Multiple stressors in north temperate streams: lessons from linked forest–stream ecosystems in northern Japan

Abstract

SUMMARY1. Streams are highly connected to their landscapes and so are easily altered by multiple stressors that affect both uplands and riparian zones, and the streams themselves. These include dams and diversions, channelisation, deforestation, water pollution, biological invasions and climate change. 2. We review research conducted in Hokkaido Island, northern Japan, which measured the effects of many of these stressors on both stream food webs and fluxes of invertebrates to and from the riparian zone that feed aquatic and terrestrial consumers. About half the energy that sustains fish falls directly into streams as terrestrial invertebrates, and a quarter of the energy needs for riparian birds is supplied by adult aquatic insects emerging from the stream. 3. Single stressors in these Hokkaido streams, including deforestation, channelisation, erosion-control dams, biological invasions and climate change, have drastic effects on stream food webs, the fishes they support and riparian predators (spiders, birds, and bats). Most stressors caused 30-90% declines in foraging, growth, or abundance of aquatic or terrestrial predators. Indirect effects of stressors also cascaded throughout stream food webs and across the aquatic-terrestrial boundary. 4. Effects of individual stressors were largely concordant across spatial scale, through time during years of different productivity and among different food web components. 5. Two studies of multiple stressors revealed that each stressor alone reduced food web components like abundance of stream benthos or riparian spiders to low levels (35-83% reduction; mean 59%), beyond which an additional stressor had little effect. Synergism and antagonism are less relevant when individual stressors have such large effects. 6. Thematic implications: small streams in Hokkaido are highly sensitive to many individual stressors and have little resistance or resilience to their effects. Moreover, each stressor alone can reduce biota strongly, indicating that restoration will need to consider all simultaneously to protect biotic diversity.

Assessing contaminated sediments in the context of multiple stressors

“…The importance of effects due to stressor interactions, however, may be lessened when individual stressors have large effects [185]. In Hokkaido (Japan) streams, extensive experimentation of deforestation, channelization, erosion-control dams, biological invasions, and climate change revealed that most of these stressors caused 30 to 90% declines in foraging, growth, or abundance of aquatic or terrestrial predators [184].…”

“…The question of which organism or community is the most appropriate test species will exist in any human-dominated ecosystem when attempting to rank the relative importance of the existing stressors. We therefore reiterate the call for a more prescriptive, experimental approach to establishing stressor importance that links controlled laboratory studies with field-based ecological manipulations and studies of exposure and response relationships between key physical, biological, and chemical conditions [14,24,181,185,[196][197][198].…”

“…In general, where rainfall is predicted to decrease, we would expect reduced TSS loads, and where storm activity is set to increase, we would predict greater and more frequent sediment resuspension events. Altered flows due to drought or more frequent and severe weather will change harbor and estuarine salinity patterns, lake and marine hypoxia episodes, nutrient inputs, food availability, and reproductive windows of opportunity [129,185,197,[199][200][201][202]. In addition, climate-related changes to ecosystems (temperature, precipitation patterns, ocean acidification, nutrient cycling, food inputs, migration patterns, range expansions and contractions, pathogens, and disease) will result in novel communities, which may lead to changes in ecosystem functioning and other unexpected biological feedbacks or ecological surprises [14,25,197].…”

Crossing boundaries: Shigeru Nakano's enduring legacy for ecology

“…These experiences have forged new friendships and connections that span the globe (Fig. 6) and continue to foster reciprocal collaborations and new research and synthesis among Japanese researchers and western scientists that are advancing ecological research worldwide (e.g., Dunham et al 2008;Fausch et al 2010;Richardson and Sato 2015;Sato et al 2016).…”

Connecting silvan and lacustrine ecosystems: transport of carbon from forests to adjacent water bodies

“…These emergences can form a substantial part of benthic production and are of importance to riparian consumers such as birds, bats, lizards and spiders, and contribute 25-100% of the energy or carbon to such species (e.g. Fausch et al 2010). …”