Habitat loss and increases in habitat isolation are causing animal population reductions and extirpations in forested areas of the world. This problem extends to protected areas, which, while often well-conserved, can be too small and isolated to maintain species that exist at low densities and require large contiguous areas of habitat (e.g. some large mammals). Costa Rica has been at the forefront of tropical forest conservation and a large proportion of the country’s land area is currently under some form of protection. One such area is the northwest portion of Costa Rica, which is an extremely biodiverse region with several noteworthy national and privately-owned protected areas. However, each protected area is an isolated island in a sea of deforestation. Within Costa Rica’s existing framework of biological corridors, we propose four sub-corridors as targets for restoration and full protection. These sub-corridors would link five major protected areas in northwest Costa Rica, with all of them linking to larger protected areas in the central portion of the country, while impacting a small number of people who reside within the corridors. After natural or active reforestation of the corridors, the result would be a contiguous protected area of 348,000 ha. The proposed sub-corridors would represent a 3.7% increase in protected area size in the region and only 0.2% of Costa Rica’s total land area. Using the jaguar (Panthera onca) as a model umbrella species, we estimated that each current isolated protected area could support between 8–104 individuals. Assuming lack of dispersal between protected areas (distance between each ranges from 8.1 to 24.9 km), these population sizes are unlikely to be viable in the long term. However, the combined protected areas, connected by biological sub-corridors, could support about 250 jaguars, a population size with a higher probability of surviving. Our study shows that focusing conservation efforts on a relatively small area of Costa Rica could create a large protected area derived from numerous small isolated preserves.

biological corridors, conservation, Costa Rica, Panthera onca, protected areas

Biological corridors are important conservation tools for maintaining species diversity in places where habitat loss results in small isolated areas of natural landscapes (Taylor et al. 1993, Ewers and Didham 2006, Hilty 2012, Hilty et al. 2012) and may be vitally important for long-term survival of species under the ongoing human-induced climate change (Heller and Zavaleta 2009, Fung et al. 2017). Corridors can help facilitate dispersal (defined as movement of individuals with potential consequences for gene flow across space, Ronce 2007) and subsequently reduce extinction probabilities of some species, especially those that are unlikely to travel across large open spaces (e.g. primates, large terrestrial mammals, Weber and Rabinowitz 1996, Hilty et al. 2006, Crooks and Sanjayan 2006). While not a perfect replacement for large blocks of natural habitat, corridors often allow a number of small populations to effectively function as one large, viable population. Examples of effective corridors include those connecting a network of parks in Bhutan (Wangchuk 2007), the Terai Arc Landscape connecting 14 different protected areas in India and Nepal (Wikramanayake et al. 2004, Harihar and Pandav 2012), the Lower Rio Grande Valley National Wildlife Refuge in south-western United States (US Fish and Wildlife Service 2001) and the European Green Belt, which runs from the Finnish-Russian border all the way through the Balkans (Terry et al. 2006). Biological corridors appear to increase connectivity for a range of taxa, including vertebrates, arthropods and plants (Rosenberg et al. 1997, Bennett 1999, Haddad et al. 2003, Chetkiewicz et al. 2006).

Tropical rainforests continue to undergo rapid deforestation and fragmentation in many parts of the world (Hansen et al. 2010, Achard et al. 2014). However, some countries have slowed deforestation and have even begun to show net positive rates of forest cover (Meyfroidt and Lambin 2011, Keenan et al. 2015). Where deforestation has been largely halted and conservation efforts are strong, the opportunity exists to connect remaining blocks of isolated forest. For example, Costa Rica suffered extremely high rates of deforestation in the latter half of the 20^th century, but by the year 2000, reforestation rates were higher than deforestation rates (Keenan et al. 2015). Although the exact amount of Costa Rica’s forest cover is debatable, the country has been slowly gaining forest cover (to about 50%, Chacón-Cascante et al. 2012) since the turn of the century (Algeet-Abarquero et al. 2015), indicating that Costa Rica is going through the “forest transition,” similar to what has been seen in some countries with highly developed economies (Mather and Needle 1998, Spray and Moran 2006, Jadin et al. 2016). Considering that 27% of Costa Rica is formally protected, much of the country’s forest cover exists outside protected areas, forests that are likely necessary to conserve wide-ranging animals. Despite a strong and largely successful track record of forest conservation, many of Costa Rica’s protected areas remain small islands of habitat, surrounded by deforested or highly fragmented lands (Sánchez-Azofeifa et al. 2003). These protected areas may lack sufficient area or connectivity to support some species, particularly those with low population densities and/or large home range requirements.

The Mesoamerican Biological Corridor proposal would connect a series of large existing protected areas via biological corridors, allowing connectivity from southern Mexico to Panama (Miller et al. 2001, Independent Evaluation Group 2011, Holland 2012). In part to meet the goals of the Mesoamerican Biological Corridor, Costa Rica’s national system of conservation areas (Sistema Nacional de Areas de Conservación, SINAC 2018a) has established a system of biological corridors designed to connect protected areas throughout the country (National Program of Biological Corridors of Costa Rica, SINAC 2006, Fig. 1). However, the corridors established by SINAC are typically quite large (Chacón Cascante et al. 2012) and often include areas with large human populations (e.g. portions of the Paso de las Nubes have > 250 people/km², CIESIN 2016), extensive deforestation (greater than 50% in some corridors, Sánchez-Azofeifa et al. 2003, Morse et al. 2009, Fagan et al. 2016) and extensive economic activity, in particular agriculture (Fagan et al. 2013). These factors make true functional connectivity difficult or unlikely (i.e. connectivity that allows the successful dispersal of organisms between protected areas that increases the probability of species persistence, Rosenberg et al. 1997, Hess and Fischer 2001). Indeed, the Mesoamerican Corridor project has faced numerous political, social and ecological challenges since its inception (Vega 1994, Kaiser 2001, Miller et al. 2001, Grandia 2007, Mendoza et al. 2013). While Costa Rican efforts, directed at encouraging conservation in the large corridors, such as voluntary reforestation and ecosystem services payments, are likely to have environmental benefits, without coordinated efforts to reduce isolation of forest remnants, these activities are unlikely to produce truly contiguous areas of habitat outside of current protected areas. There have also been efforts (both ongoing projects and proposals) to create smaller biological corridors in Costa Rica that will likely provide direct connectivity between currently protected areas. Examples of these smaller corridors include: Maquenque National Park, which, when fully protected and reforested, would connect Costa Rican and Nicaraguan protected areas (Chassot and Monge-Arias 2012); the Barbilla-Destierro Corridor connecting the Central Volcanic Mountain Range to large protected areas in southern Costa Rica (Gamboa and Salom 2015); and the “Path of the Tapir” connecting protected areas in the south-western part of the country (Newcomer 2002). These corridors would be a fraction of the width of SINAC biological corridors, but would be a contiguous habitat between existing protected areas.

Figure 1.

Proposed biological sub-corridors within the protected land and Costa Rican government designated biological corridors in north-western Costa Rica. A Guanacaste – Miravalles B Miravalles – Tenorio C Tenorio – Monteverde-Arenal Bioregion D Monteverde-Arenal Bioregion – Central Volcanic Mountain Range, 1 = Arenal – Tenorio Biological Corridor (SINAC-designated), 2 = Paso Las Nubes Biological Corridor (SINAC-designated).

The north-western part of Costa Rica includes several noteworthy state- and privately-owned biological preserves such as Santa Rosa NP, Guanacaste NP, Rincón de la Vieja NP, Arenal NP, Monteverde Cloud Forest Reserve and the Children’s Eternal Rainforest. The area is characterised by low elevation terrain punctuated by a series of young volcanic ranges. Habitat diversity is extremely high and includes tropical dry forests, pre-montane forests, cloud forests and tropical wet forests. Currently, protected areas are concentrated in the major volcanic features of the Guanacaste and Tilarán mountain ranges, plus a large area of tropical dry forest (Santa Rosa and Guanacaste National Parks) that stretches to the Pacific Ocean. With the exception of connectivity in the Guanacaste region, where three large national parks are connected either directly (Santa Rosa and Guanacaste) or by a protected corridor (Guanacaste and Rincón de la Vieja), protected areas are relatively small and isolated. For example, the Monteverde-Arenal Bioregion (MAB) is described as connected to its nearest neighbours, Tenorio National Park to the west and Juan Castro Blanco National park to the east, via SINAC’s Arenal-Tenorio and Paso Las Nubes Biological Corridors, respectively. On the ground, however, functional connectivity (in terms of ability of forest-dependent organisms to move through these corridors) is less than ideal since the two biological corridors in question are composed mostly of fragments (see results) of forest with little or no formal protection. Recent monitoring of fragmented lands surrounding the MAB indicates that many large mammals found in the parks and preserves are not utilising their habitat outside the protected areas (Zamzow et al. 2018). Therefore, the MAB – which includes Arenal Volcano National Park, the Children’s Eternal Rainforest, Monteverde Cloud Forest Biological Reserve, Santa Elena Cloud Forest Reserve and Alberto Manuel Brenes Biological Reserve – remains relatively isolated. This area has extremely high species diversity and endemism and supports a number of endangered species, including jaguar (Panthera onca), spider monkey (Ateles geoffroyi) and Baird’s tapir (Tapirus bairdii). Two of the local communities surrounding the MAB, La Fortuna and Monteverde, are major ecotourism destinations (Koens et al. 2009) that generate millions of dollars of tourism revenue and support numerous ecotourism and related jobs (Aylward et al. 1996, Langholz et al. 2000, Stuckey et al. 2014).

The distance between the isolated protected areas in the north-western part of Costa Rica is relatively small (minimum: 8km, maximum: 25 km) and there are areas of natural forest cover (variety of primary and secondary growth, M. Moran and L. Stallcup, personal observation) between these protected areas. Human population density is also relatively low, making for a high potential for creating biological corridors with low impact on human communities.

The goal of this study was to determine if we could identify potential sub-corridors within the larger SINCAC biological corridors that would have parameters that allow for the movement of forest-dependent species. These sub-corridors would meet the minimum size requirement (defined here as having a minimum width of 1000 m, Lees and Peres 2008) to achieve ecological connectivity for organisms unlikely to disperse across deforested or fragmented landscapes, while also having minimum impact on local human populations. We sought to find areas with high levels of current closed-canopy forest cover, as these areas likely retain a considerable amount of biodiversity and it would be relatively cost-effective to achieve complete forest cover through natural regeneration. We assumed that protection of currently forested lands would have less effect on economic activity in the region, since these lands are not currently being utilised for intensive agriculture. By proposing the smallest effective biological sub-corridors and minimising negative impact on current human populations and economic activity, we wanted to increase functional biological connectivity, conservation of natural resources, ecosystem services and benefits to the Costa Rican people. While interested in achieving connectivity of protected areas for the benefit of biodiversity in general, we modelled the current and potential population sizes of the jaguar as an umbrella species following Thornton et al. (2016) to determine if our proposed sub-corridors would be potentially effective in supporting a viable population of this sensitive species. The jaguar is a focal conservation species in Costa Rica (SINAC 2018b) and is declining in parts of the country (Salom-Pérez et al. 2007). It is often considered a valuable proxy for conserving biodiversity (Thornton et al. 2016), in that habitats capable of supporting this top predator species are likely to be able to support many other less sensitive species. In Costa Rica, the jaguar currently exists in several important “jaguar conservation units” that are poorly connected to each other with subsequent limited dispersal opportunities (jaguar conservation unit 154 in our study region, Sanderson et al. 2002).

We downloaded boundaries of legally protected terrestrial lands (UNEP-WCMC 2018) and the biological corridor boundaries regulated by the SINAC (Oficina Nacional Forestal 2006). We used the Global Forest Watch database (Hansen et al. 2013, parameters: 30 m × 30 m resolution, images from Landsat 7, GIS layer established in 2000, most recently updated in 2017) to locate forested areas between protected areas in north-western Costa Rica that could provide cover as feasible biological sub-corridors (See Appendix 1 for data sources). Our goals in defining a prospective sub-corridor were to minimise proposed size, minimise overall linear distance (km) between established protected areas, maximise current proportion of forest cover and minimise human population density. We first focused on finding land connecting existing protected areas that was relatively short in distance and maintained at least some forest cover. In all cases, we avoided any dense human population areas (i.e. cities and towns) using visual inspection of satellite data (explained below). When deciding on which route to choose, we ultimately had to make a judgement that considered distance of corridor, amount of forest cover and avoidance of densely populated areas. A limitation to the dataset is that the forest cover maps did not differentiate between native or non-native forest, nor forest quality (except for tree cover proportion).

We then obtained recent satellite views for each of the proposed sub-corridors from Google Earth Pro™. All corridor locations had satellite imagery from the year 2017. We established that the minimum width of any point along the corridor must be at least 1000 m, since this width appears to allow movement of practically all non-flying mammals in the Neotropics (Lees and Peres 2008). Each sub-corridor boundary was adjusted manually (using the Ruler tool in Google Earth) to minimise overlap with human structures and maximise areas of current forest cover (Fig. 2). Once sub-corridor boundaries were constructed, we measured (using visual determination and the Ruler tool) the amount of land that was forest, pasture, developed or other (i.e. barren land, either natural or due to human activity). Developed landscapes were classified as those covered by human-built structures (houses, roads etc.). We counted the number of human structures, other than roads, as a proxy for how many people live in the proposed sub-corridors. If there was some doubt about land cover classification, we used local ground observations (L. Stallcup, personal observations). Proportional land cover was then calculated by dividing the various land use categories by the total corridor area. We also measured the minimum length of each corridor – in other words, the minimal distance an animal would have to travel to move from one current protected area to another via the sub-corridor. Using Arc GIS™, we constructed maps of protected areas, SINAC biological corridors and proposed sub-corridors in order to show the locations relative to other geographic features.

Figure 2.

Satellite views of the four proposed biological sub-corridors (in yellow) with adjoining protected areas (in red). Dark green areas indicate the presence of forested habitat.

We then modelled the potential population size of jaguars, a commonly utilised “umbrella” species, whose presence may indicate a well-functioning ecosystem (Barua 2011, Thornton et al. 2016). The jaguar is a species that appears to have a viable population in some areas of our study (e.g. Guanacaste and Santa Rosa NPs, SINAC 2018b) but is very rare or absent in other protected areas (e.g. MAB, Zamzow et al. 2018). Therefore, if sub-corridor protection were to be initiated, this species could also be monitored in areas where it is currently absent or rare as a measure of whether our proposal was having some conservation success. Based on density estimates of jaguar from literature of published studies of the region (Appendix 2), we calculated the mean population density per 100 km² (± 2 standard errors of the mean). These values were used as the estimate of the potential carrying capacity of the forested habitat, based on the size of each current contiguous protected area. We then estimated population sizes of jaguars in all current protected areas as if these areas were already connected by the proposed sub-corridors (using area of currently protected areas, plus our proposed sub-corridor areas). In our estimates, we assumed no jaguar mortality other than natural density-dependent population regulation due to carrying capacity of the habitat.

The four proposed biological sub-corridors in northwest Costa Rica (Fig. 1) represent relatively small areas of the country (about 0.2% of total country area) and are relatively small as compared to the protected areas they will connect (3.7%, Table 1). Each proposed sub-corridor has high percentage forest cover (much of it native, M. Moran and L. Stallcup, personal observations) and low levels of development compared to surrounding landscape (Table 2, Fig. 3). Practically all lands not forested are in pasture, presumably utilised to raise cattle (M. Moran, personal observation).

Figure 3.

Details of land use for each proposed sub-corridor. A Guanacaste – Miravalles B Miravalles – Tenorio C Tenorio – Monteverde-Arenal Bioregion D Monteverde-Arenal Bioregion – Central Volcanic Mountain Range.

The sub-corridor we propose to connect the Guanacaste complex (specifically Rincón de la Vieja National Park) to Miravalles National Park, Sub-corridor A, has the most deforested lands and contains the greatest number of human-built structures (Fig. 3A). It should be noted that areas north of proposed Sub-corridor A have a slightly higher proportion of forest cover, but also contain many more human structures (resulting in a judgement call minimising human impact for this recommended route). The corridor proposed to connect Miravalles and Tenorio National Parks, Sub-corridor B, is more than two-thirds forested and contains no visible human structures (Fig. 3B). The area north of Sub-corridor B represents a much shorter distance between protected areas and would be smaller in area, but would also pass directly through the town of Rio Naranjo (population = 1000+, INEC 2015), making it a less desirable route. The corridor connecting Tenorio and the MAB, Sub-corridor C, is also roughly two-thirds forested and contains a minimal number of human structures (Fig. 3C); however, because it connects two more distant protected areas in our study, its area and minimum distance is greater than proposed sub-corridors A and B (Table 2). The other potential locations for sub-corridors in this region between the MAB and Tenorio all have less forest cover, longer distance between protected areas and a greater number of human structures. The fourth proposed corridor, Sub-corridor D, would connect the MAB to Juan Castro Blanco National Park, thereby connecting it to the greater Central Volcanic Mountain Range (Fig. 3D). This is the largest and longest sub-corridor. Sub-corridor D falls within the existing SINAC-designated Paso Las Nubes Biological Corridor (Fig. 1), which has a high proportion of forested land in the western portion. However, the eastern section is mostly deforested and passes close to several moderate-sized cities (most notably Ciudad Quesada). However, we were able to locate a route for Sub-corridor D with fewer human structures (Table 2). Regardless of the exact route for this sub-corridor, it is traversed in a north-south direction by the construction of a new highway connecting the cities of Ciudad Quesada and San Ramón, which means that wildlife passageways would need to be included to mitigate road impacts and enable Sub-corridor D to be ecologically functional (Fig. 3D).

The 23 studies that sampled jaguar density in tropical forests of Central America found an average of 7.09 jaguars/100 km² ± 1.16 (2SE, Appendix 2). Based on this mean value, each current isolated protected area in northwest Costa Rica can support less than 100 individuals (Table 3). The Central Volcanic Mountain Range protected area can support a population of just over 100 jaguars. None of these populations would be considered viable in the long-term (assuming N = 500 as minimum, Thomas 1990) unless there is some dispersal between populations (Rabinowitz and Zeller 2010). By connecting the protected areas in north-western Costa Rica to the Central Volcanic Mountain Range using our biological sub-corridors, the total estimated jaguar population would be more than 250. Assuming a conservative jaguar population estimate, all areas combined could support almost 200 individuals; at the higher range of the estimate, the area could potentially support more than 300 individuals (Table 3).

The current protected areas of northwest Costa Rica are each too small to provide long-term survival to some species, such as the jaguar. While jaguars are present in the region (SINAC 2018b), very few isolated populations are likely to survive long-term (Quigley and Crawshaw 1992). In fact, viable jaguar populations may only occur in only two of the areas of this study: the reserves of the Guanacaste Conservation Area and the Central Volcanic Mountain Range (Sanderson et al. 2002, SINAC 2018b), although our analysis indicates these are also too small to survive in the long-term (Thomas 1990). A recent camera study of the MAB found that jaguars were very rare (and probably non-viable) in that isolated (although relatively large, about 50,000 ha) conservation area (Zamzow et al. 2018). However, if the sub-corridors proposed here were established and protected, they would increase connectivity, presumably increasing the combined population size in all protected areas from the Guanacaste region to the Central Volcanic Mountain Range (jaguar conservation unit 154 in Sanderson et al. 2002). This process would thereby improve long-term viability of jaguar populations (although still with some risk of extinction) and provide additional conservation value to other species that also require large contiguous areas of habitat. Even if these small corridors facilitate limited jaguar use, evidence suggests that even an occasional individual successfully dispersing would improve survivability of the existing isolated populations (Rabinowitz and Zeller 2010). Furthermore, efforts are underway to establish biological corridors across international boundaries; one proposed corridor would connect protected areas near La Selva Biological Station in north-eastern Costa Rica to the Indio Maíz Biological Reserve in south-eastern Nicaragua (Morse et al. 2009), while another would connect the Turrialba region of Costa Rica with the large complex of protected areas in the Talamanca Mountains of Costa Rica and Panama (Cusack and Dixon 2006). If all of these corridors, including our proposed examples, were to be established and subsequently protected, species that occur at low densities, require large territories and which need abundant prey could conceivably roam across almost all of Costa Rica’s protected areas (Redondo-Brenes et al. 2010). The proposed corridors would directly connect multiple Central American jaguar conservation units that currently have poor dispersal frequency (Sanderson et al. 2002, Rabinowitz and Zeller 2010). Therefore, species like the jaguar would have a much better chance of survival within Costa Rica, simultaneously ensuring the country’s critical role within the Mesoamerican Biological Corridor initiative. Regardless, the chance of survival for sensitive species could be vastly improved in the three-country region (Costa Rica, Panama and Nicaragua) with the addition of the proposed functional biological sub-corridors, supporting the conservation of other key species, as well as ecosystem services in the process.

Thanks to B. Hill who assisted with Arc GIS procedures. D. Hamilton, Y. Rodríguez Santamaría and H. Villalobos Sánchez provided valuable feedback with respect to Costa Rica’s current system of biological corridors. C. Chetkiewicz and J. Jaeger provided valuable comments on an earlier version of the manuscript.

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