Review of the vertebrate-mediated dispersal of the Date Palm, Phoenix dactylifera

As a major agricultural crop in the semi-arid and arid zone of North Africa and the Middle East, drupes (“fruits”) of the Date Palm Phoenix dactylifera form with their high carbohydrate content of the flesh and oils in the seed a major part of the diet of resident and migratory bird species and fruit bats. This paper reviews the range of known instances of drupe predation by volant and non-volant vertebrate vectors. While documented dispersal distances range from tens of metres to about 50 km, the efficacy of that dispersal has not been assessed in any of the papers under review. While volant animals, primarily birds, make up the greatest number of dispersal vectors and also account for the greatest number of seeds dispersed, long distance dispersal of a larger quantity of seeds per dispersal event seems to rely on terrestrial animals, primarily canids, but also bears and flightless birds, such as the Emu (Dromaius novaehollandiae).


Introduction
Although some palm species are widely recognised as plants with a high invasive potential, very little work has been carried out on vertebrate-mediated dispersal mechanisms. Only one study specifically deals with the animal-mediated dispersal of palm seeds in general (Zona & Henderson, 1989), but as the authors specifically exclude the dispersal of non-native palms as well as palms in commercial settings, still little is known on the dispersal of the horticulturally and agriculturally common species and the range of vectors that feed on them and facilitate their spread. This paper found its genesis in a systematic review of the dispersal vectors of three ornamental palm species: the Canary Islands date palm (Phoenix canariensis) (Spennemann, in press), and the California and Mexican Fan Palms (Washingtonia filifera and W. robusta) (Spennemann, unpubl.). To understand whether there might be underlying systemic issues affecting the nature and patterns of dispersal of Phoenix spp. palms and the associated vectors, it was necessary to broaden the basis and to separately examine a closely related species, the commercial Date Palm Phoenix dactylifera.
While it is acknowledged that Ph. dactylifera has larger drupes than the other three species and that, therefore, a smaller range of vectors can and will feed on its drupes, Ph. dactylifera was seen as a suitable candidate as it has a long history as a major agricultural crop in many parts of North Africa and the Middle East (e.g. Al-Khayri, Jain, & Johnson, 2015;chapters therein), In addition, the plant is grown commercially in South Asia (Grewal & Kapoor, 1986), the USA (Krueger, 2013), Mexico (Johnson, Rivera, Alcaraz, & Ríos, 2013), Chile and Peru (Escobar and Valdivia, 2013) as well as in Australia (Reilly & Reilly, 2014). It has also been argued that in the case of the Baja California it forms a culturally defined keystone species (de Grenade, 2013). that animal-mediated dispersal was only very rarely addressed. In total, 86 papers referred to drupe predation by vertebrates. Not surprisingly, given the agricultural significance of Ph. dactylifera in these areas, the Middle East (35.0%) and North Africa (32.5%) dominated the geographic spread of the papers.
The majority of the studies collated here merely notes that selected animal species feed on drupes, or they discuss seasonality in the utilisation of the fruit as part of a given species' diet (e.g. Belkacem et al., 2017). Very few studies make comments on the nature and range of seed dispersal.

Results
Date predation and dispersal. Setting aside humans who must be regarded as the primary past and present vector for the dispersal of Ph. dactylifera, there are three major groups of vectors that need to be considered: i) volant vectors, such as birds and fruit bats, and ii) arboreal vectors, such as rats, all of which have or can have access to fruit on the tree. In addition, there are iii) terrestrial vectors that can only access autogenously dropped fruit as well as fruit dropped by the other vectors.
From an agricultural cropping perspective, the first two groups are the most destructive. Consequently, the competition for ripe fruit between the date-growers and wildlife often end in conflict, with birds being netted, shot or poisoned and bats being shot or their roost caves being fumigated (Korine, Izhaki, & Arad, 1999;Hadjisterkotis, 2006). These practices to manage agricultural losses to vertebrates are less widespread today and have been generally replaced by protecting the ripening bundles of dates in bags (Abdoussalam & Pasternak, 2015;Ba-Angood, 2015;Ben Salah, 2015;Cohen & Glasner, 2015;Khairi, 2015;Sedra, 2015aSedra, , 2015b. Date predation by volant vectors. By far the largest number of predators of Ph. dactylifera are volant vectors, primarily birds (Table S1) but also fruit bats (Table S2). The volant vectors have three ways of dealing with the fruit. One is to swallow a drupe whole, digest it and at a later stage either defecate the seed and undigested elements, primarily the epicarp, or to regurgitate them as pellets. The second is to consume all or part of the fruit at the host tree, either while the fruit is still attached to the tree and while the fruit is being held in one of the claws. The third method is to pick off a drupe and take it to safety/cover in a perch tree and consume it there.
Birds with large gape sizes, such as crows (Corvus spp.), ingest the drupe as a whole. Birds with small gape sizes will peck at the fruit and extract varied amounts of pericarp. Depending on the ripeness of the fruit, such pecking may also lead to abscission of the spoilt drupe. Guezoul, Sekour, Souttou, and Doumandji (2010), for example found that up to 7% of all dates were spoiled by House Sparrows (Passer domesticus × P. hispaniolensis), with the sparrows only consuming a small amount of the pericarp. Likewise, cockatoos (Cacatuidae) and parrots (Psittaciformes) feed on drupes while on the tree. As these species have a sharper bill, they are able to cut through the skin of less ripe drupes (Reilly, 2018b). Common to the species feeding on the drupes while still on the tree is that many of the drupes fell to the ground and thus become available to terrestrial vectors. Studies in Algeria have shown that predation of date palms occurs both by resident birds, such as Brown-necked Raven Corvus ruficollis, and by transient or wintering migratory birds, such as Starlings (Sturnus vulgaris). In Central Sahara, 29.4% of the diet of the Brown-necked Raven in the breeding season is made up from Ph. dactylifera and 31.2% in the non-breeding season (Belkacem et al., 2017). Migratory Starlings are reliant on dates, as recorded from Algeria (Djennas-Merrar, Berrai, Marniche, & Doumandji, 2016). Bulbuls are regarded as one of the most important volant dispersal agents for small (<15 mm) fruits in the Oriental Region (Corlett, 2017) which are swallowed whole and defecated. Larger fruits are ingested with the seeds regurgitated. Even larger fruit, such as mature drupes of some Ph. dactylifera varieties, which are too large for the birds' gape size, are consumed while on the tree or taken to a perch.
Fruit bats have been known to feed on and thus are implicated in the dispersal of Ph. dactylifera in Australia (Augee & Parry-Jones, 1991), Israel (Izhaki et al., 1995;Kultzer, 1979), and Egypt (Kultzer, 1979). While farmers perceive these bats as a major agricultural pest (Moran & Keidar, 1993), Korine et al. (1999) have shown for Israel, that Egyptian Fruitbats (Rousettus aegyptiacus) ate mainly non-commercial fruits and also to a lesser extent leaves and pollen.
The feeding behaviour of the Egyptian Fruitbat, which occurs in much of the eastern Mediterranean from Turkey to Egypt, is comparatively well studied. By preference, this species takes a fruit from the source tree and consumes it on site, or in the case of less "habitable" trees, takes the fruit in their mouth and fly to a nearby perch or feeding roost, but does not carry food to a resting roost (Izhaki et al., 1995;Kultzer, 1979;Shehab & Mamkhair, 2004). Field studies showed that great portions of fruit were accidentally dropped in flight (Kultzer, 1979). Once the flesh is consumed, the seed is dropped, either at the tree (source or perch) or after take-off (Kultzer, 1979). Tsoar, Shohami, and Nathan (2010) documented the seed rain around source as well as perch trees, noting average diameters of 10 m. Some seed were dropped as far as 500 m from the source tree.
Among the various types of food source trees, palms such as Phoenix spp. and Washingtonia spp. were found to be less "habitable" than others, with a higher proportion of drupes carried off to perches (Kultzer, 1979). Predation by terrestrial vectors. A major group of vectors are large canid carnivores such as Grey Wolf (Canis lupus), Coyotes (Canis latrans), Dingo (Canis lupus dingo), domestic dogs (Canis canis) and the various species of fox (Vulpes vulpes, V. cana, etc.) as has been proved by stomach content and scat analysis. Even smaller carnivores such as the European Badger (Meles meles) and the Stone Marten (Martes foina) are frugivorous and have been documented as feeding on palm fruit (Table S3). Canids seem to be opportunistic foragers and consume fallen dates when available. While Red Foxes, for example, were identified as a significant disperser for the seeds of Ph. dactylifera in Spain (Cancio et al., 2017), the majority of the feeding is, by nature of the plant, seasonal.
A stomach content analysis of the Arabian Red Fox (Vulpes vulpes arabica) in Saudi Arabia showed that most Ph. dactylifera had been chewed (Walid & Basuony, 2016). Ph. dactylifera has been recorded in scats of Blanford's Foxes (Vulpes cana) in Israel (Geffen, Hefner, MacDonald, & Ucko, 1992), while a study of the Sand Fox (Vulpes rueppelli) scats in Egypt showed that 63% of the scats contained Ph. dactylifera seed (Kowalski, 1988). The home ranges of foxes depend on the productivity of the area. In Australia, these range from 45 ha in suburban areas to over 600ha in monoculture farmland (Gentle, 2006, Meek & Saunders, 2000. Studies showed that in agricultural land, the majority of animals travelled less than 3 km from their den, whereas during the dispersal of adolescents, distances of up to 13 km were travelled (Coman, Robinson, & Beaumont, 1991). Travel distances in semi-arid areas increased concomitant with the lower productivity of that ecosystem.
In the semi-arid parts of California, the Coyote (Canis latrans) is regarded a vector of Washingtonia filifera palms, dispersing the seed between springs (Cornett, 1988) with home ranges of over 25 km (Cornett, 1987). Phoenix dactylifera is also dispersed in human-managed, agricultural areas (Silverstein, 2005). In Australia, the Dingo is deemed to fulfil the same role in the semi-arid areas. Dingo scats at various spring sites show Ph. dactylifera drupes and seed in various stages of mastication and digestions (e.g. Supplementary Figure S3). In the Australian setting, canid-based dispersal is in excess of 30 km. In remote aboriginal communities in Western Australia and the north of South Australia, domestic dogs have been observed to jump and climb onto low hanging fronds of Ph. dactylifera to feed on drupes (Reilly, 2018b). Other drupe predators. In addition to canids, there is a range of other species that are known to feed on fallen drupes. By and large, however, none of the studies that mention the dispersal of Ph. dactylifera by terrestrial vertebrates other than canids make mention of dispersal ranges and frequencies.
For example, scat analysis found that Ph. dactylifera made up 55% of the diet of the 288

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Baluchistan Black Bear (Ursus thibetanus gedrosianus) (Ghadirian, Qashqaei, Ghoddousi, Soufi, & Khayat, 2012). The spatial distribution of the analysed bear scats containing Ph. dactylifera seed shows dispersal from the date palm groves into high woodlands and thickets (43.2%) as well as riverbeds and valleys (9.1%). Of interest is also that 18.2% of the scats were deposited near the entrances of the dens. Almost one third of the scats (29.5%) were found in the date groves themselves (Ghadirian et al., 2012; Supplementary Table S2). In Australia, the Emu (Dromaius novaehollandiae), a large terrestrial bird, has been documented to feed upon a range of orchard crops (Spennemann & Allen, 1998, 2000a, including Ph. dactylifera, and to defecate multiple seeds and other residue at a later stage (Supplementary Figure S4). In production landscapes, Emus have a large home range, regularly moving between orchards to feed and remnant bushland to shelter, with travelling distances of up to 50 km per day (Noble, 1975). Another factor that makes emus formidable seed disperses over a great distance is gut retention time (Calviño-Cancela, Dunn, Van Etten, & Lamont, 2006). While much of the vegetable matter is defecated after 5.5 hours (Herd & Dawson, 1984) and the majority of soft seeds after 26 hours, hard seeds tend to take much longer, being held up in the gizzard. Experiments with Emus showed that about half of the hard experimental seeds passed within two days, but that some had not been defecated after 10 days (Willson, 1989).
In addition, there are arboreal climbers, such as rats, that can feed both on fallen drupes as well as on drupes still on the tree. The Short-tailed Bandicoot Rat (Nesokia indica) is a major predator of Ph. dactylifera fruits, but in Israel it also feeds upon the stem pith of fresh fronds (Moran & Keidar, 1993) and Pakistan (Ahmed, Lathiya, Pervez, Zaheer, & Khadijah, 2007;Lathiya, Ahmed, Pervez, & Khadijah, 2010). Not surprisingly Ph. dactylifera formed a major part of the diet of N. indica encountered in date palm orchards in the Chagrai District of Baluchistan, Pakistan. Fruit and pith remains were found in the faeces all year round, but particularly between January and August, with the highest percentages in May and June (Lathiya et al., 2010). Domestic animals. Unmanaged fallen drupes may pose potential reservoirs for insect and other pest problems and thus need to be removed from the orchards. One Australian orchard uses domestic geese (Anser anser) for the purpose (Reilly, 2018a).
Given that the vast majority of Ph. dactylifera occur in managed groves and plantations in production and subsistence settings, it is not surprising that commensal livestock are allowed to forage between the palms. In the Middle East and North Africa, the main distribution area of the date palm, domestic animals are usually allowed to forage among the palm groves, where they feed on fallen dry fronds (Medjekal, Arhab, & Bousseboua, 2011) as well as fallen drupes. On record are camels, cattle, goats and sheep, which tend to digest the epicarp and pericarp but usually defecate an intact seed (Barreveld, 1993). Inferior and unripe fruits are also being fed to livestock in Tunisia (Genin et al., 2004) and Oman (Guarino, 1989). Traditionally, the seed itself, soaked in water for three days, was fed to camels in Egypt (Preston, 1847) and Saudi Arabia (Qureshi, Bhatti, & Jakhar, 2006) as well as other various livestock in the Middle East (Abd Rabou &Radwan, 2017;Genin, et al., 2004;Sadeghi & Kuhestani, 2014). On occasion, the seeds were ground to serve as cattle food in Iraq (Harry, 1936).
While the dispersal potential is low in those instances where a Ph. dactylifera seed is fed to domestic animals, it cannot be discounted altogether and thus has been included in Supplementary Table S2. Drupe consumption by foraging or anthropogenically-fed livestock will have little dispersal capacity as the bulk of the animals will exist in well circumscribed fenced, or herded, spaces. The only exception are transport animals such donkeys and camels, that may defecate en route or while at watering or resting points.

Discussion
The overwhelming majority of the papers examined for this study do not provide details on the nature dispersal beyond the fact that it occurs. While the significance of Ph. dactylifera in the seasonal diet of some species has been established, such as in the case of the Baluchistan black bear (Ghadirian, et al., 2012;Fahimi et al., 2018), or for numerous species in the Algerian setting (Beddada, 2009;Bendjoudi et al., 2015;Djennas-Merrar, et al., 2016;Guezoul, et al., 2010), there are no reliable data on the quantities of Ph. dactylifera removed, dispersal distances or the success and rates of germination. The lack of data on actual quantities removed is, to some degree, understandable as in modern production settings agricultural losses to vertebrate pests can be managed by protecting the ripening bundles of dates in bags, as is standard practice in many countries (Abdoussalam and Pasternak, 2015;Ba-Angood, 2015;Ben Salah, 2015;Cohen & Glasner, 2015;Khairi, 2015;Sedra, 2015aSedra, , 2015b. Not formal germination studies have been carried out to date and the validity of reported examples (Fahimi et al., 2018) is questionable.
Setting aside humans who must be regarded as the primary vector for the dispersal of the domesticated as well as ornamental palms, vertebrate disperses can be grouped as either those that can fly, such as birds and fruit bats (volant vertebrates) and thus can access the fruit on the tree, and those that can only access food found on the ground, either autogenously dropped fruit and fruit dropped by birds and bats (terrestrial vertebrates). Volant and terrestrial vectors can be further classified as 'resident', i.e. those that use the canopy of the palm as shelter /roost, 'local', i.e. those that have a den/roost close to the palm (i.e. within their home range / territory of defined radius); and those that are 'transient', i.e. migratory birds and mammals with a wide territory. The latter categorisation has a direct bearing on the distances seed dispersal can occur.
The ingestion pattern allows for further differentiation. Levey (1987) classified birds as 'fruit mashers,' i.e. those that peck at the flesh of the fruit and 'fruit gulpers', i.e. those that ingest the whole seed. While useful, this dichotomy is not fine enough for an examination of dispersal mechanisms. Elsewhere, Spennemann and Allen (1998) developed a four-class system of frugivores, which considered the mode of ingestion. For the purposes of this paper this has been adapted into a three class system. Frugivorous animals have three discrete modes of dealing with and ingesting fleshy fruit: i) ingestion of the entire fruit, masticated or not, and subsequent defecation of the seed (coded as ID in the tables); ii) ingestion of the whole fruit, masticated or not, and subsequent regurgitation of the seed (coded as IR); and iii) pecking/gnawing at the fruit and ingestion of the pulp / pericarp only (coded as P). All species documented as feeding on Ph. dactylifera drupes have been classified accordingly (Supplementary Tables S2-S4).
The feeding habit of a species has a direct influence on the nature and extent of seed dispersal. Birds play a primary role in a generalised model of the dispersal of palms (Figure 1). Smaller birds, such as Passer spp. feed on the pericarp of semi-ripe and ripe drupes while on the tree (1). Either the damaged drupe remains on the spikelets and dries up, or is dislodged and falls to the ground (2). The greater the ripeness of the drupe, the more easily it will abscise and drop. Most birds tend to consume only ripe drupes. Exceptions are cockatoos and parrots, whose sharp beaks can cut the epicarp of less ripe fruit. Once the partially consumed drupe has dropped to the ground (2), it can be picked up by smaller or larger mammals and marsupials, as well as flightless birds such as the Emu. These mammals which also pick up autogenously dropped fruits, can then either feed on the drupe at the drop location or can take the drupe to cover for delayed consumption (5). Larger mammals/birds can ingest it whole and carry it intestinally to their dens or other places of rest. The partially or fully digested drupes may be  dropped in their faeces at any location of rest (7) or traverse (8). Larger birds and fruitbats take an entire drupe and may consume the pericarp on the host tree. While some palm species, such as Ph. dactylifera and Ph. canariensis have well-fronded lower crowns, the tops tend to be more open. The bunches (clusters) of mature drupes tend to weigh down lower fronds, thereby creating an open environment that exposes feeding birds to raptors. Where managed agriculturally, the lower fronds are traditionally often cut away, exposing the drupe bunches even more (Supplementary Figure S2). Consequently, a number of species seem to prefer taking a drupe to a nearby perch (3) where the non-consumed part is dropped and falls to the ground (4). Depending on the amount of pericarp left, the remains may then be picked up by smaller or larger mammals (akin to the process above). Birds with a larger gape size may consume the entire drupe and then fly to a perch tree (3) or roost (6) to rest, where they eventually regurgitate the seed as part of a pellet or void it as part of their faeces. Defecation tends to be particularly common under perch trees, as many birds are known to defecate before taking off (Caro, 2005;Pike, Spennemann, & Watson, 2017) as part of the fit-for-flight hypothesis (Van der Veen and Sivars, 2000). Larger seeds devoid of pericarp may be predated by some vertebrates and birds. From the perspective of seed dispersal, such activities are destructive and can be discounted here.
In total, 64 species have been documented to feed on Ph. dactylifera (Supplemenary Tables S1-S4). These can be summarised into biological characteristics of dispersal (Table 2) or as dispersal impact. Overall, the three classes of ingestion types are equally represented (Table 2).
Plotting all documented vectors in terms of their dispersal distance as well as the quantity of seeds dispersed per dispersal event (Figure 2) it becomes obvious that there no vectors that move larger quantities of seed at one time over a short distance only. Short distance movement is caused by birds pecking at a single fruit, causing their abscission from the tree or a short distance removal of single fruit to a perch. Large volume removal is primarily associated with animals that ingest the seed and defecate it. Ideally, a third dimension ought to be added to Figure 2 which takes into account the frequency with which the vector occurs either in terms of revisits over a short period of time, or in terms of absolute numbers. From a dispersal perspective, a large quantity of vectors, such as a flock of starlings, increases the probability of successful dispersal. The majority of vectors that occur in greater numbers, disperse over shorter distances only. These vectors, however, will return to the host tree for further drupes, which may be taken to the same perch. Long-distance dispersal tends to occur with larger terrestrial vertebrates, such as the various canids, camels, bears, emus and the like.

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Of the various modes of ingestion, the effects of ingestion of the pericarp only by pecking or gnawing are most well understood. The dispersal range depends on the presence of perch sites in the vicinity of the source plant and, in the case of multiple options, on the nature of the perch, i.e. the size/height of the perch and the canopy cover (Spennemann & Allen, 1998;Spennemann & Pike, in prep.). If the drupe remains are not predated by vectors, the germination chances are determined by the nature of the substrate on which the seed falls, competing vegetation cover, as well as the moisture regime at the location. The dispersal success and range of the other two modes are less well documented, primarily as actual stomach content and scat data are difficult to come by.
Palm seed that have passed through the digestive tract tend to germinate better, as has been documented among coyotes feeding on Ph. dactylifera and Washingtonia filifera (Bullock,1980;Silverstein, 2005). In birds, the level of scarification is directly related to both gut passage time and the size of the gizzard. While as a rule, the size of the bird is related to the length of the gut passage times, passage times can be delayed in the case of birds on sub-maintenance diets (Clench & Mathias, 1992). The digestive systems of granivorous birds, in particular the stones contained in the gizzard, tend to pulp the flesh and scarify the seed, leading to better germination. As a rule, the larger the bird, the larger the stones and hence the greater the mechanical impact on the seed (see the glass-marble experiment in Emus: Davies, 1978). Based on the experiences gained with the experimental feeding of Nitre Bush Fruit (Nitraria billadieri) (Noble, 1975), the digestive system of an Emu, for example, seems to scarify the hard endocarp and thus facilitate seed germination. Larger seeds, such as those of Ph. dactylifera seem to be less affected (see Supplementary Figure S4). Other granivorous birds, such as Turkeys (Meleagris gallopora) and parrots tend to largely grind up the hard endocarp of swallowed drupes (Mort, 1949). The latter, therefore, are less likely to be sufficiently viable to germinate. Even though viable drupes of Ph. dactylifera can be dispersed by the vectors indicated, germination of the seed requires both sufficient rainfall, at least above 30mm/pa and temperatures of 30 to 35°C (Broschat, 1994). Plant survival post germination depends inter alia on the presence of competing vegetation into which the seed is dropped, on grazing pressure and on the fertility, compaction or moisture content of the soil matrix. Any defecation or regurgitation in managed agricultural landscapes will have a lesser survival chance than that which occurs in remnant bushland or roadside reserves. Waterholes (oases) in arid and semi-arid areas are a focal point for dispersal vectors and can provide suitable locations for successful colonisation due to the availability of moisture and a higher nutrient content in the soil (due decaying vegetation). Even if a Ph. dactylifera seedling survives and grows into a mature plant, the plant then requires successful pollination, low rainfall and long sun hours during the ripening period to produce viable seed (Zaid & Arias-Jimenez, 2002).
Agricultural assessments found that female Ph. dactylifera grown from seed tend to produce late maturing fruits of variable and generally inferior quality compared to palms that were cloned from offshoots (Zaid & Arias-Jimenez, 2002). As Ph. dactylifera are heterozygous, variation will occur within the progeny with no two seedling palms being alike (Zaid & Arias-Jimenez, 2002). This will result in variations in drupe size, flesh volume and seed size. This may have implications on vectors and dispersal. With other drupe-producing plant species, such as olives (e.g. Olea europea), it had been observed that non-cultivated, feral populations have smaller fruit than the cultivars, which allows a greater range of birds, especially those with smaller gape size to both ingest as well as to carry off fruit (Spennemann and Allen, 1998, 2000a, 2000b).
An aspect that has not been examined in any depth, certainly as far as Ph. dactylifera is concerned, is the germination success of seeds in situ. The limited number of germination studies that have been carried out focussed on seeds extracted from scats and planted individually in some form of seed bed (Bullock, 1980;Silverstein, 2005). It has been posited that not all seeds in a single scat would germinate, but that the concentration of seeds would increase the probability of successful germination and establish-ment of a viable plant. To date, there seem to be no collateral studies looking at the germination success of Ph.x dactylifera seeds derived from regurgitated pellets. Likewise, no research has been carried out to ascertain whether successive individual drupes dropped from perch sites have a different chance of survival than those dropped in pellets. In addition, there appear to be no studies that examine the actual colonisation success of Ph. dactylifera in remnant bushland near production landscapes. While spatial studies of scat distribution, as in the case of the Black Bear, indicate the utilisation of a territory, they do not correlate the presence of Ph. dactylifera seeds in the scat with the existence of self-seeded Ph. dactylifera in the landscape.

Disclosure Statement
No potential conflict of interest was reported by the author.