Williams, J.W., D.M. Post, L.C. Cwynar, A.F. Lotter, and A.J. Levesque.  2002.   Rapid and widespread vegitation responses to past climate change in the North Atlantic region. Geology 30:971-97

Ongoing anthropogenic perturbations to the atmosphere and biosphere increase the risk of future abrupt changes in the climate system, generating concern about the ability of natural ecosystems to respond to rapid climate change. Study of past climatic events and biotic responses can inform us about potential future change. Qualitatively fast local responses to abrupt late-glacial climate oscillations have been reported from individual records and attributed to short migration distances in areas of high topographic relief. Using quantitative time-series analyses, we show that vegetation responses to late-glacial climate change around the North Atlantic were rapid, widespread, and occurred in areas of differing relief. Cross-correlation analysis of 11 high-resolution lacustrine records in eastern North America and Europe indicates vegetation response times consistently <200 years and often <100 years, despite regional differences in physiography and species composition. Vegetation lags <200 years confirms theoretical predictions and the apparently tight coupling between vegetation and atmosphere suggests that recent climatic trends may already have begun to affect plant population abundances and distributions.

Post, D.M.  2002.  The long and short of food-chain length. Trends in Ecology and Evolution 17(6):269-277.

Food-chain length is a central characteristic of ecological communities that has attracted considerable attention for over 75 years because it strongly affects community structure, ecosystem processes, and contaminant concentrations. Conventional wisdom holds that resource availability or dynamical stability limit food-chain length; however, new studies and new techniques challenge the conventional wisdom and broaden the discourse on food-chain length. Recent results suggest that resource availability only limits food-chain length in systems with very low resource availability, and calls into question the theoretical basis for dynamical stability as a determinant of food-chain length. Evidence currently points towards a complex and contingent framework of interacting constraints that includes the history of community organization, resource availability, the type of predator-prey interactions, disturbance, and ecosystem size. Within this framework, the debate has shifted from a search for singular explanations, to a search for when and where different constraints operate to determine food-chain length.

Post, D.M.  2002.  Using stable isotopes to estimate trophic position: models, methods, and assumptions. Ecology 83:703-718.

The stable isotopes of nitrogen (Delta-15N) and carbon (Delta-13C) provide powerful tools for estimating the trophic positions of and carbon flow to consumers in food web; however, the isotopic signature of a consumer alone is not generally sufficient to infer trophic position or carbon source without an appropriate isotopic baseline. In this paper, I develop and discuss methods for generating an isotopic baseline, and evaluate the assumptions required to estimate the trophic position of consumers using stable isotopes in multiple ecosystem studies. I test the ability of two primary consumers, surface grazing snails and filter feeding mussels, to capture the spatial and temporal variation at the base of aquatic food webs. I find that snails reflect the isotopic signature of the base of the littoral food web, mussels reflect the isotopic signature of the pelagic food web, and together they provide a good isotopic baseline for estimating trophic position of secondary or higher trophic level consumer in lake ecosystems. Then, using data from 25 north temperate lakes, I evaluate how Delta-15N and Delta-13C of the base of aquatic food webs varies both among lakes and between the littoral and pelagic food webs within lakes. Using data from the literature, I show that the average trophic fractionation of Delta-15N is 3.4‰ (s.d. = 1‰) and of Delta-13C is 0.4‰ (s.d. = 1.3‰), and both, even though variable, are widely applicable. A sensitivity analysis reveals that trophic position is very sensitive to assumptions about the trophic fractionation of Delta-15N, moderately sensitive to different methods for generating an isotopic baseline, and not sensitive to assumptions about the trophic fractionation of Delta-13C when Delta-13C is used to estimate the proportion of nitrogen in a consumer derived from two sources. Finally, I compare my recommendations for generating an isotopic baseline to an alternative model proposed by Vander Zanden and Rasmussen (1999). With an appropriate isotopic baseline and an appreciation of the underlying assumptions and model sensitivity, stable isotopes can help answer some of the most difficult questions in food web ecology.

Post, D.M., M.L. Pace, and N.G. Hairston Jr. 2000. Ecosystem size determines food-chain length in lakes. Nature 405:1047-1049.

Food-chain length is an important characteristic of ecological communities: it influences community structure, ecosystem functions and contaminant concentrations in top predators. Since Elton first noted that food-chain length was variable among natural systems, ecologists have considered many explanatory hypotheses, but few are supported by empirical evidence. Here we test three hypotheses that predict food-chain length to be determined by productivity alone (productivity hypothesis), ecosystem size alone (ecosystem-size hypothesis) or a combination of productivity and ecosystem size (productive-space hypothesis). The productivity and productive-space hypotheses propose that food-chain length should increase with increasing resource availability; however, the productivity hypothesis does not include ecosystem size as a determinant of resource availability. The ecosystem-size hypothesis is based on the relationship between ecosystem size and species diversity, habitat availability and habitat heterogeneity. We find that food-chain length increases with ecosystem size, but that the length of the food chain is not related to productivity. Our results support the hypothesis that ecosystem size, and not resource availability, determines food-chain length in these natural ecosystems.

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.

Recent theoretical studies have show the potential for chaotic dynamics in simple three-species food chains. Most of these studies have focused on linear food chains even though natural food chains are seldom isolated from the surrounding food web. There is a growing awareness that food web dynamics can be strongly influenced by the behavior and movement of predators, energy and nutrients across ecosystem and sub-ecosystem boundaries. Motivated by observations from lakes, where the pelagic food web is often linked to the littoral food web by mobile predators, we constructed a simple model to evaluate the dynamics of two food chains linked by a top predator with prey preference. Linking the two food chains had no qualitative effect on model dynamics but did increase the density of the top predator. Instead, it was the prey preference of the top predator that changed system dynamics. We found a range of prey preference that could eliminate chaos, dampen oscillations and even produce point stability in a previously oscillatory system. The strength of prey preference required to produce a point attractor in a previously chaotic system was positively related to the dimension of chaos (a measure of the complexity of chaos). Our results suggest that, although chaos is possible in food webs, common processes like prey preference reduce the potential for chaos.

Hairston, N.G., Jr., W. Lampert, C.E. Cáceres, C.L. Holtmeier, L.J. Weider, U. Gaedke, J.M. Fischer, J.A. Fox, D.M. Post. 1999. Dormant eggs record rapid evolution. Nature 401:446.

Natural selection can lead to rapid changes in organisms, which can in turn influence ecosystem processes. A key factor in the functioning of lake ecosystems is the rate at which primary producers are eaten, and major consumers, such as the zooplankton Daphnia, can be subject to strong selection pressures when phytoplankton assemblages change. Lake Constance in central Europe experienced a period of eutrophication (the biological effects of an input of plant nutrients) during the 1960s-70s, which caused an increase in the abundance of nutritionally poor or even toxic cyanobacteria. By hatching long-dormant eggs of Daphnia galeata found in lake sediments, we show that the mean resistance of Daphnia genotypes to dietary cyanobacteria increased significantly during this eutrophication. This rapid evolution of resistance has implications for the ways that ecosystems respond to nutrient enrichment through the impact of grazers on primary production.

Sanderson, B.L., T.R Hrabik, J.J. Magnuson, and D.M. Post. 1999. Cyclic dynamics of a yellow perch (Perca flavescens) population in an oligotrophic lake: evidence for the role of intraspecific interactions. Canadian Journal of Fisheries and Aquatic Sciences 56:1534-1542.

Understanding the extent to which repeated oscillations in fish populations are driven by external factors or internal processes within the population is an important challenge. We document cyclic dynamics in a population of yellow perch (Perca flavescens) in oligotrophic Crystal Lake. Since 1981, we have observed three cases of cohort dominance in which two age-classes dominated the population for roughly 5 years. Young-of-the-year (YOY) perch were caught in 1981-1982, 1986-1987, and 1990-1991, whereas few to no YOY were caught during the midyears. The presence of YOY was negatively related to juvenile perch abundance and positively related to adult perch abundance. Mechanisms that may be responsible for these patterns include cannibalism of YOY by either juveniles or adults, potential for reproduction by adults, and competition between YOY and juveniles. YOY were abundant primarily in years when reproductively mature fish were in the lake, suggesting that the repeated oscillations are driven predominantly by pulses of abundant, reproductive, adult perch. As these young perch grow to juveniles, they exclude the possibility of survival by successive cohorts through cannibalistic and competitive interactions. This exclusion occurs until they themselves become reproductively mature and the cycle then repeats. Ultimately, long-term patterns in Crystal Lake suggest that cyclic dynamics are generated by intraspecific interactions.

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.

Wildlife refuges are sites of dense aggregations of wintering waterfowl. Refuge managers are concerned about local water-quality effects and the amount of birdborne nutrient load that might cause eutrophication in roosting ponds and(or) be exported to downstream systems. We initiated this research effort to test the hypothesis that daily feeding migrations by geese represented a significant source of nutrients to wetland systems at the Bosque del Apache National Wildlife Refuge in the middle Rio Grande River valley of New Mexico. We documented the role of geese in translocating nutrients through a combination of time budget and mass balance models for birds, bioassays of phytoplankton growth, and stable isotope methods that trace the source of nitrogen to food webs. Geese increased the nutrient loading rates in some wetland ponds by up to 40% for total nitrogen and 75% for total phosphorus. Bioassays revealed that nitrogen was consistently limiting to primary production by algae in the ponds. Chlorophyll levels increased in proportion to bird densities. Fish and crayfish from the ponds intensively used by birds had very low stable nitrogen signatures relative to those from the Rio Grande River. This result derived from the low 15N in alfalfa and corn, which were the primary forage for birds, and accumulated through food webs in proportion to bird use of individual ponds as roosting areas. In general, the wetland ponds functioned efficiently in retaining the birdborne, allochthonous nutrient load and only modest amounts of the total nitrogen or phosphorus were exported downstream.

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.

Dense aggregations of waterfowl, often caused by loss of native wetlands and increased waterfowl numbers, can result in the destruction of wetland vegetation and agricultural crops, increase the risk of infectious disease outbreaks, and decrease water quality. Problems related to water quality may be particularly problematic in arid regions of southwestern United States where water quality and quantity are contentious issues. Over 40,000 Lesser Snow and Ross' Geese winter annually at Bosque del Apache National Wildlife Refuge, New Mexico. Daily feeding bouts by geese move large quantities of nutrients from farm fields where they feed to managed wetlands where they roost. Using energy and mass balance models, population estimates, daily and seasonal migration patterns, and feeding behaviors, we estimated the mass and N:P ratio of nutrients loaded by geese into the wetlands of the Bosque del Apache National Wildlife Refuge. Loading rates peaked in late November 1995 at >300 kg nitrogen day-1 and > 30 kg phosphorus day-1. As feeding behaviors changed through the winter and bird densities declined, loading rates fell. Our estimates suggest that in the winter of 1995-1996, bird-borne nutrients supplied nearly 40% of the nitrogen and 75% of the phosphorus entering the primary wetland used for geese roosting. High loading rates by geese are a consequence of their colonial roosting behavior; >90% of the geese roost on 10% of the wetland area. Effects of nutrient loading could be reduced by increasing flushing rates or dispersal of roosting waterfowl. The loss of natural wetlands, however, limit options for expanding wetland area available to waterfowl and increasing flushing rates may be difficult in this arid region. Thus, management of waterfowl and the nutrients they move will continue to be an important issue for wetlands of wildlife refuges.

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.

Using 12 years of data, we evaluated the mechanisms controlling largemouth bass, Micropterus salmoides, recruitment in a lake near the northern extent of the largemouth bass range. We found that complex interactions among adult demographics, size-selective predation and overwinter mortality regulate the number of largemouth bass surviving the first year of life. The largest recruitment events required at least a moderate number of adults, but a large number of adults was not sufficient to produce a large cohort of largemouth bass. Predation was controlled by the number of both adult and juvenile bass and was not strongly correlated with reproductive output. Overwinter mortality was size-dependent, strongly affecting bass entering the winter at <50-60 mm in length, and likely the result of starvation. Predation and overwinter mortality interacted with spawning date and growth rate to produce variable but predictable patterns of first year survival. At high adult and juvenile densities, predation regulates first year survival. At low adult and juvenile densities first year survival was regulated by adult demographics and interactions among spawning date, growth rates and overwinter mortality. Although we can forecast coarse patterns of cohort survival, the survival of individual fish was more difficult to predict because length and age were not highly correlated.

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.

Piscivorous fish play an important role in regulating lake food web structure. However, most ultimately piscivorous fish pass through a period of planktivory before becoming piscivorous. In 1993 and 1994, two large cohorts of young-of-the-year largemouth bass (Micropterus salmoides) were recorded in Paul Lake, MI, USA. The 1993 and 1994 cohorts were the largest recorded in data extending back to 1984, and occurred during a period in which adult and juvenile bass biomass was low. In 1993, consumption of large-bodied zooplankton by the young-of-the-year cohort eliminated large-bodied cladocerans by the middle of August. As large cladocerans declined, small cladocerans, especially Bosmina longirostris, increased. By early September the biomass of B. longirostris was similar to that of the entire cladoceran community in previous years. Coincident with the shifts in the cladoceran community were increases in epilimnetic chlorophyll a concentrations < 35 mm. However, total epilimnetic chlorophyll a did not increase. In contrast, the 1994 cohort had no discernible effect on pelagic food web structure because production of large-bodied zooplankton rapidly exceeded predation. Our results show the potential for large cohorts of piscivorous fish to affect food web structure, at a season scale, through intense planktivory.

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.

A large cohort of age-0 fishes creates a pulse of zooplanktivory as it goes through trophic ontogeny. The magnitude and duration of this pulse of zooplanktivory depends on the species life history characteristics and trophic interactions. The life history characteristics and trophic interactions of fish species that were predominantly zooplanktivorous and ultimately piscivorous were contrasted to assess their effect on interactions between cohorts of age-0 fish and zooplankton. Ultimately piscivorous fish species were found to have a lower potential to affect zooplankton communities because of low reproductive potential and rapid trophic ontogeny. Then, using 12 years of data from a piscivore dominated system, we assessed the effects of large cohorts of ultimately piscivorous age-0 fish on zooplankton communities. Results showed that a large cohort of ultimately piscivorous fishes may have strong impacts on zooplankton communities. The potential for strong effects, however, depended on the interactions between Daphnia biomass, fish biomass, and the timing of peak cohort predation.

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.

Intra-cohort cannibalism can be an important process shaping the growth and survival of age-0 largemouth bass, Micropterus salmoides. To assess the morphological constraints on intra-cohort cannibalism in largemouth bass we evaluated the relationship of mouth width, body depth, and total length in age-0 fish. Morphological measurements predicted a threshold predator-prey relationship of: Prey Length = 0.309*Predator Length^1.109, while experimental results suggested the relationship of: Prey Length = 0.5*Predator Length. Our results indicated that intra-cohort cannibalism may begin as early as 9 days after swim-up in southern lakes and reservoirs, and 12 days in northern lakes. However, due to the length of the spawning season, slower growth rates, and male guarding, intra-cohort cannibalism may be more important in southern systems than in northern lakes.

Post, D.M., T.M. Frost, and J.F. Kitchell. 1995. Morphological responses by Bosmina longirostris and Eubosmina tubicen to changes in copepod predator populations during a whole-lake acidification experiment. Journal of Plankton Research 17:1621-1632.

Changes in zooplankton populations during the experimental acidification of Little Rock Lake provided an opportunity to examine specific mechanisms underlying morphological responses of bosminids to changing predation pressure. Two large copepods, Epischura lacustris and Mesocyclops edax, disappeared from the lake's acidified basin in 1986 and 1989 respectively while a smaller copepod predator, Tropocyclops extensus increased during later stages of acidification. The two bosminid species showed distinctly different responses coinciding with the changes in copepod predation. Bosmina longirostris exhibited a significant decrease in mucro length with the decline of M. edax and E. lacustris. Its mean body and antennule length, however, did not change. We suggest that the decoupling of B. longirostris mucro length and antennule length may have been related to the persistence of the smaller copepod predator, T. extensus. Eubosmina tubicen showed no apparent response to declines in M. edax and E. lacustris abundance in either mean mucro, antennule or body length. Allometric analyses indicated, however, that mucro length was related to size dependent copepod predation for both B. longirostris and E. tubicen.

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