Susan S Kilham

Drexel University | DU · Department Biodiversity, Earth and Environmental Science

Consumers vary in their excretion of nitrogen and phosphorus, altering nutrient cycles and ecosystem function. Traditional mass balance models that focus on dietary and tissue nutrients have poorly explained such variation in excretion. Here, we contrast diet and tissue nutrient models for nutrient excretion with predation risk, an often overlooked...

Resilience of ecosystems to the sudden decline of large‐bodied species is dependent on characteristics of surviving guild members. However, that response may also be mediated by local habitat conditions. Here, we examine the mechanisms behind the observed lack of functional compensation in the algal‐grazing guild by insect grazers following the dec...

Population variation in trophic niche is widespread among organisms and is of increasing interest given its role in both speciation and adaptation to changing environments. Trinidadian guppies (Poecilia reticulata) inhabiting stream reaches with different predation regimes have rapidly evolved divergent life history traits. Here, we investigated th...

Emerging infectious diseases can cause host community disassembly, but the mechanisms driving the order of species declines and extirpations following a disease outbreak are unclear. We documented the community disassembly of a Neotropical tadpole community during a chytridiomycosis outbreak, triggered by the generalist fungal pathogen, Batrachochy...

The development of ideas leading to the concept of ecological stoichiometry is detailed. Initial work on biogeochemistry of African lakes by P. Kilham led to the development of freshwater diatom physiology to test his silica hypothesis. Further resource experiments on P-limited diatoms by Tilman and his tests of competition led to the discovery tha...

Microscale conditions within metaphyton mats affected the distribution of diatom genera. We investigated the conditions within layers of floating filamentous algal mats and changes in diatom genera over 57 days using microprobes every 2, 4, and 6 cm down through mats held in floating nets. Mats were then collected, frozen, and sliced into 2 cm laye...

Foraging success affects reproductive output in sea turtles, and is therefore an important factor to measure in order to understand population dynamics. During 2010 and 2011, we used satellite telemetry to track the at-sea behavior of 20 post-nesting loggerhead turtles Caretta caretta from Rethymno, Crete, Greece. Nineteen transmitters provided loc...

Phenotypic plasticity is advantageous for organisms that live in variable environments. The digestive system is particularly plastic, responding to changes in diet. Gut length is the result of a trade-off between maximum nutrient absorption and minimum cost for its maintenance and it can be influenced by diet and by evolutionary history. We assesse...

Species losses are predicted to simplify food web structure, and disease-driven amphibian declines in Central America offer an opportunity to test this prediction. Assessment of insect community composition, combined with gut content analyses, was used to generate periphyton-insect food webs for a Panamanian stream, both pre- And postamphibian decl...

Species losses are predicted to simplify food web structure, and disease-driven amphibian declines in Central America offer an opportunity to test this prediction. Assessment of insect community composition, combined with gut content analyses, was used to generate periphyton-insect food webs for a Panamanian stream, both pre- and post-amphibian dec...

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...

Taxonomic and functional diversity in freshwater habitats is rapidly declining, but we know little about how such declines will ultimately affect ecosystems. Neotropical streams are currently experiencing massive losses of amphibians, with many losses linked to the chytrid fungus, Batrachochytrium dendrobatidis ( Bd ). We examined the ecological co...

A trophic niche shift can occur as an adaptive response to environmental change such as altered resource quality, abundance or composition. Alterations in digestive traits such as gut morphology and physiology may enable these niche shifts and affect the persistence of populations and species. Relatively few studies, however, have assessed how nich...

Background/Question/Methods Quantifying functional trait diversity in an assemblage is essential to understanding the potential for functional redundancy following an extinction event. We examined an apparent lack of functional redundancy by algae grazing mayflies following the disease-driven loss of stream-dwelling tadpoles from a Panamanian mon...

Information about temporal patterns of ecological responses to species losses is integral to our understanding of the ultimate effects of declining biodiversity. As part of the Tropical Amphibian Declines in Streams ( TADS ) project, we quantified changes in algal biomass and N cycling in algae in upland Panamanian streams following the widespread...

1. Information about temporal patterns of ecological responses to species losses is integral to our understanding of the ultimate effects of declining biodiversity. As part of the Tropical Amphibian Declines in Streams (TADS) project, we quantified changes in algal biomass and N cycling in algae in upland Panamanian streams following the widespread...

Predicting the ecological consequences of declining biodiversity is an urgent challenge, particularly in freshwater habitats where species declines and losses are among the highest. Small-scale experiments suggest potential ecosystem responses to losses of species, but definitive conclusions require verification at larger scales. We measured ecosys...

Background/Question/Methods As species are extirpated, ecosystem productivity will likely decline with concomitant declines in food web structure as linkages are removed. Catastrophic amphibian extirpations in Central America can affect ecosystem function, but have unknown consequences on food web structure. We empirically assessed periphyton-ins...

Background/Question/Methods Disease epidemics can act as extinction filters, changing community structure, species niche breadth, and species co-occurrence patterns. Rarely do data exist that are collected systematically before and after an epidemic documenting species losses and changes in community composition. We examined spatial and temporal...

Despite the typically high taxonomic and functional diversity of tropical habitats, little is known about the roles of individual consumers in their ecosystem structure and function. We studied the trophic basis of production in a tropical headwater stream by identifying major sources of energy, measuring energy flow through consumers and character...

Neotropical streams are losing dominant consumer groups as a result of disease-driven amphibian declines. The herbivorous tadpoles of Lithobates warszewitschii were once abundant in the Rio Maria in the Eastern Cordillera Central of Panama, where they consumed algae and organic matter. The decline of this once abundant grazer has the potential to a...

Habitat degradation and species introductions are two of the leading causes of species declines on a global scale. Invasive species negatively impact native species through predation and competition for limited resources. The impacts of invasive species may be increased in habitats where habitat degradation is higher due to reductions of prey abund...

Plant species documented during the 2010 resource availability surveys. Plant species found only at FM and SLNC are on the left and right, respectively, while species found at both wetlands are in the center. We documented 31 species at FM and 51 species at SLNC. (DOCX)

The effect of consumers on their resources has been demonstrated in many systems but is often confounded by trophic interactions with other consumers. Consumers may also have behavioral and life history adaptations to each other and to co-occurring predators that may additionally modulate their particular roles in ecosystems. We experimentally excl...

Statistical methods for split-plot design and analytical framework for structural and process responses. (DOC)

Benthic invertebrate tallies for control and electrified treatments in three reaches. (DOC)

1. Metaphyton mats typically originate as benthic algal biofilms that experience higher solar radiation and temperatures, and reduced access to nutrients, once they reach the water surface, but the impacts of these physicochemical changes on metaphyton condition and community composition have received little attention. 2. Using microprobes position...

Background/Question/Methods When animals experience environmental change or habitat shifts, they can be exposed to different resources or increased competition. In such circumstances, altering their trophic niche can provide a fitness advantage and help them coexist with other organisms. Such dietary shifts can generate strong selection for trait...

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 thi...

Waste streams from municipal solid waste landfills may potentially be used as feedstocks to cultivate algae for the production of biodiesel. This article describes a feasibility study designed to test the potential for using landfill leachate (and eventually landfill gas) as growth media for algae, a critical first step required to convert landfill...

1. Life histories evolve as a response to multiple agents of selection, such as age-specific mortality, resource availability or environmental fluctuations. Predators can affect life-history evolution directly, by increasing the mortality of prey, and indirectly, by modifying prey density and resources available to the survivors. Increasing survivo...

1. Of the relatively few studies that have examined consequences of amphibian declines on stream ecosystems, virtually all have focused on changes in algae (or algal-based food webs) and little is known about the potential effects of tadpoles on leaf decomposition. We compared leaf litter decomposition dynamics in two neotropical streams: one with...

Background/Question/Methods Prokaryotic organisms can have complex relationships with their eukaryotic hosts, which can, in turn, impact their ecology and evolution. We are studying the interplay among gut bacteria and diet within the Trinidadian guppy, Poecilia reticulata. Guppies from stream habitats with higher predation pressure exhibit faste...

Growth rates for five freshwater algae, Asterionella formosa, Stephanodiscus hantzschii, Scenedesmus sp., Cryptomonas ozolini, and Anabaena flos-aquae, in media ranging in pH from 5.0 to 7.5 using three zwitterionic buffers, MES, PIPES, and TES, were determined. These buffers supported similar growth rates when compared with each other in treatment...

The hypothesis that phytoplankton species assort themselves along resource ratio gradients according to their relative competitive abilities for the potentially limiting resources was tested using natural communities from Lake Michigan. Algae were grown in six continuous cultures for 46 d on a gradient consisting of four silicon to phosphorus (Si:P...

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...

1. We quantified production and consumption of stream‐dwelling tadpoles and insect grazers in a headwater stream in the Panamanian uplands for 2 years to assess their effects on basal resources and energy fluxes. At the onset of our study, this region had healthy, diverse amphibian populations, but a catastrophic disease‐driven decline began in Sep...

Background/Question/Methods Guppy (Poecilia reticulata) populations in streams with different predation regimes exhibit marked differences in their life-history traits. Differences have been attributed to direct effects of predation, but predation may also play a role through its indirect effects on guppy population structure and density and ther...

Background/Question/Methods Gut bacteria play important physiological roles in their animal hosts, influencing metabolic processes either by altering host gene expression or by directly breaking down digested food items. Because gut microbes play a significant role in energy extraction, it is important to assess what factors influence the gut mic...

As part of the Tropical Amphibian Declines in Streams (TADS) project, we estimated macroinvertebrate production and constructed quantitative food webs for four headwater stream reaches in the Panamanian uplands: two that had experienced massive amphibian declines and two with unaffected amphibian populations. As expected for forested headwaters, al...

Global declines of amphibian populations are well documented, yet effects of these declines on freshwater ecosystem structure and function are poorly understood. Here we examine responses of algal primary producers to tadpole extirpation over differing spatial and temporal scales. We experimentally excluded tadpoles from artificial substrata within...

Allochthonous inputs of detritus represent an important energy source for streams in forested regions, but dynamics of these materials are not well studied in neotropical headwater streams. As part of the tropical amphibian declines in streams (TADS) project, we quantified benthic organic matter standing stocks and organic seston dynamics in four P...

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 stabl...

The objective of this study was to examine the importance of nutrient status of a diatom (Stephanodiscus minutulus) to the uptake of PCB congener #54 (2,2',6,6'-tetrachlorobiphenyl) and the subsequent transfer of PCB to a pelagic grazing zooplankton (Daphnia pulicaria). The algae, which were grown under different nutrient treatments, were then fed...

Phosphorus uptake dynamics were examined in two small streams in the Valley Creek watershed, located in an urbanizing area approximately 30 km west of Philadelphia, Pennsylvania, USA. The goal of this research was to examine how phosphorus uptake is influenced by temporal changes in bed sediment characteristics and transient storage in streams with...

Resource availability constrains energy allocations to competing functions such as growth and reproduction, and thus influences animal life-histories and population dynamics. Leatherback turtles Dermochelys coriacea are critically endangered and exhibit physiological and life-history traits unique among marine reptiles. In addition, leatherback nes...

Oceanographic sampling is often limited to local and temporally concise assessments of complex, transient, and widespread phenomena. However, long-lived, migratory pelagic vertebrates such as leatherback turtles (Dermochelys coriacea, Vandelli 1761) can provide important integrated information about broad-scale oceanographic processes. Therefore, t...

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 imper...

Amphibians can be important consumers in both aquatic and terrestrial habitats and may represent an impor- tant energetic link between the two, particularly in the tropics, where amphibian species richness and abun- dance are high. In the past 20 years, amphibian populations have declined dramatically around the world; numbers have decreased catast...

Organic seston represents food for filter feeders and a mechanism for downstream transport of energy and nutrients. As part of a study assessing the ecological impacts of stream-breeding anuran extirpations, we examined seston dynamics in 2 stream reaches with tadpoles (El Cope) and 2 without (Fortuna) in the Panamanian uplands. All reaches are hig...

Amphibian populations are declining globally in uplands. Stream-dwelling tadpoles are potentially important herbivores, and their loss is expected to cause significant changes in structure and function of lotic ecosystems. This study is part of a collaborative effort to measure changes in trophic structure associated with amphibian extirpations. We...

Global declines of amphibians are well documented, yet effects of these dramatic losses on ecosystem structure and function are poorly understood. As part of a larger collaborative project, we compared two upland Panamanian streams. Both streams are biologically and geologically similar; however, one stream (Fortuna) has recently experienced almost...

Diatom clones of seven species used in the studies were isolated from large lakes of the Greater Yellowstone Ecosystem prior to the experiments. Bioassays were performed using a semicontinuous dilution technique for 21 to 24 days, reaching a steady state in total biovolume. The growth rate was 0.05-0.08 day1 and cultures were maintained at 10 °C on...

The effect of temperature on the silicon limited growth and nutrient kinetics of Stephanodiscus minutus Grun. was examined using batch and semicontinuous culture methods. Short-term batch culture methods gave maximum growth rates which were essentially constant over the temperature range of 10° to 20°C (μ3= 0.71–0.80 d−1). The half-saturation const...

Valley Creek watershed is a small stream system that feeds the Schuylkill River near Philadelphia, Pennsylvania. The watershed is highly urbanized, including over 17 percent impervious surface cover (ISC) by area. Imperviousness in a watershed has been linked to fish community structure and integrity. Generally, above 10 to 12 percent ISC there is...

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 sewag...

Examines the role of nutrients and spatial and temporal fluctuations in controlling the species composition, diversity and seasonal succession of planktonic algal communities. The combined net effect of limiting nutrients, major ions, pH and other physical factors ultimately determines the reproductive rate of a local algal population. The theory o...

Phytoplankton associations generally are identified by nonrandom co-occurrence of phytoplankton species, and are considered to be a result of similar species response to properties of the environment. Because an association is characterized by co-occurrence of species across samples, or synchrony of the abundance of species over time, we proposed t...

SUMMARY 1. Dietary phosphorus can be limiting for aquatic animals such as suspension-feeders. However, our understanding has been limited by the difficulty of manipulating dietary P without altering other aspects of food quality. We microencapsulated various forms of bioavailable P with carbohydrate to manipulate dietary C : P ratio for suspension-...

We analyzed sediment elemental composition (C, N, P, and Si) and planktonic diatom fossils from the top 32 cm of a short core (55 cm) from Yellowstone Lake (Wyoming), tracking deposition from about 1650 to 1998. Asso- ciations between fossil communities and sediment chemistry were measured by rank-order correlation to gauge decadal scale relationsh...

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...

Biological and environmental contrasts between aquatic and terrestrial systems have hindered analyses of community and ecosystem structure across Earth's diverse habitats. Ecological stoichiometry(1,2) provides an integrative approach for such analyses, as all organisms are composed of the same major elements (C, N, P) whose balance affects product...

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 cu...

We assessed phytoplankton dynamics in three lakes in the Greater Yellowstone Ecosystem to better understand the connections between changing environmental conditions and aquatic communities. This work primarily describes the connections between resource availability and phytoplankton seasonal succession in these lakes. We hypothesized that algal sp...

We assessed the phytoplankton communities and the relevant aquatic chemistry in three large lakes in the Greater Yellowstone Ecosystem. While N limitation of phytoplankton is most common, it appears that a recent regional increase in atmospheric N deposition is causing both P and Si limitation to occur to some degree. N additions in semi-continuous...

We assessed the phytoplankton communities and the relevant aquatic chemistry in three large lakes in the Greater Yellowstone Ecosystem. While N limitation of phytoplankton is most common, it appears that a recent regional increase in atmospheric N deposition is causing both P and Si limitation to occur to some degree. N additions in semi-continuous...

In order to conduct experiments on interactions between animals and food organisms, it is necessary to develop a medium that adequately supports the growth of both algae and zooplankton without the need to alter the medium to accommodate either the algae or the animals. We devised a freshwater medium, named COMBO, that supports excellent growth of...

1. Cell size and volume changed as a function of the type of resource limitation, with nitrogen-limited cells being smaller and less dense and phosphorus-limited cells being larger and more dense than non-limited cells. 2. The major biochemical constituents of the green alga Ankistrodesmus falcatus varied as a function of nitrogen or phosphorus lim...

1. Food quality was at least as important as food quantity for both fecundity and population growth responses of the cladoceran Daphnia pulicaria fed the green alga Ankistrodesmus falcatus grown under N limitation, P limitation, or non‐limited condition. 2. The fecundity of D. pulicaria was reduced under conditions of low food quality (low N or low...

1. The quantity of seston was measured and the elemental carbon, nitrogen and phosphorus (C, N, P) and biochemical composition (carbohydrate, protein, lipid) of the < 53 μm size fraction in three temperate lakes during one year was analysed. The lakes differed in nutrient concentration and were characterized as oligotrophic, mesotrophic and eutroph...

Resource-based physiology of the eight important planktonic diatom species in the large lakes of the Yellowstone region can be used to explain their relative abundances and seasonal changes. The diatoms are ranked along resource ratio gradients according to their relative abilities to grow under limitation by Si, N, P, and light. Hypotheses based o...

The growth rates of two diatoms, acidophilic Asterionella ralfsii and circumneutral A. formosa , were differentially affected by varying pH, Al, and EDTA in chemically defined media. Free Al ion concentration increased as pH and EDTA concentration decreased. Free trace metal ion concentration decreased as EDTA levels increased but increased by orde...

1. Fossil diatom assemblages deposited in more than a dozen African lakes roughly 9500 years BP were dominated by a single planktonic species, Stephanodiscus astraea (Ehrcnb.) Grun. (although realistically this is likely to be a species complex). These diatoms flourished when lake-levels were maximal. Data are included from many of (he large Africa...

Three diatom species, Stephanodiscus hantzschii (Ehr.) Grun., Asterionella formosa Hass. and Fragilaria crotonensis Kitt. Hass. were isolated from Lake Maarsseveen where they are dominant and show a successional sequence. The physiological responses of each species to temperature and limitation by silicon and phosphorus were determined over the tem...

Elemental ratios can be very useful tools for characterizing phytoplankton communities in lakes. Appropriate measures of the carbon, nitrogen, silicon, and phosphorus in the various particulate fractions have generally not been collected in a systematic fashion in the past. Resource ratios and turnover rates have been shown to be important factors...