-
Disturbance−diversity models: what do they really predict and how are they tested?
-
The intermediate disturbance hypothesis (IDH) and the dynamic equilibrium model (DEM) are influential theories in ecology. The IDH predicts large species numbers at intermediate levels of disturbance and the DEM predicts that the effect of disturbance depends on the level of productivity. However, various indices of diversity are considered more commonly than the predicted number of species in tests of the hypotheses. This issue reaches beyond the scientific community as the predictions of the IDH and the DEM are used in the management of national parks and reserves. In order to compare responses with disturbance among measures of biodiversity, we used two different approaches of mathematical modelling and conducted an extensive meta-analysis. Two-thirds of the surveyed studies present different results for different diversity measures. Accordingly, the meta-analysis showed a narrow range of negative quadratic regression components for richness, but not evenness. Also, the two models support the IDH and the DEM, respectively, when biodiversity is measured as species richness, but predict evenness to increase with increasing disturbance, for all levels of productivity. Consequently, studies that use compound indices of diversity should present logical arguments, a priori, to why a specific index of diversity should peak in response to disturbance.
Located in
Resources
/
Climate Science Documents
-
Divergent global precipitation changes induced by natural versus anthropogenic forcing
-
As a result of global warming, precipitation is likely to increase in high latitudes and the tropics and to decrease in already dry sub-tropical regions (1). The absolute magnitude and regional details of such changes, however, remain intensely debated (2,3). As is well known from El Nino studies, sea-surface-temperature gradients across the tropical Pacific Ocean can strongly influence global rainfall (4,5). Palaeoproxy evidence indicates that the difference between the warm west Pacific and the colder east Pacific increased in past periods when the Earth warmed as a result of increased solar radiation (6–9). In contrast, in most model projections of future greenhouse warming this gradient weakens (2,10,11). It has not been clear how to reconcile these two findings. Here we show in climate model simulations that the tropical Pacific sea-surface-temperature gradient increases when the warming is due to increased solar radiation and decreases when it is due to increased greenhouse-gas forcing. For the same global surface temperature increase the latter pattern produces less rainfall, notably over tropical land, which explains why in the model the late twentieth century is warmer than in the Medieval Warm Period (around AD 1000–1250) but precipitation is less. This difference is consistent with the global tropospheric energy budget (12), which requires a balance between the latent heat released in precipitation and radiative cooling. The tropospheric cooling is less for increased greenhouse gases, which add radiative absorbers to the troposphere, than for increased solar heating, which is concentrated at the Earth’s surface. Thus warming due to increased greenhouse gases produces a climate signature different from that of warming due to solar radiation changes.
Located in
Resources
/
Climate Science Documents
-
Divergent phenological response to hydroclimate variability in forested mountain watersheds
-
Mountain watersheds are primary sources of freshwater, carbon sequestration, and other ecosystem services. There is significant interest in the effects of climate change and variability on these processes over short to long time scales. Much of the impact of hydroclimate variability in forest ecosystems is manifested in vegetation dynamics in space and time. In steep terrain, leaf phenology responds to topoclimate in complex ways, and can produce specific and measurable shifts in landscape forest patterns. The onset of spring is usually delayed at a specific rate with increasing elevation (often called Hopkins’ Law; Hopkins, 1918), reflecting the dominant controls of temperature on greenup timing. Contrary with greenup, leaf senescence shows inconsistent trends along elevation gradients. Here, we present mechanisms and an explanation for this variability and its significance for ecosystem patterns and services in response to climate. We use moderate-resolution imaging spectro-radiometer (MODIS) Normalized Difference Vegetation Index (NDVI) data to derive landscape-induced phenological patterns over topoclimate gradients in a humid temperate broadleaf forest in southern Appalachians. These phenological patterns are validated with different sets of field observations. Our data demonstrate that divergent behavior of leaf senescence with elevation is closely related to late growing season hydroclimate variability in temperature and water balance patterns. Specifically, a drier late growing season is associated with earlier leaf senescence at low elevation than at middle elevation. The effect of drought stress on vegetation senescence timing also leads to tighter coupling between growing season length and ecosystem water use estimated from observed precipitation and runoff generation. This study indicates increased late growing season drought may be leading to divergent ecosystem response between high and low elevation forests. Landscape-induced phenological patterns are easily observed over wide areas and may be used as a unique diagnos- tic for sources of ecosystem vulnerability and sensitivity to hydroclimate change.
Keywords: drought deciduousness, hydroclimate variability, landscape phenology, MODIS NDVI, topoclimate gradient
Located in
Resources
/
Climate Science Documents
-
Diverse Connections for Diverse Watersheds
-
Save the Date: 8th Annual Tennessee River Basin Network Conference.
Located in
News & Events
/
Events
/
Upload New Events
-
Diverse pollinator communities enhance plant reproductive success
-
Understanding the functional consequences of biodiversity loss is a major goal of ecology. Animal-mediated pollination is an essential ecosystem function and service provided to mankind. However, little is known how pollinator diversity could affect pollination services. Using a substitutive design, we experimentally manipu- lated functional group (FG) and species richness of pollinator communities to investigate their consequences on the reproductive success of an obligate out-crossing model plant species, Raphanus sativus. Both fruit and seed set increased with pollinator FG richness. Furthermore, seed set increased with species richness in pol- linator communities composed of a single FG. However, in multiple-FG communities, highest species richness resulted in slightly reduced pollination services compared with intermediate species richness. Our analysis indicates that the presence of social bees, which showed roughly four times higher visitation rates than solitary bees or hoverflies, was an important factor contributing to the positive pollinator diversity–pollination service relationship, in particular, for fruit set. Visitation rate at different daytimes, and less so among flower heights, varied among social bees, solitary bees and hoverflies, indicating a niche complementarity among these pollinator groups. Our study demonstrates enhanced pollination services of diverse pollinator communities at the plant population level and suggests that both the niche complementarity and the presence of specific taxa in a pollinator community drive this positive relationship.
Located in
Resources
/
Climate Science Documents
-
Djajasasmita 1982.pdf
-
Located in
Resources
/
TRB Library
/
DIN-ECO
-
Do alternative energy sources displace fossil fuels?
-
A fundamental, generally implicit, assumption of the Intergov- ernmental Panel on Climate Change reports and many energy analysts is that each unit of energy supplied by non-fossil-fuel sources takes the place of a unit of energy supplied by fossil- fuel sources (1–4). However, owing to the complexity of economic systems and human behaviour, it is often the case that changes aimed at reducing one type of resource consumption, either through improvements in efficiency of use or by developing substitutes, do not lead to the intended outcome when net effects are considered (5–9). Here, I show that the average pattern across most nations of the world over the past fifty years is one where each unit of total national energy use from non- fossil-fuel sources displaced less than one-quarter of a unit of fossil-fuel energy use and, focusing specifically on electricity, each unit of electricity generated by non-fossil-fuel sources displaced less than one-tenth of a unit of fossil-fuel-generated electricity. These results challenge conventional thinking in that they indicate that suppressing the use of fossil fuel will require changes other than simply technical ones such as expanding non-fossil-fuel energy production.
Located in
Resources
/
Climate Science Documents
-
DO CARBON OFFSETS WORK? THE ROLE OF FOREST MANAGEMENT IN GREENHOUSE GAS MITIGATION
-
As forest carbon offset projects become more popular, professional foresters are providing their expertise to support them. But when several members of the Society of American Foresters questioned the science
and assumptions used to design the projects, the organization decided to convene a task force to examine whether these projects can provide the intended climate benefits.The authors emphasize the carbon-storage benefits of using wood products in place of nonrenewable, energy-intensive materials and using woodbased
energy instead of fossil fuels.
Located in
Resources
/
Climate Science Documents
-
Do small tributaries function as refuges from floods? A test in a salmonid-dominated mountainous river
-
Excerpts from the text: On 8–10 August 2003, a powerful typhoon hit Hokkaido Island, Japan, accompanied with heavy rain, which allowed us to investigate the potential role of tributaries as refuges from flooding. We had just completed annual population census in four small tributaries of a river system 1–2 days before the typhoon.... Overall, our results did not support the hypothesis that many large fishes immigrate to small tributaries during floods. ... Despite the lack of evidence of mass movement, our result suggested a few immigrants from the main stem (i.e., juvenile white-spotted charr, sculpin and a few relatively large Dolly Varden). Because more than 100 small tributaries exist in the Shiisorapuchi River (Koizumi 2011), only a few individuals escaping to each tributary should accumulate to a great number enough to re-colonise main stem habitats even if fishes in the main stem were extirpated. Multiple refuges at different spatial scales should increase resistance and ⁄ or resilience of fish populations (Sedell et al. 1990; Pearsons et al. 1992). Thus, the roles of tributaries as refuges would deserve further attention
Located in
Resources
/
Climate Science Documents
-
Documents
-
Located in
Resources
