Rapid and ongoing change creates novelty in ecosystems everywhere, both when comparing contemporary systems to their historical baselines, and predicted future systems to the present. However, the level of novelty varies greatly among places. Here we propose a formal and quantifiable definition of abiotic and biotic novelty in ecosystems, map abiotic novelty globally, and discuss the implications of novelty for the science of ecology and for biodiversity conservation. We define novelty as the degree of dissimilarity of a system, measured in one or more dimensions relative to a reference baseline, usually defined as either the present or a time window in the past. In this conceptualization, novelty varies in degree, it is multidimensional, can be measured, and requires a temporal and spatial reference. This definition moves beyond prior categorical definitions of novel ecosystems, and does not include human agency, self-perpetuation, or irreversibility as criteria. Our global assessment of novelty was based on abiotic factors (temperature, precipitation, and nitrogen deposition) plus human population, and shows that there are already large areas with high novelty today relative to the early 20th century, and that there will even be more such areas by 2050. Interestingly, the places that are most novel are often not the places where absolute changes are largest; highlighting that novelty is inherently different from change. For the ecological sciences, highly novel ecosystems present new opportunities to test ecological theories, but also challenge the predictive ability of ecological models and their validation. For biodiversity conservation, increasing novelty presents some opportunities, but largely challenges. Conservation action is necessary along the entire continuum of novelty, by redoubling efforts to protect areas where novelty is low, identifying conservation opportunities where novelty is high, developing flexible yet strong regulations and policies, and establishing long-term experiments to test management approaches. Meeting the challenge of novelty will require advances in the science of ecology, and new and creative. conservation approaches.

1. Migratory animals can enhance ecosystem productivity through the delivery of material subsidies. Among fish, Pacific salmon are well known to deliver large quantities of nutrients to streams as they die after spawning, but the input pathways by which iteroparous species provide nutrient subsidies have not been resolved. 2. Our objective was to determine the importance of excretion, eggs and carcasses as nutrient sources from a large migration of longnose suckers into a stream draining a moderately agricultural catchment. Additionally, we evaluated nutrient limitation in the stream using nutrient-diffusing substrates and determined the timing of nutrient releases during egg decomposition using a microcosm experiment. 3. Eggs were the largest component of the nitrogen (N, 57%) and phosphorus (P, 76%) inputs from the migration, followed by excretion by live adults (40% N, 16% P). Carcasses were a minor component of inputs. 4. Estimated P inputs from fish were over three times larger than the observed export of dissolved P in this P-limited stream during the 66-day sampling period. In contrast, sucker N inputs were <2% of dissolved N export, which was dominated by NO 3 . However, the dynamics of NH 4 concentrations through the course of the migration were closely associated with estimated NH 4 inputs from excretion and eggs. 5. Eggs and excretion constitute significant nutrient inputs during migrations, even in catchments with elevated nutrient loads from agriculture. Mass mortality is not required for migratory fish to enhance nutrient availability in their spawning habitats. 6. Given that sucker excretion rates and female reproductive investment are typical for freshwater fish, our results suggest that spawning migrations of iteroparous species in rivers around the world may deliver important nutrient subsidies when migrations are large.

Urbanisation is widely associated with a suite of physical, chemical and biological degradation of stream ecosystems, known as “urban stream syndrome.” It is unclear whether urban stream syndrome is applicable to oceanic islands, where marine dispersal of larvae enables diadromous species to continuously recolonise even highly degraded urban streams. The depauperate native fauna of oceanic island streams can be entirely composed of diadromous species, but urban streams food webs are often dominated by introduced predators, competitors and functional groups derived from continental systems. Despite these challenges, some native species appear to thrive in urbanised catchments. Here, we test for urban stream syndrome on oceanic islands by quantifying catchment land use, nutrient concentrations and fish community composition for 37 streams across the Hawaiian archipelago. To assess how native species adapt to food webs altered by species introductions, we quantified trophic responses by examining stomach contents, nitrogen stable isotopes and body condition of Awaous stamineus (an omnivorous goby) in each stream. Urbanisation was consistently associated with nitrogen pollution and replacement of native species with more tolerant exotics. Population densities of three of five native goby species declined sharply with urbanisation, whereas the two other native gobies species were resilient. The trophic position of the omnivore A. stamineus was elevated in urban streams compared to forested catchments, reflecting a shift in stomach contents from algae to greater reliance on exotic aquatic and terrestrial invertebrates. Comparable body condition and resilient population density of A. stamineus across the urbanisation gradient suggest that dietary flexibility buffers this species against environmental degradation. Our findings indicate that the concept of urban stream syndrome is applicable to oceanic islands, yet A. stamineus shows striking resilience. Flexibility in diet, life history and habitat use of this native goby appear to buffer it against the effects of urbanisation compared to most other amphidromous fishes in Hawaiian streams.

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