In Chapters 3 through 6 we have elaborated on how the City of Melbourne’s public green spaces support insect biodiversity. And in Chapter 7 we introduced the idea that this insect biodiversity does not stand in isolation, but is interconnected in a complex network of ecological interactions.
In the present chapter we use our targeted survey data to illustrate:
(1) how insect species and their life history strategies and interactions are directly linked with urban ecological processes, and
(2) how these processes are linked to ecosystem services that benefit human city-dwellers.
Here, we follow the classification of urban ecosystem services reported in Gómez-Baggethun et al. (2013), who grouped ecosystem services in four categories: (i) regulating, (ii) provisioning, (iii) cultural, and (iv) supporting and habitat. We only documented the first two being delivered by
insects here. Furthermore, we will illustrate how the life history strategies of some insect species are also linked to a series of ecosystem disservices that may affect urban inhabitants.
Regulating ecosystem services
We begin by exploring the links between insects, their feeding strategies, the ecological processes these strategies generate, and the regulating ecosystem services that these processes have the potential to deliver (Figure 8.1). Our findings indicate that a total of 71 links connect the insect groups investigated with the feeding strategies they employ to complete their life cycles (left part of Figure 8.1). The most dominant feeding strategy amongst insect in the City of Melbourne’s was herbivory, accounting for 46% of all links. Herbivores may be further sub-divided in seven feeding specialisations (dot-margined box in Figure 8.1): (1) exudativores,
specialising on plant and/or insect exudates such as sap, gum and honeydew; (2) folivores, specialising on leaf tissue; (3) graminivores, specialising in grasses; (4) nectarivores, specialising in pollen; (5) palynivores, specialising in pollen; (6) granivores, specialising in seeds; and (7) xylovores, specialising in wood. Of these, the folivores were the dominant group, accounting for 46% of all herbivory links.
The second and third most dominant feeding strategies were predation and parasitoidism, representing 19% and 17%, respectively, of all links. The less dominant strategies were detritivory (ie, consumption of decomposing organic matter), scavenging (ie, consumption of dead organic matter), fungivory (ie, consumption of fungi) and elaiosome consumers (ie, consumption of nutritious structures attached to seeds). These strategies together accounted for 18% of all links.
Our results also show that the feeding strategies employed by insects within the City of Melbourne’s public green spaces may be linked with at least four key ecological processes occurring within these urban ecosystems (green box in Figure 8.1):
(1) transformation of nutrients into biomass, a process whereby insect-consumed plant, fungi and
animal nutrients are assimilated and metabolised, guaranteeing the flow of matter and energy through the food chain; (2) nutrient re-cycling, a process whereby scavengers, detritivores and xylovores facilitate decomposition, and thus the movement of nutrients back into the soil and water; (3) biotic pollination, a process whereby nectarivores and palynivores, but also other flower-visiting insects, enable plant fertilisation by transferring pollen grains to the plant’s female reproductive organs; and (4) ant-mediated seed dispersal, a process whereby adult ants transport elaiosome-bearing seeds away from their parent plants. Our insect data suggests that this later process is being carried out in the City of Melbourne’s public green spaces by ant species of Aphaenogaster, Iridomyrmex and Rhytidoponera (A Andersen, personal communication).
Finally, our findings indicate that the ecological process mediated by insects in public green spaces within the City of Melbourne may contribute to deliver at least four regulating ecosystem services (blue box in Figure 8.1): (1) biological pest control, which is delivered when insect natural enemies (also called biological control agents) regulate populations of insect pests, noxious weeds and
Figure 8.1 Relationships between insect orders, feeding strategies, ecosystem processes and regulating ecosystem services. DIP: Diptera;
HEM: Hemiptera; HYM: Hymenoptera; LEP: Lepidoptera; IS: Immature stage; A: Adults. The black down-pointed arrows indicate that the
plant diseases; (2) soil fertility, which is delivered when insects contribute to retain in the soil basic plant nutrients such as nitrogen and phosphorous, as well as soil-improving organic matter; (3) pollination of crop and ornamental plants, which is delivered when insect specialised pollinators and other flower-visiting insects contribute to fertilise urban crops (eg, fruits and vegetables) and plants that are grown for decorative purposes;
and (4) persistence of myrmecochorous plants (ie, plants that are naturally dispersed by ants), which is delivered when the viability of plants with elaiosome-bearing seeds is increased through ant-mediated seed dispersal.
Provisioning ecosystem services
Provisioning ecosystem services are goods that are obtained directly from ecosystems, for example food, water, wood and medicines (Gómez-Baggethun et al. 2013). Our results indicate that the City of Melbourne’s insects may supply at least two types of food: honey and lerps. Honey is mostly produced by social bees (Hymenoptera:
Apidae). In our study, we documented only one species of honey-producing bee, namely the
non-native European honey bee Apis mellifera (Panel 6 and Panel 30). Major initiatives are presently being conducted in the City of Melbourne to raise awareness of the importance of honey-producing bees such as Apis mellifera (see for example Melbourne City Rooftop Honey 2015).
Lerps are crystallised protective structures made out of the sugar-rich liquid honeydew exudated by the immature stages of jumping plant lice (Hemiptera:
Psyllidae). Although not that well-known as a food source to most people, lerps are one of the main types of sweet foods gathered and consumed by Aboriginal Australians (Turner 1984).
Ecosystem disservices
Next, we explore the links between insects and the ecosystem disservices that they may cause (Figure 8.2). Our findings indicate that insects found in the targeted insect survey may potentially cause one or more of the following six ecosystem disservices (red box in Figure 8.2): (1) human discomfort, for example a skin rash produced by a mosquito’s bite; (2) allergic reactions, which for example may follow the injection of venom from a wasp’s sting;
Figure 8.2 Relationships between insect orders, life history strategies and ecosystem disservices. DIP:
Diptera; HEM: Hemiptera; HYM: Hymenoptera; LEP: Lepidoptera; IS: Immature stage; A: Adults.
(3) transmission of human diseases, for example diseases carried by blood sucking insects such as mosquitoes; (4) plant damage, for example the English elm leaf ‘skeletonisation’ (ie, the whole leaf is eaten except for its veins) caused by the leaf elm beetle Xanthogaleruca luteola (Figure 6.1);
and (5) damage to stored products, for example when the red-rust flour beetle Tribolium castaneum (Coleoptera: Tenebrionidae) infests stored cereal grain. At least two types of insect behaviours, namely feeding and defensive (orange box in Figure 8.2), are associated with causing ecosystem disservices. For example, hematophagous insects such as mosquitoes may cause discomfort, allergies and/or vectorise a disease in humans by the simple act of feeding on its preferred source of food (ie, human blood). On the other hand, the non-native European wasp Vespula germanica may on occasions feel threatened by people, for example if a person inadvertently gets to close to its source of food. This action may trigger the wasp’s innate defense mechanisms, which may lead them to attack and sting the person.