User:Greenmuffinsonly/Plagioclimax community

A Plagioclimax community is a stable biotic community that arose as a consequence of direct or indirect human interference. The term is most often used to describe vegetative communities, but can apply to any habitat impacted by human action. This process occurs when human activity interferes with succession of the community- either directly or indirectly. Ecological succession is a process wherein the structure of a biological community changes over time, sometimes taking decades or more to reach a mature climax community. As environments mature into climax communities, they become more biodiverse, accumulating species that are often more complex with specialized niches. Instead of reaching the climax stage naturally, plagioclimax communities arise from a succession pathway that has been either arrested entirely or deflected towards a different path. Anthropogenic activities that have historically disrupted ecosystem progression include:


 * Clear cutting existing forests and vegetation
 * Construction of infrastructure
 * Altering lakes or rivers by means of concrete retaining walls or dams
 * Controlled burning as a means of forest clearance
 * Agricultural planting of non-native trees, crops, or other ornamental vegetation
 * Grazing and husbandry of domesticated livestock
 * Harvesting of planted crops

In the case of deflected succession, a climax community is still formed. The resulting climax community may still be composed entirely of native species, but the exact community would have never occurred naturally without human interference. This community may have fewer species present than it was projected to have, a simpler food web, or skewed species compositions- sometimes dominated by certain plants or animals. In some cases, even when the pressures that caused the deflection (like clear cutting or grazing) have ceased, the ecosystem will never be able to revert and reach its original climax potential.

Arrested succession describes a biological community that is not yet fully stable, but instead has been stalled in a natural 'immature' mid-point of succession. Areas like these should be able to continue on their natural succession trajectory once the disturbance pressure has been lifted.

Examples
Overgrazing by livestock is a very widespread cause of both altered and deflected succession, and can have critical impacts on the development of ecosystems that are being used as pastures. Grasslands are often immature stages of ecosystems where the mature system is a forest, and these are favored settings for farmland and pasture. For grazing ungulates, taste and nutritional preferences mean that different species of grass are consumed at disproportionate rates, altering the community composition of the grassland. The inedible or least favored species have a competitive advantage, and slowly takeover the local ecosystem, ultimately resulting in a field with low nutritional value and a decreased carrying capacity for cattle. The grazing pressure also prevents the slow growth and establishment of seedlings and saplings that would naturally have taken over. Overgrazing can also eventually result in desertification.

The highlands of Northern England were once covered by deciduous woodland. Heather, a native flowering plant, was historically present, although in small populations. Gradually, forests in the area were removed during the early Middle Ages for timber and to create space for agricultural practices. The quality of the soil was continually deteriorating as a result, and heather began to outcompete other vegetation and dominate the local plant communities. Sheep grazing is a major agricultural disturbance in the area, and grazing pressure by these browsing ungulates prevents the re-growth of native forests by consuming young saplings. In addition, the high nutritional value of heather has resulted in controlled burning to prevent forest regrowth and to maintain heather pastures for grazing.

Overfishing can present a similar issue for aquatic ecosystems, altering the species compositions of entire coastlines or reef systems. In California, commercial fisheries in the 1940s and 1950s reduced the stock of sardines so rapidly that anchovies were able to competitively exclude the remaining populations, slowing their rebound. Sardines are an important food source for many larger ocean fish and megafauna, so the reduction in their numbers had implications for organisms throughout the entire food web. [https://www.nature.com/articles/s41598-018-35505-1#:~:text=Predation%20pressure%20occurs%20as%20a%20result%20of%20predation%20frequency%20and%20prey%20vulnerability.&text=We%20then%20identified%20septal%20structures,mean%20lifetime%20by%20approximately%2020%25. Predation pressure] increased for other small species of fish, and increased competition for food could result in starvation for cetaceans, pinnipeds, and even sea birds.

The Lasting Effects
One of the most damaging results from deflected or arrested ecological succession is the loss of local biodiversity. In a rich, natural climax community, there should be a high number of distinct species with high intra-species genetic diversity.

Species diversity has a multitude of beneficial impacts-


 * Disease resistance- If a species is more genetically diverse, the population will be less vulnerable to pathogens.
 * Prevents inbreeding- A large number of genetically diverse individuals within a species prevents inbreeding between closely related individuals.
 * Buffering against invasive species- If every available niche in an ecosystem is filled, it is more difficult for invasive species to establish and reproduce.
 * Prevention of ecosystem collapse- A complex ecosystem is more resilient to natural disturbances like floods or fires.
 * Maintaining ecosystem services- the availability of many natural medicines, food products, wild game, and natural resources are impacted by declining ecosystem health. Healthy ecosystems also filter water, cycle carbon, and dense vegetation uptakes anthropogenic carbon dioxide.

When human disturbance decreases the biodiversity, ecosystems are left vulnerable to a multitude of often irreversible issues. Unfortunately, increasing human population and demand for food means continual expansion of livestock and agricultural practices. Roughly 51% of the world's habitable land has been converted for either farming or grazing use, ultimately causing a 60% decline in the number of vertebrates worldwide since the 1970's.