- Organization and the adaptation of aquatic laboratory ecosystems
to resource availability, exploitation, and a toxicant were examined in
a 34-month study. Sixteen 560-liter microcosms including prey,
predator, and competitor populations were employed over a wide range
of energy and habitat resource availability and exploitation of the top
predator. Chronic exposure to the pesticide dieldrin was used to perturb
systems near steady states. Empirical generalizations from
ecological theory, productivity theory, and fishery exploitation theory
were incorporated to explain in part such performances as development,
structure, and persistence of the laboratory ecosystems, their
communities, populations, and individual organisms.
Dynamic (developmental) and near steady-state community structure
and organization were detailed for guppy, amphipod, snail,
planaria, and algae populations and for a benthic detritus and microorganism component. Near steady-state population performances
including density, production, yield, and size-specific growth
and reproduction were determined for the guppy population in order
to demonstrate the concordance of life history, population, and community
level performances with changes in environmental conditions. A
system of isoclines on a series of interrelated resource-utilizer and
competition phase planes was employed to gain a better understanding
of the structure and apparent organization of the laboratory systems.
The 16 systems were established with a 0.6 gram/day alfalfa
ration, 20 percent of each tank bottom covered with gravel for invertebrate
habitat, and 0, 10, 20, or 40 percent guppy exploitation per
month. Exploitation directly affected the size of the guppy populations
near steady state (through observed changes in production, yield,
growth, and reproduction) and indirectly affected the size of snail and
amphipod populations which responded to competition and/or predation
from guppies. Following many months of near steady-state system
behavior (i. e. restricted fluctuations of population biomasses), eight
systems were shifted to a 4.0 grams/day alfalfa ration and 95 percent
gravel cover. These high energy and habitat level systems were
characterized by relatively complex trophic and habitat resource partitioning.
These systems developed different population interactions
and community structure and had much higher population densities.
At the same time, four low energy and habitat level systems (one at each guppy exploitation rate) with well established near steady states
began continuous exposure to 1.0 ppb dieldrin in the water. The response
of the laboratory systems to the toxicant was determined by the
levels of prevailing environmental conditions as well as by the system's
organization and the capacity of the populations to adapt and to
persist. There were both density-dependent (via exploitation) and
time-dependent components to the response to toxicant perturbation.
In general, dieldrin reduced the growth and reproduction of the guppies,
this resulting in smaller population biomasses. Amphipod biomasses
increased in response to reduced predation and competition.
The ecosystem with 0 percent exploitation (i.e. the largest guppy biomass)
responded immediately with apparent dieldrin induced mortalities
of mature fish. However, eventually the population recovered its
lost biomass. There was no apparent initial response at 40 percent
exploitation (i. e. smallest guppy biomass), but after 15 months of exposure
the guppy population went extinct apparently from the. combined
stress of exploitation and toxicant.
Community organization and its expression in community development,
structure, and persistence involved the adaptation of species
populations to each other, to available energy and materials, to habitat,
to climatic conditions including water quality and temperature, to
exploitation, and to the introduction of the toxicant. Manipulating
energy and habitat availability, exploitation levels, and toxicant presence altered near steady-state community, population, and individual
organism performances. In these responses to environmental
conditions, there was a certain concordance of community, population,
and life history patterns that constitutes adaptation of the community
and its subsystems.