120
J.H. Lawton
first ecological scientists to suggest that loss of species from ecosystems via local,
regional or global extinctions may impair ecosystem processes. Their hypothesis
lay essentially untested, despite its importance, for at least ten years. For example,
in a book published in 1992 (arising from a symposium on Biodiversity and Global
Change that took place at the Royal Netherlands Academy of Sciences, in Amsterdam
in 1991, during the 24th Assembly of the International Union of Biological Sciences), the distinguished winner of the 1998 International Prize for Biology wrote:
" ... the fact is that we have very little quantitative evidence regarding the postulated
negative consequences [of the loss of biodiversity]. We would like to see whether
there is enough evidence to maintain that a certain level of diversity is essential for
the function of biota and human society .... is there enough redundancy in the system that it can be argued that the system will continue to function [even with loss of
species]?" (Solbrig 1992, page 15). Some six years later, we can provide detailed
and definitive experimental answers to these questions.
I start by reviewing a rapidly emerging body of theory that now underpins Ehrlich
and Ehrlich's insight. Then I briefly summarise the published empirical evidence
addressing this theory, before presenting a brief summary of new results from a
European-wide experiment called BIODEPTH. To date, much more attention has
been paid, experimentally and theoretically, to single trophic-level systems, particularly plants. I finish by considering the roles of animal diversity in ecosystem
processes, and touch briefly on microbial and soil-faunal diversity. Linking the
diversity of micro-organisms and animals (meshed in webs of complex community
interactions) to ecosystem processes is a theoretically challenging and unresolved
problem.
2 Species Richness and Ecosystem Processes
2.1 Theoretical Relationships
2.1.1 Simple 'Pictorial' Possibilities
Theory and data have, until very recently, focussed on the relationship between
plant species richness and ecosystem processes. Taking this state of affairs as our
starting point, within anyone site or location, we can imagine ecosystem processes
responding in one of three ways to reductions in plant species richness (Vitousek
and Hooper 1993; Lawton 1994; Naeem et a1. 1995) (Fig. 1).
(i) The redundant species hypothesis suggests that there is a minimal diversity
necessary for ecosystem functioning, but beyond this minimum, most species
are redundant in their roles (Walker 1992; Lawton and Brown 1993); above
this minimum, adding or deleting species has no detectable effect on the process or processes in question (Fig. la)
(ii) In marked contrast to (i), the rivet hypothesis postulates that all species are
important (by analogy with the rivets holding an aeroplane together - Ehrlich
J.H. Lawton
first ecological scientists to suggest that loss of species from ecosystems via local,
regional or global extinctions may impair ecosystem processes. Their hypothesis
lay essentially untested, despite its importance, for at least ten years. For example,
in a book published in 1992 (arising from a symposium on Biodiversity and Global
Change that took place at the Royal Netherlands Academy of Sciences, in Amsterdam
in 1991, during the 24th Assembly of the International Union of Biological Sciences), the distinguished winner of the 1998 International Prize for Biology wrote:
" ... the fact is that we have very little quantitative evidence regarding the postulated
negative consequences [of the loss of biodiversity]. We would like to see whether
there is enough evidence to maintain that a certain level of diversity is essential for
the function of biota and human society .... is there enough redundancy in the system that it can be argued that the system will continue to function [even with loss of
species]?" (Solbrig 1992, page 15). Some six years later, we can provide detailed
and definitive experimental answers to these questions.
I start by reviewing a rapidly emerging body of theory that now underpins Ehrlich
and Ehrlich's insight. Then I briefly summarise the published empirical evidence
addressing this theory, before presenting a brief summary of new results from a
European-wide experiment called BIODEPTH. To date, much more attention has
been paid, experimentally and theoretically, to single trophic-level systems, particularly plants. I finish by considering the roles of animal diversity in ecosystem
processes, and touch briefly on microbial and soil-faunal diversity. Linking the
diversity of micro-organisms and animals (meshed in webs of complex community
interactions) to ecosystem processes is a theoretically challenging and unresolved
problem.
2 Species Richness and Ecosystem Processes
2.1 Theoretical Relationships
2.1.1 Simple 'Pictorial' Possibilities
Theory and data have, until very recently, focussed on the relationship between
plant species richness and ecosystem processes. Taking this state of affairs as our
starting point, within anyone site or location, we can imagine ecosystem processes
responding in one of three ways to reductions in plant species richness (Vitousek
and Hooper 1993; Lawton 1994; Naeem et a1. 1995) (Fig. 1).
(i) The redundant species hypothesis suggests that there is a minimal diversity
necessary for ecosystem functioning, but beyond this minimum, most species
are redundant in their roles (Walker 1992; Lawton and Brown 1993); above
this minimum, adding or deleting species has no detectable effect on the process or processes in question (Fig. la)
(ii) In marked contrast to (i), the rivet hypothesis postulates that all species are
important (by analogy with the rivets holding an aeroplane together - Ehrlich