The importance of terrestrial-aquatic linkages was evaluated by a large-scale, 3-year exclusion of terrestrial leaf litter inputs to a forest stream. Exclusion of leaf litter had a strong bottom-up effect that was propagated through detritivores to predators. Most invertebrate taxa in the predominant habitat declined in either abundance, biomass, or both, compared with taxa in a nearby reference stream. However, fauna in moss habitats changed little, indicating that different food webs exist in habitats of different geomorphology. Thus, the ecosystem-level consequences of excluding detrital inputs to an ecosystem were demonstrated. Inputs of riparian detritus are essential for conservation or restoration of diverse stream food webs. Detritus, or dead organic matter (I), is the major carbon pathway in most ecosystems: 70 to 90% of all primary production eventually enters the detrital food web (2). The addition of detritus to food webs complicates classical plant-herbivore-predator relationships (3). Indeed, in most headwater streams draining forests in eastern North America, inputs of detritus from the surrounding forest exceed within-stream primary production (4). One of the basic tenets of stream ecology for more than two decades has been the importance of terrestrial-aquatic linkages (5). Although details about linkages between detritivores and detritus processing in streams are well known (2, 6, 7), there is little direct evidence supporting the importance of terrestrial detrital inputs and ecosystem productivity (8) and it is limited to short-term studies in artificial channels (9). It has also been suggested that several generations of consumers are required to detect responses to detrital manipulations (10). We studied the role of detritus in ecosystem productivity by excluding inputs of terrestrial litter to a 180m-long headwater stream, using an overhead canopy and a lateral fence for 3 years (11). We evaluated the impact of the basal resource (terrestrial litter inputs) in this forest stream on abundance, biomass, and production of animals. In addition to examining numerical abundances of populations, we calculated secondary production as the flow (or flux) of mass • area" 1 • time" 1 , which incorpo

The diversity of life in headwater streams (intermittent, first and second order) contributes to the biodiversity of a river system and its riparian network. Small streams differ widely in physical, chemical, and biotic attributes, thus providing habitats for a range of unique species. Headwater species include permanent residents as well as migrants that travel to headwaters at particular seasons or life stages. Movement by migrants links headwaters with downstream and terrestrial ecosystems, as do exports such as emerging and drifting insects. We review the diversity of taxa dependent on headwaters. Exemplifying this diversity are three unmapped headwaters that support over 290 taxa. Even intermittent streams may support rich and distinctive biological communities, in part because of the predictability of dry periods. The influence of headwaters on downstream systems emerges from their attributes that meet unique habitat requirements of residents and migrants by: offering a refuge from temperature and flow extremes, competitors, predators, and introduced species; serving as a source of colonists; providing spawning sites and rearing areas; being a rich source of food; and creating migration corridors throughout the landscape. Degradation and loss of headwaters and their connectivity to ecosystems downstream threaten the biological integrity of entire river networks.

We examined the importance of terrestrial detrital inputs to secondary productivity of a headwater stream. Following a year of pretreatment studies on two headwater streams, we excluded terrestrial litter inputs (=treatment) to one stream while using the other as a reference. We excluded litter for 3 yr followed by 1 yr of small woody debris (≤10 cm diameter) removal and litter exclusion. Monthly benthic samples were collected from dominant mixed substrate (cobble, pebble, and sand‐silt) as well as from moss‐covered bedrock outcrop substrates. We used randomized intervention analysis (RIA) to test the null hypotheses that no change in abundance or biomass of functional feeding groups or specific taxa occurred in the treatment stream relative to the reference stream. Benthic organic matter was significantly lower in mixed substrate habitats of the treatment stream; however, small woody debris did not show a significant reduction prior to manual removal during year 4. At the end of the treatment period, total benthic invertebrate abundance in mixed substrates in the treatment stream was less than one‐tenth of that in the reference stream, and biomass in the treatment stream was one‐sixth of that in the reference stream. Biomass and abundance of shredders, gatherers, total primary consumers, and predators displayed significant treatment effects (P < 0.012–0.00001, RIA) in the mixed substrate habitats during the 4‐yr treatment. Only scraper and filterer functional groups failed to show differences between streams during the 4‐yr treatment. Twenty of the 30 taxa comprising >90% of total secondary production displayed a significant decrease in abundance, biomass, or both, in the treatment stream relative to the reference stream (P < 0.05, RIA). Total secondary production in mixed substrate habitats declined to 22% of pretreatment values by the fourth year of treatment and is among the lowest reported for streams. Removal of small woody debris resulted in an additional 47–50% decrease in abundance, biomass, and production of the benthic fauna compared to the third year of litter exclusion. In contrast, fauna of moss‐covered bedrock substrates displayed no significant differences between streams in terms of any functional group, or in terms of abundance and biomass of individual taxa. Furthermore, secondary production on bedrock outcrops remained similar between streams. Results strongly suggest that food webs of bedrock habitats are not as closely linked to immediate allochthonous inputs from the surrounding forest as those of mixed substrates, although they may be linked in the long term. Using a 9‐yr record, we determined the relationship between organic matter standing crop and invertebrate abundance, biomass, and production in the treatment stream. There is a strong relationship between leaf litter standing crops and secondary productivity in mixed substrate habitats. In contrast, bedrock outcrop fauna showed a stronger relationship to fine benthic organic matter (FBOM) standing crop than to leaf litter. Despite a la...

Although the effects of nutrient enrichment on consumer-resource dynamics are relatively well studied in ecosystems based on living plants, little is known about the manner in which enrichment influences the dynamics and productivity of consumers and resources in detritus-based ecosystems. Because nutrients can stimulate loss of carbon at the base of detrital food webs, effects on higher consumers may be fundamentally different than what is expected for living-plant-based food webs in which nutrients typically increase basal carbon. We experimentally enriched a detritus-based headwater stream for two years to examine the effects of nutrient-induced changes at the base of the food web on higher metazoan (predominantly invertebrate) consumers. Our paired-catchment design was aimed at quantifying organic matter and invertebrate dynamics in the enriched stream and an adjacent reference stream for two years prior to enrichment and two years during enrichment. Enrichment had a strong negative effect on standing crop of leaf litter, but no apparent effect on that of fine benthic organic matter. Despite large nutrient-induced reductions in the quantity of leaf litter, invertebrate secondary production during the enrichment was the highest ever reported for headwater streams at this Long Term Ecological Research site and was 1.2-3.3 times higher than predicted based on 15 years of data from these streams. Abundance, biomass, and secondary production of invertebrate consumers increased significantly in response to enrichment, and the response was greater among taxa with larval life spans < or = 1 yr than among those with larval life spans >1 yr. Production of invertebrate predators closely tracked the increased production of their prey. The response of invertebrates was largely habitat-specific with little effect of enrichment on food webs inhabiting bedrock outcrops. Our results demonstrate that positive nutrient-induced changes to food quality likely override negative changes to food quantity for consumers during the initial years of enrichment of detritus-based stream ecosystems. Longer-term enrichment may impact consumers through eventual reductions in the quantity of detritus.