Giraffe Conservation Demography

Giraffe (Giraffa camelopardalis) are immensely charismatic animals that also indicate the health of African savanna ecosystems, home to the greatest ungulate diversity on Earth. Despite their popularity, giraffe have declined by an estimated 30% in just the last decade due to extensive habitat loss, habitat fragmentation, and poaching.  Giraffe are large (830-1000 kg), long-lived, browsing ruminants that eat leaves, twigs and fruits of Acacia and other species of woody vegetation. The main predator of giraffe is lions, which preferentially select giraffe. Giraffe provide an unusual opportunity to examine juvenile survival due to their year-round breeding cycle (birth flow reproduction as opposed to birth pulse). Giraffe are characterized as asynchronous breeders, but there is some evidence for a seasonal birth peak that coincides with peak protein content of new Acacia tree leaves during the dry season.

Development of effective conservation and management measures for at-risk species such as giraffe requires a thorough understanding of demography. Demography is births, deaths, and movement rates which together dictate overall population fitness with a quantity called ‘lambda’. Surprisingly little is known about the demography of giraffe and how they are responding to rapidly changing land-use patterns. To compound the problem, Giraffe Skin Disease (GSD) is a newly observed disease affecting skin on the legs of giraffes in Tanzania. Causative agent, rate of spread, and mortality effects of GSD are yet to be established, but wildlife managers are concerned about possible increased risk of predation and secondary infections.

Large, long-lived mammals such as giraffe have life-histories characterized by low productivity, delayed maturity, and relatively high adult survival probabilities. The elasticity of adult survival on population trend is substantially greater than that of equivalent relative changes in fecundity or juvenile survival. However, the survival rate of adults, particularly prime-aged females, tends to be highly stable in long-lived large mammals, while juvenile survival shows wide annual variability. Accordingly, variable juvenile survival could in practice be the factor mostly responsible for fluctuations in population abundance.

Studies of juvenile survival rate and the factors that influence variation in this important parameter can provide insights into population dynamics, life history theory, individual fitness, and the selective forces that shape evolution. Quantifying juvenile survival rate and the relative influence of the various potential selective forces is difficult. Constraints of most field studies of large mammals typically limit investigators to estimating a single juvenile survival rate for the first year of life, or from birth to recruitment in species with delayed sexual maturity.

Like most African ungulates, giraffe reside in heterogeneous landscapes that are experiencing major losses and degradation of habitat quality due to changing land uses. Especially problematic for ungulates in the Maasai country of east Africa is a recent increase in settlement farming in formerly pastoral lands. Changing land use means subpopulations of wildlife face different habitat quality conditions and anthropogenic threats that can influence birth, death, and movement rates. A subpopulation may contain many animals but have low survival and reproduction, acting as a sink that consumes surplus animals from a smaller, healthier source subpopulation. Thus, the number of animals in an area does not tell the complete story of how that subpopulation contributes to the species’ overall welfare. Targeting conservation towards the sink subpopulation with the mistaken belief that it is more important than the smaller source subpopulation, ultimately will result in the loss of both populations. Therefore, site-specific demographic data (“lambda landscape maps”) are essential for successfully conserving imperiled wildlife and for developing effective management and sustainable-use models like those that exist for temperate species such as deer and elk.

The Tarangire Ecosystem encompasses 35,000 sq. km. including a significant portion of the remaining Maasai giraffe range ( G. c. tippelskirchi). Land management in the Tarangire Ecosystem is divided among National Parks, Game Control Areas, private and village lands, and a private conservancy. Aerial wildlife surveys have been conducted only every 4–5 years because of their considerable cost. To date, our understanding of the status of giraffe populations has been based upon simple counts of animals from aerial surveys, which provide little information about spatial heterogeneity of population declines, and are not useful for estimating survival, reproductive, or movement rates because they do not identify individuals.

Giraffe Skin Disease was first recorded in November 2000 in Ruaha National Park in central Tanzania. Currently the disease has spread to all parts of Ruaha, and has been observed in Tarangire and Lake Manyara national parks in the Tarangire Ecosystem. An international effort has been initiated to understand this disease, its dynamics, and its effects on giraffe populations involving wildlife veterinarians from Tanzania, the U.S.A. and the U.K., and population biologists from Dartmouth College and Wild Nature Institute in the U.S.A. However, limited capacity in Tanzania has been a major challenge hindering successful identification of GSD pathogens, its dynamics, and mortality effects.

Project Actions:

We have developed and validated a computer-assisted photographic mark-recapture method to estimate population parameters for giraffe. As technology and research ethics have evolved, animal marking techniques have progressed towards less invasive approaches. Placing a visible mark is still the most common method, but marking an animal induces acute effects of capture and chronic effects of carrying the mark. Computer-assisted photographic mark-recapture is noninvasive and low cost, and enables large sample sizes which enhance cost-effectiveness and animal welfare.

Our software analyzes digital images of giraffe spot patterns that we acquire in the field and compares them to previously acquired images in our existing database. Mark-recapture data are the raw materials for estimating population size, survival, recruitment, and movement rates. We have an existing image database acquired in 2008 and 2009. We propose to collect an additional 3 years of photographic mark-recapture data with sampling of giraffe 4 times per year in each land-management category. These data provide us with the replication necessary to rigorously estimate temporal effects, habitat quality, disease transmission and mortality rates, and human impacts on giraffe population dynamics.

Mark-recapture data allow us to estimate giraffe demographic rates at a fine spatial scale, and create a map of giraffe fitness in this landscape. These rates are being examined among different land management units (e.g. to compare survival rates between national parks and game control areas), to compare survival and reproduction in areas with differing vegetation (from existing GIS databases). These data also allow us to track movement of individuals within and among management units to identify potential critical dispersal corridors. These analyses help us to discriminate high-quality source habitats from low-quality sink habitats. We can also determine GSD infection rates, whether GSD significantly raises mortality or decreases fecundity, and if so, which areas are most affected.

We are using CMR data to estimate quarterly survival rates of dependent juvenile, independent juvenile, and adult giraffe to investigate effects of habitat attributes on those survival probabilities. Ungulate mortality is generally attributed to predation or food limitation. We are modeling giraffe survival in each age class as a function of vegetation variables (quantity and quality), giraffe density, lion density, and human density within each site to determine the relative importance of each factor.

Vegetation quantity are measured using remotely sensed data in a GIS layer developed by the Tarangire NP GIS lab and systematic point-centered quarter methods. To quantify spatio-temporal differences in browse quality, we are measuring vegetation quality in all sites in all seasons using a portable chlorophyll meter at random stratified plots. We are measuring foliar nitrogen concentration of tree species browsed by giraffe using a SPAD 502 portable chlorophyll meter (Minolta Camera Co. Ltd., Osaka, Japan). The SPAD meter was demonstrated to be a useful tool for nondestructively assessing foliar Nitrogen status, particularly for relative comparison purposes, for several tree species.

Predation pressure is quantified according to source. Human poaching pressure is quantified by human population density from census records and village house counts. Lion predation is quantified by collaborating with the Tarangire Lion Project to obtain site- and season-specific indices of lion predation pressure. Both food and predation functions may be affected by giraffe density in a given site. We are using line transect distance sampling to estimate site- and season-specific giraffe, wildebeest, and zebra density to determine how these quantities interact with food and predation.

Our results and conclusions will be disseminated via the annual Tanzanian Wildlife Research Institute (TAWIRI) conference, peer-reviewed publications in scientific journals, and informal meetings with officials from TAWIRI, Tanzanian National Parks, and conservation NGOs such as Manyara Ranch Conservancy, African Wildlife Foundation, and Wildlife Conservation Society. These site-specific data are exactly what are needed to inform conservation decision-making by these agencies.

Project Goals:

This project is being conducted with Dr. Doug Bolger of Dartmouth College.  Derek Lee is a PhD Fellow working with Dr. Bolger studying giraffe demography.  Our project’s ultimate goal is to improve our understanding of ungulate population dynamics by adding robust estimates of survival, reproductive success, and movement for a tropical ungulate species to the existing knowledge base of temperate species. Our proximate goal is to understand giraffe population dynamics and quantify habitat quality in order to successfully conserve the species and their savanna habitat. Our lambda landscape map will identify the highest-quality habitat areas and most productive subpopulations in order to focus conservation efforts on the engines of population growth.