Linking Species And Ecosystems: Different Biotic Assemblages Cause Interstream Differences In Organic Matter - Statistical Data Included

Ecology, Sept, 1999 by C. M. Pringle, Nina Hemphill, W. H. McDowell, Angela Bednarek, James G. March

INTRODUCTION

There is a critical need for integration of two major subdisciplines of ecology: population/community ecology and ecosystem ecology (e.g., Jones and Lawton 1995). Species, populations, and communities embedded within ecosystems can alter ecosystem function (Chapin et al. 1997), and consideration of their effects is a powerful way to conceptualize and analyze ecological systems (Huntly 1995). A major challenge for ecologists is to determine how different biotic assemblages can determine ecosystem processes within the landscape. In aquatic systems, very little is known concerning the role of omnivorous consumers (defined here as organisms that consume both plant and animal material) compared to our knowledge of how predators and herbivores influence ecosystem functioning (e.g., Carpenter et al. 1987, Carpenter 1988).

In stream ecosystems, invertebrate consumers can have important impacts on organic matter processing (Short and Maslin 1977, Wallace et al. 1982, Webster 1983, Wallace et al. 1987). It is not known, however, to what extent different consumer assemblages can influence differences in the quality and quantity of organic material among streams (but see Wallace et al. [1991]). Several studies have documented that bottom-feeding fishes and shrimps reduced sediments and organic material in depositional stream environments (e.g., Power 1984, Pringle et al. 1993, Flecker 1996, Pringle and Hamazaki 1998). Studies have also indicated that large densities of filter-feeding insects reduced downstream transport of suspended particulate organic matter (e.g., Maciolek and Tunzi 1968). Omnivorous filter-feeding hydropsychid caddisflies (Trichoptera: Hydropsychidae) have been found to select high quality food items such as diatoms and animals from stream drift (Benke and Wallace 1980, Petersen 1985, 1989, Georgian and Thorp 1992); such selectivity suggests that they might impact the quantity and type of organic matter in suspension (Wallace and Webster 1996). It thus follows that differences in biotic assemblages may result in interstream differences in levels of fine benthic particulate organic material and concomitant differences in the availability of this important food resource and associated nutrients. Accordingly, elimination of 90% of the insect biomass in a south Appalachian headwater stream (through whole-stream insecticide treatment) reduced leaf litter breakdown and the export of coarse particulate organic material compared to adjacent untreated reference streams (Wallace et al. 1982, Cuffney et al. 1990, Wallace et al. 1991).

In many streams, the manner in which consumers affect ecosystem properties and processes remains to be tested using techniques with adequate power to discriminate ecologically meaningful effects. For example, while it has been shown that invertebrate consumers can have important influences on nutrient cycles, primary productivity, decomposition, and the translocation of materials in streams, most previous studies have been constrained by the use of enclosures or exclosures that didn't allow for sufficient exchange with the stream environment, the use of unnatural densities of consumers, and/or the lack of consideration of abiotic forces such as hydrologic regime (see review by Wallace and Webster [1996]). In this study, we incorporate the natural background hydrologic regime into our experiment by using an electric exclusion technique that is resistant to high stream discharge (Pringle and Blake 1994, Pringle and Hamazaki 1997, 1998). This technique allows us simultaneously to exclude macrobiota and to quantify their effects on the benthic depositional environment, without the artifact of high sedimentation that accompanies the use of cage exclosures.

The Caribbean National Forest (CNF) in northeastern Puerto Rico [ILLUSTRATION FOR FIGURE 1 OMITTED] provides an ideal study area to examine the role of biotic assemblages in dictating differences in organic matter quality and quantity among streams, given the distinct differences in macrobiotic communities that have been observed among streams. Nine major streams drain the CNF [ILLUSTRATION FOR FIGURE 1 OMITTED]. Some streams are dominated by an abundance of omnivorous shrimps (Atyidae and Xiphocarididae: Decapoda) but lack predaceous fishes, while others have relatively few shrimps and contain fishes that prey upon shrimps (e.g., Covich and McDowell 1996, Pringle 1996; Hemphill and Garcia, unpublished manuscript). This study is an outgrowth of previous studies, which have shown that shrimps (Atya and Xiphocaris) are omnivorous and play a variety of ecological roles. Shrimps process and assimilate fine particulate organic matter through both filtering (Covich 1988) and gathering (Pringle and Blake 1994) feeding modes, reduce organic and inorganic materials in benthic environments through direct ingestion and potentially through bioturbation (Pringle et al. 1993), reduce algal biomass and alter algal community composition through grazing activities (Pringle 1996), and decrease densities of sessile insects through disturbance and/or direct ingestion (Pringle et al. 1993). None of these aforementioned studies assessed how differences in macrobiotic assemblages among streams can affect interstream differences in both the quality and quantity of organic matter. Using cage enclosures/exclosures, Pringle et al. (1993) found that shrimps reduced sediment accrual on benthic substrata. However, cages experienced unnaturally high sedimentation, and quantitative measurements (mass of sediment per square meter) reflective of natural conditions were not possible. Subsequent short-term experiments, using electricity to exclude shrimps (Pringle and Blake 1994), allowed quantification of effects of natural densities of atyid shrimps on sediment cover. However, these experiments were confined to one stream over an 8-d period and did not assess shrimp effects on the quality of benthic sedimentary material.