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Is prolonged isolation driving the Critically Endangered White-bellied heron to extinction?

  • Category: Environment
  • Subcategory: Nature
  • Pages: 6
  • Words: 2679
  • Published: 14 May 2019
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Introduction

World over the loss of biodiversity has increased exponentially in recent years and according to BirdLife International (2018), around 1469 bird species are threatened with extinction. Of these 222 species are considered Critically Endangered. Biodiversity loss due to human-driven land-use changes results in loss of species especially in the tropics where both species diversity and human pressures on natural environments are high (Dirzo and Raven, 2003). More biodiversity is being lost in some habitats as compared to others leading to skewed geographic distribution of threatened species.

While some species have received attention of conservationists (e.g. Spoon-billed Sandpiper Calidris pygmeus and Great Indian Bustard Ardeotis nigriceps; Government of India has prepared guidelines for the recovery programme of GIB: see www.indiaenvironmentportal.org), others for example, Jerdon’s Courser Rhinoptilus bitorquatus, Himalayan Quail Ophrysia superciliosa and Manipur Bush-Quail Perdicula manipurensis, in India have received little attention in spite of the fact that first two are Critically Endangered and the third is Endangered (BirdLife International, 2013). Dedicated surveys (Jeganathan et al., 2002 for Jerdon’s Courser; Choudhury, 2006 and Baral et al., 2013 for Himalayan Quail) have not helped to improve our understanding of their status.

The White-bellied Heron Ardea insignis (WBH) has received little attention until recently, with the preparation of a single species conservation action plan (Price and Goodman, 2015). According to IUCN Red List, it is Critically Endangered (BirdLife International, 2017). Figure 1 shows the historical and recent distribution of the WBH; previously the heron was restricted to undisturbed swamps, reed beds and marshes in eastern Nepal and the Sikkim terai, Bihar (north of river Ganges), Bhutan duars to northern Assam, East Pakistan (Bangladesh), Arakan and northern Burma (= Myanmar) (Ali and Ripley, 1987; and Walters 1976).

In the last two decades, it has been reported in Assam, from Kaziranga National Park (Barua and Sharma 1999), Jamjing and Bordoloni of Dhemaji district (Choudhury 1990, 1992, 1994), Dibru-Saikhowa National Park (Choudhury 1994), and Pobitara Wildlife Sanctuary (Choudhury 1996a). In Arunachal Pradesh, it has been reported from Namdapha Tiger Reserve, especially from its fast-flowing rivers namely Noa-Dihing, Namdapha and Debannalla (Choudhury 1996b, Maheswaran 2007, 2008). The population in Namdapha is estimated to be seven individuals (Mondal and Maheswaran, 2014).

In northern Myanmar, the WBH had been described as common along the Mali Kha river in Putao (Smythies 1953) and on the Irrawady river (Stanford & Ticehurst 1939). However, records in Myanmar as far southeast as Toungoo and Pegu suggest a migration or other post-breeding dispersal (Hancock and Kushlan, 1984). King et al. (2001) during their surveys have mentioned that the herons were sighted in 1998 and 1999 along a large river near the village of Ziyardum (north-west of Putao) and Nan Lam River (Wasadam) on a shingle bed. It is possible that these sightings were the first sightings of this species in Myanmar for many years.

Figure 1: Map showing the global distribution (recent and historical) of WBH (Map downloaded from: BirdLife International, 2017)

The population of the heron has remained low for many decades and probably from early 19th century, as few sightings have been recorded from anywhere across the species range (Ali & Ripley, 1987); although this may have been a result of limited access of ornithologists to these remote protected areas and security challenges.

According to Kushlan (2007), 1% conservation population threshold of WBH remains at 6 individuals and global population at 250-1000. BirdLife International (2017) has estimated the WBH population at 70-400 individuals. However, Price and Goodman (2015) has estimated the global WBH population at less than 60 individuals. This number is highly prone to extinction, if not protected adequately with immediate conservation actions.

According to Szabo et al. (2012), reasons for extinction of bird species are many and cannot be pinned to a single cause, but extinction often occurs when renewed threats emerge from outside the evolutionary experience of species (Brook et al. 2008). The WBH has not been recorded in Nepal since the 19th century and is considered to be ‘Regionally Extinct’ (Inskipp et al., 2016). During early 1960s, many national and international agencies assisted the Nepalese in eradicating malaria. Many wetlands were drained only to eliminate the malaria mosquito, while the remaining wetlands were lost as settlements proceeded (Harris 1992) over the years. This could be one reason why herons have disappeared along with Sarus Crane (Grus antigone) from southwestern Nepal terai, though the later continue to be present in significant numbers in south-central lowlands.

It should be noted that there has been only limited access of ornithologists to remote areas in NE India or across the border in northern Myanmar, with access to Namdapha only from 1980s. Poor road networks, rugged terrain and logistical difficulties were reasons attributed to poor surveys targeting avifauna and other wildlife of the region (Srinivasan et al. 2010). Very little is known about the breeding habits of this species, with recent information from Bhutan (RSPN, 2007). Observation on a pair of adult herons that was courting and nesting within Namdapha Tiger Reserve, Arunachal Pradesh, India (Mondal and Maheswaran, 2014) was the only sole record from the country till date.

We observed White-bellied herons for 858.45 hours from 2013-2016 while foraging in the rivers such as Noa-Dihing and Namdapha within Namdapha. Continuous focal sampling method (Altman, 1974; Martin and Bateson, 2007) was used to record various foraging activities of herons. During foraging observations the starting and ending time of each foraging bout, number of foraging attempts (defined as the number of times the heron strikes the water with its bill in search of prey; Figure 3) and the number of fish caught were recorded. The herons were seen striking (Figure 4) the water for 1121 times, out of which only 403 attempts were successful. As the results suggest, herons made fewer successful attempts (64%) against 36% of unsuccessful ones. Moreover, the species had a strike rate of 1.3 strikes/hour and a capture rate of 0.5 fishes/hour. In Bhutan, herons’ captures were infrequent and almost similar as recorded in India i.e., 11 fishes in 78 hours (0.45 fishes/hour) and also similar to Goliath herons (0.332). In Bhutan, the strike rate was at 1.2 strikes per capture (RSPN, 2011). This implies that the herons had easy access to fish in Bhutan than their counterparts at Namdapha, India.

Reasons for isolation

From recent observations, the WBH appears to have restricted its distribution and foraging habitats to fast-flowing rivers across its range, including Myanmar, Bhutan and India (Pradhan et al. 2007; Maheswaran, 2007), ranging from 60 m to 1300 m in altitude (Choudhury, 2000).

It is an established fact that in the last decade or so, habitat destruction and alteration have become rampant in the Indian landscape as human population (for example, Assam in Brahmaputra flood plains has human density of 400/km2; World Bank, 2014) has increased many fold. Expanding agriculture and reclamation of natural wetlands for agriculture, especially rice, have diminished the habitats of many native fauna, including waterbirds that were directly dependent on these habitats. Other studies (Seitre and Seitre 1992; Milberg and Tyrberg 1993; Butchart et al. 2006; Blackburn et al. 2004) reaffirm that habitat loss due to agricultural expansion and overexploitation has affected species world over and this single reason has remained as one of the major drivers of extinctions and sometimes acted simultaneously (Didham et al. 2005). In Assam and Arunachal Pradesh, tropical moist forest cover, which is the most diverse and occurs at lower altitudes, decreased at 1.38% per year between 1994 and 2002 when compared to tropical evergreen and sub‐tropical evergreen forests (Kushwaha and Hazarika, 2004). With the loss of wetlands in the plains, the herons may have been forced to remain within the tropical and sub-tropical evergreen forests in north-eastern India and the more temperate forests of Bhutan as these ecosystems unlike the plains took longer time to witness severe habitat changes such as road/dam construction and jhum cultivation. Declaration of some important habitats as protected areas including national parks and sanctuaries, has provided refuge for species such as White-winged Duck (Cairina scutulata) and WBH from direct impact of habitat alteration, hunting and fishing, as these actions are prohibited inside protected areas.

The heron’s preference in current times of selecting isolated rivers in the region as its preferred habitat may have helped it to remain unaffected by hunting pressure exerted by the locals on other species outside protected areas. However, it becomes difficult to establish the earlier statement as no data has ever been collected on inter- and intra-species interactions from India and elsewhere. When habitats surrounding major cities and towns in northeast India started to feel the pressure of human interventions post 1990s, species like WBH may have remained in remotest corners of major forest areas in northeast India especially in Arunachal Pradesh and Assam but only inside protected areas.

Also, competition with other fish-eating species for space and food apart from disturbance from humans in the remaining wetlands (outside protected areas) in northeast India may have pushed the species further to a habitat that remained as their present permanent habitat. Thick forests with fast-flowing rivers of Namdapha provides feeding (Figure 2) and breeding space for WBH in India, possibly this habitat seldom preferred by other fish-eating birds (Maheswaran, 2008). Possible explanation to as to why other fish-eating birds do not prefer Namdapha could be that the open wetlands in the plains are far away and wandering birds in search of food find it increasingly difficult to locate any suitable habitat other than fast-flowing rivers inside thick patches of evergreen forests which in turn not suitable for other colonial fish-eating species found in the region (except cormorants).

The WBH has a large body size, coupled with low fecundity rate (RSPN, 2007) and likely shyness to human presence even within protected areas where human interventions are minimal. Collectively these reasons may have attributed to the species being pushed further toward isolation.

Effects of isolation on the heron

Maccarone et al. (2012) hypothesized that once a bird reaches a remote site, it faces less competition from other birds and was able to attend to prey capture for the entire time. In case of WBH of Namdapha, success rate is less in spite of the fact that the reserve is remote and has no other flock foragers, except Large Cormorants Phalacrocorax carbo (Maheswaran, 2008). On a few occasions (n=4) we had seen herons chasing the cormorants whenever the later approached herons very close especially while the herons foraging actively. However, once we had see a cormorant chasing the heron that was with fish its bills. Cormorant tried to steal the fish from heron but failed in its attempt. Furthermore, herons were seen close to Black storks but both of them ignored each other. Reason why herons and cormorants showed aggressive behaviour at each other could lies with the reason that both preferred almost similar sized fishes, whereas the Black Stork always foraged in the edges of the main river and seen capturing small fishes (4-9 cm). In Bhutan, on two occasions, while two foraging herons were approached closely by Large Cormorants, herons flew away from the spot in search of another site (RSPN, 2011). It shows that Large cormorants are one of the potential competitors of herons in India and Bhutan than any other species of fish-eating bird.

Among many animal groups, aggregations occur especially while foraging (Krause and Ruxton 2002), including among wading birds (e.g., Erwin 1983b, Kersten et al. 1991, Master et al., 1993). Several hypotheses have been proposed to explain how individuals benefit from foraging in mixed-species aggregations. There is evidence that mixed-species aggregations of wading birds often form in areas of high prey density (Kushlan, 1976b, Erwin et al. 1985, Smith 1995). Whereas, other studies have shown that wading birds locate their feeding grounds by using the presence of other foragers; this is often referred to as “local enhancement” (Krebs 1974, Kushlan 1976, Caldwell 1981, Erwin 1983b).

Similarly, foraging success may also be enhanced through attaining added information about the location of prey availability within a patch (Valone, 1989, Valone and Giraldeau 1993), or may be learning foraging tactics from other forager’s novel prey types (Krebs and Inman, 1992, Beauchamp et al. 1997). It has been demonstrated elsewhere that some species of wading birds simply act as focal or target members that attract others to mixed-species foraging congregations (Kushlan 1977, Smith 1995, Strong et al. 1997). Dark-plumaged WBH do not attract other species like their white-plumed counterparts. Similarly, by foraging in groups, some species benefit significantly as they spend less energy while finding food for themselves (Hafner et al. 1982, Master et al. 1993).

The WBH appears to be a largely solitary feeder although it has been recorded in small parties of four to five; with a single record of six individuals in a marsh lined with elephant grass (Choudhury 2000; Mondal and Maheswaran, 2014). Additionally, it has been seen with Grey Heron (Ardea cinerea) and other waterfowl (Choudhury 2000), although no mention is made of any interactions. In Namdapha we recorded Black Stork (Ciconia nigra) foraging close to WBH but no interaction happened between two species. Thus, it is difficult to evaluate how any socially stimulated aspects may enhance heron’s foraging success.

Fast-flowing rivers of Namdapha in its upper reaches though support flock-foraging cormorants and solitary Black Storks (Ciconia nigra) beside herons, other long-legged fish-eating birds (egrets, other species of herons) were not recorded (Maheswaran, 2008). Then, why White-bellied heron do not leave Namdapha to seek alternative sites outside this protected area remains vague and it may reveal crucial information about the survival strategy of the species especially in India. Although known to be a fish eater, little is published about its food preferences which may provide more clues about its distribution pattern.

We speculate that having vacated their former feeding territories, herons now stick to undisturbed riverbanks inside evergreen forests in India and Myanmar. The reason why some individuals are still present in fast-flowing rivers of Bhutan may be due to less human disturbance and limited fishing and hunting pressure there. From breeding studies there, mortality is high mainly due to collision with power lines and forest fires (while nesting as nest trees are damaged in fire) (Pradhan et al. 2008). This may serve as another important reason for the continued decline of this species.

Conclusion

Extreme shyness towards human presence (Mondal & Maheswaran, 2014; RSPN, 2011) and habitat alterations pushed the species to rivers within forests of protected areas. Having studied the foraging behavior of White-bellied heron in Arunachal Pradesh, India for three years, we suspect that the prolonged isolation by associating with one habitat (fast-flowing freshwater rivers) for long may be driving the species to near extinction in India. It curtails herons from gaining any experience as of other flock foragers in finding suitable alternative sites to enhance their survival. Elsewhere, it has been demonstrated that the colonial foragers can gain maximum energy by sharing experience and information about food patch that enhances their survival. It is also possible across its current and limited range in Myanmar and Bhutan.

Furthermore, the WBH has so far did not get the attention of conservationists in India and Myanmar and to some extent world over. For example, Butchart et al. (2006) in their article did not consider WBH as top target for birders though they touched upon all other threatened species that ought to get the attention of the conservationists. Surveys to ascertain current status has ever been undertaken in India and this is urgently needed to cover all potential localities in north-east India and Myanmar. This would give complement a long-term study (Pradhan et al. 2007) underway in Bhutan, the only country in the region that has a significant population of herons (c. 30 individuals). Of late, we launched our first study to find out more about the foraging habits of this species and we expect this will provide valuable information to understand more about the causes for its restricted distribution and decline. If herons continue to remain in isolation within protected areas, finding alternative sites in case of any exigencies will become difficult.

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