Colorado Front Range GK-12 : connecting kids & ecology



Aquatic Secondary Production Lab

Stream Ecology Background

In the western US, native cottonwood-willow floodplain forests are being replaced by exotic species such as tamarisk (Tamarix sp.) and Russian olive (Elaeagnus angustifolia) (Everitt 1998; Lessica and Miles 1999). Riparian vegetation occurs on <1% of the western North America landscape, yet it provides habitat for more species of birds than all other vegetation types combined (Knopf et al. 1988). Riparian vegetation influences stream communities by shading, contributing leaf litter, and stabilizing the stream banks. Stream nutrients are transferred to riparian ecosystems when terrestrial predators, such as riparian birds and lizards, consume emergent aquatic mayflies, such as mayflies (Nakano et al. 1999; Sabo and Power 2002).

Headwater and mid-order streams are often forested, heavily shaded, and often light limited (Vannote et al. 1980). Because these streams have low levels of primary production, as much as 70-90% of a headwaters stream's energy budget comes from terrestrially-derived nutrients (Minshall 1967; Hanson et al. 1984). The majority of allochthonous (external) material (49-98%) that falls into streams is leaf litter (Abelho 2001). After nutrients leach from leaves, the leaves are colonized by microbes and then fragmented and consumed by macroinvertebrates. A drastic change in leaf litter quality or quantity has the potential to alter stream invertebrate communities.

Shredder Ecology

Macroinvertebrate shredders convert large organic plant substrates (coarse particulate organic matter - CPOM) into smaller particles (FPOM), either by fragmenting CPOM through chewing and tearing or by ingestion. Approximately 60% of food ingested by shredders is converted to feces which are utilized by other invertebrate groups, such as filter feeders and collector/gatherers (Cummins et al. 1989). Shredders typically represent about 20% of the total invertebrate biomass (or 10% of the numerical abundance) of headwater streams (Petersen et al. 1989), while collector/gatherers compose the remaining 80% (Vannote et al. 1980).

Cummins (1974) proposed the analogy that the bacteria and fungi that colonize a leaf are like peanut butter on a cracker; most of the energy is derived from the peanut butter. Shredders have been shown to consume leaf litter that has been colonized by a biofilm over sterile leaf litter (Cummins and Klug 1979) and invertebrate assimilation is higher for associated microorganisms (50-90%) than for detritus (6-35%) (Berrie 1976 in Cummins and Klug 1979). Feeding trials have shown that shredders growth rate is frequently correlated with leaf nutritional value, but not perfect (Allen 1995).

Invertebrate Secondary Production

Production is a useful invertebrate response variable in streams because it incorporates invertebrate density, biomass, individual growth rate, reproduction, survivorship and development time (Benke 1996). "For the individual organism, production is the growth of its own body. From the standpoint of the functioning of an ecosystem, production is the means by which energy is made available for transmission from one trophic level to the next" (Waters 1977). Secondary production is the accumulation of heterotrophic biomass through time (Benke 1996). Invertebrate production can be represented by:

P = A - R - E

where P = production, A = assimilation (ingestion - egestion), R = respiration, and E = excretion. Assimilation depends on the food quality and how efficiently food is transformed into tissue; assimilation ranges from 5% for detritivores to almost 90% for carnivores (Benke 1996).
The most commonly used method for determining secondary production in streams is the size-frequency method (Benke 1993). Briefly, the method requires that a stream is sampled several times over the course of a year and that the number and size (mass and length) of all individuals in a population is recorded (Benke 1996). Biomass is calculated for each sampling date (mass x number of individuals) and the amount of biomass lost between sampling is estimated (average mass at two sampling dates x number of individuals lost between sampling dates). The lost biomass is used to estimate production. Typical production/biomass (P/B) ratios range from 1-10 but can be as high as 100 for some diptera and mayflies (Benke 1993).

Given the model proposed by Cummins et al. (1989), one would expect that shredders would grow faster on high quality "fast" decomposing litter than on low quality "slow" litter. Growth rates have been measured for a number of invertebrate shredders on a variety of leaf species in order to determine the effect of food quality (Benke 1993).

Results from Tests of Secondary Production in Other Systems

North American and European willows have been planted along streams in Australia in order to increase stream production, because the native eucalyptus leaf litter decomposes slowly and contains nasty phenolic compounds that make them inedible to some invertebrates. Yeates and Barmuta (1999) compared the growth rates two shredders on exotic willow (Salix fragilis) and native eucalyptus (Eucalyptus viminalis) in Australia. They found that both the snail (Physastra gibbosa) and the caddisfly (Leptoceridae: Notalina sp.) grew faster on the green willow than any other leaf type (senescent willow, green eucalyptus, senescent willow). They suggested that the thicker biofilm on green willow leaves may have been responsible for the observed results.

Eucalyptus, native to Australia, has been widely planted for wood production in Spain and Portugal. Canhoto and Graça (1995) compared the growth of cranefly larvae (Tipula lateralis) on eucalyptus and native (alder, chestnut, and oak) leaf litter in experimental chambers. They found that larvae preferred the alder to the other three species. Tipulids grew the fastest on alder and did not grow at all on eucalyptus. Survivorship was best on chestnut and alder (126 days), intermediate on eucalyptus (91 days), and lowest on oak (63 days). Canhoto and Graça (1995) hypothesized that wide scale replacement of native forest by eucalyptus would have deleterious effects on stream communities.


Are shredder growth rates different on native and exotic leaf litter?

Our task: We would like to compare the growth rates of shredders on native and exotic leaf litter.

Experimental Design: We will use a full factorial design

Bugs: We have four shredders to choose from: isopods (Caecidotea communis), amphipods (Hyalella azteca), stoneflies (Pteronarcys californica) and crane flies (Tipula sp.). We (Angie) will collect the invertebrates from the Poudre River before we run the experiment. We can measure shredder growth in one of three ways:

      1. We could develop a size-mass relationship by measuring the invertebrates (e.g. length or head width) and then weighing them. We would use size instead of mass to determine growth rates. This has the advantage of probably being more accurate than wet weight but requres that the bugs hold still while we measure them. Also it requires that we have adequate instruments for measuring the bugs (e.g. microscope with ocular micrometer, caliper).
      2. We could use the wet weight of the shredders throughout the experiment. Generally the bugs are patted dry with a paper towel and weighed on a lab balance. This method has the advantage of being quicker than length measurements but requires that we have a really good scale. However, this method varies with the wetness of the invertebrates and how much food they have in their guts.
      3. We could photograph the invertebrates with a digital camera and compare the photos. I have never heard of anyone doing this, but it could work if we had a set up to photograph the bugs in the same way each time and placed them on the same background each time (e.g. graph paper?).

Leaves: We have Russian olive, tamarisk, cottonwood, and/or willow leaves. We can choose to use all four leaves or increase replication with fewer leaf types. Many researchers use a standard mass of leaves for tests of invertebrate secondary production. Other researchers use a standard area of leaves (e.g. leaf disks). It is also common to have control disks that are kept in aquariums without shredders to determine decomposition due to microbes and fungi alone.

Results: We will compare the shredder growth rates with standard t-tests and analysis of variance (2-way ANOVA). If we choose to use leaf controls we can also compare the leaf decomposition rates.



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    3. Allen, J. D. 1995. Stream Ecology: the structure and function of running waters. Kluwer Academic Publishers.
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This project material is based upon work supported by the National Science Foundation under Grant No. DGE0086443. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.


Principal Invesitigator for the Colorado Front Range GK12 is John Moore and Co-PI's are Dave Swift, Bill Hoyt, Carol Seemueller, and Ray Tschillard. For more information contact Kim Melville-Smith.

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