TABLE OF CONTENTS
PREFACE
Ecosystem management of arthropods, what does it entail? .. .... 3
Literature search
How many arthropods are there? .... 4
Synopsis of literature search submitted . .... 5
The nature of the arthropod information in the literature .. ..... 6
PART ONE: Ecosystem process sustainability (top-down approach)
Soil food web structure .. ....10
Critical environmental determinants of soil arthropod density and diversity ...16
Critical ecosystem processes provided by soil arthropods . ...18
Information gaps
Top-down approach for monitoring . .....22
Highlighting biogeographically significant higher taxa;
analysis of species distribution patterns .. .. .23
Why monitor with arthropods? ......24
The most pressing needs ....27
Why focus on arthropods? .....28
Monitoring priorities for focal ecosystem priorities .... ..30
PART TWO: Individual arthropod species and endemism (bottom up approach)
Introduction ... .33
Why is Klamath/Siskiyou region unique? .....34
Individual potentially sensitive species: are there landscape patterns of
co-occurrence? ... .. ..40
Information gaps
Notion and utility of indicator species ... 47
Long-term validation monitoring .. ... ......49
Landscape diversity and habitat management .. .....54
Highest priority information gaps (bottom-up) . ... .56
Summary .. ..62
PART THREE: Specific prioritized research recommendations
Soil nutrient recycling .. ..63
Trophic pyramids; arthropods as vertebrate food .. 64
Pollination interrelationships of low-elevation forest species . . .67
Characterization of arthropod communities ...69
Summary . ...72
FOOTNOTES .. ..73
LITERATURE CITED ..74
APPENDIX ONE: Glossary of terms ....84
APPENDIX TWO: What the original arthropod advisory panel to FEMAT said ....85
APPENDIX THREE: Bee (Apoidea) species of conservation concern (BSHT) .. 92
3A. Tentative listing of Vancouveran bioregion endemic BSHTs ..102
3B. Tentative listing of BSHT bees further south ...107
APPENDIX FOUR: Biogeographically significant higher taxa (Coleoptera,
Hymenoptera, Lepidoptera (butterflies), Diptera) . .. ....109
APPENDIX FIVE: Example of BSHT presentation . .. 124
APPENDIX SIX: Tentative listing of Collembola of the PNW ..separate document
APPENDIX SEVEN: Tentative listing of Diptera of PNW ....separate document
APPENDIX EIGHT: Tentative listing of Oribatida of PNW ..separate document
APPENDIX NINE: Tentative listing of Carabidae of PNW ...separate document
APPENDIX TEN: Tentative listing of spiders of the PNW . ...separate document
APPENDIX ELEVEN: Tentative listing of Diplopoda of PNW ....separate document
APPENDIX TWELVE: Tentative listing of Cerambycidae of PNW .separate document
APPENDIX THIRTEEN: Tentative listing od Scolytidae of PNW ...separate document
APPENDIX FOURTEEN: Tentative listing of Apoidea of PNW . .separate document
FIGURES AND TABLES:
Figure 1. Structure of the invertebrates in a typical soil foodweb 12
Table 1. The usual components of the soil foodweb in nearly all ecosystems ...13-14
Figure 2. Harpaphe haydeniana ... ... 19
Figure 3. Zootermopsis angusticollis . ..23
Figure 4. Amaurobiidae distribution of species . . .35
Figure 5. The distribution patterns of arthropods inhabiting the PNW ... 36
Figure 6. Grylloblatta campodeiformis ... 37
Figure 7. Agulla ... 37
Figure 8. Bequaertomyia jonesi . .. 38
Figure 9. Deuterophlebia inyoensis .....38
Figure 10. Tanypteryx hageni .. .39
Figure 11. Eulonchus tristis .. 39
Figure 12. Snow scorpionfly, Boreus .. .60
Figure 13. Glacier fly, Chionea ....60
Table 2. Comparison of standardized insect traps ....71
PREFACE
Ecosystem management and arthropods, what does it entail?
We were asked to review the available information and provide recommendations which would allow the USDA Forest Service and USDI Bureau of Land Management to protect the ecological functions of forest soil and litter arthropods and coarse woody debris chewers. We were also asked to focus on information gaps that could affect future policy and to comment on any potential problems relevant to current policy, particularly as it relates to the soil, litter, and coarse woody debris components of ecosystem processes. We will confine our remarks to those issues judged to be of the highest priority and broadest applicability only.
Ecosystem management is an approach based on maintaining large-scale biological processes (FEMAT 1993). In theory, efficiently maintained processes within the realm of naturally occurring variability should insure a healthy forest that is maximally buffered against perturbations that would adversely affect both short-term resource production and long-term forest sustainability. Ecosystem processes (such as primary production, nutrient cycling, pollination, succession, predation) are carried out by multitudinous arrays of individual species; arthropods play important roles in each of these processes. In a complex circular argument (supported by most ecologists, though with little experimental evidence), stable ecosystems permit the evolution of multiple species with similar but not identical functionsthis redundancy of species then feeds-back positively on the system stability as a whole by permitting more efficient substitution and dampening the variability in ecosystem processes when the ecosystem is stressed. Hence, forest health can be interpreted to mean the preservation at relatively natural population densities of all the species within a forest ecosystem. Since it would be difficult to monitor all the arthropods within a forest ecosystem, priority must be placed on establishing the identity of species which participate in different forest processes and which measures of diversity are appropriate to indicate both the natural levels of variability and warnings when the system is being severely stressed.
There are probably in excess of 10,000 species of arthropods within the region encompassed by the Northern Spotted Owl (west of the Cascade Mountain crest from the Canadian border to and inclusive of northern California). Most of these arthropods are intimately associated with the soil/litter for at least part of their life cycles. No comprehensive listing exists of these species, nor is there any systematic compilation of the relevant literature. This diffuse state of information forms the setting of everything else to be stated below. In order to stimulate a useful collation of this material, the Forest Service requested that we assemble pertinent literature on these soil/litter-dwelling groups, especially with reference to basic ecological process studies and inventory techniques. To that end we were provided a list of 1262 references produced by a search of an archival database; the search keywords were an abbreviated list of the families of litter/soil dwelling arthropods known to occur in the Andrews Experimental Forest (Parsons and others 1991). This list was examined, several hundred references selected and annotated (either with official abstracts or a personal evaluation). Since the original list provided was severely limited, we initiated a preliminary literature search based on all the genera of arthropods known to occur in the Pacific Northwest (since the large majority of them are soil-associated) based on the relevant library databases. This process, to be subsequently accompanied by production of a faunal list of all the Pacific Northwest (PNW) species, will take several more years to complete. For the purposes of this contract though, we have searched the databases (tens of thousands of additional references) on major taxonomic groups: spiders (Araneae), turtle-mites (Oribatida), carabid ground-beetles (Carabidae), longhorned-beetles (Cerambycidae), bark-beetles (Scolytidae), flies (Diptera), springtails (Collembola), bees (Apoidea). Additionally, we searched the database for inventory techniques and soil ecological terms in order to compile an annotated bibliography of the most pertinent references (most recent continually updated version subsequent to this one may be found at http://www.osu.orst.edu/dept/entomology/moldenka). The current bibliography is over-representative of such topics as bark-beetle ecology and ground-beetle toxicology studies. All of the references have been given additional keywords to facilitate use by subsequent workers (Appendix 15-16).
In order to prioritize topics or locations of special concern, a subset of the resident species (indicator species) are chosen to permit comparative analysis of multiple issues. Since there is currently no complete listing of resident arthropod species for this region, how can one evaluate the possibility of using a restricted set of designated indicator species for inventory studies? How can one determine hot-spots of biodiversity for special conservation attention? Is it possible to prioritize a short-list of sensitive species of special concern? The answers will necessarily contain a significant amount of archival bias; better-known species provide more extensive ability to interpret the results. Even after our faunal listing and literature search is completed in several years, the impediments will basically remain the same since the ecological knowledge base for species varies significantly (including no knowledge for many taxa). The appended professional judgments are based on communications with a great number of entomologists and ecologists.
Literature Search
How many arthropods are there? Which ones are most worthy of attention?
What are the landscape level patterns of arthropod richness and rarity?
We have
conducted an extensive search in the literature for information pertinent to
these multi-faceted issues in the Northwest. We started with a list of 1262
references, provided by the Forest Service and originally generated by the HJ
Andrews arthropod inventory list of families
of arthropods (Parsons and others 1991). We searched all the usual web
databases (e.g., Zoo Record, World Cat, Biosis, Agricola (FirstSearch and
Webspirs), Biol & Ag Index, CAB Abstracts) for listings involving these
suggested taxa. These searches generated tens of thousands of references; we
reduced the number of especially pertinent references to several hundred,
obtained abstracts for any in this category that were not provided on the web,
and categorized them into groupings that would make them far easier to use.
Since it was apparent that the original list did not include many families of
arthropods known to be associated with the soil or coarse woody debris, and
since the most useful sorts of references were frequently found by doing
generic-level searches instead of family-level searches, we initiated a
multi-year attempt to broaden this search of the literature to all the genera
of certain groups of largely soil-associated arthropods known to live in the
PNW. This search is on-going and any information resulting from it will be
available to any management agency that requests it (see
http://www.osu.orst.edu/dept/entomology/moldenka/). All the genera of
Oribatida, Scolytidae, Cerambycidae, Diptera, Apoidea, Collembola and Araneae
inhabiting the PNW have already been searched. A similar endeavor focusing on
understory-dwelling herbivorous arthropods was compiled by Pacific Analytics
(2001) and is available on the US Forest Service web page (http://www.fs.fed.us/r6/nr/fid/
pubsweb/litsurvey/index.html).
First drafts of species lists of the Apoidea, Araneae, Collembola, Oribatida, Carabidae, Cerambycidae, Diplopoda, Scolytidae and Diptera of the Pacific Northwest are in Appendices 6-14. The majority of species in each of these groups are soil- or CWD-associated).
Synopsis of literature search submitted
The dataset (SoilArth) was
compiled by searching the literature databases for 1) genera of arthropods inhabiting the Pacific Northwest, 2) for
various sampling methodologies, and 3) for other relevant terms: soil
arthropods, fire, forest underburning, clearcutting, compaction (soil
compaction, forest soil compaction), Pacific Northwest forest, woody (woody
soil, woody forest, woody forest soil, coarse woody debris, woody debris
removal), litter (soil litter, forest soil litter, litter removal), herbicides
(forest herbicides), canopy removal, non-target effects, fertilizers,
pesticides, forest management, endemism, trapping methods of various kinds
(pitfall, etc.), soil biology, soil ecology, forest environment, forest
ecology, forest ecosystem management, biodiversity, Klamath, Siskiyou, and any
other permutations or phrases that came up as categories while doing the
searches. For example, if the word
fire was used in a specific way in the databases, e.g., as fire ecology,
then that phrase was entered and researched as well. Thus one word could lead to a number of
offshoots, each of which was followed as long as deemed relevant.
The dataset was entered in the software program, ProCite, and contains 2964 references separated into two groups, those with abstracts and those without. Descriptive keywords have been added to each reference for quick identification. Specific fields have been selected on the ProCite workform (under the View menu > Configure Record List) to show author, title, date, and keyword. Other pertinent information is also present for each reference, such as source, notes, abstract, etc. ProCite gives one the option of opening a larger preview pane for each entry that can be pre-selected (View menu > Configure Record List) to show some of these fields, such as abstracts. The source of the reference (e.g., journal title) automatically appears in the preview pane.
You may use the bibliography in the following manner. If you have ProCite5 and are interested in finding references about Dendroctonus bark-beetles, simply click on the tab below the spreadsheet marked Search. In ProCite4 Search would be on the menu above the spreadsheet. Type Dendroctonus in the search for window, making sure that all records shows in the look in window. If using more than one search word, and, or, not or some other operator must be included between each word. (Look under the Operators menu above this window for options). For example, in our dataset, we generated 788 records when the word Dendroctonus was typed into the search window; Scolytidae generated 462 records; and barkbeetle 1. Bark and beetles yielded 219, bark and beetles or Dendroctonus yielded 902, bark and beetles or Dendroctonus or Scolytid yielded 916, and bark and beetles or Dendroctonus or Scolytid or Scolytidae yielded 1086 records, etc.
You may also use the keyword column in which each reference is labeled by subject. By tapping the grey Keywords column header, the list will automatically sort itself alphabetically, and if you tap it again, it will sort itself in the opposite order (true of all the column headers). If the keyword list is not showing on the screen, go to the View menu > Configure Record List, and put a checkmark by keywords. This column will then appear on screen. If your interests pertain to a broader subject, such as fire ecology, just insert fire and ecology into the search window, or look under fire ecology in the keywords list.
The
nature of the arthropod information in the literature
To understand patterns of abundance, species diversity and endemism of soil-associated arthropods on a landscape basis, we focused on 3 groups of soil and coarse woody debris (CWD)-associated species: spiders (excluding the poorly-known Linyphiidae), Carabidae, and Apoidea. (The Apoidea are all soil- or CWD-associates for 50 weeks out of the year; during their 2-week adult activity period they are seldom further than a few inches away from the soil.) We confined our analysis to these three higher taxa since they are: (1) each highly diverse, (2) each generally well-known taxonomically and (3) each represents a group the senior author is personally quite familiar with from previous research over the past four decades. We felt that we were able to both assemble and interpret the information on these three groups more adequately than any other arthropod groups (Moldenke 1976b, 1979c). The Oribatida and Collembola are so poorly scientifically documented as to render in depth analysis of beta and gamma diversity futile. The specific numbers relevant to distribution patterns, rarity and endangerment we report are based on the most recent group we analyzed (the Apoidea), but the general patterns apply equally well to all three groups. Both spider and carabid distributional analyses suffer from imprecise habitat information, and are (at best) overlapping map patterns for each species, each with poor resolution. Hence, separation of the alpha and beta components of diversity is problematic. The gamma diversity component of the elevated biodiversity of the Klamath/Siskiyou region is apparent for both of these groups, as is the general North/South gradient of biodiversity. For both the Carabidae and the Amaurobiidae (spiders) the Klamath/Siskiyou region is known worldwide as a center of biodiversity (Leach 1972, David Kavanaugh, unpublished data on Nebria [13]). None of the diversity patterns we have documented for these three groups come as a surprise to us or differ from patterns assumed by the FEMAT panel of experts, but we felt it necessary to quantify these general distributional patterns due to the gravity of the questions being asked.
To use the entomological literature, it is necessary to understand the fundamental nature of the taxonomic database. Taxonomy usually proceeds in three distinct phases. The first phase is akin to Wow, this looks different! Lets call it a new species, since no one has collected in this region before. This leads to a common pattern of many different scientific names for the same widespread species. The second phase is analysis of specimens from throughout the entire extent of the species distribution; the emphasis is on basic similarities rather than minor differences. Such taxonomic revisions, as they are called, lead to long lists of synonyms and a de-emphasis on local differentiation. The third phase is an in-depth analysis of the landscape patterns of variability, separating out the forms that are geographically (or otherwise) isolated from those that are maintained within breeding populations. Though most plants and vertebrates have passed through all three phases of taxonomic development, few insects have. Perhaps butterflies (Hinchliff 1994) and tiger-beetles (Willis 1968; Pearson and Cassola 1992) are the only examples in the Pacific Northwest to have entered phase #3. Other arthropod groups are split between phase #1 and phase #2, with most groups still in phase #1.
The third phase is critical for ensuring scientific credibility; variants of a species are appropriately recognized for their potential contribution to long-term evolutionary potential of the overall species. Long-term evolutionary potential, in this case, means divergence of new forms or variants in unique environments or in peripheral locations. For example, Gavrilets (1999) found that Rapid speciation is most likely for populations that are subdivided into a large number of small subpopulations. Thus, to maintain the evolutionary potential of a species that displays local variants, one might strive to formally identify those variants (i.e., the third phase mentioned above) and then subsequently conserve them.
The taxonomic status of bees (Apoidea) provides a good example of the status of arthropod taxonomy. As a whole, the group is relatively well-known in the Pacific Northwest. Nearly all of the genera have been revised during the past 40 years, and identification keys (for use by the trained specialist) are generally available. A list of all the species known to occur, or suspected of possibly occurring, in the Pacific Northwest is appended (Appendix 14). However, even within this well-known group as a whole, the second, third and fourth most speciose genera have not yet been revised. The second largest (Nomada) and the third largest (Dialictus) are comprised of more species without names than with names. The fourth largest (Osmia) undoubtedly contains so large a number of synonyms as to be intractable. And yet, I state without chance of refutation, that bees are a well-studied group relative to the total range of arthropods. The most speciose genus in the Pacific Northwest is Andrena; the database records 174 species in the region. During the phase #2 revision of just these northwestern species, 297 names were placed in synonymy (nearly twice as many as there are valid names!). At the same time, 31 of them were described as new to science. This is the state of the arthropod database.
One of the major procedural difficulties that faces arthropod ecology is the process of identification. Even when recent monographic treatments of a group are available, it is difficult to impossible for a novice to use the keys due to either specialized vocabulary or a lack of comparative identified museum material. In an attempt to relieve this procedural problem and allow easier identification of field-collected specimens, we have been working on a user-friendly computer key COMTESA COMputer-assisted Taxomony and Ecology of Soil Arthropods (www.ent.orst.edu/comtesa). The goal is to provide illustrated keys in a database matrix format which allows rapid identification of specimens and subsequent access to literature citations and ecological data. The process of programming has been largely completed, and portions can be viewed as a test case. Instructions in Appendix 17 will allow the reader to visualize how the identification and information-retrieval functions will operate. The entire process is still under construction, but instructions relevant to identifying a bumblebee specimen are included in Appendix 17.
PART ONE: Ecosystem process sustainability vis-ΰ-vis arthropods
(top-down approach)
Although there are many components within the structure of any ecosystem, ecologists generally focus first on the structure of the energy flow through the trophic pyramid as the prime emphasis in their descriptive models. As such, ecologists are interested in the quantity of energy/biomass that comprises the various subcomponents of a trophic pyramid as well as the regulatory processes that determine the energy flow through the entire pyramid (Schowalter 2000).
In PNW conifer forests it is well established (1) that the majority of primary production is sent to the roots both for root growth and (primarily) for mycorrhizal functioning; and (2) that the majority of the photosynthetic tissue produced on an annual basis ends up in the litter (and is not consumed by typical herbivores). This signifies that the soil, litter and coarse woody debris portion of the conifer forest ecosystem is a very large, if not the most significant component (especially if standing dead woody material is included). Though most of this biomass or nutrient potential is encompassed within dead plant matter, bacteria, fungi and protozoa arthropods do comprise a significant portion. The major significance of arthropods in this environment is neither their standing crop biomass nor their respiratory rates, but their role as regulators of microbial growth, nutrient decomposition and plant growth. Though some of these effects are direct (i.e., fungivory, or major source of prey items for secondary consumers), most are indirect (i.e., shredding plant material) and contribute to the process of buffering soil health and building the soil matrix itself. Hence, the biological processes taking place in the soil are as significant to a forest as those taking place in the canopy.
The biomass and nutrients encompassed in arthropods is passed on to the next higher level of the trophic pyramid as well. The majority of species of vertebrates are insectivorous, or primarily insectivorous during periods when their young are growing the fastest. Examination of these energy transfers demonstrates how integrated the forest ecosystem is. Since both the soil arthropods and vertebrate predators may be highly mobile, there can be very significant transfer between ecosystem components. For instance, between 26-53% of the diet of terrestrial birds and aquatic fishes can be made up of arthropods coming from the opposing environment during the course of the year (Nakano and Murakami 2001).
It is known that the soil organic matter, litter, coarse woody debris and their component arthropod faunas vary enormously through the region inhabited by the Northern Spotted Owl. Though this has not been adequately quantitatively documented for arthropods, it is universally assumed to be true since fundamental changes have been documented in the plant community structure and quantified as well in terms of soil nutrient capacity and depth of organic layers. It is also known that the arthropod fauna of concern varies significantly through successional time at any one locality, since the vertical structure of the plant community as well as the herbivore-resistence of the leaves changes greatly. Through the seasonal limitations of low temperature and evapotranspiration, total soil metabolic activity is strongly bimodal which ultimately controls most arthropod activity as well. Hence, the soil arthropod communities no doubt vary tremendously across both spatial and temporal aspects.
The large organic component of the soil and its widespread heterogeneity produces a high level of species richness in the soil, litter and coarse woody debris faunas. As a general rule, the soil is the most diverse component of any ecosystem, especially in so far as arthropods are concerned. This diversity of arthropods can be functionally subdivided into a rather large number of relatively separate functional guilds, each of which is characterized by species belonging to rather distinct taxonomic groups (see Moldenke and others 1999 for analysis of PNW conifer soil fauna).
Soil food web
structure (see
Lewandowski 1999)
The basic structure of all natural soil food webs requires
a minimum of four components; (1) the living plant root, which is incapable of
enzyme production, but obtains nutrients for the plant passively through water
absorption; (2) the soil bacteria and fungi, which are in most circumstances
fuel (carbohydrate)‑limited, but are
capable of a bewildering diversity of degradative enzymatic decomposition processes; (3) the fauna which
graze upon the microbial biomass, thereby releasing, as part of their waste
products, water‑soluble (and hence
"useful") forms of plant nutrients; and (4) the predator fauna which
keep the population of microbial grazers below levels which might limit the
microbes in their pivotal role as the "biological sponge" which prevents loss of nutrients from an ecosystem.
The cycle of growth and decomposition is driven by
photosynthesis. Plants send more than 50% of the carbon fixed by photosynthesis
directly to the roots to be excreted in order to facilitate microbial growth.
This fuel is shared with both specific symbiotic fungi (mycorrhizae) and the
general microbial population of the rhizosphere (the volume of soil within 1 to
2 mm of a living root tip). Bacteria are important as local foci of intense
metabolic activity on a scale of several millionths of a pinch of soil. Fungi
are capable of slower metabolic activity, but because their basic body form,
similar to a railroad system, allows the simultaneous exploitation of widely
spaced, yet different, required nutrients, activity can be maintained
continually. Both bacteria and fungi are relatively incapable of movement; they
require insects and other arthropods to bring them to sites of new resources or
to physically mix the resources currently available. Arthropods that graze upon
the fungi are seldom able to consume them entirely, resulting in stimulation of
rapid regrowth (like pasture plants subjected to grazing by cattle). The result
of all of this microbial growth is the immobilization of nutrients into living
biomass. The long-term health of any ecosystem is intimately dependent upon
minimizing the rate of nutrient loss: the greater the biomass of soil microbes,
the slower the rate of nutrient loss. The progressive decrease in soil organic
matter (both living and dead) in agricultural soils in North America is a major
factor leading to chemical pollution in waterways and aquifers in the United
States.
The grazing by soil fauna leads to
momentary availability of soluble nutrients for two reasons. (1) The ratio of
carbon to nitrogen in the tissues of bacteria and fungi is less than that of
animals. As a consequence, an organism feeding on a bacterium or fungus must
excrete any nitrogen in excess of its own requirements (or become poisoned by
it). (2) As an animal locomotes in search of
food, it burns (respires) carbohydrates for
energy. As CO2 leaves the body, other nutrients (nitrogen, etc.)
must leave it as well in order to maintain the ratio within living cells. Some
of the nutrient release by the fauna is immediately recaptured by microbes in
their vicinity, while a significant portion is released close enough to a root
to be swept into the plant by the inward rush of water caused by transpiration
from the leaves. Since most faunal activity is
within the rhizosphere, faunal activity does
result in nutrient uptake (but generally only when the stomata are open). A
generalized foodweb of the diversity of soil‑
and litter‑associated invertebrates is represented in Figure 1. The arthropod groups comprising each of these categories
are presented in Table 1.
Soil is physically and chemically heterogeneous at the
scale of microbes and arthropods. The movement of arthropods aerates the soil
and transports microbial inocula to regions of nutrient availability. Microbial diversity, though fundamental, is difficult to
measure.
Arthropod diversity is far easier to monitor and is likely to reflect microbial
diversity. Soil degradation is associated with a decrease in soil fauna
diversity (Coleman and Crossley 1996; Benckiser 1997). Some forms of
degradation are more easily measured structurally or physically, for instance
the depth of litter or the resistance to a penetrometer. But complex soil
chemical changes may best be monitored through arthropod biodiversity. Any change in invertebrate diversity should be interpreted not
just in isolation, but as an integral part of the basic functioning of the soil
foodweb, which supports all plant life.
There are indistinct boundaries on
the definition of soil-associated arthropods;
the herbivorous fauna on the grass, forb and
sometimes shrubby vegetation is a major component of the food resource of epigeic
(soil-surface-dwelling) macropredators. Though many
invertebrate predators (i.e., Calosoma
sycophanta) routinely ascend tall trees to hunt,
they represent exceptions and are not considered in this discussion. Far more
important, are all the species of truly arboreal herbivores, which descend to
the ground to pupate. Such species not only provide food resources to soil‑dwelling
predators, but they represent important additional linkages between potential
canopy health and prescribed burning protocols.
Generally, the biomass of organisms
high in the trophic pyramid is relatively
insensitive to shifts in species composition of the lowest trophic levels and is theoretically sensitive only to the total
primary production of the ecosystem. However, the species comprising the higher
levels of the trophic pyramid are very
sensitive to changes in management policies. To understand whether land
management practices will affect community structure, it is necessary to
differentiate the producer biomass into separate components (i.e., within
column I) and to quantify how efficiently this biomass is transformed into the
different groups of primary consumers (i.e., column II). The majority of the
biomass and species richness of any arthropod community is contained within the
primary consumers. Any (and perhaps, every) major change in management policy
will alter the percentages of arthropod biomass between boxes of the primary
consumers, and consequently within the secondary consumers as well. Such
changes are
Table 1
not
necessarily "bad", but it is important to recognize such a change
(and quantify it) when it occurs. The primary consumers are the engines that
drive these 20 separate processes within an ecosystem. In general, increases
either in the biomass of resources or their spatial heterogeneity will favor
increased species richness within each separate process.
The actual dynamics of the soil foodweb in any Pacific Northwest community is far more complex than the one outlined in Figure 1. The foodweb structures among differing community types (forest, desert, grassland) are likely to differ significantly, both in terms of the relative biomass included within the boxes and the rates of energy flow between them. No foodweb analysis of invertebrates has ever been attempted in the Pacific Northwest on other than a very generalized basis. Figure 1 represents: (1) a structure into which most communities can be accommodated, and (2) a structure that is adapted for invertebrate bio-indicator use.
The farther to the right one proceeds in Figure 1, the
larger the individual organisms become and the easier they are to collect and
monitor. However, the further to the right one proceeds, the lower the species
richness (and the more generalized the behavior and, therefore, the less useful
it is at indicating basic changes in energy functioning within the ecosystem).
There are more than 10,000 species
of arthropods (though many of them do not even have scientific names yet)
within the Pacific Northwest. Though the specific identity of the members of
any one box in Figure 1 will vary between community types and between sites, at
the family‑level of taxonomic resolution there is considerable
consistency. We present the taxa in Table 1,
as a checklist for comparing different sites. This list is certainly not all‑inclusive,
but it attempts to encompass the great majority of arthropod taxa found associated with the soils of the Pacific Northwest.
It is not yet possible to enumerate all the
species of arthropods at any one site, but it is certainly logistically
practical to monitor several of the component groups when most of the species
are collected in a particular trapping system. For instance, epigeic
macropredators, macroherbivores and macrodetritivores can be collected in pitfall traps and compared between
treatments (Niemela and others 1993). The insect canopy fauna of different age
stands can be compared to see if there are qualitative as well as quantitative
differences (Schowalter 1989, 1995). The trapped specimens can be weighed to
get an estimate of biomass, and analysis at the resolution of morphospecies of
the component groups can easily indicate significant changes in species
richness. These taxa are listed in the table,
not as an attempt to facilitate a one‑species‑at‑a‑time
approach to conservation/monitoring, but rather as an educational device. Most
ecosystem managers are unaware of the functional roles played by invertebrates,
and even if they are aware of the roles, they have never learned who does what.
Managing our natural resources for the long‑term will require ensuring
the persistence of all the functional links in the community.
Critical environmental
determinants of soil arthropod density and diversity
The overall level of soil arthropod
biomass and diversity is regulated by a number of factors under natural
conditions. The relative importance of these factors vary in different environments
within the Pacific Northwest, and consequently affect the soil arthropod fauna
substantially.
- GrazingIn general, any photosynthetic
structures that end up in the stomach of a vertebrate grazer are
subtracted from the resource base for invertebrates. A reduction in heavy
grazing by mammals will also increase habitat structure and refugia.
- DroughtPrecipitation
is generally highly unpredictable for certain critical periods in the
year. Total annual precipitation coupled with periodicity determines gross
primary productivity in large part. Primary productivity is the most
important determinant of secondary productivity.
- Habitat RefugiaEvery species
requires refugia of some sort: the
more numerous and diverse the potential refugia,
the more individuals and species will prosper. In all habitat types a
premium must exist on physical means that promote vertical circadian migration patterns. In forested
systems, consolidated clay soils are frequent, which limits vertical
movement to those few taxa that have
the actual ability to burrow, or to those fauna that can utilize the
burrows created by other species. Very few soil invertebrates are
physiologically capable of true burrowing; most of them, such as anecic earthworms and geophilomorph centipedes are absent from
drier soils. Cicadas, millipedes, gryllotalpids
and stenopelmatids may
exercise 'ecosystem engineer' keystone roles in these environments. In
unconsolidated soils, true burrowing is not required and many/most taxa may be able to bury themselves
sufficiently to prevent desiccation.
- Surface compaction/cementationAny
change leading to a decreased ability of the fauna to pass through the
boundary layer or a decreased infiltration of precipitation is generally
detrimental. In many environments a cyanobacterial
crust forms on the surface of the soil. The relationship that this
crust has on the total community is unclear; one would expect it to
facilitate faunal diversity due both to its nutrient biomass and ability
to absorb water.
- Litter depthIn
forest environments the litter layer is apparently positively correlated
with increased arthropod density and diversity (Madson
1997). Litter serves as both a food source and in a three‑dimensional
habitat capacity. In scrublands a
definitive litter layer is generally absent, but individual detrital pieces are the center of soil
invertebrate activity, especially after they are anchored in place by
fungal hyphae and are no longer
subject to wind displacement. Early in the morning vertical migration of
the microfauna creates an evanescent
hot spot of activity within the litter (Wallwork
1970).