Age structure as an indicator of poaching pressure: Insights from rapid assessments of elephant populations across space and time

Abstract

51Detecting and monitoring illegal harvesting pressure on wild populations is 52 challenging due to the cryptic nature of poaching activities. Although change in 53 population age structure has been suggested as an indicator of harvesting pressure, 54 few studies have tested its validity when based on short-term field surveys. Using data 55 from rapid demographic assessment surveys carried out in 2009 at six sites in 56Tanzania, we examined whether African elephant populations experiencing 57 contrasting levels of poaching pressure showed significant differences in their age 58 structure, operational sex ratio (i.e. adult males to adult females), dependent 59 individual to adult female ratio at the group level, and proportion of tuskless 60 individuals. We also compared similar metrics between the population sampled in 61Ruaha National Park in 2009 and again in 2015 following a suspected increase in 62 poaching. Elephant populations experiencing medium and high levels of poaching in 63 2009 were characterised by fewer calves and old individuals, a reduced number of 64 adult males relative to adult females, and a lower ratio of calves to adult females 65 within groups. We also found a higher proportion of tuskless individuals in poached 66 populations (> 6 %). Changes in age structure in the Ruaha population between 2009-67 15 were similar to those observed across sites in 2009. Our findings are consistent 68 with previous work documenting how the loss of older individualstargeted for their 69 larger tusksdecreases recruitment and survival of elephant calves. Illegal killing for 70 ivory is a huge threat to the survival of African elephants. In this context, the present 71 study contributes towards validating the use of age structure as an indicator of 72 poaching pressure in elephant populations, but also in other wildlife populations 73where illegal offtake is targeted at specific age classes. 74 75 KEYWORDS: age structure, operational sex ratio, rapid demographic assessment, 76Ruaha National Park, Tanzania, tusklessness. 77 78

Importance of old bulls: leaders and followers in collective movements of all-male groups in African savannah elephants (Loxodonta africana)

“…All observed probabilities (blue squares) fell within range of randomly permuted probabilities of occupying the rear of groups (boxplots). www.nature.com/scientificreports/ alterations to sexual and social interests and behaviours 63 , and (ii) male elephants over this age are the prime targets of trophy hunting and poaching activity due to the weight of their ivory 41,[56][57][58] . Elephants walked in single file along elephant pathways, making order of travel easy to quantify (Fig.…”

Age structure changes indicate direct and indirect population impacts in illegally harvested black rhino

“…Overharvesting affects both population size and growth as a result of the direct mortalities suffered, and can have profound consequences on population trajectories [ 5 ]. Continued removals over time, as is often seen with illegal killing, can also profoundly alter the structure of a population [ 6 ]. Large mammal populations exhibit defined age and sex structures, often with class-specific survival rates [ 7 ]; as a result, changes in population structure can severely alter temporal dynamics and the population-level response to stochastic environmental variation [ 8 ].…”

Zoo elephant research: contributions to conservation of captive and free‐ranging species

“…Both African elephants – Savanna elephants Loxodonta africana and Forest elephants Loxodonta cyclotis – and Asian elephants Elephas maximus are threatened because of habitat loss and fragmentation (Evans et al ., ), and poaching for ivory (Breuer et al ., ; Ihwagi et al ., ) and, more recently, skins (Sampson et al ., ). These activities disrupt social hierarchies (Jones et al ., ) and increase stress, which may decrease resilience (Vijayakrishnan et al ., ). African elephants in zoos often have difficulty reproducing (Brown et al ., ), and many working Asian elephant populations are not sustainable (Leimgruber et al ., ).…”