ESA 2015 conference talk: Managing species across vast spatial areas: does one size fit all?

Understanding how the spatial and temporal dynamics of populations vary across the landscape is fundamentally important to managing and conserving species. For example, populations may fluctuate in synchrony, or exhibit other forms of spatial sub-structuring, due to intrinsic population parameters or to influences from environmental factors. Importantly, synchronous populations may be at greater risk of extinction if all populations are decreasing to zero at the same time, thereby reducing rescue through colonisation. Different species may not have similar dynamics, even if they share the same environment and thus unravelling the spatial dynamics of multiple species provides vital information about what scale to apply management actions.

MARSS models
MARSS framework is hierarchical and allows modelling of different spatial population structures and parameters, such as density dependence, while including both process and observation variability. Process variability represents temporal variability in population size due to environmental stochasticity. Observation variability includes sampling error. The process component is a multivariate first-order autoregressive process and is written in log-space:
MARSS process eqnwhere X = matrix of all m sub-populations at time t
B = density dependence
u = mean growth rate of the sub-population
w = process errors, assumed to be independent and to follow a multivariate normal distribution with a mean of 0 and variance-covariance matrix Q.
The observation component, written in log-space:
MARSS obs eqnwhere Y = a matrix of observations of all sub-populations at time t,
a = the mean bias between sites
Z = a matrix of 0’s and 1’s that assigns observations to a sub-population structure.
v = observation error, assumed to be uncorrelated and follow a multivariate normal. distribution, with a mean of 0 and a variance-covariance matrix R

Using long-term data (17‑22 years) across a large-scale study region (8000 km2) in arid central Australia, we test for regional synchrony in a population driver, annual rainfall, across nine sites (>20 km apart). We then draw from examples from small mammal and reptile populations and investigate if each species exhibits synchrony. For species that did not exhibit synchrony, we used multivariate autoregressive state-space (MARSS) models to explore four other sub-population structures. We also use the MARSS models to identify important drivers that may regulate populations of these species.

We show that species exhibited different spatial population structuring and respond to extrinsic factors in different ways. We conclude that investigating how the spatial connections among populations interact with their temporal dynamics and eventual persistence or decline, is important for determining the appropriate scale to implement management actions and that “one size does not fit all”.

More on population dynamics of small mammals, MARSS models and Moran:

EcoTas 2013: Spatial and temporal synchrony in small mammal populations

 

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My PhD journey comes to an end: the role of ecological interactions

Ecology is often described as the web of life. Many complex interactions occur across this web, including between individuals of the same or different species (biotic) or between the environment and a species (abiotic). Understanding ecological interactions, such as the role of predators in an ecosystem or how climate influences the abundance of a species, are fundamental for describing how this web of life works.

After working for over 10 years in central Australia I had become hooked on the Simpson Desert. The sand dunes of the Simpson Desert that lie in south-western Queensland and towards the border of the Northern Territory are like a second home to me. The red parallel dunes roll across the landscape like waves across the ocean, carrying spinifex, a spiky grass that form odd-looking donuts floating the red surface. There is something about the red sand that gets under your skin and keeps drawing you back. This setting was where I would tackle the question—what are the relative roles of both biotic and abiotic interactions for driving an ecosystem? The Simpson Desert, like many areas of Australia, has another characteristic I could take advantage of: every trip to the Simpson is different. A small storm may have gently swept rain across the sand and marked out a patch of green for all to see against a canvas of red. Further down the road, the vegetation may lay twisted and grey, prostrate as if it collapsed from the effort of begging for rain. This high variability across time and the landscape must be important for explaining how species live in such a hostile environment. Furthermore, the earth’s climate is changing. How will climate change affect the ecology of central Australia, a region of Australia where the extinctions of native animals has already hit the hardest? What lessons can this remote and understudied part of the globe teach us about our world? It was from these questions pressing on my mind that my PhD was born.

panopan

Small storms can fall through-out the year in the Simpson Desert turning patches green, but other areas remain dry. This variability added an extra dimension to my PhD project. Photo by Aaron Greenville.

I found that the operation of both biotic (species‑species) and abiotic (species‑environment) interactions were important for the ecology of the Simpson Desert (see figure below). More interestingly, interactions are not fixed and can change across the landscape and through time. For example, the relative influence of predation compared to increases in food from large rainfall events varied over the years. How this affected the role of the dingo in supressing smaller predators, such as the red fox and feral cat, and the implications for feral predator management were highlighted in an earlier article.

Chapter 8-fig1

Figure 8.1 from my thesis: Summary of thesis findings on biotic and abiotic interactions in an arid environment. Here we see how interactions are context-specific and can change over the landscape and time. Grey arrows are possible future interactions to explore. Diagrams by A. Foster.

Fierce debates raged in 1950s and 1960s the about the importance of biotic verses abiotic factors for populations after some Australian ecologists argued that the established dogma of the northern hemisphere ecologists may not be the whole story. The upstarts, H.G. Andrewartha and L.C. Birch, published a seminal text: The distribution and abundance of animals, which used empirical evidence, rather than the largely theoretical evidence from the northern hemisphere, to demonstrate that the environment (abiotic factors) could influence the population abundance of a species. Today, findings such as mine are showing that both biotic and abiotic factors are important. This kind of understanding is crucial if we are going to try and predict how species will be affected by climate change.

Over the past 100 years the climate of central Australia has changed. The deserts have warmed and the frequency of extreme rainfall events have increased. This may effect populations of native species. For example, extreme rainfall events (>95th quantile, or >400 mm, for the Simpson Desert) were required for rodent populations to increase, resulting in a boom in population numbers. One example was long-haired rat which I wrote about here . However, not all small mammals responded to the same drivers, as dasyurid marsupials (such as dunnarts) were not influenced by extreme rainfall events. These interactions are becoming increasing important to understand, as extreme weather events, such as tropical cyclones, heat waves and flooding rains, are increasing in magnitude and frequency. Increases in extreme weather events may change species’ populations through increases in mortality rates, decreases in reproduction, and by facilitation of invasive species or novel interactions. In addition, extreme weather events, such as flooding rains, are important not only for driving food pulses in arid systems, but also other abiotic events such as wildfire.

Not only do different species show a different response to rainfall over time, they also show different responses across the landscape. Native rodents, like the spinifex hopping mouse, exhibit a high level of population synchrony across the study region. Even though each population was separated by at least 20 km, thus preventing dispersal between populations from being a big factor (the rodents I was studying are only 10‑35 grams and the reptiles are even smaller), large rainfall events that occurred through-out the region could drive all the rodent populations up in the same way. The highs and lows of each population were in synchrony, dancing in uniform across an 8000 km2 study area. The dunnarts did what dunnarts do best: whatever they want. Each population I looked at was dancing to its own beat. Contrasting strategies in responses to rainfall in a water-limited environment. Such information is vital for knowing how best to conserve each species. A “one size fits all” approach to conservation management will not work for the small mammals and reptiles I studied.

Species do not only respond to biotic and abiotic factors differently over the landscape or across time. We also need to look at multiple factors, such as rainfall and wildfire, simultaneously when trying to predict how species will interact with each other and their environment. Projecting my findings that the frequency of extreme rainfall events had increased into the future (the next 100 years), populations of rodents did not increase as much as expected, as increases in wildfire frequency and current levels of introduced predators limited rodent populations. After removing introduced predators and keeping dingoes in the system, rodents could take advantage of increases in extreme rainfall events.

IMG_20150224_145658_1

The final product after 3.5 years of work: a thesis and several scientific and popular science articles.

I am truly indebted to all the help and support I have had in this journey. Particularly to my supervisors Professors Glenda Wardle and Chris Dickman, my wife, friends and family. Funding was provided by the Australian Research Council, an Australian Postgraduate Award and the Australian Government’s Terrestrial Ecosystems Research Network. Perhaps it is best to end this post with the last paragraph of my thesis:

“This thesis has attempted to take advantage of over 20 years of ecological research focused on the ecology of central Australia. Of this history, I have been fortunate enough to have been part of the last 15 years of the endeavour, working in various capacities. Even though the hand of extinction has been brought down most strongly in Australia’s arid regions, studies such as this elucidate the complexity of the current ecology of central Australia and the sense of wonder that this environment conveys. This wonder feeds curiosity and surely curiosity, the most noble of human traits, needs to be conserved.”

 

Further reading:

Andrewartha, H. G. and L. C. Birch. 1954. The distribution and abundance of animals. University of Chicago Press, Chicago, USA.

Dickman , C., G. M. Wardle, J. Foulkes, and N. de Preu. 2014. Desert complex environments, Pages 379-438 in D. Lindenmayer, E. Burns, N. Y. Thurgate, and A. Lowe, editors. Biodiversity and environmental change: monitoring, challenges and direction. CSIRO Publishing, Vic.

Greenville, A. C. 2015. The role of ecological interactions: how intrinsic and extrinsic factors shape the spatio-temporal dynamics of populations. PhD Thesis. University of Sydney, Sydney.

Greenville AC, Wardle GM, Tamayo B, Dickman CR (2014). Bottom-up and top-down processes interact to modify intraguild interactions in resource-pulse environments. Oecologia, 1-10.

Greenville, A.C., Wardle, G.M. and Dickman, C.R. (2013). Extreme rainfall events predict irruptions of rat plagues in central Australia. Austral Ecology, 38: 754–764.

Greenville A. C., Wardle G. M., Dickman Christopher R. (2012). Extreme climatic events drive mammal irruptions: regression analysis of 100-year trends in desert rainfall and temperature. Ecology and Evolution, 2, 2645-2658.

Greenville A. C., Dickman C. R., Wardle G. M. & Letnic M. (2009). The fire history of an arid grassland: the influence of antecedent rainfall and ENSO. International Journal of Wildland Fire, 18, 631-639.

Top Dog: How Dingoes Save Native Animals. Australasian Science, November 2014.

 

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New paper: Ecosystem risk assessment of Georgina gidgee woodlands

Authors: Glenda Wardle, Aaron Greenville, Anke Frank, Max Tischler, Nathan Emery and Christopher R Dickman.

Published in: Austral Ecology Special Issue: Ecosystem Risk Assessment

Abstract:

Ecosystems across the world, and the biodiversity they support, are experiencing increasing anthropogenic pressure, and many will not persist without intervention. Given their complexity, the International Union for Conservation of Nature has adopted an international standard for ecosystem risk assessment that builds on the strengths of the species-based Red List criteria.

A Gidgee stands against a smoky sunset. Photo by Aaron Greenville

A Gidgee (Acacia georginae) stands against a smoky sunset, Simpson Desert, Qld. Photo by Aaron Greenville

 

We applied this protocol to the relatively understudied Georgina gidgee woodland ecosystem, which has a patchy but widespread distribution in central Australia. To address the extensive knowledge gaps, we gathered data to provide the first description of the characteristic biota, distribution of dominant species and the processes that support the ecosystem. Criteria evaluated include historical, current and future declines in spatial distribution, the extent and area of occupancy, and disruptions to abiotic and biotic processes. Future declines in suitable habitat were based on key climatic variables of rainfall, temperature and soil substrate. We also quantified the uncertainty in bioclimatic models and scenarios as part of predicting degradation of the abiotic environment.

Overall, we assessed the risk status of Georgina gidgee woodlands as vulnerable based on the degradation of abiotic and biotic processes. Bioclimatically suitable habitat was predicted to decline by at least 30% in eight scenarios over the period 2000 to 2050. Predicted declines in overall suitable habitat varied substantially across all scenarios (7–95%). Pressures from grazing, weed encroachment and altered fire regimes further threaten the ecosystem; therefore, vulnerable status was also recorded for future declines based on altered biotic processes. Accurate mapping and monitoring of the study ecosystem should receive priority to inform conservation decisions, and sustainable grazing practices encouraged. Our findings focus attention on other patchily distributed ecosystems that may also have escaped attention despite their contribution to supporting
unique biodiversity and ecosystem services. It is timely that environmental monitoring  and policy account for these natural assets.

Georgina gidgee woodlands snake through the swales in the Simpson Desert. Photo by Aaron Greenville.

Georgina gidgee woodlands snake through the dune swales in the Simpson Desert. Photo by Aaron Greenville.

 

Reference:

Wardle, G. M., Greenville A. C., Frank A. S. K. , Tischer M., Emery N. J. & Dickman C. R. (2015). Ecosystem risk assessment of Georgina gidgee woodlands in central Australia. Austral Ecology 40: 444-459.

Related reading:

Dickman C. R., Greenville A. C., Tamayo B. & Wardle G. M. (2011). Spatial dynamics of small mammals in central Australian desert habitats: the role of drought refugia. Journal of Mammalogy 92, 1193-209.

Frank A. S. K., Wardle G. M. , Dickman C. R. & Greenville AC (2014). Habitat- and rainfall-dependent biodiversity responses to cattle removal in an arid woodland-grassland environment. Ecological Applications, 24:2013–2028.

 

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New paper: On the validity of visual cover estimates for time series analyses

Authors: Vuong Nguyen, Aaron Greenville, Chris Dickman and Glenda Wardle.

Journal: Plant Ecology

Spinifex (Triodia basedowii) in the Simpson Desert, Qld. Photo by Aaron Greenville.

Spinifex (Triodia basedowii) in the Simpson Desert, Qld. Photo by Aaron Greenville.

Abstract:

Changes in vegetation cover are strongly linked to important ecological and environmental drivers such as fire, herbivory, temperature, water availability and altered land use. Reliable means of estimating vegetation cover are therefore essential for detecting and effectively managing ecosystem changes, and visual estimation methods are often used to achieve this. However, the repeatability and reliability of such monitoring is uncertain due to biases and errors in the measurements collected by observers. Here, we use two primary long-term monitoring datasets on spinifex grasslands, each established with different motivations and methods of data collection, to assess the validity of visual estimates in detecting meaningful trends.

The first dataset is characterised by high spatial and temporal coverage but has limited detail and resolution, while the second is characterised by more intensive sampling but at fewer sites and over a shorter time. Using multivariate auto-regressive state-space models, we assess consistency between these datasets to analyse long-term temporal and spatial trends in spinifex cover whilst accounting for observation error. The relative sizes of these observation errors generally outweighed process, or non-observational errors, which included environmental stochasticity. Despite this, trends in the spatial dynamics of spinifex cover were consistent between the two datasets, with population dynamics being driven primarily by time since last fire rather than spatial location. Models based on our datasets also showed clear and consistent population traces.

We conclude that visual cover estimates, in spite of their potential uncertainty, can be reliable provided that observation errors are accounted for.

Reference:

Nguyen, V., Greenville A., Dickman C., and Wardle G. 2015. On the validity of visual cover estimates for time series analyses: a case study of hummock grasslands. Plant Ecology, 1-14.

 

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New paper: Resolving the value of the dingo in ecological restoration

Authors: Thomas M Newsome, Guy-Anthony Ballard, Mathew S Crowther, Justin A Dellinger, Peter J S Fleming, Alistair S Glen, Aaron C Greenville, Chris N Johnson, Mike Letnic, Katherine E Moseby, Dale G Nimmo, Michael Paul Nelson, John L Read, William J Ripple, Euan G Ritchie, Carolyn R Shores, Arian D Wallach, Aaron J Wirsing and Christopher R Dickman.

Published in: Restoration Ecology.

Abstract:

There is global interest in restoring populations of apex predators, both to conserve them and to harness their ecological services.

In Australia, reintroduction of dingoes (Canis dingo) has been proposed to help restore degraded rangelands. This proposal is based on theories and the results of studies suggesting that dingoes can suppress populations of prey (especially medium- and large-sized herbivores) and invasive predators such as red foxes (Vulpes vulpes) and feral cats (Felis catus) that prey on threatened native species.

The dingo is one of Australia's top-predators. Photo by Bobby Tamayo.

The dingo is one of Australia’s top-predators. Photo by Bobby Tamayo.

 

However,the idea of dingo reintroduction has met opposition, especially from scientists who query the dingo’s positive effects for some species or in some environments. Here,we ask ‘what is a feasible experimental design for assessing the role of dingoes in ecological restoration? ’

We outline and propose a dingo reintroduction experiment — one that draws upon the existing dingo-proof fence — and identify an area suitable for this (Sturt National Park, western New South Wales). Although challenging, this initiative would test whether dingoes can help restore Australia’s rangeland biodiversity, and potentially provide proof-of-concept for apex predator reintroductions globally.

Newsome TM, Ballard G, Crowther MS, Glen AS, Dellinger JA, Fleming PJS, Greenville AC, Johnson CN, Letnic M, Moseby KE, Nimmo DG, Nelson MP, Read JL, Ripple WJ, Ritchie EG, Shores CR, Wallach AD, Wirsing AJ, Dickman CR (2015) Resolving the value of the dingo in ecological restoration, Restoration Ecology, 23: 201–208.

This paper generated a lot of interest in the media. See here for a list.

Let’s move the world’s longest fence to settle the dingo debate, The Conversation, February 2015.

 

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ESA2014 conference talk: the web of arid life

The web of arid life: biotic and abiotic interactions in a changing world.

Below is my abstract and slides for the Ecological Society of Australia annual conference. My talk is a summary of the last three years on my PhD work. This year we are in Alice Springs, central Australia. More information on the conference can be found here.

Interactions are important in driving the composition and functioning of ecological assemblages and in maintaining diversity. How both biotic and abiotic interactions drive assemblages are fundamental questions in ecology, but complex systems with multiple species are difficult to study. Climate is a major abiotic driver for species in central Australia and the central desert regions are not immune from climate change, with higher temperatures and an increase in the frequency and magnitude of extreme rainfall events already recorded over the last 100 years. Wildfire return intervals are also predicted to decrease from climate change, making it imperative that we understand how both biotic and abiotic interactions shape ecological systems.

Here we use structural equation modelling to integrate remote camera trapping and live-trapping of vertebrates with long term (>15 years) vegetation data from the Simpson Desert to investigate interactions between the biota and with rainfall and fire. We then use these models to predict how changes in rainfall and wildfire events, in-line with future climate scenarios, will permeate up the trophic levels and interact with top-down effects from mammalian carnivores during both boom and bust resources periods in central Australia.

 

More information:

Greenville A. C., Wardle G. M., Dickman Christopher R. (2012). Extreme climatic events drive mammal irruptions: regression analysis of 100-year trends in desert rainfall and temperature. Ecology and Evolution, 2, 2645-2658.

Greenville A. C., Dickman C. R., Wardle G. M. & Letnic M. (2009). The fire history of an arid grassland: the influence of antecedent rainfall and ENSO. International Journal of Wildland Fire, 18, 631-639.

Greenville A. C., Wardle GM, Tamayo B, Dickman CR (2014). Bottom-up and top-down processes interact to modify intraguild interactions in resource-pulse environments. Oecologia, 1-10.

Greenville, A.C., Wardle, G.M. and Dickman, C.R. (2013). Extreme rainfall events predict irruptions of rat plagues in central Australia. Austral Ecology, 38, 754–764.

Popular science articles:

Of mice and dogs

Predicting rat plagues in the heart of the continent

More on population dynamics of small mammals:

EcoTas 2013: Spatial and temporal synchrony in small mammal populations

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A 25 year commitment to digging for answers in the sand

Remote regions of Australia are rarely studied, but one research group from the School of Biological Sciences has been heading to the Simpson Desert for the last 25 years. This long-distance relationship has endured droughts, floods, fires and flies, but:

“The iron that gives the sands their brilliant, rusty red appearance must have magnetic qualities – you just keep getting pulled back!” says Prof. Chris Dickman.

panoNo3dune copy

The red sand and blue skies get under your skin and you want to learn more about the Simpson Desert. Photo by Aaron Greenville.

Prof. Chris Dickman and A/Prof Glenda Wardle head up the Desert Ecology Research Group, who along with Bobby Tamayo, Chin-Liang Beh, David Nelson, myself, students and volunteers make the 2.5 day drive from Sydney to our field sites in south-western Queensland.

“The possibility of new discoveries and a genuine curiosity for how a mostly dry environment can support abundant life keeps us keen for more.” says A/Prof Glenda Wardle.

The red sand gets under your skin and we gladly leave city comforts behind and head into the outback to pursue new research questions. Over the last 25 years we have expanded our efforts across the eastern Simpson Desert. The first trip in January 1990 consisted of one 4WD with four people to survey the small mammals, reptiles and vegetation at one site, but now crews head out several times per year. Our annual ‘big trip’ requires four 4WDs and 20 people to survey 12 sites across an area of 8000 km². More recently, the project has grown with collaborations, such as the Nutrient Network, an international effort to study how nutrients change productivity and diversity in grasslands around the world. Our studies form part of the Long-Term Ecological Research Network and we are collaborating with AusPlots Rangelands and the remote sensing facility, AusCover, all national infrastructure facilities of the Terrestrial Ecosystem Research Network.

under the southern sky

We call this site Main Camp and we have been returning back to camp under the trees for the past 25 year. Photo by Aaron Greenville

When you think of a desert, many people think desolate, but over the past 25 years we have discovered that the Simpson Desert is the most diverse place in the world for reptiles and insectivorous mammals. “The abundant plant life supports hundreds of species of pollinators such as the native bees and wasps, many of them still undescribed. The world below ground is just as rich with burrowing frogs, termites and importantly the seeds that bring new life to the desert after big rain events” says A/Prof Glenda Wardle. In addition, “Australian deserts do not ‘behave’ in the same way that other world deserts do, and are especially different from the once-paradigmatic deserts of North America; the high unpredictability of the rainfall regime has probably been a key driver of many of the biological adaptations that characterise our desert biota; large rainfall events can be – paradoxically – very bad for many native species and communities because they provide windows of opportunity for invasion by weeds and pest species. You need to be there long-term to document and understand the changes that occur” says Prof. Chris Dickman.

AaronGreenvilleAfter-the-fire

Our long-distance relationship with the desert has endured droughts, floods, fires and flies. Photo by Aaron Greenville.

 

Even after 25 years there is still more to learn from our relationship with the desert. “We still have to discover how to use the desert regions in a sustainable manner, how to effectively manage the threats to their biological riches, such as from introduced predators and feral herbivores, how (and where) some of the key species persist during the long periods of rainfall deficit, and what will be the effects of climate change on the character and composition of deserts in future” says Prof. Chris Dickman.

The many people who have joined us over the years have contributed to the work and the social life of our desert trips and we thank them for enriching our experience and helping us to deliver new knowledge to help sustain this important ecosystem.

 

This article was also published in Biology News July 2014.

 

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