Benefiting from a migratory prey: spatio-temporal patterns in allochthonous subsidization of an Arctic predator

1.?Flows of nutrients and energy across ecosystem boundaries have the potential to subsidize consumer populations and modify the dynamics of food webs, but how spatio-temporal variations in autochthonous and allochthonous resources affect consumers' subsidization remains largely unexplored. 2.?We studied spatio-temporal patterns in the allochthonous subsidization of a predator living in a relatively simple ecosystem. We worked on Bylot Island (Nunavut, Canada), where arctic foxes (Vulpes lagopus L.) feed preferentially on lemmings (Lemmus trimucronatus and Dicrostonyx groenlandicus Traill), and alternatively on colonial greater snow geese (Anser caerulescens atlanticus L.). Geese migrate annually from their wintering grounds (where they feed on farmlands and marshes) to the Canadian Arctic, thus generating a strong flow of nutrients and energy across ecosystem boundaries. 3.?We examined the influence of spatial variations in availability of geese on the diet of fox cubs (2003-2005) and on fox reproductive output (1996-2005) during different phases of the lemming cycle. 4.?Using stable isotope analysis and a simple statistical routine developed to analyse the outputs of a multisource mixing model (SIAR), we showed that the contribution of geese to the diet of arctic fox cubs decreased with distance from the goose colony. 5.?The probability that a den was used for reproduction by foxes decreased with distance from the subsidized goose colony and increased with lemming abundance. When lemmings were highly abundant, the effect of distance from the colony disappeared. The goose colony thus generated a spatial patterning of reproduction probability of foxes, while the lemming cycle generated a strong temporal variation of reproduction probability of foxes. 6.?This study shows how the input of energy owing to the large-scale migration of prey affects the functional and reproductive responses of an opportunistic consumer, and how this input is spatially and temporally modulated through the foraging behaviour of the consumer. Thus, perspectives of both landscape and foraging ecology are needed to fully resolve the effects of subsidies on animal demographic processes and population dynamics.     

Strength of asymmetric competition between predators in food webs ruled by fluctuating prey: the case of foxes in tundra

In food webs heavily influenced by multi‐annual population fluctuations of key herbivores, predator species may differ in their functional and numerical responses as well as their competitive ability. Focusing on red and arctic fox in tundra with cyclic populations of rodents as key prey, we develop a model to predict how population dynamics of a dominant and versatile predator (red fox) impacted long‐term growth rate of a subdominant and less versatile predator (arctic fox). We compare three realistic scenarios of red fox performance: (1) a numerical response scenario where red fox acted as a resident rodent specialist exhibiting population cycles lagging one year after the rodent cycle, (2) an aggregative response scenario where red fox shifted between tundra and a nearby ecosystem (i.e. boreal forest) so as to track rodent peaks in tundra without delay, and (3) a constant subsidy scenario in which the red fox population was stabilized at the same mean density as in the other two scenarios. For all three scenarios it is assumed that the arctic fox responded numerically as a rodent specialist and that the mechanisms of competition is of a interference type for space, in which the arctic fox is excluded from the most resource rich patches in tundra. Arctic fox is impacted most by the constant subsidy scenario and least by the numerical response scenario. The differential effects of the scenarios stemmed from cyclic phase‐dependent sensitivity to competition mediated by changes in temporal mean and variance of available prey to the subdominant predator. A general implication from our result is that external resource subsidies (prey or habitats), monopolized by the dominant competitor, can significantly reduce the likelihood for co‐existence within the predator guild. In terms of conservation of vulnerable arctic fox populations this means that the likelihood of extinction increases with increasing amount of subsidies (e.g. carcasses of large herbivores or marine resources) in tundra and nearby forest areas, since it will act to both increase and stabilize populations of red fox.     

Pulses of movement across the sea ice: population connectivity and temporal genetic structure in the arctic fox

Lemmings are involved in several important functions in the Arctic ecosystem. The Arctic fox (Vulpes lagopus) can be divided into two discrete ecotypes: “lemming foxes” and “coastal foxes”. Crashes in lemming abundance can result in pulses of “lemming fox” movement across the Arctic sea ice and immigration into coastal habitats in search for food. These pulses can influence the genetic structure of the receiving population. We have tested the impact of immigration on the genetic structure of the “coastal fox” population in Svalbard by recording microsatellite variation in seven loci for 162 Arctic foxes sampled during the summer and winter over a 5-year period. Genetic heterogeneity and temporal genetic shifts, as inferred by STRUCTURE simulations and deviations from Hardy–Weinberg proportions, respectively, were recorded. Maximum likelihood estimates of movement as well as STRUCTURE simulations suggested that both immigration and genetic mixture are higher in Svalbard than in the neighbouring “lemming fox” populations. The STRUCTURE simulations and AMOVA revealed there are differences in genetic composition of the population between summer and winter seasons, indicating that immigrants are not present in the reproductive portion of the Svalbard population. Based on these results, we conclude that Arctic fox population structure varies with time and is influenced by immigration from neighbouring populations. The lemming cycle is likely an important factor shaping Arctic fox movement across sea ice and the subsequent population genetic structure, but is also likely to influence local adaptation to the coastal habitat and the prevalence of diseases.     

The marine side of a terrestrial carnivore: intra-population variation in use of allochthonous resources by arctic foxes

Inter-individual variation in diet within generalist animal populations is thought to be a widespread phenomenon but its potential causes are poorly known. Inter-individual variation can be amplified by the availability and use of allochthonous resources, i.e., resources coming from spatially distinct ecosystems. Using a wild population of arctic fox as a study model, we tested hypotheses that could explain variation in both population and individual isotopic niches, used here as proxy for the trophic niche. The arctic fox is an opportunistic forager, dwelling in terrestrial and marine environments characterized by strong spatial (arctic-nesting birds) and temporal (cyclic lemmings) fluctuations in resource abundance. First, we tested the hypothesis that generalist foraging habits, in association with temporal variation in prey accessibility, should induce temporal changes in isotopic niche width and diet. Second, we investigated whether within-population variation in the isotopic niche could be explained by individual characteristics (sex and breeding status) and environmental factors (spatiotemporal variation in prey availability). We addressed these questions using isotopic analysis and Bayesian mixing models in conjunction with linear mixed-effects models. We found that: i) arctic fox populations can simultaneously undergo short-term (i.e., within a few months) reduction in both isotopic niche width and inter-individual variability in isotopic ratios, ii) individual isotopic ratios were higher and more representative of a marine-based diet for non-breeding than breeding foxes early in spring, and iii) lemming population cycles did not appear to directly influence the diet of individual foxes after taking their breeding status into account. However, lemming abundance was correlated to proportion of breeding foxes, and could thus indirectly affect the diet at the population scale.     

Structure of arctic fox (Alopex lagopus beringensis) colonies in the northern extremity of Bering Island

Data on the spatial structure of an Arctic fox (Alopex lagopus beringensis) colony were obtained in July-August 1995, using walk counts and observations near living dens around the Northern rookery of the northern fur seals located on Bering Island (Commander Islands). The home ranges of 31 Arctic fox families (61 adults and 145 pups inhabiting 66 dens) were found over 27 km of the coastline. Sixty individuals (3 adults and 57 pups) were marked by color ear-tags. Among adult foxes, 24 (39.3%) were recognized as females and 12 (19.7%) as males; the sex of 25 (41.0%) foxes was not recognized. Among 57 marked cubs, 26 (45.6%) were females and 31 (54.4%) were males. The best studied families (13) had 3–11 pups (6.7 ± 0.7, on average). The survival of cubs at an age younger than 2.0–2.5 months was 82.5%; 30.8% of the families consisted of more than two adults. The distribution of the Arctic fox dens and home ranges along the coastline has been studied; specific features of the location of dens have been described. In the studied area, Arctic foxes have been foraging on birds (67.6% of dens with food remains), northern fur seals (40.5), other marine mammals (13.5), Pacific salmon (29.7), and reindeer (2.7%), as well as on amphipods and voles. Rich constant food sources (rookeries, marine bird colonies, and spawning places of the blueback salmon) were found in 7 home ranges of the Arctic fox; 6 home ranges included temporary food sources (spawning streams of the humpback salmon); and 18 home ranges were poor in food resources. Arctic foxes whose home ranges lie within 6–7 km around a “food patch” used the concentrated food resources together. Food resources are supposed to become important only after the raised pups turn to self-feeding. Differences in the use of space, foraging and breeding of the two Arctic fox subspecies (A. l. beringensis and A. l. semenovi), and arrangement of colonies around the northern fur seal rookeries are discussed.     

Arctic fox Vulpes lagopus population structure: circumpolar patterns and processes

Movement is a prominent process shaping genetic population structure. In many northern mammal species, population structure is formed by geographic distance, geographical barriers and various ecological factors that influence movement over the landscape. The Arctic fox Vulpes lagopus is a highly mobile, opportunistic carnivore of the Arctic that occurs in two main ecotypes with different ecological adaptations. We assembled microsatellite data in 7 loci for 1834 Arctic foxes sampled across their entire distribution to describe the circumpolar population structure and test the impact of (1) geographic distance, (2) geographical barriers and (3) ecotype designation on the population structure. Both Structure and Geneland demonstrated distinctiveness of Iceland and Scandinavia whereas low differentiation was observed between North America–northern Greenland, Svalbard and Siberia. Genetic differentiation was significantly correlated to presence of sea ice on a global scale, but not to geographical distance or ecotype designation. However, among areas connected by sea ice, we recorded a pattern of isolation by distance. The maximum likelihood approach in Migrate suggested that connectivity across North America–northern Greenland and Svalbard was particularly high. Our results demonstrate the importance of sea ice for maintaining connectivity between Arctic fox populations and we therefore predict that climate change will increase genetic divergence among populations in the future.     

Local variation in arctic fox abundance on Svalbard,Norway

Arctic fox (Alopex lagopus) numbers vary greatly, with cyclic fluctuations often associated with fluctuations in microtine rodents. However, in areas where small prey mammals are absent, such as Iceland and Svalbard, such cyclic fluctuations are lacking. Annual fluctuations in the density of the arctic fox population on the Brøggerhalvøya peninsula and Kongsfjorden region on Svalbard, Norway, were studied from 1990 to 2001 by using indices of fox abundance. All indices showed similar trends; fox numbers were low in 1990, increased until 1995 whereupon they decreased sharply, before increasing again and levelling off in 2001. Increasing numbers of foxes during the first part of the study paralleled increasing numbers of Svalbard reindeer (Rangifer tarandus platyrhynchus) carcasses in winter and increasing numbers of nesting barnacle geese (Branta leucopsis) in summer. This study shows that the number of arctic foxes varies greatly even in areas without fluctuating microtine rodents.     

The effect of summer feeding on juvenile arctic fox survival - a field experiment

The arctic fox Alopex lagopus L population in Sweden is small and its numbers fluctuate widely with food availability, l e rodent populations This fluctuation is mediated through differences in recruitment rates between years The recruitment can be divided into three phases number of litters born, number of cubs per litter and cub survival rates The number of litters and their sizes have been shown to depend on food availability during winter and spring To examine cub survival during the summer and how it relates to food availability, we conducted a feeding experiment m northern Sweden during 1990, a year of low rodent density, involving six occupied arctic fox dens Feeding at dens lowered cub mortality rates However, condition and growth rates of juveniles were not influenced by supplementary feeding at dens, nor were they related to the probability of survival for an individual Thus arctic foxes seem to minimize risks rather than maximize growth The juvenile mortality from weaning and over the next 6 wk was 21%, mostly due to starvation Only 82% survived from weaning to the first breeding season Of the one-year-old foxes, 50% survived their second year Supplementary feeding of juveniles had no effect on the final survival rates over these two years However, the immediate, positive effect on cub survival could be used in a long-term, extensive management programme if combined with winter feeding     

Bottom-up and top-down processes interact to modify intraguild interactions in resource-pulse environments

Top predators are declining globally, in turn allowing populations of smaller predators, or mesopredators, to increase and potentially have negative effects on biodiversity. However, detection of interactions among sympatric predators can be complicated by fluctuations in the background availability of resources in the environment, which may modify both the numbers of predators and the strengths of their interactions. Here, we first present a conceptual framework that predicts how top-down and bottom-up interactions may regulate sympatric predator populations in environments that experience resource pulses. We then test it using 2 years of remote-camera trapping data to uncover spatial and temporal interactions between a top predator, the dingo Canis dingo, and the mesopredatory European red fox Vulpes vulpes and feral cat Felis catus, during population booms, declines and busts in numbers of their prey in a model desert system. We found that dingoes predictably suppress abundances of the mesopredators and that the effects are strongest during declines and busts in prey numbers. Given that resource pulses are usually driven by large yet infrequent rains, we conclude that top predators like the dingo provide net benefits to prey populations by suppressing mesopredators during prolonged bust periods when prey populations are low and potentially vulnerable.     

An avian terrestrial predator of the Arctic relies on the marine ecosystem during winter

Top predators of the arctic tundra are facing a long period of very low prey availability during winter and subsidies from other ecosystems such as the marine environment may help to support their populations. Satellite tracking of snowy owls, a top predator of the tundra, revealed that most adult females breeding in the Canadian Arctic overwinter at high latitudes in the eastern Arctic and spend several weeks (up to 101 d) on the sea‐ice between December and April. Analysis of high‐resolution satellite images of sea‐ice indicated that owls were primarily gathering around open water patches in the ice, which are commonly used by wintering seabirds, a potential prey. Such extensive use of sea‐ice by a tundra predator considered a small mammal specialist was unexpected, and suggests that marine resources subsidize snowy owl populations in winter. As sea‐ice regimes in winter are expected to change over the next decades due to climate warming, this may affect the wintering strategy of this top predator and ultimately the functioning of the tundra ecosystem.     

Is polecat (Mustela putorius) diet affected by “mediterraneity”?

Carnivores in Mediterranean ecosystems respond to the inherent heterogeneity of these systems by tracking the spatial and temporal availability of food resources. This feeding strategy, however, has been associated primarily with generalist carnivores and little is known for specialist species such as the European polecat. We collected polecat scat to determine the diet of this species, how it matches the seasonal availability of food resources, and how it is affected by population spatial structure and anthropogenic disturbance. Polecats were present in only 34% of the surveyed area and were clumped into three main population nuclei. Despite the spatial segregation of the populations, they had no significant differences in food items consumed. Polecats mostly fed on mammals (percentage of occurrence (P.O.)=43%) and arthropods (P.O.=49%). Biomass intake was also mostly from mammals (percentage of biomass (P.B.)=96%), followed by birds (P.B.=3%), with arthropods contributing less than 1%. Lagomorphs were the most consumed prey (P.O.=25% and P.B.=87%), which is consistent with the marked spatial overlap between scat with high content in lagomorphs and the areas with high wild rabbit availability. These results indicate that polecats are specialists in the consumption of wild rabbits, spatially track the availability of this prey, and may be affected by the decrease in abundance of the prey populations. Future conservation of polecats in Mediterranean regions of southern Portugal may be achieved through the restoration of hunted and diseased wild rabbit populations.     

Movement tactics of a mobile predator in a meta‐ecosystem with fluctuating resources: the arctic fox in the High Arctic

Animal movement is a fundamental process shaping ecosystems at multiple levels, from the fate of individuals to global patterns of biodiversity. The spatio‐temporal dynamic of food resources is a major driver of animal movement and generates patterns ranging from range residency to migration and nomadism. Arctic tundra predators face a strongly fluctuating environment marked by cyclic microtine populations, high seasonality, and the potential availability of sea ice, which gives access to marine resources in winter. This type of relatively poor and highly variable environment can promote long‐distance movements and resource tracking in mobile species. Here, we investigated the winter movements of the arctic fox, a major tundra predator often described as a seasonal migrant or nomad. We used six years of Argos satellite telemetry data collected on 66 adults from Bylot Island (Nunavut, Canada) tracked during the sea ice period. We hypothesized that long‐distance movements would be influenced by spatio‐temporal changes in resource availability and individual characteristics. Despite strong annual and seasonal changes in resource abundance and distribution, we found that a majority of individuals remained resident, especially those located in an area characterized by highly predictable pulse resources (goose nesting colony) and abundant cached food items (eggs). Foxes compensated terrestrial food shortage by commuting to the sea ice rather than using long‐distance tracking or moving completely onto the sea ice for winter. Individual characteristics also influenced movement patterns: age positively influenced the propensity to engage in nomadism, suggesting older foxes may be driven out of their territories. Our results show how these mammalian predators can adjust their movement patterns to favor range residency despite strong spatio‐temporal fluctuations in food resources. Understanding the movement responses of predators to prey dynamics helps identifying the scales at which they work, which is a critical aspect of the functioning and connectivity among meta‐ecosystems.     

Adaptations to Stochastic Environmental Variations: The Effects of Seasonal Temperatures on the Migratory Window of Svalbard Arctic Charr

The circumpolar Arctic charr, Salvelinus alpinus, is ideal for studying how environmental factors affect life history in fishes. Charr populations demonstrate a tremendous ecological plasticity and adaptations to harsh environments. Arctic charr is the only freshwater fish on Svalbard, including anadromous, resident, and landlocked stocks. Freshwater lake systems on Svalbard are characterized by very low water temperatures, long-term or even permanent ice cover, and low levels of nutrients. Food is thus limited and may lead to growth stagnation and early maturity in Arctic charr. The individual growth pattern may alternatively follow a sigmoid-shaped curve, caused by a shift to either cannibalistic or anadromous (migration to sea) behaviour. In lake systems that include migratory charr, the population may consist of a mixture of parr, postsmolt, and adult migratory individuals, as well as small-sized resident, large-sized resident (cannibals), and large formerly resident individuals transformed to anadromy. Our study in the Lake Dieset watercourse (79°N), Svalbard, demonstrates that the annual water flow in the outlet river is strongly correlated to air temperatures and provides a passage to the sea, allowing the charr access to the nutrient-rich seawater environment, during at most two months each year. During one of the years studied, the youngest and small-sized part of the sea-going stock was prevented from ascending the river and probably suffered mortality during winter. The migratory window of the Arctic charr in Lake Dieset is therefore highly variable among years and thus unpredictable. We hypothesize that in worst case scenarios (cold years, low water discharge), climatic variations may occasionally prevent charr from migrating upstream in Svalbard lake systems in late autumn, resulting in high mortality in the population.     

Seasonal pulses of migratory prey and annual variation in small mammal abundance affect abundance and reproduction by arctic foxes

We examined how large seasonal influxes of migratory prey influenced population dynamics of arctic foxes and how this varied with fluctuations in small mammal (lemming and vole) abundance—the main prey of arctic foxes throughout most of their range. Specifically, we compared how arctic fox abundance, breeding density and litter size varied inside and outside a large goose colony and in relation to annual variation in small mammal abundance. Information-theoretic model selection showed that (1) breeding density and fox abundance were 2–3 times higher inside the colony than they were outside the colony and (2) litter size, breeding density and annual variation in fox abundance in the colony tracked fluctuations in lemming abundance. The influence of lemming abundance on reproduction and abundance of arctic foxes outside the colony was inconclusive, largely because fox densities outside the colony were low, which made it difficult to detect such relationships. Lemming abundance was, thus, the main factor governing reproduction and abundance of arctic foxes in the colony, whereas seasonal influxes of geese and their eggs provided foxes with external subsidies that elevated breeding density and fox abundance above that which lemmings could support. This study highlights (1) the relative importance of migratory prey and other foods on the abundance and reproduction by local consumers and (2) how migratory animals function as vectors of nutrient transfer between distant ecosystems such as Arctic environments and wintering areas by geese thousands of kilometres to the south.     

Adjustment of offspring sex ratios in relation to the availability of resources for philopatric offspring in the common brushtail possum

The local-resource-competition hypothesis predicts that where philopatric offspring compete for resources with their mothers, offspring sex ratios will be biased in favour of the dispersing sex. This should produce variation in sex ratios between populations in relation to differences in the availability of resources for philopatric offspring. However, previous tests of local resource competition in mammals have used indirect measures of resource availability and have focused on sex-ratio variation between species or individuals rather than between local populations. Here, we show that the availability of den sites predicts the offspring sex ratio in populations of the common brushtail possum. Female possums defend access to dens, and daughters, but not sons, occupy dens within their mother's range. However, the abundances of possums in our study areas were determined principally by food availability. Consequently, in food-rich areas with a high population density, the per-capita availability of dens was low, and the cost of having a daughter should have been high. This cost was positively correlated with male bias in the sex ratio at birth. Low per capita availability of dens was correlated with male bias in the sex ratio at birth.     

Historical and ecological determinants of genetic structure in arctic canids

Wolves (Canis lupus) and arctic foxes (Alopex lagopus) are the only canid species found throughout the mainland tundra and arctic islands of North America. Contrasting evolutionary histories, and the contemporary ecology of each species, have combined to produce their divergent population genetic characteristics. Arctic foxes are more variable than wolves, and both island and mainland fox populations possess similarly high microsatellite variation. These differences result from larger effective population sizes in arctic foxes, and the fact that, unlike wolves, foxes were not isolated in discrete refugia during the Pleistocene. Despite the large physical distances and distinct ecotypes represented, a single, panmictic population of arctic foxes was found which spans the Svalbard Archipelago and the North American range of the species. This pattern likely reflects both the absence of historical population bottlenecks and current, high levels of gene flow following frequent long-distance foraging movements. In contrast, genetic structure in wolves correlates strongly to transitions in habitat type, and is probably determined by natal habitat-biased dispersal. Nonrandom dispersal may be cued by relative levels of vegetation cover between tundra and forest habitats, but especially by wolf prey specialization on ungulate species of familiar type and behaviour (sedentary or migratory). Results presented here suggest that, through its influence on sea ice, vegetation, prey dynamics and distribution, continued arctic climate change may have effects as dramatic as those of the Pleistocene on the genetic structure of arctic canid species.     

Contrasting consequences of climate change for migratory geese: Predation,density dependence and carryover effects offset benefits of high‐arctic warming

Climate change is most rapid in the Arctic, posing both benefits and challenges for migratory herbivores. However, population‐dynamic responses to climate change are generally difficult to predict, due to concurrent changes in other trophic levels. Migratory species are also exposed to contrasting climate trends and density regimes over the annual cycle. Thus, determining how climate change impacts their population dynamics requires an understanding of how weather directly or indirectly (through trophic interactions and carryover effects) affects reproduction and survival across migratory stages, while accounting for density dependence. Here, we analyse the overall implications of climate change for a local non‐hunted population of high‐arctic Svalbard barnacle geese, Branta leucopsis, using 28 years of individual‐based data. By identifying the main drivers of reproductive stages (egg production, hatching and fledging) and age‐specific survival rates, we quantify their impact on population growth. Recent climate change in Svalbard enhanced egg production and hatching success through positive effects of advanced spring onset (snow melt) and warmer summers (i.e. earlier vegetation green‐up) respectively. Contrastingly, there was a strong temporal decline in fledging probability due to increased local abundance of the Arctic fox, the main predator. While weather during the non‐breeding season influenced geese through a positive effect of temperature (UK wintering grounds) on adult survival and a positive carryover effect of rainfall (spring stopover site in Norway) on egg production, these covariates showed no temporal trends. However, density‐dependent effects occurred throughout the annual cycle, and the steadily increasing total flyway population size caused negative trends in overwinter survival and carryover effects on egg production. The combination of density‐dependent processes and direct and indirect climate change effects across life history stages appeared to stabilize local population size. Our study emphasizes the need for holistic approaches when studying population‐dynamic responses to global change in migratory species.     

Terrain structure and selection of denning areas by arctic foxes on Svalbard

We examined the relationship between the distribution of arctic fox dens and the occurrence of rugged terrain on Svalbard, Norway, using indices of terrain ruggedness (TRI) based on contour characteristics from topographic maps in 240 grid cells, each 4 km². The distribution of rugged terrain co-varied with occurrence of arctic fox dens. Moderately rugged terrain (TRI=1.5–3.0) constituted only 21% of the total study area, but contained 77% of all natal dens recorded in the study area. Large clusters (8–36 km²) of moderately rugged terrain were generally preferred for location of den sites compared to smaller scattered clusters of rugged terrain. The importance of rugged terrain is discussed in relation to snow cover, exposure, soil conditions and distribution of prey species. This simple and non-invasive analysis of terrain ruggedness may be used to predict the distribution of potential arctic fox denning areas across landscapes. Accepted: 6 September 2000     

Dynamics of Reciprocal Pulsed Subsidies in Local and Meta-Ecosystems

Temporally variable and reciprocal subsidies between ecosystems are ubiquitous. These spatial flows can generate a suite of direct and indirect effects in local and meta-ecosystems. The focus of most subsidy research, however, has been on the response of consumers in recipient ecosystems to constant subsidies over very short or very long time scales. We derive a meta-ecosystem model to explicitly consider the dynamic feedbacks between local ecosystems coupled through reciprocal pulsed subsidies. We predict oscillating reinforcing and dampening effects of reciprocal pulsed herbivore flows. Maximum reinforcing effects between reciprocal pulsed herbivore flows occur when these flows are in phase with the dynamics of neighboring predators. This prediction is robust to a range of pulse quantities and frequencies. Reciprocal pulsed herbivore subsidies lead to spatial and temporal variability in the strength of trophic cascades in local and meta-ecosystems but these cascading effects are the strongest when reciprocal pulsed subsidies are temporally concentrated. When predators demonstrate a behavioral response to prey abundance, reciprocal pulsed subsidies dampen the strength of local trophic cascades but lead to strong trophic cascades across local ecosystems. The timing of reciprocal pulsed subsidies is a critical component that determines the cascading effects of spatial flows. We show that spatial and temporal variabilities in resources and consumers can have a significant influence on the strength of cascading trophic interactions; therefore, our ability to detect and understand trophic cascades may depend on the scale of inquiry of ecological studies.