Larry B. Crowder

Habitat Structural Complexity and the Interaction Between Bluegills and Their Prey

Structural complexity of the habitat often reduces predatory efficiency by reducing prey capture rates. Prey density is often positively correlated with habitat structure because it pro- vides food and substrate to the prey as well as a relative refuge from predators. Dense structure inhibits foraging, allowing abundant, highly profitable prey to coexist with predators. Sparse structure allows efficient foraging and generally contains few highly profitable prey. This suggests that feeding rates of predators may be maximized at intermediate structure. If this is true, we might also expect predator growth rates to be higher in intermediate structure habitats. Since diet breadth is thought to be related to rates of encounter with profitable prey, we also expect diets of predators to be narrower at intermediate structure than in either sparsely or densely structured habitats. Bluegill sunfish (Lepomis macrochirus) restricted to experimental ponds varying in vegetation density grew better and consumed more prey at intermediate macrophyte density than fish held at either low or high macrophyte densities. Fish at low macrophyte density had narrower diets than expected due to high initial prey availability relative to prey available at intermediate and high macro- phyte density. Fish at high macrophyte density ate fewer, but larger, prey and thus had a narrower diet than expected. Fish predation reduced total prey biomass as well as mean prey size and altered the prey community structure by removing large active invertebrate predators and herbivores with subsequent release of smaller invertebrate predators and herbivores. These changes in prey com- munity structure were also mediated by habitat structure. Habitat structure-food density interactions may be added to temperature and presence of predators as variables that influence the use of resources by fishes.

A Stage‐Based Population Model for Loggerhead Sea Turtles and Implications for Conservation

Management of many species is currently based on an inadequate under- standing of their population dynamics. Lack of age-specific demographic information, particularly for long-lived iteroparous species, has impeded development of useful models. We use a Lefkovitch stage class matrix model, based on a preliminary life table developed by Frazer (1983a), to point to interim management measures and to identify those data most critical to refining our knowledge about the population dynamics of threatened log- gerhead sea turtles (Caretta caretta). Population projections are used to examine the sen- sitivity of Frazers life table to variations in parameter estimates as well as the likely response of the population to various management alternatives. Current management practices appear to be focused on the least responsive life stage, eggs on nesting-beaches. Alternative protection efforts for juvenile loggerheads, such as using turtle excluder devices (TEDs), may be far more effective.

LIFE HISTORIES AND ELASTICITY PATTERNS: PERTURBATION ANALYSIS FOR SPECIES WITH MINIMAL DEMOGRAPHIC DATA

Elasticity analysis is a useful tool in conservation biology. The relative impacts of proportional changes in fertility, juvenile survival, and adult survival on asymptotic population growth λ (where ln(λ) = r, the intrinsic rate of increase) are determined by vital rates (survival, growth, and fertility), which also define the life history characteristics of a species or population. Because we do not have good demographic information for most threatened populations, it is useful to categorize species according to their life history characteristics and related elasticity patterns. To do this, we compared the elasticity patterns generated by the life tables of 50 mammal populations. In age-classified models, the sum of the fertility elasticities and the survival elasticity for each juvenile age-class are equal; thus, age at maturity has a large impact on the contribution of juvenile survival to λ. Mammals that mature early and have large litters (“fast” mammals, such as rodents and smaller carnivores) also...

An index to assess the health and benefits of the global ocean

The ocean plays a critical role in supporting human well-being, from providing food, livelihoods and recreational opportunities to regulating the global climate. Sustainable management aimed at maintaining the flow of a broad range of benefits from the ocean requires a comprehensive and quantitative method to measure and monitor the health of coupled human–ocean systems. We created an index comprising ten diverse public goals for a healthy coupled human–ocean system and calculated the index for every coastal country. Globally, the overall index score was 60 out of 100 (range 36–86), with developed countries generally performing better than developing countries, but with notable exceptions. Only 5% of countries scored higher than 70, whereas 32% scored lower than 50. The index provides a powerful tool to raise public awareness, direct resource management, improve policy and prioritize scientific research.

Sustainability and Global Seafood

Tight coupling to ecosystems and dependence on common-pool resources threaten fisheries and aquaculture. Although seafood is the most highly traded food internationally, it is an often overlooked component of global food security. It provides essential local food, livelihoods, and export earnings. Although global capture fisheries production is unlikely to increase, aquaculture is growing considerably. Sustaining seafoods contributions to food security hinges on the ability of institutions, particularly in developing countries, to protect and improve ecosystem health in the face of increasing pressures from international trade.

Guiding ecological principles for marine spatial planning

The declining health of marine ecosystems around the world is evidence that current piecemeal governance is inadequate to successfully support healthy coastal and ocean ecosystems and sustain human uses of the ocean. One proposed solution to this problem is ecosystem-based marine spatial planning (MSP), which is a process that informs the spatial distribution of activities in the ocean so that existing and emerging uses can be maintained, use conflicts reduced, and ecosystem health and services protected and sustained for future generations. Because a key goal of ecosystem-based MSP is to maintain the delivery of ecosystem services that humans want and need, it must be based on ecological principles that articulate the scientifically recognized attributes of healthy, functioning ecosystems. These principles should be incorporated into a decision-making framework with clearly defined targets for these ecological attributes. This paper identifies ecological principles for MSP based on a synthesis of previously suggested and/or operationalized principles, along with recommendations generated by a group of twenty ecologists and marine scientists with diverse backgrounds and perspectives on MSP. The proposed four main ecological principles to guide MSP--maintaining or restoring: native species diversity, habitat diversity and heterogeneity, key species, and connectivity--and two additional guidelines, the need to account for context and uncertainty, must be explicitly taken into account in the planning process. When applied in concert with social, economic, and governance principles, these ecological principles can inform the designation and siting of ocean uses and the management of activities in the ocean to maintain or restore healthy ecosystems, allow delivery of marine ecosystem services, and ensure sustainable economic and social benefits.

Habitat structure and predator—prey interactions in vegetated aquatic systems

Many common features of aquatic habitats provide complexity in an otherwise unstructured water column. Abiotic elements include consolidated and unconsolidated sediments as well as many types of rock, whereas biotic elements include woody debris, coral and oyster reef formations and emergent and submersed vegetation. Special attention has been focused on submersed vegetation and its role in the functioning of aquatic systems, because of its very frequent occurrence in both freshwater (ponds and streams) and coastal marine ecosystems (estuaries and lagoons) and, more importantly, because animal abundances in vegetated habitats are frequently several orders of magnitude greater than in nearby unvegetated areas (cf. Rosine, 1955; Gerking, 1957; Harrod, 1964 for freshwater sites; Tabb et al., 1962, Thayer et al., 1975; Livingston, 1975 for marine sites). In fact, over 70 years ago Victor Shelford (1918) recognized the importance of plant surface as a factor governing the species composition of the freshwater fauna (cited in Rosine, 1955). At about the same time, Peterson (1918) described the richness of the fauna associated with European seagrass meadows.

Forage Fishes and Their Salmonid Predators in Lake Michigan

Abstract Alewife Alosa pseudoharengus and rainbow smelt Osmerus mordax dominate the planktivorous fish fauna of Lake Michigan and are now the primary food of lake trout Salvelinus namaycush and introduced salmonids. Their fluctuations in abundance have been a concern due to their effects on native species and their present role as forage species. Each has been implicated as an important factor in the local reduction or extinction of important native species. Mechanisms for these interactions include competition for food and predation on eggs and larvae. Bioenergetic modeling simulations of alewife consumption by stocked salmonids suggest that as much as 20 to 33% of the annual alewife production may be consumed in some years. Increasing stocking rates of salmonids in Lake Michigan yield a predator-prey system in which predator numbers become relatively independent of prey dynamics. This suggests possible declines in alewife production, changes in major forage available to predators, and perhaps destabiliz...

Ecosystem impacts of three sequential hurricanes (Dennis, Floyd, and Irene) on the United States' largest lagoonal estuary, Pamlico Sound, NC

Three sequential hurricanes, Dennis, Floyd, and Irene, affected coastal North Carolina in September and October 1999. These hurricanes inundated the region with up to 1 m of rainfall, causing 50- to 500-year flooding in the watershed of the Pamlico Sound, the largest lagoonal estuary in the United States and a key West Atlantic fisheries nursery. We investigated the ecosystem-level impacts on and responses of the Sound to the floodwater discharge. Floodwaters displaced three-fourths of the volume of the Sound, depressed salinity by a similar amount, and delivered at least half of the typical annual nitrogen load to this nitrogen-sensitive ecosystem. Organic carbon concentrations in floodwaters entering Pamlico Sound via a major tributary (the Neuse River Estuary) were at least 2-fold higher than concentrations under prefloodwater conditions. A cascading set of physical, chemical, and ecological impacts followed, including strong vertical stratification, bottom water hypoxia, a sustained increase in algal biomass, displacement of many marine organisms, and a rise in fish disease. Because of the Sounds long residence time (≈1 year), we hypothesize that the effects of the short-term nutrient enrichment could prove to be multiannual. A predicted increase in the frequency of hurricane activity over the next few decades may cause longer-term biogeochemical and trophic changes in this and other estuarine and coastal habitats.

Models to Evaluate Headstarting as a Management Tool for Long‐Lived Turtles

Most turtle species suffer highly mortality in their first year, have a long juvenile period, and can live for decades once they reach adulthood. Conservationists have implemented a number of recovery plans for threatened turtle populations, including experimental headstart programs. Headstarting involves the captive rearing of hatchlings from eggs collected in the wild. The hatchlings are held for several months to help them avoid high mortality in their first year. It is hoped that these turtles survive and grow like wild turtles after release. The purpose of our study was to evaluate headstarting as a management tool for threatened turtle populations. We critically examined the population-level effects of headstarting with a series of deterministic matrix models for yellow mud turtles (Kinosternon flavescens), a non-threatened, well-studied species, endangered Kemps ridley sea turtles (Lepidochelys kempi). We show that management efforts focused exclusively on improving survival in the first year of life are unlikely to be effective for long-lived species such as turtles. Population projections for both turtles predict that head-starting can augment increasing populations when adult survival is returned to or maintained at high levels, provided that headstarted juveniles are as vigorous as wild turtles. However, when subadult and adult survival is reduced, headstarting cannot compensate for losses in later stages. Proportional sensitivity (elasticity) analyses of stage-based matrix models indicated that annual survival rates for subadult and adult turtles are most critical; small decreases in the survival of older turtles can quickly overcome any potential benefits of headstarting. In general, the biological benefits of headstarting programs may be overestimated for turtles, and a careful examination of stage-specific mortality sources, demography, and life history can guide us toward more effective management strategies.