Bees and flies

Insects and other invertebrates are often under-appreciated even though many species perform important ecological roles to help sustain eco-systems. One such important role is pollination. Many species of flowering plants rely on insects to help them reproduce by transporting pollen from one flower to another. This is an example of a mutualistic relationship between flowering plants and insects where both individuals benefit from the exchange. The flowering plant benefits because the insect takes its pollen to another plant of the same species to help it to reproduce and as a reward the insect gets a reward of food in the form of nectar.

When we think about pollination we tend to think of bees and butterflies flying around from plant to plant drinking the nectar from flowers while transporting pollen with them to each new plant they visit but they aren’t the only insects that pollinate flowering plants. Many other insects also pollinate plants including species of beetles, moths, wasps and flies. 

Many of New Zealand’s native pollinators are quite specialised and tend to only pollinate one or a few closely related species and the survival of these species is heavily affected by habitat loss, climate change and introduced species. Habitat loss is especially an issue for specialised species because as they start losing the plants they survive on their population numbers decrease meaning there are less of them to pollinate the remaining plants. Also specialised species have to deal with competition from introduced species that can pollinate the same plants as them meaning there will be less opportunity for them to gain the food reward from the plant they pollinate. Many species are adapted to specific environmental conditions so climate change can have a huge impact on their survival because if the conditions that are ideal for their survival change too much they can die.

The relative importance of solitary bees and syrphid flies as pollinators of two outcrossing plant species in the New Zealand alpine is a paper that investigates insect pollination of flowering alpine plants which hasn’t really been studied before. It was believed that many of New Zealand’s alpine plants are autogamous meaning they self-fertilise and don’t rely on pollination which is likely because more than 70% of NZ’s alpine plants have white or near-white flowers and insects are typically attracted to brightly coloured flowers.

This paper investigated pollinator importance by measuring the amount of pollen grains deposited by on the stigma by the insect and how often the insect visited that species of plant. The results showed that Hyloeus matamoko, a species of short tongued solitary bee, was the primary pollinator of the two plant species investigated (Ourisia glandulosa and Wahlenbergia albomarginata) having done 90% of the pollinations. Syrphid flies also visited one of the plant species the same amount of times as the solitary bee but only did 10% of the pollinations meaning it was a much less important pollinator than the bee species.

Without insects around many of the flowering plants that rely on them for pollination would begin to disappear and go extinct making the world around us a bit less beautiful.

Lepidoptera

Before NZ was colonised by Europeans it was isolated from the rest of the world millions of years and evolved without the presence of mammalian predators. The long isolation and lack of terrestrial predators allowed for unique characteristics, including flightlessness, to evolve in many species. Also without the presence of many of the species evolved defence mechanisms that are only effective against their natural predators which are mostly birds. The introduction of mammals into this country has been devastating to numerous species of native/endemic flora and fauna including many invertebrates.

Due to the lack of defence mechanisms against many naturalised predators NZ has a high number of threatened and at risk species. Also many animal and plant species have to deal with competition from introduced generalist species that thrive in our environment. This is a huge problem because many of New Zealand’s invertebrate species are specialised to survive on one or a few species of plant so a decline in plant species means a decline in invertebrate species.

One of the largest orders of insects in New Zealand, after Coleoptera and Diptera, is Lepidoptera; the moths and butterflies. It is estimated that there are around 1800 species present in New Zealand with 1600 of them being endemic that’s a whole 89% of NZ Lepidopterans that are only found in New Zealand! Of the 1800 Lepidoptera species only 26 of them are butterflies (17 endemic/native) while the rest of them are moths.

The idea that the loss of plant species is affecting the loss of invertebrate species was put forward as one of the most likely explanations for why more Lepidoptera species are becoming endangered in a 2012 paper: The Conservation status of New Zealand Lepidoptera. In the paper they established that of the species they investigated in 2007, 49 taxa are threatened, 69 taxa are in danger of becoming endangered and 56 were data deficient meaning they could find enough information on the species to establish populations’ numbers. No known taxa have yet become extinct but this could easily change in the future.

As stated before it is likely that the increase in threatened/ at risk species is due to the decrease in plant species they live on. Much of NZ’s once abundant forest has been destroyed and fragmented and many species lost. It was established in the investigation that many of the Lepidopteran species lived in restricted habitats which restricts their poulation numbers. Also many species have become quite specialised including 11% of threatened/ at risk taxa which only feed on a single species of plant and another 20% that only feed on a single genus of plant.

The restrictions on the Lepidopteran species’ habitat and diet aren’t the only factors contributing to the high number of threatened/ at risk species but they are two of the most obvious. Other possible contributing factors are competition and susceptibility to parasitism/ predation which links back to the fact that many NZ species don’t have the right defense mechanisms to protect them against introduced predators and parasites and since many species are specialists, they are easily out-competed by generalist species.

Alpine scree weta

New Zealand is a unique and beautiful country that evolved for millions of years without the presence of terrestrial mammals meaning. Because of this many of New Zealand’s species had to evolve to fill the niches that mammals would normally fill causing NZ to have some unusual and wonderful fauna including the weta which is a member of the Orthopteran order. Weta are an iconic insect that has the unique role off being a seed disperser for plants with fleshy fruits; a role that is usually carried out by mammals or birds. The more commonly known seed dispersers are ants but unlike ants weta disperse the seeds after consuming the fruit so the seeds travel through the gut passage.

The alpine scree weta (Deinacrida connectens) has a mutualistic relationship with fruit bearing plants including the mountain snowberry plant (Gaultheria depressa) where the weta, in exchange for food (the plant’s fruit), will disperse its seeds. Of course since there is no such thing as true mutualism; many of the plants seeds are predated on especially by smaller weta.  

This mutualistic relationship was investigated in the paper Seed dispersal effectiveness increases with body size in New Zealand alpine scree weta (Deinacrida connectens) where snowberry seeds were fed to weta in cheese to see how many would pass through the gut passage intact. The weta were of varying sizes and it was found that the larger the weta the more seeds consumed and dispersed whereas the smaller weta acted more as seed predators and dispersed only a small number of seeds. Also the larger weta were able to disperse the seeds over greater distances meaning that the seeds can be further from the parent plant and are unlikely to compete with it for resources.

Since this investigation was carried out under lab conditions the results may not reflect what actually happens but many seeds were recovered from the captured weta before the actual investigation began. This shows that the weta do have a role in seed dispersal but the extent of that role in the wild hasn’t been studied.

A similar investigation was done with Wellington tree weta (Hemidenina crassidens) and tree fuchsia (Fuchsia excorticata) which came to the same conclusion as the other investigation that weta can be important seed dispersers in New Zealand and that the larger the weta the more seeds they can disperse over longer distances.