The time scale of change refers to the time required for the effects of a perturbation of a process to be expressed. Time scales relevant to ecosystems and their services are shown in Figure 7.1. Inertia refers to the delay or slowness in the response of a system to factors altering their rate of change, including continuation of change in the system after the cause of that change has been removed. resilience refers to the amount of disturbance or stress that a system can absorb and still remain capable of returning to its predisturbance state.
Time Scales and Inertia
Many impacts of humans on ecosystems (both harmful and beneficial) are slow to become apparent; this can result in the costs associated with ecosystem changes being deferred to future generations. For example, excessive phosphorus is accumulating in many agricultural soils, threatening rivers, lakes, and coastal oceans with increased eutrophication. Yet it may take years or decades for the full impact of the phosphorus to become apparent through erosion and other processes (S7.3.2). Similarly, the use of groundwater supplies can exceed the recharge rate for some time before costs of extraction begin to grow significantly. In general, people manage ecosystems in a manner that increases short-term benefits; they may not be aware of, or may ignore, costs that are not readily and immediately apparent. This has the inequitable result of increasing current benefits at costs to future generations.
Different categories of ecosystem services tend to change over different time scales, making it difficult for managers to evaluate trade-offs fully. For example, supporting services such as soil formation and primary production and regulating services such as water and disease regulation tend to change over much longer time scales than provisioning services. As a consequence impacts on more slowly changing supporting and regulating services are often overlooked by managers in pursuit of increased use of provisioning services (S12.ES).
The inertia of various direct and indirect drivers differs considerably, and this strongly influences the time frame for solving ecosystem-related problems once they are identified (RWG, S7). For some drivers, such as the overharvest of particular species, lag times are rather short, and the impact of the driver can be minimized or halted within short time frames. For others, such as nutrient loading and, especially, climate change, lag times are much longer, and the impact of the driver cannot be lessened for years or decades.
Significant inertia exists in the process of species extinctions that result from habitat loss; even if habitat loss were to end today, it would take hundreds of years for species numbers to reach a new and lower equilibrium due to the habitat changes that have taken place in the last centuries (S10). Most species that will go extinct in the next several centuries will be driven to extinction as a result of loss or degradation of their habitat (either through land cover changes or increasingly through climate changes). Habitat loss can lead to rapid extinction of some species (such as those with extremely limited ranges); but for many species, extinction will only occur after many generations, and long-lived species such as some trees could persist for centuries before ultimately going extinct. This “extinction debt” has important implications. First, while reductions in the rate of habitat loss will protect certain species and have significant long-term benefits for species survival in the aggregate, the impact on rates of extinction over the next 10–50 years is likely to be small (medium certainty). Second, until a species does go extinct, opportunities exist for it to be recovered to a viable population size.