Deep Sea Research Part II: Topical Studies in Oceanography
Ecological niche modeling of sympatric krill predators around Marguerite Bay, Western Antarctic Peninsula
Introduction
The structure and function of ocean ecosystems are affected in large part by spatial and temporal variability in the physical environment, which produces areas of enhanced biological activity. These areas, where critical linkages between trophic levels exist, are considered ‘biological hot spots’ (Sydeman et al., 2006). These regions attract higher trophic level organisms, as a result of physical features (static) and forcing (episodic) mechanisms that act in concert to enhance prey availability (Ainley et al., 1998, Bost et al., 2009). At evolutionary time scales, species have adopted life history strategies and foraging behaviors to take advantage of particularly persistent and profitable regions (Costa, 1993, Etnoyer et al., 2004). Biological hot spots have been described across a broad range of spatial scales: from broad upwelling zones to meso-scale frontal boundaries and smaller episodic eddies (see Palacios et al., 2006). In fact, it could be argued that the entire Southern Ocean below the Polar Front is in and of itself an entire biological hot spot (Tynan, 1998, Bost et al., 2009).
Within the Southern Ocean, physical, biological, and chemical processes combine to create an ecosystem dominated by the annual advance and retreat of sea ice and seasonal primary productivity (Knox, 2007). This in turn has lead to a zooplankton community dominated by abundant euphausiids, particularly Antarctic krill (E. superba). The Southern Ocean supports an unprecedented number of upper trophic-level predators, including whales, seals, penguins, seabirds, and fish, which feed primarily on the large biomass of Antarctic krill (Knox, 2007). Baleen whales (humpback (M. novaeangliae), minke (Balaenoptera bonaerensis)), crabeater seals (L. Carcinophagus), and Adélie penguins (P. adeliae) are all abundant in the nearshore waters along the Western Antarctic Peninsula (WAP). They all feed primarily on Antarctic krill, and yet have extremely varied foraging strategies and abilities to access prey (Costa and Crocker, 1996, Fraser and Trivelpiece, 1996, Ducklow et al., 2007). Humpback whales are seasonal residents, migrating between tropical breeding and calving grounds to feed along the WAP in summer and autumn months (Laws, 1985). Minke whales have been observed throughout winter months around the WAP, indicating that they are year-round residents (e.g. Thiele et al., 2004). Some portion of the minke whale population may migrate seasonally, but little is known regarding this behavior and what proportion of the population this represents. Crabeater seals are resident to the Antarctic and are pagophilic; they rarely come to shore, and use sea ice as their primary haul-out platform and breeding and pupping substrate (Siniff, 1991). Adélie penguins breed on rocky shores along the WAP (Fraser and Hofmann, 2003). They are considered central-place foragers that they must come and go to their breeding colonies to provision chicks while they are being raised in spring and summer months. They also utilize sea ice as a haul-out substrate throughout the year between foraging bouts.
Given the broad size ranges and energetic requirements of these krill predators, each has developed unique foraging behaviors and feeding strategies to maximize efficiency (Costa, 1991). While Adélie penguins feed mainly in the upper 100 m of the water column (Chappell et al., 1993), crabeater seals forage as deep as 450 m (Burns et al., 2004). And while both penguins and seals feed on individual krill, baleen whales (including humpback and minke whales) engulf large quantities of krill-rich water to consume as many prey as possible at once. Considering these feeding and life history constraints, the distributions of each species should reflect locations that offer the ability to satisfy both their physical and energetic requirements for survival.
Over the past 50 years, significant climate warming has been detected along the WAP (Vaughan et al., 2003). This warming is believed to have affected the amount of sea ice and its interannual variability (Murphy et al., 2007, Stammerjohn et al., 2008) and the abundance of Antarctic krill around in the region (Atkinson et al., 2004, Ducklow et al., 2007). These changes have, in part, led to decrease in population trends in some krill predator populations (e.g. Adélie penguins, see Fraser and Hofmann, 2003, Ducklow et al., 2007, Forcada et al., 2008). Given the changes in both the physical environment and availability of prey resources, assessing the likelihood or degree of ecological interactions between krill predators is necessary to better understand future impacts of climate-driven changes to the Antarctic marine ecosystem (Costa et al., 2010). The purpose of the present study is to: (1) better understand how physical and biological features, and variability among these as observed in 2001 and 2002, affect the distribution of sympatric krill predators around Marguerite Bay, WAP; and (2) describe the amount of niche overlap among and between krill predators during autumn months and how these relationships are affected by environmental variability. We address these objectives by determining the combination of environmental variables (derived largely from data collected during the Southern Ocean Global Ecosystems Dynamics (SO GLOBEC) field seasons in 2001 and 2002) that best predict the habitat and account for the distribution of different krill predators using a presence-only habitat modeling technique called maximum entropy modeling, Maxent. Maxent estimates a species' probability distribution by finding the probability distribution of maximum entropy (i.e., closest to uniform), subject to a set of constraints derived from available information about the species' environmental relationships (Phillips et al., 2006). We also explore the extent of ecological overlap and niche partitioning of these habitat models across predator taxonomic groups and between years using established niche overlap assessment techniques.
Section snippets
Time frame, study region and data sources
We use data collected as part of the Southern Ocean GLOBEC program in April–May 2001 and 2002 (Julian days 90–150) in and around Marguerite Bay, WAP (see Hofmann et al., 2004; Fig. 1). We integrate a suite of physical and biological environmental variables collected by a combination of underway continuous and station-based sampling, as well as remotely sensed imagery. Hydrographic data were collected from the RVIB Nathaniel B. Palmer in both 2001 and 2002. The hydrographic variables in this
Ecological niche modeling
The number of observations or occurrences for each species in each year is shown in Table 1. Initial plots of predator occurrence reveal several “hot spots” (Fig. 2). In 2001, the area along the southeastern corner of Adelaide Island and near the northwestern corner of Alexander Island had the highest rates of occurrence. In 2002, the same general area around Adelaide Island (as well as along the northern coast of the island) contained the most predator occurrences. Predator occurrence is
Discussion
The results of our analyses indicate several important and novel aspects of the distribution and amount of niche overlap between krill predators in Antarctica. We use presence-only data to generate ecological niche models, which (1) describe the concurrent habitat preferences, (2) model the amount of niche overlaps between, and (3) measure variability in modeled ecological niches in 2 years for several major groups of krill predators in autumn around Marguerite Bay, WAP. We find that there are
Acknowledgments
We are grateful to the ships' crews and RPSC employees for their assistance. We would like to thank all of the members of the field and survey teams that helped with data collection. We would especially like to thank Eileen Hofmann for her guidance and leadership during the SO GLOBEC project. Our manuscript was greatly improved by the comments and thoughts of Christine Ribic and one anonymous reviewer. All animal handling protocols were authorized under National Marine Fisheries Service permit
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