United States
Environmental Protection
Agency
Environmental Research
Laboratory
Duluth MN 55804
Research and Development
EPA-600/S3-82-050 Dec. 1982
Project Summary
Organization and Adaptation of
Aquatic Laboratory
Ecosystems Exposed to the
Pesticide Dieldrin
William J. Liss, Daniel M. Weltering, Susan E. Finger, Michael L Kulbicki, and
Becky McClurken
A system of generalizations was
formulated about the organization,
development and persistence, adapta-
tion, and productivity of ecological
systems and their response to toxic
substances. Laboratory ecosystems
composed of persistent populations of
guppies, amphipods, snails and various
micro-invertebrates were used in
examining the generalizations fortheir
utility and conformity with field
observation. Guppy populations in the
ecosystems were exploited at different
rates to simulate fishing, and the
systems were provided with different
levels of habitat availability and
energy input rates. The laboratory
communities developed different
steady-state structures (population
densities) at different guppy exploita-
tion rates and different levels of
habitat availability and energy input.
One part per billion (ppb) of dieldrin
was continuously introduced into four
ecosystems, one at each guppy exploi-
tation rate, at the low level of habitat
availability and energy input. It was
determined in ancillary experiments
that 1 ppb of dieldrin probably directly
affected only the guppy populations.
As exploitation rates increased, gup-
pies exhibited increased growth and
reproduction. Oieldrin altered life
history patterns by reducing survival,
growth, and reproduction. Thus, the
toxicant may have caused the extinc-
tion of the heavily exploited popula-
tion by effectively preventing it from
exhibiting the life history pattern that
adapted it to persist at high exploitation
rates.
In aquarium experiments conducted
in conjunction with the laboratory
ecosystem studies, concentrations of
dieldrin similar to those used in the
laboratory ecosystems had effects on
life history patterns similar to those
observed in the ecosystems. It is less
evident how the diversity of effects on
guppy populations observed in the
ecosystems—ranging from perturba-
tion and recovery to extinction—could
have been predicted from the aquarium
experiments.
This Project Summary was devel-
oped by EPA's Environmental Re-
search Laboratory, Duluth, MN, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The performances of individual or-
ganisms, populations and communities
are in continuous change. Any perfor-
mance, such as structure, development,
replication, or persistence is determined
by the system's capacity and its envi-
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ronment. The capacity of such a system
depends on its organization, which
includes subsystems whose capacities
and performances are concordant with
the capacities and performances of their
level-specific environments. Systems
with a given capacity exhibit different
performances under different sets of
environment conditions. Development
and evolution alter the capacities of
these systems and thus also lead to
different performances. Competition,
predation, human exploitation and toxic
materials alter the capacities of these
systems and thus also lead to different
performances. Competition, predation,
human exploitation and toxic materials
alter organization and therefore the
adaptive capacities of both populations
and communities. These factors are
considered in setting up a system of
generalizations.
Determinants of community structure
and organization include level and kinds
of energy and materials available,
patterns of climatic conditions, colo-
nization opportunities, species interac-
tions (predation, competition, mutual-
ism, habitat distribution), and the spatio-
temporal distribution of primary phys-
ical habitats.
In order to gain a more general
theoretical and empirical understanding
Low Med High
150E
of some of the primary determinants of
community organization, an effort has
been made to couple interactions of
populations within an ecological system
with one another and with environ-
mental conditions and to display the
outcomes of these interactions graph-
ically by systems of isoclines on phase
planes. Systems of populations are
assumed to be multisteady-state sys-
tems. In Figure 1 a single steady-state
point is shown on each phase plane for
each set of environmental conditions.
The set of these points defines the
steady-state structure for a given
environment. Changing conditions such
as rate of exploitation or energy input
bring about a change in system struc-
ture. Thus, at each energy input rate,
increased rate of exploitation (E) reduces
the steady-state biomassof C, increases
H, reduces P, and increases R. Increases
in I shift the steady-state relationships
between predator and prey to the right
on each phase plane, essentially
increasing the biomass of C, H, P and R.
In Figure 1, individual species popula-
tions of carnivores, herbivores, plants
and plant resources form the subsystems
that are incorporated into the system.
The position and form of isoclines on
phase planes (and therefore steady-
state points) are determined by the
Low Med' High
I3
8
=8/1-
characteristics of the interacting popu-
lations. A complete set of isoclines on all
phase planes provides at least a partial
view of the structure and organization of
a system of populations faced with
varying rates of exploitation and energy
or plant resource input.
Toxic substances can alter the struc-
ture and organization of systems of
populations. The response of systems to
toxic substances is affected by conditions
in their environments such as rate of
energy input and level of exploitation of
their populations. In Figure 2, the
carnivore population is able to persist at
a low rate of energy input (low I) when it
is heavily exploited (90E) if a toxic
substance (T) is not present (the
predator isocline identified by 90E, OT
intersects the prey isocline identified by
low I). But under these same environ-
mental conditions at a toxicant concen-
tration of 2T, the carnivore population is
driven to extinction (C trajectory). The
carnivore is able to persist, although at
reduced biomass, at a toxicant concen-
tration of 2T at low I when the carnivore
is unexploited (OE), or at medium and'
high I when C is heavily exploited (90E).
But toxicant at a concentration of 2T so
altered carnivore adaptive capacity that
it was no longer adapted to persist when
heavily exploited at low I.
Low Med High
5
OH
1.9H
2.2H
3H
3.5H
1234
Herbivore biomass (H)
'01 234
Plant biomass (P)
1234
Plant resource
Figure 1. Phase planes and isocline systems representing the inter-relationships between populations in a system represented as
E *-^ C * \ * H . '^ P i + " R * + I, where C,H,P and R comprise the system andE. exploitation rate, and I, rate of
energy or plant resource input, are factors in the environment of the system. Predator biomass is plotted on the y-axis of
each phase plane and prey biomass is plotted on the x-axis. On each phase plane, the descending lines identified by
different rates of plant resource input, I, are prey isoclines. Each prey isocline is defined as a set ofbiomasses of predator
and prey where the rate of change of predator biomass with time is zero. Each intersection of a predator and prey isocline
is a steady-state point where the rate of change of both predator and prey biomass with time is zero. The positions and
forms of the isoclines can be reduced from response functions representing the rates of gain and loss of each of the
populations. At a particular level of I and E, a single steady-state point exists on each phase plane, the set of these points
defining the steady-state biomasses of C, H, P, R at Med I and OE (circles), 30E (squares), 90E (triangles) and 150E
(hexagons) are shown.
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Thegoalofthisstudyistoadvancethe
understanding of adaptive and other
capacities and performances of individ-
ual organisms, populations and sys-
tems of populations exposed to dieldrin.
Laboratory ecological systems composed
of guppies, amphipods, snails, and
various microinvertebrates were ex-
posed to different environments—
different exploitation rates, levels of
habitat availability and energy input and
exposure to the toxicant. Under each set
of conditions, systems were allowed to
reach steady-states. The specific objec-
tives are as follows:
1) Determine and explain, under
different sets of environmental
conditions, the impact of dieldrin
on the persistence, structure and
organization of laboratory eco-
systems in terms of concordance of
the capacities and performances
of the incorporated populations.
2) Determine and explain the impact
of dieldrin on the adaptive capacity,
life history patterns, production
and yield, as well as density of the
exploited population, and relate
these to community structure and
organization and environmental
conditions.
3) In a separate aquarium experi-
ment, determine and explain the
5 Low I Med I High I
impact of dieldrin on maturation,
growth and reproduction of indi-
viduals of the exploited population,
and relate these results to effects
on life history patterns and popu-
lation density of the exploited
population and on system structure
and organization observed in the
laboratory ecological systems.
Figure 3 summarizes the relation-
ships of predation and/or competition
observed in the laboratory ecosystems.
Exploitation and dieldrin, which in part
decreased the survival of young, result
in a decreased guppy population, as in-
dicated by the negative signs. Alfalfa
and light energy were both introduced
into the ecological systems, as indicated
by the positive signs.
Conclusions and
Recommendations
1. The system of generalizations devel-
oped by the authors broadly conforms
with observations of the laboratory
ecosystems. Communities composed
of persistent populations of guppies,
snails, amphipods and assorted
microinvertebrates established near
steady-states. Conditions in the
environments of the communities
such as rate of exploitation of their
5 Low l! Med I High I
populations, level of habitat avail-
ability and energy input, and exposure
to dieldrin in part determined the
steady-state structure that the
system developed.
2. The capacities of ecological systems
depend upon the way they are
organized. Trophic organization
entails interactions between popu-
lations such as competition, pre-
dation, mutualism and commen-
salism. Population interactions can be
represented on phase planes, as
summarized in Figure 4. In the
laboratory ecosystems, populations
of guppies, snails and amphipods
competed for a common food source,
the organic sediment. Amphipods
were also prey of guppies. As a
consequence of this organization,
increases in exploitation rate resulted
in reductions in near steady-state
guppy biomass and increases in the
near steady-state biomasses of their
snail competitor and amphipod
competitor/prey. Increases in habitat
availability and energy input in-
creased the biomasses of guppies,
amphipods and snails at each ex-
ploitation rate.
3. One ppb of dieldrin continuously
introduced into the laboratory eco-
systems at the low level of habitat
Low I High I
Med I \ ^QH
1H,2.5H
3.5H
3-
1-
H
Figure 2. Phase planes and isocline systems illustrating a possible effect on different concentrations of a toxicant (T) on the
structure of a simple community. In this example the toxicant directly affects only the carnivore population. Steady-
state structure at high I and OE, OT (circles); high I and OE. 27 (squares); low I and 90E, 2T (triangles) is shown.
Trajectories of biomasses of carnivore 1C), herbivore (H), plant (P). and plant source (R) converging on each of these
steady-states are shown. Introduction of toxicant at a concentration of 27 reduced carnivore biomass, increased
herbivore biomass, reduced plant biomass and increased plant resource at high I, OE (compare circles and squares on
all phase planes). At low I, in the absence of toxicant (0 T), the carnivore is able to persist at 90E (prey isocline identified
by low I intersects predator isocline identified by 90E, OT). But, at a concentration of 27, the carnivore population is
driven to extinction at 90E (prey isocline identified by low! does not intersect predator isocline identified by 90E, 2 T).
At higher I the carnivore is able to persist at 90E, 27.
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Exploitation
— Dieldrin
Planar/a *-
Snails
Organic sediments and
associated microorganisms
Alfalfa ration
Attached
algae
Light energy
Figure 3.
Kinetic diagram representing inferred trophic interrelations in the laboratory
ecosystem. Population interactions are designated as predation fp) or
competition (c). Exploitation, dieldrin, alfalfa ration, and light energy
are variable.
availability and energy input directly
affected only the exploited guppy
populations. Other populations in
the systems were affected only
indirectly as a result of changes in
guppy biomass. The response of the
system to dieldrin ranged from initial
reduction in biomass and subsequent
recovery of the unexploited guppy
population to extinction of the
heavily exploited guppy population.
Amphipods, a competitor and prey of
guppies, increased in biomass as a
result of the toxicant-induced reduc-
tion of guppy biomass.
4. Different near steady-state com-
munity structures were generated by
different rates of exploitation and
levels of habitat availability and
energy input. Guppies developed
different near steady-state life
history patterns at different exploita-
tion rates and levels of habitat and
energy input. Thus, there was a cor-
respondence between guppy life
history pattern and community
structure. At each level of habitat
availability and energy input rate,
increased exploitation rate reduced
guppy population density and induced
the following changes in life history
pattern: reduced lifespan, reduced
number of clutches per lifetime, in-
creased growth, increased size of
first reproduction, increased fecun-
dity. Increases in growth and repro-
duction of individuals can be inter-
preted as life history adaptations en-
abling the population to persist at
high levels of exploitation (or mortal-
ity), where longevity and clutch
number are reduced. Guppy produc-
tion and yield bear dome-shaped re-
lationships to population biomass.
Increases in habitat availability in-
crease the magnitude of these
curves.
5. Dieldrin altered guppy life history
patterns by reducing survival, growth
and fecundity. These alterations in
guppy life history resulted in reduc-
tions in the magnitude of guppy pro-
duction and yield curves. At the 40
percent exploitation rate, dieldrin
may have caused population extinc-
tion by effectively preventing indi-
viduals from exhibiting life history
patterns—more rapid growth, higher
fecundity, increased offspring survi-
val—that adapt the population to
persist at high exploitation rates.
Such heavily exploited populations,
where rapid growth, high fecundity
and good juvenile survival are essen-
tial for persistence, may be more
"sensitive" to reductions in growth,
fecundity and survival caused by
toxicants than population exploited
at lower rates. The density of the
unexploited guppy population was
reduced by exposure to dieldrin but
recovered to pre-dieldrin levels
while toxicant was still being intro-
duced. This population was appar-
ently able to adapt evolutionary to
the pesticide, with natural selection
favoring individuals with more "re-
sistant" life histories. The "re-
covered" population may have been
composed of individuals with quite
different life history capacities than
the population prior to dieldrin intro-
duction.
6. Separate aquarium experiments
were conducted to evaluate effects
of dieldrin on guppy life history pat-
terns at different food rations.
Dieldrin concentrations in these ex-
periments were similar to the con-
centration to which the guppies in
the laboratory ecosystems were
exposed. Guppy life history patterns
observed at different food rations
were broadly similar to near steady-
state life history patterns observed at
different exploitation rates and com-
munity structures in the laboratory
ecosystems. Thus, different food
rations in aquarium experiments
generated life history patterns similar
to those occurring at different exploi-
tation rates. Perhaps life history pat-
terns observed in these simple
experiments can be thought of, in the
broadest sense, as steady-state life
history patterns that would exist in
some steady-state community struc-
ture generated by some fixed set of
environmental conditions. Thus,
using ecological theory, perhaps
meaning can be written onto the
results of such simple experiments.
7. Effects of dieldrin on life history
patterns observed in aquarium ex-
periments resembled effects ob-
served in laboratory ecosystems—
reduced juvenile survival, decreased
growth and reproduction. However,
it is not as evident how the diversity
of effects of population abundance,
ranging from perturbation and re-
covery at zero percent exploitation to
extinction at 40 percent exploitation,
could have been predicted from
these simple aquarium experiments.
Perhaps, with appropriate theory,
such effects would have become
apparent. Interpretation of simple
aquarium experiments is not a mat-
ter of direct extrapolation to complex
systems. Ecological theory should be
used as a "vehicle" of extrapolation.
4
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and it is in the context of such theory
that results of simple aquarium
experiments can be given meaning.
1
.o
•c
X*
o
!
\ Alter, prey
\ Tox. 20E
'/yx/v
//' A /X
Higher E
. 40E, TOX.
Prey biomass
Figure 4. Generalized phase plane and isocline systems representing the interaction
between predator and prey, and some possible effects of energy input rate
(I), competition (comp.), toxic substances (tox.), plant nutrients (nut.),
exploitation (E), and alternative prey (alter, prey). Steady-state points are
indicated at th intersections of predator and prey isoclines. At each rate of
energy input, prescence of a competitor, toxic substances, or decrease in
plant nutrients can shift the prey isocline to the left on the phase plane and
can cause reduction insteady-state predator and prey biomass at each
exploitation rate, presence of alternative prey can increase and toxic
substances lower a predator isocline as shown at 20E. If a predator isocline
identified by a particular level of environmental factor (e.g. higher E or 40E,
TOX.) does not intersect a prey isocline identified by another environ-
mental factor or factors (e.g. low energy input with competitor present -
dashed prey isocline), then the predator population cannot persist under
that set of environmental conditions (solid hexagon).
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William J. Liss. Daniel M. Weltering, Susan E. Finger. Michael L Kulbicki, and
Becky McClurken are with the Oak Creek Laboratory of Biology of the
Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR
98331.
John G. Eaton is the EPA Project Officer (see below).
The complete report, entitled "Organization and Adaptation of Aquatic Labora-
tory Ecosystems Exposed to the Pesticide Dieldrin," (Order No. PB 82-219
122; Cost: $12.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, V'A 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
Duluth, MN 55804
•&U. S. GOVERNMENT PRINTING OFFICE: 1983/659-095/556
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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Fees Paid
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