Detrital Fruit Processing in a Hawaiian Stream Ecosystem¹

Abstract

In temperate forested streams, fruit from riparian trees is generally a minor and seasonal component of the allo‐chthonous detritus. In contrast, riparian fruit input to tropical streams is often high and continuous. Detrital fruit is abundant in some forested Hawaiian streams compared to other forms of riparian detritus, and rates of leaf litter processing by macroscopic invertebrates are very low. These observations suggested that fruit is an important food resource for detritivores. A microcosm system was used to measure the rates at which two common detrital fruits, guava and mango, were processed by two common detritivores, the prawn Macrobrachium lar and the gastropod Tarebia granifera. Comparisons of fruit weight loss rates normalized by detritivore weight indicated that M, lar processed guavas at significantly higher rates than T. granifera, differences in rates of mango processing by M. lar and T. granifera were not significant. Microcosms containing both M. lar and T. granifera were used to test for interactions between the invertebrates that affected rates of mango processing. No interspecific interactions were detected. A field study was conducted in Kaiwiki Stream, Island of Hawaii, to determine rates of detrital fruit input and export. Detrital fruit was supplied to the study area year‐round, with peaks corresponding to summer and autumn fruiting seasons. Guavas and mangos accounted for 85 percent of the fruit biomass entering the stream and 92 percent of the fruit exported from the stream. Mean daily export rates of guava were 7 percent of input, and export rates of mango were 5 percent of input. These measurements suggested that most of the fruit entering the stream is retained and comprises a substantial food resource for detritivores. Comparisons of the biomass‐specific rates at which M. lar and T. granifera processed mangos and guavas with the rates at which mangos and guavas entered Kaiwiki Stream suggested that these invertebrates can process most of the detrital fruit in the stream.

Modeling C and N flows through a stream food web: an inverse approach*

“…Results from Kaiwiki Stream are consistent with this pattern; modeled C flows from algae to metazoans were 23 as high as C flows from riparian plant material to metazoans, and modeled N flows from algae and bryophytes to metazoans were 30% higher than N flows from riparian plant material. This finding also corroborates an earlier observation that Kaiwiki Stream functions largely as a conduit for riparian CPOM; rates of input and turnover are very high, but rates of processing by invertebrates are low (Larned 2000, Larned et al 2001, 2003.…”

“…We used Kaiwiki Stream on the island of Hawaii for our demonstration models. Previous work at Kaiwiki Stream provided some empirical data, with a focus on coarse POM (CPOM) dynamics (Larned 2000, Larned et al 2001, 2003. Our study represents an advance in foodweb analysis in 3 respects.…”

“…Detritus ingestion rates were measured from April to September 1998 in streamside microcosms (Larned et al 2001(Larned et al , 2003. Ingestion rates of detrital leaves and fruit were estimated by mass loss in the presence of macroinvertebrates, corrected for mass loss in control microcosms with no macroinvertebrates.…”

Stable Isotope Evidence for Dietary Overlap between Alien and Native Gastropods in Coastal Lakes of Northern KwaZulu-Natal, South Africa

“…Tarebia granifera has previously been classified as both detritivore and grazer [31], [63]. This study shows that T. granifera has a generalist diet which includes a wide variety of food sources.…”

Do the tropical freshwater fishes feed on aquatic fungi?

“…The importance of nutrient-and energy-rich allochthonous organic matter inputs for fishes and invertebrates in tropical streams is not clearly understood (Wantzen and Junk, 2000;Larned et al, 2001;Wantzen et al, 2008).…”