1.1 Two contrasting views on the relationship between resource availability and
population dynamics
The abundance or carrying capacity of animal populations is often determined by top-down forces like predation pressure or bottom-up forces like resource availability (Anne & Rudy, 1997; Berryman, 2004; Hopfenberg, 2003; Melis et al., 2009; Rutz &
Bijlsma Rob, 2006; Schluter & Repasky, 1991; Walankiewicz, 2002), both of which can be influenced by environmental conditions. The role of bottom-up forces extends the view of resource-constrained populations proposed by the economist Thomas Malthus over two centuries ago (Malthus, 1798). Not only is Malthus’ view on
resource-constrained population dynamics still widely held in ecology (Gotelli, 2008; Lomnicki, 1988; May & McLean, 2007; Molles, 2016), his view on the human struggle for
existence remains central to the theory of evolution by natural selection (Darwin, 1859).
After the industrial revolution, however, the growth of the world’s population prompted economists to reconsider the role of resources in human population dynamics (Brown, 1954; Cépède, Houtart, & Grond, 1964; Cohen, 1995). More than a half century ago, the economist Esther Boserup (Boserup, 1965) further proposed that high population density stimulated human cooperation in order to improve agricultural efficiency, thereby increasing resource supply to match the needs of a growing population. In
contrast to the views of Malthus, Boserup hypothesized that human populations can overcome resource constraints and thrive through cooperation. Whether human
populations can actually escape from resource limitation by cooperating remains a topic of great debate to this day (Decker & Reuveny, 2005; Demont, Jouve, Stessens, &
Tollens, 2007; Lipton, 1989; Richerson & Boyd, 1997; Urdal, 2005).
1.2 Intraspecific cooperation: an overlooked “lateral force” acting within
populations
As an extension of Boserup’s ideas, intraspecific cooperation can be considered to
be a “lateral force” that acts within populations and interacts with external top-down and bottom-up forces to regulate population size. Although the role of cooperation has been widely discussed in studies of human population dynamics (Ellis, Magliocca, Stevens, & Fuller, 2018; Hamilton et al., 2009), it is rarely considered in studies of population dynamics in other animals. One exception comes from studies of microbes (de Vargas Roditi, Boyle, & Xavier, 2013; Gore, Youk, & van Oudenaarden, 2009;
Sanchez & Gore, 2013) that have explored the impact of cooperation on population growth (Gore et al., 2009; Rainey & Rainey, 2003) or the interaction between cooperation and population dynamics (Sanchez & Gore, 2013). Yet, given that harsh environments are thought to favor cooperation in microbes (Bottery, Wood, &
Brockhurst, 2016; Frost et al., 2018; Yurtsev, Chao, Datta, Artemova, & Gore, 2013), as they do in other social animals (Firman, Rubenstein, Moran, Rowe, & Buzatto, 2020;
Jetz & Rubenstein, 2011; Lukas & Clutton-Brock, 2017; Rubenstein & Lovette, 2007), how environment-associated cooperation affects population dynamics remains largely unstudied in any organism.
1.3 Brief review on the relevant theoretical studies
Although initial theoretical work on cooperative behavior tended to focus on the emergence and maintenance of cooperation by studying the dynamics of cooperators and free riders in populations of fixed size (Axelrod & Hamilton, 1981; Ohtsuki, Hauert, Lieberman, & Nowak, 2006; Traulsen & Nowak, 2006; Weitz, Eksin, Paarporn, Brown, & Ratcliff, 2016), more recent studies have begun to consider populations that vary in size (Epstein, 1998; Zhang & Hui, 2011). Indeed, eco-evolutionary feedbacks between cooperative behavior and population dynamics often induce coexistence of cooperators and defectors (Hauert, Holmes, & Doebeli, 2006; Sanchez & Gore, 2013).
Yet, the role of resource availability in driving these eco-evolutionary feedbacks remains poorly known. Environmental harshness, which reduces resource availability (Allison, 2005; Wang & Goldenfeld, 2011) and increases mortality (Yurtsev et al., 2013;
Zhang & Hui, 2011), tends to favor cooperation (Smaldino, Schank, & McElreath,
2013). Yet, clarifying the interactions between resource availability (a bottom-up force) and intraspecific cooperation (a lateral force) on population dynamics remain a
challenge for theoretical biologists. Simultaneous consideration of the relationships among population dynamics, cooperation, and resource availability is necessary to more fully understand how social species and populations respond to resource constraints and other environmental challenges.
1.4 Research aims
To capture the essence of a social population and formulate the theory about the
relationships among population dynamics, cooperation, and resource availability, it is necessary to construct a model to address this subject. Individuals’ cooperative
behaviour and their interactions with each other and with other elements of their environment play an important role in impacting the properties of a social population.
Most importantly, individuals are adaptive, and adaptation, from which population-level properties emerge, occurs at level, not higher levels. Therefore, individual-based modeling is a suitable approach to explore the emergent properties at the population level in which we are interested. Here, we model how environmental
conditions and the benefits of cooperation shape the evolution of asexual and structured populations with overlapping generations. We assume that there are two types of
individuals in the population: free-riders, who do not contribute to creating group benefits, and cooperators, who invest in creating benefits that are shared by all group members at the cost of decreasing the cooperator’s own fitness. Groups interact in the population through random offspring dispersal. Population size is not externally assumed, but instead emerges from the dynamics of birth and death processes that are influenced by both environmental conditions and individual behavioral strategies (Fig.
1). We first consider environments of differing quality (harsh environments with low resource availability versus benign environments with high resource availability) to elucidate the relationship between environmental quality and the evolution of
cooperation. To tease out the ecological consequences of cooperation, we then compare the population size and the niche breadth of social and non-social populations.
Ultimately, we model population dynamics in a fluctuating environment with varying levels of resources to explore how social populations respond to environmental changes.
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