Trophic Dynamics in Urban Communities

Abstract

Articles Ecologists have long debated what factors control the trophic (feeding) structure and function of ecosystems. This is more than just a matter of determining "who eats whom"; ecologists have pondered whether there are fundamental rules for determining (a) how many trophic levels an ecosystem can support, (b) how much primary production is consumed by herbivores, and (c) whether resources from the bottom of the food chain, or consumers from the top, control biomass, abundance, and species diversity in food webs. These questions are not only fundamental to ecology but essential for conservation and management. For example, the loss of a top predator in a food web that is largely controlled by top-down forces may drastically alter biodiversity and ecosystem function (e.g., nutrient cycling), whereas the same loss may have little effect in a resourcecontrolled (i.e., bottom-up) food web.To answer these questions, ecologists have expended an enormous effort to understand the relative importance of predation or parasitism (and, to a lesser extent, mutualism) and competition for resources in trophic organization. Three basic models of control of trophic structure have emerged from this endeavor. The first of these, the energetic model of food webs, holds that energy supply (from the bottom of food webs), in concert with the relative efficiencies of consumers, limits the number of trophic levels and the relative biomass of each level in natural ecosystems (Lindeman 1942). The second model, commonly known as the "green world" hypothesis (Hairston et al. 1960), states that predators and parasites exert top-down control on herbivore populations. According to this model, herbivores do not generally compete with each other, and plant resources are not limiting because herbivore population densities remain low as a result of top-down control. The third model (Menge and Sutherland 1987) hypothesizes that the relative effects of predation on species diversity vary as a function of environmental stress (e.g., exposure, desiccation, extreme temperatures) and productivity. Specifically, the Menge-Sutherland model suggests that predation should be more important at low and intermediate levels of stress, because high stress limits the abundance of predators more than it limits herbivore competitors. Competition for resources should be more important at high levels of stress (and low levels of productivity). Various modifications and elaborations of these three basic models of food webs and trophic structure have proliferated in the past several decades (Oksanen et al. 1981, Power 1992.Empirical tests of the food web models, and modifications thereof, have been conducted mostly in non-humandominated ecosystems ranging from marine environments to freshwater lakes and streams, tundra, deserts, forests, and grasslands, each test often producing a different answer (Connell 1983, Schoener 1983, Sih et al. 1985. Empirical tests and the development of theory for food web dynamics have historically involved human-dominated ecosystems...

Global urban signatures of phenotypic change in animal and plant populations

“…Urban development creates new opportunities and challenges for species competition and predation, both as exotic species are introduced and as invasive species migrate in, taking advantage of poorly integrated communities and patches. This might result in colonization, as more frequent introductions of exotic species translate into invasions (23). For example, McDonnell and Hahs (24) found higher levels of earthworm biomass and abundance in urban forests compared with rural ones, likely because of introduced species.…”

“…We used an information-theoretic approach to rank statistical models and conduct multimodel inference, based on AICc (23,55). AICc favors model fit (minimizing deviance) while avoiding model overfitting (penalizing for the number of estimated parameters, K ), and was the basis for enforcing the parsimony principle given our sample sizes (1,663 rates nested in 175 study systems).…”

Does urbanization explain differences in interactions between an insect herbivore and its natural enemies and mutualists?

“…Variation in responses within and between trophic groups may cause restructuring of communities through changes in competitive, bottom-up and top-down control effects (Van der Putten et al 2004). Any given species is affected by interactions with other species, therefore understanding how changes in species interactions potentially affect food web structure and function in urban habitats may help us to succeed when planning conservation strategies (Faeth et al 2005;Faeth et al 2011). To our knowledge, our work presents the first effort to address how interrelated multi-trophic interactions composed by herbivory, predation, parasitism and mutualism behave in urban habitats, with predation the most affected by the increase of urban features in the habitat.…”

“…There are also some dramatic physical changes from increased densities of roads, buildings and other sealed structures and microclimatic changes such as the urban heat island effect (Bradley and Altizer 2007;Faeth et al 2011). Together, these changes affect the likelihood of encountering species at higher trophic levels (Faeth et al 2005;Egerer et al 2017). Understanding how such extreme anthropogenic habitat changes may affect patterns of ecological interactions is perhaps most tractable with arthropod model systems (McIntyre 2000;Bang and Faeth 2011), but experimental studies in urban ecosystems are scarce.…”

Past and projected rural land conversion in the US at state, regional, and national levels

“…During this time, approximately 680,000 ha of rural land were converted to developed uses annually (USDA NRCS, 2007). Net loss and fragmentation of rural lands have many potential implications for the goods, services, and functions of natural resources provided by such landscapes (e.g., Alberti et al, 2003;Arnold and Gibbons, 1996;Collins et al, 2000), the species that use them and their habitat (e.g., Faeth et al, 2005;McKinney, 2002;Riley et al, 2003), and the ability of invasive species to establish themselves (e.g., Holway, 2005;Lambropoulos et al, 1999;Yates et al, 2004). Past research also has suggested that the addition of homes and other structures into rural landscapes can increase the probability of wildland fire and complicate fire management efforts (e.g., Berry and Hesseln, 2004;Cardille et al, 2001;Gebert et al, 2007;Sturtevant and Cleland, 2007), can have deleterious effects on water quality (Atasoy et al, 2006;Pijanowski et al, 2002b;Tang et al, 2005), and possibly can reduce the propensity for forest management and timber harvest Munn et al, 2002;Wear et al, 1999).…”