest – some human activities alter canopy structure, or focus disproportionately on particular species and specific components of their populations.
Thus, useful measures of forest capacity for biodiversity preservation are likely to address the amount of forest remaining, its configuration or integrity and its ‘naturalness’, or lack of anthropogenic disturbance. However, developing measures of disturbance and making them operational is not straightforward. Precise histories of forest disturbance and its intensity are rarely available, especially over broad geographical scales (Kapos & Iremonger 1998). The problem of describing and measuring human-induced change in ecosystems has been summed up by Groombridge (1992, p. 250) in a review of habitat/ecosystem classification:
. . . Just as it is impossible to define rigidly the limits of any given ecosystem or habitat, so it is impossible to determine how much a given area of ecosystem or habitat has to change before it can be considered destroyed or converted. The problem is compounded by the fact that the natural environment is not static but rather dynamic, sometimes highly so, on a time scale ranging from hours to millions of years. It is thus difficult even to define an undisturbed ecosystem or habitat as a standard against which to measure degree of disturbance.
There is a wide range of problems to be grappled with in successfully formulating methods for measuring the impact of humans in forest ecosystems. Difficulties arise from:
• Different interpretations of the basic form and function of ecosystems – Some concerns focus on ecosystem processes, while others emphasise composition. Such different conceptual approaches may lead to very different conclusions as to appropriate sets of information for the description and measurement of change (O'Neill et al.,. 1986).
• Sensitivity to observation and process scale – The scale at which ecological phenomena are observed and measured will have a major bearing on the conclusions drawn (Goodall 1974; Allen et al.,. 1987; Allen and Hoekstra 1990; 1992; Noss 1991), and the uses to which they can be put.
• Ambiguity in identifying benchmark conditions – The notion of naturalness depends on a clear distinction between the presence and impact of human activity and natural ecological patterns and processes. This can be problematic if, as in much of Europe, there is little or no reference forest with little human influence to provide a basis for comparison
• Establishing whether human-induced change represents a fundamental shift in organisation or change within normal limits of forest dynamics and ecosystem processes;
• Uncertainty regarding the place of humans in the environment.
These difficulties pose significant technical problems and can give rise to potentially contradictory answers to questions concerning ecological change.
Two principal strategies can be adopted to address these issues and assess the naturalness and ecological integrity of forest ecosystems at broad geographic scales in biodiversity-relevant terms:
1. Use indicators to assess key aspects of forest structure and/or function. This strategy has the obvious advantage of directly addressing the primary concern, which is the state of the ecosystem, but is subject to the difficulties described above concerned with the defining and measuring human-induced change in ecosystems. Attention must focus on parameters that can be evaluated in a globally consistent manner at broad geographical scales and are clearly related to the status of component biodiversity within the forests. Fragmentation of forests and parameters that describe it are a promising avenue for developing such indicators.
2. Measure the driver of ecosystem change - human activity, rather than the response - ecological change. This approach avoids fundamental problems associated with describing and measuring ecosystem response to human activity. Separation of the driver (cause) and response (effect) components also allows for greater precision and flexibility in analysis. General indicators of naturalness or ecological integrity can be developed on the assumption that the greater the amount of ecosystem exposure to human activity the greater the potential for human intervention in these ecosystems.
Indicators are measurements that convey information about more than just themselves. They provide means for quantifying and simplifying information on complex issues. They are purpose-dependent, almost always open to various interpretations, and never tell the whole story. Indicators are needed because assessing and monitoring everything is impossible and because what is known needs to be conveyed to non-experts in policy-relevant form.
Good indicators are:
• scientifically valid, i.e. they relate appropriately to what they are supposed to represent;
• based on easily available data;
• responsive to change;
• easily understandable;
• relevant to focal issues and users’ needs;
• subject to target or threshold setting.
This document presents some approaches to generating indicators of forest condition in relation to biodiversity that could be used to conduct a globally consistent assessment. It focuses primarily on indicators relating to forest fragmentation and exposure to human activity.