Conversion of Natural Habitats
The driving force for most landscape conversion/land-use change is human population growth. With 40% of the American population living in coastal counties occupying only 10% of the nation’s land area, coastal land use conversion is an acutely important factor in managing our estuarine waters and resources. Between 2000 and 2019, populations have increased in Alabama’s coastal counties, Baldwin and Mobile, by 57.9% and 3.3%, respectively, with a total increase of 94,895 persons. The addition of this many people cannot be accommodated without significant conversion of previously natural landscapes to residential, commercial, and industrial land uses. The following figures represent changing land uses historically in Alabama’s two coastal counties from 2001 to 2006 to 2011 to 2016 and the areas of increased urban.
Conversion of natural landscapes, like forests, savannas, and wetlands, to agricultural or urban uses eliminates the ecosystem services – or benefits humans gain, free of charge, from properly functioning ecosystems – they once provided. For example, land use changes reduce primary production (conversion of carbon dioxide by plants using sunlight to form glucose and oxygen), rainwater infiltration, and water purification and retention. Natural landscape conversion is a common source of coastal ecosystem stressors, factors or phenomena that negatively impact waters or habitats, like habitat fragmentation and loss and increasing volumes and velocities of stormwater runoff, streambank erosion, and sedimentation.
Habitat Fragmentation. A primary result of converting natural landscapes, fragmentation of habitats is the single stressor impacting most of the coastal habitat types. When natural landscapes are cleared for development or farming, large continuous tracts of natural landscape are divided into smaller, separate, “leftover” islands isolated from each other by cropland, pasture, or pavement. These patches can only support smaller populations of species with less generic diversity and vigor. Populations maintain genetic diversity through migration, which is also disrupted by fragmentation. Habitat fragmentation reduces biodiversity and forces animals into smaller habitat patches, leading to overcrowding and intense competition for food, space, and other needs.
Streambank Erosion and Sedimentation. Urban development transforms portions of natural landscapes to hard surfaces, like rooftops, driveways, roads, parking lots, patios, sidewalks, compacted soils, etc., to which we collectively refer as “impervious cover.” Naturally vegetated landscapes protect soils from raindrop impact, increase infiltration of rainfall (thereby decreasing runoff volumes), reduce the velocity of stormwater runoff, and hold soils in place with root structures. Through the process of evapotranspiration, water present in the soil is pulled up through plant roots and evaporated from leaves into the atmosphere, increasing the soil’s capacity to hold water and reduce runoff. In the natural, undisturbed environment, rain that falls is intercepted by trees and other vegetation and/or infiltrates, or soaks, into the soil. When permeable soils are present, stormwater runoff usually only occurs with significant precipitation events.
When a landscape is urbanized, large areas are covered with impervious surfaces. With increased imperviousness, runoff occurs even during small precipitation events, which normally would have been absorbed by soils and vegetation. Cumulative impacts of land cover changes result in altered hydrology, producing increased runoff volumes and peak runoff velocities, greater connectivity between impervious areas, and loss of soils and vegetation that previously slowed or reduced runoff prior to development. Degradation of aquatic ecosystems occurs when hydrology of a drainage area is altered by large increases in impervious cover. The collective force of the increased stormwater runoff scours streambeds, erodes and collapses streambanks, and causes large quantities of sediments and associated pollutants to enter streams every time it rains. This sediment impacts downstream wetlands and fishery nursery habitats, like submerged aquatic vegetation (SAV)/seagrass beds.
Dredging/Filling. Dredging involves removal of accumulated sediment from waterway bottoms so vessels can navigate and operate. Filling is used primarily to convert wetlands or open water to upland residential, commercial, or industrial land uses. Together, these processes destroy wetlands, marshes and important aquatic edge habitats. They impact shorelines, and benthic (bottom) habitats by decreasing sediment where it has been lost, and, in the process, impacting benthic invertebrates, or by increasing sedimentation and covering/smothering benthic habitats and communities.