Changes to Biodiversity

As discussed, biodiversity has a very powerful impact on the health of an ecosystem. The greater the biodiversity, the greater the resilience and the better ecosystems are able to withstand disruptions. However, disruptions can still happen, and biodiversity levels can change.

Indicator Species

Indicator species are organisms whose abundance, or even presence vs absence, reflects (or "indicates") environmental conditions and can relay information on the health of the ecosystem.

As an example, take a look at the image above. These are indicators for the pollution level of freshwater ecosystems. As the pollution level increases, different populations begin to be negatively affected. By observing and monitoring their population sizes, it is possible to determine how the particular condition has changed.

Disruptions to Ecosystems

When discussing disruptions to ecosystems, it is most common to imagine human-caused disruptions. While these are significant and can have drastic impacts on biodiversity and ecosystem health, they will be discussed elsewhere. There are a variety of natural events that can impact ecosystems, with some of them having even greater effects than many of the human impacts.

The processes that result in these disruptions can occur with a range of frequencies.

Periodic disruptions occur with a regular frequency. An example of this is the yearly rainy and dry seasons seen in African safaris.
Episodic disruptions occur occasionally, but with an irregular frequency. An example of this is El Niño or La Niña, which occur roughly every 2-7 years.
Random disruptions occur, well, randomly - they do not have a regular frequency. This is like when asteroids crash into Earth.

One of the largest disruptions that can happen is a change in Earth's climate. What we are currently experiencing is anthropogenic (human-caused) climate change, and it is occurring faster than it has before, which results in more severe effects and more damage to species (as is discussed here), but it has occurred due to natural events in the past. The graph below shows changes in climate over the past 800,000 years - as you can see, the spike we are currently witnessing in atmospheric carbon is significantly greater than any other experienced in the past 800,000 years.

The climate has changed over Earth's history for a variety of reasons.

The gravitational pull from other celestial bodies in our solar system can alter Earth's tilt and orbit, which affects the amount of solar insolation received.

As continental drift has occurred due to the movement of tectonic plates, ocean currents have shifted. This can also have drastic effects on local climates due to moving habitats closer to or farther from the equator, changing whether an area is landlocked or coastal, as well as the creation of geologic structures such as rifts and mountains.

In addition, the amount of atmospheric CO2 has varied over Earth's history.

All of these can have profound impacts on climate, which greatly affects the species living in the changed climate. As different species of plants have different ranges of tolerance, habitats can change due to altered dominant plant species.

The amount of glacial ice present on Earth has changed drastically with the climate. During ice ages, there is significantly more ice. As the Earth warms, the ice melts and sea levels rise. Along with impacting species that rely on the ice, this can alter habitats as well. Coastal habitats can be flooded, estuaries will increase in salinity, and wetlands can be displaced.

Various other disruptions can impact the geologic landscape and alter habitats, such as meteor impacts, volcanoes, and earthquakes. The first two can affect the climate in the short-term by blocking the sun, and they can all create new landscapes and damage existing ones.

Many organisms migrate, and several due it normally, such as the regular wildebeest migration that matches the wet/dry season in their habitat. Ecosystem disruptions can lead to new or altered migrations. One big example of this is when organisms have extended ranges. If global temperatures are increasing, tropical species will be able to survive in areas that were formerly too cold for them.

Many species act based on seasonal cues, such as temperature. These "actions" include things such as migration, mating, hibernation, foraging, etc. An alteration of migration patterns, even if only by a few days or weeks, can have major effects on the ecosystem, such as by causing a mismatch in organisms and their food source.

Ecological Succession

After a disturbance, it is extremely common to witness massive changes to the community. Ecological succession is the series of changes that a community undergoes, in which the species present are replaced by other species in a series of such changes. This leads to changes in the biomass, species richness, and NPP of the community.

Think about the word succession in terms of royal families and a monarchy. The king's successor becomes the next king after him. Simba succeeded Mufasa as the leader of the Pride Lands. In ecological succession, a community is replaced by the next one - just like how the king is replaced by the next king when he is succeeded.

Primary Succession

Sometimes, this process occurs in an area that has not yet been colonized before. Imagine an area of bare rock, such as a new volcanic island or moraine (rocks and sediment) left after the retreat of a glacier. The most common pioneer species - the first to colonize the area - is lichen, a composite organism of fungi and algae that works to break down the rock and create soil. As it takes a long time to create soil, depending on the weathering of rock and decomposition of organic matter, primary succession takes a long time, often hundreds to thousands of years.

Secondary Succession

If an existing community has been destroyed, but the soil is still present (such as after a wildfire), the process is much faster. Wind-dispersed or animal-dispersed seeds typically result in the earliest plants that colonize the area. This process may result in a climax community that resembles the original one before the disturbance, but it is possible for it to be significantly different as well.

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