9 Rats, Bats, and Birds

The Role of Non-Human Ecosystem Engineers in Pre-European Polynesian Agriculture

Seth Quintus, Jennifer Huebert, Jillian A. Swift, and Kyungsoo Yoo

Abstract

Agricultural practices modify the environment, and that modified environment is, in turn, inherited by subsequent generations. Humans are not the only animals that transform the environment, however, and humans often use ecosystems that depend on the engineering and services of other animals for maintained functionality. In this way, non-human animals also influence trajectories of agricultural change, as we demonstrate empirically using case studies from East Polynesia focused on Rapa Nui, the Marquesas, the Cook Islands, and the Gambier Islands. Through the discussion of ecosystem engineering and ecological inheritance, we show how non-human animals bring about incremental changes to the environment, which are then inherited by human populations and come to affect the ways food production and other cultural practices intersect and cascade through time.

Historically random processes can produce novel opportunities and constraints for cultural practices (Bintliff 1999). For instance, the cumulative outcomes of land use can carry significant and sometimes unintended ramifications for the behaviors of successive generations (van der Leeuw 2013). Such impacts are especially apparent in sequences of agricultural change when people create the environmental conditions and social context of food production through long-term activity of agricultural infrastructure, anthropogenic soils, and land tenure (Morrison 2006). All practices of cultivation modify the environment, even in subtle ways, and that modified environment is transmitted to future generations in that location. As the modification and inheritance of the environment create a different context for cultivation, they often lead to changes in the way production is practiced.

Understanding the scope of human modifications to the environment and their recursive influence on human societies has profoundly influenced discussions of human agency in landscape evolution (e.g., Balée 2006; Balée and Erickson 2006). However, humans are not the only animals that modify their environments, and these other animals live alongside and become entangled with humans, as noted also by Ammerman (chapter 10), Bishop (chapter 8), and Tomášková (chapter 11, all this volume). Such multi-agent landscape modification is accentuated when humans and other organisms migrate or are introduced to new regions. These impacts are especially apparent in the relatively small, isolated islands of Oceania, where human voyagers brought animals intentionally (e.g., pigs, dogs, chickens) and unintentionally (e.g., rats, land snails, earthworms) to islands already inhabited by birds, bats, lizards, and other organisms. Habitation in new environments by migrating or introduced organisms created a catalyst for changes and disruptions to organisms long in place.

We structure our discussion of non-human agency around the concepts of niche construction and ecosystem engineering. These concepts highlight the capacity of non-human animals to reshape environments in ways that impact material and energy flows (Jones et al. 1994), as well as the evolution of other species (Odling-Smee et al. 2003). Ecosystem engineers often play disproportionately larger roles than other organisms within social-ecological systems, given their involvement across a wide range of services including provisioning services (e.g., food), regulating services (e.g., decomposing wastes and maintaining water quality), cultural services (e.g., recreational and spiritual benefits), and supporting services (e.g., soil formation, photosynthesis, and nutrient cycling; MEA 2005). In this way, the activities of non-human animals can influence human agricultural development by contributing to the socioecological context of production each generation encounters. To empirically investigate the contributions of non-human actors in sequences of agricultural development, in particular those that are non-domesticated, we use four archaeological case studies from Polynesia (Rapa Nui, the Marquesas, Mangareva, and Mangaia) that illustrate ecosystem services or engineering activities provided by several key fauna: native birds and bats and the human-introduced Polynesian rat (Rattus exulans).

Niche Construction, Ecosystem Engineering, and Ecological Inheritance

Humans and other organisms impact the evolution of themselves and other species through the modification of their environment and the transmission of that modified environment, a process now referred to as niche construction (Odling-Smee 2003). This process can be described by two separate mechanisms: ecosystem engineering and ecological inheritance. Ecosystem engineering describes the actions of organisms to modify their environments in such a way that it changes the availability and distribution of resources for themselves and others. In turn, changes to resource availability and distributions alter or create new habitats and transform nutrient and energy flows, thereby altering the selective pressures within a particular ecosystem (Jones et al. 1994, 1997). The modified ecosystems and novel selective pressures are transmitted to subsequent generations in that same location, a process referred to as ecological inheritance. Within a niche construction framework, this inheritance serves as the crucial link between the engineering behavior of past organisms and the viability and development of future organisms in the same location.

Humans are thought of colloquially as the ultimate ecosystem engineers, and humans have certainly had substantial impacts on their environments (ArchaeoGlobe 2019; Boivin et al. 2016; Ellis 2015). Significant ecosystem engineering effects are also made by non-human and non-domesticated animals. This can be most easily appreciated for large animals such as African elephants and other megafauna that transport large amounts of sediment and nutrients from one location to another (Doughty et al. 2013; Haynes 2012). Yet animals do not have to be large in stature or body mass to make significant alterations to their surrounding ecosystems (Wilby 2002). Earthworms, ants, termites, and gophers all modify landscapes through the movement of soil (Lavelle et al. 2006; Johnson et al. 2005; Yoo et al. 2005), constructing landscape features such as mounds that are evident within landscapes of human activity (McKey et al. 2010; Zangerlé et al. 2016). Burrowing animals also substantially affect water and sediment fluxes, not only vertically within soil profiles (Capowiez et al. 2014) but also laterally at landscape scales (Wackett et al. 2018). These behaviors extend outside of terrestrial environments as well. Beavers are the best-documented example, as their construction activity contributes to the modification of stream stability, channel width, and the texture of stream bank sediments (Polivi and Sarneel 2018). Modification of surrounding environments by non-human animal agents thus contributes to both stasis and change in ecosystems (see Laland and Boogert 2010). Today, the recognized importance of ecosystem engineering in the creation of habitats is exemplified by the deliberate introduction or reintroduction of some animals by conservationists to increase or manage biodiversity (Burney and Burney 2016; Byers et al. 2006; Derner et al. 2009).

Ecological inheritance is unique compared to genetic and cultural inheritance because it can occur among different types of organisms. For example, an insect-modified ecosystem will be inherited by subsequent generations of humans and vice versa. In this way, animal impacts on both human populations and other organisms become more complex through time. Ecological inheritance creates the potential for enhanced feedback and cascading interactions, creating more complex causal scenarios (see Laland et al. 2011). The process is cumulative and gives rise to far-reaching influences on evolutionary pathways in which the process of ecological inheritance may result in unexpected or unforeseen consequences of ecosystem engineering, which are just as important in structuring future behaviors as those that are intended or expected. While the time depth of such processes is generally unknown, Douglas H. Erwin (2008) has suggested that some effects of ecosystem engineering and other sources of niche construction activities can be felt across geological time on the scale of hundreds of thousands or millions of years.

Anthropologists have recognized the effects of these processes on humans, and Timothy Ingold (1995) specifically grappled with the implications of niche construction in his concept of “dwelling.” He drew attention to the potential for long-term entanglements between humans and non-human animals through their mutual dwelling activities in the same location. Both contribute to the continual construction of the environment in which they dwell. Both respond to the impacts of the other, over short and long timescales. At any given time, one might be considered to be more important, but such importance is fleeting and difficult to untangle. It is within this context that we turn to the activity of agriculture and the complex contemporary entanglements recognized therein.

Animal Agents in Contemporary Agriculture and Plant Growth

While the influence of anthropogenically modified environments on the long-term evolution of production systems is recognized (e.g., Kirch 1994), the impact of non-human ecosystem engineering remains under-appreciated. This is curious given the role played by a range of organisms in modern agricultural production systems. Landesque capital forms of agricultural intensification are classically defined as investments in long-lasting geomorphic alterations to the environment that improve production or reduce labor expenditure (Blaikie and Brookfield 1987). However, more recent theorizing has included reference to enduring biotic transformations as well (Morrison 2014). These modifications are inherited and manipulated by successive generations of producers. As such, ecosystem engineering and ecological inheritance are at the center of agricultural practice, and the combination of the two underpins the concept of landesque capital (Håkansson and Widgren 2014).

Few animals figure more prominently than earthworms in the vast literature that examines the effects of non-domesticated animals on modern agricultural strategies and productivity (Edwards and Bohlen 1996). For the most part, earthworm ecosystem engineering is beneficial for agriculture given its role in water regulation, soil structural stability, and nutrient cycling (Blouin et al. 2013). Earthworm activities reduce soil compaction and increase water infiltration, processes long known to be important in food production (Capowiez et al. 2014). Earthworms are also an important component of nitrogen (N) mineralization (Marinissen and Ruiter 1993). Still, research has also shown that earthworm ecosystem engineering can have a negative effect on plant productivity. Earthworm activity is known to increase soil erosion in some locations (Blanchart et al. 2004), which, especially in the tropics, could reduce biome productivity.

Birds provide another example of connections between the ecosystem engineering and ecosystem services of modern fauna and plant growth through insect predation, seed dispersal, and nutrient inputs through guano deposition. Birds are known to reduce insect species that prey on crops and other vegetation, thereby increasing crop yields by removing those pests (Johnson et al. 2010; Whelan et al. 2008). Bats also prey on insects, with bats and birds often working in tandem in tropical agroforestry landscapes (Maas et al. 2013). The avian ecosystem service of plant seed dispersal is more directly associated with environmental net primary productivity and human food production. This is especially true in the tropics, where 30 percent to 50 percent of plant species are dispersed by vertebrates (Wenny et al. 2011:3).

Removal of birds and other frugivorous animals from ecosystems can result in a reduction or risk of reduction of trees that produce large seeds or fruits, since they require large-bodied dispersers (Hansen and Galetti 2009; Meehan et al. 2002). This is apparent on remote islands where native and introduced plants rely on a small number of dispersal or pollinating agents because of the lack of agent redundancy (Kelly et al. 2010; McConkey and Drake 2015). In their study, Sandra H. Anderson and colleagues (2011) found that mammalian predation of pollinating birds in New Zealand resulted in an 84 percent reduction in seed output and a 54 percent reduction in regeneration of a native shrub (Rhabdothamnus solandri; taurepo), though generally, the impacts of mutualism disruption are still poorly understood and poorly quantified for both native and introduced species (Wenny et al. 2011).

Finally, the foraging behavior of seabirds that focus on aquatic species can redistribute key nutrients from marine to land-based ecosystems through guano deposition, especially nitrogen (N) and phosphorus (P) (Ellis 2005:234). Wendy B. Anderson and Gary A. Polis (1999) report six-fold increases in N and P due to increased seabird guano deposition in the coastal islands of California, which, in turn, increased the growth of long-lived cactus, short-lived shrubs, and annuals. Such results can be applied to the growth of economic crops. For example, an experimental study observed that maize fertilized with seabird guano resulted in higher crop yields and ¹⁵N-enriched soils (Szpak et al. 2012). The long-term effects of changing seabird abundance will often cascade throughout entire ecosystems, impacting other organisms that have become dependent on seabirds’ ecosystem services (Sánchez-Piñero and Polis 2000; Thoresen et al. 2017).

Relationships between human and animal agents are intertwined: human behavior (e.g., engineering, plant and animal introductions) impacts animal behavior, thereby disrupting, altering, or enhancing ecosystem services—which can then provide the background for changes in human behavior (e.g., agricultural practices). Such feedback loops can be positive or negative for the other species in an ecosystem; positive examples of such relationships are the human-constructed hedges or field margins in European agricultural systems, which serve as havens for myriad organisms that interact with and provide services for crop growth—including birds that pollinate plants, disperse seeds, and prey on field pests (Marshall and Moonen 2002). From the human perspective, the creation of artificial habitats promotes more stable long-term production by attracting organisms that provide ecosystem services beneficial for plant growth. However, some anthropogenic botanical modifications can reshape ecosystem energy flows in ways that are ultimately detrimental to human populations. For instance, the propagation of economic coconuts (Cocos nucifera) reduces N and P nutrient availability in tropical environments because the trees are avoided by nesting seabirds (Young et al. 2010). Similarly, limited comparisons between palm- versus non-palm-dominated ecosystems documented 10 to 20 times more N and 10 to 18 times more P in the non-palm ecosystems (Young et al. 2017), presumably because of differential guano deposition. Based on these differences, the long-term effects of introduced palms potentially include reduced productivity of arable land due to nutrient declines.

While scholars have long recognized the modern connections between animal activities and agriculture, the complexity of these relationships and their significance for archaeological study are not well explored, especially in terms of non-domesticated animals. Given the important and often complex relationships observed today, non-domesticated, non-human animals surely influenced the developmental pathways of food production systems in the past as well (see Leppard 2018).

Oceanic Cases

Islands serve as model systems for the investigation of human-environment relationships (e.g., DiNapoli and Leppard 2018; Fitzpatrick and Erlandson 2018; Kirch 2007). Due to a degree of isolation and less complex biology and geology relative to continental systems, the effects of human-animal-environment interactions are clearer or more apparent in these settings. In fact, islands have long been regarded as useful spaces to examine niche construction activities (Kirch 1980). This is especially true of the relationship between the environment and agricultural practices, where the bounded nature of islands presents opportunities for investigations of agricultural sequences that evolved—to some extent—independent of other island groups. The impact of species introductions (including humans) is often immediately visible on islands and can provide information on how the ecosystem engineering activities of introduced species resulted in impacts on long-term ecosystem functioning. Here, we focus on four case studies that demonstrate the importance of this theme: Rapa Nui, the Marquesas Islands, Mangaia (Cook Islands), and Mangareva (Gambier Islands). All of the islands in these case studies are located in the cultural area of East Polynesia (figure 9.1), and all were settled by humans in the last 1,000 years.

Figure 9.1. Distribution of case-study islands across East Polynesia (Rapa Nui, the Marquesas, Mangareva, and Mangaia) in relation to other islands and groups of islands. Arrows denote the timing and direction of initial island settlement in years BP (before present).

Rapa Nui: Behavioral Outcomes of Rat-Influenced Forest Decline

Rapa Nui, also referred to as Easter Island, is one of the most remote islands in the Pacific and one of the last to be settled (ca. twelfth–thirteenth centuries CE; Hunt and Lipo 2008). The island is the most archaeologically investigated in the region, owing to its monolithic statues (moai) and accompanying narratives of the past that have long captivated archaeologists, ecologists, and the general public (see Diamond 2005). Substantial environmental changes occurred on Rapa Nui after human arrival (Flenley 1979; Flenley et al. 1991; Mann et al. 2008). Although Rapa Nui’s pre-human environment was dominated by trees (Mieth and Bork 2010), only small trees and shrubs existed by the time Europeans arrived in the area (e.g., González et al. 1908:101). Significant forest declines such as this can have many cascading effects: the removal of tall vegetation increases wind prevalence and speed and also changes the environmental water balance. In some world regions, deforestation correlates with increased aridity (Cook et al. 2012), in part because the removal of trees decreases plant transpiration, which can result in reduced regional rainfall and water availability (Laurance and Williamson 2001).

The causes of environmental change are more nuanced than originally thought, such that the classic narrative of human-induced environmental and societal “collapse” on Rapa Nui is unlikely based on archaeological evidence (e.g., Hunt 2007; Mulrooney 2012). Forest contraction was a protracted process instead of a rapid event, spanning four centuries from roughly 1250 to 1650 CE (Hunt and Lipo 2018). The declines were most likely the result of multiple overlapping factors that included climate change and human land-use practices (Mieth and Bork 2010), but the Polynesian rat probably also played a role. As described by Terry L. Hunt (2007), rat populations had few natural constraints, and the apparent prevalence of palm seeds provided an attractive and ready food source. In fact, there is archaeological evidence of rats preying on native Rapa Nui palm tree seeds (Jubaea sp., syn. Paschalococos disperta) (Hunt 2007). Rat impacts on vegetation are known or hypothesized for other locations at other times, including case studies from the Pacific prehistorically (Athens 2009; Athens et al. 2002) and historically (Campbell and Atkinson 2002). In addition to their effects on vegetation, the introduction of rats had further cascading effects, as rat populations prey on seabird nests (Atkinson 1985). Decreasing seabird populations, evident in the island’s archaeological record (Steadman et al. 1994), were followed by changes in nutrient cycling on the island, resulting in poorer soil fertility that necessitated alternative strategies of nutrient input (Hunt and Lipo 2009:605).

On Rapa Nui, human settlers developed a variety of cultivation techniques, including pit and walled cultivation (manavai) and lithic mulch gardens (Stevenson et al. 2002; Wozniak 1999). The latter technique is particularly extensive and forms an important component of the productive landscape (Ladefoged et al. 2013). The use of these techniques increased soil moisture retention and reduced variation in soil temperatures. These functions counteract the constraints of the Rapa Nui environment, which includes aridity caused by low precipitation and high wind speeds (Louwagie et al. 2006) made worse by deforestation. On the other hand, the inclusion of basalt in lithic gardens likely increased soil nutrient availability through weathering (Vitousek et al. 2014), and soil fertility was perhaps improved by human-mediated inputs of seabird guano (Commendador et al. 2013; Jarman et al. 2017:358).

Rapa Nui was always a challenging environment for cultivation, even before deforestation. Because of this, Terry L. Hunt and Carl P. Lipo (2009:606) argued that deforestation was not the only factor influencing the development of agricultural infrastructure because people likely practiced some moisture-saving techniques. Still, there are temporal correlations between the expansion of lithic mulch gardens and the sequence of deforestation for the island that hint at a causal relationship. The earliest dated examples of lithic mulch gardens on Rapa Nui were constructed in the fourteenth and fifteenth centuries (Bork et al. 2004; Stevenson 1997), with the most intensive use evident in the sixteenth to eighteenth centuries (Ladefoged et al. 2013; Stevenson et al. 2006). Based on this chronological sequence, Hans-Rudolf Bork and colleagues (2004:12) explicitly connected the development of lithic gardens to ecological change on Rapa Nui (see also Hunt 2007:498); and Christopher M. Stevenson and coauthors (2006) hypothesized a sequence that began with forest decline, was followed by open field cultivation, and was then replaced with the use of lithic mulches as aridity increased with forest contraction. The addition of nutrients through lithic mulches (Vitousek et al. 2014) may even have offset declines in nutrient inputs caused by seabird reductions through time.

While a lot remains unknown, at least some of the conditions that led to the development, use, expansion, and persistence of lithic gardens are documented for Rapa Nui. They include the impacts of static environmental conditions (Stevenson et al. 2015) and previous land-use decisions inherited by later generations of agricultural communities (Stevenson et al. 2006). The archaeological evidence from Rapa Nui (Hunt 2007; Hunt and Lipo 2009) and other Polynesian islands (Athens 2009; Athens et al. 2002) suggests that rats played a partial role in forest contraction by restricting the reproduction of vegetation, likely through predation on both palm seeds and seabird nests. As such, agricultural pathways were partially influenced by the ecosystem engineering capabilities of introduced rats. Important in this equation is the fact that the ecological impacts of rats accumulated through time and that the effects inherited by subsequent human population were exacerbated by other factors, including climatic conditions and human deforestation. Inheritance is the key mechanism by which rat impacts and their entanglement with broader ecological factors accumulated to influence change in human cultural practices. The sum of these modified ecosystems called for a response by human populations that led to the innovation and expansion of specific agricultural strategies, such as enclosures and mulching.

Marquesas Islands: What Occurs When Seed Dispersal Becomes Restricted?

The Marquesas Islands, initially settled ca. eleventh–twelfth centuries CE (Allen 2014), form part of the eastern boundary of Polynesia. These islands are rugged and known for their food production systems centered on the cultivation of introduced tree crops, particularly breadfruit (e.g., Crook 2007:73–74; Forster 1777:27; Krusenstern 1813:124–125). Archaeological research over the past twenty years illustrates the temporal sequence of forest transformations that resulted in these intensive arboricultural landscapes, particularly on the island of Nuku Hiva. The indigenous Marquesan lowland forests were irrevocably altered within two to three centuries of Pacific Islander arrival, and there is evidence that the productive tree-cropping systems observed at Western contact were in place by 1650 CE (Huebert and Allen 2016). While the creation of agroforest landscapes generated numerous anthropogenic environmental transitions, the crucial role played by birds in Marquesan vegetation change has also been recognized (Huebert 2014:265–267, 2015; Huebert and Allen 2020).

The development of an agronomic system focused on arboriculture helped maintain the forest structure compared to more open field cultivation systems created in other regions of the Pacific and, in turn, continued to provide a habitat for native fauna, including land birds. Even so, there are indications of drastic reductions in some avifaunal species due to hunting, particularly frugivorous native land birds such as pigeons and doves (Steadman and Rolett 1996). Once lost, these birds do not recover because recruitment from other islands is not feasible (Steadman 1995). These reductions would have inevitably led to further changes in forest composition. For example, Jennifer M. Huebert (2015) has found that arboreal members of the Sapotaceae vegetation family were extirpated early in the Marquesan cultural sequence. Genera of this family provide food for frugivorous birds in Polynesia (McConkey and Drake 2002:385) as well as in other regions (Snow 1981), and the fruits might have once made important contributions to the diets of larger Marquesan land birds. These plants might also have relied on birds as dispersal agents with few alternatives, with the declines in both organisms reflecting the dissolution of an important interconnected relationship.

The removal or persistence of avian ecosystem services had several implications. First, the reduction of native land birds may have decreased the spread of tree crops introduced by human populations, especially those with fleshy fruits. The importance of birds and bats in seed dispersals in modern tropical agroforestry systems (Maas et al. 2013) hints at this possibility. For example, Cordyline fruticosa (tī plants) and Morinda citrifolia (noni) are present in birds’ diets elsewhere in Polynesia (McConkey et al. 2004:371; Steadman and Freifeld 1999). Bats, too, are known to act as dispersal agents and to play a role in the reproduction of economic trees such as Tahitian chestnut (Inocarpus fagifer) and Pandanus (McConkey and Drake 2002). In consideration of these activities, the retention of avian ecosystem services may have facilitated a more rapid expansion of human-introduced economic vegetation.

A follow-on effect is that the elimination of native vegetation communities, which may have been the outcome of the disruption of dispersal agents (e.g., land birds), may have facilitated more rapid establishment of novel forests, especially those propagated by humans. The opportunities provided by vegetation extirpations, such as the reduction and elimination of certain species of shrubs and trees, likely factored into forest transformation in the Marquesas. The loss of land birds may have also impacted soil nutrient cycling and hence the fertility of some spaces. Land birds, if they consume the fruits of land plants—which, in turn, take up nutrients from deep subsoil—contribute to moving deep subsurface soil nutrients to the surface soil. That mechanism is lost when those land birds become extinct, and this may have led to declines in soil nutrients. Finally, the observed economic importance of Sapotaceae species as fuel wood factored into its decline—a situation compounded by the loss of important dispersal agents. Over time, these declines necessitated the use of other plants as a source of fuel. The loss of land birds’ ecosystem services, therefore, would have influenced the economic behaviors of human populations, which, in turn, had other long-term effects on the structure of forests. These processes are not unique to the Marquesas, as bird extinctions are well documented throughout the Pacific (Steadman 2006), and we expect that the reduction or retention of bat- and bird-associated ecosystem services played a role in the formation and continued maintenance of novel forests across the region.

A related concern regarding plant-animal interactions in this region is the impact of Polynesian rat predation on arboreal fruits and seeds, which, as previously discussed, has been attributed to the suppression and possibly even the extirpation of some plant species (rats can act as dispersers as well, but much more is known about their role as predators). Today, trees such as sandalwood (Santalum spp.) and Nesoluma nadeaudii (the latter a Sapotaceae) are severely impacted in French Polynesia by several types of rats; although researchers have concluded that they are probably not the main driver of modern-day extinctions, one study documents rats consuming up to 99 percent of sandalwood fruits (Meyer and Butaud 2009).

Mangaia and Mangareva: What Occurs When Bird Ecosystem Services Are Restricted?

Mangaia (Cook Islands) and the “almost-atoll” of Mangareva (Gambier Islands) are geologically old islands in Central East Polynesia, roughly 69 million years and 5 million to 6 million years in age, respectively (Kirch 2017; Kirch et al. 2010). Humans settled there in the eleventh century CE (Kirch et al. 2010; Niespolo et al. 2019). Extensive archaeological research conducted in both locations since the 1990s provides a substantial dataset from which to understand human-environment interactions. Each location features a complex local ecological history, but important similarities exist. Most notably, each region has a record of bird extinctions (including seabirds) and extensive vegetation change following settlement (Kirch 2007; Kirch et al. 1995). For each case, researchers have argued that prior to human settlement, birds provided a key ecosystem service by maintaining native vegetation through deposition of guano and associated nutrients (Conte and Kirch 2004:154; Kirch 2007:94; Kirch et al. 2015:36; Swift et al. 2017).

For the land mass of Mangareva, Jillian A. Swift and colleagues (2017) applied stable carbon (C) and nitrogen (N) isotope analysis of archaeological rat bone as a proxy for nutrient flows through anthropogenic food webs. Their results demonstrate a decline through time in δ¹⁵N values across three of Mangareva’s islands. They associated these declines with reductions in seabird populations, caused by a combination of human and rat predation as well as contraction of native forest habitat. Elimination of seabirds from Mangareva would have had a twofold impact on rat δ¹⁵N: directly, through the removal of seabirds from the Pacific rat diet, as well as indirectly from the removal of ¹⁵N-enriched seabird guano, which would lower baseline δ¹⁵N values across the islands. While not detectable through rat bone isotope studies, Patrick V. Kirch (2007:94) similarly suggested that seabirds were essential for the maintenance of terrestrial ecosystems through inputs of P, a situation that exists elsewhere (see also Anderson and Polis 1999). While reductions in seabird populations generally are associated with human and rat predation, Kirch and colleagues (2015:39) also suggest that the removal of bird nesting habitat through the expansion of economic crop plants may have also contributed to seabird declines. Seabird reductions and nutrient losses are also evident for Mangaia, although they are not quantified. The record of seabird exploitation does, however, continue through much of the cultural sequence, and there was never a complete loss of seabirds on the island (Kirch 2017). Even though birds continued to be present, their reduction in the later temporal periods is thought to have reduced the net productivity of the environment (Kirch 2007).

The nutrient cycling transformations and loss of P and N through the decline in seabirds would have been particularly catastrophic for the geologically old and nutrient-leached islands of Mangaia and Mangareva. In combination with initial human-induced forest decline, the use of shifting cultivation, erosion, and reductions in avian-generated nutrient inputs would have resulted in lower agricultural productivity and limited capacity for forest regrowth (Kirch 2007:94; Swift et al. 2017). The most notable impact in each case was the loss of the viable productive landscape of the interior hillslopes because of the reduction of the ecosystem services of birds. It is not just the direct impacts of avian-derived nutrients that are significant, however, as such scenarios also had cascading impacts on cultural sequences.

While certainly lowering theoretical carrying capacity, landscape change did create some agricultural opportunities (see Walter and Reilly 2010:353, 367 for Mangaia). For example, the loss of soil-stabilizing tree roots on hillslopes resulted in the accumulation of sediments in valleys (Kirch 1994:282, 1996; Leppard 2018:57). In both cases, the suitability of valley ecosystems was improved for food production, especially irrigated agriculture and arboriculture (Kirch 2007:91, 95). Productive resource zones became more centralized, which resulted in differential productive capacity for different groups. Kirch (1994:276–279) and colleagues have argued that these ecological changes on Mangaia created conditions for chiefly control, resource defensibility, competition, and warfare manifesting in an interconnectedness between food production and warfare unlike anywhere else in Polynesia. Endemic warfare developed in Mangareva as well, related to the circumscribed nature of the productive environment following nutrient declines (Kirch 2007). In both locations, therefore, the reduction in seabird ecosystem services resulted in cascading effects that created contexts for circumscribed production and for political systems based on warfare and competition.

Mangaia and Mangareva provide case studies that demonstrate the wide-ranging effects of the removal of bird-associated ecosystem services. These case studies highlight the potential importance of these animals in other production systems in the region. If the removal of these birds had such an impact on nutrient dynamics and paths of agricultural change in these regions, it stands to reason that the inclusion of these birds is an important source for increased yields in environments where birds are still present (see Fukami et al. 2006).

Discussion and Conclusion

A complete understanding of any agricultural system requires a holistic evaluation of the pathways and processes of agricultural change (Morrison 2006). Often, such systems are the long-term outcome of socioecological circumstances that create novel landscapes and accumulate incremental change across generations. Human settlers encountered and created selective pressures that resulted in some agricultural techniques becoming more viable or less useful than others. These same human inhabitants counteracted pressures by actively constructing their environments, which resulted in intended and unintended consequences feeding back into a sequence of cause and effect (Quintus and Cochrane 2018). Humans are affected by the behaviors of other organisms as well, especially when those organisms modify or disrupt the environmental context around human activities. The reality that such modification should have an effect on human behavioral strategies is predicted by niche construction theory and supported by modern analysis of ecosystem servicing.

In each case analyzed here (table 9.1), the impacts of non-human agency were significant because of their cascading effects across ecosystems as well as their pronounced influence on important ecosystem services on which humans rely. Islands tend to have low species redundancies in which few species perform the same ecosystem services as others, such that even small-scale species reductions or losses can lead to dramatic consequences (McConkey and Drake 2015). In addition, disruption of plant-animal mutualisms can lead to escalating changes that compound to impact human populations. The importance of these animal impacts becomes more visible as temporal scales increase and as the webs of organism connectivity that maintain ecosystem functionality become more historically entangled. The accumulation of effects over time and their interaction with other ecological and anthropogenic changes create opportunities for incremental ecological shifts to influence the direction and rate of human behavioral strategies such as crop rosters, techniques of cultivation, and other social and ecological approaches.

Table 9.1. Summary of non-human impacts on agricultural trajectories in Polynesia

Animals	Ecosystem Service/Activity	Compounding Factors	Outcome	Case Study Highlighted
Rats	Predation of vegetation and avifauna	Low static soil fertility, human forest removal	Deforestation, changing nutrient cycle, human agricultural innovation	Rapa Nui, Marquesas, Mangaia, Mangareva
Seabirds	Loss of bird-derived nutrient deposition	Human predation, rat predation, human agroforestry	Deforestation, changing nutrient cycle, circumscription of production	Rapa Nui, Mangaia, Mangareva
Land birds	Negation or persistence of seed dispersal and loss of nutrient cycling	Human predation, rat predation, human agroforestry	Forest restructuring, manipulation of mutualisms, changing nutrient cycling	Marquesas
Bats	Negation or persistence of seed dispersal and loss of nutrient cycling	Human predation, human agroforestry	Forest restructuring, manipulation of mutualisms, changing nutrient cycling	Marquesas

Niche construction is a multi-agent process that is cumulative and emergent, although the specific consequences of niche constructing behavior on any organism are largely unpredictable. Environments extend beyond the sphere of humans, and the agency of other animals need not be seen only in their interactions with humans but also in their interactions with other animals, whether indigenous or introduced. For example, all four case studies presented here highlight the role rats played in transforming ecosystems through seabird predation (see also Leppard 2018), which is a well-documented process in contemporary times (Jones et al. 2008). These rats played a part in removing key ecosystem services through their predatory behavior on birds, which then modified processes of nutrient cycling important for ecosystem functioning (Fukami et al. 2006) and human strategies of resource procurement.

While ecosystem engineering can be difficult to quantify on archaeological timescales, recent isotopic analyses of commensal fauna present opportunities for tracing and measuring the long-term impacts of human-animal-environment interactions on island food webs and nutrient flows (Swift et al. 2018). In the Marquesan case study, birds and bats dispersed seeds in ways that affected the distribution of trees and other vegetation. Impacts on this process might be measured by examining the distribution and development of novel economic landscapes in places where those ecosystem services are still intact and comparing that to locations where they are lost. If ecosystem services such as bird-mediated seed dispersal played a critical role in these sequences, variation in final outcomes should be identifiable.

The environment, including its flora and fauna, is often viewed as a backdrop of human resource exploitation, including cultivation. However, as Brendan J. Doody and colleagues (2015:126) and Harper Dine and coauthors (chapter 7, this volume) have shown for weed species in gardens, the presence and impacts of flora and fauna often play important roles in constraining and presenting opportunities for certain behaviors among all the species participating in the system. It is the characteristics of these relationships formed among humans, plants, and non-human animals that dictate the nature of agency. The concepts of niche construction, ecosystem engineering, and ecosystem services are manifested in actions by all parts of an ecosystem. Engaged actions of farming, hunting, and resource collection on the landscape scale create human, plant, and animal entanglements, in addition to the sedentary actions of what Timothy Ingold (1995) has called “dwelling.” The resultant persistence of ecosystem functionality, on which humans and other organisms rely, is contingent on webs of relationships that are at the core of understandings of agencies (see Murdoch 1997). Recognizing these entanglements is important to appreciate how agency is materialized as well as how that materialization is embedded within and persists through landscapes to influence future generations of the same and other organisms, including but not limited to humans.

Pathways of agricultural change are the product of multiple factors that interact temporally and spatially. No single factor directs these trajectories, though they may influence strategies in part, as historical contingencies are one component of causation. An important consideration is the role non-human animals play in constructing the environments within which cultivation is conducted. As demonstrated here, various non-domestic animals both in contemporary times and in the deep past influenced pathways of agricultural change and agricultural strategies. This was largely accomplished by making the practice of one technique more likely than another through ecological change, which was subsequently inherited by future generations.

Acknowledgments. We wish to extend our gratitude to the editor of this volume, Monica Smith, as well as to two anonymous reviewers. Comments from these individuals on earlier drafts of this chapter have certainly improved the final product.

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