Resource tracking by fish and wildlife: scientific progress and management implications

An emerging body of research clearly shows that many species move in concert with pulses of resource quality that propagate across space. By tracking these resource waves, which are often dictated by phenology (e.g., fruiting, flowering, spawning), mobile consumers can obtain more energy than consuming resources based on abundance alone. Traditional habitat management has generally assumed that habitats do not change very fast (i.e., on the order for years to decades). Yet, habitats often provide important food resources for only days or weeks within a year, forcing animals to move among habitat patches to meet their energy needs. For instance, rainbow trout in Alaskan streams are limited to a roughly three week period when they have access to the eggs of spawning sockeye salmon. Individual trout that move among multiple sockeye salmon populations spawning at different times grow 2.5 times faster than stationary trout. Similarly, migratory ungulates adjust the time and duration of their migrations to track spatial variation in vegetation phenology, and such behavior can result in increased fat gain compared to resident individuals. The objectives of this symposium are to: 1) review research on resource tracking across fish and wildlife; 2) introduce the latest advances in resource tracking science; 3) provide examples of how managers are updating management regimes for species that track resource waves; and 4) peer into the future to assess how species that track resource pulses will fair in the Anthropocene. This symposium brings together researchers and managers who work on both fish and wildlife.

1:10PM Continental-Level Forcing of Large Herbivore Movements By Resource Phenology
  Ellen Aikens
Why some animals migrate while others remain resident is an unresolved question in ecology. Moving in sync with the progression of spring green-up across the landscape (i.e., green-wave surfing) is a likely precursor to the development of migration in ungulates, however, surfing may not be profitable in all environments. Patterns of spring green-up influence how well individuals surf, suggesting that landscape-level gradients in plant phenology should dictate where migration emerges as a dominant behavior. Using a cross-continental dataset of 1,558 GPS-collared individuals from four ungulate taxa across 58 populations, we found that surfing is facilitated in landscapes with strong phenological gradients (i.e. rapid green-up rate, sequential green-up order and long green-up duration). Across all species, migrants consistently surfed better than residents, but residents experienced similar exposure to high-quality forage because they occupied landscapes with prolonged green-up. Thus, migration emerges as a strategy to track fleeting resources in ungulates, highlighting the importance of the temporal availability of resource in shaping animal movement strategies. Diminished connectivity and altered resource phenology hold potential to fundamentally change how animals move and thereby meet resource requirements in a changing world.
1:30PM Accounting for Phenological Diversity in Ecosystem-Based Management: A Case Study of Bears, Salmon Fisheries, and Watershed Development
  Jonny Armstrong
A key challenge in ecosystem-based management is to understand how habitat alteration combines with species exploitation to mediate how energy flows through food webs. Here we modeled bears foraging on salmon to explore how watershed development affects the capacity for coastal ecosystems to support both commercial fisheries and inland food webs. We found that salmon abundance has saturating effects on bears, such that fisheries can often harvest large fractions of salmon runs without substantially impacting energy flows to bears. However, eroding watershed complexity and associated phenological diversity in salmon spawning shortened the duration of consumer foraging opportunity and strongly reduced the capacity for salmon to support bears. Contrary to the predictions of typical ecosystem-based management, we found that habitats with the lowest resource abundance (small streams harboring small populations of salmon) contributed the most to bear foraging opportunity because they offer salmon at unique moments in time and render them vulnerable to predation. By accounting for empirically corroborated effects of intra-specific prey diversity on bear foraging, we show that coastal food webs may be highly vulnerable to watershed development even if salmon abundance is not substantially reduced.
1:50PM A Test of the Effects of Resource Pulse Timing on Life History Diversification of a Stream Salmonid Fish in a Seasonally-Linked Forest-Stream Ecosystem
  Takuya Sato
Life-history diversity can stabilize populations in the face of environmental fluctuation, but maintenance mechanisms of the life-history diversity are not well-known. Resource pulses can dramatically increase growth and/or reproduction of consumers, but magnitude of the consumer responses depend largely on its seasonal timing. A resource pulse occurring at a given season may more effectively allow some individuals adopt a fast life history (i.e., fast growth and early maturation) than the resource pulse occurring at the other seasons. As a consequence, maintenance of life-history diversity within a consumer population may depend on the resource pulse timing. To test those hypotheses, we conducted a large-scale field experiment, in which we directly manipulated the timing of the terrestrial invertebrate input into stream (Early pulse: June-August vs. Late pulse: August-October), keeping constant the total amounts of the invertebrate input. We found large increase in the proportion of individual masu salmon (Oncorhynchus masou ishikawae) adopting the fast-life, in response to the early pulse, but not so to the late pulse. Since the slow-life was also maintained to some degree in the early-pulsed treatment, the diversity of life history as a measure of Shannon’s diversity index was highest in the early-pulsed treatment. Our results emphasize the need to acknowledge timing-dependent consumer responses in understanding the effects of the resource pulses on individual life history and their diversity. Testing how resource tracking is utilized by individual consumers and contributes to maintain life-history diversity would be an important avenue to better understand population dynamics in spatially and seasonally coupled ecosystems.
2:10PM Herring Spawn and Sea Ducks: A Resource Tracking Phenomenon Linking Marine and Terrestrial Biomes
  Dan Esler
Pacific herring (Clupea pallasi) spawn in large aggregations in intertidal and shallow subtidal marine habitats, although occurrence of spawn is highly constrained in both space and time. The concentrated fish and large quantity of eggs are known to attract an abundance of predators. Predators include a number of sea duck species, which feed on the spawned eggs during pre-migratory and spring migration stages of their annual cycles. We conducted a series of studies to: (1) determine spatiotemporal relationships between sea ducks and herring spawn to understand the strength of resource tracking by these migratory birds at local and continental scales, (2) evaluate the degree to which herring eggs constitute part of marine diets and are incorporated into tissues, and (3) consider the importance of herring spawn resource pulses on behavior, energetics, and investment in reproduction. We found that surf scoter (Melanitta perspicillata) distributions were closely linked with herring spawn events, both locally during pre-migratory periods and as a distinct resource tracking phenomenon during spring migration. We also determined that surf and white-winged (M. fusca) scoters and harlequin ducks (Histrionicus histrionicus) feed heavily on herring spawn when available and that spawn contributes to pre-migratory and pre-reproduction lipid storage. Finally, our work indicates that herring spawn feeding results in distinct changes in behavior and presumably contributes to costs of reproduction when sea ducks reach their terrestrial breeding grounds. Sea ducks that track the “silver wave” of herring spawn during their spring migration north receive a number of benefits that presumably confer greater fitness.
2:30PM Identifying Drivers of Migration Phenology across Terrestrial and Aquatic Systems
  Briana Abrahms
In terrestrial systems, the Green Wave Hypothesis posits that migrating animals can enhance foraging opportunities by tracking phenological variation in high quality forage across space (i.e., ‘resource waves’). To track resource waves, animals may rely on proximate cues and/or memory of long-term average phenologies. Although there is growing evidence of resource tracking in terrestrial migrants, such drivers remain unevaluated in migratory marine megafauna. In this talk we explore unifying drivers of migration phenology across systems and present a case study testing the Green Wave Hypothesis in marine migrants. Specifically, we examined blue whale (Balaenoptera musculus) migrations and investigated resource tracking both contemporaneously (response to proximate cues) and based on climatological conditions (memory). Blue whales closely tracked the long-term average phenology of forage availability, but did not track contemporaneous conditions, and foraging locations were characterized by low interannual variability. Results indicate that memory of long-term average conditions may have a previously underappreciated role in driving migratory movements of marine megafauna, and suggest that these animals may struggle to respond to rapid deviations from historical mean environmental conditions. Results further highlight that resource tracking occurs in both marine and terrestrial systems, but this process is likely modulated by the degree of variability in the environment. Understanding the drivers of animal migration is critical for assessing how environmental changes will affect highly mobile fauna at a global scale and we discuss key conservation implications of our findings.
2:50PM Refreshment Break
3:20PM Resource Waves for Juvenile Fish in Stream Food Webs Result from Abiotic Heterogeneity on Complex Landscape
  Matthew Dunkle
The food web pathways that energy takes from basal resources to top consumers varies across landscapes and through seasons. On complex landscapes, the structure and dynamics of food webs can be diverse leading to asynchronous waves of resources for mobile consumers. As landscapes are homogenized by humans, resource waves may become synchronized, resulting in fewer incentives for resource tracking by consumers. In Southeast Alaska, juvenile salmon, trout, char, and sculpin rear in river networks that may contain a mosaic of glacial-, snow- and rain-fed streams with distinct flow, thermal, and nutrient regimes. We hypothesize that heterogeneity in abiotic conditions provides a template for a diversity of unique food webs that exhibit asynchronous waves of resource availability that are important for sustaining fishes that can move among streams in search of more suitable foraging conditions. However, climate change is homogenizing the hydrology of river networks in Southeast Alaska. As glaciers melt and precipitation shifts from winter snow to rain, the unique characteristics of glacial- and snow-fed watersheds may be lost, which in turn, could reduce the diversity of food webs and synchronize resource waves that support fishes. Here, we present model simulations and empirical data that explores variation in productivity and feeding relationships in glacial-, snow- and rain-fed rivers in Southeast Alaska. We use this information to explore potential resource tracking by juvenile salmon, and the implications of climate-induced homogenization on the relative benefits of resource tracking when the presence of glacial- and snow-fed streams diminish. Our findings suggest that streams peak in optimal growth conditions for fish at different times of the year stemming from variation in thermal- and invertebrate prey- and primary-producer dynamics and that climate change may lead to more productive, but less diverse food webs, with reduced incentives for mobile consumers to track resources across the landscape.
3:40PM Spatially Mixed Adult Aquatic Insects from Various Stream Habitats Provide Prolonged Resource Subsidy to Riparian Spiders and Enhance Their Growth
  Hiromi Uno
Phenological variation of mobile prey would help providing prolonged resources to less mobile predators. Emerging aquatic insects from streams are important food sources for riparian predators, yet their availability is seasonally limited. While the web-spinning riparian spiders are not highly mobile, they may take advantage of the phenological variation of the mobile prey, emerging aquatic insects. Here I introduce a case that spatial heterogeneity in stream water temperature spatially desynchronize the emergence timing of aquatic insects, and prolong their flight period, potentially enhancing consumer growth. While a mayfly Ephemerella maculata emergence lasted for 12–22 d in local sites along a river, mayflies emerged 19 days earlier from warmer than cooler sites. Therefore, the overall emergence of E. maculata from the river lasted for 37 d, and adult swarms were observed over that same period in an adjacent reproductive habitat. A feeding experiment with the riparian spider Tetragnatha versicolor showed that a prolonged subsidy, as would occur in a heterogeneous river, led to higher juvenile growth than a synchronous pulsed subsidy of equal total biomass, as would typify a more homogeneous river. Restoring spatial heterogeneity in streams may benefit not only stream communities but also riparian predators.
4:00PM Migratory Coupling between Predators and Prey
  Nathan B. Furey
Animal migrations represent mobile pulses of energy across landscapes. Predators are often observed aggregating in response to and exploiting migrant prey, but the movements taken by these consumers are less studied. The range of movements by predators induced by prey migrations range from no movement (i.e. no response) to large-scale movements that are potentially migratory. We define when migrant prey induce large-scale movements of predators as migratory coupling. Migratory coupling can have ecological consequences for the participating prey, predators, and the communities they traverse across the landscape. We review examples of migratory coupling and provide hypotheses regarding conditions favorable for their occurrence. We also provide a framework for interactions induced by migratory coupling and demonstrate their potential community-level impacts by examining other forms of spatial shifts in predators. Migratory coupling integrates the fields of landscape, movement, food web and community ecologies, and represents an understudied frontier in ecology that could affect conservation and management in the Anthropocene.
4:20PM Coursing the Mottled Wave: Generalist Predators Track Pulses in Availability of Neonatal Ungulates
  Katey Huggler
Spatial and temporal variation in resources across a landscape is the foundation in which animal movement and behavior operates. Although spatial distribution of resources may affect where individuals occur most consistently, temporal pulses in resources should encourage plasticity in movement strategies to capitalize on short windows of valuable resources. Exploitation of resource pulses allows consumers to expand foraging opportunity, while maximizing energy intake and minimizing energy expenditure. Therefore, animals should synchronize their movements to capitalize on resource pulses as they proliferate across the landscape. We used GPS data of 27 coyotes (Canis latrans) and 50 female mule deer (Odocoileus hemionus) to evaluate the hypothesis that coyotes exhibit resource tracking behavior and exploit pulses in availability of neonatal mule deer. We expected coyotes to select areas used by female mule deer before, during, and after peak parturition, however, coyotes should select areas used by female deer most strongly during peak parturition when neonates are most available and most vulnerable. Secondly, we expected coyotes to exhibit search behavior within areas used by female deer as a mechanism to maximize detection of neonates. Coyotes selected strongly (odds ratio = 27.2, 95% CI = 22.16, 40.85) for areas used by female mule deer before and during peak parturition, and avoided (odds ratio = -15.61, 95% CI = -10.23, -20.96) areas used by female deer after peak parturition—supporting the notion that coyotes may be tracking pulses in availability of neonatal mule deer. Further, coyotes exhibited more complex paths and longer residency time with increasing probability of use by female deer. Our results provide further explanation as to how coyotes are capable of acting as a primary predator to neonatal ungulates. Our analyses merges expectations of resource tracking and optimal foraging theory to reveal the behavioral mechanisms by which generalist predators may affect ungulate populations.
4:40PM Resource Tracking in Bears and Implications for Management
  William Deacy
Mangers and researchers generally assume that abundance of interacting species drives their trophic interactions. However, recent work has shown that attributes besides abundance, such as phenology, can strongly mediate species interactions. Dr. Deacy will present data showing how Kodiak brown bears are influenced by, and respond to, the phenology of their foods. Individual brown bears exploited variation in sockeye salmon spawning phenology by tracking salmon runs at two scales: along a single spawning stream and across a 2,800 km2 region of Kodiak Island. At the local scale, data from time lapse cameras show bears tracking salmon spawning from the upper to lower sections of a tributary. At the regional scale, data from 40 GPS collared brown bears show that bears visited multiple spawning sites to consume salmon for much longer than is possible at a single site. Second, Dr. Deacy will explore how bears responded to asymmetric phenological shifts between its primary foods, sockeye salmon and red elderberries. In years with anomalously high spring air temperatures, elderberry fruited several weeks earlier and became available during the period when salmon spawned in tributary streams. Bears departed salmon spawning streams, where they typically kill 25-75% of the salmon, to forage on berries on adjacent hillsides. This prey switching behavior attenuated an iconic predator-prey interaction and likely altered the many ecological functions that result from bears foraging on salmon. These results show how climate-induced shifts in resource phenology can alter food webs through a mechanism other than trophic mismatch.

Organizers: Jonathan Armstrong, Jerod Merkle, William Deacy

Location: Reno-Sparks CC Date: October 3, 2019 Time: 1:10 pm - 5:00 pm