Freshwater invertebrates

In the laboratory experiments, amphipod Gammarus pseudolimnaneus prefer leaves of maple, Acer saccharum, conditioned by two fungi, Humicola grisea and Heliscus lugdunensis, over uncoditioned leaves (Bärlocher & Kendrick, 1975). Two species of caddisfly larvae (Trichoptera), Hesperophylax magnus and Psychoglypha sp., show feeding preferences to leaves of aspen, Populus tremuloides, colonized by fungi, Flagellospora curvula, Alatospora acuminata and more, over stream detrirus. (Arsuffi & Suberkropp, 1986). Amphipod G. pulex is most attracted to the aufwuchs on the conditioned discs made of poplar (Populus canadensis) leaves and less to the leaves themselves, fungi and bacteria are more important than green algae Scenedesmus obliquus (De Lange et al., 2005).

Generally, feeding preferences in macroinvertebrates are associated with the leaf decomposition rate that, in turn, is related to fungal colonization and protein content in conditioned leaves (e.g., Kaushik & Hynes, 1971). Freshwater amphipods and isopods exhibit feeding preferences in the same order as the leaf decomposition rate, approximately in the following sequence: elm > maple > alder > oak > beech.

In particular, in streams in the Western Oregon, USA, newly fallen leaves of alder, Alnus sp., are quickly colonized by larvae of Leindostonta quercina (Lepidostomatidae), other caddis larvae and snails (Oxytrema) (Anderson & Grafius, 1975). According to these authors, the relatively high consumption rate of the unconditioned leaves by L. quercina demonstrates an exceptional palatability or attractiveness of Alnus leaves.

Graça et al. (2001) have studied feeding preferences in caddisfly larvae, Nectopsyche argentata and Phylloicus priapulus from tropical Venezuela, Sericostoma vittatum from Portugal, as well as amphipod G. pulex from Germany. In general, all shredders exhibit the same high preference for conditioned leaves over unconditioned ones (beech, Fagus sylvatica, alder, Alnus glutinosa, and more), irrespective of the geographical origin of the trees or shredder species.

However, freshwater detritivore-shredders do not per se prefer leaf litter being be able to select actively other food items such as filamentous green algae or macrophytes. Friberg & Jacobsen (1994) have studied feeding preferences of the trichopteran shredder, Sericostoma personatum, and the amphipod shredder, G. pulex, in springbrook with the major food source represented by beech (F. sylvatica) litter. Six food items have been tested: conditioned beech leaves, conditioned alder (A. glutinosa) leaves, conditioned Sitka spruce (Picea sitchensis) needles, fresh beech leaves, fresh macrophyte (Potamogeton perfoliatus) and fresh filamentous green alga (Microspora sp.). Conditioned leaves and needles have been collected directly in the springbrook. Both shredders prefer conditioned Alnus leaves and fresh Microspora, conditioned Fagus leaves and Picea needles are less preferred food items. For larval populations of another trichopteran shredder,  Anabolia nervosa, from the two streams with the different food availability, conditioned Alnus leaves are the most consumed food item, then fresh pondweed P. perfoliatus (Jacobsen & Friberg, 1994). Larvae from the Alnus shaded stream prefer conditioned Alnus leaves over all other food items, while larvae from the stream with the abundant submerged macrophyte do not clearly discriminate between conditioned Alnus leaves and fresh Potamogeton. These data indicate the high attractiveness of conditioned alder leaves for many freshwater detritivore-shredders and their feeding plasticity.

Fish

Rinses of decaying leaf detritus are highly attractive to elvers of American eel, Anguilla rostrata, regardless of where detritus is collected (Sorensen, 1986). In contrast, rinses of living and fallen leaves collected from the forest floor are not attractive.

Coral reef clownfish, Amphiprion percula, and other reef fish use terrestrial chemical cues (leaf litter of beach tree called almond, Terminalia catappa) to find island homes (Dixson et al., 2008, 2011).

Basic References

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Contributions of bacteria, fungi and detritivorous macroinvertebrates (shredders) to leaf litter breakdown in freshwater are studied by Hieber & Gessner (2002), in Germany. They have enclosed alder (Alnus glutinosa) and willow (Salix fragilis) leaves in coarse-mesh bags (5 g dry mass), placed them in the stream during peak leaf fall and retrieved periodically to study. Leaves have been retrieved after 1, 3, 7, 14, 28 and 55 days of submersion. In is shown that leaves decompose rapidly with the exponential breakdown coefficients k of 0.035 per day (alder) and 0.027 per day (willow). Leaves are also quickly colonized within the first 4 weeks of decomposition, when shredder biomass reach 263 and 141 mg dry mass per litter bag, respectively. Maximum bacterial numbers (5,6 and 4,8 x 10¹⁰ g^-1 detrital dry mass) are attained after 8 weeks and corresponded to biomass of 3,6 (alder) and 3,1 (willow) mg dry mass g^-1. This value is less than 5% of the maximum fungal biomass (77 and 70 mg dry mass g^-1, respectively). Aquatic hyphomycetes are equivalent to the daily conidial production of 9.4 and 2.9 mg dry mass g^-1 on alder and willow, respectively. Shredder feeding rates indicate that shredders account for the largest portion of overall leaf mass loss (64 % and 51 % on alder and willow leaves, respectively). According to Hieber & Gessner (2002), fungi contribute at least 15 % and 18 % to leaf litter background, bacterial contribution is estimated at the substantial (7 % and 9 %) levels.

As mentioned above, Hieber & Gessner (2002) have found that conditioned leaves of alder, A. glutinosa, decompose faster (92 % of initial dry mass after 8 weaks) than leaves of willow, S. fragilis (74 %). According to Gessner & Chauvet (1994), leaf decomposition rate (in stream in the French Pyrenees) decreases in the following order: ash (Fraxinus excelsior) (k = 0,059), wild cherry (Prunus avium), alder (A. glutinosa), hazel (Corylus avellana), sycamore (Platanus hybrida), beech (Fagus sylvatica) and evergreen oak (Cuercus ilex) (findings for sycamore and oak are given for comparison). Decomposition rates (in stream in the Northern Appenins, Italy) of conditioned leaves of maple, Aser pseudoplatanus (0,054), and elm, Ulmus minor (0,036), are also high (Gazzera et al., 1993).

Numerous papers have been published that directly examine decomposition in leaf mixtures as well as in the component species decaying alone (for review, see Gartner & Cardon, 2004). From these litter-mix experiments, it is clear that decomposition patterns are not always predictable from single-species dynamics. Non-additive patterns of mass loss are observed in 67 % of tested mixtures, mass loss is often but not always increased when litters of different species are mixed.

In the experiments by Hieber & Gessner (2002), macroinvertebrates have been represented chiefly by stonefly larvae Nemoura, Protonemura, Amphinemura and Leuctra, caddisfly Potamophylax as well as amphipod Gammarus fossarum. Collector-gatherers and scrapers are mayfly larvae such as Baetis and Leptophlebiidae, together accounting for an additional 44% of total numbers but only 19% of biomass. Maximum densities of macroinvertabrates have been observed to 4 weeks after leaf submersion, with an average of 708 (alder) and 422 (willow) animals per 5 g leaf pack (Hieber & Gessner, 2002).

For comparison, leaves of beech, F. sylvatica, are colonized in Irish rivers by macroinvertebrates of 76 taxa (Murphy et al., 1998).

Tiegs et al. (2008) have manipulated the quantity of leaf litter to test whether: 1) greater litter quantity promotes microbial leaf decomposition (through greater microbial inoculum potential), and 2) reduced litter quantity enhances decomposition by leaf-shredding invertebrates (because shredders aggregate on rare resource patches). Decomposition rates and macroinvertebrate colonization of alder leaves placed in coarse- and fine-mesh litter bags, an approach intended to allow or prevent access to leaves by leaf-shredding macroinvertebrates, are also determined. Tiegs et al. (2008) have found that the effects caused by manipulations of litter quantities on leaf decomposition and macroinvertebrate colonization are relatively weak, such large and highly mobile shredder as amphipod G. fossarum can play an instrumental role in causing differences in decomposition in response to litter manipulations.