As I have been recently wondering about pathogen and parasite transmission among species (Could large predators reduce parasite and pathogen load in the entire ecosystem?), I also considered my experiences with fleas and ticks while walking in the wilder areas.
One of my observations was that there were fewer fleas and ticks in places with more diverse vegetation and with more diverse woody debris composition (both fine and coarse woody debris of different species and different sizes, with and without root systems attached etc.).
Plant biodiversity effects on parasites and pathogens might be rather obvious.
For example, species such European badgers supposedly even collect bedding material that includes forbs with antibacterial and antiparasitic properties (such as Allium spp.).
There are also recommendations to plant native herbs or cultivated herbs (mints, yarrow, oregano, sage, lavender etc.) near garden paths in order to repel fleas (natural pet protection method).
Accordingly, in natural ecosystems there would also be plant species that produce essential oils or other chemical defense substances which have antibacterial or antiparasitic effects in the vicinity of the growth or when collected, ingested, rolled in by the animals.
On the other hand, parasites and pathogens that affect animal species frequently use dead or living plant material as host plants or host substrate.
If there is too much host material in one spot, the respective organisms would also become more abundant.
Biodiversity has the regulatory capacity to control parasite and pathogen loads through the simple mechanism of introducing heterogeneity (host plants do not grow in one spot and they have to be travelled to increasing energetic costs and decreasing reproduction, longevity etc. in parasite/pathogen species).
Also, in any diverse enough patch there might be plants and substrates that the parasites and pathogens would rather avoid further limiting their distribution.
Lack of plant biodiversity and proliferation of few neutral or beneficial species (from the parasite and pathogen perspective) could lead to:
- overabundance of parasites and pathogens;
- lack of natural repellents;
- inability by wildlife species to apply natural remedies (through ingestion, bedding collection, ‘skin care products’ etc.).
The diversity of woody debris substrates could also lead to increased parasite and pathogen loads because some species inhabit finer debris (twigs, branches, smaller detached pieces of bark etc.) while other species might frequent larger debris (fallen trees, standing dead trees, root systems of uprooted trees, stumps etc.).
Predators are often larger than prey species and many beetles are associated with coarse woody debris which are necessary for their living conditions or alternative foraging resources, ovipositioning or simply for suitable micro-climate conditions.
Fine debris can be more frequently disturbed than larger debris and it might not always be as suitable for overwintering.
Thus in places where only fine debris exists but larger wood is removed (such as any logging site), there could be a mismatch between the abundance of parasites and their predators (or parasites and their parasites).
Many timber harvesting operations tend to leave fine debris while removing larger materials and it would be important to study if this has any effects on the distribution and abundance of key species (e.g., parasites and pathogens) that bear impact on other wildlife.
Leaving any sort of deadwood is better than leaving no deadwood at all.
However, finer deadwood without larger elements might have some adverse effects and if that is so, requirements to leave at least one bole etc. per a specific area in clearings should be put forth.
Increase in pathogen and parasite load can prove detrimental in the long term because animals suffer reduced immunity and higher stress levels which can make the species more vulnerable to extreme events or prolonged periods of suboptimal conditions.
Thereby, biodiversity might be essential to ensure conservation measures for species that suffer from parasites and pathogens which are hosted by plants or in the substrate (and all species have such associations).
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Additionally, I am very curious if ungulate species ever choose to bed in sites that have certain types of plant species.
I would imagine that their bedding site choice is mainly determined by exposure (to wind, precipitation, predators etc.) but I have not come by any studies that evaluate other types of selectivity factors.
Antiparasitic and antibacterial properties of the plants in the bedding site vicinity might be one such factor (or absence of parasite host plants).
While ungulates tend to make bedding by scraping off vegetation, the surrounding vegetation can still touch them and affect them through smell or through reduction in vigilance potential (dense growth can reduce visibility both for predators and prey).
Perhaps ungulates, too, (where there is a choice) prefer bedding near softer plants that offer some medicinal purposes or that can be leisurely foraged on.
Notes by Ieva 