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Pictures of landscapes past, part 2

The kind of things plant ecologists often measure in the field – the height and percentage foliage cover of different plants, species lists – are difficult to obtain from looking at a photograph of the landscape because of distortions due to perspective. Or, in other words: “The geometry of the photographs inhibits quantification”.  Even mapping boundaries between vegetation types can be difficult because the process requires translating from an oblique 3-dimensional perspective to a 2-D aerial view. Further complicating matters, photos taken at different times may not be at exactly the same location, or in different seasons, or the lighting may be very different.

MI rephoto Langdon Pt
1980 (L) and 2014 (R) photographs of coastal terrace and escarpment near Langdon Point, Macquarie Island. Photographer: Jenny Scott.

While trying to quantify the magnitude of changes from repeat photographs is fraught, it is normally straightforward to determine the direction of change. This is the approach we are using with a time series of Macquarie Island landscape photographs. Three time periods have been chosen for analysis: 1980 (black and white film photographs digitized by scanning), 2009 and 2014 (colour digital images). These correspond with the end of a period of highest recorded rabbit population (1974-1980), the end of next highest population cycle (2004-2010), and the beginning of the post-rabbit phase (mid-2011 onward).

If there is a rabbit influence on the vegetation we would expect to see it strongly in 1980 (after several years of high rabbit numbers peaking at an estimated 350,000), less evident in 2009 (when the preceding rabbit population peak is estimated at 200,000), and some signs of vegetation recovery in 2014 (2.5 years after rabbit eradication). Of course, there may be other factors involved, notably climate.

The method involves viewing the photographs side by side on a screen, selecting distinct vegetation features which can be reliably identified in each of the three time series photographs and drawing these on a printed version. Key plant species or vegetation types such as feldmark, bare ground, moss, Pleurophyllum, tussock grass are scored as increase, decrease or no change.

plateau photos1_2_cr
Photographs of Whisky Creek from 1980, 2009, 2014 with annotations of visible vegetation change. Photos: Jenny Scott.

The centre-point of each of the zones identified from the photographs is then geo-located on a map of Macquarie Island. With all of the photograph locations recorded using GPS, these points can be uploaded to a GIS which allows the viewshed of each point to be determined. The viewshed shows which parts of the landscape are visible from a given point which helps to narrow down the features visible in the photographs taken from that point. A satellite image and contour lines provide enough information to confidently locate most of the zones in the photographs to a point on the map.

masked
Satellite image of Macquarie Island overlaid with contour lines. Shading represents the viewshed from one of the photograph locations (red dots). Orange + symbols indicate locations of vegetation zones identified from photographs.

Combining observations of particular types of change (or, equally importantly, no change) with a geographic location allows us to look for spatial patterns in vegetation trends. Does aspect, elevation, landform, wind exposure or drainage influence changes in vegetation over a 25 year period? These questions will be answered after hundreds of observations are completed.

 

Pictures of landscapes past, part 1

Repeat photography, or rephotography, is an established method for studying landscape change. A big advantage of this technique is that it can provide a record going back decades wherever there is old photographs of scenes which can be relocated. Another advantage is the compelling visual story of change that a time series of photographs can present, for example dramatic images of glaciers retreating. These advantages tend to compensate for the rather limited quantitative data that can be obtained using this method.

Grinnell Glacier3
Glacial retreat 1938-2009. Source: http://www.usgs.gov

While historical rephotography projects document changes in streetscapes, vegetation and landforms using photographs from incidental sources such as old postcards, more recent rephotography projects are intentionally set up with precisely located camera positions (using GPS or marker posts) to capture a baseline photograph and subsequent images.

 

Walls of Jerusalem 1993-2009 Rudman rephoto
Subalpine vegetation in 1993 and 2009 at Walls of Jerusalem National Park, Tasmania. Photographer: Tim Rudman. Source: http://www.flickr.com/groups/tas_rephoto

On Macquarie Island several long-term vegetation monitoring plots provide a detailed record of changes in plant species composition and abundance, but for large parts of the island we have no such data. Aerial photography and satellite images with useful resolution extend back to the late 1980s and early 1990s. To go further back in time we can turn to the relatively old technology of the handheld camera.

Veteran Macquarie Island expeditioners have repeatedly photographed the same scenes over many years resulting in time series photographs which offer an insight into changes in the vegetation and landscape from around 1980 to the present. Around 100 landscape scenes have been systematically rephotographed at various time providing a valuable visual record of change – and stability – in the Subantarctic environment.

My research group is currently analyzing these repeat photographs to build up a story of change over time and space on Macquarie Island.

MI rephoto Whisky Ck
Whisky Creek on Macquarie Island in 1980 and 2014. Photographer: Jenny Scott.

Plan B

When the French ship L’Astrolabe departs Hobart for Macquarie Island later today I won’t be aboard. The passenger capacity of the ship is only enough for essential logistical operations (like refuelling and provisioning the research station) and personnel changeover. The week long sojourn at Macquarie Island ordinarily provides an opportunity for researchers such as botanists, geomorphologists and zoologists to undertake intensive fieldwork. The Aurora Australis has ample capacity, with 116 passenger berths, to accommodate round-trip research. Unfortunately, the Aurora was damaged during a severe blizzard in the Antarctic and the smaller replacement vessel can carry only 50 passengers.

Macca NF-22
Instead of traipsing through giant tussock grasses and Subantarctic bogs in bright yellow waterproofs, I’ll be sitting in front of a computer screen with dry feet.

So rather than collecting new data on the Macquarie Island vegetation I am going to Plan B, which involves methodically examining hundreds of photographs of Macquarie Island, dating from 1980 through until last year. This, hopefully, will reveal some long-term trends in vegetation change as well as a good picture of the current status of the vegetation. Combined with detailed data from long-term vegetation monitoring plots and analysis of satellite images covering the whole island at different time periods I will be asking what grows where and why? (or why not?).

waveheight2016-03-24_114324

The upside is that I won’t be on the notoriously seasickness-inducing L’Astrolabe in the 8 metre plus swells predicted south of Tasmania tomorrow!

 

 

Hurry up and wait…

The Aurora Australis recently arrived back in Australia and is currently in Fremantle for repairs to the hull which was damaged when the ship ran aground in Antarctica last month. Meanwhile, alternative plans for the season’s fourth and final Australian Antarctic voyage are being developed. With the Aurora out of action, another ship needs to be commissioned for the Macquarie Island voyage. I imagine finding a suitable ship at short notice is no small task. Consequently the departure date has been delayed and, depending on the capacity of the replacement ship, there may be some expeditioners who miss out.

Hurry-Up-and-Wait

It’s a case of “Hurry up and wait”. This catchphrase of military origins is popular in the Antarctic and Subantarctic. Typically, it’s waiting for the weather to improve and being prepared to take advantage of that weather window, if and when it occurs.

I’ve been waiting two years for that window to get an automatic weather station installed on the summit of Mt Elder (385 m.a.s.l.) during a resupply voyage when helicopters are available. Most of the weather data for Macquarie Island is from sea level, yet the weather can be very different at higher elevations. For example, there is often low cloud on the peaks which makes helicopter operations unsafe. Even if I do miss out on this trip I do hope the weather station installation goes ahead.

So far the only weather data I have collected is air temperature using a dozen miniature temperature loggers at different locations on the island. After 18 months in the field, these loggers will be returned to Australia on the upcoming voyage.

Wpt 10 Mt Elder
This temperature logger (on post on RHS) on Mt Elder has recorded air temperature for the past 18 months. Hopefully a complete automatic weather station will be installed here soon. Photo: Chris Howard.

 

The A-Factor

In Antarctic slang it’s called the “A-factor”. Experienced expeditioners will tell you that things rarely go according to plan in the Antarctic and Subantarctic. It is wise to expect the unexpected and to have a string of contingency plans. If your plans are inflexible, you’re setting yourself up for disappointment.

DCIM100GOPRO
The Aurora Australis in the Southern Ocean, March 2013.

This advice has been reinforced by my own experiences over the past three years as I look toward another potential trip to Macquarie Island with dwindling optimism. My scheduled departure from Hobart on the Australian Antarctic Division’s icebreaker Aurora Australis in under three weeks time is now in doubt. Last week the Aurora ran aground at Mawson station on the Antarctic coastline during a blizzard. Thankfully the crew and expeditioners are safe and the ship has been refloated. Hopefully the Aurora is seaworthy to travel back to Australia for repairs to the damaged hull.

Under this best case scenario the Macquarie Island resupply voyage will be delayed. Macquarie Island is far north of the sea ice zone which limits sea access to the research stations on the Antarctic continent, so voyages to Macca are scheduled in the shoulder season, book-ending the Summer window of Antarctic operations. The March-April voyage to Macquarie Island is therefore vulnerable to any delays which accumulate during the Antarctic voyages.

Macca April2015 sml-39
Macquarie Island under snow, April 2015. Not good for looking at plants.

Such was the case two years ago, at the very beginning of my PhD project, as I watched my departure date repeatedly postponed. A string of delays due to the most extensive sea ice ever recorded hampering resupply efforts and the attempted rescue of the Akademik Shokalskiy which become stuck in sea ice meant that the Aurora Australis completed its Antarctic operations several weeks late. By late April 2014 the decision was made to employ the French vessel L’Astrolabe to undertake the Macquarie Island resupply. With around half the passenger capacity of the Aurora, all non-essential resupply personnel (that’s me) missed out on that voyage.

Spseaice_extent_2013_chart.jpg
Antarctic sea ice cover grows in autumn and winter, and shrinks again each spring and summer. In 2013 (black line) and 2012 (red line), the ice reached the highest extents ever recorded, but it was only slightly above the historical average (blue line). Source: NASA via Wikipedia.

The following year I had better luck: after only minor delays to the departure we arrived at Macquarie Island in mid-April 2015 for a planned six days of fieldwork. However after a couple of days lost due to snow and an early return to the ship due to impending bad weather, we managed to squeeze as much work as we could into just two days, before spending three days on the ship waiting for the storm to blow over and another three days travelling home.

So after achieving two days fieldwork in the first two years of my PhD, I am lucky enough to get a berth on the 2016 voyage… which is now in doubt. It may well be a repeat of two years ago, where another ship is tasked with the resupply mission. If I’m in luck, the Aurora Australis will be repaired and make a late trip to Macquarie Island.

Southern Ocean NickFitzgerald -92 sml
The long route to Macquarie Island: icebergs and adelie penguins near Commonwealth Bay, Antarctica.

The Aurora Australis visits Macquarie Island every six months to resupply the research station and transfer personnel. Consequently the Macquarie Island fieldwork season is typically a six month stay from Spring to Autumn. Alternatively, researchers take a round trip, providing around six days in the field (if you’re lucky!) while the resupply operation is in progress. Sometimes tourist ships visiting the island during Summer transport researchers, providing a little more flexibility in scheduling field work.

My first trip to Macquarie Island was on one of these tourist ships – and things did not go according to plan. The planned trip was to take six days, including transiting via New Zealand. Instead it took 23 days to get to Macquarie Island via Antarctica and stopping back home in Hobart! This was partly due to undertaking a dramatic rescue in the Southern Ocean. The six weeks I experienced on the island in early 2013 inspired me to undertake this PhD project. At least I went into it well aware of the A-factor.

Southern Ocean rescue NickFitzgerald -11_sml
French solo sailor Alain Delord is rescued by the MV Orion after three days in a tiny liferaft in the vast Southern Ocean, January 2013.

 

First glimpses of a post-feral future

Macca fieldwork April2015 sml 014
The battle for dominance is underway on the West Coast as the creeping Acaena (foreground and middle ground) struggles to compete with the increasingly taller grasses while the megaherb Stilbocarpa gains a foothold.

In April 2015 the short grasslands of Macquarie Island are at the end of their summer growing season. The plants are the tallest they have been for years, probably for many decades. There is virtually no bare ground visible as the lush growth covers dirt previously laid bare by rabbits. These changes were predictable consequences of the rabbit eradication nearly four years earlier. It is too early to tell for sure, but it does look like other predicted changes are starting to occur, such as the ascendancy of the megaherbs.

While historical information about Macquarie Island’s vegetation is quite limited, better data are available from the 1980s. One of several long-term ecological monitoring programs on the island is a set of six vegetation monitoring sites established by Geof Copson in 1980. These include locations on the west coast, east coast and plateau, all with some form of short grassland vegetation.

Whinam et al 2014 site map
Location of Tasmanian Parks and Wildlife Service vegetation monitoring sites on Macquarie Island. Source: Whinam et al. (2014).

Each site comprises five randomly located 20 x 20 metre quadrats. These 30 quadrats are typically surveyed every 2 to 3 years by a pair of botanists who record the vascular plant species present and estimate the foliage cover of each species.

Rabbits kept these short grassland ecosystems very short. There was little point in measuring the height of the plants in what resembled a mown lawn. Now the short grassland is getting taller. And that is something worth measuring. Measurements of the maximum foliage height of each major species have been made during the past two surveys. Not surprisingly, there is a consistent increase in height across all species and sites.

Fest height
Height of native grass species Festuca contracta at four study sites has increased as much as three-fold between summer 2013 and summer 2015.

Since the peak in the rabbit population around 2006 some changes in the extent of plant species are evident in the monitoring sites. The two largest plants on the island, Stilbocarpa polaris and Poa foliosa, have declined. Both these megaherbs are known to be highly palatable to rabbits so it is likely they suffered during the rabbits’ final rampage and have yet to recover. They are still present in very small numbers at all sites so it seems that the rabbit browsing did not wipe them out locally and, therefore, there is capacity for recovery. Given what we know about these species it is quite possible they will increases from less than 1% foliage cover on some sites to nearly 100% over the next few years.

megaherbs
Average foliage cover class of the megaherb Stilbocarpa polaris and tussock grass Poa foliosa at six sites since 1981.

Plants which have benefited from disturbance by rabbits, such as Poa annua and Acaena magellanica, display a very different trend. These disturbance-lovers have expanded since the mid-2000s. It is reasonable to assume that without rabbits these plants will become less abundant as they compete with other plants. Unless there has been a state-shift in the ecosystem or there are other factors at play which make such simplistic predictions wrong – as is often the case in ecology!

rabbit lovers
Average foliage cover class of the introduced grass Poa annua and the native disturbance-colonizing herb Acaena magellanica at six sites since 1981.

 

 

Feral rabbits and the baseline problem

Rabbits were already locally abundant on Macquarie Island by the time the first descriptions of the island’s vegetation were made by Scott (1880) and A. Hamilton (1884). Consequently there is no information to give us a baseline of the vegetation prior to the impact of rabbits (there are no native grazing animals on the island). Another problem is that these early vegetation records of Scott, A. Hamilton and, later, his son H. Hamilton (1911-14) provide only basic information with no precise details. Hamilton noted in 1884 that:

The tussocks and the Stilbocarpa become smaller as you ascend and, at about 300 feet, you gain a plateau so swept by the Antarctic gales that vegetation is reduced to compact, closely growing mosses, small Uncinias and the conspicuous cushion-like masses of Azorella selago.

An exclosure plot showing the difference between ungrazed (inside the fence) and grazed vegetation (everything else) around 18 months after rabbits were removed.
An exclosure plot showing the difference between ungrazed (inside the fence) and grazed vegetation (everything else) around 18 months after rabbits were removed.

The first detailed scientific treatment of the Macquarie Island vegetation was by Taylor, who spent some months on the island in 1950. He noted that rabbits were in “plague proportions” in some areas. Rabbit population estimates are problematic since rabbit populations are highly variable over space and time. Rabbits would reach great numbers in certain areas, until the food resource was reduced to a point where the rabbits would either starve or move elsewhere. Data collected from the 1970s onward provides a good indication of trends in rabbit populations.

Population trends of rabbits on Macquarie Island. Red line is smoothed population estimate based on models, with 95% confidence intervals (dotted lines).
Population trends of rabbits on Macquarie Island, 1974-2012. Red line is smoothed population estimate based on models, with 95% confidence intervals (dotted lines). Source: Terauds et al. (2014) Polar Biology.

Following the successful introduction of the biocontrol agent myxoma in 1978 rabbit populations declined by around 90% and remained relatively low until an increase in the early 2000s likely resulting from removal of cats and cessation of the myxoma program. Vegetation recovery was observed during the 1980s and 1990s following the damage caused by high rabbit populations in the 1970s on both steep coastal slopes and on other grassland sites (coastal terraces, plateau and inland valleys). Vegetation change consistent with heavy grazing pressure was subsequently observed in the recent period of high rabbit numbers which peaked around 2006.

A rabbit skull in 2013 is a reminder of the now-extinct feral species which thrived on Macquarie Island.
A rabbit skull in 2013 is a reminder of the now-extinct feral species which thrived on Macquarie Island.

After more than 130 years of running rampant in the subantarctic ecosystem, the story of rabbits on Macquarie Island ends in 2011 with the successful efforts of the Macquarie Island Pest Eradication Project. So we would expect the vegetation to now respond to the absence of grazing…

The mysterious case of the Subantarctic Bedstraw

What is Macquarie Island’s rarest plant? One contender for the title is Subantarctic bedstraw or Antarctic bedstraw (Galium antarcticum), a small creeping herb with tiny delicate pinkish flowers. The species was first recorded in 1983, growing near Skua Lake around halfway along the 34 km long Subantarctic island.

It was not until 2013, following listing as Critically Endangered on the Commonwealth Environment Protection and Biodiversity Conservation Act, and after years of doubt about its persistence on Macquarie Island, that Subantarctic bedstraw was rediscovered in the same location. Where had it been hiding for 30 years?

Subantarctic bedstraw growing on a mossy bank at Skua Lake.
Subantarctic bedstraw growing on a mossy bank at Skua Lake.

The single known population of around 500 plants occurs on a mossy bank in short grassland vegetation. At around 140 metres above sea level on the plateau which covers most of Macquarie Island, the short grassland habitat in which the species occurs is typical of much of the island. Why it is not more widespread is a mystery. Perhaps it was preferentially grazed by the rabbits which have had a dramatic impact on the vegetation of Macquarie Island. It may be a formerly more widespread species nearly grazed to extinction by rabbits or, as appears to be the case with some other localised species such as Carex trifida, a recent immigrant yet to expand its range on the island.

Searching for Galium antarcticum at Skua Lake.
Searching for Galium antarcticum at Skua Lake.

Macquarie Island is a young and very remote landmass with an unusual cold maritime climate. Consequently its flora is the result of long-distance dispersal and composed largely of Subantarctic specialists. Galium antarcticum has a wide distribution around the Subantarctic, suggestive of dispersal by seabirds. It occurs in Patagonia, South Georgia, the Falklands, Crozet and Kerguelen islands. The species was first described from the Falkland Islands by J.D. Hooker in 1846.

Subantarctic bedstraw grows to a few centimetres in height and width, with creeping stems and solitary flowers. What appears to be whorls of four fleshy red- or purple-tinged leaves, are actually a pair of leaves and a pair of nearly identical stipules. Galium is a large and widespread genus in the Rubiaceae with several native and introduced species in Australia including the familiar garden weed Galium aparine (cleavers or sticky weed). Many species have distinctive bristly hairs whereas G. antarcticum is hairless.

Subantarctic bedstraw
Subantarctic bedstraw

The future prospects for Galium antarcticum on Macquarie Island are uncertain. It may benefit from the recent eradication of rabbits, expanding its range, or it may struggle to compete with taller growing plants as the short grassland transitions to a more closed vegetation community.

And the other rare vascular plants on Macca? The tiny filmy fern Hymenophyllum falklandicum is confined to a few boulders in mountain-top feldmark. The large sedge Carex trifida occurs in only one small population on the coastal terrace near Handspike Point.

An earlier version of this article first appeared in Australian Plants journal, issue 221.

A one-way ticket to a remote island

Where did Macquarie Island’s plants come from? Being hundreds of kilometres from its nearest neighbours, all of the plants must have been transported to this tiny speck of land in the ocean since it rose from the seabed less than a million years ago.

The chances that a seed might, for example, get stuck in mud on the feet of a bird in Tasmania, which is then blown into the Southern Ocean by a storm and eventually lands 1500 km away on Macquarie Island seem pretty low. But over thousands of years such unlikely scenarios probably will occur.

A giant petrel with several globular heads of Acaena seeds stuck to its feathers. Each Acaena seed has small barbs on it which hook firmly onto fur feathers (or socks!). The inventor of Velcro was inspired by plants with similar seed dispersal mechanisms.
A giant petrel with several globular heads of Acaena seeds stuck to its feathers (hundreds of seeds in total). Each Acaena seed has small barbs on it which hook firmly onto fur or feathers (or socks!). The inventor of Velcro was inspired by plants with similar seed dispersal mechanisms.

Not only does a propagule (a seed or spore or piece of plant capable of regenerating) have to find its way to the island, to successfully become established it needs to survive and reproduce.

Most of Macca’s vascular plant species are found elsewhere in the Subantarctic, which suggests that they are well adapted to long-distance dispersal. Few seeds survive in sea water for any length of time, so the obvious vectors for seed transport are wind and birds. Around half of Macquarie Island’s plants have seeds small enough to travel by wind. The strong westerly winds which predominate at these latitudes provide a mechanism for distributing small seeds and spores within the subantarctic.

Sea birds such as albatrosses, petrels and skuas travel widely, easily covering distances of hundreds or thousands of kilometres. Some seeds such as the hook sedges (Uncinia spp.) and ‘buzzies’ (Acaena spp.) are purpose designed for attaching to fur or feathers. Other seeds may be swallowed by birds, or stick to their feet or feathers in mud.

Uncinia species have distinctive seeds with a barb attached, hence the common name 'hook sedge'.
Uncinia species have seeds with a distinctive barb attached, hence the common name ‘hook sedge’.

Leptinella plumosa, a common coastal plant on Macquarie Island and many other subantarctic islands, was not known from Heard Island until 2004 when a single plant was found. This species has sticky seeds which can adhere to a bird, the most plausible explanation for this dispersal event. Similarly on Macquarie Island a large sedge, Carex trifida, is confined to a single location on the island and absent from other areas of similar habitat, suggesting it is a relatively recent arrival (though present since early botanical collections).

Leptinella plumosa is a common coastal plant on Macquarie Island and many other subantarctic islands yet only arrived on Heard Island this century.
Leptinella plumosa is a common coastal plant on Macquarie Island and many other subantarctic islands yet only arrived on Heard Island this century.

Rather than seeds, ferns and mosses produce spores which are far tinier and easily float on wind currents, or hide in mud attached to a bird. These types of plant are widespread in the Subantarctic and many species have extensive natural ranges. They are so well dispersed that very few species are endemic to a single location.

Exotic seeds occasionally turn up on Macca, transported by birds or ocean currents, but do not become established. A study of pollen in peat cores from a lake on the island reveals the presence of a diverse range of exotic pollen, from Australia and New Zealand, showing that strong northerly or northwesterly airflows can transport things to Macquarie Island. A small European bird, the redpoll, arrived on Macquarie Island in 1912, apparently having been carried by one of these weather systems from New Zealand where it is an introduced species. Similarly, starlings arrived in 1930.

Carex trifida, a large sedge known from only a single location on Macquarie Island, appears to be a relatively recent immigrant.
Carex trifida, a large sedge known from only a single location on Macquarie Island, appears to be a relatively recent immigrant.

However, the main vector for transporting plants to subantarctic islands now is humans. Seeds are frequently found in the clothing and equipment of researchers and tourists. Grass seeds are the most common stowaways and Velcro fastenings on clothing are the most likely place for them to hide. Procedures are in place to reduce the risk of importing these ‘subantarctic hitchhikers’ to the Macquarie Island Nature Reserve. Tiny grass seeds can easily evade detection. Two exotic species of grasses were discovered growing next to a track on Macquarie Island in 2014. Since rabbits and rodents have been exterminated, weeds now pose the number one threat to the naturalness of Macquarie Island’s ecosystem.