Anne K. Salomon

Ecosystem ecology: size-based constraints on the pyramids of life

Biomass distribution and energy flow in ecosystems are traditionally described with trophic pyramids, and increasingly with size spectra, particularly in aquatic ecosystems. Here, we show that these methods are equivalent and interchangeable representations of the same information. Although pyramids are visually intuitive, explicitly linking them to size spectra connects pyramids to metabolic and size-based theory, and illuminates size-based constraints on pyramid shape. We show that bottom-heavy pyramids should predominate in the real world, whereas top-heavy pyramids indicate overestimation of predator abundance or energy subsidies. Making the link to ecological pyramids establishes size spectra as a central concept in ecosystem ecology, and provides a powerful framework both for understanding baseline expectations of community structure and for evaluating future scenarios under climate change and exploitation.

Global regime shift dynamics of catastrophic sea urchin overgrazing

A pronounced, widespread and persistent regime shift among marine ecosystems is observable on temperate rocky reefs as a result of sea urchin overgrazing. Here, we empirically define regime-shift dynamics for this grazing system which transitions between productive macroalgal beds and impoverished urchin barrens. Catastrophic in nature, urchin overgrazing in a well-studied Australian system demonstrates a discontinuous regime shift, which is of particular management concern as recovery of desirable macroalgal beds requires reducing grazers to well below the initial threshold of overgrazing. Generality of this regime-shift dynamic is explored across 13 rocky reef systems (spanning 11 different regions from both hemispheres) by compiling available survey data (totalling 10 901 quadrats surveyed in situ) plus experimental regime-shift responses (observed during a total of 57 in situ manipulations). The emergent and globally coherent pattern shows urchin grazing to cause a discontinuous ‘catastrophic’ regime shift, with hysteresis effect of approximately one order of magnitude in urchin biomass between critical thresholds of overgrazing and recovery. Different life-history traits appear to create asymmetry in the pace of overgrazing versus recovery. Once shifted, strong feedback mechanisms provide resilience for each alternative state thus defining the catastrophic nature of this regime shift. Importantly, human-derived stressors can act to erode resilience of desirable macroalgal beds while strengthening resilience of urchin barrens, thus exacerbating the risk, spatial extent and irreversibility of an unwanted regime shift for marine ecosystems.

Global patterns of kelp forest change over the past half-century

Significance Kelp forests support diverse and productive ecological communities throughout temperate and arctic regions worldwide, providing numerous ecosystem services to humans. Literature suggests that kelp forests are increasingly threatened by a variety of human impacts, including climate change, overfishing, and direct harvest. We provide the first globally comprehensive analysis of kelp forest change over the past 50 y, identifying a high degree of variation in the magnitude and direction of change across the geographic range of kelps. These results suggest region-specific responses to global change, with local drivers playing an important role in driving patterns of kelp abundance. Increased monitoring aimed at understanding regional kelp forest dynamics is likely to prove most effective for the adaptive management of these important ecosystems. 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 in kelp abundances currently exists. Here, we build and analyze a global database of kelp time series spanning the past half-century to assess regional and global trends in kelp abundances. We detected a high degree of geographic variation in trends, with regional variability in the direction and magnitude of change far exceeding a small global average decline (instantaneous rate of change = −0.018 y−1). Our analysis identified declines in 38% of ecoregions for which there are data (−0.015 to −0.18 y−1), increases in 27% of ecoregions (0.015 to 0.11 y−1), and no detectable change in 35% of ecoregions. These spatially variable trajectories reflected regional differences in the drivers of change, uncertainty in some regions owing to poor spatial and temporal data coverage, and the dynamic nature of kelp populations. We conclude that although global drivers could be affecting kelp forests at multiple scales, local stressors and regional variation in the effects of these drivers dominate kelp dynamics, in contrast to many other marine and terrestrial foundation species.

Principles for managing marine ecosystems prone to tipping points

Abstract As climatic changes and human uses intensify, resource managers and other decision makers are taking actions to either avoid or respond to ecosystem tipping points, or dramatic shifts in structure and function that are often costly and hard to reverse. Evidence indicates that explicitly addressing tipping points leads to improved management outcomes. Drawing on theory and examples from marine systems, we distill a set of seven principles to guide effective management in ecosystems with tipping points, derived from the best available science. These principles are based on observations that tipping points (1) are possible everywhere, (2) are associated with intense and/or multifaceted human use, (3) may be preceded by changes in early‐warning indicators, (4) may redistribute benefits among stakeholders, (5) affect the relative costs of action and inaction, (6) suggest biologically informed management targets, and (7) often require an adaptive response to monitoring. We suggest that early action to preserve system resilience is likely more practical, affordable, and effective than late action to halt or reverse a tipping point. We articulate a conceptual approach to management focused on linking management targets to thresholds, tracking early‐warning signals of ecosystem instability, and stepping up investment in monitoring and mitigation as the likelihood of dramatic ecosystem change increases. This approach can simplify and economize management by allowing decision makers to capitalize on the increasing value of precise information about threshold relationships when a system is closer to tipping or by ensuring that restoration effort is sufficient to tip a system into the desired regime.

Persistent millennial-scale shifts in moisture regimes in western Canada during the past six millennia.

Inferences of past climatic conditions from a sedimentary record from Big Lake, British Columbia, Canada, over the past 5,500 years show strong millennial-scale patterns, which oscillate between periods of wet and drier climatic conditions. Higher frequency decadal- to centennial-scale fluctuations also occur within the dominant millennial-scale patterns. These changes in climatic conditions are based on estimates of changes in lake depth and salinity inferred from diatom assemblages in a well dated sediment core. After periods of relative stability, abrupt shifts in diatom assemblages and inferred climatic conditions occur approximately every 1,220 years. The correspondence of these shifts to millennial-scale variations in records of glacial expansion/recession and ice-rafting events in the Atlantic suggest that abrupt millennial-scale shifts are important to understanding climatic variability in North America during the mid- to late Holocene. Unfortunately, the spatial patterns and mechanisms behind these large and abrupt swings are poorly understood. Similar abrupt and prolonged changes in climatic conditions today could pose major societal challenges for many regions.

SERIAL DEPLETION OF MARINE INVERTEBRATES LEADS TO THE DECLINE OF A STRONGLY INTERACTING GRAZER

We investigated the relative roles of natural factors and shoreline harvest leading to recent declines of the black leather chiton (Katharina tunicata) on the outer Kenai Peninsula, Alaska (U.S.A.). This intertidal mollusk is a strongly interacting grazer and a culturally important subsistence fishery for Sugpiaq (Chugach Alutiiq) natives. We took multiple approaches to determine causes of decline. Field surveys examined the significant predictors of Katharina density and biomass across 11 sites varying in harvest pressure, and an integrated analysis of archaeological faunal remains, historical records, traditional ecological knowledge, and contemporary subsistence invertebrate landings examined changes in subsistence practices through time. Strong evidence suggests that current spatial variation in Katharina density and biomass is driven by both human exploitation and sea otter (Enhydra lutris) predation. Traditional knowledge, calibrated by subsistence harvest data, further revealed that several benthic marine invertebrates (sea urchin, crab, clams, and cockles) have declined serially beginning in the 1960s, with reduced densities and sizes of Katharina being the most recent. The timing of these declines was coincident with changes in human behavior (from semi-nomadic to increasingly permanent settlement patterns, improved extractive technologies, regional commercial crustacean exploitation, the erosion of culturally based season and size restrictions) and with the reestablishment of sea otters. We propose that a spatial concentration in shoreline collection pressure through time, increased harvest efficiency, and the serial depletion of alternative marine invertebrate prey have led to intensified per capita predator impacts on Katharina and thus its recent localized decline.

Ancient Clam Gardens Increased Shellfish Production: Adaptive Strategies from the Past Can Inform Food Security Today

Maintaining food production while sustaining productive ecosystems is among the central challenges of our time, yet, it has been for millennia. Ancient clam gardens, intertidal rock-walled terraces constructed by humans during the late Holocene, are thought to have improved the growing conditions for clams. We tested this hypothesis by comparing the beach slope, intertidal height, and biomass and density of bivalves at replicate clam garden and non-walled clam beaches in British Columbia, Canada. We also quantified the variation in growth and survival rates of littleneck clams (Leukoma staminea) we experimentally transplanted across these two beach types. We found that clam gardens had significantly shallower slopes than non-walled beaches and greater densities of L. staminea and Saxidomus giganteus, particularly at smaller size classes. Overall, clam gardens contained 4 times as many butter clams and over twice as many littleneck clams relative to non-walled beaches. As predicted, this relationship varied as a function of intertidal height, whereby clam density and biomass tended to be greater in clam gardens compared to non-walled beaches at relatively higher intertidal heights. Transplanted juvenile L. staminea grew 1.7 times faster and smaller size classes were more likely to survive in clam gardens than non-walled beaches, specifically at the top and bottom of beaches. Consequently, we provide strong evidence that ancient clam gardens likely increased clam productivity by altering the slope of soft-sediment beaches, expanding optimal intertidal clam habitat, thereby enhancing growing conditions for clams. These results reveal how ancient shellfish aquaculture practices may have supported food security strategies in the past and provide insight into tools for the conservation, management, and governance of intertidal seascapes today.

CASCADING EFFECTS OF FISHING CAN ALTER CARBON FLOW THROUGH A TEMPERATE COASTAL ECOSYSTEM

Mounting evidence suggests that fishing can trigger trophic cascades and alter food web dynamics, yet its effects on ecosystem function remain largely unknown. We used the large-scale experimental framework of four marine reserves, spanning an oceanographic gradient in northeastern New Zealand, to test the extent to which the exploitation of reef predators can alter kelp carbon flux and secondary production. We provide evidence that the reduction of predatory snapper (Pagrus auratus) and lobster (Jasus edwardsii) can lead to an increase in sea urchins (Evechinus chloroticus) and indirect declines in kelp biomass in some locations but not others. Stable carbon isotope ratios (delta13C) of oysters (Crassostrea gigas) and mussels (Perna canaliculus) transplanted in reserve and fished sites within four locations revealed that fishing indirectly reduced the proportion of kelp-derived organic carbon assimilated by filter feeders in two locations where densities of actively grazing sea urchins were 23.7 and 8.3 times higher and kelp biomass was an order of magnitude lower than in non-fished reserve sites. In contrast, in the two locations where fishing had no effect on urchin density or kelp biomass, we detected no effect of fishing on the carbon signature of filter feeders. We show that the effects of fishing on nearshore trophic structure and carbon flux are context-dependent and hinge on large-scale, regional oceanographic factors. Where cascading effects of fishing on kelp biomass were documented, enhanced assimilation of kelp carbon did not result in the magnification of secondary production. Instead, a strong regional gradient in filter feeder growth emerged, best predicted by chlorophyll a. Estimates of kelp contribution to the diet of transplanted consumers averaged 56.9% +/- 6.2% (mean +/- SE) for mussels and 33.8% +/- 7.3% for oysters, suggesting that organic carbon fixed by kelp is an important food source fueling northeastern New Zealands nearshore food webs. The importance of predators in mediating benthic primary production and organic carbon flux suggests that overfishing can have profound consequences on ecosystem functioning particularly where pelagic primary production is limiting. Our results underscore the broader ecosystem repercussions of overfishing and its context-dependent effects.

Regional ecological variability and impact of the maritime fur trade on nearshore ecosystems in southern Haida Gwaii (British Columbia, Canada): evidence from stable isotope analysis of rockfish (Sebastes spp.) bone collagen

The maritime fur trade (1785–1840s) led to the local extinction of sea otters (Enhydra lutris) in many parts of the northeast Pacific. On the basis of studies of extant sea otter populations, it has been established that they have a disproportionate effect on nearshore ecosystems by limiting sea urchin abundance and facilitating the establishment of nearshore kelp forests; in the absence of sea otters, a local reduction in kelp-derived carbon is therefore expected. We measured the isotopic composition (δ13C and δ15N) of rockfish (Sebastes spp.) bone collagen from late Holocene archaeological sites in southern Haida Gwaii, BC, Canada, using δ13C as a proxy for kelp-derived carbon in the diet and δ15N as a proxy for trophic position. We observed significant spatial variability in rockfish kelp-derived carbon (δ13C), but not trophic level (δ15N). Kelp-derived carbon varied largely as a function of site characteristics (wave exposure), suggesting that local oceanographic conditions are important factors with respect to consumer tissue isotopic compositions. Kelp-derived carbon decreased in post-European contact rockfish relative to pre-European contact rockfish, likely as a result of the reduction of kelp forests associated with the local extirpation of sea otters. Although we detected a reduction in kelp-derived carbon in rockfish diets, we found no shift in trophic level at sites occupied following the maritime fur trade. This implies a shift in local ecosystems, and particularly in carbon sources, following the maritime fur trade, likely due to a trophic cascade resulting from the local extirpation of sea otters. Stability in rockfish trophic levels, however, implies that rockfish continued to feed at similar trophic levels, consuming prey with similar nitrogen isotopic compositions.

Ancient Shellfish Mariculture on the Northwest Coast of North America

While there is increasing recognition among archaeologists of the extent to which non-agricultural societies have managed their terrestrial ecosystems, the traditional management of marine ecosystems has largely been ignored. In this paper, we bring together Indigenous ecological knowledge, coastal geomorphological observations, and archaeological data to document how Northwest Coast First Nations cultivated clams to maintain and increase productivity. We focus on “clam gardens,” walled intertidal terraces constructed to increase bivalve habitat and productivity. Our survey and excavations of clam gardens in four locations in British Columbia provide insights into the ecological and social context, morphology, construction, and first reported ages of these features. These data demonstrate the extent of traditional maricultural systems among coastal First Nations and, coupled with previously collected information on terrestrial management, challenge us to broaden our definition of “forager” as applied to Northwest Coast peoples. This study also highlights the value of combining diverse kinds of knowledge, including archaeological data, to understand the social and ecological contexts of traditional management systems.