Estimating population sizes to evaluate progress in conservation of endangered golden lion tamarins (Leontopithecus rosalia)

Abstract

Efforts to reverse the decline of endangered golden lion tamarin monkeys have been relatively successful because the Brazilian organization dedicated to the species' conservation (Associação Mico-Leão-Dourado, AMLD) relies on science-based computer modeling to determine the number of tamarins necessary to achieve demographic and genetic goals, and a process of strategic planning to achieve those goals. Accurate estimates of the numbers of tamarins in forest fragments are essential to evaluate progress in achieving goals and adapt strategies as necessary. In this report we present the results of a new method to survey the number of tamarins in the wild, a modification of the point transect with lures procedure. Using this method, we estimate that in 2014 there were approximately 3,700 golden lion tamarins in 41,400 hectares of Atlantic Forest. Of these, 59% are from remnant wild populations, 34% are descendants of captive-born reintroduced animals and 7% are descendants of wild translocated groups. The number of tamarins and amount of forest estimated in this survey exceeded values necessary to meet AMLD's definition of a viable population, determined to be 2,000 tamarins in 25,000 hectares of connected and protected forest. However, the seven forest blocks and their tamarin populations are not yet adequately connected and protected. AMLD's strategic plan to achieve a viable population of golden lion tamarins includes 12 strategies that mitigate these and other threats or contribute directly to the conservation goal. The point transect with lures survey method provides a way to evaluate progress in achieving that goal and adapt strategies as appropriate.

Fig 1. Map of current geographic distribution of golden lion tamarins.

Map showing the current geographic distribution of golden lion tamarins. Colored polygons represent blocks of connected forest fragments (as we understood connectivity in 2014) which we refer to as management units with GLT populations. Dark blue is a body of water, green shows forest fragments not used by GLTs. Estimates of GLT population sizes and forest area are shown for each management unit.

Fig 2. Locations of experimental and survey playbacks in the São João river basin.

São João river basin, geographic range of most golden lion tamarins. Forest fragments (green) were identified using Landsat satellite images. Orange squares indicate randomly selected, 48 ha or 120 ha quadrats sampled in the playback survey. Observers played recorded GLT vocalizations at points 200 m apart along transects in each quadrat, noting responses by GLTs. Yellow polygons are locations of experimental playback trials to estimate the detection function. Density of tamarins in fragments bordered by red were obtained from complete counts or home range data.

Fig 3. Plot of response against distance of GLT group from the playback point for 40 trials.

The range of values along the Y-axis, 1 and 0, represent probability of detecting GLTs during the playback trial. Also shown is the estimated probability of detection as a function of distance from the playback point (black line), and the estimated probability of detection as a function of distance from the playback point conditional to the fragment size (Small fragment (< 60 ha) = red line; Large fragment (3,606 ha, 5,584 ha) = blue line).

Fig 4. Conceptual model of the 2016–2025 strategic plan for conservation of golden lion tamarins (Version 18 October 2018, translated from Portuguese).

Conceptual model showing elements of AMLD’s strategic plan to achieve a viable population of GLTs including the project scope (green rectangle) containing two conservation targets (ovals) and AMLD’s desired conservation outcome (blue rectangle), direct human-caused threats (purple rectangles), contributing factors such as indirect threats or opportunities (small brown rectangles), strategies with actions that have a common focus (yellow hexagons), and benefits to humans that result from GLT conservation (large brown rectangle). Arrows represent presumed causal relationships. The diagram was produced using Miradi project management software.