No abstract

Studies of the effects of cross-habitat resource subsidies have been a feature of food web ecology over the past decade. To date, most studies have focused on demonstrating the magnitude of a subsidy or documenting its effect in the recipient habitat. Ecologists have yet to develop a satisfactory framework for predicting the magnitude of these effects. We used 115 data sets from 32 studies to compare consumer responses to resource subsidies across recipient habitat type, trophic level, and functional group. Changes in consumer density or biomass in response to subsidies were inconsistent across habitats, trophic, and functional groups. Responses in stream cobble bar and coastline habitats were larger than in other habitats. Contrary to expectation, the magnitude of consumer response was not affected by recipient habitat productivity or the ratio of productivity between donor and recipient habitats. However, consumer response was significantly related to the ratio of subsidy resources to equivalent resources in the recipient habitat. Broad contrasts in productivity are modified by subsidy type, vector, and the physical and biotic characteristics of both donor and recipient habitats. For this reason, the ratio of subsidy to equivalent resources is a more useful tool for predicting the possible effect of a subsidy than coarser contrasts of in situ productivity. The commonness of subsidy effects suggests that many ecosystems need to be studied as open systems.

In this paper we review, develop, and differentiate among concepts associated with environmental patterning (patch,-division, and heterogeneity), spatial and temporal scales of ecological proaesses (ecological neighborhoods), and responses of organisms to environmental patterning (relative patch size, relative patch duration, ielative patgh isolation" and grain lespoqse), We gencralizc the concept of ecological neighborhoods to represent regions-of activity or influence during periods of t'ime appropri.aleto-particular ecological processes. Therefore, there is no single ecol6gical neighborhood for any given organism, but rather a number of neighborhoods, each appropriate to different processes. Neighborhood sizes can be estimated by examining the cumulative distribution of activity or influence of an organism as a function of increasingly. large spatial units. The spatial and temporal dimensions of neighborhoods provide the scales necessary for assessing environmental patterning relaiive to particular ecological processes for a given species. Consistent application of the neighborhood-concept will assist in the choice of appropriate study-units, comparisons among different studies, and comparisons between empirical studies and-theoretical postulates.

Summary1. Riparian trees regulate aquatic ecosystem processes, such as inputs of light, organic matter and nutrients, that can be altered dramatically when these trees are harvested. Riparian buffers (uncut strips of vegetation) are widely used to mitigate the impact of clear-cut logging on aquatic ecosystems but there have been few experimental assessments of their effectiveness. 2. Forests along 13 headwater stream reaches in south-western British Columbia, Canada, were clear-cut in 1998, creating three riparian buffer treatments (30-m buffer, 10-m buffer and clear-cut to the stream edge), or left as uncut controls, each treatment having three or four replicates. 3. We predicted that periphyton biomass and insect consumers would increase as buffer width decreased, because of increased solar flux. We used two complementary studies to test this prediction. 4. In one study, we compared benthic communities before and after logging in all 13 streams; a second study focused on periphyton and insect colonization dynamics over 6-week periods in each of four seasons in four streams, one in each treatment. 5. Photosynthetically active radiation, and mean and maximum water temperature, increased as buffer width narrowed. 6. Periphyton biomass, periphyton inorganic mass and Chironomidae abundance also increased as buffer width narrowed, with the largest differences occurring in the clearcut and 10-m buffer treatments. 7. Photosynthetically active radiation, water temperature, periphyton biomass and periphyton inorganic mass were significantly greater in the 30-m buffer treatment than in controls during some seasons. 8. Synthesis and applications. We have shown that a gradient of riparian buffer widths created a gradient in light and temperature that led to non-linear increases in periphyton biomass and insect abundance. For example, Chironomidae abundance was generally greater in the 10-m and 30-m buffer treatments than in controls, whereas this was not always the case in the clear-cut treatment. This pattern may be due to the high sediment content of the periphyton mat in the clear-cut treatment, which potentially limited the response of some insects to increased food resources. Overall, our results indicate that uncut riparian buffers of 30-m or more on both sides of the stream were needed to limit biotic and abiotic changes associated with clear-cut logging in headwater, forested watersheds.

Many species of detritivorous invertebrates in small streams depend almost entirely on inputs of leaf litter for their nutritional requirements, however the concentration of this resource varies considerably seasonally. An experiment designed to test the hypothesis that productivity of coarse particle detritus feeders (shredders) is seasonally food limited was performed using replicate streamside channels that received one of three input rates of whole leaf detritus (rates equal to those naturally falling into montane streams, and two levels of increased inputs). Seven of the nine common shredder species attained significantly higher adult mass, higher densities, or both when food was supplemented. Larval densities of Malenka spp. and Brillia retifinis were significantly higher when additional food was added. Brillia retifinis densities were more than 10 x greater in high food additions than in the "natural" treatment; B. retifinis apparently fills a role as a fugitive in this system and is better able to track shifts in resource abundance by virtue of its short generation time. Zapada cinctipes and Z. haysi were more dense in high and intermediate food input treatments during the last half of the summer. Other taxa had nonsignificant trends to higher densities when extra food was provided. Most of the change in benthic densities can be attributed to decreased rates of emigration with increased food supply. Six of the eight taxa for which adult mass at emergence was measured were significantly more massive (4—46%) when food was added. Both sexes exhibited this increase in mass, but females gained proportionally more in most species. There were no detectable changes in the timing of adult emergence due to food manipulations. The biomass of most taxa increased in proportion of the overall increase in biomass of the common coarse—detritus consumers. The main exceptions to that pattern were a disproportionate increase in the percentage of the biomass represented by Brillia retifinis and a decrease in the representation of Zapada cinctipes. The responses of this community to food supplementation demonstrate food limitation of detritivores. The exponential increase in benthic biomass under food addition shows the scope for productivity if food were not limiting. These coexisting species benefitted from enhanced food supply on a spatial scale that is relevant for population level processes. The large seasonal variation in resource abundance under natural conditions creates bottlenecks during periods of low food supply, which constrain subsequent production even during periods when food is abundant.

Headwater streams make up a large proportion of the total length and watershed area of fluvial networks, and are partially characterized by the large volume of organic matter (large wood, detritus, and dissolved organic matter) and invertebrate inputs from the riparian forest, relative to stream size. Much of those inputs are exported to downstream reaches through time where they potentially subsidize river communities. The relative rates, timing, and conversion processes that carry inputs from small streams to downstream reaches are reasonably well quantified. For example, larger particles are converted to smaller particles, which are more easily exported. Also, dissolved organic matter and surface biofilms are converted to larger particles which can be more easily intercepted by consumers. However, the quality of these materials as it affects biological activity downstream is not well known, nor is the extent to which timing permits biological use of those particles. These ecological unknowns need to be resolved. Further, land uses may disrupt and diminish material transport to downstream reaches by removing sources (e.g., forest harvest), by affecting transport and decomposition processes (e.g., flow regulation, irrigation, changes in biotic communities), and by altering mechanisms of storage within headwaters (e.g., channelization). We present conceptual models of energy and nutrient fluxes that outline small stream processes and pathways important to downstream communities, and we identify informational gaps that, if filled, could significantly advance the understanding of linkages between headwater streams and larger rivers. The models, based on empirical evidence and best professional judgment, suggest that navigable waters are significantly influenced by headwater streams through hydrological and ecological connectivities, and land use can dramatically influence these natural connectivities, impacting downstream riverine ecosystems.

Abstract. Artificial light at night is gaining attention for its potential to alter ecosystems. Although terrestrial ecologists have observed that artificial light at night may disrupt migrations, feeding, and other important ecological functions, we know comparatively little about the role artificial light might play in disrupting freshwater and riparian ecosystems. We identify and discuss four future research domains that artificial light may influence in freshwater and associated terrestrial ecosystems, with an emphasis on running waters: (1) dispersal, (2) population genetics and evolution, (3) ecosystem functioning, and (4) potential interactions with other stressors. We suggest that future experimental and modeling studies should focus on the effects of different spectral emissions by different light sources on freshwater organisms, the spatial and temporal scale over which artificial light acts, and the magnitude of change in light at night across the landscape relative to the distribution of running and standing waters. Improved knowledge about the effects of artificial light on freshwater ecosystems will inform policy decisions about changes to artificial light spectral emissions and distributions.

Fluxes of resource subsidies, such as terrestrial leaf litter to streams and adult aquatic insects to riparian predators, are examples of important links between adjacent ecosystems. The importance of these cross-ecosystem resource flows from donor systems to recipient consumers is increasingly recognized. Streams, especially small streams with their high edge ratio with the terrestrial system, provide excellent models for the study of subsidies and a large portion of this literature has been produced by aquatic scientists. Field experiments manipulating flows between small streams and their riparian areas (e.g. leaf litter, terrestrial invertebrates, and adult aquatic insects to riparian areas) have indicated that consumers in streams and riparian areas are highly dependent upon such subsidies and the value of the subsidies are further modified by patterns of retention and pathways of use. Experiments typically indicate rapid growth or demographic responses by consumers, indicating these populations are resource limited or at levels of incipient population limitation, and can capitalize on short-term resource pulses. More press manipulations are still necessary to determine the dynamical consequences of subsidies for recipient communities. The nature of the subsidy (e.g. species of litter or invertebrates) and its timing are also important details that need further study. Finally, there are opportunities to consider the evolution of life cycle timing (modelling), interception strategies by recipient populations and short-term and long-term responses of communities.