Europe has experienced an increase in temperature by more than 1.2 °C so far (IPCC, 2007), with a further increase of 1.0–5.5 °C expected by the end of the 21st century (Christensen et al., 2007). Already southern Europe has experienced extremely dry weather conditions, with rainfall decreasing by up to 20 % during the 20th century. In northern European countries, meanwhile, precipitation increased by 10–40 %. The frequency of extreme weather conditions is expected to increase (EEA‑JRC‑WHO, 2008). Europe's snow cover has decreased by 1.3 % per decade during the past 40 years. And the average duration of ice cover on lakes and rivers in the northern hemisphere has been decreasing at a rate of 12 days per hundred years (EEA‑JRC‑WHO, 2008).An indicator based on observed populations of 122 common bird species across 18 European countries alongside climatic envelopes shows that rapid climate change in Europe in the past 20 years has strongly impacted these bird populations. Three‑quarters of the populations declined as a result of climate change, A tipping point is a situation in which an ecosystem experiences a shift to a new state, with signif changes to bd and the services to people it underpins, at a regional or global scale. These can rarely be predicted with precision.
The mounting pressures on bd risks pushing some ecosystems into new states, with severe ramifications for human wellbeing as tipping points are crossed. Wile the precise location of tipping points is difficult to determine, once an ecosystem moves into a new state it can be very difficult – if not impossible – to return it to its former state. (GBO-3 page 72; CBD)A tipping point is defined, for the purposes of this Outlook, as a situation in which an ecosystem experiences a shift to anew state, with significant changes to biodiversity and the services to people it underpins, at a regional or global scale.Tipping points also have at least one of the following characteristics:✤ The change becomes self-perpetuating through so-called positive feedbacks, for example deforestation reducesregional rainfall, which increases fire-risk, which causes forest dieback and further drying.✤ There is a threshold beyond which an abrupt shift of ecological states occurs, although the threshold point can rarelybe predicted with precision.✤ The changes are long-lasting and hard to reverse.✤ There is a significant time lag between the pressures driving the change and the appearance of impacts, creatinggreat difficulties in ecological management.
Competition between grasses and woody vegetation in a semiarid environment. Suppose that either the grass or the woody vegetation has an advantage when at high densities relative to the other. In such a case, the system has stable equilibria that correspond to high levels of grass and woody vegetation, respectively. The competition is also influenced by the stocking rate of cattle, which consume grass but not woody vegetation. We shall regard the two plant forms as the dynamic state variables, and the stocking rate as a slowly varying parameter. Imagine starting with high levels of grass and low levels of woody vegetation. At low levels of stocking, there is only a small difference from the ungrazed system: if the system starts out with grass dominant, grass will continue to dominate. As stocking increases, the competition may favor woody vegetation. Eventually, there may be a collapse of the grass, and woody vegetation will dominate. Thus, the effect of grazing is to move the system from a state in which grass dominates to one in which woody vegetation dominates. Even when grazing pressure is relaxed, there may be little change in composition, because of the advantage enjoyed by woody vegetation over grass when the former is dominant. The effect of grazing is to move the system into the domain of attraction of woody vegetation for the ungrazed system. If one plots grass density vs. the stocking level, the behavior may appear to be inexplicable: the grass level declines as grazing increases, but does not return to former levels when grazing returns to its former level. The apparent paradox is resolved if we realize that the density of grass depends not only on the stocking level, but also on competition with woody vegetation.
A Forest Ecosystem will reorganize after a fire if there are areas in the landscape not hit by a fire containing ecological memory