Food as the main habitat feature

All folivorous animals acquire energy and nutrients through foraging but this takes time, consumes some energy itself and exposes the individuals to risk. Clearly an animal must trade off one gain against potential loss.
Nutritious, healthy food is the most important component of any ringtail habitat. Without it even the most secure sanctuary will not sustain a viable population. This sounds like a no-brainer, but how do we ascertain that the trees we see provide adequate food?

Herbivorous marsupials such as ringtail possums belong to the single largest dietary category in the fauna of Australia. Ringtails are browsing marsupials whose digestive system is well adapted to a diet of leaves of low nutritional value. (Dickman and Vieira, 2006)

Barbara Jones pioneered research into peppermint leaf quality in the 90s but we are still far from able to determine food quality in ringtail habitat.
Foliage nutrient levels are a major factor in explaining variation in abundance in ringtails and also a key factor influencing fecundity. Nitrogen levels are the most important determinant of browse quality and habitat suitability for possums. (Jones et al, 1994a, Wayne et al, 2005)      
Nitrogen is an essential building block for amino acids, proteins and DNA. No plant, animal or human being can live without it. Even though our air contains a lot of nitrogen gas, this cannot be used by plants; they have to take it from the soil in the form of ammonia, which is also the main ingredient of our fertilisers. Animals including us humans have to eat plant materials or other animals.

Eucalyptus has been far better researched but there is no doubt that Agonis flexuosa (peppermint) is also characterised by high levels of indigestible fibre and lignin and low levels of essential nutrients, such as phosphorus, potassium and nitrogen. (Tyndale-Biscoe, 2005)
Peppermint foliage is the largest component of the western ringtail possum diet in coastal areas where the trees are still plentiful or even represent the dominant vegetation. (Jones et al, 1994a)
In inland regions myrtaceous leaves and in particular jarrah are the stable food.  (Jones et al, 1994a, Wayne et al, 2005).
Ringtails are specialised folivores and their low metabolic rate and efficient digestive system enable them to live on a diet of almost exclusively leaves. (Clarke, 2011)

Herbivorous marsupials can be divided on the basis of the main location of microbial fermentation into foregut fermenters (e.g. kangaroos) and hindgut fermenters. The latter can be divided further into colon fermenters (e.g. wombats) and caecum fermenters such as koalas and our ringtail possums. (Tyndale-Biscoe, 2005, Hume, 1999)

The caecum of a ringtail possum is greatly enlarged as bacterial fermentation is a slow process and the food must be stored for a long period of time to gain sufficient nourishment for the daily metabolic needs when fermenting plant material.
Obtaining nourishment from leaves necessitates the breakdown of cell walls, disposing of the indigestible fibres (cellulose or lignin) and detoxification of the plant’s chemicals. Small folivores such as our ringtails break the cell walls and reduce the particle size by grinding their food to a very fine pulp. In the caecum fine and coarse particles are separated and while larger particles are excreted during foraging at night as hard, dry faeces, those fine leaf particles and fluids are selectively retained. (Hume, 1999) During the day those moist fine particles that are still high in nutrients such as microbial protein are passed as soft faeces only to be immediately re-ingested by the animal. This coprophagy (eating of faeces) or caecotrophy (ingestion of material form the caecum) maximises protein intake due to double-processing of food and ingestion of microbial amino acids and B vitamins. Thus through caecotrophy ringtails gain far greater access to the protein and products of bacterial fermentation that occur in the caecum and the species could not exist on a diet of leaves without practicing it. (Chilcott, 1984, Hume et al, 1984, Hume, 2004)
The fact of caecotrophy in ringtails has only been discovered in the 1960s by Mervyn Griffiths. (Tyndale-Biscoe, 2005)

The bacteria in ringtails’ digestive system can synthesise proteins from urea produced by the host animal which conserves nitrogen and water and some may also help detoxify plant secondary compounds. (Tyndale-Biscoe, 2005)

Water requirements are higher in ringtails than e.g. in koalas or greater gliders (Hume, 1999). They obtain a high percentage of their water requirements from foliage but need access to fluids from more than leaves! (Hume, 2006)
This finding is not new but seems to not have found its way into newer research.

In woodland areas water is only available as dew, rain on leaves or collected in tree hollows.
Water is constantly lost through urine and faeces and at hot temperatures by evaporative cooling for thermoregulation.

Those caecotrophes passed during the day still contain 3 times the water as those hard faeces passed during the night. (Hume, 2006) Re-ingestion saves a lot of water from being excreted as waste.

Leaves have low nutritional value but many potentially toxic compounds that protect the tree from being (overly) browsed by leaf-eating marsupials. (Wiggins et al, 2003; Wiggins et al, 2006).Those secondary metabolites (e.g. tannins, terpenes) need to be detoxified by the animal.

Ringtails require a certain amount of food to fulfil their nutritional needs; however e.g. lignin and tannins interfere with digestion of nutrients. Tannins can make protein less available to the animal while elevating nitrogen requirements (nitrogen is the major element of protein) while lignin reduces the availability of carbohydrates and increases energetic costs of digestion.
The process of detoxification uses up a high percentage of the energy gained from the leaves and takes time. It is not possible for the animals to just eat more or without breaks to make up for the bad quality of the food. (Foley et al, 2004, Tyndale-Biscoe, 2005)

1,8-cineole is a dietary terpene and major constituent of the essential oil in eucalypt species (Wiggins et al, 2003), however it is also associated with peppermint (Agonis flexuosa).
The presence of 1,8-cineole is also an indication that the even more toxic secondary metabolite sideroxylonal is present in the foliage. (Harris, 2012) Sideroxylonal can only be tolerated by ringtails in low concentrations while strangely enough brushtail possums can tolerate more than 3 times that concentration.  (Tyndale-Biscoe, 2005)

Ringtail possums and other folivores have the ability to smell the presence of secondary metabolites such as 1,8-cineole.  If concentrations of toxins are too high they will completely reject the foliage. (Foley et al, 2004)
Judy Clarke observed frequently that ringtails sniffed peppermint foliage before moving on to another tree without eating. (Clarke, 2011)

Laura Harris tested and proved in her study that the abundance and distribution of western ringtail possums will increase with decreasing concentrations of 1,8-cineole in the prominent vegetation. However, this relationship was by far not as pronounced as the relationship between the basal area of trees and abundance of ringtails. Also, there was no indication that larger trees contain lower 1,8-cineole concentrations. (Harris, 2012)

In general, ringtail possums prefer young, soft leaves. Those are usually higher in nitrogen, have lower lignin levels and are easier to digest. (Ellis et al, 1992, Hume, 1999, Clarke, 2011)
The downside is that they contain higher concentrations of tannins, which make the protein less available. (Cork et al, 1984)

Secondary metabolite concentrations tend to decrease in a tree with its age increasing. Low levels of nitrogen and potassium are usually paired with higher levels of toxins and stress on a plant can also increase toxin levels. (Harris, 2012)
Foliage toxicity may also vary seasonally or through times of drought. (Clarke, 2011)
Whatever the reason for high foliage toxicity, palatability is reduced and nutritional stress on ringtail populations is increased.

Just like eucalyptus trees  (Wiggins et al, 2006) individual peppermint trees (A. flexuosa) also vary widely in the levels of secondary metabolites and nutrients in their foliage and therefore in their palatability.

Barbara Jones  observed as early as in the 90s that an abundance of A. flexuosa did not necessarily mean a large ringtail population. There were significant differences in levels of - most importantly - nitrogen but also phosphorus in the foliage of peppermint between occupied and unoccupied sites. (Jones et al,
Occupied sites had higher nutrient levels in late summer when nitrogen and phosphorus are particularly low in general.
Nutrient levels also vary seasonally with tree maturity, leaf maturity, and even leaf position within the tree canopy. (Wayne et al, 2005

The only time when foliar nutrient levels in peppermint have been measured was in 1990 to 1992 during Jones’s study. At Leschenault Peninsula – an area that was earmarked as a translocation site – she found that nitrogen and potassium levels in February were lower than at sites with a current ringtail population. (Jones et al, 1994a)
No other translocation site has ever been researched that way.

Common ringtails are known to feed on leaves of shrubs, shoots and flowers in addition to eucalyptus foliage, but only as a supplement as those are insufficient on their own (Clarke, 2011).
Western ringtail possums however can have peppermint or Jarrah leaves as their staple food and supplement this with other eucalyptus varieties, melaleuca and acacia leaves and flowers for example. It seems that neither of the other food sources (apart from peppermint and jarrah) are sufficient as a staple. Jarrah leaves were deemed comparable to peppermint leaves (from the Busselton area) in their phosphorus level but lower in nitrogen and potassium. (Wayne et al, 2005)

As the only comparative study ever done was just between Busselton and Leschenault and the latter turned out nutritionally inferior, it is likely that other areas away from core habitat around Busselton are lower in their nutritional values too and as a consequence in their carrying capacity.

Laura Harris investigated the other limiting aspect for ringtail abundance – secondary metabolites - and found that the peppermint leaves in Locke Nature Reserve were considerably lower in those toxins than for instance in Tuart Forest National Park. Both are located on an older dune system which has severely reduced potassium levels compared to young dune systems such as the Quindalup dune system around Busselton. (Harris, 2012)

Another interesting finding  was that abundance and distribution of ringtails will increase with an increase in the basal area of trees. At all her study sites (Locke NR, Tuart NP, Kookaburra and Siesta Park caravan parks) ringtails preferred those peppermint trees with a large basal area. (Harris, 2012)

Food quantity and quality does not only influence the carrying capacity of areas and the abundance/density of ringtails but also seasonal patterns in body condition and mortality of animals. The highest mortality coincides with times of low leaf growth.   (Wayne et al, 2005)