1 Introduction

Species endangerment and population decline are pressing global issues that threaten biodiversity and ecosystem stability. The primary drivers of these phenomena include habitat loss, climate change, pollution, and overexploitation, which are often exacerbated by human activities (Selwood et al., 2015). Habitat destruction remains the most significant ultimate cause of extinction, as it directly impacts the availability of resources and suitable living conditions for species (Hernández et al., 2013). Additionally, climate change poses a significant threat by altering habitats and affecting species' physiological tolerances and interactions (Cahill et al., 2013). The decline of species such as amphibians and tropical insectivorous birds highlights the complex interplay of proximate and ultimate factors, including environmental pollutants and evolutionary vulnerabilities (Hayes et al., 2010; Sherry, 2021). Understanding these causes is crucial for developing effective conservation strategies and mitigating the ongoing loss of biodiversity.

A comprehensive theoretical framework is essential to unravel the complex mechanisms driving species endangerment and population decline. Current research indicates that both proximate causes, such as demographic changes and species interactions, and ultimate causes, like habitat fragmentation and climate change, play critical roles in these processes (Selwood et al., 2015). However, the interactions between these factors are not fully understood, necessitating a more integrated approach to study them. For instance, while habitat loss is a well-documented ultimate cause, the proximate causes leading to the final extinction of species, such as stochastic events or reduced genetic diversity, require further investigation. Developing a robust framework will enable researchers to predict extinction risks more accurately and design targeted conservation interventions.

This study attempts to identify common patterns and research gaps in current knowledge by analyzing a wide range of research literature, in order to gain a clearer understanding of the near and end causes of species endangerment and population decline. The study will emphasize the importance of ecological and genetic factors in the risk of extinction, and emphasize the need for comprehensive conservation strategies to simultaneously address multiple threats. It is expected to provide reference for future research directions and policy decisions, promote more effective conservation actions, and maintain global biodiversity.

2 Proximate Causes of Species Endangerment

2.1 Habitat loss and fragmentation

Habitat loss and fragmentation are widely recognized as the most significant proximate causes of species endangerment. The destruction and division of natural habitats due to human activities such as agriculture, urban development, and deforestation lead to reduced living spaces for species, disrupting their ecological niches and leading to population declines. This fragmentation not only reduces the available habitat but also isolates populations, making it difficult for species to maintain genetic diversity and increasing their vulnerability to extinction (Hernández et al., 2013).

2.2 Overexploitation: hunting, fishing, and wildlife trade

Overexploitation through hunting, fishing, and wildlife trade is another critical proximate cause of species endangerment. These activities directly reduce population sizes and can lead to the depletion of species faster than they can reproduce. The unsustainable harvesting of wildlife for food, medicine, and other purposes has been a significant driver of declines in many species, particularly those with high economic value or those that are easily accessible (Kotiaho et al., 2005).

2.3 Pollution and environmental contaminants

Pollution and environmental contaminants pose significant threats to species by altering their habitats and directly affecting their health. Contaminants such as pesticides, heavy metals, and industrial chemicals can lead to physiological stress, reproductive failures, and increased mortality rates in wildlife. For instance, amphibians are particularly vulnerable to pollutants, which can disrupt their endocrine systems and increase susceptibility to diseases (Hayes et al., 2010).

2.4 Invasive species and competition with native fauna

Invasive species can outcompete native fauna for resources, leading to declines in native populations. These non-native species often have no natural predators in their new environments, allowing them to proliferate unchecked and disrupt local ecosystems. The introduction of invasive species can lead to competition for food, habitat, and other resources, often resulting in the decline or extinction of native species (Hayes et al., 2010).

2.5 Climate change-induced physiological and behavioral disruptions

Climate change is increasingly recognized as a proximate cause of species endangerment, affecting species through physiological and behavioral disruptions. Changes in temperature, precipitation patterns, and extreme weather events can alter habitats and food availability, leading to stress and reduced survival rates. Species with limited physiological tolerance to changing climates or those unable to migrate to more suitable habitats are particularly at risk (Selwood et al., 2015; Cahill et al., 2013).

3 Ultimate Causes of Population Decline

3.1 Anthropogenic pressures and habitat modification

Anthropogenic pressures, including habitat loss and fragmentation, are significant ultimate causes of population decline. Habitat destruction due to land development, agriculture, and urbanization leads to the loss of critical living spaces for many species, thereby reducing their populations and increasing extinction risk (Hernández et al., 2013). Additionally, climate change, driven by human activities, exacerbates these pressures by altering habitats and affecting species' survival and reproduction (Cahill et al., 2013; Selwood et al., 2015).

3.2 Loss of genetic diversity and reduced adaptive potential

The loss of genetic diversity is another ultimate cause of population decline, as it reduces a species' ability to adapt to changing environmental conditions. Small populations are particularly vulnerable to genetic drift, inbreeding depression, and the fixation of deleterious mutations, which can lead to reduced fitness and increased extinction risk. Hybridization with non-adapted gene pools and selective pressures from human activities further contribute to the loss of genetic variability (Kotiaho et al., 2005).

3.3 Evolutionary constraints and ecological specialization

Species with narrow ecological niches and specialized evolutionary traits are more prone to extinction. These species often have limited geographic ranges and specific habitat requirements, making them vulnerable to environmental changes and habitat loss. For example, tropical insectivorous birds are highly specialized and sensitive to habitat fragmentation and other anthropogenic changes, which threaten their survival.

3.4 Disruptions in reproductive strategies and life history traits

Disruptions in reproductive strategies and life history traits can significantly impact population viability. Factors such as reduced recruitment, altered reproductive timing, and changes in life cycle events due to environmental stressors can lead to population declines (Hayes et al., 2010). These disruptions are often exacerbated by climate change, which can affect critical windows in species' life cycles, leading to disproportionate impacts on demographic rates.

3.5 Ecosystem changes and the breakdown of ecological interactions

Ecosystem changes, including the breakdown of ecological interactions, are critical ultimate causes of population decline. Alterations in species interactions, such as predator-prey dynamics and competition for resources, can destabilize ecosystems and lead to declines in species populations (Cahill et al., 2013; Sherry, 2021). Climate change and habitat modification further disrupt these interactions, leading to cascading effects on biodiversity and ecosystem function.

4 Case Study: The Decline of Large Carnivores in Fragmented Habitats

4.1 Selection of the case: the impact of habitat fragmentation on big cats

Habitat fragmentation is a significant threat to large carnivores, particularly big cats, as it disrupts their natural habitats and leads to population declines. Fragmentation results in isolated habitat patches, which can severely impact species that require large territories for hunting and breeding. For instance, pumas in the Santa Cruz Mountains experience reduced survival rates due to habitat fragmentation, which creates source-sink dynamics detrimental to their population stability (Figure 1) (Nisi et al., 2023). Similarly, the fragmentation of habitats in agricultural landscapes has been shown to decrease species numbers and disrupt ecological interactions (Kruess and Tscharntke, 1994).

4.2 Proximate causes: human-wildlife conflict, habitat reduction, and prey depletion

The proximate causes of large carnivore decline in fragmented habitats include human-wildlife conflict, habitat reduction, and prey depletion. Human encroachment into carnivore territories often leads to direct conflicts, resulting in increased mortality rates for these animals (Farris et al., 2015; Nisi et al., 2023). Habitat reduction due to urban development and agriculture further exacerbates these conflicts by limiting the available space for carnivores to roam and hunt (Murphy et al., 2017). Additionally, prey depletion, often a consequence of habitat fragmentation, reduces the food availability for large carnivores, further threatening their survival (Ripple et al., 2014).

4.3 Ultimate causes: genetic bottlenecks and reduced reproductive success

The ultimate causes of decline in large carnivore populations include genetic bottlenecks and reduced reproductive success. Fragmented habitats lead to isolated populations, which can suffer from reduced genetic diversity and increased inbreeding, as seen in the Florida black bear subpopulation (Murphy et al., 2017). This genetic bottleneck can result in lower reproductive success and increased vulnerability to diseases and environmental changes (Lino et al., 2019). The loss of genetic diversity is a critical concern for the long-term viability of large carnivore populations in fragmented landscapes (Ramírez-Delgado et al., 2021).

4.4 Conservation efforts and management strategies for large carnivores

Conservation efforts for large carnivores in fragmented habitats focus on maintaining habitat connectivity and reducing human-wildlife conflicts. Strategies include creating wildlife corridors to connect isolated habitat patches, thereby facilitating gene flow and reducing genetic bottlenecks (Cosgrove et al., 2018). Additionally, conservation easements and government land acquisitions can help preserve critical habitats (Murphy et al., 2017). Effective management also involves community engagement to mitigate human-wildlife conflicts and promote coexistence (Gálvez et al., 2018). Restoration of habitat matrices and improving matrix quality are essential for reducing the negative impacts of fragmentation on carnivore populations (Ramírez-Delgado et al., 2021).

5 Mechanisms Linking Proximate and Ultimate Causes

5.1 Synergistic effects of multiple threats on species decline

Species decline is often driven by the synergistic effects of multiple threats, which can amplify the risk of extinction beyond the impact of individual threats. For instance, habitat destruction and overexploitation can lead to abrupt species loss, but the final descent to extinction is frequently accelerated by interacting processes such as climate change and invasive species (Hayes et al., 2010). Experimental studies have shown that the combination of environmental warming, overexploitation, and habitat fragmentation can lead to population declines up to 50 times faster than when these threats act independently (Mora et al., 2007). This highlights the importance of addressing multiple threats simultaneously to mitigate biodiversity loss effectively.

5.2 The role of ecological thresholds and population viability

Ecological thresholds play a critical role in determining population viability under changing environmental conditions. For example, the midday gerbil experienced a significant population decline due to landscape transformation, which altered the ecological balance and increased mortality rates, particularly among males (Tchabovsky et al., 2019). Similarly, rapid climate warming has been linked to declines in terrestrial bird and mammal populations, with more pronounced effects in areas experiencing faster temperature increases (Spooner et al., 2018). These examples underscore the importance of understanding ecological thresholds to predict and manage population viability in the face of environmental changes.

5.3 Long-term evolutionary impacts of rapid environmental changes

Rapid environmental changes can have profound long-term evolutionary impacts on species. The genetic and cultural evolution of unsustainability suggests that human-driven environmental changes are accelerating due to multi-level selection for niche construction and ecosystem engineering (Snyder, 2020). This evolutionary perspective highlights how rapid changes can outpace the adaptive capacity of species, leading to unsustainable ecological shifts. Additionally, the evolutionary history of tropical insectivorous birds makes them particularly vulnerable to Anthropocene activities, as their specialized feeding and poor dispersal capacities limit their ability to adapt to rapid changes (Sherry, 2021). Understanding these evolutionary impacts is crucial for developing conservation strategies that account for both immediate and long-term threats to biodiversity.

6 Conservation Strategies to Address Proximate and Ultimate Causes

6.1 Habitat protection and restoration initiatives

Habitat protection and restoration are critical strategies for conserving biodiversity and preventing species endangerment. The loss and fragmentation of habitats due to human activities such as urbanization and agriculture are major threats to species survival. Initiatives that focus on habitat remediation and managed connectivity can significantly reduce the risk of population extirpation by enhancing genetic diversity and adaptability (Lamka and Willoughby, 2023). Establishing protected areas and connectivity corridors are traditional methods that help maintain population connectivity and size, which are essential for genetic diversity.

6.2 Sustainable management of wildlife resources

Sustainable management of wildlife resources involves creating and maintaining insurance populations in breeding centers and private ranches. This approach, exemplified by the Conservation Centers for Species Survival (C2S2), focuses on producing genetically diverse source populations that can support research and reintroductions (Wildt et al., 2019). By leveraging both public and private sector resources, this strategy ensures the survival of endangered species through a combination of in situ and ex situ conservation efforts.

6.3 Genetic rescue and assisted evolution approaches

Genetic rescue and assisted evolution are innovative strategies aimed at enhancing the genetic diversity and evolutionary potential of endangered species. These approaches involve the deliberate movement of genotypes to initiate or enhance gene flow, thereby increasing species fitness and adaptability (Hoffmann et al., 2020). Genetic rescue through translocation has been shown to preserve genetic diversity and mitigate the effects of inbreeding depression, although it requires careful consideration of potential risks such as outbreeding depression (Kardos, 2021).

6.4 Climate change adaptation and mitigation strategies

Climate change poses a significant threat to biodiversity, necessitating adaptation and mitigation strategies to prevent species extinctions. Active interventions such as assisted gene flow and artificial selection are increasingly being integrated into conservation efforts to address the rapid loss of biodiversity due to climate change (Gaitán-Espitia and Hobday, 2020). Understanding the proximate causes of climate-change-related extinctions, such as changes in species interactions and food availability, is crucial for developing effective adaptation strategies (Cahill et al., 2013).

7 Future Directions in Species Conservation and Population Recovery

7.1 Integrating genomics and conservation biology

Integrating genomics into conservation biology offers a promising avenue for enhancing species conservation efforts. Genomic tools can help identify genetic diversity and population structure, which are crucial for understanding species' resilience to environmental changes and anthropogenic pressures. For instance, genetic factors such as inbreeding depression and loss of genetic variability are significant contributors to extinction risk, especially in small populations. By leveraging genomic data, conservationists can develop more effective strategies to manage genetic diversity and improve the adaptive potential of endangered species.

7.2 The role of community-based conservation approaches

Community-based conservation approaches emphasize the involvement of local communities in conservation efforts, recognizing their role as stewards of biodiversity. These approaches can be particularly effective in addressing the ultimate causes of species decline, such as habitat loss and fragmentation, by promoting sustainable land-use practices and enhancing local livelihoods (Hernández et al., 2013). Engaging communities can also help mitigate proximate threats like poaching and overexploitation, as local stakeholders are more likely to support conservation initiatives that align with their economic and cultural interests.

7.3 Predictive models for population viability and extinction risk

Developing predictive models for population viability and extinction risk is essential for proactive conservation planning. Such models can incorporate ecological characteristics, demographic rates, and environmental variables to forecast species' responses to changing conditions (Kotiaho et al., 2005; Selwood et al., 2015). For example, understanding the effects of climate change and land-use change on demographic rates can help predict which species are most vulnerable to extinction. These models can guide conservation priorities by identifying species at high risk and informing targeted management actions.

7.4 Global collaboration and policy implementation

Global collaboration and effective policy implementation are critical for addressing the multifaceted challenges of species conservation. International cooperation can facilitate the sharing of knowledge, resources, and best practices, enhancing the capacity to tackle global threats like climate change and habitat destruction (Cahill et al., 2013; Moritz and Agudo, 2013). Policies that integrate scientific insights into legislative frameworks can help ensure that conservation efforts are both effective and sustainable. For instance, aligning conservation policies with genomic research findings can improve the management of genetic resources and support species recovery efforts.

Acknowledgments

The authors extend sincere thanks to two anonymous peer reviewers for their feedback on the manuscript.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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