EcoTrends Topics
Biogeochemistry
The biogeochemistry of ecosystems involves the movement or cycling of elements (e.g., sulfur, carbon, nitrogen) and compounds (e.g., water) through the biotic (e.g., plants, animals, microbes) and abiotic (e.g., soils, atmosphere) components of an ecosystem. All elements and compounds cycle through the Earth system, although at different rates and by different pathways that depend on their chemical characteristics and the extent to which they are utilized by organisms. Cycling involves inputs to and losses from different pools or standing stocks and the transformations of major and trace elements. Inputs include atmospheric deposition that can be either wet (i.e., in precipitation) or dry (i.e., as gases or particles), and weathering of rocks and minerals. Losses can occur either through gaseous emissions to the atmosphere or drainage below the soil surface or from the land to the oceans. Pools include the accumulations of elements within the ecosystem, such as in soil, sediments, and vegetation. Important internal transformations of elements include litter inputs, mineralization of organic matter, uptake of nutrients by vegetation, and the retention or release of material in soil or sediments.
Ecologists measure these pools and fluxes in order to learn critical information about the function of ecosystems. Because it can take hundreds to thousands of years for a molecule to move completely through an ecosystem, long-term data provide one of the few means to estimate how ecosystems use and respond to changes in nutrients and toxic substances. Long-term data are used to characterize the average size and variability in pool sizes and the rates of flow between pools. Monitoring biogeochemical indicators provide useful insight on the response of ecosystems to chronic change, such as climate change, land use change, the introduction of invasive species or changes in air pollution, or abrupt change such as fire. Many important ecosystem services, such as the supply of clean air and water, ecosystem productivity, and carbon sequestration, are closely coupled to the biogeochemistry of ecosystems.
The biogeochemistry of ecosystems can be characterized by a wide array of variables, but the current web site is limited to those variables that represent key components of ecosystems measured at a number of sites. As a result, we focus on wet deposition and precipitation chemistry (e.g., nitrogen, sulfur, calcium, chloride, phosphate) through data available in the National Atmospheric Deposition Program (NADP; http://nadp.sws.uiuc.edu/) and surface water chemistry (e.g., sulfate, nitrogen [ammonium, nitrate], and chloride) collected by each site. All of these solutes are changing in precipitation in response to changes in emissions of air pollutants, and have water quality and ecosystem implications.
Two measures of atmospheric chemistry are commonly obtained from precipitation (rain, snow) collected at a site: concentration expressed as milligrams per liter (mg/L) is measured on a subsample of the precipitation and converted to the total volume collected (i.e., volume-weighted concentration), and total amount collected in a precipitation sample is converted to an area basis (i.e., deposition expressed as kg/ha-year). In both cases, samples are collected frequently (e.g., daily or weekly) and converted to a mean value for the entire year. Data obtained from the NADP website were obtained as mean annual values from that web site whereas data from sites that do not participate in the NADP were converted to annual values after obtaining the data. Mean surface water export data on an annual basis (mg/L-year) for nitrogen, sulfer and chloride were obtained directly from sites that had long-term monitoring programs; no national network was available for these data.
Biotic structure
Trends in biotic structure have been of interest since the establishment of the Division of Biological Survey in the US Department of Agriculture in the late 1890’s. Changes in biotic structure can serve as a bellwether for quantifying the impacts of climate change, landuse change and the spread of exotic species, as well as the loss of rare and endangered species. Indeed, considerable evidence suggests that changes in biotic structure can have significant consequences for ecosystem functioning and the provisioning of ecosystems goods and services. Biotic structure can be characterized by a wide array of variables, but the current web site is limited to those variables that represent key components of ecosystems measured at a number of sites.
Primary production: One of the most important variables in all ecosystems is net primary production (NPP), the accumulation of biomass over a specified time period, usually seasonally or annually. NPP represents the amount of energy fixed by producers (e.g., vascular plants, algae) that can be used for their growth and reproduction, and that is available for consumption by herbivores. Life on Earth depends on this conversion of inorganic compounds to organic molecules and the release of oxygen; thus NPP is a critical variable for all ecosystems even though the primary producers vary from vascular plants on land to algae and phytoplankton in the lakes and oceans.
At most sites, net primary production is summarized in comparable units (e.g., grams/m2/year) despite a variety of measurement techniques. For terrestrial ecosystems, most sites only estimate long-term aboveground net primary production (ANPP); difficulties in obtaining accurate and cost-effective estimates of belowground net primary production (BNPP) result in very few, if any, long-term data sets of this variable. Repeated clipping of herbaceous biomass or estimations of changes in plant sizes are often used in grasslands and deserts to estimate ANPP. Diameter or basal area increment (DBH or BAI) and annual litterfall are most often used in forests. Chlorophyll content or measurement of either O2 or CO2 consumption or production in light and dark bottles can be used as surrogates for NPP in aquatic systems. Although the methods in terrestrial and aquatic systems are highly disparate, all measurements can be converted to the common units of g/m2 making cross-system comparisons possible. At very large spatial scales, satellite data can be used to estimate “greenness” which can be correlated with NPP, at least in freshwater and marine systems and mesic terrestrial vegetation. More information on remotely sensed data can be found at many of the individual research site web pages.
Biomass, cover, and density of key species and groups of similar species (i.e., functional groups) that represent each ecosystem are of particular importance in ecosystems. Biomass is the mass per unit area of living material (plants, animals, microbes) typically measured as grams per square meter (g/m2) or kilograms per hectare (kg/ha). Changes in biomass over time can be used to calculate NPP. Biomass is important in its own right as a measure of stored energy (e.g., wood, sugar cane, corn) and carbon that is sequestered from the atmosphere. Cover is the amount of surface area occupied by plants or animals that is often represented as a percentage (e.g., m2/m2 * 100). Density is the number of individuals found in an area, such as number per m2 or number per ha.
The abundance of organisms or material and species composition (the percentage that each species contributes to each measurement) can be determined from biomass, cover, or density. Species richness, the number of species in an area (e.g., number per m2), is an important measure of biodiversity that is available for some sites, although differences in sampling area result in difficulties in comparing across sites.
Climate and physical variability
Climate (i.e., average weather conditions over a period of time) is a primary driver of ecological systems. Important climatic factors for ecosystems include water, temperature, ice cover, and sunlight that affect resources available to plants, animals, and microbes, and act as environmental constraints on the suitable habitat for organismal reproduction, growth, and survival. Changes in the seasonal and annual climate patterns can have important consequences for key ecosystem properties, such as species composition and diversity, trophic interactions, rates of nutrient cycling, and net primary production.
Long-term data are required to differentiate directional climate trends from short-term pulses and natural variability. In addition to this background of progressive long-term change, there are multi-decadal scale variations associated with phenomena such as the Pacific Decadal Oscillation and North Atlantic Oscillation, as well as interannual variations dominated by the El Niño-Southern Oscillation (ENSO). Understanding ecological responses to climatic change is difficult because of the interactions of climate drivers on these multiple time scales. In addition, ecological systems respond to multiple drivers such as climate and land use change simultaneously, and the responses are often nonlinear.
Climate has been monitored throughout the U.S. since President Grant started the National Weather Service in 1870 (http://www.nws.noaa.gov/). Numerous standardized measurement locations exist within the continental U.S. and the coastal ocean. The volume edited by Greenland et al. (2003) is a valuable resource that addresses climate variability and ecological responses at many LTER sites.
The EcoTrends website contains data for many climate variables, including air temperature, precipitation, the Palmer Drought Severity Index (PDSI). It also includes physical environmental data such as wind speed, water temperature, and sea level.
Disturbance
A disturbance is defined as an external event with the capacity to alter the structure, functioning, and species composition of an ecosystem. The effects of a particular disturbance depend on its duration (short or acute vs. chronic or long-term) and intensity, how large an area it affects, the state of the ecosystem at the time of impact (whether the system is mature or young, in active growth or dormant), and the frequency of return of the disturbance. Some disturbances occur frequently but at low intensity (like annual fires that move quickly through forests), some are very infrequent but of high intensity such as volcanic eruptions or category 5 hurricanes, and others exhibit a wide range of frequency and intensity combinations, such as the size and frequency of landslides on forested landscapes.
Ecologists recognize four major types of disturbances: climatic, physical, biological, or anthropogenic. Each disturbance type has different effects on ecosystems. For example, windstorms are climatic disturbances that mechanically alter the structure of forests, and transfer biomass from the forest canopy to the soil surface where it can be processed by microorganisms. In contrast, fires are physical disturbances that consume organic matter and release ash plus carbon dioxide gas into the atmosphere. Other types of disturbances include those that affect ecosystems biologically, such as insect attacks on trees or defoliation by herbivores. Human-caused, anthropogenic disturbances include the clearing of trees or cultivation of agricultural land as well as atmospheric warming and ozone pollution.
Human population and economy
Human activities have profound influences on ecosystems, both directly through land use change, spread of invasive species, and increases in air and water pollution, and indirectly through increases in atmospheric CO2 and trace gases that modify climate and weather patterns. Rapid growth in human populations globally during the last century, from 1.6 billion in 1900 to 6.7 billion in 2008, has increased demands for resources with subsequent effects on biotic (plants, animals, microbes) and abiotic (soils, atmosphere, water) properties of ecosystems. These changes in ecosystem properties result in modifications to the goods and services provided to human populations. Thus, there is a feedback loop between human populations and their natural environment that makes it imperative that trends in human populations be examined as both a key driver to changes in ecosystems, and as a key responder to changes in those same systems.
For the U.S., the best source for long-term population and economic data is the census raw data (http://www.census.gov/) and the summarized census data in the Geolytics database (http://www.icpsr.umich.edu/index.html). Due to funding constraints, we focused on collecting key population and economic variables for counties selected to represent each LTER site. Since 1790, the Census Bureau has collected information every 10 years on the population and economic characteristics of the country. Sites east of the Appalachian Mountains typically have census data dating to 1790, while most areas west of the Rocky Mountains have data starting after 1860 and Alaska has data since 1970. Census data are not available for the two sites in Antarctica or one site in French Polynesia; thus a total of 23 LTER sites are included in the current analysis (Table III.D.1).
We tabulated census data for three population variables for each county in each year of the census: total population (number of people per county), density (number of people per km2), and the percentage of the population living in urban areas. Density was used as a way to standardize the data because counties differ in their area covered. The total population size of a county was divided by the area of that county to obtain an average density value. We were also interested in trends in the percentage of people living in urban areas through time. We tabulated one economic variable for each county: the percentage of the population employed by one of four sectors of the economy: commercial industries, farming, manufacturing, and service industries.
