How hurricane severity affects extinction probabilities for howler monkeys

Howler on Barro Colorado Is. Photo: Vince Smith

Howler monkeys are known for their long, bellowing calls that can be heard in the forests of Central and South America. There are five subspecies, the taxonomic rank below species, of howler monkeys. Alouatta palliata mexicana is a critically endangered subspecies found in parts of Mexico and Guatemala. Ameca y Juárez and colleagues had the idea to examine how hurricanes affect population numbers of this arboreal, New World primate. Using data from 1988 to 2002, the authors modeled how hurricane intensity affects extinction likelihood of local populations of this New World primate. They also wanted to know which human disturbance, hunting or habitat loss, had a greater impact when combined with different intensities of hurricanes.

Previous studies have examined how other species of howler monkeys have been impacted by hurricanes on a short-term basis, but this study is the first to look at the effects over a long period of time. The area in which A. p. mexicana inhabits is experiencing more hurricanes lately (Manson et al., 2009; Portilla-Ochoa et al., 2006), thus this study is of increasing relevance.

Studying A. p. mexicana living on Agaltepec Island in Mexico, which is isolated and predator-free (including human predators),  Ameca y Juárez and colleagues estimated predator, hunting, and habitat loss pressures from other A. p. mexicana populations in the area. They used population viability analysis (PVA) to determine the impact of different hurricane intensities on howler monkeys' quasi-extinction risk, or the probability of reaching a population size in which additional threats could cause the loss of the entire population.

The lowest degree hurricane disturbance result in a 28% quasi-extinction risk forty years later. Baseline estimates of hurricane intensity result in a 74% risk. Overall, the authors found that hurricanes have the ability to exceed the risk of human impacts. The combination of hurricanes and habitat loss have a substantially faster effect than when hurricanes and hunting are paired together.  It is the decrease in survival of adults that most affects the quasi-extinction rates, with adult males having an especially significant effect on the population.

The authors do point out that recurrent exposure to disturbances, including hurricanes, can be expected to affect the adaptability over time, thus reducing the likelihood the species will go extinct from the disturbance. Thus, the relationship between quasi-extinction risk and hurricane intensity won't be lineal in reality, as populations frequently exposed to hurricanes over time will adapt to lessen or moderate the negative effects.

As more frequent and intense hurricanes are expected in the future, studies such as this one allow us to determine how primate populations might be affected. Future modelling of hurricane intensity and frequency may incorporate other factors likely to affect population numbers, such as inbreeding.

Food for thought:
How might conservation biologists use information from this study to protect A. p. mexicana?
How might this study apply to other species of primates and other animal species in general?

Links of interest:
IUCN's page on A. palliata
How animals survive hurricanes
Why we should expect more extreme weather
Howler monkeys' call


Works cited:
Ameca y Juárez EI, Ellis EA, Rodríguez-Luna E. 2015. Quantifying the severity of hurricanes on extinction probabilities of a primate population: Insights into “Island” extirpations. American Journal of Primatology 77:786-800.

Manson RH, Jardel Pelaez E, Jimenez Espinosa M, et al. 2009. Perturbaciones y desastres naturales: impactos sobre las ecorregiones, la biodiversidad y el bienestar socioeconomico, en: Capital natural de Mexico, vol II: Estado de conservacion y tendecias de cambio. Conabio, Mexico, pp. 131-184. 

Portilla-Ochoa E, Sanchez-Herdandez AI, Hernadez-Meza D. 2006. El impacto de los huracanes en la biodiversidad del estado de Veracruz. In: Inundaciones 2005 en el estado de Veracruz. pp. 101-119. Universidad Vercruzana, Veracruz Mexico.