Biodiversity is most commonly thought of as the number of species in an area. There are other ways of expressing biodiversity that take into account the relative abundance of different species. Biodiversity also exists at two other important levels: genetic diversity within a population*, and diversity in the types of communities* in a larger area or ecosystem.
The extant biodiversity of ecosystems is a legacy of life on earth that should be preserved for practical as well as ethical reasons. At the simplest level, biodiversity represents the reservoir of options that a system possesses to respond to changes over time, and a reservoir of information about life itself. We do not understand the role of everything today. Seemingly minor species or hidden aspects of biodiversity may play surprisingly important, but still unknown, roles in today’s ecosystems. Moreover, we cannot predict what aspects of today’s biodiversity may emerge as important in the future. At a higher level, patterns of diversity determine how systems function–for example, in storing and recycling nutrients, fixing inorganic carbon, producing fish, and supporting other species of special interest. In addition to these important roles in "life support", biodiversity presents us with opportunities for new and potentially important discoveries (for example, in the area of medicine), and a wealth of aesthetic wonders.
Understanding and conserving biodiversity are goals that integrate across many human needs and values. Knowledge of biodiversity and the ways that it influences ecosystem functions are thus central to the development of ecosystem approaches to management.
Because threats to biodiversity are increasing globally, the need to understand what is there and how biodiversity contributes to the functioning of ecosystems is urgent, but it is not straightforward. How do we measure biodiversity in a useful way? How do we conserve biodiversity when so much of it remains unknown to us?
It may be best to think of the relationships in a hierarchical fashion. Genetic diversity may be selectively neutral, may represent the capacity of a population to adapt to changes over time, or may result from selective pressures we do not yet understand. Species diversity may add resiliency to systems because different species offer alternative pathways for energy flow in systems. Within these alternative pathways, functional redundancy may dampen system-wide shifts at higher levels by maintaining energy flow through guilds of organisms that function in similar ways (pelagic predators, benthic suspension-feeders, etc.). A diversity of communities sustains a greater total diversity of organisms in an ecosystem, and may equip it to adapt more quickly to externally forced changes. On the other hand, because many communities are limited by unique habitat requirements, how flexible are they to system change? Understanding these various relationships would help us understand patterns of biodiversity and would prepare us to do a better job of managing human activities in sustainable ways.
A simple schematic of relationships between natural and anthropogenic forcing, biodiversity and ecosystem function is shown in the figure below. In this illustration, the relationships are integrated at the community level. This is not the only way to study the system, but it is useful because we know how to describe communities of organisms, and it is possible to quantify the goods and services that they provide to the natural system and to human society. Simple examples of such services include the recycling of nutrients; metabolism of pollutants; storage and transfer of energy and material between various pools; support of finfish and shellfish production; harboring of rare species; provision of bio-active compounds; and the feeding of seabirds, shorebirds and other migrating taxa. Indeed, organisms do not live outside of communities, so this is a useful unit for studying ecosystem functions. The effects of genetic and species diversity can be considered in the context of their effects on the dynamics of populations and communities. This hierarchy is convenient for mapping possible human impacts on ecosystems. It is also a strategy for conserving biodiversity, much of which remains unknown but is contained within well-maintained communities. Conserving chemical and physical habitats is another strategy for protecting "hidden" biodiversity. There are instances where we might point to direct links between losses of biodiversity and changes in ecosystem functions, especially for dominant species, but such direct linkages are probably greatly outnumbered by situations where the role of biodiversity is unknown and much harder to discern. Not all diversity will map out neatly into identifiable human benefits, but it is not possible to know soon what everything is doing, and it is impossible to predict what components of today’s biodiversity will emerge as significant in the future.
The scientific requirements for understanding the pathways illustrated in the figure are substantial, but many are not new. They include, for example, an understanding of metapopulation structure and spatial requirements. How much habitat change can be allowed? How must communities be distributed to ensure recruitment and the maintenance of genetic and species diversity? What levels of protection are needed for various species and communities? How do these choices intersect with larger-scale processes such as oceanographic currents and biogeographic boundaries that can shift as a result of climate and other system changes? Answers to these questions will take time to generate. To support the development of ecosystem-based management, tentative answers must be devised through a combination of existing knowledge; new and ongoing observations and experiments; theoretical development; modeling; and cautious standard-setting. Society, through its exercise of policies, must be adaptive to new knowledge, including feedback from the ecosystem itself.
Ecosystems are defined by a number of characteristics that are useful in distinguishing an area from its surroundings, but the boundaries are open to debate and open to exchange with neighboring systems through transport and migrations. Operationally, we have to accept these complications, but it requires that we define what we mean when we are talking about them. Practical considerations also mean that management units and ecosystem boundaries will not always coincide, but the functions that transcend boundaries must be understood.
Human Activities, Biodiversity, and Ecosystem Function
Human activities and natural forcing ultimately affect ecosystem functioning through direct and indirect effects on communities. Major human impacts include (1) species introductions; (2) regional changes in species composition or gene pools; (3) physical and chemical alterations of habitats; and (4) local alterations to communities. Environmental conditions affect communities through (5) species introductions (transport or migrations); (6) habitat changes (due to changes in production, temperature, turbulence, sedimentation, sea-level change, circulation); and (7) physiological impacts. This figure does not include feedback from the ecosystem functions.
*Communities – Assemblages of interacting or cohabiting organisms, including mobile, non-mobile and passively transported taxa.
*Population – A group of organisms of a single species living in a geographic area and interacting reproductively.