A Hierarchical Model of Lotic Ecosystems

To understand the background of the strong variation and recent decline of stocks and production of mussels (Mytilus edulis) on tidal flats of the Wadden Sea, we analysed long-term (twice-annual for 26 years) and multi-station (15 sites) estimates of numbers, mean individual weights, biomass, and annual production on Balgzand, a 50-km2tidal-flat area in the westernmost part of the Wadden Sea (The Netherlands). Somatic production was estimated from summed growth increments of soft tissues per half-year period and expressed in ash-free dry mass (AFDM). In adults, positive values in spring/summer regularly alternated with negative values in autumn/winter, when up to ∼25% (mean: 14%) of individual weight gains in the preceding season were lost. No weight losses were observed during the first winter of the life of mussels. The 26-year mean of net somatic tissue production P amounted to 5.5 g AFDM m−2 a−1at a mean biomass B of 3.2 g AFDM m−2; the ratioP/Bvaried strongly with age composition of the mussel population and ranged between 0.5 and 3.0 a−1(mean: 1.7). Within the restricted areas of mussel beds, mean biomass and annual production values were two orders of magnitude higher. In the Wadden Sea, mussel beds cover a typical 1% of extensive tidal flat areas. Numerical densities of recruits showed straight-line relationships with subsequent life-time year-class production. Once recruits had reached an age of ∼10 months, their numbers predicted subsequent production within narrow limits. Production per recruit averaged 0.21 g AFDM for 10-mo recruits and was not related to recruit density. Local variation in annual production varied strongly, with maximal values between mid-tide and low-tide level, where recruitment was also maximal. Production per recruit was higher at low than at high intertidal levels. Frequently failing recruitment is indicated as the main cause of declining mussel stocks in the Wadden Sea. As in other bivalve species, a declining frequency of the occurrence of cold winters appears to govern declining recruitment success and consequently declining production.

The impact that predators have on benthic, macroinvertebrate prey density in streams is unclear. While some studies show a strong effect of predators on prey density, others show little or no effect. Two factors appear to influence the detection of predator impact on prey density in streams. First, many field studies have small sample sizes and thus might be unable to detect treatment effects. Second, streams contain two broad classes of predators, invertebrates and vertebrates, which might have different impacts on prey density for a variety of reasons, including availability of refuge for prey and prey emigration responses to the two types of predators. In addition, predatory vertebrates have more complex prey communities than predatory invertebrates; this complexity might reduce the impact that predatory vertebrates have on prey because of indirect effects. I conducted a meta-analysis on the results of field studies that manipulate predator density in enclosures to determine (1) if predators have a significant impact on benthic prey density in streams, (2) if the impacts that predatory invertebrates and vertebrates have differ, and (3) if predatory vertebrates have different impacts on predatory prey versus herbivorous prey. The results of the meta-analysis suggest that on average predators have a significant negative effect on prey density, predatory invertebrates have a significantly stronger impact than predatory vertebrates, and predatory vertebrates do not differ in their impact on predatory versus herbivorous invertebrate prey. Three methodological variables (mesh size of enclosures, size of enclosures, and experimental duration) were examined to determine if cross correlations exist that may explain the differences in impact between predatory invertebrates and vertebrates. No correlation exists between mesh size and predator impact. Over all predators, no correlation exists between experimental duration and predator impact; however, within predatory invertebrates a correlation does exist between these variables. Also, a correlation was found between enclosure size and predator impact. This correlation potentially explains the difference in impact between predatory invertebrates and predatory vertebrates. Results of the meta-analysis suggest two important areas for future research: (1) manipulate both types of predators within the same system, and (2) examine their impacts on the same spatial scale.

PREDATION is widespread in the pelagic zone of lakes and both vertebrate and invertebrate predators are abundant and diverse in their feeding habits. Vertebrate predators are often size-selective and can virtually eliminate larger prey items1–3. These dramatic effects resulting from fish predation have led several workers to conclude that vertebrates contribute the largest percentage of the total predation intensity on freshwater zooplankton. Particular invertebrate predators, however, have also been shown to crop a substantial proportion of their prey populations4–6. I report here a study showing that invertebrate predation is more intense than vertebrate predation in the pelagic zone of Gull Lake, Michigan, with the first in situ evaluation of both types of predation on a single zooplankton prey assemblage. The significance of identifying major predation pathways in lakes is basic to many contemporary discussions of nutrient and energy flow as well as considerations of community structure and stability.

A zooplankton community was established in outdoor experimental ponds, into which a vertebrate predator (topmouth gudgeon: Pseudorasbora parva) and/or an invertebrate predator (phantom midge larva: Chaoborus flavicans) were introduced and their predation effects on the zooplankton community structure were evaluated. In the ponds which had Chaoborus but not fish, small- and medium-sized cladocerans and calanoid copepods were eliminated while rotifers became abundant. A large-sized cladoceran Daphnia longispina, whose juveniles had high helmets and long tailspines as anti-predator devices, escaped from Chaoborus predation and increased. In the ponds which had fish but not Chaoborus, the large-sized Daphnia was selectively predated by the fish while small-and medium-sized cladocerans and calanoid copepods predominated. In the ponds containing both Chaoborus and fish, the fish reduced the late instar larvae (III and IV) of Chaoborus but increased the early instar larvae (I and II). Small- and large-sized cladocerans were scarcely found. The former might have been eliminated by predation of the early instar larvae of Chaoborus, while the latter was probably predated by fish. Consequently, the medium-sized cladocerans, which may have succeeded in escaping from both types of predator, appeared abundantly. The results suggest that various combinations of vertebrate and invertebrate predators are able to drive various kinds of zooplankton community structure.

A widespread perception is that carnivores are limited by the amount of prey that can be captured rather than their nutritional quality, and thus have no need to regulate macronutrient balance. Contrary to this view, recent laboratory studies show macronutrient-specific food selection by both invertebrate and vertebrate predators, and in some cases also associated performance benefits. The question thus arises of whether wild predators might likewise feed selectively according to the macronutrient content of prey. Here we review laboratory studies demonstrating the regulation of macronutrient intake by invertebrate and vertebrate predators, and address the question of whether this is likely to also occur in the wild. We conclude that it is highly likely that wild predators select prey or selectively feed on body parts according to their macronutrient composition, a possibility that could have significant implications for ecological and foraging theory, as well as applied wildlife conservation and management.

Terrestrial invertebrates: An underestimated predator guild for small vertebrate groups

We conducted experiments at the National Bison Range, Montana to compare grasshopper densities in cages with and without spiders, and inside and outside avian exclosures to examine the impacts of the most common invertebrate and vertebrate predators. Spider predation did not reduce grasshopper populations. Avian predation reduced the abundance of large- and small-bodied grasshopper species, but total grasshopper abundance increased in the presence of birds, because medium-sized grasshopper species (250-500 mg adult size) became more abundant. The decrease in abundance of large-bodied grasshoppers was due to birds preferentially killing them; however, predation did not explain declines in small-bodied grasshoppers or increases in medium-sized grasshoppers

In this study, we asked whether different predatory assemblages (i.e., flying invertebrates, crawling invertebrates, and birds, representing vertebrate predators) in a temperate forest impose significantly different levels of predation on larvae of Orgyia leucostigma (J. E. Smith), the whitemarked tussock moth and also whether the size of whitemarked tussock moth larvae influence invertebrate and vertebrate predation. Predation by species in vertebrate and invertebrate predator assemblages on two sizes of O. leucostigma larvae on box elder, Acer negundo (L.) (Sapindales: Aceraceae) was compared using exclusion cages. Cages covered with mesh of different sizes and sticky barriers were used to exclude different kinds of predators (i.e., birds, flying invertebrates, and crawling invertebrates). Five small and five large larvae were placed on box elder saplings. Predation by birds was the greatest source of mortality of large larvae when compared with that caused by flying and crawling invertebrates. Predation played an insignificant role in the disappearance of small larvae whose disappearance was associated with their dispersal behavior.

In forested streams, litter patches are important microhabitats for macroinvertebrates, and the nature of litter patches can affect structure and function of macroinvertebrate assemblages. We examined whether litter patch types with different characteristics could be predicted by their location within stream pools (pool middle, alcove, edge) and, if so, whether patch types had different macroinvertebrate assemblages. Mean mass of leaves per unit area of streambed was 2 to 3x higher in edge patches than in other patches, whereas mean mass of wood and small litter particles was 2 to 6x higher in middle patches. Densities of nemourid stonefly taxa were higher in edge patches than other patches, with density of Nemoura being highly correlated with leaf mass, whereas densities of lepidostomatid caddisfly taxa were higher in middle patches, with density of Goerodes complicatus being highly correlated with mass of small litter particles. Mean biomass and annual secondary production of shredders, collect...

I assessed the impact of both vertebrate and invertebrate predators on a lotic benthic community in a 1-month-long experiment, using enclosures containing cobble/gravel bottoms, with large-mesh netting that allowed invertebrates to drift freely. Brown trout (Salmo trutta) and leeches (Erpobdella octoculata) were used as predators and four treatments were tested: a predator-free control, leeches only, trout only, and leeches and trout together. A density of 26.7 leeches/m2 (20 leeches/enclosure) and 1.3 trout/m2 (one trout per enclosure) was stocked into the enclosures. The total biomass of invertebrate prey was significantly lower in the trout and trout plus leech treatments than in the leech and control treatments, which were due to strong negative effects of trout on Gammarus. On the individual prey taxon level, both trout and leeches affected the abundance of Asellus , Baetis and Ephemerella, whereas the abundance of Gammarus was only affected by trout, and the abundance of Orthocladiinae and Limnephilidae was only affected by leeches. In the treatment with trout and leeches together, the abundance of Ephemerella and Baetis was higher than when trout or leeches were alone, which was probably due to predator interactions. Leeches and trout had no effects on prey immigration but did affect per capita emigration rates. Both trout and leeches indirectly increased periphyton biomass in enclosures, probably due to their strong effects on grazers. Both trout and leeches were size-selective predators, with trout selecting large prey, and leeches selecting small prey. Size-selective predation by trout and leeches affected the size structure of five commonly consumed prey taxa. Trout produced prey populations of small sizes owing to consumption of large prey as well as increased emigration out of enclosures by these large prey. Leech predation produced prey assemblages of larger size owing to consumption and increased emigration of small prey. These results suggest that in lotic habits, predatory invertebrates can be as strong interactors as vertebrate predators.

Everglades periphyton mats are tightly-coupled autotrophic (algae and cyanobacteria) and heterotrophic (eubacteria, fungi and microinvertebrates) microbial assemblages. We investigated the effect of water column total phosphorus and nitrogen concentrations, water depth and hydroperiod on periphyton of net production, respiration, nutrient content, and biomass. Our study sites were located along four transects that extended southward with freshwater sheetflow through sawgrass-dominated marsh. The water source for two of the transects were canal-driven and anchored at canal inputs. The two other transects were rain-driven (ombrotrophic) and began in sawgrass-dominated marsh. Periphyton dynamics were examined for upstream and downstream effects within and across the four transects. Although all study sites were characterized as short hydroperiod and phosphorus-limited oligotrophic, they represent gradients of hydrologic regime, water source and water quality of the southern Everglades. Average periphyton net production of 1.08 mg C AFDW−1 h−1 and periphyton whole system respiration of 0.38 mg C AFDW−1 h−1 rates were net autotrophic. Biomass was generally highest at ombrotrophic sites and sites downstream of canal inputs. Mean biomass over all our study sites was high, 1517.30 g AFDW m−2. Periphyton was phosphorus-limited. Average periphyton total phosphorus content was 137.15 μg P g−1 and average periphyton total N:P ratio was 192:1. Periphyton N:P was a sensitive indicator of water source. Even at extremely low mean water total phosphorus concentrations ( ≤ 0.21 μmol l−1), we found canal source effects on periphyton dynamics at sites adjacent to canal inputs, but not downstream of inflows. These canal source effects were most pronounced at the onset of wet season with initial rewetting. Spatial and temporal variability in periphyton dynamics could not solely be ascribed to water quality, but was often associated with both hydrology and water source.

In streams the impact that predators have on prey density is unclear. A recent review suggests that predators have on average a small to moderate negative effect on prey density and that predatory invertebrates have a stronger impact than predatory vertebrates. We suggest that a reason for this difference between predatory invertebrates and vertebrates is differences in prey emigration responses to the two types of predators. If prey increase their emigration rates in the presence of predators, predator impacts on local prey density will be strong; conversely, if prey decrease their emigration rates in the presence of predators, prey density might actually build up in patches containing predators and predator impact on prey density will be weak or even positive. We reviewed 22 studies that examined the impact that predatory vertebrates and invertebrates have on prey drift and activity (crawling and rates of emergence from refuge). Our review revealed that predatory invertebrates cause prey to increase drift rates more often than expected by chance. Predatory vertebrates had variable effects on prey drift rates (sometimes increasing and sometimes decreasing prey drift rates), but they caused prey to decrease activity more often than expected by chance. These results provide a potential mechanism for the relatively large impact that predatory invertebrates have on prey density as compared to the impact that predatory vertebrates have. We suggest directions for future research that include examining the impact that both types of predators have on prey emigration behavior and benthic density in the same system

The main purpose is to analyse the modelling of gravel bed streams with movable bed models, comparing with same modelling of sand bed streams. The stream processes are the same for gravel and sand bed streams. The critical point is to develop a sound physical theory to be applied to basic stream processes, using simultaneously laboratory, numerical, analytical, and field observation efforts.

In terms of biomass, the three dominant fish species in the Huizache‐Caimanero lagoon system during 1975–76 were the mullet Mugil curema, the anchovy, Anchoa panamensis and the catfish, Galeichthys caerulescens. Peaks in the abundance of the secondary predators occurred at the end of the wet season (September‐October), while pelagic forms and members of the centropomid‐gerreid association (Warburton 1968a) were most common during November‐February and December‐February respectively. Twelve species, including the ten having the highest overall mean biomass, were selected for growth and production studies. Lagoon‐specific differences in the patterns of growth and recruitment of several species were observed. The total estimated annual fish production in Caimanero lagoon was 34·48 g m−2 y−1, with M. curema contributing 9·36 g m−2, A. panamensis 9·24 g m−2 and G. caerulescens 6·15 g m−2 (maximum estimate). Mean turnover ratios (annual production: mean biomass) were 3·25 (demersal species), 8·44 (pelagic species) and 4·48 (all species). Implications for possible fishery and aquacultural expansion are discussed.

Spatial variation in Spanish populations of brown troutSalmo truttawas studied in 10 streams of contrasting environmental and biological characteristics based on data compiled over 7 years (1992–1998). Three of the streams had soft water (mean alkalinity as CaCO3= 19.3 mg/L) supplied by granite catchments at elevations around 1,250 m above sea level and had a low abundance of macroinvertebrates (mean density = 598 individuals/m2; mean biomass = 0.63 g/m2). The remaining streams had hard water (mean alkalinity = 253.6 mg/L) flowing over limestone at 850–1,400‐m elevations and possessed a greater benthic faunal abundance (mean density = 2,433 individuals/m2; mean biomass = 2.76 g/m2). Mean brown trout population characters varied significantly throughout the study area (density = 1,567–5,594 fish/ha; biomass = 56.6–240.2 kg/ha; annual production = 47.0–182.0 kg/ha, and the ratio of annual production to mean biomass = 1.01–1.56). A stepwise multiple regression analysis revealed a significant relationship between brown trout production and chemical features indicative of high water productivity, which accounted for 61% of the variance explained by the model. A broader spatial analysis, based on a review of the available European work, corroborated that annual brown trout production in streams flowing over limestone bedrock was greater (mean = 121.6 kg/ha; range = 30.0–253.3 kg/ha) than that in streams flowing over siliceous bedrock (mean = 76.6 kg/ha; range = 3.5–234.0 kg/ha). Data from brown trout populations throughout Europe showed a significant positive correlation between production and alkalinity according to the model log10(production) = 1.41 + 0.31·log10(alkalinity), which explained 53% of the variance. Our findings support previous evidence on the influence of water fertility on salmonid production and expand knowledge of the factors that influence brown trout production within the native range of the species.