Susan S. Kilham

Nutritional constraints in terrestrial and freshwater food webs

Biological and environmental contrasts between aquatic and terrestrial systems have hindered analyses of community and ecosystem structure across Earths diverse habitats. Ecological stoichiometry provides an integrative approach for such analyses, as all organisms are composed of the same major elements (C, N, P) whose balance affects production, nutrient cycling, and food-web dynamics. Here we show both similarities and differences in the C:N:P ratios of primary producers (autotrophs) and invertebrate primary consumers (herbivores) across habitats. Terrestrial food webs are built on an extremely nutrient-poor autotroph base with C:P and C:N ratios higher than in lake particulate matter, although the N:P ratios are nearly identical. Terrestrial herbivores (insects) and their freshwater counterparts (zooplankton) are nutrient-rich and indistinguishable in C:N:P stoichiometry. In both lakes and terrestrial systems, herbivores should have low growth efficiencies (10–30%) when consuming autotrophs with typical carbon-to-nutrient ratios. These stoichiometric constraints on herbivore growth appear to be qualitatively similar and widespread in both environments.

Environmental and ecological factors that shape the gut bacterial communities of fish: a meta‐analysis

Symbiotic bacteria often help their hosts acquire nutrients from their diet, showing trends of co‐evolution and independent acquisition by hosts from the same trophic levels. While these trends hint at important roles for biotic factors, the effects of the abiotic environment on symbiotic community composition remain comparably understudied. In this investigation, we examined the influence of abiotic and biotic factors on the gut bacterial communities of fish from different taxa, trophic levels and habitats. Phylogenetic and statistical analyses of 25 16S rRNA libraries revealed that salinity, trophic level and possibly host phylogeny shape the composition of fish gut bacteria. When analysed alongside bacterial communities from other environments, fish gut communities typically clustered with gut communities from mammals and insects. Similar consideration of individual phylotypes (vs. communities) revealed evolutionary ties between fish gut microbes and symbionts of animals, as many of the bacteria from the guts of herbivorous fish were closely related to those from mammals. Our results indicate that fish harbour more specialized gut communities than previously recognized. They also highlight a trend of convergent acquisition of similar bacterial communities by fish and mammals, raising the possibility that fish were the first to evolve symbioses resembling those found among extant gut fermenting mammals.

The effects of amphibian population declines on the structure and function of Neotropical stream ecosystems

Amphibians can be important consumers in both aquatic and terrestrial habitats and may represent an important energetic link between the two, particularly in the tropics, where amphibian species richness and abundance are high. In the past 20 years, amphibian populations have declined dramatically around the world; numbers have decreased catastrophically in protected upland sites throughout the neotropics, usually resulting in the disappearance of over 75% of amphibians at a given site, particularly those species that breed in streams. Most studies of amphibian declines have focused on identifying causes and documenting changes in adult abundance, rather than on their ecological consequences. Here, we review evidence for the potential ecological effects of catastrophic amphibian declines, focusing on neotropical highland streams, where impacts will likely be greatest. Evidence to date suggests that amphibian declines will have large-scale and lasting ecosystem-level effects, including changes in algal com...

EFFECT OF NUTRIENT AVAILABILITY ON THE BIOCHEMICAL AND ELEMENTAL STOICHIOMETRY IN THE FRESHWATER DIATOM STEPHANODISCUS MINUTULUS (BACILLARIOPHYCEAE)

The objective of this study was to examine the differences in the biochemical and elemental stoichiometry of a freshwater centric diatom, Stephanodiscus minutulus (Grun.), under various nutrient regimes. Stephanodiscus minutulus was grown at μmax or 22% of μmax under limitation by silicon, nitrogen, or phosphorus. Cell sizes for nutrient‐limited cultures were significantly smaller than the non‐limited cell sizes, with N‐limited cells being significantly smaller than all other treatments. Compared with the nutrient‐replete treatment, both carbohydrates and lipids increased in Si‐ and P‐limited cells, whereas carbohydrates increased but proteins decreased in N‐limited cells. All of the growth‐limited cells showed an increase of carbohydrate and triglyceride, and a decrease of cell size and polar lipids as a percentage of total lipids. The non‐limited cells also had a significantly higher chl a concentration and galactolipids as a percentage of total lipids than any of the limited treatments, and the low‐Si and low‐P cells had significantly higher values than the low‐N cells. The particulate C concentrations showed significant differences between treatments, with the Si‐ and P‐limited treatments being significantly higher than the N‐ and non‐limited treatments. Particulate Si did not show a strong relationship with any of the parameters measured, and it was the only parameter with no differences between treatments. The low‐Si cells had a significantly higher P content (about two times more) than any other treatment, presumably owing to the luxury consumption of P, and a correspondingly high phospholipid concentration. The elemental data showed that S. minutulus had a high P demand with low optimum N:P (4) and Si:P (10) ratios and a C:N:P ratio of 109:16:2.3. The particulate C showed a positive relationship with POM (r = 0.93), dry weight (r = 0.88), lipid (r = 0.87) and protein (r = 0.84, all P < 0.0001). Particulate N showed a positive relationship with galactolipids (r = 0.95), protein (r = 0.90), dry weight (r = 0.78), lipid (r = 0.75), and cell volume (r = 0.64, all P < 0.0001). It is evident that nutrient limitation in the freshwater diatom S. minutulus has pronounced effects on its biochemical and elemental stoichiometry.

LIMITING RESOURCES AND THE REGULATION OF DIVERSITY IN PHYTOPLANKTON COMMUNITIES

Species diversity is a key concept in ecology, yet the mechanisms regulating diversity in most systems are not completely understood. To address this issue, we analyzed the relationship between phytoplankton diversity and limiting resources (N, P, Si, and light) over two summers in three lakes in the Yellowstone (Wyoming, USA) region. Diversity was highly variable along temporal and spatial axes within lakes. We discovered a strong positive correlation between diversity and the number of resources at physiologically limiting levels. Consistent with resource-competition theory, we found the highest diversity to occur when many resources were limiting. Conversely, the lowest diversity occurred when few resources were measured at limiting levels. Sensitivity analyses demonstrated that threshold levels of resources (below which growth is limited and above which there is saturation for the resource) appear to exist in the natural environment, and that diversity is regulated in part by absolute levels of resources available to phytoplankton in aquatic systems. Threshold levels are generally in agreement with those determined to be limiting in prior physiological experiments. Consistent with previous observations, among-lake comparisons yielded a significant negative relationship between species diversity and system productivity. This result tentatively supports the hypothesis that the proximity of individual phytoplankton plays a role in the strength of competitive interactions. Our results generally suggest that, even in dynamic environments, where equilibrium conditions are rare, resource competition among phytoplankton is a mechanism by which communities are continually structured.

ELEVATED δ15N IN STREAM BIOTA IN AREAS WITH SEPTIC TANK SYSTEMS IN AN URBAN WATERSHED

Anthropogenic inputs of nitrogen from human sewage are a central concern in urban watershed management. However, identifying the locations of these inputs, whether from improperly functioning or ill-maintained septic tanks or from leaking sewer lines, is difficult. We used nitrogen stable-isotope analysis of aquatic food webs as indicators of sewage-derived nitrogen in Valley Creek watershed. Stable nitrogen isotope analysis revealed elevated δ15N in all trophic levels at stations located downstream of the divide between sewered and nonsewered (septic tanks) neighborhoods in the watershed. Stations not located in the septic system area or on the other branch of the creek did not show this elevated level of δ15N. Allochthonus inputs, which do not derive nitrogen from aquatic sources, such as detrital leaf material, showed no difference in δ15N between Valley Creek and Little Valley Creek. Particular fish species that are found throughout both branches, such as blacknose dace and creek chub, had as much as ...

Divergence across diet, time and populations rules out parallel evolution in the gut microbiomes of Trinidadian guppies

Diverse microbial consortia profoundly influence animal biology, necessitating an understanding of microbiome variation in studies of animal adaptation. Yet, little is known about such variability among fish, in spite of their importance in aquatic ecosystems. The Trinidadian guppy, Poecilia reticulata, is an intriguing candidate to test microbiome-related hypotheses on the drivers and consequences of animal adaptation, given the recent parallel origins of a similar ecotype across streams. To assess the relationships between the microbiome and host adaptation, we used 16S rRNA amplicon sequencing to characterize gut bacteria of two guppy ecotypes with known divergence in diet, life history, physiology and morphology collected from low-predation (LP) and high-predation (HP) habitats in four Trinidadian streams. Guts were populated by several recurring, core bacteria that are related to other fish associates and rarely detected in the environment. Although gut communities of lab-reared guppies differed from those in the wild, microbiome divergence between ecotypes from the same stream was evident under identical rearing conditions, suggesting host genetic divergence can affect associations with gut bacteria. In the field, gut communities varied over time, across streams and between ecotypes in a stream-specific manner. This latter finding, along with PICRUSt predictions of metagenome function, argues against strong parallelism of the gut microbiome in association with LP ecotype evolution. Thus, bacteria cannot be invoked in facilitating the heightened reliance of LP guppies on lower-quality diets. We argue that the macroevolutionary microbiome convergence seen across animals with similar diets may be a signature of secondary microbial shifts arising some time after host-driven adaptation.

A stable isotope study of a neotropical stream food web prior to the extirpation of its large amphibian community

Rapid and massive amphibian population declines have been reported throughout upland areas of the Neotropics. The abundance and species richness of Neotropical amphibian communities suggest that losses of this magnitude are likely to have strong effects at the ecosystem level. To improve understanding of the implications of their loss we used stable isotope analysis to examine trophic relationships in an ecosystem in which amphibians are dominant in a second-order forest stream at 750 m asl in Parque Nacional Omar Torrijos Herrera, Panama. We analysed δ 13 C, δ 15 N and C:N ratios of major biotic components (basal resources, invertebrates, amphibians, fish and reptiles) in the stream and of the adjacent riparian food web. Tadpoles (mean δ 15 N = 4.49%o) and adult amphibians (mean δ 15 N = 5.45‰) were intermediate links in the aquatic and terrestrial food web respectively. High δ 15 N signatures identified fish as top predators in the aquatic food web and snakes and the toad Bufo as top predators in the terrestrial food web. Isotopic signatures clearly distinguished between trophic groups of tadpoles: microbial feeders (Centrolenidae, δ 15 N range = 1).91-3.05‰), herbivores (Rana and Hyla, δ 15 N range = 4.74-5.15‰) and neuston feeders (Colostethus, δ 15 N range = 5.31-6.40%o). Dependence on autotrophic production was indicated by enriched signatures of carbon isotopes in pool dwellers versus those that reside in faster-flowing sections of the stream. High nitrogen concentrations in detrital matter (average 0.8%, C:N = 10.3) suggested that grazing tadpoles enhanced nitrogen fluxes and improved the quality of organic matter available to detritivores.

Effects of urbanization and land use on fish communities in Valley Creek watershed, Chester County, Pennsylvania

Valley Creek watershed, located in southeastern Pennsylvania, is a small, fourth-order stream that empties into the Schuylkill River at Valley Forge National Historic Park, thirty-five kilometers northwest of Philadelphia. The 64 km2 watershed has been under extreme urbanization pressure over the past 30 years, resulting in rapidly increasing impervious surface cover and decreasing open space. The purpose of this study was to document some of the effects of urbanization on fish assemblages by quantifying the fish communities at fifteen sites throughout the watershed. Long-term effects of continued urbanization were identified, as data from the present study were compared to similar work completed nearly ten years earlier. There has been a shift in species composition from intolerant, coldwater species to more tolerant, eurythermal species. Currently, Valley Creek is supporting a naturally reproducing population of brown trout, but there has been a marked decline in relative abundance and range since 1993. Increased stream temperature from urban run-off is one of the primary issues in Valley Creek. Species composition was unique at each of the 15 stations owing to the effect of local land use in each station’;s drainage area. Fish assemblages revealed a patchy, non-continuous pattern of fish distribution.

Stream invertebrate responses to a catastrophic decline in consumer diversity

Abstract Tadpoles are often abundant and diverse consumers in headwater streams in the Neotropics. However, their populations are declining catastrophically in many regions, in part because of a chytrid fungal pathogen. These declines are occurring along a moving disease front in Central America and offer the rare opportunity to quantify the consequences of a sudden, dramatic decline in consumer diversity in a natural system. As part of the Tropical Amphibian Declines in Streams (TADS) project, we examined stream macroinvertebrate assemblage structure and production for 2 y in 4 stream reaches at 2 sites in Panama. One site initially had healthy amphibians but declined during our study (El Copé), and 1 site already had experienced a decline in 1996 (Fortuna). During the 1st y, total macroinvertebrate abundance, biomass, and production were generally similar among sites and showed no consistent patterns between pre- and post-decline streams. However, during the 2nd y, tadpole densities declined precipitously at El Copé, and total macroinvertebrate production was significantly lower in the El Copé streams than in Fortuna streams. Functional structure differed between sites. Abundance, biomass, and production of filterers generally were higher at Fortuna, and shredders generally were higher at El Copé. However, shredder production declined significantly in both El Copé reaches in the 2nd y as tadpoles declined. Nonmetric dimensional scaling (NMDS) based on abundance and production indicated that assemblages differed between sites, and patterns were linked to variations in relative availability of basal resources. Our results indicate that responses of remaining consumers to amphibian declines might not be evident in coarse metrics (e.g., total abundance and biomass), but functional and assemblage structure responses did occur. Ongoing, long-term studies at these sites might reveal further ecological consequences of the functional and taxonomic shifts we observed.