How does ecosystems change over time

Nelson Hairston 's lab focuses on freshwater environments, especially lakes and ponds, where some of the species present respond to environmental change with decreases in their numbers, even to the point of extinction, while others may benefit to excess, becoming so dominant that they present problems, as in the case of harmful algal blooms stimulated by nutrient enrichment or climate warming.

Hairston's lab studies how individual species, food webs, and whole ecosystems are altered when the environment changes. One way that some freshwater organisms respond to environmental change is to evolve rapidly. A marked change in the environment favors some characteristics of plants, animals and microbes over others. These character differences are often genetically based so that favored characteristics may increase in the next generation. The shorter the generation time, the faster this evolutionary change can occur.

For example, tiny but abundant plankton, eaten by fish and other larger animals, can become adapted to the changed environment within a few years because their generation time is only a few days. The speed at which a driver reacts strongly influences how quickly related ecosystem problems can be solved once they are identified.

For some drivers, such as the overharvest of particular species , lag times are rather short and the impact of the driver can quickly be reduced or stopped. Nutrient loading and, especially, climate change have much longer lag times and the effects of these drivers cannot be reduced for years or decades.

The extinction of species due to habitat loss also has a significant lag time. Even if habitat loss were to end today, it would take hundreds of years for species numbers to reach a new, lower, equilibrium in response to the habitat change that took place in the last centuries.

For some species this process can be rapid, but for others, like trees, it may take centuries. Consequently, reducing the rate of habitat loss might only have a small impact on extinction rates over the next half century, but lead to significant benefits in the long term.

Time lags between habitat reduction and extinction provide an opportunity for humans to restore habitats and rescue species from extinction. Most changes in ecosystems and their services are gradual and incremental, making them, at least in principle, detectable and predictable. However, many examples exist of non-linear and sometimes abrupt changes in ecosystems. A change may be gradual until a particular pressure on the ecosystem reaches a threshold , at which point rapid shifts to a new state occur.

Some non-linear changes can be very large and have substantial impacts on human well-being. Capabilities for predicting non-linear changes are improving, but in most cases science can not yet predict the exact thresholds. Ecosystem are resilient to disturbances until a certain threshold , meaning that they are able to withstand them or to recover from them.

Changes in ecosystems caused by humans may reduce this resilience and increase the likelihood of abrupt changes in the system, with important consequences for human well-being. The species of an ecosystem belong to different functional groups. But reality is not like that! Most of the animals in an ecosystem move during the day and some of them only appear at night.

Plants produce different edible parts depending on the season. The entire ecosystem can even change due to catastrophes like forest fires. Not to mention, we rarely even imagine the diversity that happens in the soil under our feet.

Of course, soil biodiversity also changes with time, although not necessarily the same way changes happen aboveground. First, movement is certainly more difficult in the soil. Earthworms, insect larvae, mole crickets also moles, but we are going to focus on small invertebrates , and many other tiny creatures must dig with their mouths, claws, or legs. Smaller creatures move throughout the soil mainly using tiny air-filled spaces called soil pores. Soil inhabitants are not limited to the typical horizontal movements of surface animals.

Soil invertebrates can also move up and down beneath the same surface area, which is called vertical migration. Vertical migrations can occur during a single day, or across seasons. Enchytraeids, very tiny worms, are one of the few types of soil-dwelling animals that have been observed to migrate during the day. Enchytraeids move deeper into the soil to escape from dry surface conditions at midday and return from the deep in the evening, when their favorite moist conditions are reestablished.

This migration is the basis of one of the most-used methods to study soil mesofauna. Many soil invertebrates can exist in resistant forms that allow them to survive harsh conditions for a long time. Ground pearls, small, rounded, and very interesting insects, are a perfect example. But when delicious roots are available, the cysts develop and become voracious adults. If conditions are really good, many ground pearl species can clone themselves to profit as much as possible from favorable conditions.

An unlucky vineyard farmer may not see the tiny ground pearls 1 year, but find his crops infested with adults the next. Some of these nonlinear changes can be very large in magnitude and have substantial impacts on human well-being. Capabilities for predicting some nonlinear changes are improving, but for most ecosystems and for most potential nonlinear changes, while science can often warn of increased risks of change, it cannot predict the thresholds where the change will be encountered C6.

Numerous examples exist of nonlinear and relatively abrupt changes in ecosystems:. There is established but incomplete evidence that changes being made in ecosystems are increasing the likelihood of nonlinear and potentially high-impact, abrupt changes in physical and biological systems that have important consequences for human well-being C6, S3, S The increased likelihood of these events stems from the following factors:. The growing bushmeat trade poses particularly significant threats associated with nonlinear changes , in this case accelerating rates of change C8.

SDM, C Growth in the use and trade of bushmeat is placing increasing pressure on many species , particularly in Africa and Asia. While population size of harvested species may decline gradually with increasing harvest for some time, once the harvest exceeds sustainable levels, the rate of decline of populations of the harvested species will tend to accelerate.

This could place them at risk of extinction and also reduce the food supply of the people dependent on these resources. Finally, the bushmeat trade involves relatively high levels of interaction between humans and some relatively closely related wild animals that are eaten.

Again, this increases the risk of a nonlinear change, in this case the emergence of new and serious pathogens. Given the speed and magnitude of international travel today, new pathogens could spread rapidly around the world. A potential nonlinear response, currently the subject of intensive scientific research, is the atmospheric capacity to cleanse itself of air pollution in particular, hydrocarbons and reactive nitrogen compounds C.

Once an ecosystem has undergone a nonlinear change, recovery to the original state may take decades or centuries and may sometimes be impossible. For example, the recovery of overexploited fisheries that have been closed to fishing is quite variable. Although the cod fishery in Newfoundland has been closed for 13 years except for a small inshore fishery between and , there have been few signs of a recovery, and many scientists are not optimistic about its return in the foreseeable future C On the other hand, the North Sea Herring fishery collapsed due to overharvesting in the late s, but it recovered after being closed for four years C This summary is free and ad-free, as is all of our content.

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