Biodiversity is a cornerstone of human health and well-being. However, while evidence of the contributions of nature to human health is rapidly building, research into how biodiversity relates to human health remains limited in important respects. In particular, a better mechanistic understanding of the range of pathways through which biodiversity can influence human health is needed. These pathways relate to both psychological and social processes as well as biophysical processes. Building on evidence from across the natural, social and health sciences, we present a conceptual framework organizing the pathways linking biodiversity to human health. Four domains of pathways-both beneficial as well as harmful-link biodiversity with human health: (i) reducing harm (e.g. provision of medicines, decreasing exposure to air and noise pollution); (ii) restoring capacities (e.g. attention restoration, stress reduction); (iii) building capacities (e.g. promoting physical activity, transcendent experiences); and (iv) causing harm (e.g. dangerous wildlife, zoonotic diseases, allergens). We discuss how to test components of the biodiversity-health framework with available analytical approaches and existing datasets. In a world with accelerating declines in biodiversity, profound land-use change, and an increase in non-communicable and zoonotic diseases globally, greater understanding of these pathways can reinforce biodiversity conservation as a strategy for the promotion of health for both people and nature. We conclude by identifying research avenues and recommendations for policy and practice to foster biodiversity-focused public health actions.

Sperm competition is thought to be responsible for the tremendous inter- and intraspecific variation in sperm number¹ and size². But why several animals produce a range of sperm types³,⁴, some of which are incapable of fertilizing the female's eggs⁴, has remained unexplained for nearly 100 years⁵. We have found that non-fertile sperm protect a male's reproductive investment by delaying female remating in the polyandrous green-veined white butterfly, Pieris napi (Pieridae).

Males of many insects eclose with their entire lifetime sperm supply and have to allocate their ejaculates at mating prudently. In polyandrous species, ejaculates of rival males overlap, creating sperm competition. Recent models suggest that males should increase their ejaculate expenditure when experiencing a high risk of sperm competition. Ejaculate expenditure is also predicted to vary in relation to sperm competition intensity. During high intensity, where several ejaculates compete for fertilization of the female's eggs, ejaculate expenditure is expected to be reduced. This is because there are diminishing returns of providing more sperm. Additionally, sperm numbers will depend on male's ability to assess female mating status. We investigate ejaculate allocation in the polyandrous small white butterfly Pieris rapae (Lepidoptera). Males have previously been found to ejaculate more sperm on their second mating when experiencing increased risk of sperm competition. Here we show that males also adjust the number of sperm ejaculated in relation to direct sperm competition. Mated males provide more sperm to females previously mated with mated males (i.e. when competing with many sperm) than to females previously mated to virgin males (competing with few sperm). Virgin males, on the other hand, do not adjust their ejaculate in relation to female mating history, but provide heavier females with more sperm. Although virgin males induce longer non–receptive periods in females than mated males, heavier females remate sooner. Virgin males may be responding to the higher risk of sperm competition by providing more sperm to heavier females. It is clear from this study that males are sensitive to factors affecting sperm competition risk, tailoring their ejaculates as predicted by recent theoretical models.

Purpose of review: Biodiversity underpins urban ecosystem functions that are essential for human health and well-being. Understanding how biodiversity relates to human health is a developing frontier for science, policy and practice. This article describes the beneficial, as well as harmful, aspects of biodiversity to human health in urban environments. Recent findings: Recent research shows that contact with biodiversity of natural environments within towns and cities can be both positive and negative to human physical, mental and social health and well-being. For example, while viruses or pollen can be seriously harmful to human health, biodiverse ecosystems can promote positive health and well-being. On balance, these influences are positive. As biodiversity is declining at an unprecedented rate, research suggests that its loss could threaten the quality of life of all humans. Summary: A key research gap is to understand—and evidence—the specific causal pathways through which biodiversity affects human health. A mechanistic understanding of pathways linking biodiversity to human health can facilitate the application of nature-based solutions in public health and influence policy. Research integration as well as cross-sector urban policy and planning development should harness opportunities to better identify linkages between biodiversity, climate and human health. Given its importance for human health, urban biodiversity conservation should be considered as public health investment.

The butterfly spermatophore is a complex structure containing both sperm and accessory substances. Most of the sperm are anucleated `apyrene' sperm, which do not fertilize the eggs. The fertilizing `eupyrene' sperm comprise only 10-15% of the total sperm number transferred to the female at mating. In addition to sperm, some butterfly species also provide the female with nutrients that are used for egg production and/or somatic maintenance. Many butterflies eclose with most resources available for reproduction already present. Consequently, resources have to be allocated to the different spermatophore components over consecutive matings. This study is the first to quantify eupyrene and apyrene sperm numbers in relation to spermatophore mass, mating history and larval diet. Spermatophore mass was significantly reduced on the male's second mating. However, both eupyrene and apyrene sperm numbers were dramatically increased, as was the proportion of eupyrene sperm. Theory predicts that sperm numbers should increase under sperm competition. On males' second matings, the probability of encountering virgin females is reduced. Males may therefore maximise fertilization success by providing these females with large numbers of sperm. Simultaneously, it may be advantageous for males to reduce their nutrient donation because of their lower paternity assurance. Larval diet affected mass of the first spermatophore but had no effect on sperm number. This suggests that males maintain sperm numbers when resources are limited, prioritising fertilization success over investment in offspring.