Natural disasters have continuously perturbed the Earth’s biosphere and the various ecosystems within it. These adverse naturally occurring events include floods, cyclones, wildfires, earthquakes, landslides, volcanic eruptions, tsunamis, and droughts (Ayyam et al., 2019). Such disturbances result in profound modifications to the structure and functioning of an ecosystem that persist for long periods. Examples of these modifications are alteration of species composition, increased mortality rate of intertidal fauna, removal of vegetation cover, loss of predators allowing for an expansion of the population of certain species (Masuda et al., 2016), and loss of habitats and habitat boundaries resulting to hybrid communities (Roeder et al., 2018). The effects of natural disasters can be extensive as seen in the case of the Great East Japan earthquake that caused an ecosystem resetting (Siddle, 2017). Moreover, the frequency of these disturbances increased from 1975 to 2008 fourfold due to climate change reaching the global level. The years 2013 to 2020 showed a more alarming rate and impact of natural disasters, including the recent Australian wildfire and the eruption of Mount Taal. Both the extent and frequency are direct factors in the recovery of the affected ecosystem. Ecosystems have self-regulating abilities that allow them to naturally recover back to their near-equal state after environmental stress. However, having a higher rate of disturbance than its recovery time can hinder the system from reaching its previous state of equilibrium. For a recovery to fully occur, the rate of environmental changes should be slower than the innate processes of the ecosystem (Jamarillo, 2012). This paper discusses the recovery mechanisms that occur within an ecosystem after natural perturbation including its indicators and endpoints.
Response of Ecosystems to Natural disasters
The deviance from a balanced state due to stress refers to the response of the ecosystem to the disturbance. This response can be broken down into the effects on components, and on the processes within the ecosystem. According to Roeder et al. (2018), events such as floods cause a decrease in component diversity favoring those with morphological and behavioral adaptations. Examples are millipedes and bristletails that produce eggs resistant to flood. In their study, most of the taxonomic classes attained before the flood were non-existent afterward. Moreover, the species richness within the invertebrate communities declined. In another study by Jamarillo (2012), similar changes in the abundance of representative species were observed after an earthquake and tsunami resulting in local extinctions. A significant change in fauna such as the loss of sand dollars was also observed by Seike et al. (2017) after the tsunami. In addition, changes in endangered species distribution resulted from these disturbances (Sidle, 2017).
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The processes within the ecosystem also respond to the stress they are exposed to. An example is the eruption of Mount Pinatubo wherein the plant succession was reset due to the shifting surfaces and unstable canyon walls (Marler and del Moral, 2011). Landslides, that cause the mixing of deep and surface soils, affect nutrient cycling and deplete the carbon stocks in grasslands (Sidle, 2017). These responses of an ecosystem are characterized to indicate if an ecosystem has been altered by a disturbance. They also vary across ecosystems and stresses. Other examples of response are changes in carbon metabolism, altered autotrophic species composition, changes in secondary productivity of grazers, and rate of nitrification by sediment bacteria. Moreover, these responses range in importance. An example is the loss of a keystone predator that has a vital role in structuring the ecosystem than the loss of a species of fungi that have redundant species to replace it. The response will also differ whether an ecosystem is highly resistant or not. This is due to the mechanism of an ecosystem to adapt to periodic disturbances that are absorbed within the system rather than cause change to its overall basic function. In addition, chronic disturbances such as prolonged floods that do not mimic the frequency of other natural disturbances can alter the ability of the ecosystem to absorb damage and in turn, alter its properties. A prolonged flood can cause death to riparian forests than brief intermittent flooding. Once the response is characterized to have a significant change within the system, the recovery can then be observed (Dey and Schweitzer, 2014).
Recovery of an Ecosystem
To determine whether an ecosystem has started to recover, indicators are observed. These are variables that characterize the ecosystem status and reflect the biological aspects of the ecosystem. These indicators also assess the current condition of the ecosystem as well as monitor trends over time (Palmer et al., 2016). There are various recovery indicators however most are physicochemical or biological including microorganisms. A study by Rojas et al. (2016) employed soil properties and microbial communities as indicators of the recovery of wildfire-ravaged ecosystems. Specifically, the soil organic carbon (SOC), pH, and available phosphorus (P) were the parameters used to indicate environmental change due to its linked functions to plant growth and nutrient supply. The microbial indicators e.g. soil microbial activity and structure were also used because of their rapid response to disturbance compared to other living organisms and are highly suitable for detecting environmental changes. The results showed higher values of soil pH immediately after the disturbance and the year after. A decrease was then observed in the following years. The pH change correlated to the organic matter combustion and ash production, and the pH decreased as ashes were rapidly removed by wind. The same trend was observed for SOC and available P having a high value following the fire then decreased after five years. The microbial indicators showed a higher abundance of Gram-bacteria and Pseudomonas immediately and a year after the fire due to the rapid increase of nutrients from the ash in the soil following a wildfire. This resulted in an increase in microbial activity and was in unison with the above-ground vegetation recovery of grasslands. These indicators, though sensitive to changes after the fire, are more adequate for short or medium-term recovery.
In another study by Borja et al. (2010), the long-term recovery of the estuary and coastal waters along with their indicators was observed. After the disturbance, the biological indicators, specifically mobile soft-bottom macroinvertebrates, progressively increased in richness and diversity over 15 years in the inner part of the estuary. For the coastal waters, hard-bottom macroalgae recovery took 14 years. Further indicators of recovery were the colonization of demersal fishes four years after the initial recovery of the soft-bottom macroinvertebrates. This implied more complex biological interactions developing and the near complete recovery was achieved in 10 years. Similar results were seen in the study by Masuda et al. wherein the abundance of surfperch black rockfishes, and fish assemblages stabilized years after the occurrence of a tsunami. These key species have long been important indicators of ecological recovery and have specific requirements on physical and chemical variables indicating whether they are within their preferred limits. Vegetation is also an adequate indicator because it is directly correlated with the environment state and the well-being of the organisms needing quality habitats.
The recovery processes of an ecosystem are highly dependent on its resilience. An ecosystem’s resilience refers to its ability to recover its structural and functional integrity after disturbances. Natural disturbances can decrease ecosystem resilience and can cause abrupt shifts to new states (Jones and Schmitz, 2009). A more resilient ecosystem has a greater abundance of species' functional traits. Moreover, the more diverse the species are in an ecosystem, the higher the capacity to maintain function. However, when the resilience is exceeded, the system can shift to a new state with different functions and the probability of returning to its previous state is low (Walker et al., 2016). A complete recovery to the previous state of equilibrium is rare but ecosystems do improve in biodiversity and ecosystem after disturbance. According to Jones et al. (2018), aquatic ecosystem recovery averages 10 years while terrestrial ecosystems take about 42 years. Higher trophic levels are correlated with faster recovery than those in lower levels.
After an ecosystem’s recovery process, it then reaches an endpoint. Endpoints are values of biological or chemical variables measured to determine whether an ecosystem has restored its structure, composition, and function before significant ecological loss. The endpoint has been reached if the pre-disturbance flora and fauna are present, healthy, and productive. However, the time from recovery to the endpoint varies throughout an ecosystem and this may not completely indicate an end to the recovery. These endpoints are based on pre-impact data to serve as the basis for the recovery success. The index of biotic integrity (IBI) is one of the important endpoints of recovery that verifies if the affected area has restored its biological integrity parallel to the reference condition from the same ecoregion or areas of similar geological history, soil, and natural vegetation. The basis of this index is the species composition, presence of indicator species, trophic feeding dynamics, and the abundance of individuals. Another endpoint uses macroinvertebrates and fishes that asses the aquatic macroinvertebrate assemblages along with their structure and function. These include recovery to average individual size, recovery to former density, recovery of species richness, recovery of total biomass, and return to a relatively stable population level. Biological endpoints provide more reliable measures of ecological recovery because they integrate conditions around them including the changes in their environment which also explains the use of invertebrates and key indicator species (Simon, 2002).
Irreparable Effects of Natural Disasters
An ecosystem subjected to large-scale and frequent disturbance may not attain full recovery resulting in an abrupt shift to a state. An example is the 2011 tsunami in Japan that removed all the seagrass and the substrate in the area. This resulted in the bottom being covered in fine silt sediment and no stable vegetation grew even five years after the disturbance. The fish community adapted to these effects as shown in the increase in fish abundance and biomass wherein the seagrass-dependent species were replaced with less dependent species. Wildfires also cause irreparable impact on forests and drought can further hinder the growth of new trees. The ecosystem adapts to this replacing the moisture-dependent trees with drought-tolerant shrubland and coniferous trees regenerating in higher elevations where there is more moisture availability (Rumann et al., 2018).
Conclusion
Recovery mechanisms after natural disasters vary across different ecosystems. This includes the recovery time, indicators, and endpoints. Studying these innate recovery processes can aid in implementing effective restoration strategies. Furthermore, allowing ecosystems to repair themselves to gauge their ability to recover followed by identifying when active restoration with human intervention is implemented will be most effective in restoration efforts.
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