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For interspecific competition to occur species must use the same limited resources (e.g. food, habitat) and overlap in time and space. Bison (Bison bison) were reintroduced to southwestern Yukon, Canada, where they are sympatric with resident caribou (Rangifer tarandus), moose (Alces americanus) and thinhorn sheep (Ovis dalli). Concerns from local communities regarding potential competition between reintroduced bison and resident ungulates prompted us to test their spatial distribution for co-occurrence. In late-winter, we conducted multiple aerial surveys (n = 1–5) of 12.2 km2 cells (n = 779) to develop a presence—absence matrix (4 species × 779 cells) of the spatial distribution of ungulates. Randomization procedures were used to conduct a null model analysis of co-occurrence. Community-wide null model analysis indicated that community members were neither segregated nor aggregated; rather, their spatial distribution was random because they did not differ from simulated null communities (n = 50,000). Similar analyses conducted on sub-matrices for each species pair also did not find evidence for segregation or aggregation among any species pairs except caribou and sheep, who were spatially segregated. We conclude that the overall potential for competition between reintroduced bison and resident ungulates during late-winter is low, based on spatial overlap. However, further investigations on the potential for competition among other niche dimension axes (e.g. food, habitat use) are recommended. Even though bison are reintroduced, these species had interacted for thousands of years and have probably co-evolved mechanisms to partition resources and co-exist on a shared landscape.
High-latitude lakes are usually transparent, due to their low productivity and low concentration of dissolved organic matter (DOM), but large variations in lake optical properties can be found within and between regions. We investigated the light regimes in relation to DOM in 18 oligotrophic, high-latitude lakes across mountain birch woodland, shrub tundra and barren tundra in north-west Finnish Lapland. In 12 lakes >1% of photosynthetically active radiation (PAR) reached the lake bottom, while 1% UV-B depth ranged from 0.1 to >12 m. Lakes located in barren tundra had highest transparency, lowest dissolved organic carbon (DOC) concentration and lowest DOM absorption (a440) (mean values: Kd PAR 0.3m-1, DOC 2.1mg l-1, a440 0.4m-1), while lakes in shrub tundra and mountain birch forest were less transparent (DOC 4.7 mg l-1, a440 1.4 m-1). Solar attenuation and lake transparency was best explained by a440. Our survey emphasizes the importance of catchment type on DOM characteristics and lake optics. We predict that even small changes in DOM quality may largely change the UV radiation exposure of lakes while changes in PAR may have smaller biological effects in these shallow lakes that are already illuminated to the bottom.
KEYWORDS: oasis, spatio-temporal changes, Heihe River basin, human development, landscape change, arid ecosystems, Bassin de la rivière Heihe, changements des paysages, changements spatiaux et temporels, développement humain, écosystèmes arides, oasis
Artificial oases have flourished as human activities have intensified. Changes in oases in the Heihe River Basin are typical for the arid areas of China. Based on images from 14 separate periods during 1963–2013 that were compiled from multi-sensors, data on the boundaries of oases were extracted using the methods of object-oriented image segmentation and Normalized Difference Vegetation Index thresholds as well as visual interpretation. Based on extracted data, the spatio-temporal changes in oases were analyzed using models i.e. grid transformed model for rate of change. The drivers were analyzed based on data from Statistics Yearbooks and field surveys. The results show that from 1963 to 2013 sprawl dominated oasis evolution and occurred not only in the surroundings but also in interior patches. Oasis evolution patterns of “unchanged,” “expanding,” “shrinking” and “oscillating” were observed. The development exhibited three stages, the unstable (1963–1980), the steady development (1980–2002) and the rapid expansion (2002–2013), which correspond to the Planned Economy Period, the Commodity Economy Period and the Market-oriented Economy Period, respectively, in China. Oasis expansion was mainly determined by the human instincts for survival and for human well-being and was governed by population growth, agricultural policies and economic development.
We investigated the forest health of red fir (Abies magnifica) and how it compared with commonly-associated species Jeffrey pine (Pinus jeffreyi), lodgepole pine (Pinus contorta) and white fir (Abies concolor) in the upper montane forests of California. We evaluated tree mortality rates, changes in the density of recently-dead trees and the amount of insect and disease damage on live trees from comprehensive forest inventories. The annual mortality rate for red fir was 1.8%, while the rates for Jeffrey pine, lodgepole pine and white fir were 1.9, 1.1 and 3.0%, respectively. The proportion of recently-dead red fir trees increased over time, suggesting an increase in mortality; however, stage-transition models suggested the current population structure of red fir is stable. Dwarf mistletoe and drought-stressed sites were significant predictors of red fir mortality. Trees with substantial damage had a higher probability of experiencing mortality in five years. Our results are consistent with others, but the timeframe is too short to make conclusions about long-term declines. Our results suggest that the most significant of multiple biotic factors involved in red fir mortality processes is dwarf mistletoe, while tree age and the proportion of forest in old-growth may also influence mortality rates.
The postglacial history of vegetation at a regional scale is mainly governed by climate. However, the action of other environmental factors can lead to differentiation of the long-term vegetation dynamics in locations that are in proximity. In this context, the vegetation history of Covey Hill, at the northern tip of the Adirondack Mountains, was reconstructed using pollen analysis of a sediment core collected in a bog near the hilltop. This history was then compared with that of other sites in the Adirondacks, in the St. Lawrence Lowlands and in the Appalachian foothills to determine whether differentiations had occurred between sites across the landscape. Regionally, climate seems to have been the main driver of vegetation development. Differences between sites are most pronounced in regard to relative abundance of aspen and alder, and mostly occurred during the Late Glacial and early Holocene. Unlike other sites, fires occurred frequently on Covey Hill for several millennia, and most probably allowed the long-term maintenance of a rare pine barren. Finally, drier conditions are probably partly responsible for the hemlock decline evident around 5100 cal BP.
The widespread decline of the European brown hare (Lepus europaeus) in Europe has been attributed to both bottom-up and top-down factors, as well as climate change. Few studies have attempted to study the relative importance of these factors considered simultaneously. In this study we tested the hypotheses that hare population density is regulated by bottom-up (food), top-down (predation) or abiotic factors including tidal floods and climatic conditions. We related data on hare population density on a relatively isolated island to changes in surface area of suitable vs. unsuitable vegetation for forage, predator densities, flooding parameters and climatic variables. During the study period (1996–2012), hare numbers decreased from 580 to 219. Estimated population density was positively correlated with the cover of short, intermediate successional vegetation types and was negatively correlated with the cover of tall, late successional vegetation types. These findings corroborate results from earlier experimental studies that reported a strong aversion of hares to tall vegetation. Additionally, we found indications that raptor population density and unusually high floods also exerted negative effects on hare population density. We conclude that bottom-up factors (the availability of suitable forage) are the main regulators of the studied hare population. This suggests that the importance of bottomup effects has been underestimated and could explain leporid population decline in areas that have experienced a similar increase in tall, unsuitable vegetation.