Global rarity of high-integrity tropical rainforests for threatened and declining terrestrial vertebrates

Abstract

Structurally intact native forests free from major human pressures are vitally important habitats for the persistence of forest biodiversity. However, the extent of such high-integrity forest habitats remaining for biodiversity is unknown. Here, we quantify the amount of high-integrity tropical rainforests, as a fraction of total forest cover, within the geographic ranges of 16,396 species of terrestrial vertebrates worldwide. We found up to 90% of the humid tropical ranges of forest-dependent vertebrates was encompassed by forest cover. Concerningly, however, merely 25% of these remaining rainforests are of high integrity. Forest-dependent species that are threatened and declining and species with small geographic ranges have disproportionately low proportions of high-integrity forest habitat left. Our work brings much needed attention to the poor quality of much of the forest estate remaining for biodiversity across the humid tropics. The targeted preservation of the world's remaining high-integrity tropical rainforests that are currently unprotected is a critical conservation priority that may help alleviate the biodiversity crisis in these hyperdiverse and irreplaceable ecosystems. Enhanced efforts worldwide to preserve tropical rainforest integrity are essential to meet the targets of the Convention on Biological Diversity's 2022 Kunming-Montreal Global Biodiversity Framework which aims to achieve near zero loss of high biodiversity importance areas (including ecosystems of high integrity) by 2030.

Keywords: forest cover; forest integrity; habitat quality; intact forests; terrestrial vertebrates.

Fig. 1.

Structural condition and integrity of tropical rainforests worldwide for forest-dependent terrestrial vertebrates. (A) Tropical rainforest structural condition (SCI) and (B) integrity (FSII) maps (10, 11). The SCI is a remotely sensed measure combining data on canopy cover, tree height, and time since disturbance to distinguish taller, older, more structurally complex closed-canopy forests from degraded forests. The FSII is a cumulative measure of structural condition and major human pressures as captured by the global human footprint (13). Tropical rainforests largely span the latitudes between 23.5° N and 23.5° S (indicated by the horizontal dashed lines on each map) but extend into subtropical latitudes in some areas. Bar plots in each panel show the proportion of humid tropical range for forest-dependent mammals, birds, reptiles, and amphibians encompassed by rainforest in high, moderate, and low structural condition and integrity. Proportions are averaged across the species in each taxonomic group; SI Appendix, Table S1 for sample sizes.

Fig. 2.

Variation in the proportion of tropical rainforest with low, moderate, and high integrity in the ranges of forest-dependent terrestrial vertebrate groups in relation to their IUCN threatened status and population trend. Points denote the proportion of forest in each forest integrity and threat or population trend category for individual species and are jittered horizontally to limit overlap. Box plots show the median, 25th, and 75th percentiles. Filled circles represent the mean proportion of forest predicted by the linear mixed models fitted to the data and the associated black error bars are the predicted 95% credible intervals (CRIs). Horizontal dotted lines show the median proportion forest cover (i.e., without considering forest integrity) calculated across all species in each panel. SI Appendix, Table S2 for model estimates.

Fig. 3.

Variation in the proportion of tropical rainforest with low, moderate, and high integrity in the ranges of forest-dependent terrestrial vertebrate groups in relation to the various IUCN Red List categories (in order of decreasing species conservation concern): Critically Endangered (CR), Endangered (EN), Vulnerable (VU), Near Threatened (NT), and Least Concern (LC). Points denote the proportion of forest in each forest integrity and threat category for individual species and are jittered horizontally to limit overlap. Box plots show the median, 25th, and 75th percentiles. Filled circles represent the mean proportion of forest predicted by the linear mixed models fitted to the data and the associated black error bars are the predicted 95% CRIs. Horizontal dotted lines show the median proportion forest cover calculated across all species in each panel. SI Appendix, Table S3 for model estimates.

Fig. 4.

Comparisons across biogeographic realms: variation in the proportion of tropical rainforest with low (L), moderate (M), and high (H) integrity in the ranges of forest-dependent terrestrial vertebrate groups in relation to their IUCN threatened status and population trend. Orange, yellow, and green areas on the maps of each realm show the distribution of low-, moderate-, and high-integrity rainforest respectively. Points denote the proportion of forest in each integrity and threat or population trend category for individual species and are jittered horizontally to limit overlap. Box plots show the median, 25th, and 75th percentiles. Filled circles represent the mean proportion of forest predicted by the linear mixed models fitted to the data and the associated black error bars are the predicted 95% CRIs. SI Appendix, Table S5 A–D for model estimates.

Fig. 5.

Relationships between forest integrity, geographic range size of forest-dependent terrestrial vertebrate groups, and their threatened status or population trend. (A) Species with larger ranges had a greater proportion of moderate- (dashed lines) and especially high-integrity forest (solid lines), and a lower proportion of low-integrity forest (dotted lines) than species with smaller ranges. (B) Larger-ranged species that are not threatened and not declining had greater proportions of high-integrity forest and lower proportions of low-integrity forest within their ranges than threatened or declining species. Points represent the proportion of forest for species as a function of range size and threat status or population trend and are jittered vertically and horizontally to reduce overlap. Circle, triangle, and square point types denote low-, moderate-, and high-integrity forest respectively. The lines show the mean relationships between the forest integrity gradient, range size, and threat or population trend predicted by the linear mixed models fitted to the data and the shaded areas of the lines are the predicted 95% CRIs. Dotted, dashed, and solid line types denote predicted relationships for low-, moderate-, and high-integrity forest respectively. SI Appendix, Tables S6 and S7 for model estimates.