Invasive Species Rewire the Hidden Highways of Energy Between Ecosystems

Every ecosystem on Earth leaks. Streams spill insects into forests, oceans wash nutrients onto islands, and rivers deliver organic matter to the sea. Ecologists call these cross-boundary flows of energy, nutrients, and materials resource subsidies, and for decades they have studied them as the invisible threads that stitch separate habitats into a single functioning planet. A new systematic review, published in the journal NeoBiota, argues that one of the most powerful and least understood forces acting on these threads is the spread of non-native species. Drawing on 143 publications and 212 documented case studies, the review provides the first comprehensive synthesis of how invasive plants, fish, mammals, and other organisms can add, reduce, or fundamentally rewrite the movement of resources between ecosystems, often with consequences that ripple far beyond the places where they first took hold.

The scale of the blind spot is striking. Research on resource subsidies has historically focused on native species, particularly a handful of conspicuous actors such as spawning salmon, storm-driven seaweed, and emerging swarms of aquatic insects. Meanwhile, thousands of studies have catalogued the impacts of non-native species on biodiversity, yet almost none have asked a deceptively simple question: what happens to the flow of energy between ecosystems when an invader arrives? The answer, the new review shows, is that invaders can do everything from delivering pulses of marine nutrients into Patagonian forests to severing the supply of insects that feed spiders along stream banks. Because these effects are often indirect and rarely the original focus of ecological research, the true footprint of biological invasions may be substantially underestimated.

To build the synthesis, the researchers systematically searched the Scopus database for studies combining the concepts of trophic, spatial, or resource subsidies with non-native, invasive, alien, or introduced species, screening every article published up to the end of 2024. From an initial pool of 307 papers, supplemented by backward reference searches, 143 met the criteria and yielded 212 individual case studies. For each case, the team recorded the continents involved, the donor and recipient ecosystems, the taxonomic identity of the invading species, the affected native biota, the methods used, and the biological and chemical variables measured. The result is a global map of invasion-mediated resource flows that reveals both what science knows and, more revealingly, what it has ignored.

The geographic and taxonomic biases are stark. North America dominates the literature, followed by Europe, where only a handful of countries, including the United Kingdom, Germany, France, Spain, and Portugal, account for most of the output. Africa is severely understudied, contributing just 2.8 percent of all publications, with Asia at 7.7 percent. Oceania presents a particular paradox: despite hosting some of the world’s most threatened endemic biodiversity and suffering some of the worst invasion impacts, only 12 studies from the region examined invasion-mediated connectivity. The tropics, which harbor the greatest biodiversity and face accelerating invasions, remain largely absent from the record. Taxonomically, the picture is equally lopsided. Non-native plants account for 28.8 percent of case studies, fish for 25.5 percent, and mammals for 21.7 percent, while fungi represent a mere 0.9 percent and bacteria 2.4 percent, despite their fundamental roles in decomposition and nutrient cycling.

What the existing cases do show is remarkable in its variety. Non-native species can add entirely new subsidies to ecosystems. Introduced Pacific salmon in Patagonia transport marine-derived nutrients into otherwise nutrient-poor streams and forests when they spawn and die, enriching food webs from invertebrates to birds and mammals. Mass die-offs of the invasive Asian clam Corbicula fluminea generate carrion pulses that feed decomposer communities in both aquatic and terrestrial systems. The nitrogen-fixing albizia tree injects nitrogen into invaded Hawaiian ecosystems, fueling bottom-up changes that cascade through the food web. In some cases, invaders even create spatial connections that never existed before, such as the non-native melaleuca tree in Florida, which forces panthers to travel farther for resources, linking habitats that were previously unconnected.

Just as often, invaders cut existing subsidies off. On tropical islands, non-native rats prey on seabird chicks and eggs, collapsing colonies and halting the flow of marine-derived guano that fertilizes soils and coastal waters. Where rats strip islands of seabirds, the surrounding coral reefs lose nutrients, and reef fish biomass declines. In riparian zones, non-native fish such as brook trout consume terrestrial invertebrates that fall into streams, reducing food available to native species, while also preying on aquatic insect larvae before they can emerge as adults to feed spiders, birds, and bats on land. The green alder sawfly and European bird cherry strip or replace native riparian vegetation, cutting the supply of terrestrial insects to streams. Even behavioral disruption plays a role: freshwater crocodiles in Australia, poisoned by invasive cane toads, abandon terrestrial foraging altogether, severing an aquatic-terrestrial energy pathway.

A third category of impact is subtler: modification. Invaders rarely just add or subtract; they change the character of what flows. The river red gum, an Australian tree introduced elsewhere, sheds leaf litter that leaches more dissolved compounds and breaks down differently than native vegetation, altering the detrital base of streams. Non-native bivalves such as Corbicula fluminea release nitrogen through their biodeposits at rates that reshape nutrient cycling in stream sediments. Armored catfish in nutrient-limited streams create biogeochemical hotspots through their dense daytime aggregations and excretion, changing the quality and availability of nutrients for other organisms. Even marine wrack washing ashore differs in nutritional value depending on whether it comes from native or non-native algae, shifting the composition of the invertebrate communities that depend on it.

The review frames these effects through four critical dimensions: the quantity, quality, timing, and duration of subsidies. Quantity matters because ecosystems have thresholds. Low levels of an invader’s subsidy may boost productivity, but excessive inputs can trigger eutrophication, oxygen depletion, and abrupt ecological collapse, a pattern documented after mass mortalities of invasive bivalves. Quality determines whether a subsidy is assimilated or toxic: some non-native plants leach allelochemicals that harm water fleas and amphibian larvae, while others, like the high-nitrogen Himalayan balsam, support denser and more diverse invertebrate communities than native plants. Timing can desynchronize resource availability from the life cycles of dependent organisms, and duration determines whether effects are transient pulses or chronic pressures that permanently restructure ecosystems. The review notes that these thresholds have never been formally assessed in invaded systems, representing a critical gap.

The methodological landscape reflects the field’s youth. Field-based studies dominate at 43 percent of case studies, followed by stable isotope analysis at 17 percent and diet content analysis. Controlled experiments, which allow manipulation of environmental variables while retaining ecological realism, account for 10 percent. Emerging tools remain rare: DNA analysis appears in just 1.2 percent of cases, remote sensing in 0.4 percent, and field-monitoring techniques such as telemetry and camera trapping in 3.6 percent. The authors argue that this is precisely where the field must go next. Environmental DNA and metabarcoding are revolutionizing the detection of non-native species across ecosystems, while combining molecular tools with fatty acid and stable isotope tracers could reveal the fine-scale mechanisms by which invaders redirect energy flows. Machine learning and artificial intelligence offer the capacity to process large datasets, model invasion pathways, and predict which ecosystems are most vulnerable before invasions occur.

Ultimately, the review’s central message is that biological invasions cannot be understood, managed, or costed without accounting for their cross-ecosystem reach. An invader established in one habitat can extend its ecological footprint well beyond the boundaries of that habitat, affecting stakeholders and ecosystems that may appear entirely disconnected from the site of introduction. As global change drivers such as climate warming, land-use transformation, and pollution continue to alter both the quality and quantity of natural resource flows, the added layer of invasion-mediated disruption becomes increasingly consequential. The authors call for long-term monitoring, multilingual and multi-database literature searches to correct geographic biases, explicit study of overlooked groups such as fungi, bacteria, and amphibians, and the integration of local ecological knowledge in data-poor regions. Without these steps, the global network of invasion-altered resource flows will remain only partially mapped, and conservation strategies will continue to underestimate the true reach of the species humans have moved across the planet.

Subject of Research: How non-native species mediate resource subsidies across ecosystem boundaries

Article Title: Resource subsidies mediated by non-native species: a review

Article References: Resource subsidies mediated by non-native species: a review. (n.d.). https://doi.org/10.3897/neobiota.109.193175

Image Credits: AI Generated

DOI: 10.3897/neobiota.109.193175

Keywords: invasive species, resource subsidies, ecosystem connectivity, biological invasions, cross-ecosystem flows, nutrient cycling, food webs, NeoBiota, trophic ecology, seabirds, riparian ecosystems, eDNA