Introduction

Habitat fragmentation poses serious threats to many species and to global biodiversity1,2,3, including reductions in population viability4,5,6. Animal species absent or rare in small habitat patches are called ‘area-sensitive’ species7,8,9. Researchers have documented the ‘area-sensitive’ species in almost all taxonomic groups of wildlife, including insects10,11, fish12, reptiles and amphibians13, mammals5,14 and birds15,16.

The most effective way to conserve area-sensitive species is to maintain habitat patches large enough for the persistence of local populations. Therefore, estimating habitat patch area requirements17,18 is an essential component of conservation plans. The Minimum Area Requirements (MAR) of a species is defined as the amount of space (suitable habitat) that is required for the long-term persistence of a population19. Recently, an increasing number of studies have focused on MAR for protecting animals and highlighted the importance of this measure for conservation decisions12,20,21,22. Despite the conservation significance of MAR, empirical studies estimating it remain limited19.

The giant panda (Ailuropoda melanoleuca) is regarded as one of the most imperiled mammals in the world23. Populations of giant panda originally extended throughout most of southern and eastern China, northern Myanmar, and northern Vietnam24. Giant panda habitat is dramatically degrading due to increasing human activities and natural catastrophes25,26,27,28. Current remnant populations are restricted to six separate regions scattered throughout rugged mountain ranges at the eastern edge of the Tibetan Plateau29,30. Within those regions, the habitats are highly fragmented31,32,33,34, which drives further decreases in total giant panda numbers35.

Many studies have been conducted on the habitat of giant panda, and most of them focused on habitat selection36,37,38, habitat quality assessment39,40,41 and the impacts of human activity27,42,43. However, little work has been done on the effects of habitat patch size on giant panda populations. Giant pandas are generally regarded as an area-sensitive species in previous studies44, but empirical evidence is still lacking. This study seeks to understand how giant panda population presence depends on patch size and to test whether the giant panda are area-sensitive. We use extensive empirical datasets on giant panda presence and environmental variables to evaluate the MAR for giant panda in five mountain ranges, covering more than 78.7% of giant panda habitat and supporting more than 74.4% of giant pandas remaining in the wild45. We also examine the characteristics of remnant habitat patches in these mountain ranges, which collectively comprise the most important giant panda habitat in the world.

Results

Using the giant panda presence records and environmental variables, we estimated habitat suitability index (HSI). The area under the curve (AUC) value of 0.843 indicated that the Maximum Entropy (MaxEnt) model of giant panda presence had a good discriminatory ability, while the true skill statistic (TSS) score of 0.772 indicated an excellent discriminatory ability of MaxEnt. By the cut-off value that maximized the TSS score, we identified 5569 habitat patches with a total area of 15475.53 km2. The habitat patches clustered in five mountain ranges: Minshan Mountains (MS Mountains), Qionglaishan Mountains (QLS Mountains), Daxiangling Mountains (DXL Mountains), Xiaoxiangling Mountains (XXL Mountains) and Liangshan Mountains (LS Mountains) (Table 1).

Table 1 Count and area of giant panda habitat patches in five mountain ranges.
Full size table

A logistic regression model for the response of occupancy probability to patch area showed a positive association between occupancy probability and habitat area (P < 0.001) (Table 2, Fig. 1). We tested whether the giant panda was area-sensitive by testing whether the occupancy probability of giant panda in a patch of a given size was greater than the probability driven only by passive sampling, in which large patches have a higher probability of being occupied at random than smaller patches46,47,48. We used a curve, for the response of occupancy rate to patch area only due to passive sampling, as the null response curve. Occupancy probability to patch area exceeded the null response curve after 40.4 km2 (Fig. 1), suggesting area-sensitivity of giant panda.

Table 2 Logistic regression model of occupancy probability of giant panda against habitat patch size.
Full size table
Figure 1
figure 1

Relationship between the probability of giant panda presence and habitat size.

Open circles are patches without indicators of giant panda presence and filled circles are patches where giant pandas were present. The black line is the logistic regression fit, and the grey region shows 95% confidence intervals. The red line represents the null response curve derived from pure passive sampling.

Full size image

We calculated the value of EDp, i.e. the patch area at which it is likely that pandas were present with probability of p%. Both counts and total sizes of habitat patches with size over EDp decreased with the increasing of p, but they did not change from ED80 to ED99 (Fig. 2). At the 90% effective dose (ED90, indicating the patch area at which it is 90% likely that pandas were present), the value of MAR was estimated as 114.7 km2. Ten habitat patches exceeded the estimated value of MAR, which covered 86.1% of the habitat of giant panda. Six habitat patches greater than the MAR were in the MS and QLS Mountains, including the four largest patches. Two patches whose sizes exceed the MAR threshold were located in the LS Mountains, and another two patches in the DXL Mountains just reached the MAR threshold. In contrast, the habitat in the XXL Mountains was highly fragmented, consisting of 1,326 habitat patches, none of which exceeded the MAR threshold (Fig. 3).

Figure 2
figure 2

The function EDp (1% ≤ p% ≤ 99%) against p%, shows the effects of criterion choice on the amount of habitat patches that match the criterion.

The solid line with filled circles is the patches count; the dashed line with open circles represents the change in patch area with increasing p%.

Full size image
Figure 3
figure 3

Distribution of habitat patches in the five mountain ranges.

Yellow regions show the schematic of the mountain ranges. Habitat patches greater than MAR are green and patches smaller than MAR are grey. The map is made by ArcGIS 9.1 software, http://www.arcgis.com/features.

Full size image

Discussion

Using the latest and most complete empirical data of giant panda presence and land cover, we analyzed the response of occupancy probability of giant panda against habitat patch size. The occupancy probability of giant panda was positively associated with habitat patch areas, and the changes in occupancy as a function of patch area exceeded effects due only to sampling. These results suggest that giant panda is indeed area-sensitive, implying that the size of individual habitat patches is important to giant panda conservation, as is total habitat and population sizes.

Though area-sensitive species are believed to be absent from small habitat fragments, the area-sensitivity of species are mostly identified by the relationship between the patch-specific density of individuals and patch size49,50,51,52. For sparsely distributed species, where calculated densities can be strongly influenced by small numbers of individuals, occupancy rather than density is the preferred metric for assessing area-sensitivity. Generally, low probability of occurrence for wildlife in small patches is attributed to demographic stochasticity53,54, environmental stochasticity and catastrophic events55,56, inbreeding and loss of heterozygosity57, edge effects58, food shortages in small patches15, and landscape structure59. Our study went further, comparing the occurrence-patch size curve with a null response curve, demonstrating that area-sensitivity in giant pandas is real and not just an effect of patch size sampling.

The MAR was estimated to be 114.7 km2 using a logistic regression of habitat patch size on giant panda occupancy. MAR can also be estimated from Population Viability Analyses (PVAs)19. A MAR of 156–248 km2 for giant panda was estimated from its PVA (40 individuals)60 and home range size (3.9–6.2 km2)29. The MAR value estimated from PVA and home range size is higher than that estimated from occupancy patterns and patch area. This discrepancy may arise because giant panda home ranges often overlap61,62, which will inflate estimates of required space in PVAs. MAR estimated from occupancy patterns are believed to be sensitive to transient dynamics, while MAR estimated using the PVA-based approach enable us to consider time horizon and extinction probability19. As a mechanism-driven model, the plausibility of the PVA-based approach is dependent upon the reality of the modeling assumptions and the robustness of model behavior when population and environment parameters cannot be accurately determined, both of which are, often untested. Meanwhile, occupancy data is an empirical synthetic result of all ecological factors and processes in the research area. MAR estimated from occupancy data, together with that estimated by a PVA-based approach, can produce more applicable conservation planning and policy.

Giant panda habitats are large and continuous in the MS and QLS Mountains, preserving approximately 45 and 36% of giant panda habitat in the five mountain ranges, respectively. Residential areas and farmland along roads and valleys segment the habitat into several patches, of which Patch A, B, C and D (in Fig. 3) are large enough to hold relatively viable populations. The gaps between Patch A, B and D are narrow, therefore establishing corridors could further enhance the situation of populations in those patches. It is interesting that giant panda have not been found in patch C at least since the 1980s, even though this patch exceeds the MAR threshold and has a high habitat suitability index. This absence of giant panda may occur because patch C is isolated from other habitat patches in the MS and QLS Mountains by the Minjiang River (Fig. 3), a major tributary, and by human communities along those rivers. Patch E accounted for about 85% of the total area of habitat patches in the QLS Mountains. Several habitat patches, each smaller than the MAR but still containing giant panda presence records, were distributed around Patch E. Those records may stem from ‘spillover’ of giant panda from patch E rather than separate viable populations63. Therefore, we urge connection of these small habitat patches to Patch E to enhance the persistence of giant panda in the region.

Though habitats are fragmented in the DXL and LS Mountains44, four habitat patches (Patches G-J) exceed the giant panda’s MAR. Establishing corridors to link Patches G and H with other patches in the QLS Mountains could be particularly beneficial for giant panda by creating a single patch substantially larger than the MAR. Such corridors may be possible because there is no major river or other barrier separating the patches in question. In contrast, habitat patches in the LS Mountains have been isolated from adjacent giant panda habitat in the DXL and XXL Mountains due to the Dadu River and a wide stretch of human settlements. As a result, connecting patches in the LS Mountains to those in other mountain ranges appears infeasible. Nevertheless, within the LS Mountains, the gap between Patches I and J is very narrow, suggesting it might be possible to link the patches with corridors.

Of all habitats in the five mountain ranges, those in the XXL Mountains are the most fragmented. No patch in the XXL Mountains exceeds the MAR, and the largest patch therein was a mere 81.7 km2. Unsurprisingly, the population of giant panda in the XXL Mountains is the smallest and most endangered of all. To help improve this population, giant panda have been translocated from captivity to the XXL Mountains64. However, Veitch65 suggested that revealing and repairing the original cause of population degrading is the most important prerequisite to successful translocation. Our MAR analyses imply that these translocation efforts are insufficient to guarantee the long-term survival of local populations, unless efforts are also made to reduce habitat fragmentation. Two parallel efforts would be beneficial. One effort should focus on restoring habitats to increase patch size and suitability for giant pandas. The other should focus on creating ‘corridor groups’66 to connect highly fragmented patches inside the XXL Mountains. Together these two restoration strategies would consolidate several small patches into a bigger one, thereby helping to relieve the problem of giant panda habitat fragmentation.

This study has examined only the effects of habitat patch area on giant panda occupancy. Habitat isolation is another key factor that can influence occupancy patterns in other systems5,67,68,69. However, we did not explore the effects of isolation on giant panda occupancy here because there was a strong negative correlation between patch size and distance to the nearest patch. Nevertheless, isolation should be considered in future studies of giant panda occupancy that explicitly adopt a more mechanistic perspective on giant panda dispersal.

We determined the value of MAR based on the response of giant panda occurrence probability against habitat patch size. The landscapes and forest habitats in which giant panda live have changed considerably in the five mountain ranges over the last few decades, and the impacts of these changes on populations may lag behind the physical changes themselves. For example, because wild giant pandas may live as long as 13.3 years70, a shrinking habitat patch can still be occupied by giant pandas for many years beyond when the patch size falls below the true MAR, due to extinction debt71 or just because of the stochastic nature of extinction72,73. This time lag effect introduces a downward bias on the estimated MAR because the occurrence patterns have not equilibrated to habitat changes. In contrast, though rare, landscapes where dispersal is sufficient to maintain long-term connections between nearby patches (e.g., landscapes where big river gorges, farming areas, communities and roads introduce transient but not permanent barriers to connectivity; for example, Patch C) would bias the estimated MAR upwards. Because time lags, rare dispersal events, and other factors will influence giant panda occupancy patterns over the long-term, our calculated MAR values are merely estimates of the relationship between giant panda occupancy and patch size. Nevertheless, these estimates provide helpful guides to assist in identifying priority areas for restoration efforts.

We identified habitat patches using HSI, but this does not negate the importance of areas outside the patches for the viability of giant panda. Almpanidou et al.74 suggest that conservation planning should consider the entire habitat rather than isolated patches of high quality, because less suitable areas can be indispensable connections among habitat patches. That connection is essential for the viability of populations, especially those in small patches75. Future work should analyze habitat connectivity and plan corridors to enhance the viability of giant panda populations76, particularly in southern mountain ranges where most habitat patches are smaller than the MAR. Because fragmentation of habitats may be aggravated in the future due to possible global climate change76,77,78, it is necessary to evaluate their status within that context and investigate potential solutions.

Methods

Data and data sources

All of the presence records (feces, footmark and forage traces) for giant panda in the five mountain ranges (i.e. Minshan Mountains, Qionglaishan Mountains, Daxiangling Mountains, Xiaoxiangling Mountains and Liangshan Mountains) were obtained from the Fourth National Giant Panda Survey (NGPS4) in Sichuan province. The survey was carried out between 28°10′ N and 33°48′ N, 101°50′ E and 105°28′ E, from 2011 to 2013, and covered about 330,000 km2.

Giant panda habitat patches were identified by 29 spatially explicit environmental variables and classified into two categories, namely geography and land use (Supplementary Information). Elevation data were obtained from the Digital Elevation Model (DEM) (30 m × 30 m), provided by the International Scientific & Technical Data Mirror Site, Computer Network Information Center, Chinese Academy of Sciences (http://www.gscloud.cn). Other geographic variables (slope, curvature, topography position index (TPI), aspect, solar radiation index, and latitude) were derived from the DEM. Information on land use was obtained from the Second National Forest Inventory (NFI2) and revised by the NGPS4 dataset.

Habitat patch identification

The habitat suitability index was evaluated using MaxEnt modeling79,80. This technique, which has its origins in statistical mechanics81, builds a map of a species’ likelihood distribution by estimating the probability distribution of maximum entropy82,83. The presence records of giant pandas were thinned randomly to consolidate presence records from sites separated by less than 1,125 m. The distance of 1,125 m was roughly determined by the value of the smallest home range of giant pandas (3.9 km2)29. Overall, 4,224 records of giant panda presence were consolidated to 1,421 valid presence sites.

We did not try to reduce the collinearity by eliminating strongly correlated environmental variables, because high collinearity is not a significant problem for machine learning methods if the goal is predicting presence rather than interpreting the response of presence to environmental variables84. Moreover, eliminating variables may discard critical information for prediction, even when those variables are strongly correlated with other variables. For example, the variable of slope is strongly correlated with altitude (r = 0.8469, P < 0.01) at the scale of the five mountain ranges, whereas slope is apparently indispensable for predicting giant panda presences at finer scales.

We measured model performance using the area under the receiver-operator curve (AUC)85 and the True Skill Statistic (TSS)86. A value of AUC greater than 0.90 was considered to be excellent, 0.8 to 0.9 good, 0.7 to 0.8 fair, and 0.6 to 0.7 poor87. For TSS values, greater than 0.75 was considered to be excellent, 0.4 to 0.75 good, and less than 0.4 poor88.

Using the reduced set of valid presence recodes and environmental variables, we performed a 10-fold cross-validation procedure to create MaxEnt models. The averaged AUC and TSS across the 10-fold test sets were calculated to determine the predictive power of the models. We delineated the averaged HSI into suitable and unsuitable habitat based on the threshold value that would maximize the TSS score, i.e., the sensitivity-specificity sum maximization approach89,90. There are many approaches to determining the thresholds to transform results of species distribution modeling from suitability for species occurrence to presences/absences. Liu et al.91 compared 12 methods and suggested that sensitivity-specificity sum maximization, which we adopted, was a good approach.

Those adjacent suitable habitat cells were joined together as a habitat patch following an 8-cell rule. The 8-cell rule considers all 8 adjacent cells, including the 4 orthogonal and 4 diagonal neighbors. A habitat patch is considered occupied if any presence record of giant panda (valid or not) is located in it.

Area-sensitivity identification

Identifying whether a species is area-sensitive is tricky. An occupancy rate increasing with patch size does not necessarily indicate area-sensitivity because occupancy of a species may be higher in bigger habitat patches than in smaller ones due to passive sampling. Specifically, if a patch has an area of a, and the occupancy rate of a species in that patch is q, then in a bigger patch with an area of na, the occupancy rate will be 1 − (1 − q)n in the absence of any area-related ecological effects. In contrast, a species that is truly area-sensitive should have an occupancy curve that increases faster than that of passive sampling as patch size increases. A variety of mechanisms contribute to this effect. For example, bigger patches can increase the availability of ‘core’ habitat area unaffected by edge effects or human disturbance, and thus are favorable for species that require interior habitat92. Therefore, an area-sensitive species should show a higher occupancy rate than that driven by passive sampling alone.

To test whether the occupancy rate of giant panda in patches of given size is higher than that driven by passive sampling, we compared the observed logistic occupancy curve with a null response curve. The null response curve is defined by the function , where ab (corresponding to the minimum home range size of a giant panda29, namely 3.9 km2) and a (a > ab) is the patch size, qb is the occupancy rate in a patch with size of ab. The null response curve corresponds to the probability of a habitat patch of size a being occupied by giant panda, provided that patch is ecologically and functionally equivalent to independent patches with areas of ab and shows no area-related ecological advantages over the set of small patches.

Estimating MAR

We progressed from occurrence estimates to estimates of giant panda’s area needs by leveraging dose-response curves from pharmacology93,94 in which the key measure is the effective dose (ED) that yields a particular probability. Thus, in the present study, EDpis the patch area at which it is likely that pandas were present with probability of p%. We used the ED90 as the value of MAR. The ED90 was derived from a logistic regression of probability of occurrence against patch size. The interpretation of ED90 is that 90% of patches of that size will support pandas. The ED90 is sometimes used as an estimation of the maximal effective dose in drug development95. Though we used a threshold of 90% to determine MAR, the choice of threshold may vary among conservation programs depending on specific objectives. Therefore, we plotted the function EDp (1% ≤ p% ≤ 99%) against p%, to show the effects of criterion choice on the amount of habitat patches matching the criteria.

Additional Information

How to cite this article: Qing, J. et al. The minimum area requirements (MAR) for giant panda: an empirical study. Sci. Rep. 6, 37715; doi: 10.1038/srep37715 (2016).

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.