Mark Novak

Oregon State University | OSU · Department of Integrative Biology

A long-standing debate concerns how functional responses are best described. Theory suggests that ratio dependence is consistent with many food web patterns left unexplained by the simplest prey-dependent models. However, for logistical reasons, ratio dependence and predator dependence more generally have seen infrequent empirical evaluation and th...

The community matrix is among ecology’s most important mathematical abstractions, formally encapsulating the interconnected network of effects that species have on one another’s populations. Despite its importance, the term “community matrix” has been applied to multiple types of matrices that have differing interpretations. This has hindered the a...

Intraspecific variation in ecologically relevant traits is widespread. In generalist predators in particular, individual diet specialization is likely to have important consequences for food webs. Understanding individual diet specialization empirically requires the ability to quantify individual diet preferences accurately. Here we compare the cur...

Many populations consist of individuals that differ substantially in their diets. Quantification of the magnitude and temporal consistency of such intraspecific diet variation is needed to understand its importance, but the extent to which different approaches for doing so reflect instantaneous vs. time-aggregated measures of individual diets may b...

Intraguild predation theory centres on two predictions: (i) for an omnivore and an intermediate predator (IG-prey) to coexist on shared resources, the IG-prey must be the superior resource competitor, and (ii) increasing resource productivity causes the IG-prey's equilibrium abundance to decline. I tested these predictions with a series of species-...

Understanding coexistence within community modules such as intraguild predation (IGP), where an omnivore both preys on and competes with an intermediate consumer for a shared resource, has provided insight into the mechanisms that promote the persistence of complex food webs. Adaptive, predator-specific defense has been shown theoretically to enhan...

Significance Kelp forests are declining worldwide due to varied combinations of environmental change and the trophic downgrading of urchin-controlling predators. These processes have increased the frequency and extent of rapid, nonlinear shifts to so-called urchin barrens whose ecological functioning and services are reduced relative to those of ke...

Historical resurveys of ecological communities are important for placing the structure of modern ecosystems in context. Rarely, however, are surveys alone sufficient for providing direct insight into the rates of ecological processes that underlie how communities function, either now or in the past. In this study, I used a statistically-reasoned ob...

While the use of networks to understand how complex systems respond to perturbations is pervasive across scientific disciplines, the uncertainty associated with estimates of pairwise interaction strengths (edge weights) remains rarely considered. Mischaracterizations of interaction strength can lead to qualitatively incorrect predictions regarding...

The assessment of relative model performance using information criteria like AIC and BIC has become routine among functional-response studies, reflecting trends in the broader ecological literature. Such information criteria allow comparison across diverse models because they penalize each model's fit by its parametric complexity—in terms of their...

Stable isotopes are increasingly being used to unlock the wealth of information contained in specimens preserved in museum collections. However, preservation methods that employ formalin may confound ecological interpretations. To quantify the effects of formalin fixation and subsequent fluid storage in ethanol on the isotopic signatures of small m...

Collective behaviour is common in bacteria, plants and animals, and therefore occurs across ecosystems, from biofilms to cities. With collective behaviour, social interactions among individuals propagate to affect the behaviour of groups, whereas group-level responses in turn affect individual behaviour. These cross-scale feedback loops between ind...

The assessment of relative model performance using information criteria like AIC and BIC has become routine among functional-response studies, reflecting trends in the broader ecological literature. Such information criteria allow comparison across diverse models because they penalize each model’s fit by its parametric complexity — in terms of thei...

1. Theory suggests that intraspecific trait variation will alter species interaction strengths through nonlinear averaging when interaction strengths are nonlinear functions of individuals' traits. This effect is expected to be widespread, yet what factors mediate its magnitude in nature and hence its potential effects on ecosystems and communities...

Functional responses are a cornerstone to our understanding of consumer–resource interactions, so how to best describe them using models has been actively debated. Here we focus on the consumer dependence of functional responses to evidence systematic bias in the statistical comparison of functional‐response models and the estimation of their param...

Functional responses relate a consumer's feeding rates to variation in its abiotic and biotic environment, providing insight into consumer behaviour and fitness, and underpinning population and food‐web dynamics. Despite their broad relevance and long‐standing history, we show here that the types of density dependence found in classic resource‐ and...

1) Although parasites are increasingly recognized for their ecosystem roles, it is often assumed that free‐living organisms dominate animal biomass in most ecosystems and therefore provide the primary pathways for energy transfer. 2) To examine the contributions of parasites to ecosystem energetics in freshwater streams, we quantified the standing...

Functional responses are a cornerstone to our understanding of consumer-resource interactions, so how to best describe them using models has been actively debated. Here we focus on the consumer dependence of functional responses to evidence systematic bias in the statistical comparison of functional-response models and the estimation of their param...

Functional responses relate a consumer's feeding rates to variation in its abiotic and biotic environment, providing insight into consumer behavior and fitness, and underpinning population and food-web dynamics. Despite their broad relevance and long-standing history, we show here that the types of density dependence found in classic resource- and...

To support conservation practices, societal demand for understanding fundamental coastal ocean ecosystem mechanisms has grown in recent decades. Globally, these regions are among the world’s most productive, but they are highly vulnerable to extractive and non-extractive stresses. In 1999, we established the Partnership for Interdisciplinary Studie...

A major goal of the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) has been to understand the impacts of climate change and variability on the coastal ecosystems of the inner shelf of the California Current Large Marine System in particular, and other marine and even nonmarine systems more generally. Insights can result from de...

Despite progressive policies and continued advances in ocean management, numerous shifts associated with global changes have been observed in marine ecosystems in recent years, including warming, ocean acidification, and deoxygenation. As global change accelerates, science is needed to inform evidence-based management strategies for continued ecosy...

As the reality of climate change became obvious during the late 1900s, the need for understanding ecosystem pattern and dynamics at large scales and for long periods became increasingly evident. This realization inspired the genesis of the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO). PISCO research aimed to quantify intertid...

Species interactions in food webs are usually recognized as dynamic, varying across species, space and time due to biotic and abiotic drivers. Yet food webs also show emergent properties that appear consistent, such as a skewed frequency distribution of interaction strengths (many weak, few strong). Reconciling these two properties requires an unde...

Successfully predicting the future states of systems that are complex, stochastic and potentially chaotic is a major challenge. Model forecasting error (FE) is the usual measure of success; however model predictions provide no insights into the potential for improvement. In short, the realized predictability of a specific model is uninformative abo...

Intraspecific variation is increasingly recognized as an important factor in ecological interactions, sometimes exceeding the role of interspecific variation. Few studies, however, have examined how intra- versus interspecific variation affect trophic interactions over time within a seasonally dynamic food web. We collected stomach contents from 2,...

Species interactions in food webs are usually recognized as dynamic, varying across species, space and time due to biotic and abiotic drivers. Yet food webs also show emergent properties that appear consistent, such as a skewed frequency distribution of interaction strengths (many weak, few strong). Reconciling these two properties requires an unde...

Successfully predicting the future states of systems that are complex, stochastic and potentially chaotic is a major challenge. Model forecasting error (FE) is the usual measure of success; however model predictions provide no insights into the potential for improvement. In short, the realized predictability of a specific model is uninformative abo...

We consider the goal of predicting how complex networks respond to chronic (press) perturbations when characterizations of their network topology and interaction strengths are associated with uncertainty. Our primary result is the derivation of exact formulas for the expected number and probability of qualitatively incorrect predictions about a sys...

Describing the mechanisms that drive variation in species interaction strengths is central to understanding, predicting, and managing community dynamics. Multiple factors have been linked to trophic interaction strength variation, including species densities, species traits, and abiotic factors. Yet most empirical tests of the relative roles of mul...

Estimates of predator feeding rates are important for understanding trophic dynamics. One common method for quantifying feeding rates in fishes combine mass-based diet data with gastric evacuation times to estimate prey mass consumed per predator. An alternative approach is to estimate rates of prey individuals consumed using prey identification ti...

Considerable effort has been devoted to the estimation of species interaction strengths. This effort has focused primarily on statistical significance testing and obtaining point estimates of parameters that contribute to interaction strength magnitudes, leaving the characterization of uncertainty associated with those estimates unconsidered. We co...

In diverse tropical webs, trophic cascades are presumed to be rare, as species interactions may dampen top-down control and reduce their prevalence. To test this hypothesis, we used an open experimental design in the Galápagos rocky subtidal that enabled a diverse guild of fish species, in the presence of each other and top predators (sea lions and...

Locations of research sites in the Galápagos Islands. (PDF)

Summary of parameters of Beddington-DeAngelis functional response model fitted to pencil urchin survivorship in the trophic cascade experiment (Fig 5A) to quantify hogfish and top predator interference rates. The parameters a and h respectively represent the per capita attack rate and handling time with which triggerfish consume urchins, and the pa...

Photo sequence of hogfish interference with blunthead triggerfish foraging on a pencil urchin during the Eucidaris TC experiment on June 25, 2012. Photos are taken at 1 second intervals. Triggerfish has an urchin in mouth in A, but then drops it in C after a close pass from the circling hogfish. Circular bases are 0.31 m2 area for scale. (TIFF)

Photos of top predator effects. A-C. represents a time series taken at 1 second intervals during the Eucidaris TC experiment showing interference effects of a diving sea lion. Note that the school of zooplanktivorous scissortail damselfish (Chromis atrilobata) are high above the sea floor in A, but start to descend in B, as the sea lion above them...

List of 16 species of known sea urchin predators in the Galápagos Islands. The source of diet information is listed, * representing a direct feeding observation made by J. Witman or F. Smith., 1 = www.fishbase.org, 2 = Grove JS, Lavenberg RJ. 1997, 3 = Humann P, DeLoach N. 2002 4 = Dee LE, Witman JD, Brandt. 2012, 5 = http://www.iucnredlist.org/, 6...

Observations of behavioral interactions between adult hogfish Bodianus diplotaenia and triggerfish preying on tethered pencil urchins Eucidaris galapagensis during overnight trials (2008). Data from time lapse images taken at 2 minute intervals. Urchin survivorship data from these trials are plotted in Fig 2C. (PDF)

Photos of predation on pencil urchins either during the tethering (A-E) or trophic cascade experiments. A & B illustrate adult hogfish attacking small pencil urchins during tethering experiments at Baltra South (A) and Rocas Gordon site (B). A finescale triggerfish is preying on a large Eucidaris urchin in C. during a trial at Baltra South on June...

Temperature regime during the trophic cascade experiments. Temperatures were recorded at 5 minute intervals throughout each TC experiment by an Onset Tidbit data logger (Onset Computer Corporation, Pocasset, Massachusetts, USA, +/- 0.01°C precision) attached to one of the control bases. The temperature record for the pencil urchin TC experiment ran...

Video illustrating hogfish interference with triggerfish predation on a pencil urchin (E. galapagensis) during the trophic cascade experiment. It was made from 80 seconds of a triggerfish attack on a pencil urchin in an open (fence) treatment, where an adult hogfish enters the area 25 seconds after the triggerfish. The hogfish swims closely around...

As the contribution for long-term ecological and environmental studies (LTEES) to our understanding of how species and ecosystems respond to a changing global climate becomes more urgent, the relative number and investment in LTEES are declining. To assess the value of LTEES to advancing the field of ecology, we evaluated relationships between cita...

One of the twenty-first century's greatest environmental challenges is to recover and restore species, habitats and ecosystems. The decision about how to initiate restoration is best-informed by an understanding of the linkages between ecosystem components and, given these linkages, an appreciation of the consequences of choosing to recover one eco...

A longstanding debate concerns whether functional responses are best described by prey-dependent versus ratio-dependent models. Theory suggests that ratio dependence can explain many food web patterns left unexplained by simple prey-dependent models. However, for logistical reasons, ratio dependence and predator dependence more generally have seen...

Kelp forests (Order Laminariales) form key biogenic habitats in coastal regions of temperate and Arctic seas worldwide, providing ecosystem services valued in the range of billions of dollars annually. Although local evidence suggests that kelp forests are increasingly threatened by a variety of stressors, no comprehensive global analysis of change...

We consider the goal of predicting how complex networks respond to chronic (press) perturbations when characterizations of their network topology and interaction strengths are associated with uncertainty. Our primary result is the derivation of exact formulas for the expected number and probability of qualitatively incorrect predictions about a sys...

Habitat loss, overexploitation, and numerous other stressors have caused global declines in apex predators. This “trophic downgrading” has generated widespread concern because of the fundamental role that apex predators can play in ecosystem functioning, disease regulation, and biodiversity maintenance. In attempts to combat declines, managers have...

All ecosystems are subjected to chronic disturbances, such as harvest, pollution, and climate change. The capacity to forecast how species respond to such press perturbations is limited by our imprecise knowledge of pairwise species interaction strengths and the many direct and indirect pathways along which perturbations can propagate between speci...

Considerable effort has been devoted to the empirical estimation of species interaction strengths. This effort has focused primarily on statistical significance testing and on obtaining point estimates of parameters that contribute to interaction strength magnitude, leaving characterizations of estimation uncertainty and distinctions between the de...

Considerable effort has been devoted to the empirical estimation of species interaction strengths. This effort has focused primarily on statistical significance testing and on obtaining point estimates of parameters that contribute to interaction strength magnitude, leaving characterizations of estimation uncertainty and distinctions between the de...

Much of the focus in evolutionary biology has been on the adaptive differentiation among organisms. It is equally important to understand the processes that result in similarities of structure among systems. Here, we discuss examples of similarities occurring at different ecological scales, from predator-prey relations (attack rates and handling ti...

Knowing how time is distributed within a fossil record is fundamental to paleobiology. Many efforts to quantify temporal resolution have estimated rates of specimen decay from the frequency distribution of specimen ages in near-surface assemblages. The implicit assumption has been that the shape of these distributions is invariant with depth and th...

University of Chicago PhD Dissertation 2008

Ecological network models and analyses are recognized as valuable tools for understanding the dynamics and resiliency of ecosystems, and for informing ecosystem-based approaches to management. However, few databases exist that can provide the life history, demographic and species interaction information necessary to parameterize ecological network...

Background/Question/Methods The complexity of ecological networks has long plagued attempts to predict species responses to perturbations. The more speciose and connected the network, the lower the confidence in model predictions, unless accurate estimates of species interaction strengths are known. Low predictive certainty results from the many...

Background/Question/Methods Species introductions can drastically alter population, community and ecosystem-level properties of invaded systems. As predators play a central role in prey population dynamics, it is essential to determine whether novel predators alter predation-mediated regulatory mechanisms, potentially destabilizing prey population...

San Nicolas Island is surrounded by broad areas of shallow subtidal habitat, characterized by dynamic kelp forest communities that undergo dramatic and abrupt shifts in community composition. Although these reefs are fished, the physical isolation of the island means that they receive less impact from human activities than most reefs in Southern Ca...

Background/Question/Methods Animal populations often consist of individuals that vary substantially in their diets. Quantification of the strength and temporal consistency of such intraspecific diet specialization is needed to understand its effects on predator-prey dynamics and community structure. Two approaches – stable isotope ratios and gut...

Background/Question/Methods Many ecological communities exhibit alternative states characterized by fundamentally different species abundances, with one state often being less desirable than the other. The threat of tipping points between these states -- whereby even small, incrememental changes in a driving variable can lead to large-scale reorg...

Patterns of species interactions affect the dynamics of food webs. An important component of species interactions that is rarely considered with respect to food webs is the strengths of interactions, which may affect both structure and dynamics. In natural systems, these strengths are variable, and can be quantified as probability distributions. We...

Predators sometimes provide biotic resistance against invasions by nonnative prey. Understanding and predicting the strength of biotic resistance remains a key challenge in invasion biology. A predator's functional response to nonnative prey may predict whether a predator can provide biotic resistance against nonnative prey at different prey densit...