Out with Density, In with Demography

When we hear the word “endangered”, we immediately begin to think of the mentioned species in terms of numbers. How many individuals have died? How many are left? How many more will die? How much is the population growing? But this mindset hasn’t seemed to benefit some endangered species very much; in fact, conservation efforts seem to be solely directed to species that exhibit lower numbers of individuals. In a recent study published in Public Library of Science (PLOS One), researchers have found that for the Northern Muriqui, a count of the individuals in a population of the species actually provides misleading information in regard to the species’ actual health.

The Northern Muriqui of Brazil is an excellent example of a critically endangered species that has been affected in many more ways than simply population size. The population of primates was studied for twenty-eight years in its isolated habitat in the forests of Minas Gerais, Brazil. An initial glimpse at its population growth over the years shows growth from 60 individuals in 1983 to 288 individuals in 2010 and seems to suggest that the population is successfully recovering from its near-extinction experience. Analyzing this change alone, conservation efforts might not seem necessary since the population has evidently grown in the past twenty-eight years. However, when we take a deeper look at the demographics of the Muriqui, it is clear that the population is not recovering as impressively as it seems.

Karen B. Strier (University of Wisconsin-Madison) and Anthony R. Ives (University of Wisconsin-Madison), sought to observe the underlying demographic changes resulting from the near-extinction period the Muriqui population experienced. Focusing on three specific demographic rates, Strier and Ives extensively analyzed fertility rates, mortality rates, and birth-sex ratios recorded throughout the years. Results showed that although the population continued to grow, demographic changes suggested that stresses were still affecting the population despite the apparent recovery.

The first finding was that fertility rates continued to grow throughout the years despite the stable habitat size. It would be expected that limited resources would eventually cause fertility rates to stabilize over time; however, this did not occur in the Muriqui population. The second observation was that mortality rates of prime-aged adult males also increased over time, which is interesting since an increase in mortality rates does not often correlate with an increase in population size. The last finding was that birth sex ratios continued to skew towards males, so much so that the percentage of females born dropped from 67% to 34% throughout the 28 years. This was inconsistent with the growing fertility rates and population growth, since it would mean that fewer females were present in subsequent generations to give birth. Thus, demographic changes in the Northern Muriqui portrayed a much different image of the population’s recovery success than did population growth alone.

An important issue surrounding this study is the possibility that the unusual demographic rates observed could be attributed to factors other than population stresses. For example, it is notable that throughout the years, the primates’ vertical niche had expanded, and individuals could be found on the forest ground in later years as opposed to solely in the treetops. This could have provided greater nutritional resources for females, which could have allowed fertility rates to grow. Another effect the expansion of niche could have had is the increased prime-aged male mortality, which could be due to males increased time on the ground exposed to predators compared to females, who usually stayed in treetops. Careful consideration of such possibilities must be made when determining population stresses.

Strier and Ives’s findings demonstrate the importance of detailed demographic observations that may reveal unapparent stresses in a population. A holistic view is necessary to determine whether species are in risk of endangerment and to monitor recovering species that may still be endangered in order to allocate appropriate conservation efforts to populations in need

Work Cited


Strier KB, Ives AR (2012) Unexpected Demography in the Recovery of an Endangered Primate Population. PLoS ONE 7(9): e44407. doi:10.1371/journal.pone.0044407



This entry was posted in Conservation Biology Posts, Conservation Blogs 2012-2013. Bookmark the permalink.

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