Colin T. Kremer

Environmental variability influences the evolution of phenotypic plasticity and the ecological interactions among species in communities, but it is unclear how these processes interact to affect ecological systems across scales. We investigate models describing how plasticity evolves in competitive communities exposed to seasonal variation in abiotic conditions. We conducted eco-evolutionary analyses to identify communities of coexisting species with evolutionarily stable plasticity traits. These analyses revealed that the presence of interspecific competitors constrained the evolution of plasticity in some species, which instead evolved to be specialists. Plasticity often impaired species coexistence by allowing individual species to control more niche space. Communities containing plastic species were usually less species-rich but more productive and less variable. Similar results were obtained when plasticity rate was evolutionarily labile, but plasticity rate altered the patterning of temporal niche partitioning. This work shows how plasticity and competition combine to shape ecological and trait responses to environmental variability.

The significant contribution of phytoplankton to global primary production is regulated by several abiotic and biotic factors, which are often difficult to account for in natural systems. To address these challenges, we perturbed a summer plankton community from Narragansett Bay, Rhode Island, USA, by manipulating the temperature and nutrient conditions in a controlled short‐term incubation experiment and tracked changes in phytoplankton community structure in response to fluctuations in phytoplankton physiology and microzooplankton grazing. Water was incubated at the in situ temperature (22°C) and at deviations from that temperature (± 4°C) with both macronutrient amendments (N, P, and Si addition) and unamended controls. We found nutrient availability and microzooplankton grazing to have pronounced and opposite impacts on phytoplankton size composition, with nutrient amendments shifting the phytoplankton community toward larger cells and grazing rates correlated with smaller phytoplankton communities and low‐nutrient conditions. Nutrient amendments also altered cellular elemental ratios by increasing chlorophyll : C. Conversely, temperature was not found to have a direct impact on size or elemental stoichiometry, but did influence community composition. These findings paralleled prior observations from lab and field studies in Narragansett Bay which together suggest that over shorter timescales, nutrient availability may have a greater impact on phytoplankton community composition than temperature or grazing, altering phytoplankton nutritional value for higher trophic levels and as a result, secondary production. Thus, understanding underlying nutrient dynamics will be necessary to decipher how short‐term changes in temperature or grazing may impact phytoplankton communities and the ecosystems they support.

Natural enemies are thought to promote coexistence of competing victim species. Although existing theory suggests victim coexistence increases with enemy specialization, the dynamics and potential extinction of enemies is generally discounted. Where enemy dynamics have been considered, empirically atypical linear functional responses have been studied. These limitations could over-simplify inferences about enemy-mediated coexistence. We studied the dynamics of two competing victim species and two enemy species with a deterministic model. We derived equilibrium points, and used linear stability analysis, numerical simulations and Floquet theory to determine the influence of enemy specificity and non-linear functional responses on coexistence in this victim-enemy community. We found greater specificity could drive enemy equilibrium points to infeasible values. We found only accelerating enemy functional responses result in stable equilibrium point coexistence of otherwise equivalent competitor victims, in which case greater specificity results in greater stability. Linear and saturating responses produce complex dynamics (neutral or limit cycles, chaos) or extinction, with limit cycle stability highest at intermediate specificity. Our results indicate strict specificity may not maximize coexistence, and enemy functional response critically influences whether enemies promote victim coexistence. They highlight the need to incorporate enemy dynamics into the growing body of theory regarding enemy-mediated diversity maintenance.

Temperature and resources are fundamental factors that determine the ability of organisms to function and survive, while influencing their individual and population growth. Major bodies of ecological theory have emerged, largely independently, to address temperature and resource effects. It remains a pressing challenge to unite these ideas and determine the interactive effects of temperature and resources on ecological patterns and processes, and their consequences across ecological scales. Here, we propose a simple, physiologically motivated model capturing the interactive effects of temperature and resources (specifically, inorganic nutrients and light) on the growth of microbial ectotherms over multiple ecological scales. From this model we derive a set of key predictions. At the population level, we predict (1) interactive effects of resource limitation on thermal traits (parameters describing effects of temperature on growth), (2) consistent differences in the temperature sensitivity of auto‐ and heterotrophs, and (3) the existence of specific trade‐offs between traits that determine the shape of thermal performance curves. At the community level, we derive predictions for (4) how limitation by nutrients and light can change the relationship between temperature and productivity. All four predictions are upheld, based on our analyses of a large compilation of laboratory data on microbial growth, as well as field experiments with marine phytoplankton communities. Collectively, our modeling framework provides a new way of thinking about the interplay between two fundamental aspects of life—temperature and resources—and how they constrain and structure ecological properties across scales. Providing links between population and community responses to simultaneous changes in abiotic factors is essential to anticipating the multifaceted effects of global change.

Ecosystems are warming alongside shifts in other abiotic factors, leading to interactive effects on populations and communities. This underscores the importance of studying how organisms respond to multiple environmental changes simultaneously. In pelagic ecosystems, as surface waters warm, longer and stronger periods of thermal stratification lead to changes in resource (light and nutrient) availability. We investigate the combined effects of temperature and resource availability on the growth rates of 19 populations (comprising 17 species) of freshwater phytoplankton in order to examine how temperature influences the minimum resource requirements (and Monod parameters) for light, nitrogen, and phosphorus. We also evaluate how resource availability affects each population's thermal traits (i.e., thermal performance curve ‐ TPC ‐ parameters). When averaged across all populations, the requirements for light and phosphorus tended to display a U‐shaped relationship along temperatures. Individual populations varied greatly in their responses to temperature, leading to shifts in the identity of the best competitor across the thermal gradient, particularly for nitrogen and phosphorus. TPC responses to resource limitation were highly variable, but thermal optima and maxima of individual populations often decreased with resource limitation, and thermal breadths (range where growth is 80% or more of its maximum) often increased due to a flattening of TPCs. Across all populations and resource types, the maximum optimum temperature across resource levels (maximum T _opt ) tended to be positively correlated with the temperature at which populations had the lowest resource requirements (minimum R *). However, the temperature at which populations were the best competitors tended to be ~5°C colder on average than the temperature at which they grew the fastest. The populations with the highest thermal optima also had the lowest minimum resource requirements. Our findings reveal trait associations suggesting that some taxa already exhibit trait combinations that would support high performance under future warm and resource‐limited conditions.

Ecosystems are warming alongside shifts in other abiotic factors, leading to interactive effects on populations and communities. This underscores the importance of studying how organisms respond to multiple environmental changes simultaneously. In aquatic ecosystems, as surface waters of lakes and oceans warm, longer and stronger periods of thermal stratification lead to changes in resource (light and nutrient) availability. We investigate the combined effect of temperature and resource availability on 19 populations (comprising 17 species) of freshwater phytoplankton to examine how temperature influences the minimum resource requirements (and Monod parameters) for light, nitrogen, and phosphorus. We also evaluate how resource availability affects each population’s thermal traits (i.e. thermal performance curve -TPC-parameters). When averaged across all populations, the requirements for light and phosphorus tended to display a U-shaped relationship along temperatures. Individual populations varied greatly in their responses to temperature, leading to shifts in the identity of the best competitor across the thermal gradient, particularly for nitrogen and phosphorus. TPC responses to resource limitation were highly variable, but thermal optima and maxima of individual populations often decreased with resource limitation, and thermal breadths (range where growth is 80% or more of its maximum) often increased due to a flattening of TPCs. Across all populations and resource types, the maximum optimum temperature across resource levels (maximum Topt ) tended to be positively correlated with the temperature at which they had the lowest resource requirements (minimum R* ). However, the temperature at which populations were the best competitors tended to be ∼5 °C colder on average than the temperature at which they grew the fastest. The populations with the highest thermal optima also had the lowest minimum resource requirements. Our findings reveal trait associations suggesting that some taxa already exhibit trait combinations that would support high performance under future warm and resource-limited conditions.

The growth of populations and organisms often depends on their previous history of environmental exposure: a phenomenon referred to as “phenotypic memory.” The field of ecology presently lacks a mechanistic theory describing phenotypic memory and, as such, evaluating the ecological consequences of this phenomenon is a major challenge. Here, we show that internal nutrient storage connects past thermal experience to current growth in phytoplankton. We develop a mechanistic model showing that delays in the response of nutrient stores to changing temperatures produces phenotypic memory. By testing this model against experimental data of phytoplankton growth rates following temperature perturbations, we find general patterns in the population consequences of phenotypic memory: Prior exposure to warm temperatures depletes nutrient stores, and, in doing so, slows growth during subsequent temperature exposure and restricts the breadth of the thermal niche (i.e., the range of acute temperature exposures yielding a positive growth rate). Our model reveals how phenotypic memory produces temporal variation in critical thermal minima and maxima and predicts that the thermal niche is constricted by long-term exposure to warm temperatures (e.g., during summer months), but that high frequency temperature fluctuations can expand a population’s thermal niche. This work provides a mechanistic framework for considering the ecological implications of phenotypic memory.

Driven by anthropogenic pressures, antibiotic resistance has become more prevalent within urban bacterial communities over recent decades. In general, resistance is expected to spread from highly impacted areas into surrounding environments as the spatial patterns of resistance reflect the balance between the ability of dispersal and horizontal gene transfer to spread resistance, and the effects of dilution or selection driven by other environmental or ecological factors that change along these gradients. However, the strength and scale of this effect is less clear, especially in coastal environments. We investigated antibiotic resistance of bacterial communities along a transect from a freshwater site in the Los Angeles River (surrounded by high-density urban development) out into the San Pedro Basin in the Pacific Ocean. We determined the minimum concentration of twelve different antibiotics needed to inhibit the growth of bacterial communities across eight sites. We also examined community composition and the prevalence of known genes related to antibiotic resistance from anthropogenic sources in each sample using 16S rRNA sequencing and qPCR. In contrast to our expectations, marine bacteria communities displayed higher resistance than the freshwater LA River community. Rather than the presence of known resistance genes or proximity to urban impacts, patterns of resistance were more strongly associated with the higher species diversity found in marine communities. These unexpected results open new opportunities to determine why marine bacterial communities possess high resistance and the repercussions for potentially pathogenic bacteria and their impacts on the health of humans and wildlife.

Temperature and resources are fundamental factors that determine the ability of organisms to function and survive, while influencing their development, growth, and reproduction. Major bodies of ecological theory have emerged, largely independently, to address temperature and resource effects. It remains a major challenge to unite these ideas and determine the interactive effects of temperature and resources on ecological patterns and processes, and their consequences across ecological scales. Here, we propose a simple, physiologically motivated model capturing the interactive effects of temperature and resources (including inorganic nutrients and light) on the growth of microbial ectotherms over multiple ecological scales. From this model we derive a set of key predictions. At the population level, we predict (i) interactive effects of resource limitation on thermal traits, (ii) consistent differences in the temperature sensitivity of auto- and heterotrophs, and (iii) the existence of specific tradeoffs between traits that determine the shape of thermal performance curves. At the community level, we derive predictions for (iv) how limitation by nutrients and light can change the relationship between temperature and productivity. All four predictions are upheld, based on our analyses of a large compilation of laboratory data on microbial growth, as well as field experiments with marine phytoplankton communities. Collectively, our modeling framework provides a new way of thinking about the interplay between two fundamental aspects of life — temperature and resources — and how they constrain and structure ecological properties across scales. Providing links between population and community responses to simultaneous changes in abiotic factors is essential to anticipating the multifaceted effects of global change.

The extent and ecological significance of intraspecific functional diversity within marine microbial populations is still poorly understood, and it remains unclear if such strain-level microdiversity will affect fitness and persistence in a rapidly changing ocean environment. In this study, we cultured 11 sympatric strains of the ubiquitous marine picocyanobacteriumSynechococcusisolated from a Narragansett Bay (RI) phytoplankton community thermal selection experiment. Thermal performance curves revealed selection at cool and warm temperatures had subdivided the initial population into thermotypes with pronounced differences in maximum growth temperatures. Curiously, the genomes of all 11 isolates were almost identical (average nucleotide identities of >99.99%, with >99% of the genome aligning) and no differences in gene content or single nucleotide variants were associated with either cool or warm temperature phenotypes. Despite a very high level of genomic similarity, sequenced epigenomes for two strains showed differences in methylation on genes associated with photosynthesis. These corresponded to measured differences in photophysiology, suggesting a potential pathway for future mechanistic research into thermal microdiversity. Our study demonstrates that present-day marine microbial populations can harbor cryptic but environmentally relevant thermotypes which may increase their resilience to future rising temperatures.