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Multiple Trophic Levels of a Forest Stream Linked to Terrestrial Litter Inputs

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Science  04 Jul 1997:
Vol. 277, Issue 5322, pp. 102-104
DOI: 10.1126/science.277.5322.102

Abstract

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 (1), 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 180-m-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 incorporates abundance, biomass, individual growth rates, survivorship, and development times into a single metric (12). Production provides the best measure of the relationship of animals to energy flow in ecosystems (12, 13). Benthic animal abundance and biomass were sampled from two distinct habitats: mixed substrates and moss-covered bedrock (14). We used randomized intervention analysis (15) to compare multiyear differences of animal abundance and biomass between reference and treatment streams.

As a result of litter exclusion, we observed major changes in abundance, biomass, and production of the invertebrate fauna in the treated stream. On the basis of modes of feeding [functional feeding groups (FFG) (16)], animal populations in the two stream habitats (14) responded very differently to litter exclusion (Table 1). Significant changes occurred in abundance or biomass of large- and fine-particle-feeding detritivores (shredders and collectors, respectively) and predators in the dominant mixed substrate habitats (cobble, pebble, and gravel-sand). However, on moss-covered bedrock, there were no significant differences between streams for abundance or biomass of animals belonging to any FFG.

Table 1

Average monthly abundance (number/m2) and biomass (mg AFDM/m2) ± 1 SD in mixed substrate and bedrock outcrop habitats of the reference and litter-exclusion stream before treatment (pretreatment period was September 1992 to August 1993,n = 12) and during treatment (litter exclusion period was September 1993 to August 1996, n = 36). Abundance and biomass in the mixed substrates of the litter-excluded stream declined with each successive year of treatment. 1° consumers, primary consumers; Invert. pred., invertebrate predators; Ref., reference stream; Excl., exclusion stream.

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Seventeen of the 29 major taxa in the mixed substrates, constituting 93 to 97% of the benthic production in the reference and treatment streams, displayed significant reductions in either abundance or biomass, or both, during the period of exclusion (Table2). Several invertebrates that failed to show significant decreases are those known to feed on woody debris or organic particles buried in stream sediments (17), which were present throughout the 3-year exclusion period (18).

Table 2

RIA probabilities of change in benthic populations between reference and treatment stream abundance and biomass [log (x + 1) transformed data] for specific invertebrate taxa (organized by FFG) for mixed substrates after 3 years of litter exclusion (n = 48). Insect orders are as follows: D, Diptera; E, Ephemeroptera; NI, non-insect; O, Odonata; P, Plecoptera; and T, Trichoptera. Diplectrona modesta was the only taxon that increased in the treatment stream relative to the reference stream.

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Patterns of secondary production for mixed substrates exhibited diverging trends in the litter exclusion and reference streams (Fig.1), as observed for abundance and biomass. In contrast, production on moss-covered bedrock substrates in the two streams followed similar patterns throughout the 4-year period. Despite the close proximity of mixed substrate and bedrock habitats, our results indicate that different food webs occur within these different habitats. These results also show that geomorphology, that is, the physical nature of the substratum, affects resource availability to consumers. Fauna on the steep-gradient, moss-covered bedrock habitats rely on small particles transported by streamflow or trapped within the moss (19). Fine organic particles stored within moss-covered bedrock habitats did not decrease during the 3 years of the study, and total export of fine particles from the exclusion stream did not decrease until the third year (20). These findings indicate that this food resource was largely maintained throughout the observation period. Thus, the bedrock outcrop community is less directly dependent on leaf litter than the community found in mixed substrates (Table 1 and Fig. 1).

Figure 1

Annual secondary productivity of benthic invertebrates. Year 1 was the pretreatment year followed by 3 years of litter exclusion. Data are shown for (A) mixed substrate habitats in the reference and litter exclusion streams and (B) moss-covered bedrock outcrop habitats in the same streams.

Predators also displayed strong reductions in the exclusion stream, following the trend of total primary consumers (Tables 1 and2). Invertebrate predator production in fishless, headwater streams at Coweeta usually represents about 24 to 33% of total invertebrate production (14). Predator production declined in the treatment stream during each successive year of treatment, which suggests bottom-up effects of exclusion. Some common prey also decreased in abundance and biomass during exclusion, such as midge larvae (Order: Diptera) and copepods, both of which have high growth rates in these streams (21). We found a strong positive regression between total nonpredator and predator production in the litter exclusion stream (Fig. 2) (22). Furthermore, the slope (0.36) of the relationship between total invertebrate production and that of predators is remarkably similar to values for efficiency of conversion of ingested food by invertebrate predators (23) and suggests that predators are food-limited in the exclusion stream. These data imply that invertebrate predators in these headwater streams consume most of the invertebrate production, suggesting that invertebrate predators may exert top-down effects on their prey. Over the range of productivity measured, the close agreement between predators and prey in Fig. 2 is more consistent with models based on co-limitation by resources and predators (24) rather than top-down models (25). Others (3, 26) have suggested that top-down effects occur primarily in simple plant-herbivore-predator food chains and are not observed in speciose communities with an externally subsidized detrital energy base. However, in addition to the bottom-up effects demonstrated by litter exclusion, top-down effects appear to be important in this detrital-based stream as in other systems (27).

Figure 2

Relationship between nonpredator invertebrate production and that of invertebrate predators in mixed substrates of the litter exclusion stream. Each point represents an annual value. Four of these years represent prior studies (22), and the other 4 years are pretreatment and 3 years of exclusion (three points in lower left).

Multiyear manipulations of entire ecosystems are important tools to assess environmental change and the factors controlling ecosystem-level processes (28). Bottom-up effects have been studied by adding nutrients to lakes (29) and streams (30), but ecosystem-level studies examining the effects of resource reduction on communities are rare. Yet, many natural and anthropogenic disturbances reduce terrestrial litter inputs to streams, for example, fire (31), logging (32), land-use change (33), channelization (34), and grazing (35). However, these activities often induce multiple effects that confound analyses of biotic responses to disturbance: altered hydrology; enhanced sediment, nutrient, and solar inputs; and shifts in the relative importance of detrital inputs and stream primary production. These diverse direct and indirect effects complicate analyses of animal community response to disturbance. We demonstrated the consequences of disrupting leaf litter inputs to aquatic community structure and productivity, while minimizing the indirect effects that occur with more complex disturbances.

Experimental exclusion of leaf litter has demonstrated a strong effect of detrital resource reduction propagated through detritivores to predators. Our study provides experimental evidence of the importance of terrestrial-aquatic ecotones to aquatic diversity and productivity. Human actions have resulted in worldwide loss and degradation of riparian zones (36), thereby altering the supply of leaf litter to stream ecosystems. Maintaining or reestablishing these inputs of riparian detritus is an essential element of conservation or restoration of diverse riverine food webs.

  • * To whom correspondence should be addressed. E-mail: wallace{at}sparc.ecology.uga.edu

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