The influence of climate
Of course, urban areas exist in all major climate zones, from tropical to alpine. The species that colonize and thrive on each site will be mostly determined by the prevailing climate. But in every case this climate will also be modified by urban heat island effects — these usually tend to mean that the temperature is higher than surrounding land, and the humidity lower (Arnfield 2003; see Chapter 11).
Mosses and lichen communities in particular will tend to be dominated by those species with a lower requirement for ambient humidity — some are surprisingly tolerant of arid conditions.
In addition to the urban heat island, the usual factors affecting microclimate will also interact: altitude aspect and exposure are often the most significant, cities at high altitudes can receive higher precipitation and greater cold than other sites just a few miles away; on the other hand, sites in valleys, especially with rivers nearby, experience higher humidity and may even be frost pockets also experiencing greater cold than adjacent sites.
Other localized effects may include the influence of waste heat from urban industries that may warm the environment, often watercourses, to well above the norm, allowing exotic species to thrive. This is another typical factor influencing the emerging urban ecological communities, leading to blends of native and introduced species surviving in mixed assemblages. Such mingling of genes and species has tended to be regarded with suspicion by ecologists who see it as a threat to existing biodiversity. However, some alternative voices argue that such changes are the necessary response of life to a changing world and changing climate and, even, that it is through such shifts that the best adapted biota will emerge (see for example Thomas 2017). As an example of such a shift, the ring-necked parakeet (Psittacula krameri millanensis) has escaped from captivity and colonized London in large flocks (see Chapter 33).
One effect of an overall warmer microclimate could be to interfere with vernalization of plants, which in turn may have many consequences — it may prevent some plants from flowering successfully and thus inhibiting reproduction or fruit production, with consequent reduction of resource for fauna dependent on nectar and fruits. On the other hand, warmer winter temperatures enable longer growing seasons that will help those animal species that feed on stems, leaves, and roots. The widespread belief amongst gardeners that cold winters reduce insect numbers seems only partially true as many have successful overwintering strategies but it is true that a warmer winter will enable many insect populations to grow faster than they may otherwise have done — potentially damaging for plants, possibly beneficial for insect eating animals (Bale and Hayward 2010).
In warmer climates plants are likely to transpire more and growth could be limited by soil water availability so the increased temperature may or may not enhance their growth. In addition some plant pests will be more abundant which may also offset higher productivity. Higher winter survival of bark boring beetles is implicated in epidemics threatening many tree species.
At a more fundamental level, species composition may also reflect the prevailing heat pattern. It has been noted that in many cities the successful bird populations are dominated by those of desert origins. However, it has also been well established that living in an urban heat island can lead to many detrimental health impacts for people, from heat stroke to respiratory problems; it is likely that the same will be true for other urban mammals of temperate origin.
