Density Of Native Species Research Articles

Alien Plant Invasions, Introduced Ungulates, and Alternative States in a Mesic Forest in Hawaii

Restoration by natural successional processes after removal of perturbations may not be feasible for many degraded ecosystems. Controlling major ecological threats such as non‐native ungulates is often a critical first step toward restoring native communities but past degradation, interactions with alien species and abiotic features may create conditions requiring additional intervention to ensure effective conservation. We monitored a series of fenced plots within diverse mesic forest on western Kauai in the Hawaiian Islands from 1998 to 2005 to determine the effects of ungulate removal on native and alien plant species. Relative to unfenced control plots, germination of seedlings and frequency of understory species of both native and alien species increased in the fenced plots. Density of both native and alien canopy and understory species declined more in unfenced than fenced plots, but density of native species declined more than alien species density in both fenced and unfenced plots. In fenced plots, the frequency of larger alien woody species and cover of an alien, mat‐forming fern species increased over time, indicating that fencing may encourage alien species that could interfere with regeneration of native species. Our study suggests that effective conservation of this and other remnant native Hawaiian forests will require both ungulate exclusion, removal of alien plant species with especially detrimental effects on native species, and proactive restoration programs for native species without natural sources of propagules. As the effects of invasive species continue to escalate, continental ecosystems lacking high endemism may also require similar interventions to preserve their biodiversity.

Responses of Native and Introduced Plant Species to Sucrose Addition in Puget Lowland Prairies

Nitrogen enrichment has often been demonstrated to enhance the success of introduced plant species at the expense of native species. In the south Puget lowland prairies of Washington State, invasion by Scotch broom (Cytisus scoparius), a nitrogenfixing legume, is associated with elevated soil nitrogen levels. After broom removal, the higher soil nitrogen levels may have facilitated the secondary invasion of the prairies by numerous non-native species, particularly rhizomatous pasture grasses that can interfere with native plant seedling establishment. Numerous studies have shown the potential for carbon addition to immobilize soil nitrogen and reduce the success of introduced species relative to native species. We compared the available soil nitrate, the cover of native and introduced species between sugar-addition (1000 g C m-2) and control plots on two Puget lowland prairies. Sugar treatment initially immobilized nitrate and reduced cover of introduced species compared to that on control plots, but these effects dissipated within two years. Moreover, after four years, cover of introduced species, especially that of Agrostis capillaris and Hypochaeris radicata, had rebounded to become higher in sugar-addition than in control plots. In contrast, native species showed no negative responses to sugar treatment, suggesting that where sugar or other carbon treatment is economically feasible, combining carbon with the establishment of a high density of native species might limit the potential for introduced species to rebound.

Integrating revegetation with management methods to rehabilitate coastal vegetation invaded by Bitou bush (Chrysanthemoides monilifera ssp. rotundata) in Australia

AbstractTwo multi‐year field experiments investigated the effects of integrating revegetation with invasive plant management methods to rehabilitate coastal dune and woodland vegetation invaded by Bitou bush (Chrysanthemoides monilifera (L.) Norl. ssp. rotundata (DC.) Norl.) in New South Wales, Australia. The revegetation technique used was to sow directly seeds of three native species common to coastal habitats. Management treatments involved combinations of prescribed fire, manual removal of Bitou bush and an application of herbicide. Addition of native seeds significantly increased density of native species in both habitats. The benefits of manually removing Bitou bush were observed only where densities of native species were at their lowest. Fire increased densities of some native species in the woodland, but decreased those of others in the dune. Densities of Acacia longifolia ssp. sophorae (Labill.) Court (woodland) and of Banksia integrifolia L.f. (woodland and dune) were significantly reduced within 4 months of herbicide application, alone or in combination with other treatments. The majority of these effects, however, did not persist. Manual removal in both habitats and addition of seed in the woodland were most effective in reducing Bitou bush densities when applied post‐fire. Herbicide treatment on its own or in combination with other treatments did not significantly reduce Bitou bush densities by the end of the experiments. We conclude that restoration of coastal ecosystems invaded by a major invasive plant species requires a whole‐of‐system approach involving revegetation in combination with known management methods to assist recovery of native species in the longer term.

Invasion of Pontoscolex corethrurus (Glossoscolecidae, Oligochaeta) in landscapes of the Amazonian deforestation arc

Pontoscolex corethrurus (Glossoscolecidae, Oligochaeta) is an invasive endogeic earthworm that has colonized most land transformed by human activities in the humid tropics. When installed, populations can change soil physical properties, biogeochemical processes and microbial communities. The aim of this study was to determine whether P. corethrurus establishment is a result of (1) a competitive exclusion of native earthworm species or (2) the exploitation of a new niche created by anthropogenic disturbance that native earthworm species cannot use. We tested these hypotheses by doing a survey of earthworm communities in 270 sites that represented the diversity of land use systems encountered in two contrasted regions of the Amazonian arc of deforestation located in Brazil and Colombia respectively. When present in forests, P. corethrurus had no negative effect on the native species communities that had similar (epigeic species) or even higher densities (endogeic species) in the presence of the invasive species. These results suggest the absence of competitive exclusion. The first two axes of a PCA multivariate analysis of communities represented the densities of native species (axis 1) and P. corethrurus (axis 2) respectively. This suggests that respective densities of the two groups respond to different conditions and that their variations are independent. The density of P. corethrurus co-varied with soil N content and pH in Colombian sites while the densities of other species did not. Our results thus suggest that this invasive species, unlike native species, is able to feed and develop in environments where litter resources are decreased while soils have been enriched in C and nutrients by deforestation and burning. We discuss the reasons why some primary forests in Central America have large populations of P. corethrurus.

Effects of Long-Term Livestock Grazing and Habitat on Understory Vegetation

The herbaceous understory stratum contains most of the plant diversity in ponderosa pine (Pinus ponderosa P. & C. Lawson var. scopulorum Engelm.) forests of the American Southwest and provides critical food and habitat for many wildlife species. During the last century, this stratum has been affected by livestock grazing and by increased dominance of overstory trees. We sampled a unique grazing exclosure to examine the relative importance of long-term livestock grazing (grazed or ungrazed) and habitat (park or tree) on the understory community. We sampled 3 plots of 192 contiguous quadrats (each quadrat 0.5 m2) in each of the 4 treatment combinations, for a total of 2304 quadrats. Species-area curves were generated by aggregating quadrats into nonoverlapping areas at grain sizes of 0.5 to 576 m2. The effects of habitat and grazing on species density were evident at very different scales. Species density was higher in park than tree plots at scales ≤32 m2 but did not differ between habitats at larger scales. Species density differed minimally between grazed and ungrazed treatments at small grains, but grazed plots contained more species than ungrazed plots at larger grains. Grazing treatments differed at smaller grains (to 4–8 m2) than did habitats (to 32 m2), with respect to density of native species and graminoids. Grazed plots had more exotic species than ungrazed plots at all grain sizes, though few exotics were present. Twenty-two species were identified as indicator species associated with habitats and/or grazing treatments. Evaluations of plant community response to treatments would be improved by accounting for the grain at which data have been collected and analyzed and by identifying indicator species associated with various treatments. These data would enable more-informed conservation and management decisions.

Can horse riding induce the introduction and establishment of alien plant species through endozoochory and gap creation?

The risk of spreading of alien species to protected forest habitats through recreational horse-back riding was experimentally investigated at Oulanka National Park, north-eastern Finland during 2002–2005. Levels of disturbance, horse manure and seed rain of dwarf shrubs were manipulated in genuine boreal forest habitat. Specifically we asked (i) whether the seeds of alien species can be dispersed to natural forests by horse manure and (ii) whether disturbance in soils and vegetation increases the density of alien species and decrease the density of native species. Manure addition introduced seeds of graminoid and forb species, which were absent elsewhere in the study area. Establishment of the alien species was further enhanced by the disturbance treatment. Germination of natural shrub species was enhanced by disturbance treatment, whereas manure addition had little impact on the native shrubs. The results indicate that alien species may be introduced to natural forests through recreational horse riding, if horses are fed by hay that contains germinable seeds. Soil disturbance enhances the germination of seeds. In practice, the risk of alien species to the biodiversity of natural forests may be relatively small due to the lack of continuous disturbance in these habitats. Instead, the greatest risk is caused by the possibility of alien species to spread via trails to neighbouring, extremely sensitive open habitats.

Invasive alien plants in China: diversity and ecological insights

China’s current invasive alien plant species were analyzed with regard to their floristic status, biological attributes and invasion status elsewhere. Most of the 270 species identified were annuals, followed by perennial herbs. Woody perennials made only about 10% of the species. The invasives were comprised of 59 families, the largest being Asteraceae, Poaceae, and Brassicaceae. The genera with most invasive species were Amaranthus, Ipomoea, and Solanum. Most of the species originated from the New World, notably from South America. About one-third of the species were serious invaders of natural habitats in countries other than China. The proportion of invasive alien plants in province floras ranged from 0.5 to 3.8%, absolute numbers from nine to 117 species per province. Density of invasive species was correlated positively with native species density at provincial scale. The results demonstrate that in China invasive plants are present throughout the country, with a particularly high species richness in the Southeast. The ecological diversity of invasive plants suggests wide ranging impacts which need to be assessed.

Spatial analysis of an invasion front of Acer platanoides: dynamic inferences from static data

It is an open question whether the invading tree species Acer platanoides is invading and displacing native trees within pre‐existing forest stands, or merely preferentially occupying new stands of secondary forest growth at the edges of existing forests. Several threads of spatial pattern analyses were used to assess the invasibility of A. platanoides, and to link the invasion to the structure of a plant community in the deciduous forest of the northeastern United States. The analyses were based on maps of a contiguous 100×50 m area along an A. platanoides infestation gradient. The distribution of A. platanoides was highly aggregated and the population importance value increased from 28.1 to 38.5% according to mortality estimated from standing dead trees, while the distribution of native tree species was close to random and importance value of Quercus spp. decreased from 33.4 to 26.9% over time. The size distributions of each tree species across distance indicated that A. platanoides was progressively invading the interior of the forest while the native species (including A. rubrum) were not spreading back towards the A. platanoides monospecific patch. The null hypothesis of no invasibility was rejected based on quantile regressions. There were negative correlations between A. platanoides density and the densities of native species in different functional groups, and negative correlation of A. platanoides density and the species diversity in forest understory. The null hypothesis that A. platanoides invasion did not suppress native trees or understory was rejected based on Dutilleul's modified t‐test for correlation, consistent with experimental results in the same study site. The combination of multiple spatial analyses of static data can be used to infer historical dynamical processes that shape a plant community structure. The concept of “envelop effects” was discussed and further developed.

Biotic Impoverishment and Homogenization in Unfragmented Forest Understory Communities

Abstract: Ecological change is often hard to document because of a lack of reliable baseline data. Several recent then‐versus‐now surveys of temperate forest and grassland communities demonstrate losses of local plant species, but most are based on data from a single site. We resurveyed understory communities in 62 upland forest stands in northern Wisconsin (U.S.A.) for which quantitative baseline data exist from 50 years ago. These stands are within a largely unfragmented region but vary in species composition and successional stage. We collected data on changes in (1) total and native species richness, (2) the ratio of exotic to native species, (3) the relative abundance of habitat generalists, and (4) community similarity among sites. We also compared how these rates of change varied over time. Over the past 50 years, native species density declined an average of 18.5% at the 20‐m2scale, whereas the ratio of exotic species to native species increased at 80% of all sites. Habitat generalists increased, and habitat specialists declined, accounting in part for an 8.7% rise in average similarity in species composition among sites. Most of these changes cannot be related to succession, habitat loss, or invasion by exotic species. Areas without deer hunting showed the greatest declines in native species density, with parks and research natural areas faring no better than unprotected stands. Animal‐pollinated and animal‐dispersed species also declined, particularly at unhunted sites. These results demonstrate the power of quantitative multistand data for assessing ecological change and identify overabundant deer as a key driver of community change. Because maintaining forest habitats alone fails to preserve plant diversity at local scales, local biotic simplification seems likely to continue in the region unless active efforts are taken to protect diversity.

HABITAT DISPLACEMENT OF NORTH AMERICAN LADYBIRDS BY AN INTRODUCED SPECIES

How introduced species displace native species is often poorly understood. Various studies suggest that several species of native ladybird beetles (Coleoptera: Coccinellidae) have declined in abundance in particular habitats (especially crops) and/or geographic regions following the colonization of North America by the seven-spotted ladybird beetle (Coccinella septempunctata L.). However, the factors responsible for such declines are unclear. Long-term sampling of alfalfa fields in northern Utah revealed that as C. septempunctata established itself and increased in numbers, densities of native species decreased significantly over the period 1992–2001. The decline in numbers of native ladybirds mirrored a decline in numbers of their principal prey in alfalfa, pea aphids. Field experiments with induced aphid outbreaks were conducted to test the hypothesis that dispersing native adults have shifted their foraging efforts away from alfalfa as the introduced species has increased in numbers and depressed prey availability. As predicted by the hypothesis, native ladybirds rapidly reaccumulated in alfalfa when prey became abundant. Their presence was brief, however, as they and especially the more numerous C. septempunctata quickly suppressed experimental aphid outbreaks. The ratio of densities within experimental plots with high aphid numbers to densities in the surrounding field was greater for native ladybirds than for C. septempunctata. Also, densities of native ladybirds varied more closely with aphid densities among fields in late May 2001 than did densities of C. septempunctata. Hence, native ladybirds appear especially responsive to local aphid density. C. septempunctata's impact on aphid density in alfalfa appears to have reduced significantly the degree to which the habitat retains foraging native adults. Thus, introduced species may influence the distribution and abundance of native competitors not only by reducing survivorship in local habitats, but also by altering the dynamics of dispersal and habitat use across the landscape.

Predator-Mediated Coexistence of Exotic and Native Crustaceans in a Freshwater Lake?

The predatory effects of a Dipteran insect, Chaoborus, on the competition between exotic cladoceran Daphnia lumholtzi and two natives, D. catawba and D. pulex, were studied for a period of three years in a freshwater reservoir, Lake James, North Carolina (USA). D. lumholtzi was first encountered in September 1997 and it was present only between August and October when population densities of native species were low and that of Chaoborus sp. was high. The patterns observed in the population dynamics of the exotic D. lumholtzi and two natives, Chaoborus suggest that a ‘predator mediated coexistence’ phenomenon might be taking place in Lake James. The strong positive correlation between Chaoborus and D. lumholtzi and the negative correlation between Chaoborus, D. catawba and D. pulex is supportive of this hypothesis.

Long-term influences of livestock management and a non-native grass on grass dynamics in the Desert Grassland

Density of 23 perennial grass species was measured in 25 permanent plots nine times between 1972–2000. Grass density was not related to the intensity of livestock grazing. Only one species expressed a difference between the summer rest and no summer rest with heavier stocking grazing treatments: bush muhly (Muhlenbergia porteri Scribn. ex Beal) density was less under the no summer rest with heavier stocking treatment. Beginning in 1975, the non-native Lehmann lovegrass (Eragrostis lehmanniana Nees) spread from distant seedings to one plot, and by 1991, it was the dominant species on most plots. The density of native species was not related to the length of time that the non-native lovegrass was present on a plot. In general, native species declined prior to the arrival and increase of the non-native lovegrass.

Pasture management in semi‐arid tropical woodlands: Effects on species diversity

Abstract The effects of pasture management options (sowing introduced legumes and grasses, timber treatment, applying fertilizer, cultivation before sowing, and stocking rate) on species diversity were measured at two experimental sites (Hillgrove and Cardigan) near Charters Towers, northeast Queensland. Species were divided into three groups (sown, native and exotic) and diversity was measured as species density (number of species recorded in each plot and number of species/quadrat) annually from 1982 to 1992. The responses of individual native and naturalized species to treatment were also determined. All management options affected diversity but the responses varied with site and season, and with the different measurement scales. The density of sown species either increased or was unaffected by all the management options; there were no significant decreases. The density of native species showed both positive and negative responses; it increased at high stocking rates and with tree killing at Hillgrove, and decreased with pasture sowing and cultivation. The density of exotic species increased as stocking rate was increased and decreased when pastures were sown (although not at the quadrat scale at Hillgrove). Overall the most diverse vegetation was on plots grazed at high stocking rates; at the plot scale these were native pastures but at the quadrat scale the sown pastures had more species. Among the native and naturalized species, only Portulaca spp. were more frequent on the oversown plots than the native pasture plots; 48% (Hillgrove) and 68% (Cardigan) of the species were less frequent on the oversown plots. Fertilizer application had little effect on species frequencies, while timber treatment resulted in both increases and decreases in frequency of a small number of species. The species were divided into four groups on the basis of their responses to stocking rate: a grazing‐sensitive group (e.g. Themeda triandra), two grazing‐tolerant groups which either slightly decreased (e.g. Chrysopogon fallax) or slightly increased (e.g. Sida spinosa) in frequency as stocking rate increased, and a fourth group of species which were frequent only at high stocking rates (e.g. Bothriochloa pertusa). There were no close relationships between herbage yield and species density.

Invasibility of tree plantations by native and non-indigenous plant species in Hawaii

The Hawaiian archipelago, the most isolated on Earth, has proven to be especially vulnerable to invasions by non-indigenous species. Alien species now outnumber natives and the threat they pose is in part responsible for the fact that approximately 25% of Hawaii's native flora, 90% of which is endemic, has been listed as threatened or endangered. To assess the effect of stand characteristics on the colonization of the plantations by alien and native species, we measured basal area, leaf area index (LAI), litter depth and standing litter mass in 26- to 32-year-old plantations of Eucalyptus saligna, Flindersia brayleyana and Fraxinus uhdei. The plantations are surrounded by native Hawaiian rainforest which has been invaded by numerous non-indigenous species. The basal area of the planted Flindersia was 45.2 m 2/ha, three to four times that of the other two plantation species. Nevertheless, when colonizing species were included, total stand basal area and LAI did not differ significantly across the three plantation types. Litter depth ranged from 3.7 cm for Flindersia to 4.7 cm for Eucalyptus, and litter mass ranged from 7.4 Mg/ha for Fraxinus to 10.5 Mg/ha for Eucalyptus. Of 51 species identified in the plantation understories, 23 (45%) were common to all three plantation species. Eucalyptus plantations had 42 species, seven of which were uniquely found there, Fraxinus had 39 species, six of which were unique and Flindersia had 36 species, only one of which was unique. The most abundant understory species under Fraxinus were Cibotium glaucum and Metrosideros polymorpha, two dominant components of native Hawaiian rainforest. In contrast, the most abundant understory species in the Eucalyptus plantations was the alien tree, Psidium cattleianum and the understory of Flindersia was dominated by its own offspring. Average species richness (number of species per plot) of both native species and all species combined was significantly greater under Fraxinus than under the other two plantation species, but all plantations contained approximately equal numbers of non-indigenous species. Average density (number of individuals/m 2) was greatest under Flindersia, although the density of native species alone was greatest under Fraxinus. Species richness and plant density were not correlated with litter depth or mass, but total species richness and density of native species did decrease with increasing LAI. In general, abundance of native plants in the understory decreased with increasing abundance of non-indigenous species and increasing basal area of plantation species.

Revegetation of an Arctic Disturbed Site by Native Tundra Species

(1) Succession on an organic tundra soil in interior Alaska was monitored for ten growing seasons following removal of vegetation. (2) Exotic grasses sown following disturbance became established in the first growing season, decreased in density after 3 yr, and were virtually eliminated after 5 yr. Fertilization did not affect initial density or long-term survival of exotic species, but did increase shoot density of native species 3 yr following disturbance. (3) Native sedges established on the disturbed site in 5-10 yr, producing an above-ground biomass equal to that in undisturbed tundra. (4) A procedure is outlined whereby the growth and reproduction of Eriophorum vaginatum can be manipulated by fertilization to revegetate disturbances using plants in adjacent undisturbed tundra (rather than harvested seed) as the seed source.