Download WHY LINK SPECIES AND ECOSYSTEMS?

WHY LINK SPECIES AND ECOSYSTEMS?
A PERSPECTIVE FROM ECOSYSTEM ECOLOGY
Nancy B. Grimm
Department of Zoology
Arizona State University
Tempe, AZ USA 85287-1501
In review. To appear in 1994 (pending review) as a chapter in Lawton, J.H., and C.G.
Jones, editors. Linking species and ecosystems. Proceedings of the Vth Cary
Conference. Chapman and Hall, Inc.
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Introduction
In one of the final paragraphs of his book on the history of ecosystem ecology,
historian of biology Joel Hagen wrote,
"Populations are important, but an evolutionary ecology worthy of the name must
come to grips with the question of how large communities and ecosystems are
structured. How this might be done remains unclear, for evolutionary and
ecosystem ecologists have been talking past one another for almost a generation."
Hagen refers to a split in ecology which he traced to the publication of George William's
book, Adaptation and Natural Selection. With respect to the topic of this Cary Conference,
his statement encapsulates the view that the endpoint or goal of ecological investigation
is an understanding of the structure of populations, communities, and ecosystems. In
contrast to that view, ecological investigations of ecosystem functioning often have as their
goal an understanding of the effects of ecosystem processes, interactions, and components
on their context, or on the larger system of which ecosystems are a pert.
"The loss of biodiversity should be of concern to everybody for three basic
reasons.. .perhaps the most poorly evaluated to date, is the array of essential services
provided by natural ecosystems, of which diverse species are the key working parts."
Ehrlich and Wilson (1991) use the term "ecosystem services" to describe the net output,
net effect, ecosystem function -- or more generally, what an ecosystem does in a landscape
or biosphere context.
These two quotations exemplify the contrast between questions basic to
evolutionary-population-community ecology and ecosystem-landscape ecology. To answer
questions about distribution and abundance, we need to "build up" from components. To
answer questions about processes, it is possible to attempt mechanistic explanations at the
lower level of communities or populations, or to seek consequences of those processes for
higher-level phenomena.
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The aim of this introductory presentation is to identify key issues that are important
to answering the question posed in this chapter's title: why link species and ecosystems?
The perspective is an empirical ecosystem ecologist's, who studies ecosystems (streams)
with relatively easily-defined boundaries. I will not do justice to species/ecosystem-linking
models, but acknowledge that development of theory (models) and empirical work must
go hand in hand. First I will consider the sub-disciplinary distinctions in ecology, then ask
what evidence and logic support individual species' importance in ecosystem dynamics, then
consider the role of assemblages and their interactions in ecosystems. Finally, what themes
now integrate these subdisciplines? Is there an obvious conceptual basis for linkage?
These last questions go beyond the challenge given me and are more thoroughly considered
in later chapters, but I hope to provide some insights that at least will stimulate discussion.
Subdisciplinary Distinctions
A good place to start in identifying distinctions among subdisciplines is in ecology
textbooks. Common definitions of the science of ecology (with their different foci
italicized) are these:
The experimental analysis of distribution and abundance (e.g., Krebs 1985)
The scientific study of the interactions between organisms and their environment
(e.g., Ricklefs 1990, Begon et al. 1990)
Begon et al. assert that "the ultimate subject matter of ecology is the distribution and
abundance of organisms". This and the first definition reflect the unidirectional view - that
ecology is oriented toward understanding structure - given in the Hagen (1991) quotation.
Likens (1992) has proposed a definition that is the most inclusive of those listed, and
allows activities of all subdisciplines to be called ecology: "[Ecology is] the scientific study
of the processes influencing the distribution and abundance of organisms, the interaction
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among organisms, and the interaction between organisms and the transformation and flux
of energy and matter".
Begon et al. (1990) and Krebs (1985) place ecology firmly among the biological
sciences with their contention that ecology is a science which overlaps with behavior,
genetics, physiology, and evolution. To many ecosystem ecologists, this list is not
complete: we need to add hydrology, geochemistry, geology and geomorphology, and
perhaps even social sciences. Schimel (1992) has maintained that ecology is properly
identified as an earth science. This possibility should be given some attention, if we are
to effectively explore the links between the subdisciplines of ecology as it is currently
practiced. On the other hand, population and community ecology are now more closely
allied with evolution, genetics, and behavior than is ecosystem ecology. The rich body of
evolutionary theory may be irrelevant over the short term to ecosystem processes, but in
light of rapid environmental change and assaults on natural ecosystems, it will become
increasingly germane.
What theories underlie ecosystem ecology and population-community ecology? A
strong theoretical basis for work on ecosystem energetics and biogeochemistry are the
physical laws. Any system involving biota is also subject to evolutionary laws, and in this
regard communication problems still exist. Aside from criticisms that the teiii1s ecosystem
and community are fuzzy and vague, common usage of "function" and "role", in particular,
has fueled miscommunication, at least between evolutionary ecologists and others. The
problem is that a term such as function implies to some an evolutionary origin, which must
therefore imply operation of natural selection at supraorganismic levels, yet it remains a
useful term for describing what a system does in its context. [In this chapter, I use the
term function with no intended implication of an evolutionary origin.] On the other hand,
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communication will be facilitated by generalized interest in scale, disturbance, stability, and
patchiness. These unifying themes are the areas in which development of general, multiple
theories seems most likely.
Reiner's (1986) thoughtful commentary on conceptual models for ecosystem studies
presents a view of how laws and theories may be logically organized to generate
predictable hypotheses. He outlined the logical structure of two models that underlie most
of the research in ecosystem ecology: one dealing with biogeochemistry and based on
elemental stoichiometry and one dealing with energy flow and based on thermodynamics.
Reiners argued that development of these models and a third complementary model
(dealing with population or community structure) and their ultimate linkage is needed to
facilitate progress in ecology. Reiners' model based on the axiom that groups of organisms
have regular chemical stoichiometries has driven some excellent research on links between
species and ecosystems, notably the work of Wedin and Tilman (1990 and chapter XX) and
Sterner and colleagues (Sterner et al. 1993, chapter XX).
Finally, in practice many of the data necessary for evaluating the role of species in
ecosystems is already available from typical ecosystem studies. For example, Smock et al.
(1992) determined rates of macroinvertebrate secondary production in a southeastern
stream by dividing the stream ecosystem into patches and calculating secondary production
for the whole system by weighted summation. But what information went into this
calculation? At least in streams, secondary production of macroinvertebrate consumers at
the ecosystem level is most often calculated from an extremely detailed accounting of
individual species, by microhabitat and by size class. The point is that aggregation of this
fine-grained information, first to functional groups, then to subsystems or patches, and
ultimately to the whole ecosystem, is the best way to arrive at the measure of energy flow
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through consumers. Incidentally, information on species composition, life histories, habitat
preference, population size structure, and functional redundancy is acquired. How does
this differ from a description of macroinvertebrate community structure? It differs only in
the question originally asked.
Overall, ecologists aim to understand patterns in nature, be they community
structure or gas flux across ecosystem boundaries, by resolving mechanisms explaining the
patterns at lower scales and consequences of the pattern at higher scales. Ecosystem
functioning is simply one higher-level consequence, or net result, of organismal interactions
with other organisms and the abiotic environment.
Should We Care Who's Inside the Black Box?
What is the evidence that each individual species can influence ecosystem
functioning? The basic organizational question here is, does the identity of species inside
the black box matter at the ecosystem level? The key to prediction is an understanding of
what features of species and/or ecosystems are conducive to functional redundancy (or its
opposite, keystone species). For ecosystems, we might consider such characteristics as the
number of effective trophic levels, nutrient availability and limitation, types of functional
groups, disturbance regime, and degree of spatial and/or temporal heterogeneity; for
species, competitive ability, predation efficiency, life history traits, and unique biochemical
or metabolic abilities (e.g., nitrogen fixation) may be important.
Since Paine (1969) first defined a keystone predator, the term "keystone" has been
extended to species that exert purported significant effects on ecosystems or on community
structure. When applied to ecosystems, keystone species are those species whose
contribution to ecosystem functioning is unique. If we could divide an ecosystem into
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essential functions or processes ("ecosystem services" in Ehrlich and Wilson's terminology),
then each process might be characterized by a complement of species that perform it (i.e.,
a functional group).
Essential functions may be grossly classified as trophic,
biogeochemical, and structural, and keystone species are the sole members of a trophic,
biogeochemical, or structural functional group. It is obvious that the more detailed or
specialized our definition of functions, the lower the likelihood of redundancy (i.e., the
higher the probability of keystone species).
A few examples from streams serve to illustrate the interaction between species and
ecosystem traits that determine redundancy. In sunlit inidcontinental North American
rivers, Campostoma anomalum is an important algal grazer that can become extremely
abundant. Power et al. (1985) showed clearly that Campostoma grazing in small, speciespoor streams can control macroalgal abundance and productivity. But an additional
wrinkle to this story is that its predator feeds on the minnow in pools where both occur,
either as a result of redistribution during spates, movement between pools, or experimental
introduction. When bass are added to a Campostoma pool, the algae are released from
grazing pressure and grow rapidly. In larger rivers, this interaction is less dramatic because
of greater spatial heterogeneity. The bass-Campostoma-algae story is one that
demonstrates both the key role of a grazer, a cascading trophic effect of predation, and the
importance of spatial heterogeneity. The work was also elegant in that it dealt with the
influence of disturbance.
Desert streams in southwestern USA also are strongly influenced by disturbance
(flash floods). Because these events occur frequently (albeit unpredictably) and organismal
life spans are short relative to the timeframe of ecological investigation, my colleagues and
I have been able to repeatedly describe the process of succession (for summaries, see Fisher
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1990, Grimm 1993). Ecosystem-level attributes (at the scale of 100-200-m reaches) such
as primary producer biomass, organic matter storage, and energy flow variables of gross
primary production and ecosystem respiration increase rapidly, often to an asymptote.
Periphyton communities continue to change thereafter, however, often toward
cyanobacterial dominance. In terms of the energy flow these community-level changes are
irrelevant, but because cyanobacteria are N-fixers, there is no redundancy in terms of N
cycling.
It is a frequent practice of stream ecologists to measure algal biomass using
chlorophyll a, since this allows one to separate primary producer biomass from mass of
detritus and microconsumers. Is this measure adequate to describe changes that might be
of consequence to ecosystem functioning? In other words, can we lump species as
functionally equivalent and summarize their effects at the ecosystem level using chlorophyll
a? A study of succession at the scale of single rocks (Peterson and Grimm 1992) provides
an answer to these questions. As is typical of post-flood succession at the reach scale,
primary producer biomass and net primary production rose rapidly, indicating that
assemblages on rocks are resilient. At the functional group level, however, biovolume of
nitrogen-fixing species increased rapidly while there was little successional change in
biomass of non-fixers. Chlorophyll a alone was inadequate to describe an important
feature of this succession: the nitrogen fixer functional group was important throughout
the sequence and would obviously affect nitrogen dynamics in a different way than would
non-fixers. But what can be said about redundancy at the species level? The early
dominant in the N-fixer functional group (Epithemia sorex, a diatom containing
endosyrnbiotic cyanobacteria) was replaced within about 1 month of the 3-month sequence
by Calothrix sp., which ultimately accounted for >95% of algal biovolume. Although we
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lose information about nitrogen dynamics using "chlorophyll a", the consequence of shifts
from one species to another species of N-fixer may be less important; i.e., there may be
redundancy within that functional group.
Keystone species also affect physical structure of ecosystems (see the detailed
classification of "ecosystem engineers" offered by Lawton and Jones (chapter )0)).
Biotically-mediated change in hydrologic linkage between subsystems is an example from
streams; macrophytes and beaver dams can alter patterns of water flow between streams
and hyporheic zones. Earthworm and sediment-dwelling invertebrate burrowing or
trapping of fine sediment by macrophytes in rivers change the distribution of oxygen,
which in turn alters biogeochemical cycles. Plants and even snails (Shachak et al. 1987)
influence physical structure by contributing to weathering. Finally, species can modify
disturbance regimes. Introduced grasses can fuel fires (D'Antonio and Vitousek 1992). In
desert streams, introduced bermuda grass may anchor sandy sediments and prevent
sediment movement during flash floods. Beaver dams may also ameliorate effects of spates.
A final point is that the keystone concept has often been applied to groups of species
or even functional groups, which diminishes its usefulness. It seems more sensible to
assign an interaction strength to species, as Paine (1980) has advocated. In an ecosystem
sense, interaction strength represents the potential of species to affect processes rather than
species' distribution and abundance. We need to conduct experiments at realistic (whole
ecosystem) scales in which responses of ecosystem processes (functions) to manipulations
of strong interactors are assessed (see also Lawton and Brown 1993).
Biotic Interactions and Ecosystem Functioning
I turn now to the question of linkage between biotic interactions and ecosystem
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functioning. A brief description of biotic interactions in a desert stream ecosystem,'
illustrates the kinds of questions we must ask to address this issue. The major players in
this story are: NITROGEN - the limiting nutrient, which is highly variable in space and
time; PRIMARY PRODUCERS - abundant and productive, dominated by diatoms,
Cladophora glomerata, and Anabaena sp.; MICROBIAL COMMUNITY - associated with algaderived particulate organic matter and invertebrate and fish feces;
1VIACROINVERTEBRATES - collector-gatherers with high rates of secondary production and
short life spans; "MEGAGRAZERS" -snails and omnivorous fish; FLASH FLOODS AND
DRYING - disturbances that dictate the successional and spatial context. Interactions
between processes or phenomena associated with these players are indicated in Fig. 1 as
having positive or negative effects.
A semi-static view of mid-late successional. time is this: primary producers consume
N as they grow, and in both time and space, their activities result in a decline in N
availability. As they become N-limited, they may be more susceptible to being eaten or to
losing in competition with N fixers. Fine particulates (FPOM) may be derived largely from
megagrazer feeding, but whatever its origin, this material supports a rapid accumulation
of collector-gatherer biomass. These small macroinvertebrate consumers feed on small
particles collected from the stream bottom and sediment interstices. They feed at very high
rates - up to 1/3 of the total organic matter standing crop and seven times gross primary
production each day. [This is most significant to the FPOM pool, because they do not feed
directly on macroalgae.] But as assimilation efficiency is extremely low (10%), these
organisms must be continually reingesting fecal material. Fisher and Gray (1983)
hypothesized that this represents a reasonable tactic if residence time in environment
improves food quality (e.g., N content) through bacterial colonization. Collectors'
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continual reingestion of organic matter, however, makes the FPOM poor in N, potentially
leading to N-limitation of microbial growth. In nirri, invertebrate populations "crash" in
sequences where nitrogen is especially low, perhaps owing to low food quality.
My purpose in delving in some detail into this system is to point out that such
complicated sets of interactions can have strong effects on ecosystem-level patterns of
nutrient retention and energy flow. Biotic interactions such as the replacement of nonfixers by fixers, competition for N between producers and decomposers, and grazing are
at the heart of changes in system state. But this static view does not consider the influence
of disturbances (floods and drying), which are ever-present in these systems. How does
this scenario fit into that more inclusive scheme? In other words, when and where are
biotic interactions such as these important in controlling ecosystem structure and
functioning? Fisher and Grimm (1991) presented a conceptual model in which "control"
of ecosystem processes varies temporally as a function of disturbance regime. Spatial
variation in control of ecosystem processes is equally likely, given that susceptibility to
drying and flood varies spatially (Fisher 1993). The point of both temporal and spatial
conceptual models is that the question we should be asking is not whether biotic
interactions influence ecosystem processes, but when and where - in the context of dynamic
systems and disturbance.
Are these conceptual models applicable to other systems subject to disturbance? Or
must they be restricted to streams? Bearing in mind that time frames over which
disturbance regimes must be quantified vary tremendously among ecosystems, the answer
is that such models do apply to all types of ecosystems. Other lessons from aquatic
research might also be brought to bear on the question of linking biotic interactions and
ecosystem processes. Research in aquatic ecosystems has already accomplished substantial
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integration of community and ecosystem ecology. Why is this so? At the most basic level,
boundaries are relatively easy to define and populations are confined within them. As it
is difficult to separate biotic from abiotic components, perhaps there has been more of a
tolerance of "black boxing" in aquatic ecology. Finally, in streams and intertidal zones
where biota grow attached to the substratum and are easily seen, research has emphasized
disturbance and spatial heterogeneity. These are of consequence both to biotic interactions
and community structure and to ecosystem processes.
Integrating Themes
The themes of disturbance, stability, patchiness, and scale are potentially integrating
ones, and population, community, and ecosystem ecologists share a common interest in
them. But researchers are not using the same language and therefore must carefully define
terms and variables of interest for any discourse to be meaningful. Operational definitions
of stability and the included concepts of resistance and resilience for the stream ecosystem
study discussed earlier are taken from Webster et al. (1983). But even using consistent
definitions, the benthic invertebrate assemblage in this system can be seen as resistant or
not, depending on the variable measured (community structure or total biomass; Boulton
et al. 1992). This underscores the need to identify common response variables in
comparisons of stability across levels or organization.
Landscapes and ecosystems are spatially heterogeneous, and this key feature must
be incorporated into any conceptual scheme for linking species and ecosystems. Lessons
learned from studies of spatially heterogeneous land-water units would be well-applied to
all ecological studies. A particularly good example from a biogeochemical study is the
description of patches along an arctic toposequence as nutrient sources or sinks linked by
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down-slope water movement (Giblin et al. 1991). For streams, my colleagues and I are
trophic structure and thereby the
attempting to determine how patch dynamics influencer
extent to which different patches (each with a characteristic trophic structure) are sources
or sinks for N. Such approaches might provide a means for integrating biotic interactions
and ecosystem processes - in the latter example, effects of trophic/nutrient interactions on
nitrogen retention.
In population and community ecology, models involving sources and sinks explicitly
consider different spatial units within landscapes or ecosystems. For example, sink
populations (mortality rate [d] > birth rate [b]) may persist, such as when linked by active
dispersal to source (b > d) populations (Pulliam 1988). Stream reaches with few prey and
many predators may be supported by prey immigration from upstream prey-rich reaches
(e.g., Cooper et al. 1990).
Interactions among population-community and ecosystem ecologists would be
facilitated by adopting, as a starting point, a spatially-based conception of the units of
study. I disagree with Allen and Hoekstra (1992), who contend that ecosystems are not
geographical entities (although it is certainly true that their size and boundaries may be
ill-defined). The advantage of this spatial basis is the ability to scale up or down, and in
so doing, to include all relevant, potentially interacting components (both biotic and
abiotic) of ecosystems.
Whatever the scale of the investigation, a spatially-based
perspective places species interactions (the traditional focus of community ecology) into
a context in which their effects on ecosystem processes may be assessed. Interactions
between patches may be critical to larger-scale processes, and include biotic interactions
that occur within component subsystems.
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Summary
Population-community ecologists ask different questions about nature. Why then
should we be concerned with linking species and ecosystems? For ecosystem ecologists,
the answer is clear: the activities of certain (keystone) single species and certainly the
interactions among species can have strong effects on functioning at ecosystem and
landscape levels. Resolution of key issues will be needed to answer this question, and to
begin forging such a link. First, how can theory underlying these two distinct approaches
be integrated? How important is evolution to ecosystems, and geochemistry to
populations? Second, can keystone species be identified based on traits of the ecosystem
or of the species themselves? If we adopt the terminology of interaction strength, can it be
applied to specific ecosystem functions? Third, biotic interactions clearly influence
ecosystem processes. But under what conditions, in space and time, are they most
important? Finally, what conceptual basis for linkage exists? Treatment of traditional
population, community, and ecosystem processes in a spatial context, with the patch as a
fundamental unit, may provide a useful tool for integrating species and ecosystems.
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Figure Legend
Figure 1. Effects of biotic and abiotic interactions in a desert stream ecosystem. Processes
and interactions are linked by positive arrows when the net effect is to enhance or
increase the likelihood of the recipient process, by negative arrows when the net
effect is to decrease likelihood. Disturbances (flood, drying) interact with this static
system in mid-successional time in complex ways (+/-).
N-fixers
Algae reduce
nitrogen availability ]
dominate
Microbial processes
•
nitrogen-limited
A
N-poor
egestion
Algae unable to
"outrun"
megagrazers
N-poor POM
Collectors continually
reingest organic matter
44`+‘
Grazers
consume
algae