It is simultaneously telling and disconcerting that none of the Sustainable Development Goal targets, nor any of the Aichi Biodiversity Targets, mentions reducing human population size as a pathway to achieving their goals, even though the United Nations promotes family planning as a means to empower people and develop nations51. Our finding that the strongest predictor of environmental performance among nations in Africa is population density means that countries with the most people per unit area suffered relatively more environmental degradation on average. This result brings into question the reality of the United Nations’ Sustainability Development Goals (www.un.org/sustainabledevelopment) — particularly Goal 15 (‘Sustainably manage forests, combat desertification, halt and reverse land degradation, halt biodiversity loss’), as well as the Convention on Biological Diversity’s Aichi Biodiversity Targets (www.cbd.int/sp/targets) Strategic Goals A (‘Address the underlying causes of biodiversity loss by mainstreaming biodiversity across government and society’) and B (‘Reduce the direct pressures on biodiversity and promote sustainable use’) — without dedicated, well-funded, and large-scale family planning rolled out across the African continent. Indeed, the targets for human development are becoming increasingly connected with those for natural systems and biodiversity52, and so we concur that the “… next generation of [human development and policy] scenarios should explore alternative pathways to reach these intertwined targets, including potential synergies and trade-offs between nature conservation and other development goals”52.

Combined with the stagnation of natural fertility decline in Africa compared to other developing regions of the world10, there has therefore never before been a more important time to re-invigorate the need for long-term, culturally sensitive, and meaningful family-planning measures if many African nations are to have any hope of stemming the decline of their biodiversity. This is particularly urgent for countries such as Nigeria (~187 million inhabitants in 2016; fertility = 5.5/woman; exponential rate of increase 2000–2015 = r 2000−15 = 0.39), Democratic Republic of Congo (~80 million; fertility = 6.5/woman; r 2000−15 = 0.48), South Africa (~56 million; fertility = 2.4/woman; r 2000−15 = 0.22), Tanzania (~54 million; fertility = 5.2/woman; r 2000−15 = 0.45), Kenya (~45 million; fertility = 3.9/woman; r 2000−15 = 0.39), and Ghana (~28 million; fertility = 4.2/woman; r 2000−15 = 0.38) (see also Fig. 1).

Fertility rates particularly in sub-Saharan Africa remain high, in part due to high poverty, low education53, and high child mortality10, thus resulting in a desire for large family sizes54. In Western Africa in particular, the adoption of contraception has been slow due to pervasive attitudinal resistance55, even though there is still considerable unmet demand56,57. As such, many national governments in Africa have not prioritised family-planning programs54; yet, well-designed family planning with regionally and culturally specific approaches (e.g., traditional methods, spacing designs)56 allows people to regulate their reproduction, with well-established benefits for family welfare58, national economies58, and the environment54. For example, countries like Botswana, South Africa, and Zimbabwe benefited from early adoption of population policies and family-planning programs56. One culprit for slow or stalled implementation elsewhere is that early deaths from the HIV/AIDS epidemic — while having limited demographic impact partly because of antiretroviral availability — have nonetheless shifted emphasis away from family planning54. It is therefore undeniable that African citizens and their governments would benefit from placing greater emphasis on quality family planning, a conclusion that we have also reached with respect to Africa’s environmental integrity54.

Some past investigations of the relationships between human population size/density and measures of environmental status have been equivocal22,23,24,25,26,27,28,29,30,31,32,37 suggesting that issues of spatial and temporal scale, as well as the choice of environmental indicator, have bearing on the strength of evidence arising. At the national scale in Africa, human population density most likely reflects the current state of environmental performance because of the relative uniformity among the sample of nations regarding the timing of principal environmental change, as well as the rapid recent expansion of human populations in many countries in that region9,10. A fundamental tenet of population ecology is that per-capita resources decline as populations near carrying capacity59, so the absolute pressure on the environment is dictated more by variation in a country’s ‘carrying capacity’ than absolute population size or per capita resource use per se37. Nonetheless, population density in the African context appears to be a reasonable reflection on average of an individual country’s proximity to this moving carrying-capacity target, despite localized improvements in biodiversity following fence construction60, for example.

Previous country rankings for environmental performance37 have not incorporated indices of leakage (externalizing environmental damage via pollution trading and outsourcing environmentally intensive production processes), although it is debatable whether it would make a large difference in the African context because of the relatively lower developed state of many of its nations compared to large consumers such as China, USA, and Brazil37. However, because we included each nation’s ecological footprint in our derivation of a composite environmental performance indicator, this should at least partially account for some aspects of leakage. Another potential caveat is that our modern ‘snapshot’ of the trends driving environmental degradation among African nations is likely to vary temporally, such that older comparisons could reveal alternate patterns. However, data for the variables we used to construct our analyses are largely unavailable and/or incomparable for periods vastly older than our current dataset.

It is unsurprising that per-capita wealth (GDP) had the hypothesised effect on a country’s relative environmental performance rank, especially considering that at the global scale at least, rising GDP reduces environmental performance among nations37. That same analysis37 also found no evidence to support the environmental Kuznets curve61 — the hypothesis that a U-shaped relationship exists between environmental degradation and per-capita wealth. This hypothesis predicts that beyond a certain threshold, wealthier societies begin to reduce their environmental footprints. However, the evidence for the environmental Kuznets curve is equivocal62, depending on which metrics are measured, countries examined, and periods of development history61,63,64,65,66,67,68,69,70. Examining the bivariate plot between environmental performance rank and per-capita GDP rank (Fig. 3d) might suggest a U-shaped relationship; however, examined appropriately by partialling the effects of the other socio-economic variables using a boosted regression tree approach that can identify nonlinearities, there is no evidence of a U-shaped relationship (Supplementary Information Methods and Results Section 12, Fig. S6).

It is not clear why governance quality consistently emerges as a weak predictor of environmental performance37. This conclusion exists even after using an African-specific indicator of governance quality71, possibly because governance problems in environmental custodianship might only become clear at finer spatial scales, perhaps only at regional or protected-area levels32. Alternatively, because governance quality tends to be ubiquitously low across the African continent relative to elsewhere72, the low inter-country variation in this metric likely diminishes the power to identify a correlation with environmental performance. The weak, yet statistically supported relationship between environmental rank and wealth disparity was as predicted — increasing wealth disparity leads to better environmental performance. This relationship might seem counter-intuitive, but there is evidence that when democratic processes are restricted, a less equal income distribution generates less environmental degradation73,74. The observed relationship most likely arises because greater inequality in wealth among citizens likely engenders fewer opportunities for development of natural resources, thus hindering or at least delaying environmental damage45.

In conclusion, our results strongly support the idea that a sustainable approach to biodiversity conservation in Africa over the coming decades cannot be limited by a narrow perspective that treats different development goals of well-being and environmental custodianship as separate entities if they ignore issues of sustained human population growth52. Indeed, with the mounting pressures facing Africa’s ecological systems, continued environmental degradation will impose further negative feedbacks on human well-being, because human quality of life is fundamentally tied to the healthy functioning of ecosystems52. Of course, better education, poverty alleviation, technological advances, and participation in multilateral environmental agreements could restrict land-use change and consumption rates and patterns; however, while there are many policy levers that African nations can use to improve the future state of their environments and the societies that depend on them, limiting excessive human population growth will, on average, likely facilitate better environmental custodianship.