Hopped out

Stand at the side of a mainland island reserve and the impact of humans on New Zealand’s natural environment is obvious. From a landscape naturally dominated by tall forest, agricultural ‘improvement’ rapidly moved us to a uniform, virtual biological desert. Not only are the trees and birds missing, but the lichens, fungi, insects, worms and molluscs are gone. even the bacteria and other microbes of the soil are replaced. In response we resort to counting species and prioritising conservation efforts on the scarcest and restoration effort on the rarest habitats. But, wholesale environmental changes alter no just the abundance of native species but their ecology and interactions. Ultimately, by restructuring the landscape we alter evolutionary outcomes and this has become increasing apparent as research explores biological responses to human induced climate change.

Predator fence on boundary of native forest and exotic paddock. Bushy Park sanctuary near Wanganui.

An obvious difficulty with understanding environmental change is that it is much easier to say what is, compared to what was. We are readily inured to the situation and so are accepting of the status quo. One very powerful tool that has helped biologists understand how the geographic ranges of species and population change over time is phylogeography. Simply put, this approach combines information about where individuals and populations of a species are found with information about how those individuals are related to each other. DNA sequence data reveals how closely related individuals are (their genealogy), and how genetically diverse populations are. It is this type of data that shows, for instance, how our human ancestors left Africa and migrated into Europe, then Asia before eventually colonising islands in Oceania. We now know that New Zealand was probably the last major island to have be reached by people travelling by foot and finally boat.

Genetic data from living people has revealed how their ancestors migrated from Africa around the world (values are years before present).

Since the 1990’s phylogeographic studies have revealed the influence of many environmental factors on the distribution of biodiversity. In particular, natural, global climate cycling during the last few million years of Earth’s geophysical prehistory (the Pleistocene epoch) is known to have been influential. We now know for example that in the northern hemisphere repeated extension of the arctic ice cap during ‘glacial’ episodes extinguished populations of all species in northern Europe, Asia and America; remnant populations survived in warmer southern areas. As climate alternately warmed and cooled over 10–100 thousand year cycles, the ranges of animal and plant species expanded and retracted in response.

Estimated distribution of vegetation types in New Zealand during the Last Glacial Maximum. See Wild Life New Zealand.

 

In New Zealand a related pattern of species range change has been inferred. Pollen records show where plant species once lived and genetic data show that during cold phases of the Pleistocene, forest reduced and was replaced in many areas by scrub / grassland communities. Animal species are expected to have responded to these changes tracking their preferred habitat in space and time (or going extinct), and this has been found to be the case for some. North Island tree wētā, for instance, appear to have tracked climate niche.

 

Phaulacridium marginale

A recent study examined the response of two related grasshopper species. These endemic Phaulacridium grasshoppers live in low elevation habitat, but as is typical of short-horn grasshoppers in temperate regions they require open habitat so they can gain heat by basking in the sun. That means Phaulacridium grasshoppers do not live in forest, and they do not survive above the treeline in the subalpine zone where cool temperatures prevent trees growing (other grasshoppers are adapted to those conditions). So space for Phaulacridium would have been restricted in prehuman New Zealand to scarce open areas such as coastal dunes, river flats, wetlands and semi-arid areas.  In fact, one species (Phaulacridium otagoense) occurs today only in the semi-arid McKenzie – Alexandra area of Central Canterbury and Otago. The other  species (Phaulacridium marginale) is today found in many places around the country.

Vegetation types across New Zealand before arrival of people (left) and in modern times (right).
Known occurrences of the two New Zealand Phaulacridium grasshoppers.

A small species range usually means a small population size, compared to a species with a big range; and small populations usually have a lower level of genetic variation. Low genetic diversity is documented in many endangered species such as the famous black robins of the Chatham Islands. Paradoxically, in Phaulacridium the opposite pattern exists; the species with the smallest range (pink in map) has much higher genetic diversity than the widespread more common species. The simplest explanation is that P. otagoense (pink),  had until recently a much larger range and so bigger population. Conversely, P. marginale (turquoise) appears to have expanded its range recently and has not yet had time to accumulate new genetic diversity.

Niche models for Phaulacridium otagoense indicating optimal habitat (red, orange) during the last glacial phase may have been similar to today.

It is known that global temperatures had recovered from the last cold phase of the Pleistocene by about 15,000 years ago. Perhaps P. otagoense had a much larger range in the period before that when cooler, drier conditions allowed scrub grassland to expand; similar to conditions where it occurs today? Niche modelling indicates that in current conditions the potential range of this species is bigger than the actual range in which it is found, and  taking into account estimated temperatures during the last glaciation suggests that the habitat preferred by this species had not been much more extensive.

So, probably the major change in fortunes for these Phaulacridium species relates mostly to the recent expansion of P. marginale. Climate modelling shows that the range of this species today is close to the potential occupiable range, but there is a problem. Although the climate across much of New Zealand suits this grasshopper, other factors in the environment do not. In particular, the presence of native forest excludes these little grasshoppers because they need to bask in the sun every day to warm up. How has P. marginale become so abundant and widespread?

The answer lies not in global climate change, but in recent anthropogenic changes to the environment much closer to home. By removing New Zealand native forest, humans created a landscape with the climatic conditions to allow P. marginale to increase in abundance and expand its range across the country. The addition of a mix of northern hemisphere grasses and herbs that thrive in this artificially open environment provided the nutrient-rich food for P. marginale. So that’s great! Well no.

Males and females of different species are capable of reproduction when they meet due to anthropogenic habitat change, resulting in loss of diversity. Male Phaulacridium otagoense with female P. marginale.

The increase in available habitat has meant that the spatial range of P. marginale now meets the range of P. otagoense. Where they meet, the grasshoppers makes mistakes when choosing mates resulting in gene flow. Genetic evidence shows that pure P. otagoense remain in only part of their natural ecological range. Genetic mixing is part of the natural evolutionary mill, but around the world human activity has accelerated the rate at which species meet and interact in new ways. This adds to the trends of biological homogenisation that characterises biodiversity loss in the Anthropocene.


Original science:

Bulgarella M, Trewick S, Minards NA, Jacobson MJ, Morgan-Richards M. 2014. Shifting ranges of two tree weta species (Hemideina spp.): competitive exclusion and changing climate. Journal of Bioleography 41: 524–535.

Goldberg J, Morgan-Richards M, Trewick SA. 2015. Intercontinental island hopping: colonization and speciation of the grasshopper genus Phaulacridium (Orthoptera: Acrididae) in Australasia. Zoologisher Anzeiger 255: 71–79.

Sivyer L, Morgan-Richards M, Koot E, Trewick SA. 2018. Anthropogenic cause of range shifts and gene flow between two grasshopper species revealed by environmental modelling, geometric morphometrics and population genetics. Insect Conservation & Diversity 11: 425–434.

Media:
NZ Herald – hoppers
NZ Herald – Mackenzie biodiversity

Sticky sex

New Zealand stick insects have invaded the United Kingdom, but in the process they have lost the ability to reproduce sexually. This is odd because the vast majority  (more than 99%) of  multicellular creatures (primarily eukaryotes) engage in sex during reproduction.

Sex involves two individuals with different properties. Typically one sex (the male) produces abundant small and often motile gametes that carry genetic information to the larger egg produced by the other (female). Through this process, genetic information is passed from two parents to their offspring and results in shuffling of genetic variation. The results are readily evident in the variation seen among offspring that is prominent in human families.

Stick insects are (mostly) no exception even though scientist can show that reproduction without two sexes can have a numerical advantage over sexual reproduction. Simply, females that make only self-fertile daughters leave more of their genetics to future generations.  Theoretically it seems that clonal reproduction is advantageous, as long as the environment does not vary too much; producing offspring that are not the same as the parent could make some of them less successful. It is telling then, that despite the numerical advantage of clonal reproduction, that vast majority of large organisms do use sexual reproduction. Natural selection has made its choice.


One group of New Zealand stick insects includes individuals that differ in colour, size, and shape. In particular the number and size of spines they have varies among individuals. This group (genus Acanthoxyla) includes several described species, although in this case defining species is difficult. All are female, which means all come from self-fertile eggs produced by one parent (the mother). Hatchlings grow up to look like their mums, so are effectively clones.

Among the many individuals of common and widespread Acanthoxyla (literally: prickly stick) observed in New Zealand, no male has been encountered. Yet. But recently a male belonging to this genus turned up in England.

Rare males like this emerge among all-female stick insect populations, probably as a result of a random mutation deleting one of the XX sex chromosomes that denotes a female stick insect. XO individuals are male in appearance, but are usually not reproductive.


Research on the New Zealand genus Clitarchus has been revealing about the switching between sexual and asexual (all female) reproduction. As reported in Nature a population of Clitarchus hookeri accidentally introduced to the UK about 100 years ago has lost not only its homeland but also its sex life.

Analysis of genetic variation shows that the origin in New Zealand of the UK stick immigrants was most likely in Taranaki, North Island. This agrees with historical records indicating that native plants collected in this area were shipped to England and then the nearby Isles of Scilly. In particular the Abbey Gardens on Tresco are now home to a range of New Zealand plants, and it is likely that stick insect eggs in the soil around plant specimens were accidentally transported around the world. Hatchlings that grew into adult stick insects able to produce abundant self-fertile females were likely at an advantage. The potential of this species to switch to asexual reproduction has also resulted in a pattern of geographic parthenogenesis in New Zealand.

Genetic variation (mtDNA COI) in Clitarchus hookeri across New Zealand (A, B), highlighting the mainly parthenogenetic lineage in NZ (C), and the lineage associated with the one variant found in the UK population (D).

Closer examination of two New Zealand populations of the same species add to our understanding of the drivers and mechanisms of reproduction strategy switching. The UK population lost sexual reproduction and evolved a barrier to fertilisation, which has been demonstrated by providing captive female stick insects from UK with NZ males. Meanwhile two NZ populations recently gained sexuality and genotypic data indicate this happened via two different pathways.


Original Science:
Morgan-Richards M, Langton-Myers S, Trewick S. 2019. Loss and gain of sexual reproduction in the same stick insect. Molecular Ecology

Trewick SA, Morgan-Richards M. 2018. Missing New Zealand stickman found in UK. Antenna 42: 10–13.

Comment:
Nature

Media:
Guardian
Telegraph
STUFF/Dominion Post
BBC radio