Secondary Production Lab
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.
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).
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
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
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).
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.
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:
- 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
- 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.
- 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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