Richard R. Veit

Partial Differential Equations in Ecology: Spatial Interactions and Population Dynamics

Most of the fundamental elements of ecology, ranging from individual be- havior to species abundance, diversity, and population dynamics, exhibit spatial variation. Partial differential equation models provide a means of melding organism movement with population processes and have been used extensively to elucidate the effects of spatial variation on populations. While there has been an explosion of theoretical advances in partial differential equation models in the past two decades, this work has been generally neglected in mathematical ecology textbooks. Our goal in this paper is to make this literature accessible to experimental ecologists. Partial differential equations are used to model a variety of ecological phenomena; here we discuss dispersal, ecological invasions, critical patch size, dispersal-mediated coexis- tence, and diffusion-driven spatial patterning. These models emphasize that simple organ- ism movement can produce striking large-scale patterns in homogeneous environments, and that in heterogeneous environments, movement of multiple species can change the outcome of competition or predation.

Dispersal, Population Growth, and the Allee Effect: Dynamics of the House Finch Invasion of Eastern North America

Since about 1940, when they were first released in the New York City area, house finches (Carpodacus mexicanus) have multiplied explosively and colonized much of eastern North America. We take advantage of the richly detailed documentation of this biological invasion to construct a mathematical model that predicts the rate of population spread on the basis of readily measurable demographic parameters. We seek to improve on previous models by predicting a rate of spread that accelerates following an initial period of slower growth, a pattern of spread followed by house finches as well as a variety of other invading species. We postulate that an Allee effect-disproportionately lowered fecundity below a critical threshold density of abundance-is the mechanism leading to a slower rate of spread in the early stages of the invasion. Our integrodifference equation model also emphasizes the link between long-distance dispersal and the rate of population spread.

Tracking Ocean Wanderers: The Global Distribution of Albatrosses and Petrels

Abstract The following critiques express the opinions of the individual evaluators regarding the strengths, weaknesses, and value of the books they review. As such, the appraisals are subjective assessments and do not necessarily reflect the opinions of the editors or any official policy of the American Ornithologists’ Union.

Aggregation patterns of pelagic predators and their principal prey, Antarctic krill, near South Georgia

We examined the spatial distributions of pelagic seabirds and fur seals near South Georgia, and asked to what extent the distributions of these predators were influenced by the spatial distribution of their principal prey, Antarctic krill Euphausia superba Dana. One novel aspect of our analysis is an explicit consideration of the separation in space between swarms of krill and aggregations of predators that feed upon krill. Our data were collected in February 1986, during a systematic shipboard survey of the waters surrounding Bird Island, South Georgia. Predator abundance was estimated visually using strip transects, and krill abundance was simultaneously estimated using a hull-mounted echosounder

BLACK‐BROWED ALBATROSSES FORAGING ON ANTARCTIC KRILL: DENSITY‐DEPENDENCE THROUGH LOCAL ENHANCEMENT?

Many Antarctic seabirds depend on prey that are patchy, cryptic, ephemeral, and unpredictable in location. These predators typically employ two alternative behavioral strategies for locating resource patches: direct visual or olfactory detection, and indirect detection (local enhancement) by sighting other predators that are already exploiting a patch. We developed a model of direct detection and local enhancement in seabirds that predicts how foraging success varies with behavioral strategy, seabird densities, and prey swarm density and detectability. Application of the model to Black-browed Albatrosses foraging for Antarctic krill near South Georgia suggests that local enhancement is generally a highly effective foraging strategy, and that the fraction of time albatrosses spend in feeding flocks should show strong interactions between prey and conspecific densities. To test these predictions, we analyzed survey data collected near South Georgia in January-March 1986. Our analysis suggests a strong Allee-type density dependence in foraging success that was qualitatively and quantitatively consistent with model predictions. This density dependence suggests a potential for destabilizing patterns of resource utilization and reproductive suc- cess in Black-browed Albatrosses that may have important implications for conservation of albatrosses and other Antarctic species.

Migrations and Winter Quarters of Five Common Terns Tracked using Geolocators

Abstract. Ten geolocators (light-level data loggers) were attached to Common Terns (Sterna hirundo) at a breeding site in the northeastern USA in 2007 and 2008; six were retrieved (five with useful data) in 2008 and 2009. The birds wintered in four discrete areas on the north and east coasts of South America, from Guyana (6–7°N) to northeastern Argentina (36–42°S); three remained within restricted areas for most or all of the winter, whereas two ranged more widely. They left the breeding area at various dates between 1 August and 14 September; three migrated directly from the breeding area while two first moved southwest to stage near Cape Hatteras. All five birds flew directly to the vicinity of Puerto Rico, then moved along the north and east coasts of South America, staging at scattered locations for periods of 3–11 d, before reaching their winter quarters at various dates from 6 September to 26 October. Two birds left their winter quarters on 2 March and 4 April, staged in northern Brazil for 47 and 6 d, then traveled via the Bahamas to reach the breeding site on 1 May. During breeding and post-breeding periods, the birds spent a mean of 7 min each day and virtually no time at night resting on the water, but during the rest of the year they often rested on the water for up to 6 h by day and up to 11 h at night. Leg-mounted geolocators caused several adverse effects but did not reduce survival.

Feeding Ecology of Long-Tailed Ducks Clangula hyemalis Wintering on the Nantucket Shoals

Abstract. A substantial proportion, perhaps 30%, of the North American breeding population of Long-tailed Ducks (Clangula hyemalis) winter in the vicinity of Nantucket Island, Massachusetts. These birds spend the night on Nantucket Sound and commute during daylight hours to the Nantucket Shoals, which extend about 65 km offshore from the southeastern corner of Nantucket. Strip transects done from a single-engine plane in 1997 and 1998 indicated that Long-tailed Ducks foraged over the shallower (≤ 20 m depth) portions of the Nantucket Shoals, up to 70 km offshore. Diet analyses of ten birds collected in February 1999 and five in December 2006 showed that they fed principally (106.6 +/- 42.0 individuals per crop) on Gammarus annulatus, a pelagic amphipod that often forms large aggregations, and is consumed by several species of fish and marine mammals. Our findings emphasize the importance of conservation of the Nantucket Shoals and the prevention of oil spills or other potentially harmful accidents.

Climate and changing winter distribution of alcids in the Northwest Atlantic

Population level impacts upon seabirds from changing climate are increasingly evident, and include effects on phenology, migration, dispersal, annual survivorship and reproduction. Most population data on seabirds derive from nesting colonies; documented climate impacts on winter ecology are scarce. We studied interannual variability in winter abundance of six species of alcids (Charadriiformes, Alcidae) from a 58-year time series of data collected in Massachusetts 1954-2011. We used counts of birds taken during fall and winter from coastal vantage points. Counts were made by amateur birders, but coverage was consistent in timing and location. We found significant association between winter abundance of all six species of alcids and climate, indexed by North Atlantic Oscillation (NAO), at two temporal scales: 1.) Significant linear trends at the 58-year scale of the time series; and 2.) Shorter term fluctuations corresponding to the 5-8 year periodicity of NAO. Thus, variation in winter abundance of all six species of alcids was significantly related to the combined short-term and longer-term components of variation in NAO. Two low-Arctic species (Atlantic Puffin and Black Guillemot) peaked during NAO positive years, while two high Arctic species (Dovekie and Thick-billed Murre) peaked during NAO negative years. For Common Murres and Razorbills, southward shifts in winter distribution have been accompanied by southward expansion of breeding range, and increase within the core of the range. The proximate mechanism governing these changes is unclear, but, as for most other species of seabirds whose distributions have changed with climate, seems likely to be through their prey.

Predicting the offshore distribution and abundance of marine birds with a hierarchical community distance sampling model

Proposed offshore wind energy development on the Atlantic Outer Continental Shelf has brought attention to the need for baseline studies of the distribution and abundance of marine birds. We compiled line transect data from 15 shipboard surveys (June 2012-April 2014), along with associated remotely sensed habitat data, in the lower Mid-Atlantic Bight off the coast of Delaware, Maryland, and Virginia, USA. We implemented a recently developed hierarchical community distance sampling model to estimate the seasonal abundance of 40 observed marine bird species. Treating each season separately, we included six oceanographic parameters to estimate seabird abundance: three static (distance to shore, slope, sediment grain size) and three dynamic covariates (sea surface temperature [SST], salinity, primary productivity). We expected that avian bottom-feeders would respond primarily to static covariates that characterize seafloor variability, and that surface-feeders would respond more to dynamic covariates that quantify surface productivity. We compared the variation in species-specific and community-level responses to these habitat features, including for rare species, and we predicted species abundance across the study area. While several protected species used the study area in summer during their breeding season, estimated abundance and observed diversity were highest for nonbreeding species in winter. Distance to shore was the most common significant predictor of abundance, and thus useful in estimating the potential exposure of marine birds to offshore development. In many cases, our expectations based on feeding ecology were confirmed, such as in the first winter season, when bottom-feeders associated significantly with the three static covariates (distance to shore, slope, and sediment grain size), and surface-feeders associated significantly with two dynamic covariates (SST, primary productivity). However, other cases revealed significant relationships between static covariates and surface-feeders (e.g., distance to shore) and between dynamic covariates and bottom-feeders (e.g., primary productivity during that same winter). More generally, we found wide interannual, seasonal, and interspecies variation in habitat relationships with abundance. These results show the importance of quantifying detection and determining the ecological drivers of a communitys distribution and abundance, within and among species, for evaluating the potential exposure of marine birds to offshore development.

Ecosystem Oceanography of Seabird Hotspots: Environmental Determinants and Relationship with Antarctic Krill Within an Important Fishing Ground

The discipline of ecosystem oceanography provides a framework for assessing the role of mesoscale physical processes on the formation and occurrence of biological hotspots. We used shipboard surveys over nine years to investigate environmental determinants of seabird hotspots near the Antarctic Peninsula, a region experiencing rapid climate change and an expanding krill fishery. We hypothesize that seabird hotspots are structured by mesoscale ocean conditions that reflect differences in prey distribution within oceanic and coastal waters. We used generalized additive models to quantify functional relationships of seabird hotspots with krill biomass, and a suite of remotely sensed environmental variables, such as eddy kinetic energy. The spatial organization, changes in intensity, and distribution shifts of seabird hotspots indicate different environmental drivers within coastal and oceanic domains and reflect the seasonal variability of the ecosystem. Our results indicate at least eight mesoscale hotspot zones that represent ecologically important areas where significant krill and predator biomass may be concentrated. Our ecosystem assessment of seabird hotspots identified critical foraging habitat and provided reference points to benefit research on estimating their trophic impacts on Antarctic ecosystems and potential effects from the krill fishery. Our approach is generally applicable to other pelagic ecosystems that are structured by hydrographic fronts and eddies, and containing schooling forage species shared by multiple wide-ranging predators. Furthermore, identification of biological hotspots is useful for the designation of marine protected areas most critical to potentially endangered wildlife and fisheries resources.