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David M. Post
Bio | Research | Pubs | Lab
Members | Prospective
Students |
| My research
applies empirical and theoretical methods, typically
within a food-web framework, to answer questions at
the interface of population, community, and ecosystem
ecology. I primarily work in aquatic ecosystems, but
I also study interactions and processes that link aquatic
and terrestrial ecosystems. Major research areas include:
1) Food
web structure and dynamics
2) Influence
of population dynamics on community structure and ecosystem
processes
3) Landscape
linkages
4) Stable
isotope techniques
I address questions in these areas of research by melding
theory with empirical studies using field observations,
experimentation, and modeling. My research reflects
my belief that integration is the strength of ecology
and that we must address questions at the appropriate
scale, not the convenient scale. Because of this belief,
much of my work has been and will remain collaborative,
drawing upon diverse disciplines to answer both applied
and basic research questions.
Food web
structure and dynamics
Food web structure
Food-chain length is an important characteristic of ecological
communities because it influences community structure,
ecosystem function, and contaminant concentrations in
many top predators. Conventional wisdom typically holds
that resource availability determines food-chain length.
This argument proposes that, because a diminishing amount
of energy reaches upper trophic levels, food-chain length
should increase with an increase in the amount of energy
or limiting resources available to top predators. This
idea is typically tested by looking for relationships
between primary productivity and food-chain length (although
as others have noted, variation in ecological efficiencies
could be just as important as variation in primary productivity).
These energetic based arguments were recently re-formulated
by Thomas Schoener as the productive-space hypothesis,
which predicts that food-chain length should increase
with the product of ecosystem size (area or volume) and
some measure of per-unit-size productivity (e.g., g C
m-2 yr-1). The productive-space
hypothesis can be tested by looking for a relationship
between the productive space (size * productivity) and
food-chain length; however, a significant relationship
between these two productive space and food-chain length
tells us little about the underlying determinants of food-chain
length. Alternatively, we can independently consider the
effects of ecosystem size and per-unit-size productivity
on food-chain length, which provides a better understanding
of the potential underlying determinants of food-chain
length.
The three testable predictions that arise from independent
consideration of ecosystem size and per-unit-size productivity
are that food-chain length could be determined by 1)
productivity alone (the productivity hypothesis), 2)
by ecosystem size alone (the ecosystem-size hypothesis),
or 3) by a combination of productivity and ecosystem
size (the productive-space hypothesis). The productivity
and productive-space hypotheses both derive from the
original resource availability arguments; however, the
productivity hypothesis does not explicitly include
ecosystem size as a determinant of resource availability.
The ecosystem-size hypothesis is based on the relationship
between ecosystem size and species diversity or between
ecosystem size and habitat availability and heterogeneity.
The ecosystem-size hypothesis does not explicitly include
resource availability as a direct determinant of food-chain
length, although resource availability could indirectly
contribute to variation in food-chain length.
 There
have been few empirical tests of these food-chain theories
primarily because it is difficult to estimate both ecosystem
size and food-chain length in natural ecosystem. Working
in collaboration with Mike Pace (Institute
of Ecosystem Studies) and Nelson Hairston, Jr. (Cornell
Univeristy), we overcame these obstacles by using
stable isotope techniques to determine food-chain length
and by taking advantage of the relative isolation of
lakes to estimate ecosystem size. We estimated food-chain
length in 25 north temperate lakes ranging across independent
gradients of volume (3.8x105 to 1.7x1012
m3) and per-unit-size productivity 
(measured using total phosphorus concentrations; 2.6
to 230 μg P l-1). We found that
food-chain length increased with ecosystem size, but
was not influenced by productivity. The increase in
food-chain length was caused by both the addition of
new top predators to the food web and a general increase
in the trophic position of all top predators. These
results challenge the conventional wisdom that energy
or resource availability limits food-chain length, and
identifies a new direction for research in this area
of community ecology.
Currently I am working to better understand how ecosystem
size, independent of an effect on totally productivity,
may influence food-chain length. I also continue to
explore broadly the determinants of food-chain length
in different aquatic and terrestrial systems, and to
evaluate and critique the varied methods used to describing
food-web structure.
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Relevant publications:
Post, D.M., M.L. Pace, and N.G. Hairston Jr.
2000. Ecosystem size determines food-chain length in
lakes. Nature 405:1047-1049. [Abstract;
link to Full
article]
Post, D.M. The long and short of food-chain
length. 2002 Trends in Ecology and Evolution
17:269-277 [Abstract;
Link to Full
article]
Post, D.M. Relationship between food-chain length
and attributes of ecosystem size. In Prep |
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Relevant publication:
Post, D.M., M.E. Conners, and D.S. Goldberg.
2000. Prey preference by a top predator and the stability
of linked food chains. Ecology 81:8-14. [Abstract;
link to Full
article] |
top
| Influence
of population dynamics on community structure and ecosystem
processes
 The
population dynamics of top predators have important
impacts on food web dynamics, community structure, and
ecosystem processes. For example, piscivorous fish can
have large impacts on food web structure and primary
productivity in lakes. Most piscivorous fish, however,
begin life as zooplanktivores. Theory predicts that
a large cohort of ultimately piscivorous fish (e.g.,
largemouth bass) can strongly influence pelagic community
structure, first through zooplanktivory and then through
piscivory as the cohort progresses through its trophic
ontogeny. This hypothesis has implications both for
understanding natural variation in the food web structure
of unexploited ecosystems, and for using 
piscivorous fish manipulations to influence water quality
through food web biomanipulation. My work with largemouth
bass confirms theoretical predictions that piscivorous
fish can affect food web structure at multiple temporal
scales; however, it also shows that direct effects of
ultimately piscivorous fish on zooplankton community
structure are short-lived because largemouth bass move
rapidly from feeding on zooplankton to feeding on benthic
invertebrates and fish. I have also worked to combine
long-term population data and with short-term mechanistic
observations to evaluate mechanisms influencing recruitment
of largemouth bass through multiple life history stages.
By working within a single system, at multiple temporal
scales, I was able to clearly document both the causes
of variable recruitment and the linkages between population
dynamics and food web structure.
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Relevant publications:
Post, D.M., J.F. Kitchell, and J.R. Hodgson.
1998. Interactions among adult demography, spawning
date, growth rate, predation, overwinter mortality,
and the recruitment of largemouth bass in a northern
lake. Canadian Journal of Fisheries and Aquatic
Sciences 55:2588-2600. [Abstract;
link to Full
article]
Post, D.M., S.R. Carpenter, D.L. Christensen,
K.L. Cottingham, J.F. Kitchell, D.E. Schindler, and
J.R. Hodgson. 1997. Seasonal effects of variable recruitment
of a dominant piscivore on pelagic food web structure.
Limnology and Oceanography 42:722-729. [Abstract]
Post, D.M., and J.F. Kitchell. 1997. Trophic
ontogeny and life history effects on interactions between
age-0 fishes and zooplankton. Archiv für Hydrobiologie,
Advances in Limnology 49:1-12. [Abstract]
Johnson, J.M. and D.M. Post. 1996.
Morphological constraints on intra-cohort cannibalism
in age-0 largemouth bass. Transactions of the
American Fisheries Society 125:809-812.
[Abstract] |
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| Landscape linkages
There is broad interest in better understanding
the importance of landscape linkages among what were
once thought to be isolated ecosystems. Some of the
most interesting questions relate to linkages between
aquatic and terrestrial ecosystems, within aquatic systems
such as littora pelagic linkages, and among aquatic
systems such as lakes, rivers, and estuaries. Landscape
location and arrangement, and their interaction with
ecosystem size, may have important impacts on the strength
linkages among ecosystems and food webs.
In collaboration with researchers at the University
of Wisconsin - Madison and the US Fish & Wildlife, I
studied the role of migratory waterfowl in moving nutrients
across the landscape of central New Mexico. Birds are
highly mobile vectors for nutrients. We found that over-wintering
snow geese and sandhill cranes transport nutrients from
farm fields, where they feed, to their roosting sites
in 
managed wetlands at the Bosque del Apache National Wildlife
Refuge. Nutrient inputs into these wetlands both augment
nutrient availability and change nutrient cycling through
modification of nitrogen-to-phosphorus ratios (bird
excretion has low N:P). Although loading was extremely
high in the few wetlands where the majority of geese
roost (about half of the total annual nutrient load
for those wetlands), the impacts were diluted at the
larger spatial scale of the entire wetland complex where
bird born nutrient had little effect on ambient nutrient
concentrations. This project was especially germane
in the Southwest where water quantity and quality, and
the use of water by humans and wildlife, are in potential
conflict. High nutrient inputs by birds exacerbate conservation
and management problems associated with high bird densities
and limited water supply.
I have also worked to understand how food
webs dynamics may be influenced by the movement of predators
or the flow of energy and nutrients across ecosystem
and sub-ecosystem boundaries. Using theoretical models,
I have addressed how food web linkages created by mobile
top predators may influence dynamical properties of
food webs. I am currently working on a proposal to convene
a NCEAS working group to address linkages primarily
among aquatic systems, and over the next few years I
hope to develop an whole ecosystem experimental program
to explore ecosystem and food web linkages among lentic
(flowing water; e.g., rivers and streams) and lotic
ecosystems (standing water; e.g., lakes). |
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Relevant publications:
Post, D.M., M.E. Conners, and D.S. Goldberg.
2000. Prey preference by a top predator and the stability
of linked food chains. Ecology 81:8-14. [
Abstract; link to
Full article]
Kitchell, J.F., D.E. Schindler, B.R. Herwig,
D.M. Post, M.H. Olson, and M. Oldham. 1999.
Nutrient cycling at the landscape scale: the role of
diel foraging migrations by geese at the Bosque del
Apache National Wildlife Refuge, New Mexico. Limnology
and Oceanography 44:828-836. [Abstract]
Post, D.M., J.P. Taylor, J.F. Kitchell, M.H.
Olson, D.E. Schindler, and B.R. Herwig. 1998. The
role of migratory waterfowl as nutrient vectors in managed
wetlands. Conservation Biology 12:910-920. [Abstract] |
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Stable Isotope Techniques
My research on food-chain length was
made possible by recent advances in stable isotope analysis.
In addition to using stable isotopes to document food
web structure, I am using stable isotope techniques
to evaluate the flux of carbon into and throuh lake
food webs. I hope to expand this work in the future
by including radiocarbon (14C) to better
resolve carbon sources. My lab at Yale will be an active
partner in the new
Yale Institute for Biospheric Studies, Center for Stable
Isotopic Studies of the Environment. Over the next
few yaers I plan to develop and apply compound specific
stable isotope techniques to help better resolve food
web structure in an effort to critically test a number
of basic questions in food web ecology. The following
paper is my attempt to document the models, methods,
and assumptions required to estimate trophic position
using stable isotopes |
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Relevant publication:
Post,D.M. 2002. Using
stable isotopes to estimate trophic position: models,
methods, and assumptions. Ecology 83:703-718.
[Abstract;
link to Full
article]
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