Discussion
Apex predator actively harvests prey population and suppress
mesopredator community structure through top-down mechanisms (Ripple et
al., 2014). However, there is a dearth of evidence for subordinate
predator suppression due to lack of such natural experiment study
systems. We got an opportunity to understand the impact of a solitary
apex predator on competitively inferior social predator in a natural
experiment setup.
The Asiatic wild dog/ dhole is listed under “Endangered” category by
IUCN, with about only 949-2215 mature individuals surviving globally.
Indian subcontinent harbors majority of the viable dhole population
(Kamler et al., 2015). However, the species has been exterminated from
60% of its past distributional range in India (Karanth et al., 2010)
due to decline in prey base, habitat loss, human persecution and
intra-guild competition (Hayward, Lyngdoh, & Habib, 2014; kamler et
al., 2015). In India, persistence of dholes is mostly confined to
protected areas with infrequent presence records from secondary forests
and agro-forest plantations (Srivathsa, Karanth, Kumar, & Oli, 2019).
These remnant habitat patches are also shared by other large carnivores,
thereby limiting dhole population by lethal intra-guild interactions
(Steinmetz, Seuaturien, & Chutipong, 2013). Small size of PAs and
lopsided conservation efforts can further result in over-inflated apex
carnivore densities and be detrimental for subordinate predators like
dholes (Rayan & Linkie, 2016; Kumar, Awasthi, Qureshi, & Jhala, 2019).
Therefore, to conserve the endangered dhole, it is crucial to understand
its response to tigers, itself an endangered and flagship species. A
holistic understanding of guild interactions would improve our ability
to optimize ecological triage while conserving charismatic apex
predators and mid-ranking predators.
In this study, we found that the pack size was reduced in higher tiger
density areas. This might be due to the influence of interference
competition wherein a narrower niche is available for dholes to occupy
which limits their persistence. Studies from Africa also show a similar
trend where pack sizes of African wild dogs were reduced in areas of
higher lion densities (Groom, Lannas, & Jackson, 2017). At high tiger
density site (TATR) we also observed direct intraguild predation events
i.e. tigers killing dholes (n= 5) however, no such events have been
observed in NNTR, the low tiger density site. Similar events have been
documented in African system by Creel and Creel (1996), where lions
actively chase away dogs and in certain instances even kill them.
Conversely, in low tiger density area, larger pack size is indicative of
a competitive release scenario (Groom, Lannas, & Jackson, 2017). A
similar trend has been observed in spotted hyena clan size and pack size
of African wild dog due to reduced interspecific competition in a lion
depleted system and easy availability of food subsidies (Periquet,
Fritz, & Revilla, 2014; Green, Farr, Holekamp, Strauss, & Zipkin,
2019; Groom, Lannas, & Jackson, 2017).
There was no significant difference between the individual prey
densities at both studies, however, the higher density of a large-body
size prey in NNTR is also interesting to note. This finding is in line
with the idea that low numbers of apex predator lead to an increase in
abundance of large-bodied prey in a system. The subordinate predators
are limited by body-size and cannot target such large prey and this
might be one of the possible explanations for our finding. Similar
pattern is observed in study systems where wolf (Canis lupus )
populations are controlled (Burgar, Burton, & Fisher, 2019) that not
only causes coyote (Canis latrans ) abundance to increase but also
leads to stable number of adults in caribou (Rangifer tarandus )
population since coyotes are size-limited and cannot take down larger
individuals (Lewis, Gullage, Fifield, Jennings, & Mahoney, 2017).
To examine whether reserve level patterns were concurrent with global
scale, we did a distribution-wide assessment of pack size across dhole
distributional range. We found support for our first two hypotheses,
dhole pack size were positively associated with prey density and
negatively associated with tiger densities. Statistically significant
but moderate effect size reflects that the patterns were diffused at
global scale. Patterns of subordinate predator response to apex
predators numbers do not always mirror each other over temporal and
spatial scales (Jachowski et al., 2020), pertaining to area specific
ecological settings, anthropogenic disturbances, level of protection at
reserve level and connectivity in the landscape at regional scale
(Greenville, Wardle, Tamayo, & Dickman, 2014; Newsome, & Ripple,
2015). We did not find support for our third hypothesis, dhole pack size
was not associated with high terrain ruggedness however, a low pack size
has been reported from studies of high altitude and rugged terrain
(Bashir, Bhattacharya, Poudyal, Roy, & Sathyakumar, 2014; Lyngdoh,
Gopi, Selvan, & Habib, 2014).
Taken together, our reserve level and distribution wide assessment
results are in support of the hypothesis that area specific ecological
settings such as prey composition, abundance, and co-predator density
are drivers of carnivore community structure, group size and behavior
(M’soka, Creel, Becker, & Droge, 2016). Patterns of this study also
adhere to trends that have been observed in other long-term studies from
Serengeti National Park, Ngorongoro Conservation Area and Selous Game
Reserve, Tanzania, where the group size of subordinate predators are
positively associated with availability of food resources and negatively
associated with, apex predator numbers (Creel & Creel, 1996; Creel &
Creel, 1998; Woodroffe & Ginsberg, 1999; Pole, 2000; Périquet, Fritz,
& Revilla, 2015; Green, Farr, Holekamp, Strauss, & Zipkin, 2019).
Tiger and dhole belong to the same carnivore guild because of similar
diet spectrum (Hayward, Lyngdoh, & Habib, 2014), and therefore their
persistence in an area is linked to each other (Johnsingh,1992). Studies
based on non-invasive sampling such as camera trapping and occupancy
surveys suggest that carnivores adapt to mechanisms of niche
partitioning, depending on resource availability to facilitate sympatry
(Karanth et al, 2017). Very few studies have demonstrated that intense
intra-guild competition result in reduced density and spatial
displacement of the subordinate predators (Harihar, Pandav, & Goyal,
2011; Mondal, Gupta, Bhattacharjee, Qureshi, & Sankar, 2012; Steinmetz,
Seuaturien, & Chutipong, 2013). Our study provides a new direction in
understanding how intra-guild competition can also potentially limit
pack size of dhole.
Competition from dominant carnivores remains one of the major factors
that drives densities of other sympatric carnivores in a system
(Wrangham, Gittleman, & Chapman, 1993). NNTR and TATR are a comparable
natural experiment setup that allows us to study how these two
endangered tropical carnivores interact and understand consequences of
dominance shift. We suggest that the increased pack size of a social
subordinate predator (dhole) is an outcome of low density of apex
predator and availability of wider niche. The decision to form large
packs as compared to small packs is going to be more advantageous to
limit the growth of top predator. However, future long-term studies are
suggested based on multiple pack-year data, disease prevalence,
predation, kleptoparasitism events and recruitment rate of dholes,
across a gradient of apex predator density would further validate our
hypothesis.