Spatial dynamics of keystone predation

1. I investigated the effects of dispersal on communities of keystone predators and prey. I obtained two key results. 2. First, a strong trade-off between competitive ability and predator susceptibility allows consumer coexistence over a large resource productivity range, but it also lowers the predator-susceptible superior competitor's abundance and increases its risk of extinction. Thus, unexpectedly, dispersal plays a more important role in coexistence when predator-mediated coexistence is strong rather than weak. The interplay between the trade-off, small population sizes resulting from transient oscillations, and dispersal leads to qualitatively different species distributions depending on the relative mobilities of the consumers and predator. These differences yield comparative predictions that can be tested with data on trade-off strength, dispersal rates, and species distributions across productivity gradients. 3. Second, there is an asymmetry between species in their dispersal effects: the predator-resistant inferior competitor's dispersal has a large effect, but the predator-susceptible superior competitor's dispersal has no effect, on coexistence and species' distributions. The inferior competitor's dispersal also mediates the predator's dispersal effects: the predator's dispersal has no effect when the inferior competitor is immobile, and a large effect when it is mobile. The net outcome of the direct and indirect effects of the inferior competitor's dispersal is a qualitative change in the species' distributions from interspecific segregation to interspecific aggregation. 4. The important point is that differences between species in how they balance resource acquisition and predator avoidance can lead to unexpected differences in their dispersal effects. While consumer coexistence in the absence of dispersal is driven largely by the top predator, consumer coexistence in the presence of dispersal is driven largely by the predator-resistant inferior competitor.     

Scaling from optimal behavior to population dynamics and ecosystem function

While behavioral responses of individual organisms can be predicted with optimal foraging theory, the theory of how individual behavior feeds back to population and ecosystem dynamics has not been fully explored. Ecological models of trophic interactions incorporating behavior of entire populations commonly assume either that populations act as one when making decisions, that behavior is slowly varying or that non-linear effects are negligible in behavioral choices at the population scale. Here, we scale from individual optimal behavior to ecosystem structure in a classic tri-trophic chain where both prey and predators adapt their behavior in response to food availability and predation risk. Behavior is modeled as playing the field, with both consumers and predators behaving optimally at every instant basing their choices on the average population behavior. We establish uniqueness of the Nash equilibrium, and find it numerically. By modeling the interactions as playing the field, we can perform instantaneous optimization at the individual level while taking the entire population into account. We find that optimal behavior essentially removes the effect of top-down forcing at the population level, while drastically changing the behavior. Bottom-up forcing is found to increase populations at all trophic levels. These phenomena both appear to be driven by an emerging constant consumption rate, corresponding to a partial satiation. In addition, we find that a Type III functional response arises from a Type II response for both predators and consumers when their behavior follows the Nash equilibrium, showing that this is a general phenomenon. Our approach is general and computationally efficient and can be used to account for behavior in population dynamics with fast behavioral responses.     

Markov population processes as models of primate social and population dynamics

Thanks to recently developed theory of Markov population processes, models of how an individual primate migrates from one casual social group to another or from one breeding troop to another can now deal exactly with transition rates which depend nonlinearly on the sizes of both the group (or troop) left and the group (or troop) entered. Examples of such models presented here are consistent with existing observations of primate social and population dynamics and are more plausible as explanations of these data than previous linear models.     

Microbial activity and the dynamics of ecosystem processes in forest soils

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Mortality dynamics and population regulation in Bemisia tabaci

Natural mortality is an important determinant of the population dynamics of a species, and an understanding of mortality forces should aid in the development of better management strategies for insect pests. An in situ, observational method was used to construct cohort‐based life tables for Bemisia tabaci (Gennadius) Biotype B (Homoptera: Aleyrodidae) over 14 generations on cotton in central Arizona, USA, from 1997 to 1999. In descending order, median marginal rates of mortality were highest for predation, dislodgment, unknown causes, egg inviability, and parasitism. The highest mortality occurred during the 4th nymphal stadium, and the median rate of immature survival over 14 generations was 6.6%. Predation during the 4th nymphal stadium was the primary key factor. Irreplaceable mortality was highest for predation and dislodgment, with the absence of these mortality factors leading to the greatest increases in estimated net reproduction. There was little evidence of direct or delayed density‐dependence for any mortality factor. Wind, rainfall, and predator densities were associated with dislodgment, and rates of predation were related to densities of Geocoris spp., Orius tristicolor (White), Chrysoperla carnea s.l. Stephens, and Lygus hesperus Knight. Simulations suggest that immigration and emigration play important roles in site‐specific dynamics by explaining departures from observed population trajectories based solely on endogenous reproduction and mortality. By a direct measurement of these mortality factors and indirect evidence of adult movement, we conclude that efficient pest management may be best accomplished by fostering greater mortality during the 4th stadium, largely through a conservation of predators and by managing immigrating adult populations at their sources.     

Concordant population dynamics of Lepidoptera herbivores in a forest ecosystem

It is frequently assumed that population fluctuations are largely independent within a community of trophically‐similar species, but this need not be so. If population fluctuations are partly synchronized or concordant, this will produce interannual variability in the community's aggregate abundance and generate temporal variance in ecosystem structure. We studied the community of Lepidoptera inhabiting northern hardwood forests in New Hampshire, USA, to evaluate the hypothesis that fluctuations in consumer communities can arise from concordant dynamics of constituent populations. Interannual comparisons of moth abundances for >75 species sampled at three sites over four years revealed that concordant dynamics contribute strongly to interannual variability in the abundance of consumers. A conspicuous decline in community abundance from 2004 to 2005 was the result of predominantly negative population growth rates of the component species, while an increase in community abundance from 2006 to 2007 was the result of predominantly positive population growth rates. Population dynamics most strongly linked species that feed in the early season (perhaps due to shared responses to climatic effects), but not species that might share natural enemies or host plants. The observed concordant dynamics introduced conspicuous temporal variation in the abundance of primary consumers relative to plants and secondary consumers, thereby altering the forest's trophic structure. Such variance in the aggregate abundance of forest primary consumers could generate time‐lagged fluctuations in abundances of secondary consumers and will generally have important consequences for ecosystem properties and processes that are nonlinear functions of consumer abundance, such as plant community structure and nutrient cycling.     

Transient population dynamics impede restoration and may promote ecosystem transformation after disturbance

The apparent failure of ecosystems to recover from increasingly widespread disturbance is a global concern. Despite growing focus on factors inhibiting resilience and restoration, we still know very little about how demographic and population processes influence recovery. Using inverse and forward demographic modelling of 531 post‐fire sagebrush populations across the western US, we show that demographic processes during recovery from seeds do not initially lead to population growth but rather to years of population decline, low density, and risk of extirpation after disturbance and restoration, even at sites with potential to support long‐term, stable populations. Changes in population structure, and resulting transient population dynamics, lead to a > 50% decline in population growth rate after disturbance and significant reductions in population density. Our results indicate that demographic processes influence the recovery of ecosystems from disturbance and that demographic analyses can be used by resource managers to anticipate ecological transformation risk.     

Trophic trickles and cascades in a complex food web: impacts of a keystone predator on stream community structure and ecosystem processes

Chalk streams are among the most species-rich and productive of all temperate ecosystems. Despite this, a few keystone species have the potential to exert disproportionately powerful effects on community structure and ecosystem processes. Two of these are the bullhead Cottus gobio , a small benthic fish that is an extremely abundant, voracious predator, and the freshwater shrimp Gammarus pulex , which dominates the prey assemblage and is the principal detritivore. Field experiments detected a bullhead– Gammarus –detritus trophic cascade, with detrital processing rates slowed dramatically in the presence of the predator. In addition, survey data also revealed strong negative density-dependence between bullhead and brown trout, adding a further link in the cascade. However, although bullhead also depressed the abundance of a dominant grazer, the snail Potamopyrgus antipodarum , there was no cascading effect upon algal production, suggesting that autochthonous inputs were not controlled by top–down effects. This skewed effect of the predator upon autochthonous versus allochthonous basal resources stresses the need to consider both pathways of energy flux into the food web, whereas many previous studies have potentially overemphasized the importance of predator–herbivore–primary producer cascades. The wider community food web contained 142 species and 1383 feeding links. This complex network exhibited "small world" properties, such as high clustering (unlike many other food webs) and shortest path lengths between species were small (in common with many other food webs). In particular, each of the four members of the detrital cascade could be connected to any other species by three links or fewer. Our data revealed that powerful cascading effects can be imbedded within even very complex ecological networks.     

Population dynamics of a tree-dwelling aphid: regulation and density-independent processes

1. A population of the Turkey-oak aphid ( Myzocallis boerneri Stroyan) was sampled at approximately weekly intervals on two Turkey-oak trees for 19 years.
2. On one tree (A), the aphids exhibited a distinct seasonal pattern with a spring increase, summer decrease, early autumn increase, and late autumn decline. On the other tree (B) the aphids remained at low densities after the decrease in summer.
3. On tree A, significant undercompensating density dependence occurred during all periods of the seasonal population development, and their strength varied little during the course of the season. On tree B, significant density dependence compensated exactly for increase, but appeared only after the decrease in summer when the population remained at very low densities for the rest of the season.
4. Density-independent weather variables affected the population dynamics very little. Their influence was marginally significant only at very low densities when the aphids were regulated exactly by compensating density-dependent factors.
5. The results suggest a curvilinear density dependence, with strong regulation at low densities, and weak at high densities. That is, this aphid was most regulated not at the peak but at the trough densities.     

Cell population dynamics modulate the rates of tissue growth processes

The development and testing of a discrete model describing the dynamic process of tissue growth in three-dimensional scaffolds is presented. The model considers populations of cells that execute persistent random walks on the computational grid, collide, and proliferate until they reach confluence. To isolate the effect of population dynamics on tissue growth, the model assumes that nutrient and growth factor concentrations remain constant in space and time. Simulations start either by distributing the seed cells uniformly and randomly throughout the scaffold, or from an initial condition designed to simulate the migration and cell proliferation phase of wound healing. Simulations with uniform seeding show that cell migration enhances tissue growth by counterbalancing the adverse effects of contact inhibition. This beneficial effect, however, diminishes and disappears completely for large migration speeds. By contrast, simulations with the "wound" seeding mode show a continual enhancement of tissue regeneration rates with increasing cell migration speeds. We conclude that cell locomotory parameters and the spatial distribution of seed cells can have profound effects on the dynamics of the process and, consequently, on the pattern and rates of tissue growth. These results can guide the design of experiments for testing the effectiveness of biomimetic modifications for stimulating tissue growth.     

城市化过程中北京市人口时空演变对生态系统质量的影响

应用第五、第六次人口普查和MODIS-NDVI遥感数据,从城市、区/县和街道/乡镇3个尺度,分析了2000—2010年北京市人口时空演变特征及其对区域生态系统质量的影响.结果表明: 研究期间,北京市总人口增长了43.9%,人口空间分布形成了明显的圈层特征,呈现向近郊转移聚集的趋势;北京市的年最大归一化植被指数(NDVI)呈增加趋势,在5～6环之间却显著降低.城区和远郊区的NDVI变化趋势与人口密度变化率呈显著负相关.城市化带来的人口数量增加不仅没有导致北京整体生态系统质量下降,反而通过城市功能调整和农民进城务工人口转移等方式,提高了城区和远郊区的生态系统质量.不同的功能定位和发展机制是影响北京市人口时空格局变化及其生态影响的主要原因.     

Fishing,reproductive volume and regulation: population dynamics and exploitation of the eastern Baltic cod

The relative importance of exploitation rate and environmental variability in generating fluctuations of harvested populations is a key issue in academic ecology as well as population management. We studied how the eastern Baltic cod (Gadus morhua) is affected by fishing and environmental variation by using a newly developed single species state-space model. Survey data and auxiliary environmental data were used to estimate the model parameters. The model was then used to predict future development of the eastern Baltic cod under different fishing mortalities and abiotic conditions. Abiotic condition was represented by an index: reproductive volume which is the volume of water suitable (in terms of salinity and oxygen content) for the successful development of the early life stages of Baltic cod. The model included direct density dependence, fishing, and a lagged effect of reproductive volume. Our analysis showed that fishing rate is approximately three times more important than reproductive volume in explaining the population dynamics. Furthermore, our model suggests either under- or over-compensatory dynamics depending on the reproductive volume and long term catch levels. It follows that fishing can either reduce or increase temporal oscillations of the cod stock depending on whether the dynamics is over- or undercompensatory, respectively. The sustainable level of fishing rate is however dependent on reproductive volume. Our model predicts a dual role of fishing rate, stabilizing when reproductive volume is high and destabilizing when it is low. Exploitation rate may therefore increase or decrease the risk of the population of cod dropping below a given biomass reference point depending on the environmental conditions, which has practical implications for fisheries management.     

Information,behaviour and population dynamics

Animals must make decisions such as where and when to forage based on imperfect information about their variable world. A growing number of authors have addressed the information problem and its relation to behaviour, learning, cognition and evolution. Few studies, however, have analyzed how the quality of information available for decision making affects population growth and dynamics. Here I utilize stochastic dynamic models, statistical decision theory, and projection matrices to show how information affects patch choice and how these choices in turn affect fitness and population growth. The value of information, in terms of its impact on ?tness, depends on an organism's state and changes during the organism's life span. The relationship between a population's rate of increase and uncertainty about optimal patch choice amongst its members is concave with the positive effects of reduced uncertainty diminishing rapidly as individuals become more knowledgeable about their environment. The implications of these results for understanding populations and communities of interacting species are discussed.     

The New River, Virginia, muskellunge fishery: population dynamics, harvest regulation modeling, and angler attitudes

Although muskellunge, Esox masquinongy, fisheries in northern US states and Canadian provinces are increasingly being managed by introduction of restrictive harvest regulations (e.g. 1370-mm (54′′) minimum length limits), many southern US muskellunge fisheries continue to be managed with comparatively liberal regulations (e.g. 762-mm (30′′) minimum length limits) that are implemented statewide. We studied the population dynamics of the New River, Virginia, muskellunge fishery and used predictive modeling to determine whether restrictive harvest regulations also might prove beneficial for this southern latitude fishery. A creel survey was also conducted to learn more about angler attitudes to the New River muskellunge fishery. Muskellunge grew quickly, with fish reaching harvestable lengths (762 mm, 30′′) in 2–3 years. Muskellunge fishing pressure, harvest rates, and voluntary release rates were low compared with reports for more northern areas. Most anglers, irrespective of how often they fished for muskellunge, defined “trophy” muskellunge to be approximately 1050–1100 mm (41–43′′) in length. Although angler support for restrictive harvest regulations was low, abundance of memorable-length (≥1070 mm, 42′′) muskellunge was predicted to increase under all evaluated length limits. Muskellunge yield would remain static at 914-mm (36′′) and 1016-mm (40′′) length limits, because of the rapid growth of fish, but yield would decline dramatically with a 1143-mm (45′′) length limit, because male muskellunge rarely exceeded 1100 mm (43′′). Because of rapid growth and low release rates, implementation of higher length limits (e.g. 965–1067 mm, 38–42′′) may indeed prove beneficial for augmenting “trophy” muskellunge production on the New River. Angler support for higher minimum length limits might be increased by educating anglers about the rapid growth rates of muskellunge and the expected size structure changes that will result from a length-limit increase. Size structure changes resulting from an increase in the minimum length limit may be difficult to detect because of potential increases in fishing pressure or reduced fish growth as a result of competition for food resources. Long-term monitoring of muskellunge growth and angling pressure may therefore be needed to ensure that new regulations are indeed benefitting the fishery.     

Arbuscular mycorrhizae and terrestrial ecosystem processes

Arbuscular mycorrhizal fungi (AMF; phylum Glomeromycota) are ubiquitous in terrestrial ecosystems. Despite their acknowledged importance in ecology, most research on AMF has focused on effects on individual plant hosts, with more recent efforts aimed at the level of the plant community. Research at the ecosystem level is less prominent, but potentially very promising. Numerous human‐induced disturbances (including global change and agro‐ecosystem management) impinge on AMF functioning; hence study of this symbiosis from the ecosystem perspective seems timely and crucial. In this paper, I discuss four (interacting) routes via which AMF can influence ecosystem processes. These include indirect pathways (through changes in plant and soil microbial community composition), and direct pathways (effects on host physiology and resource capture, and direct mycelium effects). I use the case study of carbon cycling to illustrate the potentially pervasive influence of AMF on ecosystem processes. A limited amount of published research on AMF ecology is suited for direct integration into ecosystem studies (because of scale mismatch or ill‐adaptation to the ‘pools and flux’ paradigm of ecosystem ecology); I finish with an assessment of the tools (experimental designs, response variables) available for studying mycorrhizae at the ecosystem scale.     

Long-term changes in amphipod population dynamics in a temperate estuary following ecosystem restoration

The Mondego estuary (Portugal) has suffered severe ecological stress over the last two decades, as manifested in the replacement of seagrasses by opportunistic macroalgae, degradation of water quality and increased turbidity. A restoration plan was implemented in 1998, which aimed to reverse the eutrophication effects, and especially to restore the original natural seagrass (Zostera noltii) community. This article explores the long-term changes in Ampithoe valida and Melita palmata (Amphipoda) populations in response to eutrophication (with consequent seagrass loss and macroalgal proliferation) and to the subsequent restoration plan (with progressive seagrass recovery and macroalgal biomass decline). Until the early 1990s, high densities of A. valida and M. palmata were recorded in the Mondego estuary, especially during the occurrence of the macroalgal bloom and during all the periods in which green macroalgae were available. After the implementation of the restoration plan, species abundance, biomass and production levels decreased considerably due to the progressive decline of green macroalgae. This implied the virtual disappearance of the amphipod population, mainly A. valida. Distinct behaviours displayed by the two species could be related to different food strategies and habitat preferences. Ampithoe valida showed feeding preferences for ephemeral softer, filamentous or bladed algae (e.g. Ulva sp.) due to its high caloric content, using the Z. noltii bed only as a habitat for protection against predators or shelter from wave action. On the other hand, M. palmata did not suffer a strong decline in its population density, biomass and production, which may indicate that this species is probably not a primary consumer of green macroalgae and may readily shift to alternative ecological niches. Handling editor: P. Viaroli