The map matters: crop-dominated landscapes have higher vector-borne plant virus prevalence

In this post Suzi Claflin discusses her paper ‘Crop-dominated landscapes have higher vector-borne plant virus prevalence

It’s been clear for some time that landscape composition—that is, land-use types and the connections between them—strongly affects the community of creatures living in a given area. When it comes to insects, the landscape has been shown to shape the communities of both the ones we like, such as the beneficial predators, the ladybirds, and the ones we dislike, such as the infamous farm pest, the aphid. But what about the effect on what those insects might be carrying? What about insect-borne disease?

That’s the crux of what we explored in our study: how landscape composition impacts the spread of an aphid-borne plant virus (specifically, potato virus Y or PVY). And what we found is both intuitive and exciting. Areas with greater amounts of cropland had greater prevalence of PVY, likely because of the effect of landscape composition on the aphid vector (disease carrier) community.

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Potato fields. Photo credit: Flickr.

That result— while novel— makes sense, when you consider the natural history of aphids. Although they are often fairly specific in their taste in host plants, they are lousy at host selection. They identify hosts haphazardly from the air based on colour contrast (if it’s green, it’s good), then descend and probe the plant to test for tastiness, often only to find they have made a terrible mistake. They then fly off to the next plant. This is the perfect mechanism for virus spread.

On the face of it, that would mean the more aphids there are, the more the disease will spread. And that’s generally true. But, it’s not quite that simple. Some aphid species are better at spreading PVY than others (some can’t spread it at all), so it’s really the more good PVY carriers there are, the more PVY will be spread. Considering that, it makes sense that the aphid community (both abundance and species richness) should have an effect on PVY prevalence.

That brings us back to the landscape’s role. As a network of resources that are more or less palatable to specific species, the landscape composition should have a profound effect on the aphid community. And that’s what we found: the amount of cropland in an area impacts aphid species richness.

What’s most exciting about these results is their scale. We found effects of landscape composition on virus prevalence from 500 to 1500 metres around our research sites, which were on working farms. That means that the risk of disease spread could potentially be controlled on even small-scale farms.

Here’s how:

  1. Isolate your potatoes. Keep them away from your other crops by at least 500 m.
  2. Insulate your potatoes with non-crop plants. Surrounding potatoes with forested and other unmanaged land areas appears particularly effective.
  3. Do not save potato seed tubers. Although a common practice on small-scale farms, saving potato seed tubers to plant in the next year greatly increases your risk of introducing the virus into your crop at planting. When you plant saved seed, you are often literally sowing the seeds for later disease spread in your crop. Instead, plant potato seed produced by a certified grower who tests for PVY.

Plant disease can seem unstoppable, something that blows through with no warning and leaves you with no recourse. Our work suggests that the problem might be perspective. This study demonstrates that, at least in the PVY system, a wider view can offer clues for how to contain the disease. It shows that even for insect-borne plant viruses, the map matters.

Climate change and food security

In this post, Adam Frew discusses his paper ‘Increased root herbivory under elevated atmospheric carbon dioxide concentrations is reversed by silicon-based plant defences

As the global climate changes the global population continues to rise, we are faced with the daunting challenge of achieving sustainable crop production to meet the increasing demand for food. Professor John Beddington in 2009, UK chief scientist at the time, highlighted this potential ‘perfect storm’ of global events and the urgent need to address these challenges.

Insect herbivores are one of the main contributors to crop losses globally, which have traditionally been dealt with, with the application of pesticides. In fact, over the last 40 years, there has been a seven-fold increase in insecticide usage (Tilman et al. 2001). However the application of insecticides, often prophylactically, is environmentally damaging, unsustainable and costly. This has led to increased restrictions on insecticide application, and highlights the need for new methods of crop protection to ensure global food security under climate change. While climate change incorporates several factors including changes in global temperature and rainfall, we were interested in the impacts of elevated atmospheric carbon dioxide concentrations (eCO2) on insect herbivory.

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Sugarcane field, Queensland.

Impacts of elevated CO2 on plants and insects

Indeed, eCO2 is known to alter plant defence mechanisms, their chemistry and physiology. Typically, as concentrations of CO2 increase, the net carbon uptake of plants increases which causes the carbon to nitrogen ratio (C:N) of plant tissue to increase. This C:N is an indicator of the nutritional value of plant material to insect herbivores, as nitrogen is typically a limiting factor in their diets. Therefore, many insects have exhibited a compensatory feeding response to an increase in C:N, as they attempt to meet their nitrogen dietary requirements. This increase in consumption suggests a possible exacerbation of damage from insect herbivores under eCO2. Of course, it is difficult to truly replicate the gradual increases in global atmospheric CO2 concentrations, and possible acclimation responses to eCO2 by plants is difficult to test experimentally, although may not be so relevant to semi-perennial/annual plant systems and agricultural crops that are regularly harvested.

There are relatively few studies that focus on climate impacts on root-feeding insects (compared to aboveground insects), yet these belowground herbivores significantly reduce yield in many agricultural systems. It is therefore important to understand how plant-insect interactions belowground will be altered by eCO2, especially in the context of novel control strategies that remediate any possible adverse effects of climate change in crop plant susceptibility to root-feeding insects.

Plant silicon defences

One promising avenue of research is plant silicon and the ecological significant of silicon in plants is only now being realised (see the Functional Ecology special issue ‘The Functional Role of Silicon in Plant Biology’ – Cooke, DeGabriel & Hartley 2016). Silicon plays a role in plant defences against pathogens and herbivores, and is known to promote growth in many plants, particularly those among the Poaceae, although this is a gross generalisation (see Katz 2014, 2015). Silicon is also known to alleviate abiotic plant stress including heavy metal toxicity, drought and heat stress (see meta-analysis by Cooke & Leishman 2016). Silicon in plants has been shown to have considerable negative effects on insect herbivores in aboveground systems (Reynolds, Keeping & Meyer 2009), yet only one study has investigated the impact of plant silicon on a root-feeding insect (Frew et al. 2016).

Study system

Considering that eCO2 dramatically alters plant-insect relationships and the importance of plant silicon defences and the effects on insect herbivores, we saw the need to investigate the efficacy of plant silicon-based defences against root feeding insects under eCO2. Sugarcane (Saccharum spp. hybrids) is a grass crop that is grown across Queensland and northern New South Wales, Australia. Larvae of the greyback cane beetle (Dermolepida albohirtum), known as canegrubs, feed on sugarcane roots and cause the sugar industry losses up to AU$40 million (~£24.4 million) when outbreaks occur. The economic significance of this pest provided a good model to investigate the impacts of plant silicon defences on a root-feeding insect under eCO2.

Study findings

Our study found that eCO2 increased plant photosynthesis and biomass. While at the same time we observed an increase in root C:N, indicative of a decrease in plant nutritional value. We found that the canegrubs increased their consumption of sugarcane roots under eCO2 in response to this decrease in nutritional value, which has been observed in many other insect herbivores. Interestingly, we also found that the canegrub growth rates increased. These responses suggest that in the future, damage inflicted by root-feeding insects to agricultural crops could be exacerbated by increases in CO2 concentrations.

However, our study also demonstrated that increasing plant silicon, by applying soil silicon fertiliser, dramatically decreases both root consumption and performance of the canegrub, under both ambient and elevated CO2 conditions. In fact, increasing soil silicon almost entirely masked the impacts of eCO2 on canegrub root consumption and growth rates. The negative effects of plant silicon on insect herbivores is largely attributed to an increase in the physical toughness of the plant tissue, reducing digestibility through mechanical protection of the parenchyma cells, where insects retrieve much of their starch and protein.

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Canegrubs.

Implications for agriculture

So what are the implications of our findings for crop production and agriculture? As current control strategies, such as the application of pesticides, are becoming more restricted and are often environmentally unsustainable, novel pest management alternatives are continually being researched. Plant silicon defences should play a central role in climate change remediation regarding pest management. While much work on the role of silicon in plants has focussed on high silicon accumulating Poaceae crops, silicon has been shown to have significant impacts in many non-grass species including horticultural crop species (Katz 2014).

Indeed, it is common for plant-available silicon to be depleted in agricultural systems, and as silicon is still not considered an essential nutrient for crops, it is not often considered when growers attempt to optimise growth and yield. Our study suggests it would be beneficial to characterise bioavailable silicon in field soils which would facilitate targeted application of silicon fertilisers, which are already commercially available to agriculture. Silicon uptake by crops can vary markedly between different varieties (Soininen et al. 2013), which highlights the potential for crop breeding programs to select for varieties with higher silicon accumulation traits. In taking these steps, the potential crop pest exacerbation by climate change could be remediated by exploiting a previously undervalued, natural plant defence.

 

 

Fatal attraction of Spotted Wing Drosophila to a yeast symbiont, for sustainable control

In this blog post Joelle Lemmen, Alix Whitener, Boyd Mori and Peter Witzgall discuss the recent paper by Boyd Mori and colleagues ‘Enhanced yeast feeding following mating facilitates control of the invasive fruit pest Drosophila suzukii

Spotted Wing Drosophila (SWD) is currently the most economically important insect in Europe and North America. SWD damages a wide range of our favourite berries and soft fruit, including blueberries, strawberries and cherries. Unfortunately, very few insecticides or biological methods are available to control this insect. SWD is a sign of our times. In the age of accelerating global trade, SWD was likely imported from Eastern Asia, to Europe and the USA, where the invasions have rapidly spread.

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SWD – Drosophila suzukii – pupae and larvae on cherry, raspberry and blueberry (Photo credit: Joelle Lemmen).

Five decades after “Silent Spring” by Rachel Carson, the most toxic insecticides have been deregulated in the USA and to a greater extent in Europe to protect consumers, beneficial animals and the environment. The drawback is that it has become more difficult for growers to control insect pests. Crop protection against insects is even more critical in a changing climate, since elevated temperatures and altered rainfall patterns facilitate insect outbreaks.

On the bright side – we are challenged to bring our knowledge of insects to practical application. An elegant approach is to use natural, non-toxic olfactory signals to manipulate insect behaviour. The “chemical ecology” research field investigates how insects use odours to find mates and host plants. Such odours or “semiochemicals” – mediators of insect behaviour – are widely used to control insects in orchards, forests and to some extent in agricultural crops.

Mori and colleagues neatly applied chemical ecology tools, showing that a yeast symbiont of SWD, acting as a natural producer and source of semiochemicals, can be used to control SWD. This work became possible through an exchange of researchers between the Chemical Ecology Unit, SLU Alnarp, Sweden and the Department of Entomology, WSU at Wenatchee, WA, USA, combining expertise in laboratory and real-world field studies of SWD chemical ecology and population control.

Many insects contain and carry symbiotic yeasts. These yeasts provide essential nutrients and protection against detrimental fungi, while insects enhance yeast growth in plants and ensure yeast dispersal.

Yeasts facilitate this collaboration by producing aroma compounds that attract insects. Mori and colleagues show how a yeast that is specifically associated with SWD attracts adult flies and elicits feeding. This yeast can be blended with a killing agent, an insecticide derived from a bacterium, which is not toxic for vertebrates. Application of the insecticide is restricted to bait drops, rather than spraying the entire crop, which eliminates contamination of the fruit. The result is an environmentally-friendly and sustainable “attract and kill” method. It may also become possible in the future to replace the insecticide component with an insect pathogen.

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SWD adult fly attracted to feed on yeast paste with a colourant. A biological insecticide could be added to this formulation for SWD control (Photo credit: Peter Witzgall).

A central idea behind this technique is that adult females are attracted to the yeast, before laying eggs in fruit. In comparison, insecticide sprays covering the entire crop mainly aim at control of larvae, the life stage producing the damage. It may be advantageous to instead target adult female flies, before they lay eggs, before the damage is done.

Research on biological techniques to control SWD is a current main effort in Europe and North America. The yeast-based attract-and-kill technique designed by Mori and colleagues is compatible with other emerging techniques, which can be combined for enhanced efficacy against this destructive pest.

Forest thinning: a bat’s friend or foe?

In this post, Rachel Blakey discusses her paper in the latest issue of Journal of Applied EcologyBat communities respond positively to large-scale thinning of forest regrowth

The world’s forests are changing. Most of the remaining forests are re-growing after being cleared, but do these regrowth forests resemble the original primary forests? Often, where large tracts of forests were cleared at once (e.g. clear-felling), they do not. Primary (or “old growth”) forest usually contains trees of variable ages with a variable structure whereas regrowth forest often has a dense uniform structure where trees have all grown at the same time. Dense stands of young trees all compete for the same resources, slowing their growth rate and leading to what foresters call “stand lock”.  This is bad for forestry, but also for biodiversity.  Mature forests provide resources that dense regrowth can’t. In Australia, tree hollows are a critical resource for a large number of endemic bats, birds and arboreal mammals. These habitat features can take centuries to form and are virtually non-existent in regrowth forests unless forest management particularly caters for their retention. For many threatened species, a century is too long to wait for habitat to return naturally and conservation managers are looking for ways to speed the transition from regrowth to mature forest.

“Early thinning” is the practice of cutting down a proportion of young trees within regrowth to allow faster growth. This technique, long used by forestry, is gaining traction as a method to speed up regeneration of forests and formation of habitat features (e.g. hollows) important for biodiversity. This has been met with healthy scepticism from conservation advocates, especially where it has been proposed in national parks, where thinning has been dubbed “logging by stealth”. However, long-term scientific research has provided evidence that thinning can improve the conservation value of forests. For example, one Australian study showed that after 42 years thinned stands produced around 20 hollow-bearing trees per hectare, whilst unthinned stands produced none. While these long-term benefits have been established, what are the shorter term effects of thinning and are they positive or negative for biodiversity?

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To answer this question, we investigated a group of animals that are incredibly sensitive to forest structure: insectivorous bats. Insectivorous bats use echolocation to navigate their surroundings and capture prey – they are constantly calling in short, regular pulses of ultrasound so they effectively “see” using sound. In a cluttered forest, bats are trying to discern the echoes from a potential prey item (an insect) from countless twigs, leaves and shrubs whilst manoeuvring around obstacles – it can be challenging. For these reasons, we expected bat activity (a measurement of bat calls we use to indicate habitat use) to increase in thinned compared to regrowth stands.

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We studied bats in a floodplain forest in south-eastern Australia where more than a century of logging and river regulation have changed the structure of forests and large stands of dense regrowth exist. It turned out that total bat activity was 60% lower in unthinned regrowth compared to thinned sites despite higher insect biomass at the regrowth sites. Additionally, bat activity in thinned stands was similar to activity in our reference sites (mature open forest). This suggests that it was forest structure and not prey availability or time since thinning that was driving bat habitat use. Synthesising our findings on the international literature on bats and thinning, we found support for a “clutter threshold” of around 1100 trees per hectare; bat activity started to decline once trees exceeded this density. Most studies report bat species with traits adapted to open areas (e.g. long thin wings for fast flying and low-frequency echolocation calls for detection of prey at long distances) receive the most benefit from thinning. However, our study showed that even bats with clutter-tolerant traits had lowest activity (up to 22 times) in regrowth compared to other forest types.

So what does this mean for management – to thin or not to thin? Like most ecological questions, the answer is complex. While the bat community in our study appear to gain foraging opportunities from thinning, at least one threatened bat species is known to prefer dense regrowth to roost in during the day. If we look to other taxa, open forest structures can increase densities of large aggressive birds thereby excluding smaller, threatened woodland birds. So retention of some regrowth patches may be important for forest biodiversity. Finally, any benefits of thinning (short or long-term) will be wasted if thinning operations remove trees that already contain limiting habitat (e.g. hollows).

So what is the solution? If we are serious about biodiversity conservation, all thinning operations must be conducted in a strategic adaptive management framework and special care must be taken to avoid damage to hollow bearing trees.  This means identifying clear objectives and targets, trialling multiple management actions (e.g. thinning, fire and environmental water management) and intensities (e.g. thinning to different tree densities and patchy vs. uniform thinning) and learning from the outcomes.

How do renewable energy installations affect wildlife?

In the first post of its kind for The Applied Ecologist’s blog, Dr Lucy Wright, RSPB Principal Conservation Scientist, discusses five articles published in the latest issue of Journal of Applied Ecology, which have been grouped into a special profile on wildlife and renewable energy. All five papers are currently free to read online.

Renewable energy is widely accepted to be a vital part of efforts to minimise climate change. Like all man-made developments, renewable energy installations alter their environment and poorly sited structures can have significant negative consequences for wildlife. Our job as applied ecologists is to understand how, why, where and when renewable energy developments have these consequences, in order to help decision makers site projects in the least damaging places, balancing the needs of climate change mitigation and nature conservation.

This issue features five papers addressing the interactions between wildlife and renewable energy structures.

Lack of sound science in assessing wind farm impacts on seabirds

The lack of direct measurements of the impacts of offshore wind turbines on seabirds is highlighted in the Practitioner’s Perspective by Rhys Green and colleagues. Scientists have developed models to predict seabird collisions with turbines, and the long-term consequences of this for their populations. Such models ought to go through an iterative process of testing (in a range of real-world situations) and refinement in order to produce robust predictions, but this has not happened; there is almost no published evidence measuring collisions or population-level impacts at offshore wind farms, so we cannot test whether the model predictions are accurate. Birds that do not collide might be affected in other ways; some species avoid the turbines but this can lead to the effective loss of important feeding sites, or increase energetic costs for seabirds commuting between nesting colonies and offshore feeding areas if flying around a wind farm adds distance to these regular journeys. These effects are even less well-studied at existing wind farms, and therefore even more poorly considered in impact assessments for new developments. Finally, the authors criticise the application of thresholds of “acceptable” levels of predicted impact to seabird populations at internationally important sites where birds are supposed to be protected. They argue that these thresholds have no biological basis and set out an alternative method for assessing impacts by comparing the predicted future population size with and without the wind farm. It is the job of policymakers to determine whether predicted impacts can be accepted and compensated for within current regulatory frameworks.

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Offshore wind turbine. Photo credit: Andy Hay RSPB Images.

In light of the lack of evidence of impacts of offshore wind farms on seabirds, it is encouraging that two of the other papers in this series directly measure the impact of renewable developments on wildlife, while the other two quantify the behaviour of seabirds in the marine environment to help understand the likely vulnerability of particular species to particular types of renewable developments.

Measuring impact directly

Seals avoid wind farms only during pile driving

Offshore wind farm foundations are usually installed on the seabed by impact pile driving, effectively giant hammers that produce intense underwater sound pulses with each blow. Scientists are concerned that marine animals that are sensitive to underwater noise, such as seals, might leave important feeding areas as a result. However, this has not previously been measured because of the difficulty of observing animals that spend much of their time underwater. Debbie Russell and colleagues solved this problem by modifying mobile phones to create GPS tracking devices that were attached to harbour seals. This novel technology allowed comparison of seals’ movements during the construction of one wind farm and the operation of another with those measured before either wind farm was constructed. The work demonstrates that seals avoided the area within 25 km of wind farm construction only during the discrete periods when pile driving was carried out – their distribution returned to normal within 2 hours after pile driving stopped, and appeared unaffected by other construction activities or wind farm operation.

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Harbour seal (aka common seal). Photo credit: Tom Marshall RSPB Images.

Raptors take different flight paths to avoid wind farms

Wind farms on land can be risky for migrating raptors and previous studies have demonstrated that collisions may occur if turbines are built on important migration routes, while it has been theorised that long rows of turbines might cause birds to alter their migration routes, potentially increasing their energetic costs. Sergio Cabrera-Cruz and Rafael Villegas-Patraca studied the response of raptors to a 7.5 km long row of wind turbines on an important migratory corridor in southern Mexico. Combining visual observations of millions of birds over several years, and radar to measure their flight trajectories, they demonstrate that raptors took different flight paths after the wind farm was constructed compared to before, thereby avoiding the turbines. This is in contrast to the findings of other studies that have suggested that raptors at some other wind farm sites do not avoid turbines, and demonstrates the importance of understanding the site- and species-specific features that may influence how animals respond to new developments.

Improving predictions of impacts

Quantifying where seabirds coincide with areas suitable for tidal stream turbines

Tidal stream turbines are installed underwater to capture energy from strong currents. There are plans to increase the use of these devices in the near future, with uncertain impacts on local wildlife. James Waggitt and colleagues measured how pursuit-diving seabirds (such as puffins, guillemots and shags) used fine-scale physical features (e.g. different types of current, seabed and water turbulence) in Orkney where tidal stream turbines are being tested. They identified where the areas used by seabirds coincide with areas suitable for turbine deployment. Puffins were most likely to be affected by turbines since they were normally found in places with the high horizontal currents necessary for turbines to work efficiently; other seabirds also used these areas. Several species were associated with downward vertical currents (which sometimes coincide with the strong horizontal currents necessary for turbines) and the authors suggest that to minimise the impact on seabirds, turbine installations should focus on locations with strong horizontal currents that do not also have downward vertical currents. Further work will be required to understand whether and how turbine installations actually affect the species thought to be at risk. It is unfortunate that a confidentiality agreement prevented the authors from accessing information about where turbines were operational at the test site during the study, as this would have allowed some preliminary measurement of how seabirds respond to these novel devices.

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Puffins were more likely than other species to be affected by tidal stream turbines. Photo credit: Andy Hay RSPB Images.

Modelling seabird flight heights to predict collision risk

The final paper in this series revisits seabird collision risk with offshore wind turbines. Flight height is critical in predicting collision risk since only birds flying at the height of turbine rotors are at risk of colliding with them. Conventional flight height estimates come from boat or aerial surveys conducted in daylight and good weather conditions, and so collision risk models must make assumptions about birds’ behaviour at other times. Viola Ross-Smith and colleagues tackle this data gap using a Bayesian approach to analyse height data from GPS tracking devices on two seabird species. Great skua flight height was relatively consistent between day and night, but lesser black-backed gulls flew lower at night than during the day, making them more likely to fly underneath the turbine rotors at night. The modelling approach used here could be applied to other tracking data sets to improve our understanding of animal movements in other situations.

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Lesser black-backed gulls flew lower at night than during the day. Photo credit: John Markham RSPB Images.

Learning from existing renewable energy installations for a better future

Together, these studies reveal the advances being made in our understanding of how some types of renewable energy installations affect some types of wild animals, with new technologies contributing to the measurement of impacts of onshore wind farms on raptors, offshore wind farms on seals and our grasp of how seabird behaviour might affect their exposure to offshore turbines both below and above the water. However, they also expose the many questions we still need to answer to fully understand the effect of these developments on wildlife, often due to a lack of appropriate research at existing sites. We must invest in sound science if we are to understand how best to manage two of the most pressing issues affecting our natural world – climate change mitigation and nature conservation.

Ecological intensification of agriculture: ideas that begin to take root – now with Spanish translation

In this post Nahuel Policelli discusses a recent paper by Timothy M. Bowles and colleagues ‘Ecological intensification and arbuscular mycorrhizas: a meta-analysis of tillage and cover crop effects

*Update: On 10 November, we added a Spanish translation of this post. Nahuel provided the translation to reach out to Spanish readers interested in this topic. Journal of Applied Ecology is dedicated to making papers more accessible for an international audience and increasing engagement with the wider ecological community. This is the first post of its kind, but we encourage authors to write dual-language posts to increase the international relevance and real-world impact.

In a context of increasing global food demands, ecological intensification of agriculture emerges as an ideal approach for land management. It combines the benefits of intensive and extensive agriculture, enhancing ecosystem services and leading to sustainable ways of production. An ecosystem under ecological intensification has high rates of internal regulation processes, moderate resource inputs, low nutrient losses and high productivity (Bender et al. 2016). Targeted manipulation of soil biota in agricultural systems is starting to be considered a key aspect of ecological intensification and a way out to face one of the main causes of biodiversity loss: soil degradation.

Within soil biota, plant-symbiotic arbuscular mycorrhizal fungi (AMF) are between the most common organisms able to facilitate a plant’s resource use efficiency. It has been suggested that more than 80% of land plants form a symbiosis with AMF. This symbiosis not only enhances plant nutrient uptake, but also improves plant growth and influences ecosystem functioning. Intensively-managed agricultural systems have replaced biological functions originally provided by these and other organisms and are now highly dependent on anthropogenic inputs (i.e. fertilizers, pesticides, irrigation). These inputs in turn restrict the diversity of AMF fungal species and consequently the functionality of ecosystems.

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Fields dedicated to agriculture.

What are the mechanisms by which below-ground interactions could be optimized if less intensive agricultural management is applied? A recent study by Timothy Bowles (University of California, Berkeley) and colleagues takes a thought-provoking approach to address this question. They conducted a meta-analysis of 54 field studies performed around the world to determine the impacts on the AMF community of the use of low-intensity tillage regimes and cover crops.  The survey assessed how these two management techniques affect AMF colonization rates on cash crop roots, also considering the change in AMF community composition.

The study found that less intensive tillage and winter cover cropping similarly increased (nearly a 30%) AMF root colonization of summer annual cash crop roots compared with intensive tillage and winter fallow periods. Intense tillage constitutes a soil disturbance with negative effects on AMF hyphal networks and consequent reductions in root colonization. Interestingly, cover crops increased AMF colonization similarly whether tillage was used or not. These results suggest that when a cover crop is used in these systems, AMF formation in the cash crop apparently can resist some tillage. From a management perspective, this may have strong implications in agricultural systems where tillage is required for weed control and organic matter incorporation into the soil. Additionally, when AMF community composition was analyzed, the authors did not find a strong effect of winter cover cropping or less intensive tillage. Although the arrival of different AMF species could be slow after a change in soil disturbance, the role of fungal resistance structures that could remain in the soil and take advantage of the disturbance to germinate is still unclear. The authors also found increased colonization even when the cover crop used was a non-AMF host, so the role of weeds might also be important. Whether or not the presence of weeds would be enough to ensure AMF colonization of cash crops and how cash crop rotation and diversification may influence AMF diversity are still unanswered questions. So far we know, according to this study, that cover cropping and reducing soil disturbance are potential strategies for farmers to increase AMF formation even across a wide range of cash crops.

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Soil degradation is one of the main causes of biodiversity loss.

The way forward

Modern agriculture will benefit from a more comprehensive consideration of below-ground ecological processes. For example, it would be relevant to know how different ecological functions are distributed within different functional groups of soil biota. Loss of some particular AMF species or group of species due to some agricultural practices may have profound changes in soil functioning.  In this sense, it is also still unclear whether or not more root colonization translates into better plant performance as there might be functional complementarity and redundancy among species within the AMF community colonizing a single root system. The advance of molecular techniques in recent years allows us to have a clearer idea of fungal composition and opens many new ecological questions with applied implications. Funding for agricultural research is also needed, especially in countries where ecosystem services are endangered due to intensive agricultural practices. The so-called underground revolution (see Bender et al. 2016) will start when we are able to integrate ecological knowledge of below-ground interactions with management applications that could reduce human impact on ecosystems. In the case of AMF ecology these ideas have begun to take root.

*Intensificación ecológica en la agricultura: Ideas que comienzan a echar raíces

En un contexto de creciente demanda de alimentos a nivel mundial, la intensificación ecológica surge como un enfoque ideal para el manejo de sistemas agrícolas. Al acentuar y hacer uso de los servicios ecosistémicos que un determinado sistema ofrece, conduce hacia formas sustentables de producción, combinado a su vez los beneficios de la agricultura intensiva y extensiva. Un ecosistema bajo intensificación ecológica tiene altas tasas de regulación interna, un aporte externo de recursos moderado, baja pérdida de nutrientes y una alta productividad (Bender et al 2016). La manipulación de la biota del suelo en sistemas agrícolas está empezando a ser considerada como un aspecto clave de la intensificación ecológica y una salida para hacer frente a una de las principales causas de la pérdida de biodiversidad global: la degradación del suelo.

Entre los organismos que componen la biota del suelo, los hongos micorrícicos arbusculares (AMF, por sus siglas en inglés) son capaces de facilitar la eficiencia en el uso de recursos por parte de las plantas. Se cree que más del 80% de las plantas terrestres establecen una relación simbiótica con AMF. Esta simbiosis no sólo facilita la absorción de nutrientes por parte de las plantas, sino que también mejora su crecimiento e influye en el funcionamiento de los ecosistemas. Los sistemas agrícolas que aplican prácticas de manejo intensivo han sustituido las funciones biológicas proporcionadas originalmente por estos y otros organismos, tornándose altamente dependientes de aportes antropogénicos (fertilizantes, pesticidas, riego artificial). Estos cambios, a su vez, han limitado la diversidad de especies de AMF y en consecuencia la funcionalidad de los ecosistemas.

¿Cuáles son los mecanismos mediante los cuales se podrían optimizar las interacciones bióticas que ocurren bajo tierra en un escenario de manejo agrícola menos intensivo? Un estudio reciente realizado por Timothy Bowles (Universidad de California, Berkeley) y sus colegas propone un enfoque novedoso para abordar esta pregunta. Los autores llevaron a cabo un meta-análisis que incluyó 54 estudios a campo realizados en todo el mundo para determinar los impactos sobre la comunidad de AMF provocados por el uso de regímenes de labranza de baja intensidad y cultivos de cobertura. El estudio evaluó de qué forma estas dos técnicas de manejo afectan las tasas de colonización de AMF en raíces de cultivos comerciales, teniendo en cuenta a su vez, el cambio en la composición de la comunidad de AMF.

El estudio encontró que la utilización de métodos de labranza menos intensivos y el uso de cultivos de cobertura durante el invierno, aumenta de forma similar (casi en un 30%) el porcentaje de colonización de AMF en cultivos comerciales anuales en comparación con la labranza intensiva y períodos de barbecho durante el invierno. El laboreo intensivo constituye una alteración del suelo con efectos negativos sobre las redes de hifas de AMF y, por consiguiente, provocaría la reducción de la colonización de la raíz en los cultivos. Llamativamente, los autores vieron que el uso de cultivos de cobertura aumenta la colonización de raíz de los cultivos comerciales de igual forma tanto si se utiliza labranza como si no. Estos resultados sugieren que cuando un cultivo de cobertura se utiliza en estos sistemas, la colonización de AMF en el cultivo comercial aparentemente puede resistir la labranza. Desde una perspectiva aplicada, esto puede tener importantes repercusiones en sistemas agrícolas que dependen de la labranza como mecanismo para el control de malezas y la incorporación de materia orgánica en el suelo. Por otro lado, no se halló un claro efecto de los cultivos de cobertura de invierno o de la labranza menos intensiva sobre la composición de la comunidad de AMF. En este sentido, aunque la llegada de diferentes especies de AMF podría ser lenta después de una perturbación en el suelo, el papel de las estructuras de resistencia fúngicas que podrían permanecer en el suelo y sacar provecho de las condiciones generadas por el disturbio para poder germinar aún no está claro. Los autores también encontraron un aumento en el porcentaje de colonización de raíz incluso cuando el cultivo de cobertura utilizado no era un huésped de AMF, por lo que el papel de las malezas también podría ser importante. Existe la posibilidad de que la presencia de malezas sea suficiente para garantizar la colonización de AMF de los cultivos comerciales. De igual forma, la rotación y la diversificación de cultivos son otras prácticas agrícolas que podrían influir en la diversidad de AMF, pero aún se desconoce de qué forma. A partir de este estudio, se puede concluir que la utilización de cultivos de cobertura y la reducción en la perturbación del suelo mediante labranza menos intensiva son posibles estrategias para que los agricultores aumenten la colonización de AMF en una amplia gama de cultivos comerciales.

El camino a seguir

La agricultura moderna se beneficiará a partir de una consideración más amplia de los procesos ecológicos que ocurren bajo tierra. Por ejemplo, sería relevante conocer cómo diferentes funciones ecológicas se distribuyen entre los distintos grupos funcionales de hongos micorrícicos. La pérdida de especies o grupos de especies de AMF debido a prácticas de manejo agrícola podría generar cambios profundos en el funcionamiento del suelo. A su vez, no está del todo claro si una mayor colonización de las raíces de un determinado cultivo se traduce necesariamente en un mejor rendimiento del mismo, ya que puede haber complementariedad funcional y redundancia entre especies de AMF que colonizan un único sistema radicular. El avance de las técnicas moleculares en los últimos años permite tener una idea más clara de la diversidad de microorganismos del suelo y posibilita la formulación de estas y nuevas preguntas con implicancias en la ecología aplicada. Igualmente, son necesarios fondos para la investigación agrícola, especialmente en los países donde los servicios ecosistémicos están en riesgo, en parte debido a las prácticas de la agricultura intensiva. La llamada “revolución bajo tierra” (“underground revolution” según Bender et al 2016) se iniciará cuando seamos capaces de integrar el conocimiento ecológico de las interacciones que ocurren en el suelo con estrategias aplicadas de gestión y manejo que puedan reducir el impacto humano en los ecosistemas. En el caso de la ecología de AMF, estas ideas ya han comenzado a echar raíces.

New techniques for Atlantic sturgeon conservation

In this post Associate Editor Verena Trenkel discusses a paper she recently handled from Michael Melnychuk and colleagues ‘Informing conservation strategies for the endangered Atlantic sturgeon using acoustic telemetry and multi-state mark–recapture models

According to the International Union for Conservation of Nature (ICUN), ten out 17 sturgeon species are currently critically endangered. Among the two species listed as least concern is Atlantic sturgeon which occurs along the east coast of North America (St. Pierre and Parauka, 2006). However, certain populations of Atlantic sturgeon have recently been listed as threatened or endangered in U.S. waters (USOFR, 2012). The continuing poor and in many cases worsening status of sturgeon species world wide is caused by overfishing, illegal catch and illegal caviar trade, as well as coastal habitat degradation and pollution (Rosenthal et al. 2014). Putting into place effective conservation measures depends on better understanding the life cycle requirements of the species, as well as identifying locations and times of bycatch by fisheries (Dunton et al. 2015).

iucn

IUCN classification of sturgeon species in 2016.

In this recent study Melnychuk and colleagues analyse with multi-state mark-recapture models acoustic telemetry data from 429 Atlantic sturgeon tagged on the southern coast of Long Island (New York State). Application of this model provides for the first time quantitative estimates of both seasonal patterns in survival and migration rates while accounting for detection probabilities of tagged fishes at receiver stations. The exciting novelty of the study lies in being able to simultaneously estimate migration and survival rates over a relatively large area as well as resolving these estimates on a seasonal basis. In previous studies either survival (Hightower et al. 2015) or movements were studied (Breece et al. 2016), but not both. Due to the large number of tagged individuals Melnychuk and colleagues could also investigate body size effects on migration and survival rates. Overall the study found that bycatch fishing mortality might be of the same order as natural mortality (estimated as 5.88% per year), which the authors consider too high for recovery. These are the bad news.

tagged-sturgeon

Tagged sturgeon being released (Photo credit: Keith Duntan).

The good news are, as this study demonstrates, that it is possible to carry acoustic telemetry studies at scales relevant for conservation of large anadromous species (~800 km coastline in this study) and use state of the art multi-state mark-recapture models for separating survival, migration and detection rates on a seasonal basis. This might inspire researchers studying the other 16 sturgeon populations to launch similar research programs, or join forces to study Atlantic sturgeon along the entire Atlantic coast.

sturgeon

Tagged sturgeon swimming away (Photo credit: Keith Duntan).

References
Breece, M. W., Fox, D. A., Dunton, K. J., Frisk, M. G., Jordaan, A., and Oliver, M. J. 2016. Dynamic seascapes predict the marine occurrence of an endangered species: Atlantic Sturgeon Acipenser oxyrinchus oxyrinchus. Methods in Ecology and Evolution, 7: 725-733.

Dunton, K. J., Jordaan, A., Conover, D. O., McKown, K. A., Bonacci, L. A., and Frisk, M. G. 2015. Marine Distribution and Habitat Use of Atlantic Sturgeon in New York Lead to Fisheries Interactions and Bycatch. Marine and Coastal Fisheries, 7: 18-32.

Hightower, J. E., Loeffler, M., Post, W. C., and Peterson, D. L. 2015. Estimated Survival of Subadult and Adult Atlantic Sturgeon in Four River Basins in the Southeastern United States. Marine and Coastal Fisheries, 7: 514-522.

Rosenthal, H., Gessner, J., and Bronzi, P. 2014. Conclusions and recommendations of the 7th International Symposium on Sturgeons: Sturgeons, Science and Society at the cross-roads – Meeting the Challenges of the 21st Century. Journal of Applied Ichthyology, 30: 1105-1108.

St. Pierre, R., and Parauka, F. M. 2006. Acipenser oxyrinchus. The IUCN Red List of Threatened Species 2006: e.T245A13046974. http://dx.doi.org/10.2305/IUCN.UK.2006.RLTS.T245A13046974.en. Downloaded on 07 October 2016.

USOFR 2012. Endangered and threatened wildlife and plants: Threatened and endangered status for distinct population segments of Atlantic Sturgeon in the northeast region. U.S. Office of the Federal Register 77:22 (6 February 2012): 5880–5912.