Insect Ecology - An Ecosystem Approach 2nd ed - Chapter 14 pps

Decomposition can be categorized into four component processes: dation, abiotic catabolism resulting from exposure to solar radiation; leaching, photooxi-the loss of soluble materials as

14 Decomposition and

Pedogenesis

I Types and Patterns of Detritivory and Burrowing

A Detritivore and Burrower Functional Groups

B Measurement of Detritivory, Burrowing, and Decomposition Rates

C Spatial and Temporal Patterns in Processing of Detritus and Soil

II Effects of Detritivory and Burrowing

A Decomposition and Mineralization

B Soil Structure, Fertility, and Infiltration

C Primary Production and Vegetation Dynamics

III Summary

DECOMPOSITION IS THE BREAKDOWN OF DEAD ORGANIC MATTER THATeventually results in release of CO2, other organic trace gases, water, mineralnutrients, and energy Pedogenesis (soil development) largely reflects the activi-ties of animals that mix organic matter with mineral soil These two processescontribute greatly to the capacity of a site to support primary production Accu-mulated organic litter represents a major pool of energy and nutrients in manyecosystems Carbon and other nutrients released through decomposition can beacquired by plants or microbes or returned to abiotic pools (see Chapter 11)

Incorporation of decay-resistant organic matter and nutrients into soil increasesfertility, aeration, and water-holding capacity Release of CO2, CH4, and othertrace gases affects atmospheric conditions and global climate

Decomposition can be categorized into four component processes: dation, abiotic catabolism resulting from exposure to solar radiation; leaching,

photooxi-the loss of soluble materials as a result of percolation of water through material;

comminution, the fragmentation of organic litter, largely as a result of detritivory;

and mineralization, the catabolism of organic molecules by microorganisms.

Vossbrinck et al (1979) found that when arthropods and microbes were excluded,

detritus lost only 5% mass, due entirely to leaching or photooxidation A variety

of macroarthropods, mesoarthropods, and microarthropods are the primarydetritivores in most ecosystems The feeding and burrowing activities of manyanimals, including ants, termites, and other arthropods, redistribute and mix soiland organic material Burrowing also increases soil porosity, thereby increasingaeration and water-holding capacity

The effects of arthropod detritivores and burrowers on decomposition andsoil development have been the most widely studied effects of arthropods on

ecosystem processes (e.g., Ausmus 1977, Coleman et al 2004, Crossley 1977,

405

Eldridge 1993, 1994, Seastedt 1984, Swift 1977, Swift et al 1979, Whitford 2000, Wotton et al 1998) Arthropod detritivores and burrowers are relatively accessi-

ble and often can be manipulated for experimental purposes Their key butions to decomposition and mineralization of litter (both fine or suspendedorganic matter and coarse woody debris) and pedogenesis have been demon-strated in virtually all ecosystems Indeed, some aquatic and glacial ecosystemsconsist of arthropod detritivores and associated microorganisms feeding entirely

contri-on allochthcontri-onous detritus (J Edwards and Sugg 1990, Oertli 1993, J Wallace

et al 1992) Effects of detritivorous and fossorial species on decomposition and

soil mixing depend on the size of the organism, its food source, type and rate ofdetritivory, volume of displaced litter or soil, and type of saprophytic microor-ganisms inoculated into litter Although most studies have addressed the effects

of detritivores and burrowers on soil processes, some have documented effects

of animal contributions to soil development and biogeochemical cycling toprimary production as well

I TYPES AND PATTERNS OF DETRITIVORY AND BURROWING

A Detritivore and Burrower Functional Groups

Functional groups of detritivorous and burrowing arthropods have been guished on the basis of principal food source, mode of feeding, and microhabitat

distin-preferences (e.g., J Moore et al 1988, J Wallace et al 1992) For example,

func-tional groups can be distinguished on the basis of seasonal occurrence, habitats,and substrates (e.g., terrestrial vs aquatic, animal vs plant, foliage vs wood, arbo-real vs fossorial) or particular stages in the decomposition process (N Anderson

et al 1984, Hawkins and MacMahon 1989, Schowalter and Sabin 1991, Schowalter et al 1998, Seastedt 1984, Siepel and de Ruiter-Dijkman 1993, Tantawi

et al 1996, Tullis and Goff 1987, J Wallace et al 1992, Winchester 1997, Zhong

and Schowalter 1989)

General functional groupings for detritivores are based on their effect on

decomposition processes Coarse and fine comminuters are instrumental in the

fragmentation of litter material Major taxa in terrestrial ecosystems include lipedes, earthworms, termites, and beetles (coarse) and mites, collembolans, and

mil-various other small arthropods (fine) Many species are primarily fungivores or bacteriovores that fragment substrates while feeding on the surface microflora.

Many fungivores and bacteriovores, including nematodes and protozoa, as well

as arthropods, feed exclusively on microflora and affect the abundance and

dis-tribution of these decomposers (e.g., Santos et al 1981) A number of species, including dung beetles, millipedes, and termites, are coprophages, either feeding

on feces of larger species or reingesting their own feces following microbial decayand enrichment (Cambefort 1991, Coe 1977, Dangerfield 1994, Holter 1979,Kohlmann 1991, McBrayer 1975)

In aquatic ecosystems scrapers (including mayflies, caddisflies, chironomid

midges, and elmid beetles), which graze or scrape microflora from mineral and

organic substrates, and shredders (including stoneflies, caddisflies, crane flies,

crayfish, and shrimp), which chew or gouge large pieces of decomposing

mate-rial, represent coarse comminuters; gatherers (including stoneflies, mayflies, crane

flies, elmid beetles, and copepods), which feed on fine particles of decomposing

organic material deposited in streams, and filterers (mayflies, caddisflies, and black

flies), which have specialized structures for sieving fine suspended organic rial, represent fine comminuters (Cummins 1973, J Wallace and Webster 1996,

mate-J Wallace et al 1992).

Xylophages are a diverse group of detritivores specialized to excavate and

fragment woody litter Major taxa include scolytid, buprestid, cerambycid and

lyctid beetles, siricid wasps, carpenter ants, Camponotus spp., and termites (Fig.

14.1), with different species often specialized on particular wood species, sizes, orstages of decay (see Chapter 10) Most of these species either feed on fungal-colonized wood or support mutualistic, internal, or external fungi or bacteria that

I TYPES AND PATTERNS OF DETRITIVORY AND BURROWING 407

FIG 14.1 Melanophila sp (Coleoptera: Buprestidae) larva in mine in phloem of

recently killed Douglas-fir tree in western Oregon The entire phloem volume of this tree has been fragmented and converted to frass packed behind mining larvae of this species, demonstrating detritivore capacity to reduce detrital biomass Please see extended permission list pg 572.

digest cellulose and enhance the nutritional quality of wood (e.g., Breznak andBrune 1994, Siepel and de Ruiter-Dijkman 1993; see Chapter 8).

Carrion feeders represent another specialized group that breaks down animal

carcasses Major taxa include staphylinid, sylphid, scarabaeid, and dermestidbeetles; calliphorid, muscid, and sarcophagid flies; and various ants Differentspecies usually specialize on particular stages of decay (see Figs 10.3 and 10.4)and on particular animal groups (e.g., reptiles vs mammals) (E Watson andCarlton 2003)

An important consequence of litter fragmentation by arthropods is increasedsurface area for microbial colonization and decomposition Microbes also arecarried, either passively through transport of microbes acquired during feeding

or dispersal or actively through inoculation of mutualistic associates, to fresh faces during feeding

sur-Many detritivores redistribute large amounts of soil or detritus during ing or feeding activities (e.g., Kohlmann 1991) However, nondetritivores alsocontribute to mixing of soil and organic matter Fossorial functional groups can

forag-be distinguished on the basis of their food source and mechanism and volume of

soil/detrital mixing Subterranean nesters burrow primarily for shelter

Verte-brates (e.g., squirrels, woodrats, and coyotes) and many inverteVerte-brates, includingcrickets and solitary wasps, excavate tunnels of various sizes, usually depositing

soil on the surface and introducing some organic detritus into nests Gatherers,

primarily social insects, actively concentrate organic substrates in colonies Antsand termites redistribute large amounts of soil and organic matter during con-struction of extensive subterranean, surficial, or arboreal nests (J Anderson 1988,Haines 1978) Subterranean species concentrate organic matter in nests exca-vated in soil, but many species bring fine soil particles to the surface and mix soilwith organic matter in arboreal nests or foraging tunnels These insects can affect

a large volume of substrate (up to 103m3), especially as a result of restructuring

and lateral movement of the colony (Hughes 1990, Moser 1963, Whitford et al 1976) Fossorial feeders, such as gophers, moles, earthworms, mole crickets

(Gryllotalpidae), and benthic invertebrates, feed on subsurface resources (plant,animal, or detrital substrates) as they burrow, constantly mixing mineral substrateand organic material in their wake

B Measurement of Detritivory, Burrowing, and Decomposition Rates

Evaluation of the effects of detritivory and burrowing on decomposition and soilmixing requires appropriate methods for measuring rates of these processes.Several methods have been used to measure rates of decomposition and soil

mixing (Coleman et al 2004).

Detritivory can be measured by providing experimental substrates and suring colonization and consumption rates K Johnson and Whitford (1975)measured the rate of termite feeding on an artificial carbohydrate source andnatural substrates in a desert ecosystem Edmonds and Eglitis (1989) and Zhongand Schowalter (1989) measured the rate of wood-borer colonization and exca-

mea-vation in freshly cut tree boles Dissection of wood samples is necessary for urement of excavated volume for small insects Radiography can be used tomeasure larger volumes (e.g., termite galleries).

meas-Detritivory often has been estimated by multiplying the per capita feeding

rate for each functional group by its abundance (N.Anderson et al 1984, Cárcamo

et al 2000, Crossley et al 1995, Dangerfield 1994) Cárcamo et al (2000) estimated consumption of conifer needle litter by the millipede, Harpaphe haydeniana, at

about 90 mg g-1animal biomass day-1, a rate that could account for processing of36% of annual litterfall Laboratory conditions, however, might not represent thechoices of substrates available under field conditions For example, Dangerfield(1994) noted that laboratory studies might encourage coprophagy by millipedes

by restricting the variety of available substrates, thereby overrepresenting this

aspect of consumption Mankowski et al (1998) used both forced-feeding and

choice tests to measure wood consumption by termites when a variety of strate types was available or restricted

sub-Radioisotope movement from litter provided early data on decompositionrate (Witkamp 1971) Stable isotopes (e.g.,13C,14C, and 15N) are becoming widely

used to measure fluxes of particular organic fractions (Ågren et al 1996, Andreux

et al 1990, Horwath et al 1996, Mayer et al 1995, Sˇantru°cˇková et al 2000, Spain and Le Feuvre 1997, Wedin et al 1995) The most widely used techniques for

measuring decomposition rates in terrestrial and aquatic ecosystems involvemeasurement of respiration rate, comparison of litterfall and litter standing crop,and measurement of mass loss (J Anderson and Swift 1983, Bernhard-Reversat

1982, Seastedt 1984, Witkamp 1971, Woods and Raison 1982) These techniquestend to oversimplify representation of the decomposition process and conse-quently yield biased estimates of decay rate

Respiration from litter or soil represents the entire heterotrophic community

as well as living roots Most commonly, a chamber containing sodalime or a tion of NaOH is sealed over litter for a 24-hour period, and CO2efflux is mea-sured as the weight gain of sodalime or volume of acid neutralized by NaOH (N Edwards 1982) Comparison of respiration rates between plots with litterpresent and plots with litter removed provides a more accurate estimate of res-piration rates from decomposing litter, but separation of litter from soil is diffi-cult and often arbitrary (J Anderson and Swift 1983, Woods and Raison 1982)

solu-More recently, gas chromatography and infrared gas analysis (IRGA) have beenused to measure CO2efflux (Nakadai et al 1993, Parkinson 1981, Raich et al 1990).

The ratio of litterfall mass to litter standing crop provides an estimate of thedecay constant, k, when litter standing crop is constant (Olson 1963) Decay ratecan be calculated if the rate of change in litter standing crop is known (Woodsand Raison 1982) This technique also is limited by the difficulty of separatinglitter from underlying soil for mass measurement (J Anderson and Swift 1983,Spain and Le Feuvre 1987, Woods and Raison 1982)

Weight loss of fine litter has been measured using tethered litter, litterbags,and litterboxes Tethering allows litter to take a natural position in the litterbedand does not restrict detritivore activity or alter microclimate but is subject toloss of fragmented material and difficulty in separating litter in late stages of

I TYPES AND PATTERNS OF DETRITIVORY AND BURROWING 409

decay from surrounding litter and soil (N Anderson et al 1984, Birk 1979,

Witkamp and Olson 1963, Woods and Raison 1982)

Litterbags provide a convenient means for studying litter decomposition(Crossley and Hoglund 1962, C Edwards and Heath 1963) Litterbags retainselected litter material, and mesh size can be used to selectively restrict entry bylarger functional groups (e.g., C Edwards and Heath 1963, Wise and Schaefer1994) However, litterbags may alter litter microclimate and restrict detritivoreactivity, depending on litter conformation and mesh size Moisture retentionbetween flattened leaves apparently is independent of mesh size Exclusion oflarger detritivores by small mesh sizes has little effect, at least until litter has beenpreconditioned by microbial colonization (J Anderson and Swift 1983, Macauley

1975, O’Connell and Managé 1983, Spain and Le Feuvre 1987, Woods and Raison1982) However, exclusion of predators by small mesh sizes can significantly affect

detritivore abundances and decomposition processes (M Hunter et al 2003).

Large woody litter (e.g., tree boles) also can be enclosed in mesh cages for mental restriction of colonization by wood-boring insects The potential inter-ference with decomposition by small mesh sizes has been addressed in somestudies by minimizing leaf overlap (and prolonged moisture retention) in largerlitterbags, using small mesh on the bottom to retain litter fragments and largemesh on the top to maximize exchange of moisture and detritivores, and mea-suring decomposition over several years to account for differences resulting from

experi-changing environmental conditions (J Anderson et al 1983, Cromack and Monk

1975, Woods and Raison 1982, 1983) Despite limitations, litterbags have been thesimplest and most widely used method for measuring decomposition rates andprobably provide reasonably accurate estimates (Seastedt 1984, Spain and LeFeuvre 1987, Woods and Raison 1982)

More recently, litterboxes have been designed to solve problems associatedwith litterbags Litterboxes can be inserted into the litter, with the open top pro-viding unrestricted exchange of moisture and detritivores (Seastedt and Crossley 1983), or used as laboratory microcosms to study effects of decomposers

(Haimi and Huhta 1990, Huhta et al 1991) Similar constructions can be porated into streams for assessment of detrital decomposition (March et al 2001).

incor-Measurement of wood decomposition presents special problems, including thelong timeframe of wood decomposition; the logistical difficulties of experimen-tal placement; and manipulation of large, heavy material Decomposition of largewoody debris represents one of the longest ecological processes, often spanning

centuries (Harmon et al 1986) This process traditionally was studied by

com-paring mass of wood of estimated age to the mass expected for the estimatedoriginal volume, based on particular tree species However, decomposition ofsome wood components begins only after lag times of up to several years, decom-position of standing tree boles is much slower than fallen boles, and differences

in chemistry and volume between bark and wood components affect overall

decay rates (Harmon et al 1986, Schowalter et al 1998).

Abundances of detritivore functional groups can be manipulated to some

extent by use of microcosms (Setälä and Huhta 1991, Setälä et al 1996),

selec-tive biocides or other exclusion techniques (Crossley and Witkamp 1964,

C Edwards and Heath 1963, González and Seastedt 2001, E Ingham et al 1986, Macauley 1975, Pringle et al 1999, Santos and Whitford 1981, Schowalter et al.

1992, Seastedt and Crossley 1983, J Wallace et al 1991) or by adding or ing detritivores in new substrates (González and Seastedt 2001, Progar et al.

simulat-2000) Naphthalene and chlordane in terrestrial studies (Crossley and Witkamp

1964, Santos and Whitford 1981, Seastedt and Crossley 1983, Whitford 1986) and

methoxychlor or electric fields in aquatic studies (Pringle et al 1999, J Wallace

et al 1991) have been used to exclude arthropods However, E Ingham (1985)

reviewed the use of selective biocides and concluded that none had effects limited

to a particular target group, limiting their utility for evaluating effects of vidual functional groups Furthermore, Seastedt (1984) noted that biocidesprovide a carbon and, in some cases, nitrogen source that may alter the activity

indi-or composition of microflindi-ora Mesh sizes of litterbags (see later in this chapter)can be manipulated to exclude detritivores larger than particular sizes, but thistechnique often alters litter environment and may reduce fragmentation, regard-less of faunal presence (Seastedt 1984)

Few experimental studies have compared effects of manipulated abundances

of boring insects on wood decomposition (Edmonds and Eglitis 1989, Progar

et al 2000, Schowalter et al 1992) Some studies have compared species or

functional group abundances in wood of estimated age or decay class, but suchcomparison ignores the effect of initial conditions on subsequent communitydevelopment and decomposition rate Prevailing weather conditions, the physi-cal and chemical condition of the wood at the time of plant death, and prior col-onization determine the species pools and establishment of potential colonists

Penetration of the bark and transmission by wood-boring insects generally itate microbial colonization of subcortical tissues (Ausmus 1977, Dowding 1984,Swift 1977) Käärik (1974) reported that wood previously colonized by moldfungi (Ascomyctina and Fungi Imperfecti) was less suitable for establishment by

facil-decay fungi (Basidiomycotina) than was uncolonized wood Mankowski et al.

(1998) reported that wood consumption by termites was affected by wood speciesand fungal preconditioning Hence, experiments should be designed to evaluateeffects of species or functional groups on decomposition over long time periodsusing wood of standard size, composition, and condition

Assessing rates of burrowing and mixing of soil and litter is even more lematic A few studies have provided limited data on the volume of soil affected

prob-through excavation of ant nests (Moser 1963, Tschinkel 1999, Whitford et al.

1976) However, the difficulty of separating litter from soil limits measurement

of mixing Tunneling through woody litter presents similar problems Zhong andSchowalter (1989) dissected decomposing tree boles to assess volume of woodexcavated or mixed among bark, wood, and fecal substrates

C Spatial and Temporal Patterns in Processing of Detritus and Soil

All, or most, dead organic matter eventually is catabolized to CO2, water, andenergy, reversing the process by which energy and matter were fixed in primary

I TYPES AND PATTERNS OF DETRITIVORY AND BURROWING 411

production Some materials are decomposed more readily than are others; someprocesses release carbon primarily as methane; and some enter long-term storage

as humus, peat, coal, or oil Moisture, litter quality (especially lignin and nitrogencontent), and oxygen supply are extremely important to the decomposition

process (Aerts 1997, Birk 1979, Cotrufo et al 1998, Fogel and Cromack 1977,

Fonte and Schowalter 2004, González and Seastedt 2001, Meentemeyer 1978,

Progar et al 2000, Seastedt 1984, Tian et al 1995, Whitford et al 1981) For

example, animal carrion is readily digestible by many organisms and decomposesrapidly (Payne 1965), whereas some plant materials, especially those composedlargely of lignin and cellulose, can be decomposed only by relatively few species

of fungi, bacteria, or protozoa and may require long time periods for complete

decomposition (Harmon et al 1986) Conifer litter tends to decompose more

slowly than does angiosperm litter because of low nitrogen content and highlignin content Low soil or litter pH inhibits decomposition Rapid burial or saturation with water inhibits decomposition of litter because of limited oxygenavailability Submerged litter is degraded primarily by aquatic gougers and scrapers that slowly fragment and digest consumed organic matter from the

surface inward (N Anderson et al 1984).

Decomposition processes differ among ecosystem types Physical factors maypredominate in xeric ecosystems where decomposition of exposed litter reflectscatabolic effects of ultraviolet light Decomposition resulting from biologicalprocesses is favored by warm, moist conditions Decomposition is most rapid inwet tropical ecosystems, where litter disappears quickly, and slowest in desert,tundra, and boreal ecosystems because of dry or cold conditions González and

Seastedt (2001) and Heneghan et al (1999) compared decomposition of a

common litter species between tropical and temperate ecosystems and strated that decomposition was consistently higher in the tropical wet forests.Nevertheless, decomposition may continue underground, or under snow intundra and boreal regions, if temperature and moisture are adequate (e.g., Santos

demon-et al 1981) As noted earlier in this section, decomposition rates may be lower in

aquatic ecosystems as a result of saturation and limited oxygen supply Lowdecomposition rates generally result in the accumulation of large standing crops

of woody and fine litter

Different groups of detritivores and decomposers dominate different tems For example, shredders and gatherers were more abundant in pools andheadwater streams, characterized by substantial inputs of largely unfragmentedorganic matter, whereas filter-feeders were more abundant in high gradient sec-tions or higher-order streams (the Little Tennessee River), characterized byhighly fragmented, suspended organic matter (Fig 14.2) Fungi and associatedfungivores (e.g., oribatid mites and Collembola) are more prevalent in forests,whereas bacteria, bacteriovores, especially prostigmatid mites and Collembola,and earthworms are more prevalent in grasslands (Seastedt 2000) Termites arethe most important detritivores in arid and semi-arid ecosystems and may largelycontrol decomposition processes in forest and grassland ecosystems (K E Leeand Butler 1977, Whitford 1986) J Jones (1989, 1990) reported that termites indry tropical ecosystems in Africa so thoroughly decompose organic matter that

ecosys-little or no carbon is incorporated into the soil Wood-boring insects occur only

in ecosystems with woody litter accumulation and are vulnerable to loss of thisresource in managed forests (Grove 2002) Dung feeders are important in ecosys-tems where vertebrate herbivores are abundant (Coe 1977, Holter 1979)

The relative contributions of physical and biological factors to pedogenesisvary among ecosystems Erosion and earth movements (e.g., soil creep and land-slides) mix soil and litter in ecosystems with steep topography or high wind orraindrop impact on surface material Burrowing animals are common in ecosys-tems with loose substrates suitable for excavation Grasslands and forests onsandy or loamy soils support the highest diversity and abundances of burrowers

Ants often excavate nests through rocky, or other, substrates, which would preclude burrowing by larger or softer-bodied animals and are the dominant burrowers in many ecosystems

Distinct temporal patterns in decomposition rates often reflect either the conditioning requirements for further degradation or the inhibition or facilita-tion of new colonizers by established groups For example, leaching of toxicchemicals may be necessary before many groups are able to colonize litter (Barzand Weltring 1985) M Hulme and Shields (1970) and Käärik (1974) reported

pre-I TYPES AND PATTERNS OF DETRITIVORY AND BURROWING 413

0 1 2 3 4 5

FIG 14.2 Annual secondary production for aquatic functional groups in bedrock outcrop, riffle, and pool habitats of upper Ball Creek, North Carolina, during July 1983–June 1984 Data from Huryn and Wallace (1987) Please see extended permission list pg 572.

that wood decay is inhibited by competition for labile carbohydrates, necessaryfor early growth of decay fungi, by nondecay fungi However, Blanchette andShaw (1978) found that decay fungus growth in wood with bacteria and yeastswas twice that in wood without bacteria and yeasts, presumably because bacte-ria and yeasts provide fixed nitrogen, vitamins, and other nutrients while exploit-ing carbohydrates from lignocellulose degradation Microbes usually requirebark penetration, and often inoculation, by insects to colonize woody litter Manysaprophagic arthropods require some preconditioning of litter by bacteria, fungi,

or other arthropods prior to feeding Small comminuters usually feed on ments or feces left by larger comminuters (O’Connell and Menagé 1983) Shred-ders in streams convert coarse particulate organic matter (CPOM) to fineparticulate organic matter (FPOM) that can be acquired by filterers (J Wallace

frag-and Webster 1996, J Wallace et al 1991) Santos frag-and Whitford (1981) reported

that a consistent succession of microarthropods was related to the percentage oforganic matter lost

Decomposition often begins long before detritus reaches the soil able detrital accumulation occurs in forest canopies (Coxson and Nadkarni 1995,

Consider-Paoletti et al 1991) Processes of decomposition and pedogenesis in these pended sediments are poorly known, but Paoletti et al (1991) reported that sus-

sus-pended soils associated with bromeliads in a Venezuelan cloud forest had higherconcentrations of organic matter, nitrogen, calcium, and magnesium and higherdensities (based on bulk density of soil) of macroinvertebrates and micro-invertebrates than did forest floor soils However, rates of litter decomposition

as measured in litterbags were similar in the canopy and forest floor Oribatidmites and Collembola are the most abundant detritivores in temperate and trop-

ical forest canopies (Paoletti et al 1991, Schowalter and Ganio 1998, Walter and

O’Dowd 1995, Winchester 1997), and many are canopy specialists that do not

occur on the forest floor (Winchester et al 1999).

Decomposition is an easily modeled process Usually, an initial period ofleaching or microbial oxidation of simple organic molecules results in a short-term, rapid loss of mass, followed by a longer-term, slower decay of recalcitrantcompounds Decomposition of foliage litter has been expressed as a single- ordouble-component negative exponential model (Olson 1963):

(14.1)where Ntis mass at time t, S0and L0are masses in short- and long-term compo-nents, and respectively; and k’s are the respective decay constants The short-termrate of decay reflects the mass of labile organic molecules, and the long-term rate

of decay reflects lignin content and actual evapotranspiration (AET) rate, based

on temperature and moisture conditions (Meentemeyer 1978, Seastedt 1984).Long-term decay constants for foliage litter range from -0.14 year-1 to -1.4 year-1, depending on nutritional value for decomposers (Table 14.1) (Laskowski

et al 1995, Seastedt 1984, Schowalter et al 1991) Decay constants for wood range

from -0.004 year-1to -0.5 year-1 (Harmon et al 1986) Schowalter et al (1998) monitored decomposition of freshly cut oak, Quercus spp., logs over a 5-year

period and found that a 3-component exponential model was necessary to

Nt=S e- kt+L e- kt

TABLE 14.1 Annual decay rates of various litter types with microarthropods present and experimentally excluded.

Decay constant (yr -1 )

Faunal

Mixed tundra grasses a

a Mean values for experiments replicated over sites (Anderson 1973, Douce and Crossley 1982) or years (Cromack unpubl., Seastedt and Crossley 1980,

1983).

b Control versus insecticide comparison.

c Medium mesh (1 mm) versus fine mesh (0.5 mm) comparison Fine mesh bags probably did not exclude all microarthropods.

From Seastedt (1984) with permission from the Annual Review of Entomology, Vol 29, © 1984 by Annual Reviews.

account for differential decay rates among bark and wood tissues.An initial decayrate of -0.12 year-1during the first year reflected primarily the rapid loss of thenutritious inner bark (phloem), which largely disappeared by the end of thesecond year as a result of rapid exploitation by insects and fungi An intermedi-ate decay rate of -0.06 year-1 for years 2–5 reflected the slower decay rate forsapwood and outer bark, and a long-term decay rate of -0.012 year-1was pre-dicted, based on the slow decomposition of heartwood.

Decomposition often is not constant but shows seasonal peaks and annualvariation that reflect periods of suitable temperature and moisture for decom-posers Patterns of nutrient mineralization from litter reflect periods of storageand loss, depending on activities of various functional groups For example,Schowalter and Sabin (1991) reported that nitrogen and calcium content of

decomposing Douglas-fir, Pseudotsuga menziesii, needle litter, in litterbags, in

western Oregon peaked in spring each year, when microarthropod abundanceswere lowest, and declined during winter, when microarthropod abundances werehighest High rates of comminution by microarthropods and decay by micro-organisms during the wet winters likely contributed to release of nutrients fromlitter, whereas reduced comminution and decay during dry springs and summersled to nutrient immobilization in microbial biomass Similarly, fluctuating con-centrations of nutrients in decomposing oak wood over time probably reflect

patterns of colonization and mobilization (Schowalter et al 1998).

II EFFECTS OF DETRITIVORY AND BURROWINGArthropod detritivores and burrowers directly and indirectly affect decomposi-tion, carbon flux, biogeochemical cycling, pedogenesis, and primary production.The best-known effects are on decomposition and mineralization (Seastedt 1984,

Coleman et al 2004) Detritivorous and fossorial arthropods are capable of

sig-nificantly affecting global carbon budgets and ecosystem capacity to store andrelease nutrients and pollutants

A Decomposition and Mineralization

An extensive literature has addressed the effects of detritivores on

decomposi-tion and mineralizadecomposi-tion rates (Coleman et al 2004) Generally, the effect of

arthropods on the decay rate of litter can be calculated by subtracting the decayrate when arthropods are excluded from the decay rate when arthropods arepresent (see Table 14.1) Detritivores affect decomposition and mineralizationprocesses, including fluxes of carbon as CO2or CH4, by fragmenting litter and byaffecting rates of microbial catabolism of organic molecules The magnitude ofthese effects depends on the degree to which feeding increases the surface area

of litter and inoculates or reduces microbial biomass

1 Comminution

Large comminuters are responsible for the fragmentation of large detrital rials into finer particles that can be processed by fine comminuters and

mate-saprophytic microorganisms Cuffney et al (1990) and J Wallace et al (1991)

reported that 70% reduction in abundance of shredders from a small headwaterstream in North Carolina, United States, reduced leaf litter decay rates by25–28% and export of fine particulate organic matter by 56% As a result,

unprocessed leaf litter accumulated (J Wallace et al 1995) Wise and Schaefer

(1994) found that excluding macroarthropods and earthworms from leaf litter ofselected plant species in a beech forest reduced decay rates 36–50% for all littertypes except fresh beech litter When all detritivores were excluded, comparablereduction in decay rate was 36–93%, indicating the prominent role of large com-

minuters in decomposition Tian et al (1995) manipulated abundances of

milli-pedes and earthworms in tropical agricultural ecosystems They found thatmillipedes alone significantly accounted for 10–65% of total decay over a 10-week period Earthworms did not affect decay significantly by themselves, butearthworms and millipedes combined significantly accounted for 11–72% of totaldecay Haimi and Huhta (1990) demonstrated that earthworms significantly

increased mass loss of litter by 13– 41% N Anderson et al (1984) noted that

aquatic xylophagous tipulid larvae fragmented >90% of decayed red alder, Alnus rubra, wood in a 1-year period.

Termites have received considerable attention because of their substantialecological and economic importance in forest, grassland, and desert ecosystems

Based on laboratory feeding rates, K E Lee and Butler (1977) estimated woodconsumption by termites in dry sclerophyll forest in South Australia Theyreported that wood consumption by termites was equivalent to about 25% ofannual woody litter increment and 5% of total annual litterfall Based on termite

exclusion plots, Whitford et al (1982) reported that termites consumed up to 40%

of surficial leaf litter in a warm desert ecosystem in the southwestern UnitedStates (Fig 14.3) Overall, termites in this ecosystem consumed at least 50% of

estimated annual litterfall (K Johnson and Whitford 1975, Silva et al 1985) N.

M Collins (1981) reported that termites in tropical savannas in West Africa sumed 60% of annual wood fall and 3% of annual leaf fall (24% of total litterproduction), but fire removed 0.2% of annual wood fall and 49% of annual leaf fall (31% of total litter production) In that study, fungus-feeding Macrotermitinae were responsible for 95% of the litter removed by termites

con-Termites apparently consume virtually all litter in tropical savannas in EastAfrica (J Jones 1989, 1990) Termites consume a lower proportion of annual litterinputs in more mesic ecosystems N M Collins (1983) reported that termites con-sumed about 16% of annual litter production in a Malaysian rainforest receiv-ing 2000 mm precipitation year-1 and 1–3% of annual litter production in aMalaysian rainforest receiving 5000 mm precipitation year-1

Accumulation of dung from domestic mammalian grazers has become aserious problem in many arid and semi-arid ecosystems Termites removed asmuch as 100% of cattle dung over 3 months in Kenya (Coe 1977), 80–85% over5–9 months in tropical pastures in Costa Rica (Herrick and Lal 1996), and 47%

over 4 months in the Chihuahuan Desert in the southwestern United States

(Whitford et al 1982) In the absence of termites, dung would require 25–30 years

to disappear (Whitford 1986) Dung beetles (Scarabaeidae) and earthworms also

are important consumers of dung in many tropical and subtropical ecosystems(e.g., Coe 1977, Holter 1979, Kohlmann 1991).

Relatively few studies have provided estimates of wood consumption by and wood-boring insects, despite their recognized importance to wood decom-position Zhong and Schowalter (1989) reported that bark beetles consumed0.1–7.6% of inner bark and wood-boring beetles consumed an additional

bark-FIG 14.3 Rate of gallery carton deposition (top) and mass loss (bottom) of creosote bush, Larrea tridentata, and fluff grass, Erioneuron pulchellum, foliage when subterranean termites were present (black symbols) or absent (white symbols) in

experimental plots in southern New Mexico Litter (10 g) was placed in aluminum mesh cylinders on the soil surface on August, 15, 1979 Vertical lines represent standard

errors From Whitford et al (1982) with permission from Springer-Verlag Please see

extended permission list pg 573.

0.05–2.3% during the first year of decomposition, depending on conifer treespecies Ambrosia beetles consumed 0–0.2% of the sapwood during the first year.

Schowalter et al (1998) found that virtually the entire inner bark of oak logs was

consumed by beetles during the first 2 years of decomposition, facilitating ration of the outer bark and exposing the sapwood surface to generalized sapro-phytic microorganisms Edmonds and Eglitis (1989) used exclusion techniques todemonstrate that, over a 10-year period, bark beetles and wood-borers increaseddecay rates of large Douglas-fir logs (42 cm diameter at breast height) by 12%

sepa-and of small logs (26 cm diameter at breast height) by 70%

Payne (1965) explored the effects of carrion feeders on carrion decay duringthe summer in South Carolina, United States He placed baby pig carcasses underreplicated treatment cages, open at the bottom, that either permitted or restrictedaccess to insects Carcasses were weighed at intervals Carcasses exposed toinsects lost 90% of their mass in 6 days, whereas carcasses protected from insectslost only 30% of their mass in this period, followed by a gradual loss of mass,with 20% mass remaining in mummified pigs after 100 days

Not all studies indicate significant effects of litter fragmentation by

macro-arthropods Setälä et al (1996) reported that manipulation of microarthropods,

mesoarthropods, and macroarthropods in litter baskets resulted in slower decayrates in the presence of macroarthropods Most litter in baskets withmacroarthropods (millipedes and earthworms) was converted into large fecalpellets that decayed slowly

A number of studies have demonstrated that microarthropods are ble for up to 80% of the total decay rate, depending on litter quality and ecosys-

responsi-tem (see Table 14.1, Fig 14.4) (Coleman et al 2004, González and Seastedt 2001, Heneghan et al 1999, Seastedt 1984) Seastedt (1984) suggested that an appar-

ent, but insignificant, inverse relationship between decay rate as a result ofmicroarthropods and total decay rate indicated a greater contribution of arthro-pods to decomposition of recalcitrant litter fractions compared to more labile

fractions Tian et al (1995) subsequently reported that millipedes and earthworms

contributed more to the decomposition of plant residues with high C : N, lignin,and polyphenol contents than to high-quality plant residues

2 Microbial Respiration

Microbial decomposers are responsible for about 95% of total heterotrophic piration in soil Arthropods generally increase microbial respiration rates andcarbon flux but may reduce respiration rates if they overgraze microbial

res-resources (Huhta et al 1991, Seastedt 1984) Several studies have documented

increased microbial respiration as a result of increased arthropod access to detrital substrate and stimulation of microbial production

Litter fragmentation greatly increases the surface area exposed for microbialcolonization Zhong and Schowalter (1989) reported that ambrosia beetle densi-ties averaged 300 m-2 bark surface in Douglas-fir and western hemlock, Tsuga heterophylla, logs, and their galleries extended 9–14 cm in 4–9 cm thick sapwood,

indicating that considerable sapwood volume was made accessible to microbescolonizing gallery walls The entire sapwood volume of these logs was colonized