Ecological Monographs

Leadley, Paul W.; Reynolds, James F.; Chapin, F. S.

doi: 10.1890/0012-9615%281997%29067%5B0001:AMONUB%5D2.0.CO%3B2pmid: N/A

A model of nutrient uptake was used to determine the factors that control the uptake of ammonium, glycine, and nitrate for an important arctic sedge, Eriophorum vaginatum, because the factors that regulate the nitrogen (N) uptake of plants in natural ecosystems are not well understood and a growing number of studies suggest that organic forms of N, including glycine, are an important source of N for E. vaginatum and other plants. E. vaginatum was selected as an exemplary system to explore nitrogen (N) uptake of a native species in situ, because it appears to be strongly N limited in the field, its N nutrition has been extensively studied, and its root growth habits make it well suited to modeling nutrient uptake. The model accounts for N supply from microbial mineralization and other sources, flux of N through the soil to the root surface, and uptake by the plant root. We included glycine in our simulations of N uptake because laboratory experiments have shown that E. vaginatum can use glycine and other low-molecular-mass amino acids as a source of N. However, the extent to which glycine contributes to the N nutrition of E. vaginatum in the field is unknown. Based on these simulations we find that: (1) Ammonium, glycine, and nitrate could all potentially make significant contributions to the N nutrition of E. vaginatum. The relative contribution of glycine is difficult to assess, because its behavior in the soil has not been characterized. However, glycine and ammonium contributed in roughly equal proportions for most model parameterizations. (2) The importance of factors that regulated modeled ammonium and glycine uptake can be ranked as follows: supply rate > ((soil factors (buffer capacity and diffusion coefficient) == root density)) > root uptake kinetics. Supply rate was the only factor that regulated nitrate uptake. These simulation results and other considerations suggest that N uptake kinetics of roots and soil-solution concentrations may not be reliable predictors of whole-plant N uptake in tundra or other natural ecosystems.

Leadley, Paul W.; Reynolds, James F.; Chapin, F. S.

doi: 10.1890/0012-9615(1997)067[0001:AMONUB]2.0.CO;2pmid: N/A

A model of nutrient uptake was used to determine the factors that control the uptake of ammonium, glycine, and nitrate for an important arctic sedge, Eriophorum vaginatum, because the factors that regulate the nitrogen (N) uptake of plants in natural ecosystems are not well understood and a growing number of studies suggest that organic forms of N, including glycine, are an important source of N for E. vaginatum and other plants. E. vaginatum was selected as an exemplary system to explore nitrogen (N) uptake of a native species in situ, because it appears to be strongly N limited in the field, its N nutrition has been extensively studied, and its root growth habits make it well suited to modeling nutrient uptake. The model accounts for N supply from microbial mineralization and other sources, flux of N through the soil to the root surface, and uptake by the plant root. We included glycine in our simulations of N uptake because laboratory experiments have shown that E. vaginatum can use glycine and other low‐molecular‐mass amino acids as a source of N. However, the extent to which glycine contributes to the N nutrition of E. vaginatum in the field is unknown.

Ricklefs, Robert E.

doi: 10.1890/0012-9615(1997)067[0023:CDONWP]2.0.CO;2pmid: N/A

This analysis uses information obtained primarily from museum collections to estimate demographic parameters of populations of thrushes (Turdus spp.) from throughout the Western Hemisphere. Adult survival rates were estimated from the proportions of individuals >1 yr old in museum collections, assuming constant population size, discrete reproductive seasons, unbiased collecting, and maturity at 1 yr old. The effects of relaxing each of these assumptions were examined. The resulting biases were found to be small compared to differences between populations, and in several cases, they were opposite to the conventional wisdom that survival rate increases towards the equator.

Ricklefs, Robert E.

doi: 10.1890/0012-9615%281997%29067%5B0023:CDONWP%5D2.0.CO%3B2pmid: N/A

This analysis uses information obtained primarily from museum collections to estimate demographic parameters of populations of thrushes ( Turdus spp.) from throughout the Western Hemisphere. Adult survival rates were estimated from the proportions of individuals >1 yr old in museum collections, assuming constant population size, discrete reproductive seasons, unbiased collecting, and maturity at 1 yr old. The effects of relaxing each of these assumptions were examined. The resulting biases were found to be small compared to differences between populations, and in several cases, they were opposite to the conventional wisdom that survival rate increases towards the equator. The annual survival rate of adults ( S ) is estimated without bias by E ( S ) == A /( A ++ I ), which has a standard error of ((( AI )/( A ++ I ) 3 )) ½½ , where A is the number of adults in a collection and I is the number of immatures. In Turdus, adult and immature birds can be distinguished by the appearance of the secondary coverts, the immature forms of which are retained until the first postnuptial molt at ≈≈1 yr old. The present analysis included 8653 specimens from 30 populations of 19 species from Alaska to southern Patagonia. Additional data tabulated from museum specimens included numbers of juveniles, evidence of breeding, worn plumage, and molt. Estimated annual survival rates varied from a mean of 0.56 in temperate North American populations, to 0.68 in subtropical South American populations, 0.76 in lowland tropical populations, and 0.80 and 0.85 in tropical montane populations in Central and South America, respectively. Samples of adults and immatures restricted to the period immediately prior to the breeding season gave results that were indistinguishable from samples summed over the entire year. Survival rate was most strongly correlated with the difference between maximum and minimum mean monthly temperatures ( r == −−0.84). Thus, survival is inversely related to seasonality of temperature. An index to fecundity ( J ) was estimated by the frequency of juvenile specimens (generally <2 mo old) relative to those of adults in museum collections. This index was inversely related to annual adult survival. A relative index of prereproductive (generally 1st-yr) survival ( S a ) was calculated as the annual adult mortality ((1 −− E ( S ))) divided by J. Estimated S a was independent of estimated S. Calculations of annual fecundity ( F ) from field studies on breeding of selected species supported both of these observations. These results, which are consistent with conclusions based upon broader comparisons among birds, suggest several hypotheses: (1) adult survival in Turdus populations is determined by physical conditions during the nonbreeding season, particularly various ecological effects of low temperature; (2) fecundity is sensitive to density-dependent feedbacks from adult population size and, possibly, to environmental factors correlated with other factors that independently affect adult survival; and (3) 1st-yr survival, age at maturity, or both, also are affected by density-dependent factors.

Wootton, J. Timothy

doi: 10.1890/0012-9615(1997)067[0045:EATOPC]2.0.CO;2pmid: N/A

Predicting the dynamics of natural food webs requires estimates of the strength of interactions among species. The ability to estimate per capita interaction strength from observational data is desirable because of the logistical difficulty of using experimental manipulations to obtain such measures for all species within complex natural communities. In this paper, I derive observational measures of per capita interaction strength having units matching those of dynamic food web models (per capita consumption and assimilation rates). I also highlight the difference between per capita interaction strength (a parameter used in theoretical models) and species impact (empirical measures of total species effect). I then use behavioral observations and population censuses in a rocky intertidal community to estimate both per capita interaction strengths and species impacts on invertebrate prey of Glaucous‐winged Gulls (Larus glaucescens), American Black Oystercatchers (Haematopus bachmani), and Northwestern Crows (Corvus caurinus). Estimated per capita interaction strengths exhibited a skewed distribution with many weak interactions and few strong interactions: mean ± 1 sd of log10(interaction strength) = −1.95 ± 1.40 (bird‐day/m of shore)−1. Per capita interaction strength correlated poorly (r2 = 0.152–0.157) and nonlinearly with both consumption rates and percentage contribution of a prey species to the diet. Using my observational estimates of per capita interaction strengths, I predicted the species impact of bird predation on different prey taxa. Predictions included strong effects of birds on goose barnacles (Pollicipes polymerus), limpets (Lottia and Tectura spp.), sea urchins (Strongylocentrotus spp.), and large starfish (Pycnopodia helianthoides and Solaster stimpsoni), but little effect on mussels (Mytilus californianus and M. trossulus), dogwhelk snails (Nucella spp.), and acorn barnacles (Semibalanus cariosus). I compared nine of the predictions with 126 results of experimental manipulations of birds. The predictions agreed both qualitatively and quantitatively with the experimental results. These findings suggest that observational measures of interaction strength that have units matching those of dynamical food web models may be reasonable to use in estimating those found in natural communities.

Wootton, J. Timothy

doi: 10.1890/0012-9615%281997%29067%5B0045:EATOPC%5D2.0.CO%3B2pmid: N/A

Predicting the dynamics of natural food webs requires estimates of the strength of interactions among species. The ability to estimate per capita interaction strength from observational data is desirable because of the logistical difficulty of using experimental manipulations to obtain such measures for all species within complex natural communities. In this paper, I derive observational measures of per capita interaction strength having units matching those of dynamic food web models (per capita consumption and assimilation rates). I also highlight the difference between per capita interaction strength (a parameter used in theoretical models) and species impact (empirical measures of total species effect). I then use behavioral observations and population censuses in a rocky intertidal community to estimate both per capita interaction strengths and species impacts on invertebrate prey of Glaucous-winged Gulls ( Larus glaucescens ), American Black Oystercatchers ( Haematopus bachmani ), and Northwestern Crows ( Corvus caurinus ). Estimated per capita interaction strengths exhibited a skewed distribution with many weak interactions and few strong interactions: mean ±± 1 sd of log 10 (interaction strength) == −−1.95 ±± 1.40 (bird-day//m of shore) −−1 . Per capita interaction strength correlated poorly ( r 2 == 0.152––0.157) and nonlinearly with both consumption rates and percentage contribution of a prey species to the diet. Using my observational estimates of per capita interaction strengths, I predicted the species impact of bird predation on different prey taxa. Predictions included strong effects of birds on goose barnacles ( Pollicipes polymerus ), limpets ( Lottia and Tectura spp.), sea urchins ( Strongylocentrotus spp.), and large starfish ( Pycnopodia helianthoides and Solaster stimpsoni ), but little effect on mussels ( Mytilus californianus and M. trossulus ), dogwhelk snails ( Nucella spp.), and acorn barnacles ( Semibalanus cariosus ). I compared nine of the predictions with 126 results of experimental manipulations of birds. The predictions agreed both qualitatively and quantitatively with the experimental results. These findings suggest that observational measures of interaction strength that have units matching those of dynamical food web models may be reasonable to use in estimating those found in natural communities.

Reusch, Thorsten B. H.; Chapman, Anthony R. O.

doi: 10.1890/0012-9615(1997)067[0065:PASOBS]2.0.CO;2pmid: N/A

At a subtidal, soft‐bottom site in the western Baltic Sea, mussel (Mytilus edulis) patches co‐occur with high predator abundances. Sea star (Asterias rubens) biomasses, in particular, exceed reported values considered sufficient for restricting mussels to the intertidal zone. To determine how mussels can persist in the face of intense predation, we decomposed patch space occupancy into the relative contributions of newly arriving individuals (recruitment) and of increases in body size of the individuals already present in the patch over 13 mo. Sea stars, as major predators, were only able to control 77% of the potential per capita recruitment rate of 91 individuals/yr in 2 m depth. The remaining recruitment rate of 21 individuals/yr was sufficient to allow patches to occupy 1.6 times more space per year. Transplantation of patches to 6 m depth, where recruitment is negligible, revealed that sea stars were also ineffective in controlling mussel coverage through consumption of larger mussels (>1 yr, >30 mm shell length). In deeper water, space occupancy of patches through increases in mussel body size was able to balance predation mortality, demonstrating that mussels attained a relative refuge in size at only 33 mm shell length. Based on the measured shell growth rates, mussels attain this size after ≈15 mo. In situ observations of Asterias feeding activity, the ratios between necessary predator sizes to attack prey of a given size, and predator size distributions suggest that sea stars were on average too small to feed effectively on adult (>1 yr) mussels. Probably, Asterias cannot respond to abundant prey and increase its maximal body size at the site because salinities are at its lower tolerable limit (12–18 g/kg). Thus, bottom‐up factors such as high prey productivity in concert with subtle size‐based ineffectiveness of the predator population allow otherwise unstable predator–prey populations of a generalist predator and its preferred prey to coexist. Although mussel predators were unable to decimate mussels to local extinction, a release of experimental mussel patches from predation with strong recruitment (2 m depth) resulted in an approximately sevenfold yearly areal increase in shallow treatments, which would lead to a 100% mussel cover at the site within 1 yr.

Reusch, Thorsten B. H.; Chapman, Anthony R. O.

doi: 10.1890/0012-9615%281997%29067%5B0065:PASOBS%5D2.0.CO%3B2pmid: N/A

At a subtidal, soft-bottom site in the western Baltic Sea, mussel ( Mytilus edulis ) patches co-occur with high predator abundances. Sea star ( Asterias rubens ) biomasses, in particular, exceed reported values considered sufficient for restricting mussels to the intertidal zone. To determine how mussels can persist in the face of intense predation, we decomposed patch space occupancy into the relative contributions of newly arriving individuals (recruitment) and of increases in body size of the individuals already present in the patch over 13 mo. Sea stars, as major predators, were only able to control 77%% of the potential per capita recruitment rate of 91 individuals//yr in 2 m depth. The remaining recruitment rate of 21 individuals//yr was sufficient to allow patches to occupy 1.6 times more space per year. Transplantation of patches to 6 m depth, where recruitment is negligible, revealed that sea stars were also ineffective in controlling mussel coverage through consumption of larger mussels (>1 yr, >30 mm shell length). In deeper water, space occupancy of patches through increases in mussel body size was able to balance predation mortality, demonstrating that mussels attained a relative refuge in size at only 33 mm shell length. Based on the measured shell growth rates, mussels attain this size after ≈≈15 mo. In situ observations of Asterias feeding activity, the ratios between necessary predator sizes to attack prey of a given size, and predator size distributions suggest that sea stars were on average too small to feed effectively on adult (>1 yr) mussels. Probably, Asterias cannot respond to abundant prey and increase its maximal body size at the site because salinities are at its lower tolerable limit (12––18 g//kg). Thus, bottom-up factors such as high prey productivity in concert with subtle size-based ineffectiveness of the predator population allow otherwise unstable predator––prey populations of a generalist predator and its preferred prey to coexist. Although mussel predators were unable to decimate mussels to local extinction, a release of experimental mussel patches from predation with strong recruitment (2 m depth) resulted in an approximately sevenfold yearly areal increase in shallow treatments, which would lead to a 100%% mussel cover at the site within 1 yr. Given that mussels can dominate both rocky substratum and soft sediment, we also studied the effect of substratum quality in factorial combination with presence/absence of predation and water depth on mussel abundance. Attachment to stable substratum did not affect recruitment to the patches or patch space occupancy, but it completely prevented patch dislodgment and subsequent drift. In contrast to rocky shores, mussel patch dislodgment may represent the major mode of patch dispersal and new patch formation in soft-bottom environments as demonstrated by a drift collector fence.

Reid, Donald G.; Krebs, Charles J.; Kenney, Alice J.

doi: 10.1890/0012-9615%281997%29067%5B0089:POPONL%5D2.0.CO%3B2pmid: N/A

Noncyclic populations of microtine rodents may be limited within a relatively constant range of densities by generalist predators with a prey base sufficiently diverse to sustain them when rodents are scarce (generalist predator hypothesis). Collared lemmings ( Dicrostonyx kilangmiutak ) at Pearce Point, Northwest Territories, Canada, are noncyclic and limited to fairly constant low densities in summer by predation, principally by red fox and Rough-legged Hawks. We tested four predictions of the generalist predator hypothesis as a possible explanation for relatively constant lemming densities: (1) predators do not show strong numerical responses to lemming density; (2) the proportion of lemming biomass in predator diets declines with declining lemming abundance, compensated for by increased consumption of alternative prey; (3) predators show a type-III functional response to lemming density; and (4) at low densities, predation on lemmings ceases. The first prediction was not satisfied by the principal predators: at very low lemming densities, Rough-legged Hawks did not settle, and breeding success of red foxes and hawks was limited by lemming abundance. However, a number of generalist predators (Golden Eagle, grizzly bear, arctic ground squirrel, Peregrine Falcon, and Gyrfalcon) did not respond numerically to the lowest lemming densities. The second prediction was partly supported: all predators consumed lemmings at a lower rate as lemming densities declined. However, Rough-legged Hawks were not able to compensate fully for the declining consumption by increasing their use of alternative prey, and red foxes were able to do so in one of three years. Regarding the third prediction, foxes showed some evidence of a type-III functional response but hawks did not. As for the fourth prediction, most predators still consumed lemmings at very low densities; lemmings lacked a secure refuge. The Pearce Point system differs from those where microtine dynamics are relatively constant and nonirruptive because of persistent predation by generalists. In terms of biomass, lemmings are the principal prey for their dominant predators. These predators (the semigeneralist red fox and the specialist Rough-legged Hawk) rely on lemmings to breed, but drive them to densities too low to sustain breeding by these same predators in the subsequent spring. In this regard, the system is similar to one driven by specialists. In some winters, however, populations recover because lemmings breed under the snow and most summer predators are absent. As a result, lemming densities in spring are often high enough for specialists and semigeneralists to initiate breeding. When winter breeding and survival fail to allow population growth, hawks and foxes may fail to breed and then leave the system. Even so, summer generalists still persist and continue to consume lemmings, curtailing potential irruptive growth. In this regard, the system is similar to one where prey are relatively constant because of generalists. Community dynamics at Pearce Point can best be understood as a combination of three dominant processes. Summer predation by specialists and semigeneralists results in destabilizing declines. Winter breeding, coupled with good survival, can lead to destabilizing growth. However, this growth is curtailed in the following summer by either destabilizing specialist predation or the stabilizing influence of generalist predation. When lemmings are scarce, the semigeneralist red fox and some generalist predators rely on arctic ground squirrels as their primary prey or their principal alternative prey. The ground squirrel appears to be the critical species maintaining this relatively diverse arctic tundra predator community and the relatively constant lemming densities.

Reid, Donald G.; Krebs, Charles J.; Kenney, Alice J.

doi: 10.1890/0012-9615(1997)067[0089:POPONL]2.0.CO;2pmid: N/A

Noncyclic populations of microtine rodents may be limited within a relatively constant range of densities by generalist predators with a prey base sufficiently diverse to sustain them when rodents are scarce (generalist predator hypothesis). Collared lemmings (Dicrostonyx kilangmiutak) at Pearce Point, Northwest Territories, Canada, are noncyclic and limited to fairly constant low densities in summer by predation, principally by red fox and Rough‐legged Hawks. We tested four predictions of the generalist predator hypothesis as a possible explanation for relatively constant lemming densities: (1) predators do not show strong numerical responses to lemming density; (2) the proportion of lemming biomass in predator diets declines with declining lemming abundance, compensated for by increased consumption of alternative prey; (3) predators show a type‐III functional response to lemming density; and (4) at low densities, predation on lemmings ceases.