Of ecological disturbance and genetic diversity

Using genetic info to better manage biodiversity in a changing world

ANU PhD student Robyn Shaw, in the Kimberley. She is working with the Australian Wildlife Conservancy to understand small mammal population dynamics in fire-prone landscapes.

ANU PhD student Robyn Shaw, in the Kimberley. She is working with the Australian Wildlife Conservancy to understand small mammal population
dynamics in fire-prone landscapes.

Environmental disturbances range from big events like bushfires, floods and volcanic eruptions to more mundane processes like a boulder being turned over on wave-pounded rocky shore. Ecologists have long realised that disturbance underpins the dynamics and diversity of many of the ecosystems of the world, yet its influence on the patterns and distribution of genetic diversity is poorly appreciated. How do changing patterns of disturbance affect genetic diversity in natural populations? This is an important question for conservation because genetic diversity influences the survival of individuals, the persistence of populations and the adaptability of species to environmental change. And regimes of ecological disturbances like fire, floods and extreme weather events are changing globally.

With colleagues I recently reviewed the challenges and opportunities of using genetics to understand the biotic impacts of disturbance (Banks et al., 2013). We found that disturbance can cause genetic changes that may affect the viability and adaptability of populations, that some species can show rapid evolutionary responses to novel disturbance regimes, and that there is considerable potential for genetic approaches, such as landscape genomics, to inform conservation management in landscapes where disturbance regimes are changing.

Evolutionary consequences of disturbance 

Disturbance is known to be an important driver of natural selection on the traits of plants and animals. Many species have evolved in response to particular regimes of ecological disturbance, and a large proportion of Australia’s plants and animals can almost be defined by their relationship to fire (eg, many of our plants can be grouped as post-fire seeders or resprouters). So the selective ‘filter’ that fire imposes on these traits contributes to the differences in the species we find in parts of the landscape with different fire regimes, such as protected gullies vs. exposed ridges.

There is considerable potential for genetic approaches, such as landscape genomics, to inform conservation management in landscapes where disturbance regimes are changing.” 

Several species of native rodent such as the pale field rat (above left) and common rock rat (right) are in decline in the Kimberley (top image). New insights from demographic and genetic research may help us tailor fire management to ensure the ongoing persistence of these species. (Photos by Sam Banks)

Several species of native rodent such as
the pale field rat (pictured) and common rock rat (next image) are in decline in the Kimberley (top image). New insights from demographic and genetic
research may help us tailor fire management to ensure the ongoing persistence of these species. (Photos by Sam Banks)

Less well understood is how the regimes of disturbances like fire influence genetic variation within species and the evolution of their traits. However, recent work provides some

interesting insights. For instance, research from Juli Pausas’s group at the University of Valencia in Spain has shown that frequent fire can actually drive the evolution of increased flammability in plants, documenting these changes at the genetic and physiological levels.

This evolutionary process occurs because these plants do very well under conditions of frequent fire. Individuals are killed by fire, but their offspring out-compete other species by highly effective post-fire regeneration from seed. Thus, traits that make the local neighbourhood more flammable are advantageous. These plants make some compromises in evolving to become more flammable, and flammability-enhancing traits appear less advantageous where fire is uncommon. Thus, evolutionary processes can be quite dynamic in response to changes in disturbance regimes, and these responses can have major impacts on the rest of the ecological community.

Disturbance, population dynamics and genetics

In addition to evolution driven by natural selection, genetic variation can be shaped by what we know as selectively-neutral processes relating to population size, mating systems and movement patterns across the landscape. We know that landscape changes like habitat fragmentation can have major genetic effects in this way, often leading to losses of genetic variation in small, isolated patch populations. Can disturbance have similar effects? Certainly, such examples have been documented. Luciano Beheregaray (Flinders University) attributed the unusually low genetic diversity of the largest population of a subspecies of Galapagos giant tortoise to a volcanic eruption that decimated the population 100,000 years ago.

However, many species that exist in the presence of regular disturbance have mechanisms to avoid the negative genetic consequences of disturbance events, such as high levels of immigration from undisturbed areas (genetic ‘rescue effects’), large-scale seed drop or soil seed banks that preserve the diversity of previous generations, or high survival of individuals in disturbed areas. For instance, we fitted radio-collars to mountain brushtail possums the week before the 2009 Black Saturday fires, and found that all survived and were perfectly healthy in the months after the fire. The population lost no genetic diversity as a result.

The common rock rat is in decline. (Photo by Sam Banks)

The common rock rat is in decline. (Photo by Sam Banks)

The more complex interactions between disturbance history, habitat suitability and population dynamics have the potential to cause major changes in genetic diversity for species in landscapes where the disturbance regimes are changing. In many vegetated ecosystems, fire history is the major determinant of whether or not habitat is suitable for some species. For instance, a number of our native rodents specialise in early post-fire conditions, and many arboreal marsupials in south-east Australian forests require long-unburnt old-growth forest for habitat.

In the latter case, the increasing frequency of fires, together with human disturbances like logging, means that suitable long-undisturbed habitat occurs in ever-smaller patches, which may shift over time in response to disturbance ongoing patterns. In the absence of very high migration capacity, this kind of ecological scenario does not favour the maintenance of genetic diversity in populations. Recent work by Sarah Brown, Paul Sunnucks and others at Deakin and Monash Universities shows that one species that fits this ‘shifting-patch suitability’ model, the mallee emu wren, displays very low genetic diversity.

This mountain brushtail possum survived the 2009 Black Saturday bushfires at Cambarville in the Victorian central highlands. Mountain brushtail possums fared surprisingly well after Black Saturday, but many other arboreal marsupials, like the endangered Leadbeater’s possum, have been completely absent from burnt sites since the fires. The increasing frequency of fires and logging in this region means that suitable long-undisturbed habitat occurs in ever-smaller patches, which may shift over time in response to disturbance ongoing patterns. The resulting extinction-recolonisation dynamics may cause major changes to the genetic diversity of such species in these landscapes. (Photo by Sam Banks)

This mountain brushtail possum survived the 2009 Black Saturday bushfires at Cambarville in the Victorian central highlands. Mountain brushtail possums fared surprisingly well after Black Saturday, but many other arboreal
marsupials, like the endangered Leadbeater’s possum, have been completely absent from burnt sites since the fires. The increasing frequency of fires and logging in this region means that suitable long-undisturbed habitat occurs in ever-smaller patches, which may shift over time in response to disturbance
ongoing patterns. The resulting extinction-recolonisation dynamics may cause major changes to the genetic diversity of such species in these landscapes.
(Photo by Sam Banks)

Maintaining genetic diversity 

So how much should we worry about this as a threatening process? We aren’t about to rush out and measure genetic diversity in every ecosystem affected by disturbance (most places on the planet), but I would suggest that it is important to understand the conditions under which disturbance might cause ‘conservation-relevant’ changes to genetic diversity. This might be where disturbance regimes are changing drastically, or where species’ responses to disturbance are constrained by other environmental changes like habitat fragmentation.

Improving our knowledge of how genetic diversity responds to disturbance might also give us some new research tools to inform better management of biodiversity. Robyn Shaw, a PhD student at the ANU, recently started a project in collaboration with Drs Katherine Tuft and Sarah Legge of the Australian Wildlife Conservancy (AWC), with the aim of developing joint demographic and genetic approaches to understand how populations of declining native rodents in the Kimberley recover from fire events.

Genetic data have not often been applied to research in disturbance-affected ecosystems because of the complex population dynamics at play. Robyn’s work uses new computer simulation tools, combined with field and lab work, to determine the genetic signatures of different population recovery processes. She aims to use this approach to understand the importance of animal survival and immigration as drivers of post-fire population recovery, and how these are affected by the intensity, season, size and frequency of fires in the Kimberley.

A recently-burnt landscape at Mornington Wildlife Sanctuary in the Kimberley. Fire has always been a part of this landscape, but changes in the season, intensity and size of fires are contributing to the ongoing decline of many mammals in this region. Pro-active fire management strategies are helping to halt these declines, and can be informed by combinations of genetic research and field-based ecology. (Photo by Sam Banks)

A recently-burnt landscape at Mornington Wildlife Sanctuary in the Kimberley. Fire has always been a part of this landscape, but changes in the season, intensity and size of fires are contributing to the ongoing decline of many mammals in this region. Pro-active fire management strategies are helping to halt these declines, and can be informed by combinations of genetic research and field-based ecology.
(Photo by Sam Banks)

Fire is one of the key drivers of native mammal declines in northern Australia, and our aim with this project is to feed the new information into the AWC’s large-scale EcoFire program that focusses on developing and applying pro-active fire management strategies for mammal conservation in the Kimberley.

In a similar manner, understanding the interaction between gene flow and disturbance regime may contribute critical information about how to effectively manage biodiversity in a rapidly changing world.


More info: Samuel Banks sam.banks@anu.edu.au 

Reference 

Banks SC, GJ Cary, AL Smith, ID Davies, DA Driscoll, AM Gill, DB Lindenmayer & R Peakall (2013). How does ecological disturbance influence genetic diversity? Trends in Ecology & Evolution 28: 670- 679. http://dx.doi.org/10.1016/j.tree.2013.08.005 

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