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For plants and other sessile organisms, the dispersion of individuals in a population can influence the strength of ecological interactions, and can have important implications for the conservation of these species. We investigated the spatial pattern in a population of the monocarpic perennial Cirsium pitcheri (Pitcher's thistle), a dune species endemic to the western Great Lakes, U.S.A., by mapping individuals with a GPS. Using a refined nearest neighbor analysis of the mapped point data combined with Monte Carlo randomization tests, we found that individual plants were clustered on the scale of about a meter, which was smaller than expected if aggregations were caused by major habitat features such as dune height or aspect. The size of clusters was consistent with reports of relatively short-distance dispersal of seeds. We found no evidence of self-thinning via nonrandom mortality, and regression analyses indicated no density dependent effects on reproductive effort at a range of ecologically reasonable scales. However, we did find a suggestion of density dependent effects on juvenile size at several scales. The neighborhood radius that maximized the variance explained was 25 cm, roughly the radius spanned by the largest juvenile individuals in our study. Incidence of herbivory was not concentrated in denser patches of C. pitcheri; in fact, we found a trend in the opposite direction: isolated individuals were more likely to have been damaged by herbivory than those with a crowded local neighborhood. Our results show that explicit attention to such individual-scale spatial patterns can lead to increased understanding and thus more effective management of local plant populations. We call for more systematic studies examining local spatial patterns in this and other threatened plant species.
Goldenseal is an uncommon woodland herb whose rhizomes are widely harvested for their medicinal properties. Goldenseal populations regenerate from vegetative propagules that are broken-off from the primary rhizome during harvesting activities. While previous studies reported significant variability in re-growth among harvested populations, it is not entirely clear what drives differences in population re-growth. One hypothesis is that goldenseal populations re-grow at greater rates when harvested during the fall compared to mid-summer. Over a 4 y period, we biennially censused the re-growth of a goldenseal population that was wild-harvested during the fall 2001 from the Wayne National Forest in southern Ohio. Data were compared to previous studies that quantified the re-growth of a goldenseal population wild-harvested during the fall in West Virginia and to goldenseal populations (n = 3) experimentally harvested during mid-summer. Among the two fall-harvested populations, ramet densities increased by twofold in the Ohio population, but remained relatively stable in the West Virginia population. In contrast, average ramet leaf size was remarkably similar 2 and 4 y post-harvest, although the Ohio population appeared to be recovering at slightly faster rates. Comparison of data between all harvested populations supports a previous hypothesis that average ramet leaf-size in fall-harvested (after September) populations may recover at faster rates than populations harvested during mid-summer. Late-summer and fall represents the period in which goldenseal rhizomes are traditionally gathered. Based on cumulative results from these harvest studies and more recent studies on goldenseal ecology, we discuss some of the implications of wild-harvesting on goldenseal population growth and persistence. Determining sustainable harvesting rates under contrasting harvester regimes warrants further research.
Many of the present day issues associated with fire management in wilderness areas are addressed by studying past interactions among fire, humans, vegetation and climate. In this paper we describe three centuries of the fire regime in the Lower Buffalo River Wilderness Area, Arkansas, USA. We reconstructed fire events from 159 tree-ring dated fire scars on 26 shortleaf pine (Pinus echinata Mill.) remnants and live trees. During the late-17th Century and early 18th Century the mean fire return interval (MFI) was 7.7 y. Fire frequency increased abruptly circa 1820 with fires burning every 2 y on average until 1920. The number of fires decreased during the 1900s as cultural values changed to favor fire suppression over multiple-use burning. Analyses of the influence of human ignitions and drought on the fire regime resulted in two important findings: (1) that fire frequency was positively correlated to human population density up to 1920 and (2) the influence of drought seemed to be masked by frequent anthropogenic fires and fire suppression. Fire events were associated with droughts only prior to Euro-American settlement. Studies of climate-fire relationships should consider the potential for anthropogenic influence and future studies should attempt to quantify the historic role of humans in the fire regime.
Invasion of exotic species threatens the stability of ecosystems and is regarded as a significant component of global ecological change. One of the most successful invasive species of the intermountain west of the United States is Bromus tectorum L. (downy brome)—an annual Eurasian grass that was first observed in North America in the late 1800s in areas of Washington, Oregon and Utah and now covers ca. 40,000,000 ha. The prevalence of B. tectorum raises concerns regarding its potential effects on ecosystem structure and function. In this study we investigated the effects of invasion of B. tectorum on an ungrazed grassland in Canyonlands National Park, Utah. Because the invasion occurred on previously established study transects, we were able to assess the short-term effects of an invasion with a known entry date. We conducted long-term soil incubations in the laboratory to obtain data on cumulative nitrogen mineralization which was used to calculate the size of the labile pool of soil nitrogen. We also measured in situ soil inorganic nitrogen and net nitrogen mineralization. We found that, 2 y following the invasion of B. tectorum, the labile pool of soil nitrogen was 50% smaller for invaded sites compared to uninvaded sites. In addition, invaded sites had significantly reduced in situ inorganic nitrogen and net nitrogen mineralization compared to uninvaded sites. The mechanisms for these changes in soil nitrogen cycling and nitrogen availability may be related to differences in the phenology, litter composition and biomass of B. tectorum compared to native grass species. These findings suggest availability of key resources is dramatically influenced by invasion of B. tectorum at these sites.
Abstract.—Drought is an important disturbance in stream ecosystems. From 1998–2002 Georgia suffered a major drought, causing many headwater streams to experience reduced flows or to dry completely. Six headwater streams in the Georgia Piedmont were selected and paired based on similarities in substrate structure (sand, bedrock or gravel substrate). Each pair consisted of a stream that dried completely during the drought and one that retained at least some surface water. Riffles were sampled with a core sampler, runs and pools were sampled with a Hess sampler and wood was sampled by randomly collecting pieces of at least 1 cm diameter. Samples were collected within 15 d of reflooding, then after 45, 75, 165, 255, 345 and 435 d. Cluster analyses were used to assess the relative effects of drought history (dried or residual water), stream condition and temporal change. In terms of drought recovery, all streams followed the same pattern of recovery, with a rapid recolonization period following the onset of surface flow. Community compositions were initially similar in most streams, but after 15 d each stream began to develop unique recovery patterns. The number of new taxa colonizing these streams began to level off around 165 d after rewetting. Neither the presence nor absence of residual water nor substrate composition appeared to significantly influence drought recovery patterns of invertebrates.
The imperiled crayfish Orconectes williamsi is found only in the upper White River drainage in Missouri and Arkansas, and is known from only nine localities in four Missouri counties. Its distribution has never been thoroughly assessed. Knowledge of distribution and habitat associations is essential to conservation and management of rare species. Our major objectives were to estimate the distribution of O. williamsi in Missouri streams and identify associations between this crayfish's presence and selected multi-scale environmental variables. We used a probabilistic method and stratified (by stream order) random design to survey 50 of 223 total stream segments in the drainage during 2002 and 2003. We sampled 21 additional segments postsurvey (2004 and 2005). This crayfish was detected at an overall rate of 0.34 (±0.12, 95% confidence interval), and at a total of 27 sites, but we failed to collect it at two historical sites. Many stream segments harboring O. williamsi were determined to be effectively isolated from similarly populated segments by reservoirs. At the microhabitat scale, O. williamsi was associated with high current velocities, shallow depths, and cobble and small boulder substrates. At the reach scale, O. williamsi was positively associated with stream channels that were narrower and deeper relative to channels at segments where we did not detect the crayfish. This species was also negatively associated with emergent macrophyte (Justicia sp.) densities. No watershed-scale variables showed statistically significant associations with this crayfish, but the variable “drainage area” appeared to be the most likely of these to influence its distribution. Study results and our survey method will focus future searches or surveys for this crayfish. Whereas threats to O. williamsi remain, we suggest that its state rank be downgraded from “critically imperiled” to “imperiled” and that conservationists consider a reduction in this species' assigned global conservation status.
In western painted turtles (Chrysemys picta bellii), males often exhibit one of two morphs: (1) a reticulated form, characterized by an intricate network of dark markings on the carapace or (2) a non-reticulated form. Although several hypotheses have been proposed to explain the adaptive significance of reticulate melanism (RM) on western painted turtles, no attempts have been made to document whether RM is linked to habitat conditions or if the presence of melanism affects heating rates. To evaluate these questions, we compared the frequency of adult male turtles with RM across three different habitats: riverine (rivers), lacustrine (lakes) and palustrine (wetland) habitats. Using manipulative experiments, we also tested the hypothesis that body heating rates are higher in turtles with RM. Reticulate melanism occurred on 99 (31%) of 320 male turtles captured in South Dakota from 2002 to 2003. Turtles with reticulate melanism were significantly larger than non-reticulated turtles; RM was not observed on male turtles with carapace lengths <15 cm or weights <360 g. The mean proportion of adult male turtles (>15 cm carapace length) with RM was similar among river (0.54), lake (0.50) and wetland (0.64) habitats, implying that RM is not a habitat-linked trait. Heating rates for turtles with RM were similar to those measured for non-reticulated individuals. Body size, however, influenced heating rates; larger-bodied turtles with lower surface area-to-volume ratio heated more slowly than smaller turtles. Whether RM is a by-product of hormonal regulation or serves an adaptive purpose remains unclear. However, other hypotheses, especially those involving communication (e.g., courtship behavior) and/or gamete protection remain untested for western painted turtles and warrant further investigation.
The North American freshwater fish family Centrarchidae is well known for extensive natural hybridization, but there are no reports of voluntary spawning between genera. We document courtship and spawning in an aquarium between two separate pairs of a male Rock Bass (Ambloplites rupestris) and a female Sacramento Perch (Archoplites interruptus). One trial resulted in a low frequency of fertilized eggs, but these did not survive beyond the blastula stage. Fossil and molecular evidence suggests that these species have been isolated for at least 15 million years, so this spawning implies that courtship among species can persist longer during species divergence than previously appreciated.
The Neosho madtom is a small, short-lived catfish species endemic to gravel bars of the Neosho River in Kansas, Oklahoma and Missouri, U.S.A. It spawns during summer in nesting cavities excavated in gravel. Although the species has survived dam construction within the Neosho River basin, its declining numbers resulted in it being added to the federal threatened species list in 1991. To test how water flow affects the reproductive behavior of Neosho madtoms, we compared activities of male-female pairs in static versus flowing-water aquaria. Using a behavioral catalog, we recorded their behavior sequences during randomly selected 5-min nighttime periods. For males and females, Jostle and Embrace were the most performed reproductive behaviors and the Jostle-Embrace-Carousel was the most performed reproductive behavior sequence. Water flow decreased the mean frequency of occurrence, percentage of time spent and mean event duration of male Nest Building. Because Neosho madtom courtship, reproduction and parental care is a complex and extended process, disturbances such as heightened river flows during the species' spawning season may negatively affect nest quality and reproductive success.
The objective of this study was to determine the relationship between fish assemblage and habitat in streams dominated by an agricultural landscape. Fishes from 20 natural and 20 channelized streams were sampled using electrofishing gear in east central Indiana. Streams that had been channelized had a lower quality fish assemblage when compared to natural streams as measured by the Indiana V. Eastern Corn Belt Plain Index of Biotic Integrity (IBI). Stream habitat was evaluated using the Ohio EPA Qualitative Habitat Evaluation Index (QHEI), which indicated that channelized streams had lower quality primarily due to a loss of heterogeneous habitat. Pearson correlation analysis relating IBI and QHEI was positive and significant and demonstrated that a reduction in riffle and pool areas associated with channelization was the most significant factor influencing the fish assemblage. Furthermore, species lost when streams were channelized were predictable and often represented environmentally sensitive species. The results of this study suggested that stream channelization has a negative influence on the fish assemblage which should be recognized before any stream development.
We documented the fate of 29 cohorts of propagated Barrens topminnows Fundulus julisia stocked as juveniles and adults (ntotal = 2770 fish) into 17 springheads and small ponds in middle Tennessee in 2003 and 2004. Annual mortality rates were calculated after estimating the number of individuals of each cohort remaining 1–18 mos after fish were stocked. Lighted larval fish traps were deployed at seven reintroduction sites and the Type Locale to determine whether topminnows could reproduce in the presence of the introduced-transplanted Western mosquitofish Gambusia affinis. At stocking sites harboring mosquitofish (n = 12), their density ranged from 0.4 to 66.3 mosquitofish per m2. Annual mortality of stocked Barrens topminnows ranged from 45 to 100% and 24 cohorts experienced annual mortality greater than 95%. Mortality was not related to mosquitofish density or the mean size at stocking. The robustness of Barrens topminnows did not differ in the presence or absence of mosquitofish, suggesting that interspecific competition for food was not occurring. Larval Barrens topminnows were collected at two reintroduction sites and the Type Locale, but juvenile recruits were produced only at sites lacking mosquitofish. The findings of this study, and concurrent laboratory studies, support the hypothesis that mosquitofish predation on larval Barrens topminnows was the primary mechanism in failed reintroductions and is the greatest threat to wild and reintroduced populations of this imperiled species.
Increased sociality in herons, family Ardeidae, is often considered to be associated with the occurrence of white plumage. If white plumage increases sociality, we hypothesized that white-plumaged birds should forage closer to one another and dark birds should remain more solitary. We tested this hypothesis by investigating the inter-individual spacing (nearest neighbor distance) in two pairs of closely related species that differ in plumage coloration: (1) Great Blue Heron (Ardea herodias)/Great Egret (A. alba) and (2) Little Blue Heron (Egretta caerulea)/Snowy Egret (E. thula). All birds observed were in adult plumage. We recorded distances to nearest conspecific, nearest congeneric, nearest white (regardless of species) and nearest dark bird to each focal bird within a given (focal) pond. Because species occurred in varying numbers, we compared the observed mean distances with randomly generated distances based on the total number of birds in the pond. All observations were conducted under steady water levels, avoiding drying ponds as this might concentrate prey. We found that Snowy Egrets were significantly farther from conspecifics and dark congenerics (Little Blue Herons) than expected under a null model of random spacing within the focal pond. Little Blue Herons also were spaced farther apart from white congenerics (Snowy Egrets) than expected. Great Blue Herons and Great Egrets exhibited no distinct social tendencies and neither avoided nor preferred foraging near other herons. Our results do not support the hypothesis that white-plumaged birds exhibit increased social tendencies relative to dark-plumaged birds within a given pond.
Both herbivory and windthrow disturbances have individually been shown to dramatically alter the structure and composition of forest communities; however, the interaction of these two factors has scarcely been examined. Windthrow disturbance creates microsites such as fallen logs and treefall mounds and pits that may hinder movement and limit visibility of large mammals, thereby reducing the impact of browsing by white-tailed deer. In an old growth Pennsylvania forest that was severely wind-damaged in 1985, a previous study based on sampling in 1994 demonstrated reduced browsing and greater Tsuga canadensis (L.) Carr seedling density on treefall mounds (Long et al., 1998). In this study we surveyed mounds to see if they continue to be effective browsing refugia 9 y later. We also determined whether other types of microsites and nearby microsites provide protection from browsing and tested for an associational density refuge, whereby seedlings of a preferred species are less browsed when surrounded by a high density of saplings. Tsuga canadensis seedlings occurred more frequently on microsites, particularly logs and mounds, whereas randomly selected points occurred most frequently off microsites. Tsuga canadensis seedlings that established off microsites were more heavily browsed than seedlings on mounds or logs. Seedling heights were greatest on mounds and shortest on logs. Although growing on a microsite acted as refugia from browsing, having multiple surrounding microsites did not. Additionally, seedling browse damage was not decreased by having multiple nearby saplings, suggesting no associational defense. Nevertheless, certain microsites continue to provide refugia from browsing 18 y after disturbance and, thereby, enhance regeneration of Tsuga canadensis.
Wind-power development is occurring throughout North America, but its effects on mammals are largely unexplored. Our objective was to determine response (i.e., home-range, diet quality) of Rocky Mountain elk (Cervus elaphus) to wind-power development in southwestern Oklahoma. Ten elk were radiocollared in an area of wind-power development on 31 March 2003 and were relocated bi-weekly through March 2005. Wind-power construction was initiated on 1 June 2003 and was completed by December 2003 with 45 active turbines. The largest composite home range sizes (>80 km2) occurred April–June and September, regardless of the status of wind-power facility development. The smallest home range sizes (<50 km2) typically occurred in October–February when elk aggregated to forage on winter wheat. No elk left the study site during the study and elk freely crossed the gravel roads used to access the wind-power facility. Carbon and nitrogen isotopes and percent nitrogen in feces suggested that wind-power development did not affect nutrition of elk during construction. Although disturbance and loss of some grassland habitat was apparent, elk were not adversely affected by wind-power development as determined by home range and dietary quality.
We examined population dynamics of two species of small mammals, Mus musculus (house mouse) and Microtus pennsylvanicus, (meadow vole), in a highly disturbed dredge disposal site in eastern Virginia. Using mark-release-recapture methods, we trapped for 2 d three times per month for 13 mo, during which density of M. musculus declined from 104/ha to 37/ha and meadow vole density gradually increased from 8/ha to 41/ha. In all, 535 small mammals of seven species were trapped, with Mus constituting 65% and Microtus 28% of marked animals. Across the study, sex ratios were unity for house mice but nearly 1.5:1 favoring males for Microtus. Both species suspended breeding in winter, Microtus for slightly longer. Four habitat types on the 0.73-ha study grid were differentially used, less so by Mus than by Microtus. Populations of both species increased more through immigration than by breeding, and lifespans on the study grid were short, suggesting high levels of mobility for both populations. These attributes are discussed in the context of the dynamic environment in which these small mammals live.
We investigated the annual dynamics of bobcat (Lynx rufus) home range and core use areas by radiotracking 23 female and 6 male bobcats from 10 January 1989 to 31 January 1998 in Mississippi. We quantified space use by measuring changes in the dispersion and central tendency of bobcat locations (i.e., radiotelemetry locations) between annual home range and core use areas. Data from 38 female and 11 male home range and core area comparisons were used to examine bobcat spatial dynamics. Mean dispersion of home range and core use areas was greater for male than female bobcats, but dispersion for these areas did not differ between years for male or female bobcats. Annual shifts in the central tendency of home range and core use areas for female bobcats likely were a space-use strategy to optimize access to prey resources. Annual shifting of core area central tendency within non-shifting home ranges of male bobcats appeared to be a response to female spatial readjustments. Shift distance, after standardizing as a proportion of dispersion, did not differ between male and female bobcat home range and core use areas. Bobcat space use may be more dynamic than previously reported, particularly at the core-area spatial scale.
Middle Tennessee populations of the Streamside Salamander, Ambystoma barbouri, delimit the southern extent of the species' range and are geographically isolated from more northerly populations. Few populations have been discovered in Tennessee, and all of them are located in the Inner Nashville Basin ecological subregion of the Interior Plateau. We surveyed for breeding activity in first- and second-order streams in the southern Inner Nashville Basin to determine the species distribution and to examine the status of and existing threats to extant populations. Streamside salamanders were found at five of 40 localities in southern Rutherford, northern Bedford and northeastern Marshall County, and at only 4 of 6 previously known breeding sites. Continued habitat fragmentation and alteration in association with the urbanization of Rutherford County threaten existing A. barbouri populations, which may represent the last remaining populations in the state. We recommend state and local agencies develop a habitat conservation plan to preserve and improve first- and second-order breeding sites and the surrounding forests used by adult Streamside Salamanders.
We visually censused Warner sucker cohorts in a low gradient stream reach and determined microhabitat use from random availability data. For above-water visual censuses (AVC), we counted aggregations while slowly wading through Honey Creek three times both years. Underwater visual censuses (UVC) were done in 1993 by snorkeling. Mean focal point velocity (FPV) during AVC significantly differed by year but not by census date, and mean depth did not differ for either. In 1993 we found suckers used microhabitats with FPV between >3 to 6 cm/s (χ12 = 3.93, P < 0.05) and depth between >20 to 40 cm (χ12 = 4.5, P < 0.05). Suckers avoided areas where FPV exceeded 15 cm/s (χ12 = 18.7, P < 0.001) and depth between >60 to 80 cm (χ12 = 8.1, P < 0.005). Aggregate abundance was significantly related to both distance to (r64 = −0.45, P = 0.0002) and percentage (r64 = 0.44, P = 0.0003) submerged vegetation. We found 97% of suckers (N/m3) in riffle/run habitat during UVC. When mean flow was used as a continuous variable, we found suckers occupied habitats >3 cm3/s and avoided habitats <2 cm3/s in a disproportionate manner relative to availability. These results suggest young suckers select vegetated areas with moderate flow and relatively shallow depths during first few months of life. This information improves our understanding of ecological habits of early life stages of a rare western catostomid during stream residence, which could be useful for conservation management in low gradient stream reaches of Warner Basin.