Plant pests and pathogens can have a devastating impact not only on plant hosts but also the wider biodiversity that use the infected plant (e.g., for food, breeding and shelter). Thus, a decline in one plant species can have cascading effects on a wide range of other species, sometimes causing severe declines in their populations.
In this project, we trialed different approaches for identifying which habitats are at greatest risk from plant pests and pathogens. Risk assessments prioritizing habitats that are thought to have low resilience (low plant species diversity), gave very different results from prioritizing habitats with plant species known to host a lot of plant pests and pathogens. This is because our knowledge of plant pests and pathogens, as listed by the Defra plant health risk register, is biased towards plants of agricultural, horticultural or forestry importance. Therefore, current lists of plant pests and pathogens are not necessarily a good way to assess risks to semi-natural habitats.
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Plant diseases caused by pests and pathogens can have a devastating impact on local plant populations. In addition, plant pests and pathogens can cause large scale population declines in associated biodiversity i.e., species that use the plant for food, breeding and shelter. For example, 955 species are known to use the Common Ash Tree and forty-five of these species are only found on ash trees. In the UK ash trees are dying due to the establishment of a non-native fungus which is causing not only a decline in the population of ash trees but also those species that use ash trees, and particularly those only found on ash trees. Declines in plant populations due to pests and pathogens can also cause changes in ecosystem services, such as decomposition and nutrient cycling.
Plant disease surveillance is key to early detection, yet it is rarely undertaken in semi-natural habitats other than forests. Most plant disease surveillance is targeted at plants in agriculture, horticulture or forestry e.g. Plant biosecurity strategy for Great Britain. As resources are limited it is critical to prioritize surveillance to those habitats that are at greatest risk, but there is currently no methodology available to help managers identify which habitats are at greatest risk from plant pests and pathogens.
Therefore, this research sought to compare two methods to identify which habitats are at greatest risk from plant pests and pathogens:
- habitats known to host the greatest number of pest/pathogens that are most likely to establish;
- habitats with low species diversity and hence low resilience.
The first method assesses risk based on the number of pests and pathogens that could establish, the second method assesses the potential impact that a pest/pathogen could have on a habitat. Habitats with low resilience (the ability to recover from a disturbance) would be impacted more by a plant pest or pathogen than habitats with high resilience that would recover quickly. Habitats with low species diversity (the number of plant species present) are suggested to have low resilience as there are fewer plant species present which can substitute for another one if a particular plant species is lost or declines due to disease.
This study’s aims were to show how these two risk assessment methods work for 12 habitats found in the UK. Our analysis utilized two existing datasets: the Defra Plant Health Risk Register and the UK National Vegetation Classification. The Defra Plant Health Risk Register had not previously been used to assess risks to semi-natural habitats of plant pests and pathogens as it is typically used to assess risks to crops, and plants used in the horticultural and forestry trades.
Our work found 916 pests and pathogens in the Defra Plant Health Risk Register that could be hosted by plant genera (a principal taxonomic category that ranks above species and below family) that occur in semi-natural habitats at more than 25% cover in the UK. Most of these 916 pests and pathogens are not currently present in the UK and their likelihood of establishing (given current biosecurity measures to minimize risk) is ranked by Defra from 1 (low) to 5 (high). Based upon this data, we identified there are 91 pests and pathogens that have a high likelihood of establishment (ranked 4 or 5) and of these, 9 could be hosted in aquatic habitats and 87 in woodland habitats (Figure 1).
In particular, Figure 1 shows the method used to identify which habitats are most at risk from plant pests and pathogens has a big impact on how the habitats are ranked (Figure 1a vs 1b).
Figure 1 Prioritization of habitats at risk from plant pests and pathogens.
- Method 1: Habitats known to host the greatest number of pests/pathogens most likely to established (likelihood of establishment taking into account action for mitigation of 4 or 5) as listed in the Defra Plant Health Risk Register.
- Method 2: Habitats with the lowest resilience, here defined as those with the lowest number of vascular plants occurring at >25% cover.
When habitats are ranked by the number of known pests/pathogens most likely to establish (Figure 1a) then woodland, open habitats; and shingle, strandline and sand-dune habitats are ranked as highest risk while aquatic communities, salt marsh communities and maritime cliff communities ranked lowest risk. The order is very different if the habitats with the lowest resilience (defined as those with the lowest number of plants, excluding mosses and lichens) (Figure 1b) are ranked as high risk. Salt marsh communities and maritime cliff communities, swamps and tall-herb fens, heathlands, mires and aquatic communities are ranked high risk. While, woodlands, mesotrophic grasslands (grasslands with an intermediate level of nutrients) and shingle, strandline and sand-dune habitats were ranked low risk.
The results show that assessing the risk based on the potential ecological impact rather than the number of known pests alters how the habitats are ranked.
In addition, these results indicate that the current lists of plant pests and pathogens are not necessarily a good way to assess risks to semi-natural habitats as: i) there may be biases in how the lists are constructed and ii) rankings of which habitats are most at risk needs to be revised every-time the list of pests is updated.
A manuscript based on this work has just been submitted and a preprint is available here. Once the full research paper is available a link will be added to this case study.
This work has highlighted for the first time the number of known plant pests and pathogens that could be hosted by plants in semi-natural habitats. This is key because such risks to semi-natural habitats, outside of woodlands, is often ignored. Consequently, semi-natural habitats are not usually monitored, and pests and pathogens may establish in these habitats un-noticed. This surveillance gap may not be noticed until a severe impact on the plant population (and possibility associated biodiversity) is observed. However, by such time it is often too late to either try to stop the spread of the pest/pathogen or to implement mitigation actions.
In particular, the results showed that the two methods tested to rank the risks to habitats gave very different results. The method using lists of known pests and pathogens has two disadvantages. It risks focusing on known pests and pathogens, such as the Defra Plant Health Risk Register, and ignores the potential of unknown pests and pathogens to impact habitats. Second, the Defra Plant Health Risk Register focuses on pests and pathogens of agricultural, horticultural and forestry value, therefore, when this list is used to consider semi-natural habitats, this prior focus biases the results. While, prioritization of habitats with low species diversity enables clearer assessment of those habitats that are likely to be least resilient and therefore the most impacted by plant pests and pathogens. It also avoids any bias from our current knowledge.
In conclusion, prioritizing habitats with low species diversity and hence low resilience may be a better way to priorities surveillance for pests and pathogens in semi-natural habitats than using existing lists of pests and pathogens. This revised approach could help with assessing pest and pathogen risk in a wider range of habitats.