Quantifying the influence of urban land use on mangrove biology and ecology: A meta‐analysis

Abstract

Aim The aim was to quantify the influence of urban land use and urban populations on mangrove systems around the world over the past four decades. Location Global. Time period 1997–2015. Major taxa studied Avicennia, Laguncularia, Rhizophora, Aves, Actinopterygii, Crustacea and Mollusca. Methods This review extracts results of mangrove studies on forest cover and structure, nutrient dynamics, sediment contamination and faunal community assemblages conducted around the world between 1997 and 2015. These observations are then correlated with surrounding social–ecological spatial characteristics pertaining to urbanization. Results Mangrove coverage in large cities is mostly decreasing, and at a greater rate than means for corresponding countries. Both expanding urbanization and existing agriculture are most strongly correlated with mangrove losses, which are primarily occurring 5–10 km from urban areas. Along a gradient of surrounding urban land use, mangrove leaf δ15N content increases, and leaf carbon to nitrogen ratios decrease. Sediment concentrations of almost all examined heavy metals are strongly and positively correlated with both surrounding urban and mangrove extent. The diversity of subsistence fisheries increases significantly with surrounding mangrove extent. Worm diversity decreases with increasing surrounding urbanness (urban index), and mangrove specialist bird diversity increases with increasing surrounding urban coverage as well as with mangrove coverage. Main conclusions Mangroves increasingly coexist within highly anthropogenic landscapes along urbanized coasts. Most ecological reports of these systems, however, lack quantified representations of the urban landscape, such as population density or impervious surface coverage. The results of this analysis suggest that some of the reported patterns of forest coverage, nitrogen dynamics, heavy metal contamination and faunal community assemblages in isolated urban mangroves are systemic across the world. These findings are particularly pertinent to 21st century tropical coastlines, which will see some of the greatest urbanization rates over the next century and will guide urban mangrove ecology towards the development of more synergistic social–ecological systems.

Mangrove leaf and root traits and their relation to urbanization

“…Percent nitrogen of roots as well as leaves in all three species increased with either nitrogen, phosphorus, or population density. This agrees with other studies on both mangroves and terrestrial plants and is hypothesized to be a response to nutrient enrichment in urban environments sourced from both sewage and roads (Kendall et al 2007;Branoff 2017b).…”

“…These forested wetlands are subjected to waters of varying temperature, salinity, and flood frequency, which can have marked influence on plant physiology and the resulting leaf and root traits (Cintrón et al 1978;Ernesto Medina 1999). Both δ 15 N and δ 13 C in mangrove leaves have been shown to respond to urbanization as predicted from studies on terrestrial systems (Branoff 2017a;Branoff 2018a). But these same metrics are also known to change in response to natural variations in nutrient limitation (McKee et al 2002), and as plants adjust their water-use efficiency in response to non-anthropogenic influences of nutrient limitation, salinity, and flooding (Lin and Sternberg 1992;E Medina and Francisco 1997;Ernesto Medina 1999;Castañeda-Moya et al 2011).…”

Mangrove forest structure and composition along urban gradients in Puerto Rico

“…Urbanization presents a particular challenge to tropical coastlines and especially Caribbean mangrove systems (Baird, 2009;McGranahan, Balk, & Anderson, 2007;Seto et al, 2012). Conversion of mangrove forests to developed lands has been observed to occur more quickly around some of the largest cities in the world (B. L. Branoff, 2017), thus urban mangrove conservation is an important priority for these systems. But understanding how the remaining forests function in their urban surroundings will also be an important component of management towards optimizing ecosystem services.…”

“…Globally, mangrove coverage in the largest cities is decreasing faster than overall rates from corresponding countries (B. L. Branoff, 2017). In some cases, this loss results in fragmented forests consisting of novel species assemblages and size classes (Benfield, Guzman, & Mair, 2005;Mohamed, Neukermans, Kairo, Dahdouh-Guebas, & Koedam, 2009;Nortey, Aheto, Blay, Jonah, & Asare, 2016;Zamprogno et al, 2016).…”

Urbanization plays a minor role in the flooding and surface water chemistry of Puerto Rico’s mangroves

“…This is corroborated for forested wetlands in general (Faulkner 2004), but mangroves are unique in their tidal connectivity and specific examples from urban systems are scarce. The influence of municipal sewage discharge on mangroves has been well documented and generally leads to nutrient enrichment, heavy metal contamination, and sometimes mortality (Clough et al 1983; Mandura 1997; Wong et al 1997; Branoff 2017). Further, a number of studies point to changes in mangrove coverage following large engineering projects that alter surrounding geomorphology, but these are not necessarily related to urbanization, or use qualitative or otherwise ambiguous definitions of “urban” (Colonnello and Medina 1998; Tian-Hong et al 2008; Sakho et al 2011; Marois and Mitsch 2017).…”