The following extracts on soil and below-ground ecology pertain to the
interior Columbia River Basin, but general
statements on ecological relations likely pertain to most of the Pacific
Northwest and beyond.
Please note that these extracts are taken from a variety of places in
the text of two major science reports; thus, the
text may not "flow" as if they were from one coherent report.
Part I: Extracts from:
Marcot, B. G., M. Castellano, J. Christy, L. Croft, J. Lehmkuhl, R.
Naney, R. Rosentreter, R. Sandquist,
and E. Zieroth. In press. Terrestrial ecology assessment.
Pp. xxx-xxx in: T. M. Quigley and S. J.
Arbelbide, eds. An assessment of ecosystem components in the
interior Columbia Basin and portions of the
Klamath and Great Basins. USDA Forest Service General Technical
Report PNW-xxx. USDA Forest
Service Pacific Northwest Research Station, Portland, OR. xxx
pp.
Role of invertebrates:
Invertebrate populations play important roles in soil development...
No study has been conducted on the total number of bacteria species
or even uniqueness of species in soils
throughout the assessment area. One study, in the Palouse however,
has discovered a small set of unique helper
bacteria that suppress weeds; and another study is exploring the role
of beneficial bacteria that aid crop plant
growth...
Soil nematodes number perhaps 10 to 20 species in an unproductive ecosystem
and perhaps 100 to 150 species in a
healthy forest (see footnote 4). Many soil nematode species that
are agriculturally important in the assessment area
are known and their distributions are fairly well understood.
Extrapolating from small soil samples, in one estimate (see footnote
4) on the order of 100,000 soil
ectomycorrhizal microfungi species occur each in forest and grassland
ecosystems. There may be roughly equal
numbers of other forms of microfungi (see footnote 4) but they are
essentially unstudied in the assessment area.
As with bacteria, soil microfungi are critical to ecosystem health.
Although no study of soil microfungi
biodiversity in the assessment area seems to be available, unique mycorrhizal
species are known to occur in larch
and mixed conifer stands of the Blue Mountains. These species
have probably adapted to enhance tree growth (see
footnote 4).
- - - -
More on role of invertebrates:
Invertebrates are critical components of many ecosystem functions and
can make excellent bioindicators of soil and
vegetation health. Invertebrates are sensitive indicators of
change, and can be used to index changes in the
environment at small, spatial and very short, temporal scales.
Detritivory and Nutrient Cycling— Contemporary studies indicate that
soil resembles a living entity. It "lives"
and "breathes" through a complex mix of interacting organisms ranging
from viruses and bacteria, fungi,
nematodes, and arthropods to ground squirrels, marmots, and badgers.
Microbial biomass alone can reach 10,000
kilograms/hectare in productive, inland, western forest soils.
Activities of all these organisms are responsible for
developing the critical properties that underlie basic soil fertility,
health, and productivity. Biologically driven
properties resulting from such complex interactions require up to several
hundreds of years to develop. No quick
fixes are available if extensive damages occur.
Productive ecosystems tend to retain nutrients. Over time, nutrients
are metabolized to forms that are less
available for plants and animals such as phytates, lignins, tannins,
and humic and fulvic acids. For nutrients to
once again become available to plants and animals, they must be mineralized
by the interaction of decomposers
and their predators. These populations and their interactions
are important to ecosystem stability including
predator and prey interactions, mutualisms, and disease.
As total ecosystem productivity increases, biodiversity within the soil
food web also increases. The greater number
of interactions between decomposers, their predators, and the predators
of those predators, the fewer the losses of
nutrients from that system. In undisturbed ecosystems ("undisturbed"
defined hereafter as sites with little
disturbance from European settlement), the processes of immobilization
and mineralization are tightly coupled to
plant growth. Following disturbance, this coupling is lost or
reduced; nutrients are no longer retained within the
area available to the plants? roots, which reduces the productivity
of the ecosystem and causes problems for systems
into which nutrients move, especially aquatic portions of landscapes.
For this reason, the soil food web has been
suggested as a prime indicator of ecosystem health. Measurement
of disrupted soil processes; decreased bacterial
or fungal activity; inappropriate change in the ratio of fungal to
bacterial biomass; decreased number or diversity of
protozoa; and altered nematode numbers, nematode community structures,
or maturity indices can indicate
problems long before the natural vegetation appears to be affected.
Earthworms—A group of invertebrates that play important roles in maintaining
soil productivity is the
earthworms. Earthworms require organic matter in various stages
of decay and in various locations. Three broad
functional groups of earthworms have been described by Bouche (1977):
epigeic, endogeic, and anecic. Epigeic
earthworms reside in leaf litter and under the bark of decaying logs.
In a forested site, epigeic earthworms would
be expected to have functional roles in organic matter comminution
(chewing), nutrient cycling, soil structural
modification, transfer of organic matter to the soil, and providing
food for other animals.
Endogeic earthworms live in the mineral soil and consume organic matter
within the soil or at the soil-litter
interface. The endogeic earthworm species occur in a wide range
of habitats: forest and savannah, grassland and
shrubland (including exotic grass pasture and seral stages following
cessation of agriculture), and cultivated land.
Endogeic species are the least known of all earthworms even though
they constitute the majority. Factors that
influence populations of endogeic earthworm species in the assessment
area are unknown. Only by assuming that
they are comparable to other earthworms can one say that soil moisture,
soil temperature, organic matter quantity
and quality, and soil pH are probably the most important factors influencing
presence, distribution, and abundance
of populations (Lee 1985).
Anecic earthworms are those that inhabit a permanent or semi-permanent
deep vertical burrow and emerge at
night to consume relatively fresh plant detritus on the surface.
Anecic earthworms are unstudied in natural
vegetation in the assessment area. Their unique contributions
may include the transfer of relatively fresh plant
litter from the surface to deep levels of the soil and the creation
of deep vertical burrows which assist water
infiltration. Anecic species also likely provide food resources
to endogeic worms by depositing of fecal organic
matter into the soil where endogeics can reach it.
The assessment area is inhabited by at least three native earthworm
species belonging to three genera. Driloleirus
americanus was considered for inclusion in the IUCN Invertebrate Red
Data Book because its habitat was
threatened and its range was not known to be very large. Available
information suggests that it may be a narrow
endemic using a threatened habitat (grassland sites with productive
soil). The other two native species,
Drilochaera chenowithensis and Argilophilus hammondi, may be somewhat
tolerant of habitat conversion to
agricultural uses.
Other Soil Organisms— The other organisms in the soil including bacteria,
fungi, protozoa, nematodes, and
microarthropods play critical roles in maintaining soil health and
fertility (Coleman and others 1992). These roles
include: (1) decomposition of plant material by bacteria and fungi;
(2) immobilization of nutrients in soil by
bacteria and fungi in their biomass and in their secondary metabolites
such as waste or defensive products; (3) soil
aggregate structure improvement, which increases water-holding capacity,
clay surface interactions with nutrients,
and plant root architecture; (4) alteration of the soil pH; (5) mineralization
of nutrients by protozoan, nematode,
and microarthropod predation of bacteria and fungi; and (6) control
of disease-causing organisms through
competition for resources and space, control of soil micronutrient
status, and alteration of root growth.
Wood decomposition— Invertebrates also play key roles in wood decomposition.
Secondary bark beetles (also
primary, or tree-killing, bark beetles) penetrate the bark of freshly
dead trees and inoculate wood with, and provide
access to, saprophytic microorganisms. They also provide attractive
aerosols, habitats, and resources for other
invertebrates such as fungivores and termites, thereby accelerating
wood decomposition (Schowalter and others
1992, Stephen and others 1993).
Taxa Associated with Litter, Coarse Woody Material, and Soil—
Most of the invertebrate species associated
with litter, coarse woody material, and soil have characteristics that
allow them to recolonize disturbed areas and
reestablish their functional roles in the ecosystem. Although
several species perform the same specific ecological
functions, some are less able or unable to survive in disturbed areas.
Ground-nesting bees, particularly those that breed in large aggregations
of thousands of nests, move large amounts
of soil while digging their main burrows and side branches. In
so doing, they help to recycle soil layers and
nutrients.
Some degree of compaction on heavier soils may be tolerable and even
beneficial to certain ground-nesting bee
species. However, bees that inhabit light soils, particularly
sandy soils, occur in higher densities and include more
endemic species than in other types of soils, and are extremely sensitive
to disturbance. Activities can also
obliterate or change the subtle landmarks adjacent to nest-holes that
bees use to find their nests when returning
from foraging trips. Disturbance early or late in the year when
bees and most plants are less active would reduce
effects. In addition to seasonal mitigation, any reduction in
the intensity and frequency of such disturbing
activities will help to maintain adequate ground-nesting habitat and
provide time for recovery and recolonization of
sites.
Three guiding principles may help to conserve invertebrates: (1)
provision of a diversity of habitat composition
and structures to maintain invertebrate biodiversity and ecosystem
functions, (2) maintenance of soil structure and
chemistry to sustain diversity and functions of the invertebrate food
web in the soil, (3) eradication or prevention of
the introduction of exotic organisms to maintain invertebrate biodiversity
and ecosystem functions.
In the forest understory, management practices that disturb flowering
plants and other ground vegetation or that
compact or mix the soil may have distinct effects on several functional
groups of organisms. In addition to the
effects of management practices on organisms with limited dispersal
capabilities or those that are in a non-mobile
stage, habitat of many functional groups of invertebrates can be disrupted.
Consequently plant and animal
communities can change, sometimes resulting in detrimental effects
to particular invertebrate species.
On the forest floor, the major structural element contributing to the
invertebrate fauna is coarse woody material.
This consists primarily of down tree boles and large branches.
This material serves as habitat for prey for
mammalian, avian, and invertebrate predators and as a carbon source
for soil biota. Various species of arthropods,
nematodes, fungi, annelids, and bacteria are responsible for the comminution
and conversion of the wood to
elements that are available to the soil. To maintain soil productivity,
sufficient coarse woody material should be
maintained through time. In grasslands, retaining native plant
species is important as they may be key to
maintaining large invertebrate communities of over 100 species representing
several functional groups.
Soil structure and soil chemistry— Maintenance of forest soil
chemistry and structure helps to sustain soil health
and fertility. This is vital to retaining forest and range productivity
and biodiversity. Chemical changes of soil
from fire and structural changes of soil from compaction or mixing
of soil layers are two results of management
practices that are of concern. Fire, whether natural or human-caused,
has the potential to consume the litter and
coarse, woody material that are the primary sources of carbon and other
elements necessary for the soil food web.
Erosion, resulting from loss of soil-binding materials, further depletes
the capacity of the soil. Fire can volatilize
nutrients found in the upper horizon of the soil and change water-retention
characteristics. Structural changes of
soil from either compaction or soil mixing can have long-lasting effects
on successional patterns and duration.
These effects are expected with multiple management entries into a
forested area.
Implications for Invertebrates Associated with Detritovory and Nutrient
Cycling- Groups of viruses, bacteria,
protozoa, rotifers, nematods, microfungi, and algae all help determine
productivity of many parts of the ecosystem,
especially soils, organic matter decomposition, and other aspects of
the below-ground environment. Most are
critical to maintaining diverse ecosystems and productive and sustainable
renewable resources. In particular,
knowledge of how land management activities affect various kinds of
soil bacteria is important because some
(desirable) bacteria species inhibit disease-causing organisms, whereas
other (undesirable) bacteria species will
cause disease.
Two areas of concern for invertebrates are the direct effects of fire
on populations and the role of fire in forest or
range succession and soil chemistry. In fire-adapted systems,
direct effects on invertebrate populations are thought
to be slight. However, in systems where large volumes of fuel
(litter and coarse woody material) are present,
higher intensity fires may pose hazards to slow-moving organisms such
as land snails. Direct effects on
invertebrates will be reduced if refugia of litter and coarse woody
material are retained.
Fire removal of organic matter has opposite effects on forest and range
succession. In the forest, loss of organic
matter may change the ratio of fungi:bacteria to favor bacteria.
This change in ratio favors grasses rather than
woody vegetation, not necessarily the direction desirable in forestry.
In grasslands, the consumption of organic
matter and the subsequent change to a bacteria-dominated food web is
beneficial to the maintenance of grasses.
The effects on invertebrates by early-season fires in cheatgrass with
its damage to native perennial grasses is
unknown.
Coarse woody material provides habitat for prey of other organisms,
serves as primary habitat for invertebrate
predators, and serves as a carbon source for the soil food web.
It is not known how much litter and coarse woody
material and what sizes are necessary to continue ecosystem functions
of associated invertebrates. It is assumed
that the standards to provide prey for vertebrate species will suffice
to continue the functions of invertebrate
populations.
Compaction or mixing of soils is an issue that has implications for
maintaining the soil food web. Compaction
occurs from use of machinery on the land and from the effects of grazing
by large herbivores. Compaction reduces
soil pore size which results in loss of nutrient retention and increases
the bacterial component of the soil based food
web. This may reverse succession in the forested environment,
causing a negative impact on cyanobacteria,
lichen, and mat-forming ectomycorrhizal fungi. With loss of the
ectomycorrhizal fungi, tree productivity declines.
Compaction also changes the community of nematodes, favoring bacteria
and root-feeding species. Root-feeding
nematodes can be highly detrimental to survival of tree and grass seedlings.
Compaction has a negative effect on all other groups of invertebrates
but especially mollusks and earthworms,
which may occupy specific habitats or which cannot disperse quickly.
Overgrazing and the congregating of
livestock near water sources can directly affect mollusks by trampling
individuals and degrading their favored
riparian habitats.
Tilling to reduce compaction and other means of physically mixing the
duff and soil can potentially adversely
affect many ecological functional groups of invertebrates. Disruption
of the duff/litter layer has immediate effects
on water and thermal relations, degrades habitat for many functional
species groups that inhabit woody debris and
litter, and alters forb and flowering plant communities. Excess
mixing of soils affects the soil food web by
breaking roots and fungal mats and by changing the water and thermal
conditions, thereby encouraging growth of
bacteria populations.
--------------
Role of lichens:
Lichens are a widespread component of forests and rangelands of the
assessment area and are primary producers in
the forest. Their biomass and nutrients contribute to the forest
litter and duff, enriching the soil and increasing soil
moisture-holding capacity.
The nitrogen-fixing lichens and cyanobacteria that are part of the microbiotic
crust improve soil fertility.
Microbiotic crusts in general increase soil stability, influence water
infiltration, and may improve seed germination
for various plant species. Intact microbiotic crusts also may
inhibit the establishment of exotic plants such as
cheatgrass. Compared to many other groups of microorganisms,
microbiotic crusts have been well studied.
Nonetheless, details of the ecological role of microbiotic crust species
as soil stabilizers and nitrogen fixers are still
being elucidated (Belnap 1994). The role of microbiotic crusts
as potential barriers to the establishment of
cheatgrass needs to be studied further and correlated with patterns
of land-use, disturbance histories, and soil
properties.
Many of the microbiotic lichens found in the assessment area are widespread
globally, yet the area they now cover
in the United States has been greatly reduced compared to historic
times. The major threats to survival of
microbiotic crusts in the assessment area include: invasion of
exotic annual grasses and associated increases in
fire frequency; conversion of rangelands to agriculture and suburban
developments; and livestock trampling.
Increased fire frequency (more than 40 times historical frequencies)
owing to the introduction of cheatgrass has led
to dramatic changes in grassland ecosystem structure (Whisenant 1990).
Much of the shrub steppe has been
converted to a dense, closed stand of annual exotic grasses to the
exclusion of microbiotic crust communities.
Areas still containing shrubs and microbiotic crusts can be grazed
by livestock when the soil is moist with little
harm to the crusts (Wicklow-Howard and Kaltenucker 1994), but many
areas have been severely affected by
livestock trampling in the dry season. One microbiotic crust
species, the woven-spored lichen (Texosporium
sancti-jacobi), is now restricted to two general localities within
the assessment area.
-----------
Role of bryophytes (mosses, liverworts):
Bryophytes of dry soil occur on exposed or shaded mineral soil at all
elevations. Primary succession on recently
disturbed soil includes many common, opportunistic species that rapidly
colonize sites, stabilize soil, and enhance
invasion by other cryptogams or vascular plants. These are often
drought-tolerant, short-lived assemblages that are
replaced by others within one or two years. At low elevation
in arid areas, bryophytes are a significant component
of soil crusts, where they harbor nitrogen-fixing cyanobacteria, stabilize
soil by trapping particles, reduce wind
erosion, and enhance water infiltration and seed germination (Anderson
and Rushforth 1976, Harper and Marble
1988). In contrast to pioneer associations on disturbed
soil, these crusts take many years to form, are long-lived,
and suffer long-term damage from fire, livestock trampling, and off-road
vehicle traffic (Beymer and Klopatek
1992, Johansen and others 1984). At middle to higher elevations,
soil-dwelling bryophytes occur in shaded forests,
on root balls of windthrown trees, and exposed soil of trails, roads,
and earth slumps. Near timberline, they occur
on glacial moraines and outwash, especially where protected by rocks
in areas where snow may accumulate
(Spence 1981). Some of these associations may also be long-lived
rather than recent immigrants. The distribution
of some soil-dwelling species is irregular and unpredictable.
Bryophytes of wet soil occur on moist to wet soils, with or without
organic content. Most species of this group are
widespread and occur wherever water is available. Habitats include
edges of lakes and streams, moist organic soil
over rocks in shrub thickets, margins of avalanche tracks, seepage
areas, and the bases of talus slopes. Sites often
have late-lying snow beds. In spite of the widespread occurrence
of this habitat, distribution of associated species
may be sporadic and unpredictable. Some species need the shaded,
moist microclimate provided by a forest canopy
and are very sensitive to desiccation. They are readily replaced
by other species in more exposed situations.
Common wet soil bryophytes in riparian areas, such as Marchantia polymorpha
and Philonotis fontana, are readily
extirpated by livestock trampling.
--------------
Role of Livestock:
Livestock grazing can be a threat to some species, beneficial to some,
and neutral to others. The degree or
intensity of grazing, both in removal of vegetation and in soil compaction,
determines the effect on most species.
Species such as ground squirrels, moles, and some shrews are adversely
affected by soil compaction and trampling
of runways by livestock.
---------------
Soil Productivity
Species with ecological functions affecting soil productivity include
a variety of plants, invertebrates, and
vertebrates in seven function categories: physical soil turnover, nitrogen
retention or uptake, soil stabilization, rock
weathering, detoxificaion of xenobiotics, metal accumulation, and vegetation
succession. Soil productivity can be
related to complex processes. For example, Seasted and Knapp
(1993) reported that grassland ecosystem
productivity can display "transient maxima," which are related to how
grazing and fire affect water, nitrogen, and
light availability.
In one sense, all plants die, decompose, and thus contribute to soil
organic matter and composition. In particular,
one bryophyte group (rock calcareous bryophytes), 2 plant groups (Carex
ephemeral meadow low-moderate group
and Carex subalpine groups), and 13 plant species, listed in the SER
database, significantly contribute to soil
formation and soil organic material, particularly in poorly developed
or nutrient-poor soils and limestone or rock
substrates. Many other species and plant groups not listed in
the SER database also contribute to this function.
Some 51 example invertebrate species or subspecies distributed over
seven families are included in the database as
examples of invertebrates that recycle soil layers and nutrients, and
influence soil structure, porosity, and
aggregation by their physical digging actions. Among vertebrates
that perform this function are a number of
fossorial mammals listed above as primary burrow excavators.
In particular, western and Woodhouse's toads,
broad-footed and coast moles (Scapanus latimanus and Scapanus orarius),
shrew-mole (Neurotrichus gibbsii), 4
ground squirrels, white-tailed antelope squirrel (Ammospermophilus
leucurus), and water vole (Microtus
richardsoni) significantly contribute to soil aeration and affect soil
structure. The water vole contributes to soil
aeration and drainage of wet soils.
Bacteria, nematodes, rotifers, and protozoa play critical roles in nitrogen
retention or uptake in soils. Groups of
competitive rhizosphere (root sphere) bacteria, bacterial pathogens,
and beneficial bacteria, are responsible for
nitrogen retention and mineralization in soils. Bacteria-feeding
nematodes (Acrobeloides spp.) engage in
mineralization of nitrogen immobilized in bacterial biomass, and may
be responsible for up to 40 percent of plant
available nitrogen. Rotifers (virtually unstudied in the assessment
area) and the bacterial-predator protozoa such
as Amoeba amoeba also engage in mineralization of nitrogen immobilized
in bacterial biomass, and may be
responsible for up to 50 percent or more of plant available nitrogen
in some agricultural systems. In addition,
saprophytic fungi enter the process, as most plant-available nitrogen
must cycle through soil fungal biomass in
forest soils.
Many plants contribute to soil stabilization, including 6 bryophyte
groups, 9 lichen groups, 22 plant groups, and at
least 1 recognized lichen species and, among the rare plants listed
in the SER database, 76 plant species or
subspecies. The specific functions performed by this diverse
group include: trapping of sediments (for example,
aquatic submerged bryophytes, wet rock bryophytes, and wet soil bryophytes);
reducing wind erosion (for example,
dry soil bryophytes); colonization of harsh sites or erodible slopes;
and stabilization of stream banks and soils
during flood events (for example, Artemesia spp., the Carex rocky streambed
species group, the Mimulus guttatus
species complex and Mimulus high-elevation wet-habitat species group,
and the Penstemon foothills to montane
meadow species group). Species that stabilize soils on post-fire
sites include several species of milk-vetch
(Astragalus spp., Chaenactis cusickii, Penstemon peckii, and others;
also see table 15). Many other plants not
listed in the SER database likely perform this function more widely
throughout the assessment area. The beneficial
bacteria Microcoleus also serve to stabilize soil and prevent erosion.
Lichens excel as soil stabilizers, with 11 lichen species groups occurring
in a wide array of environments serving to
weather rock substrates. Additionally, 7 vascular plant species
in the SER database, including penstemons, a
rockmat, an ivesia, and a phacelia, also perform this service.
Rock weathering is often a prelude to further soil
formation and provides a substrate for other colonizers needing mineral
soils.
The ectomycorrhizal mat-forming fungi Hysterangium may detoxify some
types of pesticides, herbicides, and
pollutants. The vesicular fungus Glomus mossae and the ectomycorrhizal
mat-forming fungus Hysterangium may
sequester heavy metals in fungal hyphae, preventing heavy metals from
damaging or killing plants.
By definition, vegetation succession proceeds by new plant species invading
a site. Many plant species of the
assessment area are colonizers. Some bacteria groups, exemplified
by the bacterium Rhizobium melliotii and the
beneficial bacteria group Microcoleus, aid preparation of the soil
for succession by nitrogen-fixation.
---------------
Role of Vertebrates
Among vertebrates that might aid wood decomposition are the rubber boa
(Charina bottae) and sharptail snake
(Contia tenuis), which excavate into rotten logs for cover and prey;
the pileated woodpecker (feeds more frequently
on large down logs compared to other woodpeckers) which breaks apart
logs while foraging for insects and larvae;
and the black bear and grizzly bear, which may contribute to soil nutrient
cycling by breaking apart logs in search
of food.
-----------
Bioindicators
A number of individuals or groups of species can serve as "early warning"
indicators of ecosystem health.
Presence and condition of microbiotic crusts on rangelands can act
as indicators of soil properties and grazing
effects. Impending problems of soil productivity can be indexed
by tracking disruptions of soil processes, decreases
in soil bacterial or fungal activity, decreases in soil fungal or bacterial
biomass, change in the ratio of soil fungal to
bacterial biomass, decreases in the number of soil protozoa, change
in soil nematode numbers, and soil nematode
community structure or maturity index. Butterflies and
other invertebrate herbivores can help indicate general
trends of total ecosystem diversity. Additional bioindicators
can include selected macrofungi, lichens, and
bryophytes (see Plants and Allies).
-----------
Basic Systematics of Major Species Groups
Entire taxonomic groups, particularly many of the invertebrates, still
require basic systematics research. Many
species of arthropods, particularly canopy-dwelling forest arthropods
and soil mesoinvertebrates, remain
undescribed. About 86% of arthropods, 67% of fungi, and 51% of
mollusk species estimated to occur in the
assessment area not have been studied, surveyed, or, in some cases,
even identified. Much inventory and basic
systematics work remains to be done on these groups. Soil microorganism
groups and microbiotic (soil) crusts of
the assessment area, although critical for maintaining soil productivity,
are poorly known and little studied.
Most of the soil microbiota of the assessment area is undescribed.
This biota includes viruses, bacteria, protozoa,
rotifers, nematodes, and soil microfungi, as well as algae. Even
though we consider such taxa as species groups,
basic empirical work still is needed on taxonomy, diversity, and ecological
roles of these species, particularly in
how they aid soil productivity and maintain health of forest and rangeland
ecosystems.
----------
Part II: Extracts from:
Marcot, B. G., L. K. Croft, J. F. Lehmkuhl, R. H. Naney, C. G. Niwa,
W. R. Owen, and R. E. Sandquist. In
press. Macroecology, paleoecology, and ecological integrity of
terrestrial species and communities of the
interior Columbia River Basin and portions of the Klamath and Great
Basins. General Technical Report
PNW-GTR-XXX. USDA Forest Service. XXX pp.
On dispersal of soil organisms:
Phoretic dispersal exemplifies some interesting relations that have
evolved between species. Phoretic dispersal has
appeared in a wide variety of microorganisms and invertebrates, including
bacteria, rotifers, protozoa, nematodes,
and insects (Table 10). Host species can include seeds, plants,
organic debris, other insects, birds, mammals, and
even humans. Soil bacteria are ubiquitous and disperse by means
of most of these hosts. Rotifers and soil protozoa
can hitch rides on arthropods, small mammals, birds, and even soil
nematodes. Bacterial-feeding soil nematodes
in turn can travel by arthropods, birds, small mammals, and even by
humans via tools that redistribute soil, such as
shovels.
Phoretic invertebrates play various roles in contributing to trophic
structures and ecological processes of
ecosystems. Many phoretic invertebrates consume soil microorganisms
and are prey for other consumers, and aid
soil nitrogen cycling through nitrogen-immobilization and acting as
a source for nitrogen mineralization, which
can be vital functions in forest nitrogen conservation (Miller and
others 1989, Davidson and others 1992). A few
phoretic invertebrates are parasites or pathogens, or cause disease.
Phoretic invertebrates also include soil-oriented
species whose functions aid soil structure, enhance soil stabilization,
detoxify xenobiotics, sequester (accumulate)
soil metals, and influence vegetation succession and presence and diversity
of plant species.
On influence of earthworms:
Species dispersing by digging enhance the physical structure of soil
and turnover of soil nutrients and layers.
Earthworms are a prime example (see discussion in Marcot and others
1996). In one study of 8 species of
earthworms in a Kansas tallgrass prairie, James (1991) reported that
total annual soil consumption by all
earthworms was 4-10 percent of organic matter in the top 15 cm; 100-300
percent of plant annual belowground
production passed through the earthworms each year; and mineral nitrogen
processed was 10-12 percent of annual
plant nitrogen uptake, comparable to half of the input from precipitation,
while the phosphorous processed was
equivalent to 50 percent of annual uptake. However, native and
introduced earthworm species do not necessarily
perform the same functions. James found that exotic, introduced
earthworm species had a negative effect on soil
turnover and nutrient mineralization due to the lower soil throughput
and their relative intolerance of summer soil
temperatures, as compared with the native earthworms.
On other soil excavators:
As with invertebrate soil excavators, among vertebrates primary burrow
excavators can be important to soil
turnover and mixing of organic matter. Gophers, for example,
can process an enormous quantity of soil material.
In one study in coastal San Diego County, California, Cox (1990) found
that the total soil mined by pocket gophers
(Thomomys bottae) in 10 Mima-type mounds amounted to 8.23 Mg/ha/yr
and subsurface deposition was 20.31
Mg/ha/yr, so that total soil mining equalled 28.54 Mg/ha/yr.
On mycorrhizal fungi:
Another relationship that may complicate species response to changing
climatic and disturbance regimes is that of
mycorrhizal fungi, their host tree species, and soils. Disturbances
that reduce aeration or soil organic matter
reduce mycorrhizal activity (Amaranthus and Perry 1994). Reduction
in mycorrhizae reduces growth of many
species of trees and retards productivity of soils. Some studies
suggest that sites with harsh, continental climates
(such as parts of the CRB assessment area) may have lower fungal diversity
than marine, coastal climates, and thus
may be poorly buffered against changes in fungus populations caused
by disturbance (Powers 1989) or by adverse
climate change.
Conclusions:
The soil, however, is literally at our feet and figuratively at the
hand of management, and there is much we can do
to monitor and guide its health. It is an irony of scale that
soil health is so influenced by microbes but manifests its
effect over vast geographic areas. Its complement of macro- and
microorganisms and their vast array of ecological
functions can be quickly depleted under careless management yet may
take so much longer to rejuvenate. And
results of site-specific management activities that influence soil
compaction, organic matter input, dispersal of
mycorrhizae, vitality of seed banks, and persistence of microbiotic
crusts, can be monitored with satellites.
Maintaining ecological integrity will likely include heeding the lessons
of soil by maintaining rich below-ground
biota with their full complement of ecological functions. In
soil lies the vitality of future resources and ecosystems.