Uncolonized Sites Research Articles

Resident vegetation modifies climate-driven elevational shift of a mountain sedge

Mountain plant species are changing their ranges in response to global warming. However, these shifts vary tremendously in rate, extent and direction. The reasons for this variation are yet poorly understood. A process potentially important for mountain plant re-distribution is a competition between colonizing species and the resident vegetation. Here, we focus on the impact of this process using the recent elevational shift of the sedge Carex humilis in the northern Italian Alps as a model system. We repeated and extended historical sampling (conducted in 1976) of the species in the study region. We used the historical distribution data and historical climatic maps to parameterize a species distribution model (SDM) and projected the potential distribution of the species under current conditions. We compared the historical and the current re-survey for the species in terms of the cover of important potential competitor species as well as in terms of the productivity of the resident vegetation indicated by the Normalized Difference Vegetation Index (NDVI). We found that Carex humilis has shifted its leading range margin upward rapidly (51.2 m per decade) but left many sites that have become climatically suitable since 1976 according to the SDM uncolonized. These suitable but uncolonized sites show significantly higher coverage of all dwarf shrub species and higher NDVI than the sites occupied by the sedge. These results suggest that resistance of the resident vegetation against colonization of migrating species can indeed play an important role in controlling the re-distribution of mountain plants under climate change.

Invasion pattern of Eupatorium adenophorum Spreng in southern China

This study provides a detailed analysis of the invasion by Eupatorium adenophorum Spreng (Crofton weed) from Burma and Vietnam into Southern China since the 1940s. Currently, E. adenophorum’s main colonisation area is located in the Yunnan–Guizhou Plateau of China, where it has caused prominent economic and ecological problems. Sixty-three years ago, Crofton weed appeared in Menghai county, Yunnan Province, from where it dispersed northwards and eastwards at an average speed of 20 km year−1. The invasion of E. adenophorum nonetheless showed pronounced variations in both time and space. Spread was relatively slow in the initial invasion period between 1940 and 1950, while the most rapid range expansions occurred in the 1980s. Environmental conditions at native and invaded sites were significantly different, reflecting a great adaptability of the species during colonization. These changes were greater than habitat differences between colonized and many adjacent un-colonized sites in Southern China. Therefore, immediate measures are required to stop a further northward and eastward expansion of Crofton weed.

Capture Rates of Reptiles and Amphibians on Black-Tailed Prairie Dog (Cynomys ludovicianus) Colonies and on Uncolonized Prairie in Colorado

Prairie dog eradication negatively affects dependent wildlife. Assessing the effects of prairie dog activities on reptiles and amphibians may depend upon the efficacy of trapping designs. We compared capture rates of reptiles and amphibians on black-tailed prairie dog (Cynomys ludovicianus) colonies and on adjacent uncolonized short-grass prairie, using funnel traps and pitfall traps with drift fences, as well as ground captures. We captured 152 reptiles and amphibians representing 10 species. We captured 51.3% of all reptiles and amphibians on colonies, compared to 48.7% from uncolonized sites. Overall, captures of reptiles and amphibians were similar on colonies and uncolonized sites for all capture methods combined. We achieved higher capture rates on uncolonized sites using funnel traps and pitfall traps but higher capture rates on colonies for ground captures. For all years on all sites combined, we captured significantly more animals in funnel traps (52.6%), surpassing pitfall trap captures (23.0%) and ground captures (24.3%). We recommend using multiple Y-shaped drift fence arrays ≥ 7.6 m in length, funnel traps with double-ended openings, and black-colored pitfall traps with opening diameters ≥ 30 cm.

Salvaged-Wetland Soil as a Technique to Improve Aquatic Vegetation at Created Wetlands in Wyoming, USA

Aquatic plants usually establish following wetland creation from a variety of mechanisms including animal transport, inflows from nearby wetlands, wind dispersal, and seed banks if they are available. However, at created wetlands that are isolated from natural wetlands, aquatic plant communities may not establish even after 10 or more years. One method of improving the establishment of aquatic plants is through the use of salvaged-marsh soils. Using this method, wetland soil from a donor site is collected and spread across the basin of the created wetland. When the proper hydrologic regime is reached at the created site, the seed bank from the donor soil is then present to take advantage of the uncolonized site. Over 1500 wetlands have been created in northeast Wyoming, USA from bentonite mining and most of them have not developed submersed and emergent plant communities due to isolation from plant sources. Our goal was to evaluate the effectiveness of using salvaged-wetland soil as a tool for improving plant growth at created wetlands. Our study took place at 12 newly created wetlands that were isolated from other wetlands by >5 km. Six wetlands were treated as reference wetlands, with no introductions of seeds or propagules. At the other six wetlands we spread ≈10–15 cm of salvaged soil from a donor wetland during the winter of 1999–2000. To identify the potential plants in donor soil, we collected 10 random samples from the donor wetlands and placed them within wetland microcosms in a greenhouse where they were treated to either moist-soil conditions (water at or just below the soil line) or submersed conditions (water levels maintained at 15–30 cm). Treatment wetlands were evaluated for plant growth during the fall of 2000 and 2001, whereas the greenhouse samples were grown for two growing seasons then harvested. Our results show that using salvaged wetland soil increases: (1) the number of plant species present at a wetland over time, (2) the total vegetation coverage in a treated wetland over time, and (3) the total plant biomass in a treated wetland. The species pool available in the salvaged wetland soil was limited to 10 obligate wetland species, but several of them are considered valuable to waterfowl and other wildlife. Furthermore, salvaged-wetland soil could be useful for ameliorating poor substrate conditions (i.e., bentonite) and improving conditions for the establishment of additional species. One concern with this technique is the introduction of invasive or exotic species that could form monocultures of undesirable plants (e.g., cattail [Typha spp.]); introducing more desirable species during the application of salvaged soil could reduce this probability. We believe incorporating salvaged-wetland soil during basin construction could be used to increase the value and productivity of created wetlands in this region.

Aggregates of Resident Bacteria Facilitate Survival of Immigrant Bacteria on Leaf Surfaces

The fate of immigrant bacterial cells on leaves under stressful conditions was determined as a function of the anatomical features and the local spatial density of resident cells at their landing site. Pantoea agglomerans 299R was established on bean leaves and the survival of immigrant cells of Pseudomonas fluorescens A506 and Pseudomonas syringae B728a, as well as P. agglomerans itself, was determined by epifluorescence microscopy following subsequent exposure of plants to desiccation stress. Resident and immigrant bacterial strains constitutively expressed the cyan and the green fluorescent protein, respectively, and the viability of individual cells was assessed directly on leaf surfaces following propidium iodide staining. Although only a small fraction of the immigrant cells landed on established bacterial aggregates, their fate was usually strongly influenced by the presence of indigenous bacteria at the site at which they landed. Immigrants of P. agglomerans 299R or P. fluorescens A506 that arrived as solitary cells had about double the probability of survival when landing on aggregates formed by P. agglomerans 299R than when landing on uncolonized areas of the leaf surface. In contrast, the survival of P. syringae B728a was similar irrespective of whether it landed on colonized or uncolonized parts of a leaf. The nature of plant anatomical features at which immigrant bacteria landed also strongly influenced the fate of immigrant bacteria. The fraction of immigrant cells of each species tested that landed on veins, glandular trichomes, or epidermal cells altered by P. agglomerans that died was always less than when they landed on normal epidermal cells or at the base of hooked trichomes. Depending on the process by which immigrants arrive at a leaf, only a small fraction of cells may be deposited on existing bacterial aggregates. Although uncolonized sites differed greatly in their ability to influence the survival of immigrant cells, the fate of an immigrant bacterium will depend on the nature of the leaf structure on which it is deposited, and apparently indirectly on the amount of nutrients and water available at that site to support the development of bacterial aggregates.

Front propagation for a jump process model arising in spacial ecology

We study the propagation of a front arising as theasymptotic (macroscopic) limit of a model in spatial ecologyin which the invasive species propagate by 'jumps'. The evolution of the orderparameter marking the location of the colonized/uncolonized sites is governedby a (mesoscopic) integro-differential equation. Thisequation has structure similar to the classical Fisher or KPP - equation,i.e., it admits two equilibria, a stable one at $k$ and an unstable one at $0$describing respectivelythe colonized and uncolonized sites. We prove that,after rescaling, the solution exhibits a sharp frontseparating the colonized anduncolonized regions, and we identify its (normal) velocity. In somespecial cases the front follows a geometric motion. We also consider thesame problem in heterogeneous habitats and oscillating habitats. Our methods, whichare based on theanalysis of a Hamilton-Jacobi equation obtained after a change ofvariables, follow arguments which were already used in thestudy of the analogous phenomena for the Fisher/KPP - equation.

Behavioural response of Mullus surmuletus to habitat modification by the invasive macroalga Caulerpa taxifolia

Densities of red mullet Mullus surmuletus have declined since 1996 at Cap Martin, Alpes‐Maritimes, France, following colonization by Caulerpa taxifolia. Foraging M. surmuletus were rarely observed over C. taxifolia or the seagrass Posidonia oceanica, but were observed over bare sand and endemic macroalgae. Within colonized sites (where the mean cover of C. taxifolia was 30–100%), fish were concentrated on areas with low cover of C. taxifolia(<21%). A difference in the frequency of observations of foraging groups (17%) was observed between sites with and without C. taxifolia. The observed patterns were probably due to a physical barrier to foraging that is presented by meadows of C. taxifolia. Changes to habitat structure that occur when the substratum is colonized by C. taxifolia influence the accessibility to benthic food resources, and consequently the foraging activities of fish that feed on benthic invertebrates. The decline in density of M. surmuletus at Cap Martin could be the result of fish emigration from colonized sites (unfavourable to the species) to uncolonized sites.

THE INFLUENCE OF A RIPARIAN SHRUB ON NITROGEN CYCLING IN A SONORAN DESERT STREAM

Riparian zones often act as nutrient filters, removing NO3 from water flowing through riparian soils. The role of vegetation in NO3 retention remains unclear and may be direct (uptake) or indirect (stimulation of microbial activity). We studied the riparian shrub Baccharis salicifolia (seepwillow) in Sycamore Creek (Arizona, USA), to determine (1) if sites colonized by seepwillow were sinks for NO3, and (2) the mechanism by which seepwillow causes NO3 retention. Subsurface water was sampled along flowpaths from an uncolonized gravel bar through seepwillow sites at several depths and on several gravel bars. NO3 concentration was significantly lower in seepwillow sites than in uncolonized sites, at least to 20 cm below the water table. Predictions of three hypotheses were tested to explain NO3 losses: (Hi) by plant uptake, (H2) by stimulation of denitrification by seepwillow, and (H3) a prior condition unrelated to seepwillow. Six experiments were used to test these hypotheses. Transplant experiments, plant size relationships, and root distri- bution experiments all demonstrated the importance of seepwillow (rejection of H3). Other tests involving removal of aboveground biomass, denitrification measures, and mass balance calculations showed a predominance of denitrification over uptake (rejection of Hi). We conclude that the main effect of seepwillow is to produce organic matter creating conditions favorable to denitrification and a loss of NO3 from subsurface water. Since denitrification is a permanent loss of N to the atmosphere, and uptake only temporarily retains N, the interaction between plants and microbes has important implications for the maintenance of water quality in streams and downstream reservoirs.

The Influence of a Riparian Shrub on Nitrogen Cycling in a Sonoran Desert Stream

Riparian zones often act as nutrient filters, removing NO3 from water flowing through riparian soils. The role of vegetation in NO3 retention remains unclear and may be direct (uptake) or indirect (stimulation of microbial activity). We studied the riparian shrubBaccharis salicifolia (seepwillow) in Sycamore Creek (Arizona, USA), to determine (1) if sites colonized by seepwillow were sinks for NO3, and (2) the mechanism by which seepwillow causes NO3 retention. Subsurface water was sampled along flowpaths from an uncolonized gravel bar through seepwillow sites at several depths and on several gravel bars. NO3 concentration was significantly lower in seepwillow sites than in uncolonized sites, at least to 20 cm below the water table. Predictions of three hypotheses were tested to explain NO3 losses: (H1) by plant uptake, (H2) by stimulation of denitrification by seepwillow, and (H3) a prior condition unrelated to seepwillow. Six experiments were used to test these hypotheses. Transplant experiments, plant size relationships, and root distribution experiments all demonstrated the importance of seepwillow (rejection of H3). Other tests involving removal of aboveground biomass, denitrification measures, and mass balance calculations showed a predominance of denitrification over uptake (rejection of H1). We conclude that the main effect of seepwillow is to produce organic matter creating conditions favorable to denitrification and a loss of NO3 from subsurface water. Since denitrification is a permanent loss of N to the atmosphere, and uptake only temporarily retains N, the interaction between plants and microbes has important implications for the maintenance of water quality in streams and downstream reservoirs.

Saprotrophic invasion by the soil‐borne fungal plant pathogen Rhizoctonia solani and percolation thresholds

In this paper we distinguish between invasive and noninvasive (finite) saprotrophic spread of the soil‐borne fungal plant pathogen, Rhizoctonia solani amongst discrete sites of nutrient resource. Using simple concepts of percolation theory, we predict the critical threshold distance, associated with a threshold probability, between donor (colonized) and recipient (uncolonized) nutrient sites at which R. solani can spread invasively by mycelial growth through a population of nutrient sites on a lattice. The critical distance for invasive spread is estimated from colonization profiles derived from placement experiments that summarize the probability of colonization with distance between replicated pairs of colonized and uncolonized sites. Colonization profiles were highly nonlinear, decaying sigmoidally with distance. Thresholds for invasive spread were predicted at inter‐site distances of 8.1 mm and 11.8 mm for sites of low and high nutrient agar, respectively. In population experiments with inter‐site distances below the predicted thresholds, the spread of the fungus was invasive in all replicates. At large distances (>10 mm for low, and >14 mm for high nutrient sites) the spread of the fungus was always finite, with the proportion of finite replicates decreasing sharply close to the percolation threshold. Invasive spread did not depend on the furthest extent of growth of the fungus but on distances predicted by the percolation thresholds. Invasive spread of the fungus is also examined in a more natural and variable, nonsterile system involving the growth and colonization of a lattice of poppy seeds over sand. The system is characterized by a decay in the probability of colonization between older poppy seeds, which effectively ‘quenches’ saprotrophic spread. Hence in the population experiments with poppy seeds all growth was ultimately finite. The threshold distance, corresponding to the critical percolation probability for invasive growth changed from 18 mm to 4 mm over 21d leading to a switch from invasive to finite growth. We conclude that percolation theory can be used to link the growth of individual mycelial colonies to the formation of patches that result from the colonization of particulate organic matter. The nonlinearity of the colonization profiles combined with the presence of a percolation threshold means that small changes in the distance between nutrient sites can result in large differences in final patch size. The rapid decay of particulate organic matter in a more natural system can have a profound effect on the dynamics of colonization, restricting saprotrophic invasion of the soil. The consequences of invasion thresholds for colony growth of saprotrophic and parasitic fungi in dynamical systems are briefly discussed.

A New Population of Colobanthus quitensis near Arthur Harbor, Antarctica: Correlating Recruitment with Warmer Summer Temperatures

We discovered a previously unreported population of the Antarctic vascular plant Colobanthus quitensis (Kunth) Bartl. containing 267 individuals on Gamage Point, Arthur Harbor, Anvers Island, Antarctic Peninsula (64°46'S, 64°04'W). Mean diameters of individual cushions ranged from 3.0 to 42.1 mm. Using a previously reported growth rate for this species of 3 mm yr—1, we estimate the population became established in early 1982 and has a mean age of 5 yr. This population was younger than two populations on Bonaparte Point (approximately 200 m distant) that had individuals we estimated to be >30 yr old and mean ages of 12 and 15 yr. We suspect that the Gamage Point population spread from one of the Bonaparte Point populations. Cushion recruitment per mature cushion in the Gamage Point population was positively correlated with mean summer (December-February) air temperatures, suggesting that temperature is an important factor limiting the establishment of C. quitensis in the maritime Antarctic. The presence of apparently suitable but uncolonized sites in the proximity of established populations of vascular plants, combined with increasing mean summer air temperatures on the Antarctic Peninsula, suggest that expansion of existing populations and the establishment of new populations on the Antarctic Peninsula is likely to continue.

Nutritional environment of soil and roots in and around mycelial blocks of an ectomycorrhizal fungus Tricholoma bakamatsutake in an evergreen Fagaceae forest

In the evergreen Fagaceae forests of Japan, an ectomycorrhizal fungusTricholoma bakamatsutake forms “shiros” or developing mycelial blocks. To determine the physiological characteristics of the mycelial blocks, organic acids of the soil and major nutrient elements of the soil and roots were compared at three types of sites: presently colonized mycelial blocks, previously colonized sites behind the blocks, and uncolonized sites in front of the blocks. The upper part of the mycelial blocks showed the following features compared with the uncolonized site: lower pH (5.1), higher concentrations of oxalic and gluconic acids, lower content of total nitrogen, a similar amount of total carbon, reduced total and available phosphorus, higher content of total calcium and lower content of exchangeable calcium. These findings suggested that the activity of the fungus led to soil acidification by the organic acids, an increase in C/N ratio, depletion of phosphorus and accumulation of calcium.

Grazing History, Defoliation, and Competition: Effects on Shortgrass Production and Nitrogen Accumulation

Plants of Bouteloua gracilis were collected from heavily grazed prairie dog (Cynomys ludovicianus) colonies (ON—colony) and lightly grazed uncolonized sites (OFF—colony) in Wind Cave National Park, South Dakota. Our objectives were to determine (1) how grazing history affected production, nitrogen uptake, and biomass and nitrogen allocation following defoliation and (2) how such responses were affected by competing neighbors of the same population. The growth chamber experiment was factorial with three main treatments in 12 possible combinations. Main treatments were: population (plants from lightly and heavily grazed sites), defoliation (three intensities), and competition (with or without neighbors of the same population). There were no significant interactions among treatments. OFF—colony plants produced 121% as much biomass and their N. yield was 203% as great as ON—colony plants. ON—colony plants allocated a higher percentage of biomass and N to the roots from OFF—colony plants, while the latter allocated a greater percentage of resources to leaf sheaths and reproductive structures. In response to defoliation, plants from the two populations had an average increase of 122% in N uptake per unit of root biomass and a 141% increase in total leaf—blade N yield. Heavily defoliated plants from both populations increased biomass and N allocation to blades, but total biomass production was substantially reduced. Intra—population competition reduced biomass production and N yield in individuals of both populations, but it did not affect total and aboveground N concentrations (per unit biomass) or N uptake per unit of root biomass. Our results support the hypothesis that genetically based morphological and physiological differentiation has occurred in B. gracilis as a result of strong selection pressures from grazing mammals on prairie dog colonies. We suggest the ON—colony population displays a strategy that reduces grazing severity rather than a "grazing tolerance" response to aboveground herbivory.

Herbivory tolerance of Agropyron smithii populations with different grazing histories.

The effects of defoliation on growth and nitrogen (N) nutrition were examined in populations of Agropyron smithii (western wheatgrass) collected from a heavily grazed black-tailed prairie dog (Cynomys ludovicianus) colony (ON-colony) and a nearby lightly grazed, uncolonized area (OFF-colony). Defoliated and nondefoliated plants were grown at low soil N availability with similar sized defoliated individuals of A. smithii from a grazing-exclosure population as a common competitor. Sequential harvests were made over 24 days following defoliation. Growth analysis plus biomass and N yield and distribution data were used to identify features which may contribute to plant defoliation tolerance. Defoliation reduced total production 34% across populations. Defoliated plants produced as much new blade tissue, but only 67% as much new root biomass as did nondefoliated controls. Plants from prairie dog colonies accumulated biomass at a faster relative rate than did plants from uncolonized sites, in part, because of a 250% greater mean relative growth rate of blades and more than 200% greater rate of biomass production per unit blade biomass. Total N accumulation was significantly greater in defoliated ON- than OFF-colony individuals. The mean relative accumulation rate of N was increased by defoliation in ON-colony plants, but reduced by defoliation in OFF-colony plants. The mean rate of N accumulation per unit root biomass was more than 300% greater in the ON- than OFF-colony population. Colony plants initially had a greater proportion of biomass and N remaining after defoliation in roots. Initial differences between populations in the distribution of biomass and N were eliminated as colony plants concentrated 24-day accumulation of biomass and N in aboveground structures. The data suggest that the combination of growth, N nutrition, and biomass and N distribution characteristics of the colony population likely confer a high rate of resource capture on heavily grazed prairie dog colonies.

Evaluation of Potential Herbivore Mediation of Plant Water Status in a North American Mixed-Grass Prairie

Xylem water potentials (ip) were monitored diurnally from June through August 1981 to ascertain the water status of plants of four species growing on a heavily grazed prairie dog (Cynomys ludovicianus) colony and an adjacent lightly grazed uncolonized site in southwestern South Dakota. Highest p values were observed in June and August after periods of relatively high precipitation. Lowest values occurred in July when soil moisture had been depleted. However, there were no large differences in V of grasses (Agropyron smithii, Andropogon gerardi, and A. scoparius) growing on and off the prairie dog colony, even though root biomass of plants on the colony was substantially lower. While midday leaf conductance was comparable for A. smithii tillers on and off the colony, conductance values for A. gerardi and A. scoparius tillers were generally higher (17 to 60%) on the heavily grazed site. Artemisia frigida plants had slightly higher Vp on the prairie dog town during periods of moderate or high water stress. Soil moisture content was slightly but significantly (P _ 0.05) higher on the heavily grazed prairie dog town, even though afternoon soil temperatures at the 15-cm depth averaged 2.7°C higher.

Plant-herbivore interactions in a North American mixed-grass prairie : III. Soil nematode populations and root biomass on Cynomys ludovicianus colonies and adjacent uncolonized areas.

Seasonal dynamics of soil nematodes and root biomass were examined from under western wheatgrass (Agropyron smithii) and little bluestem (Andropogon scoparius) from a heavily grazed prairie dog (Cynomys ludovicianus) colony occupied for 5 to 10 years and an adjacent lightly grazed, uncolonized area in Wind Cave National Park, South Dakota, USA. Nematodes were differentiated into classes of plant-parasitic Tylenchida and Dorylaimida and nonparasitic Dorylamida and Rhabditida. Root-feeding nematodes were generally more numerous from A. smithii than from A. scoparius, while nonparasitic populations were not different in soil from beneath the two plant species. Rhabditida, parasitic Dorylaimida and Tylenchida (from A. scoparius only) were more numerous on the prairie dog colony than from the uncolonized site, but nonparasitic Dorylaimida populations did not differ between the two areas. Mean total (live plus dead) root biomass beneath A. scoparius and A. smithii on the prairie dog colony averaged 71% and 81%, respectively, of values from the uncolonized area. Estimated consumption by root-feeding nematodes averaged 12.6% and 5.8% of annual net root production in the upper 10 cm from the prairie dog colony and uncolonized site, respectively. We conclude that, because of microhabitat modification or reductions in plant resistance to nematodes, heavy grazing by aboveground herbivores apparently facilitates grazing by belowground herbivores. Because heavily grazed plants have less roots than lightly grazed or ungrazed plants, the impact of root-feeding nematodes on primary producers is likely to be greatest in heavily grazed grasslands.

An Integrated Study of the Factors Influencing the Choice of the Settling Site of Balanus crenatus Cyprid Larvae

Settled larvae of Balanus crenatus were collected at Pointe Mitis, in the St. Lawrence Estuary, Quebec. Four substrata were sampled and observed using scanning electron microscopy (SEM): shells of Mytilus edulis, fronds of Fucus evanescens, and laminated panels immersed for periods of 4 and 8 wk. Following this treatment, 23 variables describing the substratum surface, the cover of inert and biological material, and the gregarious response of the larvae were documented for 200 larvae, and compared with 187 control sites uncolonized by larvae. Significant differences of substratum texture, detritus, and diatom abundance were observed between colonized and uncolonized sites, indicating that the selectivity of the larva is related to the heterogeneity of the substratum. On all substrata, the settling site of the larvae was free of detritus and diatoms, clean surfaces ensuring better adhesion for the individual. To rank the variables on which selection occurred, the χ2 value of a contingency table between each variable and the type of substratum was computed. Variables carrying low χ2 values represented the most constant larval choices irrespective of the type of substratum. Variables related to the microscopic characteristics of the substratum were more important for larval selection than the variables describing the already established barnacle population.

Plant-herbivore interactions in a North American mixed-grass prairie : I. Effects of black-tailed prairie dogs on intraseasonal aboveground plant biomass and nutrient dynamics and plant species diversity.

Research was conducted to determine the effects of a native, sedentary rodent of North American grasslands, the black-tailed prairie dog (Cynomys ludovicianus), on seasonal aboveground plant biomass and nutrient dynamics and plant species diversity. The study was done on a northern mixed-grass prairie site at wind Cave National Park, South Dakota.Peak live plant biomass was greatest (190 g/m2) on the uncolonized part of the study area and least (95 g/m2) on a part of the prairie dog town colonized for 3 to 8 y. Peak live plant biomass (170 g/m2) of the oldest portion of the prairie dog town (colonized >26 y) was not significantly different from that of uncolonized prairie. However, where-as graminoids composed >85% of the total biomass of the latter area, forbs and dwarf shrubs (Artemisia frigida) were >95% of the total of the former. Both standing-dead plant biomass and litter declined markedly as time since colonization increased. Total plant species diversity (H) was greatest in the young prairie dog town (colonized for 3 to 8 y).Nitrogen concentration of plant shoots varied significantly as a function of time since colonization. Shoot-nitrogen was lowest in plants from the uncolonized site and greatest in plants collected from the longest-colonized areas of the prairie dog town. Shoot-nitrogen declined significantly over the growing season and tended to be higher in C3 graminoids than in C4 graminoids. In vitro digestible dry matter showed similar trends; the differences between C3 and C4 digestibilities were greatest during the last half of the growing season.We suggest that prairie dog-induced changes in plant biomass, plant species diversity, plant nutrient content, and forage digestibility may lead to further alterations of nutrient cycling and trophic dynamics in this mixed-grass prairie ecosystem.

A Method for Describing Former Fluctuations of Voles

trapped, and when released, retreated to burrows in and around the cages. In late February 1978, almost 1 year following the first release, an ear-tagged squirrel was observed near the cages. Several days later, 2 squirrels were trapped at the study site. Neither had eartags, but their ears were torn, indicating they had been previously ear-tagged. Squirrels and fresh squirrel signs were observed at and near all cage locations. During the next 2 months, squirrels were livetrapped that had apparently never been ear-tagged. These were thought to be offspring from 1 or more of the original pairs released. Immigration from the adjacent area could not be ruled out, although this seems unlikely, because ground squirrels had not colonized the site prior to this study. The described technique succeeded in establishing a free-living ground squirrel colony at a new, previously uncolonized site. This technique may have application in zoological gardens, parks, or other natural areas. Transplanting should not be done indiscriminately, however, because ground squirrels are carriers of several diseases transmissible to humans, and there is a danger that introduced squirrels and their offspring will damage crops or property or displace other wildlife species. Acknowledgment.-This research was supported in part by the California Department of Food and Agriculture Contract Agreement 4355.