Understanding Ecosystem Services from a Geosciences Perspective
Human societies depend greatly on the natural environment in many ways: for food production, water supplies, erosion and flood control, and recreational opportunities, for example. However, the linkages between human societies and these benefits they derive from the environment have not always been considered explicitly when managing natural resources. To understand these linkages so that benefits from the environment can be more effectively managed, the framework of “ecosystem services” has emerged as a useful approach.
The benefits that society derives from the environment have been described in many ways, with ecosystem services initially classified into four distinct categories [Millennium Ecosystem Assessment, 2005]:
- Provisioning services are material benefits to humans, such as fiber, food, or timber.
- Regulating services are processes such as pollination, flood control, and disease control.
- Supporting services include nutrient cycling and soil formation.
- Cultural services are those aspects of species and ecosystems that provide humans with recreational, spiritual, or religious experiences.
An example of an ecosystem service critical to society is provision of water of sufficient quantity, timing, and quality for drinking and other human requirements. A traditional ecosystem services perspective focuses on relating active vegetation management (e.g., forest thinning) or vegetation change due to disturbance (e.g., fire, insect, or drought mortality) to water resources, often emphasizing precipitation, soil moisture, and surface water flows while not necessarily considering other influential processes [e.g., Alila et al., 2009].
Explicitly expanding assessment of the service of water provision to include geosciences perspectives would in many cases lead to more robust understanding of relevant environmental processes and how to manage them for the benefit of society [Field et al., 2015]. For example, geosciences perspectives on water resources explicitly bring into consideration other key processes such as water quantity as affected by groundwater interactions with surface water, water timing as affected by subsurface flow transit times, and water quality as affected by rate-limiting biogeochemical processes [Chorover et al., 2011; Brooks et al., 2015]. This perspective is consistent with the original definition of ecosystems as one physical system [Richter and Billings, 2015].
To further highlight the utility of incorporating geosciences perspectives into considerations of ecosystem services, we discuss an example focusing on integration of biological, physical, and chemical processes associated with evolution of the “critical zone” (CZ, extending from groundwater level to the top of the vegetation canopy) and their relevance to society in the context of ecosystem services.
The Critical Zone Perspective
The societal relevance of processes occurring at CZ scales comes primarily from their fundamental role in regulating ecosystem processes and their effects on associated services. There is growing concern that human disturbance, including intensive management for agriculture, is altering the CZ’s potential to provide essential services, to the extent that it transforms the CZ into a less active regulator of nutrients, carbon, and water.
The long-term evolution of the CZ from its bedrock source is driven by climate-sensitive ecosystems [Rasmussen et al., 2011], and the evolved structure of porous soil and bedrock, in turn, affects how an ecosystem responds to perturbation [Lin, 2010]. However, CZ development occurs over longer time scales (thousands to millions of years) than ecosystem succession (tens to hundreds of years) [e.g., Chadwick et al.,1999]. Services deriving from CZ processes, such as water purification and carbon stabilization, are sensitive to how variations in climate or rock formation characteristics (lithology) affect the long-term evolution of regolith, the surface soil and deeper rocky material that covers unweathered bedrock.
Hence, a CZ perspective of ecosystem services (Figure 1) expands the scope to include processes at time scales often not considered in ecosystem services, such as nutrient release from rock to bioavailable form based on lithology, substrate age, atmospheric deposition, nutrient retention, and loss mediated by soil development, weathering-induced carbon sequestration, aspect-induced variation in subsurface water storage, and landscape-scale water dynamics affecting plant-available water [Field et al.,2015].
Critical zone science seeks to understand these larger-scale and longer-term processes associated with evolution of the weathering profile and their effects on regulating climate, nourishing ecosystems, and controlling water quality and quantity. Such biotic-geologic couplings are particularly relevant for the assessment of regulating and supporting ecosystem services. Incorporating a broader perspective that includes couplings of biotic and geologic processes that influence the production of natural resources, such as soils, that are “nonrenewable” on human time scales can help enhance ecosystem service assessments, particularly for regulating and supporting services.
Recent developments in CZ science have revealed the importance of ecological, geomorphic, geochemical, and hydrologic processes that affect the relevant supply of services and act over larger temporal and spatial scales than those generally accounted for in the ecosystem services community (“supply chains” [Field et al., 2015]). Many ecosystem successions occur over the mean residence time of a single regolith. This regolith is therefore the primary source of lithogenic nutrients for many successions of plants and animals, even on tectonically active hillslopes. Critical zone evolution rates therefore clearly affect ecosystem function and the supply of services.
Conceptualization of CZ processes expands the context of ecosystem services temporally and spatially, providing constraints on ecosystem services associated with rate-limiting processes such as soil formation and hydrological partitioning. This conceptualization also advances the assessment and valuation of ecosystem services by providing integrated currencies that quantify the energy flux available to do thermodynamic work on the CZ [Field et al., 2015].
By incorporating CZ processes into the ecosystem services framework, society gains a broader perspective on ecosystem services and hence more refined tools to value societal benefits. We have highlighted CZ processes as an example, but the concept applies more generally to linking geosciences perspectives to ecosystem services.
Improving Valuation with Geosciences Perspectives
Geosciences perspectives expand both spatial and temporal scales of consideration affecting ecosystem services. For example, ecologists who focus on the services provided by vegetation and reefs in reducing impacts of coastal hazards could benefit from geosciences input on how geomorphology, elevation, and coastline configuration interact with the living organisms to deliver those services.
Time scales associated with plant community succession provide a more detailed example of how ecosystems services assessments can be improved through geosciences. The succession after a disturbance such as forest blowdown (high winds that topple trees) usually occurs in tens to hundreds of years, after initial colonization by short-lived species, followed by longer-lived species.
However, geosciences perspectives also consider the conversion of rock to soil and the long-term evolution of the soil profile and are on the order of thousands to millions of years. For instance, the long-term evolution of Hawaiian tropical forest ecosystems occurs on lava flows that range in age from hundreds of years on the big island of Hawaii to about 4.1 million years on Kauai [Chadwick et al., 1999].
The regolith profiles support ecosystems comprising the same forest vegetation species, but they differ dramatically in their physical-chemical properties as a result of long-term weathering processes. A core aspect of ecosystem services is to provide a means for valuing the services they provide. A geosciences perspective could prove valuable in estimating the cost of recovering lost services, for example, if we lost both types of forests and their services due to disturbances like blowdown or lava flow. In this case, we would need to consider the contrast in rate-limiting soil production processes for these two forest types.
Geoscientists’ Perspectives Needed
In summary, we need geosciences perspectives—focused over a broader range of temporal and spatial scales than is typically studied by terrestrial ecosystem scientists—to infuse our knowledge of ecological processes with geophysical and biophysical mechanisms that support them. We encourage geoscientists to partner with ecologists, economists, and social scientists to bring these larger spatial-scale and longer temporal-scale perspectives when working with stakeholders, decision makers, and policy makers.
These groups will be well served by explicitly linking geosciences with ecosystem services, building upon advances in both communities to quantify the time scales required to replenish services following a disturbance. A geosciences context enables broader perspectives for managers, policy makers, and stakeholders to effectively understand the expanded temporal and spatial scales of Earth processes required to provision ecosystem services to society.
