Craig W. Osenberg

Marine reserves: size and age do matter

Marine reserves are widely used throughout the world to prevent overfishing and conserve biodiversity, but uncertainties remain about their optimal design. The effects of marine reserves are heterogeneous. Despite theoretical findings, empirical studies have previously found no effect of size on the effectiveness of marine reserves in protecting commercial fish stocks. Using 58 datasets from 19 European marine reserves, we show that reserve size and age do matter: Increasing the size of the no-take zone increases the density of commercial fishes within the reserve compared with outside; whereas the size of the buffer zone has the opposite effect. Moreover, positive effects of marine reserve on commercial fish species and species richness are linked to the time elapsed since the establishment of the protection scheme. The reserve size-dependency of the response to protection has strong implications for the spatial management of coastal areas because marine reserves are used for spatial zoning.

Are Plant Populations Seed Limited? A Critique and Meta‐Analysis of Seed Addition Experiments

We examine the relative importance of processes that underlie plant population abundance and distribution. Two opposing views dominate the field. One posits that the ability to establish at a site is determined by the availability of suitable microsites (establishment limitation), while the second asserts that recruitment is limited by the availability of seeds (seed limitation). An underlying problem is that establishment and seed limitation are typically viewed as mutually exclusive. We conducted a meta‐analysis of seed addition experiments to assess the relative strength of establishment and seed limitation to seedling recruitment. We asked (1) To what degree are populations seed and establishment limited? (2) Under what conditions (e.g., habitats and life‐history traits) are species more or less limited by each? (3) How can seed addition studies be better designed to enhance our understanding of plant recruitment? We found that, in keeping with previous studies, most species are seed limited. However, the effects of seed addition are typically small, and most added seeds fail to recruit to the seedling stage. As a result, establishment limitation is stronger than seed limitation. Seed limitation was greater for large‐seeded species, species in disturbed microsites, and species with relatively short‐lived seed banks. Most seed addition experiments cannot assess the relationship between number of seeds added and number of subsequent recruits. This shortcoming can be overcome by increasing the number and range of seed addition treatments.

Resolving ecological questions through meta-analysis: Goals, metrics, and models

We evaluate the goals of meta-analysis, critique its recent application in ecology, and highlight an approach that more explicitly links meta-analysis and ecological theory. One goal of meta-analysis is testing null hypotheses of no response to experimental manipulations. Many ecologists, however, are more interested in quantitatively measuring processes and examining their systematic variation across systems and conditions. This latter goal requires a suite of diverse, ecologically based metrics of effect size, with each appropriately matched to an ecological question of interest. By specifying ecological mod- els, we can develop metrics of effect size that quantify the underlying process or response of interest and are insensitive to extraneous factors irrelevant to the focal question. A model will also help to delineate the set of studies that fit the question addressed by the meta- analysis. We discuss factors that can give rise to heterogeneity in effect sizes (e.g., due to differences in experimental protocol, parameter values, or the structure of the models that describe system dynamics) and illustrate this variation using some simple models of plant competition. Variation in time scale will be one of the most common factors affecting a meta-analysis, by introducing heterogeneity in effect sizes. Different metrics will apply to different time scales, and time-series data will be vital in evaluating the appropriateness of different metrics to different collections of studies. We then illustrate the application of ecological models, and associated metrics of effect size, in meta-analysis by discussing and/or synthesizing data on species interactions, mutual interference between consumers, and individual physiology. We also examine the use of metrics when no single, specific model applies to the synthesized studies. These examples illustrate that the diversity of ecological questions demands a diversity of ecologically meaningful metrics of effect size. The successful application of meta-analysis in ecology will benefit by clear and explicit linkages among ecological theory, the questions being addressed, and the metrics used to summarize the available information.

EFFECTS OF BODY SIZE ON THE PREDATOR-PREY INTERACTION BETWEEN PUMPKINSEED SUNFISH AND GASTROPODS'

Body size is known to play a critical role in determining patterns of prey selection. In this study, we examined the diets of pumpkinseed sunfish (Lepomis gibbosus) from three Michigan lakes. Pumpkinseeds have highly developed pharyngeal jaws specialized for crushing gastropods, and in our study lakes gastropods consistently contributed >80% of the prey mass in pumpkinseed diets. However, the average dietary composition and prey selection among snail taxa and size classes varied considerably among dates and sites. We hypothesized that this variation was influenced by changes in the size structure of the snail community. We used laboratory studies to quantify the effect of snail (and fish) size on three important components of the predator—prey interaction: encounter rates, attack probabilities, and capture successes. We then used these laboratory data to predict prey selection observed in the field. For most of our field situations, a simple model based on size—specific encouter rates only) explained a large percent (71%) of the observed variation in prey selection. However, in those cases where some of the snails were resistant to predation, due to crushing resistance or gape limitation, this simple model was a very poor predictor of prey selection. A more complex model (based on encounter rates and size refuges) successfully explained 46% of the variance in these cases where snails were relatively invulnerable. Finally, we compared estimates of attack probabilities with predictions from optimal foraging theory and found qualitative agreement in that fish ignored prey of low profitabilities and became more selective as the quality of the environment improved. however, the incorporation of variable attack probabilities into the foraging model resolved only a small part of the observed residual variation in selectivities because snails with low profitabilities were already underrepresented in the diet due to their low encounter rates or capture successes. This study demonstrates that predator—prey interactions in size—structured populations can create apparently complex variation in prey selection, but that this variation can be largely understood within a framework that simultaneously considers the dynamics of prey size distributions and how components of the foraging interaction scale with body size of the predator and prey.

Effect size in ecological experiments: The application of biological models in meta-analysis

Some of the most interesting and important questions in ecology require examination of the strength of different processes across environmental gradients and among organisms with different traits (Quinn and Dunham 1983; Tilman 1989; Cooper et al. 1990; Sarnelle 1992; Osenberg and Mittelbach 1996). Metaanalysis (see, e.g., Gurevitch et al. 1992; Gurevitch and Hedges 1993; Arnqvist and Wooster 1995; Curtis 1996) combines results from independent studies to examine patterns of effect across taxa or environments and, thus, may represent a powerful tool to test ecological theory. A meta-analysis requires that a common metric of effect size be extracted from each of the studies. Here, we focus on choosing a metric that best facilitates ecological inferences. We begin with a brief description of the standard definition of effect size in meta-analysis, as used in recent papers. We then discuss potential problems with this approach and suggest an alternative that is more explicitly tied to the dynamics of ecological systems. Using two examples drawn from predator-prey experiments, we then illustrate the limitations of the standard metric and the conceptual advantages of one ecologically based alternative. We conclude by discussing the link between metrics of effect size and ecological models.

Increased soil emissions of potent greenhouse gases under increased atmospheric CO2

Increasing concentrations of atmospheric carbon dioxide (CO2) can affect biotic and abiotic conditions in soil, such as microbial activity and water content. In turn, these changes might be expected to alter the production and consumption of the important greenhouse gases nitrous oxide (N2O) and methane (CH4) (refs 2, 3). However, studies on fluxes of N2O and CH4 from soil under increased atmospheric CO2 have not been quantitatively synthesized. Here we show, using meta-analysis, that increased CO2 (ranging from 463 to 780 parts per million by volume) stimulates both N2O emissions from upland soils and CH4 emissions from rice paddies and natural wetlands. Because enhanced greenhouse-gas emissions add to the radiative forcing of terrestrial ecosystems, these emissions are expected to negate at least 16.6 per cent of the climate change mitigation potential previously predicted from an increase in the terrestrial carbon sink under increased atmospheric CO2 concentrations. Our results therefore suggest that the capacity of land ecosystems to slow climate warming has been overestimated.

Marine reserves: fish life history and ecological traits matter.

Marine reserves are assumed to protect a wide range of species from deleterious effects stemming from exploitation. However, some species, due to their ecological characteristics, may not respond positively to protection. Very little is known about the effects of life history and ecological traits (e.g., mobility, growth, and habitat) on responses of fish species to marine reserves. Using 40 data sets from 12 European marine reserves, we show that there is significant variation in the response of different species of fish to protection and that this heterogeneity can be explained, in part, by differences in their traits. Densities of targeted size-classes of commercial species were greater in protected than unprotected areas. This effect of protection increased as the maximum body size of the targeted species increased, and it was greater for species that were not obligate schoolers. However, contrary to previous theoretical findings, even mobile species with wide home ranges benefited from protection: the effect of protection was at least as strong for mobile species as it was for sedentary ones. Noncommercial bycatch and unexploited species rarely responded to protection, and when they did (in the case of unexploited bentho-pelagic species), they exhibited the opposite response: their densities were lower inside reserves. The use of marine reserves for marine conservation and fisheries management implies that they should ensure protection for a wide range of species with different life-history and ecological traits. Our results suggest this is not the case, and instead that effects vary with economic value, body size, habitat, depth range, and schooling behavior.

Perturbation and resilience : a long-term, whole-lake study of predator extinction and reintroduction

This paper presents the results of a long-term study of changing predator densities and cascading effects in a Michigan lake in which the top carnivore, the largemouth bass (Micropterus salmoides), was eliminated in 1978 and then reintroduced in 1986. The elimination of the bass was followed by a dramatic increase in the density of planktivorous fish, the disappearance of large zooplankton (e.g., two species of Daphnia that had his- torically dominated the zooplankton community), and the appearance of a suite of small- bodied cladoceran (zooplankton) species. The system remained in this state until bass were reintroduced. As the bass population increased, the system showed a steady and predictable return to its previous state; planktivore numbers declined by two orders of magnitude, large- bodied Daphnia reappeared and again dominated the zooplankton, and the suite of small- bodied cladocerans disappeared. Within each cladoceran species there was a steady increase in mean adult body size as planktivore numbers declined. Total zooplankton biomass in- creased - 10-fold following the return of large-bodied Daphnia, and water clarity increased significantly with increases in Daphnia biomass and total cladoceran biomass. These changes in community structure and trophic-level biomasses demonstrate the strong impact of removing a single, keystone species, and the capacity of the community to return to its previous state after the species is reintroduced.

META‐ANALYSIS OF MARINE NUTRIENT‐ENRICHMENT EXPERIMENTS: VARIATION IN THE MAGNITUDE OF NUTRIENT LIMITATION

Nutrient bioassay experiments have been performed in many marine and estuarine environments around the world. Although protocols have been relatively uniform, these experiments have yielded mixed results, implicating nitrogen, phosphorus, silica, iron, and several other elements as factors limiting phytoplankton growth. Meta-analysis has the potential to explain much of this variation by exploring the relationship between the magnitude of limitation and various environmental characteristics. We quantified limitation with a simple metric, Δr, that estimates the change in the per unit growth rate of phytoplankton directly attributable to addition of a specific nutrient, such as nitrogen, iron, or phosphorus. Preliminary analyses indicated that experiments lasting ≤1 d exhibited time lags in the numerical response of phytoplankton to nutrient addition, while experiments lasting >7 d confounded nutrient limitation with processes such as increased grazing or depletion of other nutrients. Thus, we restricted th...

The influence of intraguild predation on prey suppression and prey release: a meta-analysis

Intraguild predation (IGP) occurs when one predator species consumes another predator species with whom it also competes for shared prey. One question of interest to ecologists is whether multiple predator species suppress prey populations more than a single predator species, and whether this result varies with the presence of IGP. We conducted a meta-analysis to examine this question, and others, regarding the effects of IGP on prey suppression. When predators can potentially consume one another (mutual IGP), prey suppression is greater in the presence of one predator species than in the presence of multiple predator species; however, this result was not found for assemblages with unidirectional or no IGP. With unidirectional IGP, intermediate predators were generally more effective than the top predator at suppressing the shared prey, in agreement with IGP theory. Adding a top predator to an assemblage generally caused prey to be released from predation, while adding an intermediate predator caused prey populations to be suppressed. However, the effects of adding a top or intermediate predator depended on the effectiveness of these predators when they were alone. Effects of IGP varied across different ecosystems (e.g., lentic, lotic, marine, terrestrial invertebrate, and terrestrial vertebrate), with the strongest patterns being driven by terrestrial invertebrates. Finally, although IGP theory is based on equilibrium conditions, data from short-term experiments can inform us about systems that are dominated by transient dynamics. Moreover, short-term experiments may be connected in some way to equilibrium models if the predator and prey densities used in experiments approximate the equilibrium densities in nature.