Salmon increase forest bird abundance and diversity

Freshwater megafauna shape ecosystems and facilitate restoration

Freshwater megafauna, such as sturgeons, giant catfishes, river dolphins, hippopotami, crocodylians, large turtles, and giant salamanders, have experienced severe population declines and range contractions worldwide. Although there is an increasing number of studies investigating the causes of megafauna losses in fresh waters, little attention has been paid to synthesising the impacts of megafauna on the abiotic environment and other organisms in freshwater ecosystems, and hence the consequences of losing these species. This limited understanding may impede the development of policies and actions for their conservation and restoration. In this review, we synthesise how megafauna shape ecological processes in freshwater ecosystems and discuss their potential for enhancing ecosystem restoration. Through activities such as movement, burrowing, and dam and nest building, megafauna have a profound influence on the extent of water bodies, flow dynamics, and the physical structure of shorelines and substrata, increasing habitat heterogeneity. They enhance nutrient cycling within fresh waters, and cross-ecosystem flows of material, through foraging and reproduction activities. Freshwater megafauna are highly connected to other freshwater organisms via direct consumption of species at different trophic levels, indirect trophic cascades, and through their influence on habitat structure. The literature documenting the ecological impacts of freshwater megafauna is not evenly distributed among species, regions, and types of ecological impacts, with a lack of quantitative evidence for large fish, crocodylians, and turtles in the Global South and their impacts on nutrient flows and food-web structure. In addition, population decline, range contraction, and the loss of large individuals have reduced the extent and magnitude of megafaunal impacts in freshwater ecosystems, rendering a posteriori evaluation more difficult. We propose that reinstating freshwater megafauna populations holds the potential for restoring key ecological processes such as disturbances, trophic cascades, and species dispersal, which will, in turn, promote overall biodiversity and enhance nature's contributions to people. Challenges for restoration actions include the shifting baseline syndrome, potential human-megafauna competition for habitats and resources, damage to property, and risk to human life. The current lack of historical baselines for natural distributions and population sizes of freshwater megafauna, their life history, trophic interactions with other freshwater species, and interactions with humans necessitates further investigation. Addressing these knowledge gaps will improve our understanding of the ecological roles of freshwater megafauna and support their full potential for facilitating the development of effective conservation and restoration strategies to achieve the coexistence of humans and megafauna.

Nutrients from spawning salmon influence leaf area, tissue density, and nitrogen-15 in riparian plant leaves

Nutrient subsidies have significant impacts on ecosystems by connecting disjunct habitats, often through long-distance animal migrations. Salmon migrations on the North Pacific coasts provide these kinds of nutrient subsidies from senescent fish at the end of their life cycle, which can have significant ecological effects on terrestrial species. This can include impacts on individuals, populations, and communities, where shifts in community composition towards plant species that indicate nitrogen-rich soils have been documented. We investigated the effects of variation in salmon spawning density on the leaf traits of four common riparian plant species on the central coast of British Columbia, Canada. We found that all plant species had higher foliar salmon-derived nitrogen on streams with a higher spawning density. Three of the four species had larger leaves, and one species also had higher leaf mass per area on streams with more salmon. However, we found no differences in leaf greenness or foliar percent nitrogen among our study streams. These results demonstrate that nutrient subsidies from spawning salmon can have significant impacts on the ecology, morphology, and physiology of riparian plants, which lends support to a mechanism by which certain plants are more common on productive salmon streams.

Relationship between salmon egg subsidy and the distribution of an avian predator

As a spatial subsidy, which is the phenomenon of transferring resources from a donor system to a recipient system, anadromous salmonids contribute to the supply of marine-derived nutrients to freshwater and terrestrial systems. Live salmon and salmon carcasses and eggs are utilized by various organisms and affect their abundance and distribution. However, the evaluation of the effect of salmon subsidies on the abundance and distribution of terrestrial animals is biased toward predators or scavengers that utilize spawning adults and carcasses, and few studies have focused on the effect of salmon eggs as a subsidy. To avoid underestimating the function of salmon subsidies, the response to the availability of salmon eggs in various systems should be investigated. Here, we investigated the abundance and feeding behavior of the brown dipper Cinclus pallasii, as a consumer of salmon eggs, based on the hypothesis that the availability of salmon eggs affects the diet composition and stream distribution of this small predator. In addition, to test whether changes in the abundance of brown dippers are determined by salmon spawning, their abundance was compared upstream and downstream of the check dams in three streams during the peak spawning period. Brown dippers used salmon eggs during the spawning season (53.7% of diet composition), and their abundance increased as the number of spawning redds increased. In contrast, this pattern was not observed upstream of the check dam. These results suggested that the abundance and stream distribution of brown dippers vary according to the variation in the spatiotemporal availability of salmon eggs.

Ecosystem services provided by marine and freshwater phytoplankton

Phytoplankton, the ecological group of microalgae adapted to live in apparent suspension in water masses, is much more than an ecosystem's engineer. In this opinion paper, we use our experience as phytoplankton ecologists to list and highlight the services provided by phytoplankton, trying to demonstrate how their activity is fundamental to regulate and sustain Life on our Planet. Although the number of services produced by phytoplankton can be considered less numerous than that produced by other photosynthetic organisms, the ubiquity of this group of organisms, and their thriving across oceanic ecosystems make it one of the biological engines moving our biosphere. Supporting services provided by phytoplankton include almost half of the global primary and oxygen production. In addition, phytoplankton greatly pushes biogeochemical cycles and nutrient (re)cycling, not only in aquatic ecosystems but also in terrestrial ones. In addition, it significantly contributes to climate regulation (regulating services), supplies food, fuels, active ingredients and drugs, and genetic resources (provisioning services), has inspired artistic and craft works, mythology, and, of course, science (cultural services), and much more. Therefore, phytoplankton should be considered in all respects a true biosphere's engineer.

Effects of spawning Pacific salmon on terrestrial invertebrates: Insects near spawning habitat are isotopically enriched with nitrogen-15 but display no differences in body size

When Pacific salmon (Oncorhynchus spp.) spawn and die, they deliver marine-derived nutrient subsidies to freshwater and riparian ecosystems. These subsidies can alter the behavior, productivity, and abundance of recipient species and their habitats. Isotopes, such as nitrogen-15 (15N), are often used to trace the destination of marine-derived nutrients in riparian habitats. However, few studies have tested for correlations between stable isotopes and physiological responses of riparian organisms. We examined whether increases in δ 15N in terrestrial insect bodies adjacent to salmon spawning habitat translate to changes in percent nitrogen content and body size. This involved comparisons between distance from a salmon-bearing river, marine-derived nutrients in soils and insects, soil moisture content, and body size and nitrogen content in two common beetle families (Coleoptera: Curculionidae, Carabidae). As predicted, δ15N in riparian soils attenuated with distance from the river but was unaffected by soil moisture. This gradient was mirrored by δ15N in the herbivorous curculionid beetles, whereas carabid beetles, which feed at a higher trophic level and are more mobile, did not show discernable patterns in their δ15N content. Additionally, neither distance from the river nor body δ15N content was related to beetle body size. We also found that nitrogen-15 was not correlated with total percent nitrogen in insect bodies, meaning that the presence of spawning salmon did not increase the percent nitrogen content of these insects. We conclude that while salmon-derived nutrients had entered terrestrial food webs, the presence of δ15N alone did not indicate meaningful physiological changes in these insects in terms of percent nitrogen nor body size. While stable isotopes may be useful tracers of marine-derived nutrients, they cannot necessarily be used as a proxy for physiologically important response variables.

Salmon subsidies predict territory size and habitat selection of an avian insectivore

The annual migration and spawning event of Pacific salmon (Oncorhynchus spp.) can lead to cross-boundary delivery of marine-derived nutrients from their carcasses into adjacent terrestrial ecosystems. The densities of some passerine species, including Pacific wrens (Troglodytes pacificus), have been shown to be positively correlated with salmon abundance along streams in Alaska and British Columbia, but mechanisms maintaining these densities remain poorly understood. Riparian areas near salmon streams could provide higher quality habitat for birds through greater food availability and more suitable vegetation structure for foraging and breeding, resulting in wrens maintaining smaller territories. We examined relationships between salmon biomass and Pacific wren territory size, competition, and habitat selection along 11 streams on the coast of British Columbia, Canada. We show that male wren densities increase and territory sizes decrease as salmon-spawning biomass increases. Higher densities result in higher rates of competition as male wrens countersing more frequently to defend their territories along streams with more salmon. Wrens were also more selective of the habitats they defended along streams with higher salmon biomass; they were 68% less likely to select low-quality habitat on streams with salmon compared with 46% less likely at streams without salmon. This suggests a potential trade-off between available high-quality habitat and the cost of competition that structures habitat selection. Thus, the marine-nutrient subsidies provided by salmon carcasses to forests lead to higher densities of wrens while shifting the economics of territorial defence toward smaller territories being defended more vigorously in higher quality habitats.

Marine subsidies mediate patterns in avian island biogeography

The classical theory of island biogeography, which predicts species richness using island area and isolation, has been expanded to include contributions from marine subsidies, i.e. subsidized island biogeography (SIB) theory. We tested the effects of marine subsidies on species diversity and population density on productive temperate islands, evaluating SIB predictions previously untested at comparable scales and subsidy levels. We found that the diversity of terrestrial breeding bird communities on 91 small islands (approx. 0.0001–3 km²) along the Central Coast of British Columbia, Canada were correlated most strongly with island area, but also with marine subsidies. Species richness increased and population density decreased with island area, but isolation had no measurable influence. Species richness was negatively correlated with marine subsidy, measured as forest-edge soil δ¹⁵N. Density, however, was higher on islands with higher marine subsidy, and a negative interaction between area and subsidy indicates that this effect is stronger on smaller islands, offering some support for SIB. Our study emphasizes how subsidies from the sea can shape diversity patterns on islands and can even exceed the importance of isolation in determining species richness and densities of terrestrial biota.