Habitat loss and fragmentation are the main threats to nature on islands.

Over 100 years ago the French ecologist de Candolle observed that “the breakup of a large landmass into smaller units would necessarily lead to the extinction or local extermination of one or more species and the differential preservation of others” (1).

There is increasing evidence for the negative impacts of urbanization on biodiversity, most directly in the form of habitat loss and fragmentation. Fragmentation experiments—some of the largest and longest-running experiments in ecology—provide clear evidence of strong and typically degrading impacts of habitat fragmentation on biodiversity and ecological processes. A major impact of the expansion of urban areas on native species is on their dispersal through changes in habitat configuration and connectivity.

Urbanization impacts biodiversity and ecosystem services both directly and indirectly.
Direct impacts primarily consist of habitat loss and degradation, altered disturbance regimes, modified soils and other physical transformations caused by the expansion of urban areas. The most obvious direct impact of urbanization on biodiversity is landcover change.
Indirect impacts include changes in water and nutrient availability, increases in abiotic stressors such as air pollution, increases in competition from non-native species (2).

Habitat loss, isolation and changes in microclimate are the main effects of fragmentation at landscape level which causes inbreeding and decreases seed production on plant populations and modify composition of plant communities (3).

Destruction and degradation of natural ecosystems are the primary causes of declines in global biodiversity. Habitat destruction typically leads to fragmentation, the division of habitat into smaller and more isolated fragments separated by a matrix of human-transformed land cover. The loss of area, increase in isolation, and greater exposure to human land uses along fragment edges initiate long-term changes to the structure and function of the remaining fragments (4).

Sources:

1. Conservation Biology, 1992, Chapter 8 Larry D. Harris and Gilberto Silva-Lopez
2. The Routledge Handbook of Urbanization and Global Environmental Change, 2016, Chapter 10 Thomas Elmqist, Wayne C. Zipperer and Burak Güneralp
3. Determinants of land degradation and fragmentation in semiarid vegetation at landscape scale, Yolanda Pueyo, Concepción L. Alados and Olivia Barrantes
4. Habitat fragmentation and its lasting impact on Earth’s ecosystems, Sciences Advances, March 2015, 24 authors including Nick Haddad, Andrew Gonzalez, Lars Brudvig