摘要:

New silivicultural strategies to sustain both ecological and human communities are being developed and implemented on federal forest lands in the Pacific Northwest (PNW) United States. Two important stand‐level components of the new silviculture regimes are rotation age and retention level of live trees in harvest units. Ecologists have suggested that canopy tree retention and longer rotations will create patterns of stand structure in managed forest that are similar to those in natural forests, and promote long‐term ecological productivity and biodiversity. Forest economists, however, are concerned that canopy tree retention and long rotations may reduce wood production, although the high value of large logs produced by these new silvicultural regime may compensate for reduced growth rates. We used the forest model ZELIG to perform a factorial simulation experiment on long‐term responses of ecological and economic variables to nine retention levels and four rotation lengths. ZELIG output on forest structure and composition was input to a forest economics model that calculated net value of wood products in 1989 dollars. The simulated stand data were also linked with regression equations to predict the densities of 17 bird species as a function of tree size class distribution. Five replicates of each treatment were run for the 240‐yr simulation period. Results indicated that stand structure under each of the canopy tree retention levels was more similar to the pre‐treatment natural forest than following clear‐cutting. Variation in tree size under intermediate levels of retention, however, did not reach the level of the natural forest during the simulation period. Tree species composition was strongly related to retention level and rotation age. Shade‐intolerant Douglas‐fir (Pseudotsuga menziesii) lost dominance to shade‐tolerant species under intermediate retention levels and longer rotations. Wood production decreased significantly with increasing retention level and rotation age, with a notable threshold between retention levels of 0 and 5 trees per hectare. Net wood products value did not decrease as rapidly with retention level, and did not differ much among rotation ages, because of the high value of large logs. Bird species responded individualistically to retention level and rotation age. Some had peak densities under short‐rotation clear‐cutting, but most were associated with structurally complex, closed‐canopy forest. Consequently, bird species richness increased significantly with retention level and rotation age. Within the assumptions and limitations of our models, this application provided knowledge on trends and thresholds that can help land managers to choose silvicultural regimes that best balance their management objectives. We concluded that retention level and rotation age strongly influence ecological and economic responses in PNW forests; efforts are needed to reduce uncertainty about these effects.

ISSN:1051-0761    

DOI:10.2307/1941965

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Ecologists have advocated retaining various densities of canopy trees in harvest units in Pacific Northwest forests. In contrast to clear‐cutting, this practice may better emulate the patterns of disturbance and structural complexity typical of natural forests in the region. Several ecological attributes, including vertebrate habitat diversity, are thought to be associated with stands of complex structure. The goal of this study was to determine bird abundance in canopy retention sites relative to other common stand types in the Pacific Northwest and to develop habitat functions for extrapolating bird abundance across current and future landscapes. We used data from five previous studies in the west central Cascades of Oregon to compare bird abundance and to develop habitat functions for forest birds across a wide range of natural and managed stand structures and ages. The 67 stands included clearcuts, retention sites, young closed‐canopy plantations, mature stands, and old‐growth stands. ANOVA revealed that 18 of the 23 species included in the analysis differed significantly in abundance among the stand types, with some species being primarily associated with each of the stand types. The habitat variables used to build habitat functions included tree density by size class, mean tree diameter, and variation in tree diameter. Linear, polynomial, and various nonlinear regression models were evaluated for each bird species. Significant habitat functions were generated for 17 of the 23 bird species. The analyses identified four habitat‐use guilds among the 17 bird species: open‐canopy; open‐canopy with dispersed large trees; structurally complex closed‐canopy; and structurally simple closed‐canopy guilds. This study is the first in the Pacific Northwest to compare bird abundances across natural stands, traditionally managed plantations, and stands managed under ecological forestry approaches. The results suggested that canopy tree retention benefits many, but not all, of the bird species we studied, Moreover, the nonlinear responses of bird abundance revealed thresholds in tree density at which bird abundance changed dramatically. Knowledge of these thresholds allow managers to design stands for specific biodiversity objectives. The habitat functions presented here can be used to predict bird abundance based on habitat measurements derived from field data, remotely sensed data, or output from computer models of forest dynamics.

ISSN:1051-0761    

DOI:10.2307/1941966

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Vertebrate herbivores can be effective agents of biological weed control in certain applications. I compared the use of domestic geese for weed control in an agricultural field with the herbicide hexazinone and with hand control. Newly planted spruce seedlings acted as a prototype crop that would be unpalatable to the geese. Trampling by geese led to as much as 47% tree seedling mortality during the 1st yr; this was reduced significantly by either limiting the amount of time the geese spent in the plots or surrounding seedlings with small wire fences. When compared with plots with no weed control, weed control by geese improved the diameter growth of the surviving seedlings by over 100% during the 1st yr of the study, but had no effect in the 2nd yr. The geese controlled a variety of weed species, but were most effective against quackgrass (Agropyron repens). However, grazing effectively selected for unpalatable weed species (including pineapple weed, Matricaria matricarioides, prostrate knotweed, Polygonum aviculare, and wild chamomile, Tripleurospermum phaeocephalum) so that by the end of the 2nd yr plots weeded only by geese had 25 times as much cover of unpalatable species as plots with no weed control. In contrast, the herbicide was ineffective against grass and effective against the unpalatable weed species. A successful integrated weed management strategy would thus require combining geese with another method of weed control, and would include measures to prevent crop trampling.

ISSN:1051-0761    

DOI:10.2307/1941967

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The conservation of species requires preservation of natural habitats. Where the integrity of natural habitats has been upset, species go extinct. All natural habitats are continuing to decline, both inside and outside of reserves. Habitat change is partly a natural process (e.g., succession), but human activities have accelerated the process of decay so that natural rates of renewal are insufficient to maintain natural habitats. We argue that our only recourse, in light of these scenarios, is to adopt a new conservation strategy that considers the importance of habitat renewal in addition to habitat preservation. Accordingly, in our management decisions we must not only choose the size of area to preserve but also the size of area that balances habitat loss with habitat renewal. We also suggest that this habitat equilibrium point, H*, needs to be decided upon urgently, otherwise many species will become extinct in the next 50 yr according to numerous predictions. There are two ways to achieve H*. The first is to set habitats aside in protected areas in perpetuity. There are two reasons why this protection alone is insufficient: (1) protected areas continue to decline, albeit at a slower rate than outside of their boundaries, and (2) achieving H* simply by setting aside protected areas is no longer an option in many areas where severe habitat degradation or fragmentation has already occurred. The other way to achieve H* is to promote the reestablishment of natural habitats, or "habitat renewal." This concept is illustrated using a simple trade‐off model that balances habitat decay and habitat renewal. We then provide examples of habitat loss outside and inside of protected areas and discuss the potential for habitat renewal to offset these losses. We conclude that continued emphasis needs to be placed on setting aside natural habitat in protected areas. However, our examples of habitat loss show that this policy alone is most likely doomed to failure, so a policy of habitat renewal is also required.

ISSN:1051-0761    

DOI:10.2307/1941968

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Recent declines in the number of breeding ducks in the Canadian prairie‐parklands have been hypothesized to be due to loss of habitat to agriculture. However, prairie‐parkland also has experienced wetland loss to drought as well as to agriculture. If habitat restoration is to be implemented and monitored successfully, it is important to separate the effects of anthropogenic changes to the landscape on duck populations from those caused by changes in climate. We used data from annual air–ground surveys and from precipitation records to develop relationships between indices of abundance of each of 10 species of ducks and indices of wetland conditions during 1955‐1974. We used these relationships to predict annual abundance of each species during 1975‐1989. We compared predicted and observed abundances over the period 1975‐1989 to distinguish declines in duck abundance greater than those accounted for by drought alone and to determine the magnitude and location of real "deficits" in duck abundance. Average annual deficits within Canadian prairie‐parkland over the period 1975‐1989 were estimated at 1.2 x 106birds for both Mallard (Anas platyrhynchos) and Northern Pintail (A. acuta), 480 000 for Blue‐winged Teal (A. discors), 190 000 for American Wigeon (A. americana), 175 000 for Northern Shoveler (A. clypeata), 50 000 for Gadwall (A. strepera), 10 000 for Green‐winged Teal (A. crecca), 40 000 for Canvasback (Aythya valisineria), 25 000 for Lesser Scaup (A. affinis), and 5000 for Redhead (A. americana). Overall, the effect of agricultural expansion in the east on prime waterfowl habitat since 1951 appears to have been negligible. There, as much as 90% had been already lost prior to 1951. In the west, however, where prime waterfowl habitat was still relatively abundant in 1951, agricultural development has encroached substantially. The relationship between the lost area of the best breeding habitats and the size of population deficits for Mallards and Northern Pintails in the entire Canadian prairie‐parkland region was significant for both species (P<0.0027 and P<0.0001, respectively). Consequently, habitat restoration programs located where the highest quality waterfowl habitat and the lowest quality agricultural lands overlap most should have the greatest potential to affect recovery of breeding duck populations in the Canadian prairie‐parklands.

ISSN:1051-0761    

DOI:10.2307/1941969

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The population structure and effective population size of the Galapagos Cormorant (Compsohalieus [Nannopterum] harrisi), a rare seabird endemic to the Galapogos Islands, Ecuador, were investigated using standard demographic techniques and data collected over nine years (1970‐1973, 1977, 1978, and 1989‐1992). The study population was located around Punta Espinosa, Fernandina Island, where cormorants bred at seven traditional places scattered along 9 km of bare rocky coastline. This population, averaging 96 ± 13 adults (mean ± 1 SD) (47 ± 8 males; 50 ± 7 females), is an estimated 10% of the total number (≈980) distributed along 350 km of coastline around Fernandina and the western coast of Isabela Island. The Galapagos Cormorant's restricted distribution and small population size probably result from its adaptation to rich upwelling systems where food is highly reliable and abundant. This adaptation apparently is also shaping the population's general breeding ecology. The population has high potential fecundity and is probably regulated through food competition by density‐dependent processes which affect reproductive success and juvenile recruitment more than adult survival. Demographic analyses and direct counts along the species' entire range showed that the population is stable both in its demographic structure (e.g., sex ratio and age structure) and number. The population is characterized by a high and relatively constant annual adult survival, long life‐span, and a generally low and variable reproductive success and annual productivity. The serial polyandrous mating system apparently has little effect on these demographic traits. Dispersal data indicated that gene flow is extensive (≈28 km per generation) and that the whole population is distributed in at least 10 relatively discrete demes or neighborhoods. The demographic analysis yielded an expected among‐deme genetic variance component (Fst) of 0.049 (4.9%), which implies that the population is not genetically substructured to a large degree. Thus, for analyses of the effect of genetic drift, the whole population can be considered as a single panmictic unit. However, the exceptionally small total population size, and its even smaller effective population number (Ne≈650), predicts a rate of loss of neutral genetic variation (ΔF = 1/2 Ne) of 0.0008 (0.08%) per generation, equivalent to 0.006% per year. The population is therefore expected to be highly homozygous (G = 0.997). These population genetic traits should be taken into account for conservation purposes. Although after a crash (e.g., in 1982) the population may rapidly recover in number and otherwise remain relatively stable, its genetically effective size barely reaches the minimum number judged to be required for long‐term persistence in the face of genetic and environmental stochasticity. Moreover, the recurrent human disturbance of the habitat where cormorants find their prey, mainly through alteration of the sea bottom for the capture of sea cucumbers, may severely affect the survival of this endemic species, one of the world's rarest seabirds.

ISSN:1051-0761    

DOI:10.2307/1941970

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The effects of periodic environmental fluctuations on population viability are examined based on the use of environmental states. The approach is applied to the Florida population of the Snail Kite, an endangered wetland hawk that feeds almost solely on one species of snail. A preliminary assessment based on stochastic population fluctuations indicated that populations became viable when initial size surpassed 300 individuals. However, changes in population size between consecutive years, nesting success, and the length of the breeding season were all highly and positively related to water level and rainfall characteristics, which are highly periodic. Low water conditions cause Snail Kites to disperse and result in low recruitment, increased adult mortality, and population declines. The effects of cyclic drought were explored using stage‐based life tables for three different water conditions or environmental states (drought, lag years following drought, and high years). Population sizes predicted by the model were closely associated with actual kite population counts. Deterministic projections indicated that kite populations would increase when intervals between droughts exceeded 3.3 yr, but stochastic simulations found that populations did not become viable unless intervals exceeded 4.3 yr. The model was sensitive to estimates of survivorship. The use of the environmental state approach is compared to standard techniques for population viability analyses (PVA), and the implications of the model for Everglades water management are discussed.

ISSN:1051-0761    

DOI:10.2307/1941971

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The stability of Northern Bobwhite (Colinus virginianus) populations in Texas, where high density polygyne red imported fire ants (Solenopsis invicta) account for>50% of all (S. invicta) colonies, has been cited as a reason to repudiate impacts by this exotic species on Northern Bobwhite. We used two approaches to investigate the relationship between red imported fire ants and Northern Bobwhite. In the first approach, we used correlation analysis to compare Northern Bobwhite abundance trends, determined from Christmas Bird Count data in 15 Texas counties, before and after fire ant infestation. Before red imported fire ant infestation, no significant trend in Bobwhite abundance existed (r = ‐0.355, P = 0.314). After fire ant infestation, Northern Bobwhite abundance declined and was highly negatively correlated with years of infestation (r = ‐0.867, P<0.001). Bobwhite populations from 16 uninfested counties in Texas revealed no trend over a 27‐yr (1966‐1992) period (r = ‐0.081, P = 0.688). In the second approach, red imported fire ant populations were reduced on five 202‐ha study areas in the Texas Coastal Bend; autumn Northern Bobwhite densities were monitored for 2 yr on those reduced areas and five untreated areas. By the 2nd yr, Bobwhite autum density was higher (P = 0.028) on areas where red imported fire ants were suppressed. We concluded that polygyne red imported fire ants were negatively impacting Northern Bobwhite in this region of Texas.

ISSN:1051-0761    

DOI:10.2307/1941972

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The California Spotted Owl (Strix occidentalis occidentalis), like the more well‐known Northern Spotted Owl (Strix occidentalis caurina), appears to be in decline over at least a part of its geographic range. Two different types of reserve design have been implemented to protect Spotted Owls. The Spotted Owl Habitat Area (SOHA) system uses a fairly large number of small reserves; each SOHA is capable of supporting 1‐3 nesting owl pairs. The Habitat Conservation Area (HCA) plan proposes a smaller number of fairly large reserves, each including 10‐20 active Spotted Owl nests. The HCA reserve design strategy has been incorporated into current conservation planning for the Northern Spotted Owl. The Technical Assessment Team of the California Spotted Owl Assessment project has recommended preserving existing SOHAs within a lightly harvested matrix as an interim policy for conservation of the California Spotted Owl. In this paper we present results from two demographic models of the California Spotted Owl. The first model is based on a simple formulation of a birth–death process; the second model is a somewhat more detailed simulation model. The models are intended to provide a comparison of the SOHA and HCA reserve‐design strategies. We are particularly interested in the ability of the two reserve designs to withstand recurring environmental catastrophes in the form of forest fires. The HCA strategy always leads to longer persistence times than the SOHA strategy. The essential difference between the two strategies appears to lie in the shape of the function that gives the probability of colonization of an empty nest site. These results have several implications for the conservation of California Spotted Owls and for conservation biology in general. (1) Some variant of the HCA reserve design strategy may be preferable to the interim strategy being proposed for conservation of the California Spotted Owl. (2) Simple, parameter‐sparse models like ours can yield results comparable to those of more complex and detailed models. (3) Models that include the effects of catastrophic environmental perturbations have great potential for application in conservation biology.

ISSN:1051-0761    

DOI:10.2307/1941973

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

An understanding of the factors governing sciurid abundance in the Pacific Northwest is essential for prescribing forest management practices for second‐growth forests where recovery of Spotted Owl (Strix occidentalis) populations and enhancement of biodiversity are objectives. We compared results of companion studies of sciurids in western Washington and Oregon and examined patterns of abundance in relation to habitat elements on the Olympic Peninsula to elucidate governing factors and make recommendations for forest management. Regional contrasts show that Glaucomys sabrinus and Tamias townsendii in Douglas‐fir forests in Oregon are 4 times more abundant than in western hemlock forests in Washington, and dietaries of Glaucomys, and the fungal communities that provide its food, are more diverse in Oregon than in Washington. Glaucomys sabrinus in old forests are 2 times more abundant than in young, managed forests without old‐forest legacies (large live trees, large snags and large, decaying fallen trees); populations in young forests with old‐forest legacies and with understory development may equal those in old growth. On the Olympic Peninsula, Glaucomys sabrinus abundance can be predicted by density of large snags and abundance of ericaceous shrubs. At least seven large snags/ha and well‐distributed patches of dense shrubs (cover within patches>24% and patches covering 40% of the total area) are necessary for high densities of Glaucomys sabrinus. Abundance of Tamias townsendii reflects size of dominant tree and well‐developed understories. Abundance of Tamiasciurus douglasii seems to reflect territoriality in concordance with food supply and was greatest where Glaucomys and Tamias were low in abundance. Patterns of abundance of the sciurids in old‐ and managed forests suggests that silvicultural manipulation of vegetation and creative snag or den‐tree management could be used in a management strategy to accelerate the development of Spotted Owl habitat in areas where old growth is lacking.

ISSN:1051-0761    

DOI:10.2307/1941974

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY