Investigating cumulative effects across ecological scales

Species, habitats, and ecosystems are increasingly exposed to multiple anthropogenic stressors, fueling a rapidly expanding research program to understand the cumulative impacts of these environmental modifications. Since the 1970s, a growing set of methods has been developed through two parallel, sometimes connected, streams of research within the applied and academic realms to assess cumulative effects. Past reviews of cumulative effects assessment (CEA) methods focused on approaches used by practitioners. Academic research has developed several distinct and novel approaches to conducting CEA. Understanding the suite of methods that exist will help practitioners and academics better address various ecological foci (physiological responses, population impacts, ecosystem impacts) and ecological complexities (synergistic effects, impacts across space and time). We reviewed 6 categories of methods (experimental, meta-analysis, single-species modeling, mapping, qualitative modeling, and multispecies modeling) and examined the ability of those methods to address different levels of complexity. We focused on research gaps and emerging priorities. We found that no single method assessed impacts across the 4 ecological foci and 6 ecological complexities considered. We propose that methods can be used in combination to improve understanding such that multimodel inference can provide a suite of comparable outputs, mapping methods can help prioritize localized models or experimental gaps, and future experiments can be paired from the outset with models they will inform.     

Spatially explicit modeling of overstory manipulations in young forests: Effects on stand structure and light

Young forests can be manipulated in diverse ways to enhance their ecological values. We used stem maps from two dense, second-growth stands in western Washington and a spatially explicit light model (tRAYci) to simulate effects of five silvicultural manipulations on diameter distribution, species composition, spatial patterning, and light availability. Each treatment removed 30% of the basal area, but differed in how trees were selected for removal. Three primary treatments were thin from below (removing the smallest trees), random thin (removing trees randomly), and gap creation (removing all trees in circles ∼1 tree height in diameter). Two additional treatments combined elements of these approaches: random ecological thin (a mixture of thin from below and random thin) and structured ecological thin (a mixture of thin from below and gap creation).     

Physiology of the digestive tract of the sedentary polychaete Terebellides stroemi

The digestive tract of the sedentary polychaete Terebellides stroemi, collected from 88 m depth on the French Catalan Coast, was studied using histological, biochemical, cytological and spectrographic techniques. The ciliated oesophagus possesses a ventral pharyngeal pouch with a muscle bulb. The stomach is divided into two parts, the fore stomach is ciliated, but the hind stomach takes the place of a gizzard with a thick peritrophic membrane and a thick muscle mantle. The digestive gland enters the digestive tract at the boundary of the two stomachal areas; it is comprised of epithelial lamellae with a glandular epithelium. The gland cells contain a spherocrystal secretion with a high calcium and phosphorus content. The cells are also absorbant due to their microvillar brush border and to the storage of glycogen and lipids in the basal epithelium. A biochemical study using the Apizym System confirmed the secretion of enzymes such as phosphatases, esterases and various carbohydrases. Tryptic and chymotryptic enzymes were visualized by biochemical hydrolysis of synthetic substrates. The epithelium of the fore intestine is similar in appearance to the epithelium of the digestive gland, but consists of a single epithelial layer. Hind intestine and rectum are only excretory systems. The presence of this digestive gland does not seem to have been described elsewhere in polychaetes but is similar to the Morren's gland of some oligochaetes. The study was carried out during 1982 and 1983.     

Understanding and predicting ecological dynamics: are major surprises inevitable?

Ecological surprises, substantial and unanticipated changes in the abundance of one or more species that result from previously unsuspected processes, are a common outcome of both experiments and observations in community and population ecology. Here, we give examples of such surprises along with the results of a survey of well-established field ecologists, most of whom have encountered one or more surprises over the course of their careers. Truly surprising results are common enough to require their consideration in any reasonable effort to characterize nature and manage natural resources. We classify surprises as dynamic-, pattern-, or intervention-based, and we speculate on the common processes that cause ecological systems to so often surprise us. A long-standing and still growing concern in the ecological literature is how best to make predictions of future population and community dynamics. Although most work on this subject involves statistical aspects of data analysis and modeling, the frequency and nature of ecological surprises imply that uncertainty cannot be easily tamed through improved analytical procedures, and that prudent management of both exploited and conserved communities will require precautionary and adaptive management approaches.     

Towards a framework for assessment and management of cumulative human impacts on marine food webs

Effective ecosystem‐based management requires understanding ecosystem responses to multiple human threats, rather than focusing on single threats. To understand ecosystem responses to anthropogenic threats holistically, it is necessary to know how threats affect different components within ecosystems and ultimately alter ecosystem functioning. We used a case study of a Mediterranean seagrass (Posidonia oceanica) food web and expert knowledge elicitation in an application of the initial steps of a framework for assessment of cumulative human impacts on food webs. We produced a conceptual seagrass food web model, determined the main trophic relationships, identified the main threats to the food web components, and assessed the components’ vulnerability to those threats. Some threats had high (e.g., coastal infrastructure) or low impacts (e.g., agricultural runoff) on all food web components, whereas others (e.g., introduced carnivores) had very different impacts on each component. Partitioning the ecosystem into its components enabled us to identify threats previously overlooked and to reevaluate the importance of threats commonly perceived as major. By incorporating this understanding of system vulnerability with data on changes in the state of each threat (e.g., decreasing domestic pollution and increasing fishing) into a food web model, managers may be better able to estimate and predict cumulative human impacts on ecosystems and to prioritize conservation actions.     

Insect herbivores, density dependence, and the performance of the perennial herb Solanum carolinense

How insect herbivores affect plant performance is of central importance to basic and applied ecology. A full understanding of herbivore effects on plant performance requires understanding interactions (if any) of herbivore effects with plant density and size because these interactions will be critical for determining how herbivores influence plant population size. However, few studies have considered these interactions, particularly over a wide enough range of densities to detect nonlinear effects. Here we ask whether plant density and herbivores influence plant performance linearly or nonlinearly, how plant density affects herbivore damage, and how herbivores alter density dependence in transitions between plant size classes. In a large field experiment, we manipulated the density of the herbaceous perennial plant Solanum carolinense and herbivore presence in a fully crossed design. We measured plant size, sexual reproduction, and damage to plants in two consecutive years, and asexual reproduction of new stems in the second year, allowing us to characterize both plant performance and rates of transition between plant size classes across years. We found nonlinear effects of plant density on damage. Damage by herbivores and plant density both influenced sexual and asexual reproduction of S. carolinense; these effects were mostly mediated via effects on plant size. Importantly, we found that herbivores altered the pattern of linear density dependence in some transition rates (including survival and asexual reproduction) between plant size classes. These results suggest that understanding the ecological or evolutionary effects of herbivores on plant populations requires consideration of plant density and plant size, because feedbacks between density, herbivores, and plant size may complicate longer-term dynamics.     

Forest conservation delivers highly variable coral reef conservation outcomes

Coral reefs are threatened by human activities on both the land (e.g., deforestation) and the sea (e.g., overfishing). Most conservation planning for coral reefs focuses on removing threats in the sea, neglecting management actions on the land. A more integrated approach to coral reef conservation, inclusive of land-sea connections, requires an understanding of how and where terrestrial conservation actions influence reefs. We address this by developing a land-sea planning approach to inform fine-scale spatial management decisions and test it in Fiji. Our aim is to determine where the protection of forest can deliver the greatest return on investment for coral reef ecosystems. To assess the benefits of conservation to coral reefs, we estimate their relative condition as influenced by watershed-based pollution and fishing. We calculate the cost-effectiveness of protecting forest and find that investments deliver rapidly diminishing returns for improvements to relative reef condition. For example, protecting 2% of forest in one area is almost 500 times more beneficial than protecting 2% in another area, making prioritization essential. For the scenarios evaluated, relative coral reef condition could be improved by 8-58% if all remnant forest in Fiji were protected rather than deforested. Finally, we determine the priority of each coral reef for implementing a marine protected area when all remnant forest is protected for conservation. The general results will support decisions made by the Fiji Protected Area Committee as they establish a national protected area network that aims to protect 20% of the land and 30% of the inshore waters by 2020. Although challenges remain, we can inform conservation decisions around the globe by tackling the complex issues relevant to integrated land-sea planning.     

Asymmetry in community regulation: effects of predators and productivity

Eutrophication and predator additions and extinctions are occurring in ecosystems worldwide. Although theory predicts that both will strongly alter the distribution of biomass in whole communities, empirical evidence has not been consolidated to quantitatively determine whether these theoretical predictions are generally borne out in real ecosystems. Here we analyze data from two types of trophic cascade studies, predator removals in factorial combination with fertilization and observed productivity gradients, to assess the role of top-down and bottom-up forces in structuring multi-trophic communities and compare results from these analyses to those from an extensive database of trophic cascade studies. We find that herbivore biomass declines and plant biomass increases in the presence of predators, regardless of system productivity. In contrast, while plants are increased by fertilization, this effect does not significantly increase herbivores in either the presence or absence of predators. These patterns are consistent among marine, freshwater, and terrestrial ecosystems and are largely independent of study size and duration. Thus, top-down effects of predation are transferred through more trophic levels than are bottom-up effects of eutrophication, showing strong asymmetry in the direction of control of biomass distribution in communities.