Basal Trophic Levels Research Articles

Effects of consumers and nitrogen availability on heterotrophic microbial activity during leaf decomposition in headwater streams

Summary Microbial processes in streams may be influenced by bottom‐up factors, such as nutrient availability, and by top‐down factors, such as the activity of upper trophic levels. The latter pathway has been well described between autotrophs and grazers, but the effects of consumers on heterotrophic basal trophic levels may also be important. Consumers may influence microbial activity through consumption of microbial biomass and by altering nutrient availability via consumer nutrient recycling (CNR). We investigated the effects of two shredder taxa, Gammarus spp. and Tipula spp., on nitrogen (N) uptake during organic matter decomposition by leaf‐associated heterotrophic microbial assemblages under conditions of ambient and enriched N availability in stream mesocosms. Colonised red maple leaves were placed in mesocosms with low (<20 μg L−1) or high (>800 μg L−1) nitrate concentration in the presence of amphipods, tipulids or no shredders. To test the importance of consumption and CNR, leaf packs were constructed from both coarse and fine mesh, which respectively allowed or prevented shredders from consuming detritus. Nitrogen enrichment generally increased red maple breakdown rates, while shredder influence on leaf breakdown was taxon specific. Feeding by tipulids increased red maple breakdown rate compared with amphipods, although not significantly. While feeding by amphipods did not directly influence breakdown rates, amphipod nutrient recycling stimulated leaf breakdown rate by the same magnitude as the N‐enrichment treatment. Fungal biomass increased with N enrichment. However, this did not lead to changes in N uptake, which was extremely variable and did not respond significantly to either treatment. In general, N uptake (per leaf mass) was greatest under enriched N conditions when tipulids were present. Our results indicate that both top‐down and bottom‐up factors influence microbial activity in heterotrophic systems and suggest that consumers may play important roles in detrital processing, not only through direct consumption but also through nutrient recycling.

Behavioural and physiological responses of limpet prey to a seastar predator and their transmission to basal trophic levels

Besides the well-documented behavioural changes induced by predators on prey, predator-induced stress can also include a suite of biochemical, neurological and metabolic changes that may represent important energetic costs and have long-lasting effects on individuals and on the demography of prey populations. The rapid transmission of prey behavioural changes to lower trophic levels, usually associated with alteration of feeding rates, can substantially change and even reverse direction over the long term as prey cope with the energetic costs associated with predation-induced stress. It is therefore critical to evaluate different aspects and assess the costs of non-consumptive predator effects on prey. We investigated the behavioural and physiological responses of an herbivorous limpet, Fissurella limbata, to the presence of chemical cues and direct non-lethal contact by the common seastar predator, Heliaster helianthus. We also evaluated whether the limpets feeding behaviour was modified by the predator and whether this translated into positive or negative effects on biomass of the green alga, Ulva sp. Our experimental results show the presence of Heliaster led to increased movement activity, increased distances travelled, changes in time budget over different environmental conditions and increased feeding rate in the keyhole limpets. Moreover, additional experiments showed that, beyond the increased metabolic rate associated with limpet increased activity, predator chemical cues heighten metabolic rate as part of the induced stress response. Changes in individual movement and displacement distances observed through the 9-day experiment can be interpreted as part of the escape response exhibited by limpets to reduce the risk of being captured by the predator. Increased limpet feeding rate on algae can be visualized as a way individuals compensate for the elevated energetic costs of movement and heightened metabolic rates produced by the predator-induced stress, which can lead to negative effects on abundance of the lower trophic level. We suggest that in order to understand the total non-consumptive effect of predators in natural communities, it is necessary to evaluate not only short-term behavioural responses, but also the costs associated with the multiple interdependent pathways triggered by predator-induced stress, and determine how individuals cope with these costs in the long term.

Current velocity influences the facilitation and removal of algae by stream grazers

The modification of flows in lotic ecosystems can have dramatic effects on abiotic and biotic processes and change the structure of basal trophic levels. In high-gradient streams, most of the biota are benthic, and decreased flow may homogenize and reduce benthic current velocity, potentially changing stream ecosystem function. Grazing by macroinvertebrates is an important component of stream function because grazers regulate energy flow from primary producers to higher trophic levels. We conducted an experiment to examine how macroinvertebrate grazers facilitated or removed algal biomass across a gradient of benthic current velocity (0–40 cm s−1). We chose three grazers (Drunella coloradensis, Cinygmula spp., and Epeorus deceptivus) from a montane stream and conducted our experiment using 24 artificial stream channels that had three treatments: no grazers (control), single-grazer, and combined-grazer treatments. In the absence of grazers, algal biomass increased with benthic current velocity. Grazer treatments differed from the control in that more algal biomass was removed at higher velocities, whereas algal accrual was largely facilitated at low velocities. The transition from facilitation to removal ranged from 4.5 to 5.9 cm s−1 for individual grazer treatments and occurred at 11.7 cm s−1 for the combined-grazer treatment. Our data suggest that velocity plays a significant role in the facilitation and removal of algae by macroinvertebrate grazers. Additionally, the patterns revealed here could have general implications for algal accrual in systems where flow is reduced.

Terrestrial organic matter and light penetration: Effects on bacterial and primary production in lakes

We investigated productivity at the basal trophic level in 15 unproductive lakes in a gradient ranging from clear‐water to brown‐water (humic) lakes in northern Sweden. Primary production and bacterial production in benthic and pelagic habitats were measured to estimate the variation in energy mobilization from external energy sources (primary production plus bacterial production on allochthonous organic carbon) along the gradient. Clear‐water lakes were dominated by autotrophic energy mobilization in the benthic habitat, whereas humic lakes were dominated by heterotrophic energy mobilization in the pelagic habitat. Whole‐lake (benthic + pelagic) energy mobilization was negatively correlated to the light‐extinction coefficient, which was determined by colored terrestrial organic matter in the lake water. Thus, variation in the concentration of terrestrial organic matter and its light‐absorbing characteristics exerts strong control on the magnitude, as well as on the processes and pathways, of energy mobilization in unproductive lakes. We suggest that unproductive lakes in general are sensitive to input of terrestrial organic matter because of its effects on basal energy mobilization in both benthic and pelagic habitats.

Intermediate‐consumer identity and resources alter a food web with omnivory

1. Omnivory is an important interaction that has been the centre of numerous theoretical and empirical studies in recent years. Most of these studies examine the conditions necessary for coexistence between an omnivore and an intermediate consumer. Trait variation in ecological interactions (competition and predator tolerance) among intermediate consumers has not been considered in previous empirical studies despite the evidence that variation in species-specific traits can have important community-level effects. 2. I conducted a multifactorial microcosm experiment using species from the Sarracenia purpurea phytotelmata community, organisms that inhabit the water collected within its modified leaves. The basal trophic level consisted of bacterial decomposers, the second trophic level (intermediate consumers) consisted of protozoa and rotifers, and the third trophic level (omnivore) were larvae of the pitcher plant mosquito Wyeomyia smithii. Trophic level number (1, 2 and 3), resources (low and high), omnivore density (low and high) and intermediate consumer (monoculture of five protozoa and rotifers) identity were manipulated. Abundance of the basal trophic level, intermediate consumers, and growth of the omnivore were measured, as well as time to extinction (intermediate consumers) and time to pupation (mosquito larvae). 3. The presence of different intermediate consumers affected both bacteria abundance and omnivore growth. At high resource levels, Poteriochromonas, Colpidium and Habrotrocha rosa reduced bacteria densities greater than omnivore reduction of bacteria. Mosquito larvae did not pupate at low resource levels except when Poteriochromonas and Colopoda were present as intermediate consumers. Communities with H. rosa were the only ones consistent with the prediction that omnivores should exclude intermediate consumers at high resources. 4. These results had mixed support for predictions from omnivory food web theory. Intermediate consumers responded and affected this community differently under different community structures and resource levels. Consequently, variation in species-specific traits can have important population- and community-level effects and needs to be considered in food webs with omnivory.

ATTENUATION OF TOP-DOWN AND BOTTOM-UP FORCES IN A COMPLEX TERRESTRIAL COMMUNITY

Carnivore (top-down) and resource (bottom-up) influences in food webs are strong and pervasive, but few studies have investigated their interactive effects in species-rich terrestrial ecosystems. This study focused on arthropods associated with the dominant tree species, Metrosideros polymorpha (Myrtaceae), in Hawaiian forests. Severe soil nutrient limitation on a 120-yr-old lava flow was removed by fertilization and combined with bird predator exclusion cages in a randomized block design. Arthropod densities were measured from clipped foliage at the outset and conclusion of a 33-mo experiment, and their biomass was estimated using regression equations. Metrosideros foliar nitrogen, tree growth, and biomass increased directly in response to fertilization but did not change with bird exclusion. Fertilization increased detritivore densities but not biomass, and both density and biomass of herbivores, while bird exclusion increased both density and biomass of carnivores. Fertilization also increased spider density and biomass, but bird exclusion increased spider numbers (15 species) only in high resource plots. Overall, trophic level biomass responses were less pronounced than density because smaller bodied individuals responded more to enrichment. Bottom-up factors controlled basal trophic levels, and detritivores comprised the largest fraction of arthropod density and biomass. Conversely, top-down impacts were apparent but variable, limited to higher order consumers, and did not cascade to the level of primary producers. These experimental results were consistent with the view that complex forest ecosystems are structured on a bottom-up template.

Simple rules yield complex food webs.

Several of the most ambitious theories in ecology describe food webs that document the structure of strong and weak trophic links that is responsible for ecological dynamics among diverse assemblages of species. Early mechanism-based theory asserted that food webs have little omnivory and several properties that are independent of species richness. This theory was overturned by empirical studies that found food webs to be much more complex, but these studies did not provide mechanistic explanations for the complexity. Here we show that a remarkably simple model fills this scientific void by successfully predicting key structural properties of the most complex and comprehensive food webs in the primary literature. These properties include the fractions of species at top, intermediate and basal trophic levels, the means and variabilities of generality, vulnerability and food-chain length, and the degrees of cannibalism, omnivory, looping and trophic similarity. Using only two empirical parameters, species number and connectance, our 'niche model' extends the existing 'cascade model and improves its fit ten-fold by constraining species to consume a contiguous sequence of prey in a one-dimensional trophic niche.

Independent and Interactive Effects of Crayfish and Darters on a Stream Benthic Community

A growing body of research suggests that direct and indirect interactions among consumers can have strong effects on the abundance of their prey. We examined the independent and interactive effects of 2 common freshwater consumers, crayfish (Orconectes propinquus) and rainbow darters (Etheostoma caeruleum), on benthic macroinvertebrates in a southern Michigan, USA, stream. A secondary goal was to assess the effects of crayfish and darters on the basal trophic level (periphyton) in the system. Presence and absence of crayfish and darters were manipulated in situ in wire mesh cages (0.2 m², 6-mm mesh) using a 2 × 2 factorial, randomized block design. Invertebrates and periphyton (for chlorophyll a analysis) were sampled biweekly for 6 wk from artificial rock substrates within these cages. Neither consumer significantly affected chlorophyll a accumulation. Darters alone had no significant effects on total invertebrate abundance. Crayfish alone caused, on average, a 58% reduction in total invertebrate abundance relative to exclosures, and most common invertebrate taxa were negatively affected. In the presence of darters, however, crayfish reduced invertebrate numbers by only 29% relative to exclosures. Daters apparently modified the ability of crayfish to suppress invertebrate abundance, although the mechanism is unclear. Evaluations of community structure based on manipulations of individual consumer species may be misleading when interaction modifications such as these occur.

Scale and Food-Web Structure: From Local to Global

A central question in food-web research focuses on the relation between species at different trophic levels and «scale» or, in more specific terms, the number of species in food webs. We show that recently advanced scale-dependence hypotheses are consistent with independent data and assumptions connecting properties of the smallest food webs with those of the largest. Consistent with scale dependence, these data and assumptions suggest that mean chain length and the fractions of species at top, intermediate, and basal trophic levels depend on the number of species within «local» (i.e., small-scaled) webs with 2 to 10 2 species. Our analysis also suggests that the fractions of species in different trophic levels become asymptotically stable for regionally to globally scaled food webs with 10 4 to 10 7 species, respectively. A new hypothesis for the behavior of mean chain length in regional to global webs is also advanced. Our analyses specify for the first time specific functional forms for the effects of scale on food-web structure that take into account recently demonstrated scale dependencies at the local scale as well as estimates of biodiversity at regional and global scales. Rates of change over specific ranges are described as well as numerical estimates of asymptotes