Readers’ wildlife photos

“Wildlife” today means the intersection of humans in Sardinia with insects, and the narrative and photos come from Athayde Tonhasca Júnior. His captions are indented, and you can enlarge the photos by clicking on them.

Giving bees a hand, the Italian way

Cagliari, the capital of the island of Sardinia, an autonomous region of Italy, shares the plight of many old European cities: most of its 154,000 residents live in tightly packed buildings crisscrossed by a maze of narrow streets and alleys. Cities like Cagliari are on the far end of the urbanisation scale, where natural habitats have been mostly eliminated or severely reduced to suit human occupation. At a first approximation, such landscapes are inhospitable to wildlife.

And yet, species’ success hinges on adaptation. Some bees take advantage of city perks such as lower pressure from pesticides and higher diversity and abundance of flowers when compared to agricultural areas, and are happy to share urban spaces with human residents. Many Cagliari citizens, perhaps even unknowingly, have done their part by creating miniature bee-friendly environments in their homes and business.

Small gardens, flower strips and even flowerpots are islands of pollinator habitats. These spaces may look tiny to us, but could supply bees and other insects with lots of pollen, nectar and possibly nesting and hibernating sites. Habitat fragmentation, the almost inevitable byproduct of habitat depletion, has negative consequences for an array of animal and plant species in which survival depends on large, continuous and pristine areas. But for many bees, flower patches work as stepping stones of interconnected networks (Hinners et al., 2012).

Females of six common European wild bee species fly from 70 to 120 m in search of food in urban gardens (Hofmann et al., 2020). The larger the species, the longer its range; the large scissor-bee (Chelostoma florisomne), with a mean body length of 5.5 mm, has a maximum foraging distance of 150 m; the violet carpenter bee (Xylocopa violacea), with a body length of 24 mm, can reach up to 1,200 m (Gathmann & Tscharntke, 2002). Bees hardly ever find themselves isolated in a city street, and a flower bed on a balcony is just one stopover to be explored.

Bees can fly for relatively long distances, but they maximise energy efficiency by keeping foraging expeditions short. This has management implications: as a general rule, based on results from a number of bee species, flower patches are best placed within a few hundred metres of each other to facilitate foraging and reduce the risk of bees running out of nectar, their fuel (Zurbuchen et al., 2010).

Small is ok: small, isolated flower patches may not attract many pollinators because the amount of food available may not be worth the effort of flying to them. But insects that take their chances may be compensated by less competition and greater energy efficiency, as they are less likely to come across flowers depleted of nectar and pollen by a previous visitor. Being in a small group can be good for plants too: each has a better chance of being visited and pollinated.

The size of a flower patch is not the main determinant of the volume of nectar available to pollinators: what matters most is the quality of plants. A variety of plant species also assures nectar availability throughout the season. For example, willows (Salix spp.) and ivy (Hedera helix) are great providers of nectar during early and late seasons, respectively, when other sources are scarce (Tew et al., 2022).

Prendergast et al. (2021) reviewed 215 studies about bees’ responses to urban landscapes and concluded that some cities with moderately disturbed areas and rich floral resources can support exceptionally diverse and abundant bee assemblages. It all depends on pollinator-friendly management of domestic gardens, parks, allotments, cemeteries, road verges, brownfield sites and other spaces.

The Mediterranean Basin is one of the ‘hot spots’ for bee diversity, and Sardinia is no exception: 336 species have been recorded on the ~24,000 km² island, although the true number is likely to be higher because the bee fauna is not well known (Nobile et al., 2021). Many of these wild bees must take advantage of sympathetic urban spaces.

A great number of bee species, particularly from the families Andrenidae and Halictidae (solitary, mostly ground-nesting bees), are oligolectic, that is, they gather pollen from only a few plants, typically from the same genus or related genera. Oligolecty may sound like a poor strategy because pollen specialists may go hungry even if surrounded by flowers, if flowers are of the wrong type. But as oligolectic species make up about half of all bees in some habitats, being a fussy eater must have selective advantages. Pollen specialists may compensate for their narrow diet by gathering more pollen and faster than polylectic species (those that collect pollen from many unrelated flower species); they may have become adapted to plants’ chemical defences, thus getting resources not available to their rivals; or they may suffer less from the effects of direct competition with other flower visitors. Whatever their causes, oligolecty and polylecty have practical consequences: to help the bee fauna, we need to provide them with an array of flowers, so that even the pickiest visitor is likely to find the food it needs.

Urban areas are spatially heterogeneous mosaics of niches such as walls, fences, wooden posts, derelict structures, bare soil and road embankments. Several bees readily occupy cavities in man-made structures and patches of exposed earth to construct their nests.

The remarkable levels of resilience and adaptation of some bees to city-living may give the impression that urbanisation is benign or neutral to wildlife. By and large, it’s neither. Urban growth and agriculture expansion are among the greatest threats to global biodiversity and natural habitats. These anthropogenic effects are inevitable, but we have tools to mitigate their impact. Creating and protecting bee-friendly urban spaces is one of them, with the added advantage of helping other types of wildlife. We too reap the benefits: a city or town that’s hospitable to wild bees is likely to be a healthy and pleasant human habitat.