Identifying thresholds and ceilings in plant community recovery for optimal management of widespread weeds

A substantial body of work underlies the theory and practice of early intervention in the management of invasive alien plants, but less attention has been paid to the strategic management of widespread weeds, especially in the context of natural asset recovery. The assumption lingers amongst some researchers and land managers that removing weeds will automatically lead to positive biodiversity outcomes, with the more weed removed, the better the outcome. However, this is often not the case, particularly for long-established weed species whose dominance has created impoverished communities with little capacity for passive recovery. A common result may be wasted investment in weed control and, in the extreme, net negative impacts upon asset values. We present a conceptual model for the management of weed-impacted assets, plus guidance for its application, with a view to improving asset recovery practice. Weed removal should be calibrated by asset recovery, which may mean not seeking to completely remove a weed at a given spatial scale. Our model focusses on weed removal that is enough to initiate asset recovery, but not more than is necessary to promote maximum expression of asset resilience, particularly in the context of secondary invasions. Optimal management efficiency will involve a proportional allocation of resources to control, monitoring and revegetation activities that is appropriate to the stage of asset recovery, as well as a willingness to revise a management goal if the original one cannot be achieved within existing constraints on resources.

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Axes

The y-axis represents a sliding scale of vegetation community condition between an average invaded state (red zone) and an average non-invaded, native reference state (green zone). The origin at point 1 represents a state 100% dissimilar to the non-invaded, native condition (e.g. a weed-dominated area that contains none or a different suite of native species to those found in non-invaded reference areas). It is assumed that a non-invaded, native state comprises higher asset condition than weed-dominated areas. Impact of invasion is measured as the magnitude of difference between the average non-invaded state (green zone) and the average invaded state (red zone), represented by interval iii.

The x-axis represents a sliding scale of weed removal. The curves A, B, C and D represent different patterns of vegetation community response to weed removal (i.e. regeneration trajectories). Vegetation community regeneration in response to weed removal will vary as a function of resilience to invasion, which is defined as the extent of recovery of the asset post invasion . Blue curves (A, B and D) represent communities with relatively high resilience to invasion, while the orange curve C represents a community with relatively low resilience (see description below). For simplicity, we have presented resistance to invasion (i.e. degree of community change in response to invasion, interval iii) as equal for all curves, hence why the level of impact is equivalent at point 1 for all curves at the maximum level of weed abundance before removal of the weed commences.

Regeneration trajectories

Curve A represents the most resilient community (the best-case scenario for managers), because the native vegetation begins to recover very soon after weed removal commences (i.e. the asset recovery threshold occurs close to point 1), any amount of weed removal facilitates native vegetation recovery, and full recovery to the reference native state is achieved. An asset recovery ceiling occurs at point 2, beyond which asset condition will not improve with any further investment in weed removal. This point can also be considered as a weed removal ceiling, the latter being causal and asset recovery its effect.

Curve B is similar initially to curve A in that removal of the weed initiates rapid, linear asset recovery near point 1. However, for curve B, the community is relatively less resilient to invasion because the response trajectory does not reach the maximum level of the desired native reference state by the time the asset recovery ceiling is reached. This results in a recovery deficit (interval i) and represents the model space in which active intervention would be required to facilitate full community recovery (e.g. planting nursery-grown seedlings or seed addition).

Curve C represents a community with a much-reduced level of resilience, since regeneration of the asset only commences after a much greater proportion of the weed is removed from the invaded site (i.e. asset recovery threshold at point 3).

Curve D represents a scenario where weed removal promotes asset recovery until a weed removal ceiling is reached, but thereafter inhibits asset recovery due to disturbance effects of the control technique being used to remove the weed. In this case point 4 represents a management impact threshold and interval ii represents the net effects of management that balances benefits of weed removal (maximum at point 4) with negative effects of control action used to remove the weed.

Panetta, F.D., O’Loughlin, L.S. & Gooden, B. (2019) Identifying thresholds and ceilings in plant community recovery for optimal management of widespread weeds. NeoBiota 42, 1-18 // PDF DOI

How practitioners integrate decision triggers with existing metrics in conservation monitoring

HIGHLIGHTS

  • There are challenges to integrating decision triggers into existing biodiversity monitoring programs.
  • Programs most frequently monitor direct measures of biodiversity assets (e.g. native species).
  • However, decision triggers are most frequently linked with measures of threats, monitored as surrogates for biodiversity.
  • Many operational barriers prevent practitioners from attaching decision triggers to direct measures.

Decision triggers are defined thresholds in the status of monitored variables that indicate when to undertake management, and avoid undesirable ecosystem change. Decision triggers are frequently recommended to conservation practitioners as a tool to facilitate evidence-based management practices, but there has been limited attention paid to how practitioners are integrating decision triggers into existing monitoring programs. We sought to understand whether conservation practitioners’ use of decision triggers was influenced by the type of variables in their monitoring programs. We investigated this question using a practitioner-focused workshop involving a structured discussion and review of eight monitoring programs. Among our case studiesdirect measures of biodiversity (e.g. native species) were more commonly monitored, but less likely to be linked to decision triggers (10% with triggers) than measures being used as surrogates (54% with triggers) for program objectives. This was because decision triggers were associated with management of threatening processes, which were often monitored as a surrogate for a biodiversity asset of interest. By contrast, direct measures of biodiversity were more commonly associated with informal decision processes that led to activities such as management reviews or external consultation. Workshop participants were in favor of including more formalized decision triggers in their programs, but were limited by incomplete ecological knowledge, lack of appropriately skilled staff, funding constraints, and/or uncertainty regarding intervention effectiveness. We recommend that practitioners consider including decision triggers for discussion activities (such as external consultation) in their programs as more than just early warning points for future interventions, particularly for direct measures. Decision triggers for discussions should be recognized as a critical feature of monitoring programs where information and operational limitations inhibit the use of decision triggers for interventions.

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Fig. 1. Representation of a ‘typical’ monitoring program discussed at the workshop, illustrating how measured variables link to management goals, what types of measures were used to link variables and goals (direct measures or surrogates), which links had decision triggers or potential triggers attached, and whether intervention or discussion activities were triggered (see Table 1 for definitions of classifications used). Our approach intentionally simplified a monitoring program down to its core components to (1) focus explicitly on how goals are informed by measures (not how measures inform other measures, or how goals inform other goals) and (2) allow comparison among dissimilar programs.

Foster*, C.N., O’Loughlin*, L.S., Sato, C.F., Westgate, M.J., Barton, P.S., Pierson, J.C., Balmer, J.M., Catt, G., Chapman, J., Detto, T., Hawcroft., A., Jones, G., Kavanagh, R.P., McKay., M., Marshall, D., Moseby, K.E., Perry, M., Robinson, D., Seddon, J.A., Tuft., K. & Lindenmayer, D.B. (2019) How practitioners integrate decision triggers with existing metrics in conservation monitoring. Journal of Environmental Management 230, 94-101 *joint lead-authors // PDF DOI

 

Higher-taxon and functional group responses of ant and bird assemblages to livestock grazing: A test of an explicit surrogate concept

Highlights

  • Surrogates of biodiversity responses to ecological treatments require explicit validation.
  • Different sub-components of biotic assemblages can respond to treatments in ways that differ from overall species richness.
  • Simple correlation between surrogate and target variables might not provide inference of matched response to a treatment.

Biodiversity monitoring programs are routinely established to quantify changes in biotic communities in response to land management. Surrogacy is implicitly used in many such monitoring programs whereby the measurement of a component of biodiversity is used to infer responses of broader biodiversity. Yet rarely is this surrogacy validated by demonstrating that measured variables and the target variable of interest have matching responses to management treatments. Here we examined the responses of higher-taxon and functional groupings of ants and birds (our surrogate variables) two years after the implementation of experimental livestock grazing treatments, and compared these with the responses of total ant and bird species richness (our target variables) to the same treatments. We found significant and strong correlations between surrogate and target variables, but this did not predict corresponding similar response to treatments. For ants, we found that the genus Monomorium had a negative response to the grazing exclusion treatment, but there was no matching response of species richness, and so no surrogacy was identified. For birds, total species richness had a weak positive response to spring/summer grazing exclusion, and the abundance of honeyeaters (Meliphagidae) showed a similar positive response, suggesting surrogacy. Our study highlights that correlations among variables do not necessarily lead to surrogacy, and indeed that different sub-components of biotic assemblages can respond in ways that contrast with overall species richness. Careful assessment of the matched responses of surrogate and target variables to management can provide a simple and robust way to critically assess biodiversity surrogacy.

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Fig. 1. A surrogate concept that incorporates the relationships between treatment, surrogate, and target, as well as covariates. (a) We quantified the effects of the grazing treatments on a range of candidate surrogate variables, as well as (b) treatment effects of the target for both bird and ant assemblages. Environmental covariates were also considered in separate models. (c) We then examined the correlations between surrogate and target variables to see if this gave any insight into which surrogate and target responses to the treatment were similar.

Barton, P.S., Evans, M.J., Sato, C.F., O’Loughlin, L.S., Foster, C.N., Florance, D. & Lindenmayer, D.B. (2019) Higher-taxon and functional group responses of ant and bird assemblages to livestock grazing: A test of an explicit surrogate concept. Ecological Indicators 96, 458-465 // PDF DOI

Are you using surrogates in your research? Almost definitely

All over ecological research, inferences are being made from related yet indirect measures.

Some of it is explicit, like in applied research where surrogate measures are used to represent a true attribute of interest that is too difficult or costly to measure directly. However, some is implicit, like in fundamental research where ‘productivity’ is almost exclusively inferred from other measures, such as chlorophyll A, leaf litter biomass, or remotely sensed ‘greenness’ metrics.

‘But hold on’ I hear you saying….. Those are the well-established measures demonstrated to accurately represent productivity.

Sure. But to generalise that there is no context-specificity, variability, uncertainty or error in how well your measure represents your target (1) mischaracterises the risks associated with indirect measures, (2) does not consider that the inference could be wrong, and (3) may result in over-simplification and a false-confidence of understanding.

Even for the measures used to represent productivity.

In an essay just published in BioScience, my co-authors and I discuss how many standard approaches for measuring ecosystem properties and processes are in fact surrogates, in order to highlight the broader usefulness of a whole body of literature for evaluating variability, error and context-dependency in surrogate-target relationships.

O’Loughlin, L.S., Lindenmayer, D.B., Smith, M.D., Willig, M.R., Knapp, A.K., Cuddington, K., Hastings, A., Foster, C.N., Sato, C.F., Westgate, M.J. & Barton P.S. (2018) Surrogates Underpin Ecological Understanding and Practice. BioScience [early view]

Ecological surrogacy is typically only considered as something for applied research on conservation monitoring and management. This is where time and resources are most limiting, both in terms of the capacity to collect information and use that information to inform management decisions. As a result, you can’t always collect the most ideal measures.  And therefore, evaluating how accurately one measure relates to a different target – along with acknowledging trade-offs, uncertainty and risk – is of critical importance.

However, thinking this way is also important in fundamental research. Evaluating surrogacy establishes the conditional boundaries under which relationships between variables exist. In our paper, we discuss how this may be under-appreciated when it comes to many ecological relationships that are considered well-established and therefore generalisable across all situations.

We know why this occurs – there is a need to simplify complexity. And of course all associations in ecology have variability, error and context specificity. The key point of our paper is that no one benefits from simply ignoring that.

Instead, improved recognition of surrogate use in fundamental research probably has some benefits. Surrogate research offers a whole literature of frameworks and approaches for accounting for uncertainty and ensuring accuracy in inferences being made.

We contend that greater recognition of surrogacy could represent a significant knowledge transfer from applied to fundamental research. Adopting the tools for evaluating surrogates will allow researchers to quantitatively formalize trade-offs between accuracy and cost-effectiveness to more clearly justify the use of surrogates, whatever the context.

Hopefully this paper will get you thinking about the measures in your own research that you may be implicitly using as surrogates. These could be almost anything. For example, are you really measuring fire severity, or are you measuring scorch-height on tree trunks and simply inferring that that accurately represents fire severity?

The important question that you should hopefully come away from our paper asking is:  do I need to contextually evaluate the measure I’m using to make this inference, to ensure it accurately represents what I’m saying it represents? 

Ultimately this comes down to whether you are making generalisations or context-specific evaluations and what level of accuracy and certainty is acceptable in either. It’s not always going to be the same.

SURROGATES AND ECOLOGICAL THEORY WORKSHOP. Participants from left to right: Melinda Smith, Kim Cuddington, Alan Knapp, Alan Hastings, Claire Foster, Micheal Willig, Chloe Sato, Philip Barton, David Lindenmayer. Absent from photo due to illness on the day: Martin Westgate

How (un)necessary​ is your response to a published article?

The time-honored tradition of writing a response (or rebuttal) to a published article is vital to the progression of science, providing active debate, pointing out flaws in papers, and ensuring the validity of claims (read more here). But are we always doing it right? And with the correct intentions?

Associate Professor Trevor A. Branch from Washington University recently posted a thread about this on Twitter (see it HERE), which I considered particularly timely as I was in the process of preparing a reply, to a response, to an article my supervisor and I had published.

Trevor’s key points were that we shouldn’t be writing responses as a way of making publication lists look numerically more impressive, and there are simple approaches that can be taken to ensure that the response is needed in the first place (i.e. are you sure you have a meaningful correction to make or have you just misunderstood something).

You can read our original paper HERE, the response letter HERE, and our reply to that HERE. The short summary of this is (1) we wrote a concept paper that detailed a unique invasion phenomenon and defined it by the term currently being used haphazardly and without definition to describe that and other dissimilar phenomena. (2) the response letter had no problem with our highlight and discussion of the invasion phenomena, but took issue with the term we used, suggesting we had redefined something, and so they were now re-redefining it. (3) we then systematically responded to their claims, again making the point that we hadn’t redefined anything because the term was undefined, and so we had actually un-undefined it.

SecondaryInvasionChain

Fun play-on-words aside, it felt like I was just writing a response letter to a Reviewer that had only fleetingly sort to understand the article. A lot of “The authors claim we didn’t consider X, when actually it was a major component of our idea with a dedicated section and 2 Figures in the original article” ….. yep.

Let’s step through Trevor’s 6-step guide to best practice responding and see how we faired shall we:

1. Email the authors expressing your concerns. You might have misunderstood the paper or have it wrong. 

I was first contacted by the journal editor with a copy of an accepted response letter to my article with an invitation to prepare a reply. Never received any direct contact from the authors of the response regarding our article at any point. Upon first read of their response, it was clear that most of the criticisms they had of our article were a misinterpretation of some of our points, that could have easily been corrected by either a more careful reading of our article, or by just shooting off an email.

2. If you found something wrong, and the original authors agree, consider writing a joint paper with the original authors that corrects the scientific record. This is much more powerful than a we-said-they-said narrative, which seldom changes people’s minds

This is an interesting idea from Trevor, and I would be interested to know how frequently this occurs. A key point I take away from this is about opening a dialogue to make sure that what you think is something wrong, is something that actually is wrong. Rather than a we-said-they-said narrative, the article-response-reply chain that I’m now a part of is really a we-said-they-claim-we-said-so-then-they-said-but-no-we-actually-said narrative…….which, yeah, is not at all useful. Just read our original paper and make up your own mind.

3. If you can’t reach agreement with the original authors, write your rebuttal, and share it with the original authors so they have time to write a response that can be published at the same time as yours. This is common courtesy.

We were invited to prepare a reply and given plenty of time to do so (and I would hope that would be the standard). But I guess there was also the option of dismissing the response as being inaccurate and not a meaningful contribution to the literature (which is essentially what our reply letter is). Which brings up a great point…..Was this letter even peer-reviewed?!? I wouldn’t have thought so given so much of it is simply mischaracterizing our original paper (which I would think would be cause for rejection). My supervisor and I prepared a 5000-word concept paper full of case studies and detailed discussion that was updated and improved by the considered comments and suggestions of reviewers. But now there is a 700-word letter designed to supersede our article (through reframing and repackaging our ideas with new terms) but without the same detail or critical oversight. Surely this is not good science. I would suggest another peer-reviewed 5000-word concept paper full of case studies and detailed discussion would be a better response ……..

4. Remember that the point of a rebuttal is not to get yourself a published paper. It is to correct the scientific record, so that flawed science does not persist.

Is the written response just about showboating or is there genuine and important criticisms? I propose a pretty simple test for determining this before you even read the content of a response….. Does the title of the response article include explicit reference to the article they are responding to?? Is it called “Something something: a response to Jones et al.” or is it just called “Something something”. I contend that if you do not reference the article in the title of your response you are being deliberately misleading by making it appear like your response is actually an original article. Did the authors of the response letter include reference to our original article in their title…….? NOPE. So we did the same thing with the title of our reply so that the chain didn’t look like their paper was the original article and we were the responders. So we were misleading in response to other misleading activity…… Do two wrongs make a right?

5. Remember that rebuttals seldom change people’s minds, get cited 1/10th as much as the original papers, and generally don’t have much effect on citations or perceptions of the original paper.

I don’t think there is any reason to ever cite the response letter or our reply letter. Unless you are looking for an example where somehow a response letter has been published that presents a key component of an original article, as an independent argument against the original article……… Our original paper on the other hand – definitely worth citing – whether you agree with the ideas or not (that’s the point of an ideas paper I thought!)

6. With these guidelines, you can at least reduce the chance of making enemies by writing rebuttals, increase the chance of changing the minds of the original authors, and sway the minds of the bystanders watching the debate. A joint correction is great science and politics.

Have the authors of the response letter ever gotten in contact with me? Nope. Have I ever gotten in contact with them? Nope (why should I make the first move……?). Are we enemies? I hope not, I find many aspects of their research really useful and I cite them regularly. Do I care? Care enough to dedicate an hour to prepare this I guess.

Mostly I’m just annoyed.

When visiting the pub is good for your science

 

I have published a letter in Science about how visiting the pub was a benefit to my research. Yeah seriously, Science! Read it here: No one is an island

OK – some context. Science has a ‘Working Life’ forum dedicated to stories about academic life. Some are positive (e.g. My children help my science), others less so (e.g. Sexual harassment and the toll it takes), but the overall idea is to share experiences that other scientists might find useful to hear (especially if they are going through something similar).

My Ph.D. involved heaps of remote field work, for long stints of time, where I was working alone, and away from my family and friends. So yeah, at times I felt a bit isolated. It wasn’t long into my first field season that I thought my research would suffer if I didn’t look after my mental wellbeing by finding myself a local support network.

So I reached to the people of Christmas Island. Integrated into their tight-knit local community. Made a bunch of lifelong friends who not only helped with my feelings of isolation, but offered practical support throughout my field work that ultimately improved much of my science.

At the centre of this story is the local pub. That was where I first started to meet new friends and where (years later) I organised a raucous rock ‘n roll gig to give back to the community that had helped me out so much.

Hopefully hearing my story is (somewhat) helpful for other students and researchers who find themselves away from their own community and feeling a bit isolated. My recommendation is to drop into the local pub as a first step to becoming part of a new community.

Also, here is a citable letter in Science that promotes going to the pub – enjoy. Maybe that might be the most useful aspect of this ……

Cheers

 

Review of historic stock routes may put rare native plants and animals at risk

TSR-Gate

Since the 19th century, Australian drovers have moved their livestock along networks of stock routes. Often following traditional Indigenous pathways, these corridors and stepping-stones of remnant vegetation cross the heavily cleared wheat and sheep belt in central New South Wales.

The publicly owned Travelling Stock Reserve network of New South Wales is now under government review, which could see the ownership of much of this crown land move into private hands.

But in a study published today in the Australian Journal of Botany we suggest that privatising stock routes may endanger vital woodlands and put vulnerable species at risk.


Read more: How ancient Aboriginal star maps have shaped Australia’s highway network


The review will establish how individual reserves are currently being used. Although originally established for graziers, the patches of bush in the network are now more likely to be used for recreation, cultural tourism, biodiversity conservation, apiary and drought-relief grazing.

This shift away from simply moving livestock has put pressure on the government to seek “value” in the network. The review will consider proposals from individuals and organisations to buy or acquire long-term leases for particular reserves.

It is likely that most proposals to purchase travelling stock reserves would come from existing agricultural operations.

A precious national resource

Travelling stock reserves across New South Wales represent some of the most intact examples of now-endangered temperate grassy woodland ecosystems.

Our research found that changing the status or use of these reserves could seriously impact these endangered woodlands. They criss-cross highly developed agricultural landscapes, which contain very limited amounts of remnant vegetation (areas where the bush is relatively untouched). Travelling stock reserves are therefore crucially important patches of habitat and resources for native plants and animals.

This isn’t the first time a change in ownership of travelling stock reserves has been flagged. Over the last century, as modern transport meant the reserves were used less and less for traditional droving, pressure to release these areas for conventional agriculture has increased.

Cattle grazing_IMG_1197_DANIEL_FLORANCE

To understand what a change in land tenure might mean to the conservation values of these woodlands, we spent five years monitoring vegetation in stock reserves in comparison to remnant woodlands on private farmland.

We found that travelling stock reserves contained a higher number of native plant species, more native shrubs, and less exotic plants than woodland remnants on private land.

The higher vegetation quality in travelling stock reserves was maintained over the five years, which included both the peak of Australia’s record-breaking Millennium Drought and the heavy rainfall that followed, referred to as the “Big Wet”.

The take-home message was that remnant woodland on public land was typically in better nick than in private hands.

Importantly, other studies have found that this high-quality vegetation is critical for many threatened and vulnerable native animals. For example, eastern yellow robins and black-chinned honeyeaters occur more frequently in places with more shrubs growing below the canopy.

superb parrot DAMIAN MICHAEL

The contrast we saw between woodlands in travelling stock reserves and private land reflects the different ways they’re typically managed. Travelling stock reserves have a history of periodic low-intensity grazing, mostly by cattle, with long rest periods. Woodland on active farms tend to be more intensively grazed, by sheep and cattle, often without any strategic rest periods.

The stock reserves’ future

The uncertain future of travelling stock reserves casts doubt on the state of biodiversity across New South Wales.

The current review of travelling stock reserves is considering each reserve in isolation. It flies in the face of the belief of many managers, practitioners and researchers that the true value of these reserves is in the integrity of the entire network – that the whole is greater than the sum of its parts.

Travelling stock reserves protect threatened species, allow the movement of wildlife, are seed sources for habitat restoration efforts, and support the ecosystem of adjacent agricultural land. These benefits depend on the quality of the remnant vegetation, which is determined by the grazing regime imposed by who owns and manages the land.

Of course, not all travelling stock reserves are in good condition. Some are subject to high-intensity livestock grazing (for example, under longer-term grazing leases) coupled with a lack of funding to manage and enhance natural values.

Changing the land tenure status of travelling stock reserves risks increasing grazing pressure, which our study suggests would reduce ecosystem quality and decrease their conservation value.

The travelling stock routes are important parts of our ecosystem, our national heritage, and our landscape. They can best be preserved by remaining as public land, so the entire network can be managed sustainably.

This requires adequate funding for the Local Land Services, so they can appropriately manage pest animals, weeds, erosion and illegal firewood harvesting and rubbish dumping.

Travelling stock reserves are more than just The Long Paddock – they are important public land, whose ecological value has been maintained under public control. They should continue to be managed for the public good.

This article was originally published on The Conversation. Read the original article.

This article was based on research published in Australian Journal of BotanyRead the full article.

Secondary invasion: When invasion success is contingent on other invaders altering the properties of recipient ecosystems

Fig4

Two examples of interacting organisms and how the invasion success of the secondary invader is contingent on the presence of a primary invader modifying a native component of the recipient ecosystem. (a) Mutualism between invasive yellow crazy ants and scale insects removes the native red crab which allows entry into the community by the giant African land snail that were previously predated upon by the native crab (Green et al., 2011). (b) The invasive green crab preferentially predates the native Nutricola clam, allowing the population release of the exotic clam, Gemma gemma, which was competitively inferior to the native clams (Grosholz, 2005). Solid and dashed lines denote direct and indirect interactions respectively. Circles and triangles denote negative and positive interactions, respectively. Clam photographs include ruler for scale (1 mm between lines). All pictures in example (b) by Dr E Grosholz

Positive interactions between exotic species may increase ecosystem-level impacts and potentially facilitate the entry and spread of other exotic species. Invader-facilitated invasion success—”secondary invasion”—is a key conceptual aspect of the well-known invasional meltdown hypothesis, but remains poorly defined and empirically underexplored. Drawing from heuristic models and published empirical studies, we explore this form of “secondary invasion” and discuss the phenomenon within the recognized conceptual framework of the determinants of invasion success. The term “secondary invasion” has been used haphazardly in the literature to refer to multiple invasion phenomena, most of which have other more accepted titles. Our usage of the term secondary invasion is akin to “invader-facilitated invasion,” which we define as the phenomenon in which the invasion success of one exotic species is contingent on the presence, influence, and impacts of one or more other exotic species. We present case studies of secondary invasion whereby primary invaders facilitate the entry or establishment of exotic species into communities where they were previously excluded from becoming invasive. Our synthesis, discussion, and conceptual framework of this type of secondary invasion provides a useful reference to better explain how invasive species can alter key properties of recipient ecosystems that can ultimately determine the invasion success of other species. This study increases our appreciation for complex interactions following invasion and highlights the impacts of invasive species themselves as possible determinants of invasion success. We anticipate that highlighting “secondary invasion” in this way will enable studies reporting similar phenomena to be identified and linked through consistent terminology.

Read the paper in-full here: DOI // PDF

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Invasibility doesn’t always equal impact: an example of a highly-abundant exotic that is not a damaging invader

File 20170722 28465 ngf5eh

The giant African land snail is a poster child of a global epidemic: the threat of invasive species. The snails are native to coastal East Africa, but are now found across Asia, the Pacific and the Americas – in fact, almost all tropical mainlands and islands except mainland Australia.

Yet, despite their fearsome reputation, our research on Christmas Island’s invasive snail population suggests the risk they pose to native ecosystems has been greatly exaggerated.

Giant African land snails certainly have the classic characteristics of a successful invader: they can thrive in lots of different places; survive on a broad diet; reach reproductive age quickly; and produce more than 1,000 eggs in a lifetime. Add it all together and you have a species recognised as among the worst invaders in the world.

The snails can eat hundreds of plant species, including vegetable crops (and even calcium-rich plaster and stucco), and have been described as a major threat to agriculture.

They have been intercepted at Australian ports, and the Department of Primary Industries concurs that the snails are a “serious threat”.

Despite all this, there have been no dedicated studies of their environmental impact. Some researchers suggest the risk to agriculture has been exaggerated from accounts of damage in gardens. There are no accounts of giant African land snails destroying natural ecosystems.

Quietly eating leaf litter

In research recently published in the journal Austral Ecology, we tested these assumptions by investigating giant African land snails living in native rainforest on Christmas Island.

Giant African land snails have spread through Christmas Island with the help of another invasive species: the yellow crazy ant.

Until these ants showed up, abundant native red land crabs ate the giant snails before they could gain a foothold in the rainforest. Unfortunately, yellow crazy ants have completely exterminated the crabs in some parts of the island, allowing the snails to flourish.

We predicted that the snails, which eat a broad range of food, would have a significant impact on leaf litter and seedling survival.

However, our evidence didn’t support this at all. Using several different approaches – including a field experiment, lab experiment and observational study – we found giant African land snails were pretty much just eating a few dead leaves and little else.

We almost couldn’t distinguish between leaf litter removal by the snails compared to natural decomposition. They were eating leaf litter, but not a lot of it.

We saw almost no impact on seedling survival, and the snails were almost never seen eating live foliage. In one lab trial, we attempted to feed snails an exclusive diet of fresh leaves, but so many of these snails died that we had to cut the experiment short. Perhaps common Christmas Island plants just aren’t palatable.

It’s possible that the giant African land snails are causing other problems on Christmas Island. In Florida, for example, they carry parasites that are a risk to human health. But for the key ecological processes we investigated, the snails do not create the kind of disturbance we would assume from their large numbers.

The assumption that giant African land snails are dangerous to native plants and agriculture comes from an overriding sentiment that invasive species are damaging and must be controlled.

Do we have good data on the ecological impact of all invasive species? Of course not. Should we still try to control all abundant invasive species even if we don’t have evidence they are causing harm? That’s a more difficult question.

The precautionary principle drives much of the thinking behind the management of invasive species, including the giant African land snail. The cost of doing nothing is potentially very high, so it’s safest to assume invasive species are having an effect (especially when they exist in high numbers).

But we should also be working hard to test these assumptions. Proper monitoring and experiments give us a true picture of the risks of action (or inaction).

The ConversationIn reality, the giant African land snail is more the poster child of our own knee-jerk reaction to abundant invaders.

Luke S. O’Loughlin, Research fellow, La Trobe University and Peter Green, Head of Department, Ecology, Environment and Evolution, La Trobe University

This article was originally published on The Conversation. Read the original article.

This article was based on research published in Austral EcologyRead the full article.

The secondary invasion of giant African land snail has little impact on litter or seedling dynamics in rainforest

In the absence of empirical evidence, invasive species are often assumed to have negative impacts because of their conspicuously high abundance. The giant African land snail Achatina(Lissachatinafulica is one such invader where its impact in natural ecosystems remains completely untested. On Christmas Island (Indian Ocean), A. fulica has become established across large tracts of rainforest following the impacts of invasive yellow crazy ant (Anoplolepis gracilipes) in mutualism with non-native scale insects. Yellow crazy ants facilitate the secondary invasion of A. fulica by extirpating native red land crabs (Gecarcoidea natalis) that are normally effective predators of A. fulica. We used a multifaceted approach to investigate some potential impacts of abundant A. fulica in invaded rainforest. Over the course of a wet season, diel activity transects showed that A. fulica consumed detrital material almost exclusively. However, stable isotope analysis did not confidently identify A. fulica as a predominantly detritivorous species. We found no statistically significant treatment effects of A. fulica exclusion on standing leaf litter and seedling recruitment processes during a 6-month manipulative field study. However, litter cover and biomass did remain slightly higher where A. fulica were excluded, albeit with overlapping confidence intervals with control plots. Our study constitutes the first empirical test for impact of A. fulica in a natural ecosystem and suggests that for Christmas Island rainforest, this species is not a damaging invader. Other studies will need to assess the impacts of A. fulica in other natural areas before these findings could be considered broadly applicable.

Littlest exotic species provide evidence for big invasion concept [reprise]

A couple of years ago I published an observational study on invasive ants facilitating exotic snails by killing native crabs. That’s the specifics that may be of limited interest, but the interactions (invader-facilitated invasion success) are broadly relevant to anyone studying the dynamics of multi-invaded complex systems. The overall conclusion is that we need to better recognise influential invasive species as intrinsic properties of recipient communities. You can quickly skim the paper by watching the video below, then I highly recommend reading the rest of this short blog that summarises the whole thing:

Humans have been really good at moving species all over the world, and in turn, exotic species have been really good at becoming abundant and influential in their new environments. However, sometimes the conditions in a new environment wont allow an exotic species to get their foot in the door and become invasive. For that to happen, conditions need to change – and who better placed to drive that change than other, previously successful, exotic species.

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It is well established that the impacts of invasive species can seriously alter ecosystem properties. For example, the invasion of a highly-flammable grass will increase fire intensity and frequency in that environment. What is less established is how these altered ecosystems can now be invaded by other, previously unsuccessful, exotic species – in short, exactly how one exotic species will indirectly help another.

The concept being described here is that of ‘secondary invasion’. Most people will have an implicit understanding of the idea based on the more everyday notion of ‘secondary inflections’. A body contracts some pathogen (successful primary invader) making the body ill (altering the conditions of the environment) which leads to the body contracting other infections it otherwise wouldn’t have caught (previously unsuccessful secondary invader).

In ecological research, secondary invasion is a key part of the ‘invasional meltdown hypothesis’ (two invaders working together that amplifies impacts and accelerates more invasions) but was not explicitly defined as such by the authors. Actually, the concept itself remains undefined in the scientific literature and in desperate need of some dedicated investigation and empirical evidence.

A heavily invaded rainforest ecosystem on Christmas Island has long provided some of the strongest evidence in support of the ‘invasional meltdown hypothesis’, and is now also lending key support to this concept of secondary invasion.

The highly abundant red land crab dominates Christmas Island (natural densities of around 1 crab every square meter) and is responsible for the unique open structure and sparse ground layer typical of the rainforest ecosystem on the island (see below). Enter the invasive yellow crazy ant, that when fueled by the sugar of exotic honeydew-producing scale insects, cause local extinctions of the red land crab. The deletion of this ecosystem-engineer causes a pulse recruitment of seedlings and the build-up and persistence of leaf litter. The influence of the crab as a predator is also removed. The end result is an environment with a completely different set of conditions that other invasive species will respond to.

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The rainforest community on Christmas Island has quite different characteristics before and after the impacts of primary invaders.

These changes were recognised early on as a great benefit to the exotic giant African land snail, which had been hanging-out on the island since WWII but had never been able to enter the rainforest without being quickly eaten by abundant red land crabs. The yellow crazy ant (primary invader) had indirectly caused the invasion success of the giant African land snail (secondary invader) by deleting the red land crab and creating enemy-free space (altering the conditions of the environment).

However, the giant African land snail is just one of many land snail species on Christmas Island – most of which are exotic and all of which are very small (20 – 1 mm in length). In a study published earlier this year in Biological Invasions, Pete Green and myself investigated whether the invasion success of these snails also depended on the yellow crazy ants altering the environment.

We found that in rainforest where the yellow crazy ant had removed the red land crab these smaller snails were an order of magnitude more abundant – the difference of only a few individuals compared to hundreds for every sample of leaf litter. When comparing intact to impacted rainforest there was no difference in total number of species (~20) or species composition as calculated from presence absence data, meaning all species were essentially at all sites.

Data

The key results shortened from O’Loughlin & Green (2015) Biological Invasions 17, 2659 – 2674. Data is from quadrats during one wet across four forest states; Intact (abundant red crabs, native control), Supercolony (abundant yellow crazy ant, no red crabs), Ghosted (no red crabs, no yellow crazy ants), Recovered (previously a supercolony that was managed and had red crabs return).

This pattern of success was markedly different from what had been initially observed for the giant African land snail. These smaller species were able to enter the rainforest where crabs were present without issue. Yet they still responded positively (increased abundance) when the invasive ants deleted the native crabs. So how does this help inform our concept of secondary invasion?

When considering what determines invasion success, we need to establish when a species goes from simply being ‘exotic’ to ‘invasive’. It is generally considered that ‘invasive’ be reserved for those species that are well established in highly abundant populations, with the capacity to quickly spread and potential to impact the environment. That means the conditions limiting a species could be at any of the earliest points on the invasion pathway – either at the transport, entry or establishment stage.

What we have here on Christmas Island – with our investigation of the entire land snail fauna – is evidence for two distinct pathways of secondary invasion. A true-entry model (the giant African land snail that could not get into the rainforest at all) and a population-release model (the other smaller exotic species that could enter the rainforest but only persist at very low abundances). Both were indirectly facilitated by primary invaders to establish high abundant populations – by definition, ‘invasive’ – but from different starting points.

Often the focus of research is at a global scale, with big data, attempting to make the broadest generalization on the nature of something (which is of course important). The results and implications of this publication demonstrates that the intricate details of specific phenomena will continue to be understood through dedicated small-scale investigations, on literally some of the littlest species.