Stanley H. Faeth

Global Change and the Ecology of Cities

Urban areas are hot spots that drive environmental change at multiple scales. Material demands of production and human consumption alter land use and cover, biodiversity, and hydrosystems locally to regionally, and urban waste discharge affects local to global biogeochemical cycles and climate. For urbanites, however, global environmental changes are swamped by dramatic changes in the local environment. Urban ecology integrates natural and social sciences to study these radically altered local environments and their regional and global effects. Cities themselves present both the problems and solutions to sustainability challenges of an increasingly urbanized world.

Trophic Dynamics in Urban Communities

Abstract Human activities dramatically change the abundance, diversity, and composition of species. However, little is known about how the most intense human activity, urbanization, alters food webs and trophic structure in biological communities. Studies of the Phoenix area, situated amid the Sonoran Desert, reveal some surprising alterations in the control of trophic dynamics. Species composition is radically altered, and resource subsidies increase and stabilize productivity. Changes in productivity dampen seasonal and yearly fluctuations in species diversity, elevate abundances, and alter feeding behaviors of some key urban species. In urban systems—in contrast to the trophic systems in outlying deserts, which are dominated by limiting resources—predation by birds becomes the dominant force controlling arthropods on plants. Reduced predation risk elevates the abundance of urban birds and alters their foraging behavior such that they exert increased top-down effects on arthropods. Shifts in control of food web dynamics are probably common in urban ecosystems, and are influenced by complex human social processes and feedbacks.

Ground arthropod community structure in a heterogeneous urban environment

Despite being conspicuous and influential features of the biosphere, urban ecosystems have been neglected in ecological research. Arthropods are abundant in urban settings, but little is known about how these animals respond to urbanization. We systematically monitored the structure of ground arthropod communities for 12 months at 16 sites representing the four most abundant forms of urban land use (residential, industrial, agricultural, and desert remnant) in a rapidly growing metropolitan area (Phoenix, AZ). Although taxonomic richness was comparable among land-use types, community composition differed, with certain taxa being uniquely associated with each form of land use. Three taxa (springtails, ants, and mites) were extremely widespread and abundant, accounting for over 92% of captures; when these three taxa were excluded from analysis, however, differences were revealed in arthropod community composition with urban land use. Trophic dynamics also varied with land use: predators, herbivores, and detritivores were most abundant in agricultural sites, while omnivores were equally abundant in all forms of land use. These community-level differences resulted from taxon-specific responses to habitat structure, which varied with land use. Because arthropod community structure is affected by habitat structure and land use, and because arthropods play key roles in nutrient cycling, organic matter decomposition, pollination, and soil aeration, the spatial heterogeneity of urban ecosystems therefore may affect ecosystem functioning.

INDIRECT INTERACTIONS BETWEEN TEMPORALLY SEPARATED HERBIVORES MEDIATED BY THE HOST PLANT

I tested the hypothesis that early season herbivory by leaf-chewing insects affects dis- tribution, densities, and survivorship of late-feeding, leaf-mining insects; early chewing may physically and chemically alter quality of leaves for later leaf miners on the shared host plant, Quercus emoryi. Proportions of intact leaves and leaves damaged by native leaf chewers on six control and six experimental trees were determined over two growing seasons. I manually damaged - 50% of leaves on experimental trees to increase the total fraction of damaged leaves to z 75%. Leaf-miner densities, distribution, survivorship, and mortality were monitored on control and experimental trees and in intact and damaged leaves within trees. Leaf miners occurred more frequently than expected by chance on intact than damaged leaves for both control and experimental trees. Leaf-miner densities did not differ between control and experi- mental trees. This result suggests ovipositing leaf miners did not discriminate between damaged and undamaged trees, but selected leaves within trees. More leaf miners survived in intact than damaged leaves in both growing seasons. Survivorship was less in damaged leaves because of significantly increased rates of parasitism on these leaves. However, death from other causes (including bacterial and fungal death) was significantly less for leaf miners on damaged leaves. This positive effect did not compensate for overall lower survivorship of leaf miners in damaged leaves due to increased parasitism. Survivorship and mortality did not differ between control and experimental trees when damaged and intact leaves were pooled within treatments. However, in 1982-1983, survival was lower and parasitism greater for leaf miners in control-damaged leaves than experimental-damaged leaves. Survival and parasitism were not different between control- intact and experimental-intact leaves. These results suggest that (1) the effect of early season herbivory on leaf miners was inversely density dependent and, (2) occurrence of leaf miners on intact leaves may be caused by avoidance of damaged leaves rather than preference for intact leaves. Early season herbivory caused localized changes in photochemistry within trees, and these chemical alterations were consistent with observed distributions and survivorship of leaf miners. Both early season herbivory and experimental damage resulted in higher levels of condensed tannins and lower protein content in damaged leaves within trees, but herbivory had no effect on between-tree chemistry differences. Leaf miner distributions corresponded to these localized chemical changes: leaf miners avoided damaged leaves within trees but showed no between-tree preferences. Parasitism of miners on damaged leaves was higher, possibly because parasitoids used physical and chemical changes as cues to locate leaf-miner hosts, or because exposure of leaf miners to para- sitoids was prolonged. However, when many leaves were damaged (experimental trees), the negative effect of parasites was mitigated because physical and chemical cues associated with damaged leaves were less effective. Lower leaf-miner mortality from other causes may be related to the bactericidal or fungicidal properties of increased tannins in damaged leaves. This study demonstrates that temporally separated guilds can interact subtly at low levels of herbivory through changes in the host plant. Current theories of within- and between-guild organization of phytophagous insects may need to be re-evaluated if such interactions are common.

Invasion, Competition, and Biodiversity Loss in Urban Ecosystems

The global decline in biodiversity as a result of urbanization remains poorly understood. Whereas habitat destruction accounts for losses at the species level, it may not explain diversity loss at the community level, because urban centers also attract synanthropic species that do not necessarily exist in wildlands. Here we suggest an alternative framework for understanding this phenomenon: the competitive exclusion of native, non-synanthropic species by invasive species. We use data from two urban centers (Phoenix and Baltimore) and two taxa (birds and spiders) to link diversity loss with reduced community evenness among species in urban communities. This reduction in evenness may be caused by a minority of invasive species dominating the majority of the resources, consequently excluding nonsynanthropic species that could otherwise adapt to urban conditions. We use foraging efficiency as a mechanism to explain the loss of diversity. Thus, to understand the effects of habitat conversion on biodiversity, and to sustain species-rich communities, future research should give more attention to interspecific interactions in urban settings.

Mutualistic asexual endophytes in a native grass are usually parasitic.

Asexual systemic fungi that live symbiotically within grasses are viewed as strong mutualists on the basis of theory and empirical studies of introduced agronomic grasses. Evolutionary theory predicts that microbial symbionts that lose sexuality and rely on propagules of their hosts for transmission should evolve to benefit their hosts. Fungal endophytes of some cultivated turf and pasture grasses are well known for increasing plant performance and competitive abilities, especially under stress, and increasing resistance to herbivores, pathogens, and root‐feeders by virtue of fungal alkaloids. The assumption of mutualism, however, has rarely been tested in native grasses, which often harbor high but variable frequencies of systemic asexual endophytes. We tested the effect of Neotyphodium infections for the native grass Arizona fescue in a 3‐yr field experiment. We strictly controlled host genotype and manipulated soil moisture and nutrients. Infection generally decreased host growth in terms of plant volume, number of tillers, and dry mass of shoots and roots. Infected plants also showed decreased reproduction in terms of number and mass of seeds, and the seeds produced by infected plants had lower germination success than plants without their endophytes, suggesting that the negative effects of the symbiont are transferred to the next generation. Plant genotype strongly influenced host’s growth and reproduction and interacted with the presence of the endophyte, but the interaction was usually in the direction of negative effects. Our results challenge the notion that systemic asexual endophytes must be plant mutualists for infections to persist in nature. We propose other hypotheses to explain the variable but usually high endophyte frequencies in natural populations of grasses.

Fungal endophytes: Common host plant symbionts but uncommon mutualists

Abstract Fungal endophytes are extremely common and highly diverse microorganisms that live within plant tissues, but usually remain asymptomatic. Endophytes traditionally have been considered plant mutualists, mainly by reducing herbivory via production of mycotoxins, such as alkaloids. However, the vast majority of endophytes, especially horizontally-transmitted ones commonly found in woody plants, apparently have little or no effect on herbivores. For the systemic, vertically-transmitted endophytes of grasses, mutualistic interactions via increased resistance to herbivores and pathogens are more common, as predicted by evolutionary theory. However, even in these obligate symbioses, endophytes are often neutral or even pathogenic to the host grass, depending on endophyte and plant genotype and environmental conditions. We present a graphical model based upon variation in nitrogen flux in the host plant. Nitrogen is a common currency in endophyte/host and plant/herbivore interactions in terms of limitations to host plant growth, enhanced uptake by endophytes, demand for synthesis of nitrogen-rich alkaloids, and herbivore preference and performance. Our graphical model predicts that low alkaloid-producing endophytes should persist in populations when soil nutrients and herbivory are low. Alternatively, high alkaloid endophytes are favored under increasing herbivory and increasing soil nitrogen, at least to some point. At very high soil nitrogen levels, uninfected plants may be favored over either type of infected plants. These predictions are supported by patterns of infection and alkaloid production in nature, as well by a manipulative field experiment. However, plant genotype and other environmental factors, such as available water, interact with the presence of the endophyte to influence host plant performance.

Urban biodiversity: patterns and mechanisms.

The patterns of biodiversity changes in cities are now fairly well established, although diversity changes in temperate cities are much better studied than cities in other climate zones. Generally, plant species richness often increases in cities due to importation of exotic species, whereas animal species richness declines. Abundances of some groups, especially birds and arthropods, often increase in urban areas despite declines in species richness. Although several models have been proposed for biodiversity change, the processes underlying the patterns of biodiversity in cities are poorly understood. We argue that humans directly control plants but relatively few animals and microbes—the remaining biological community is determined by this plant “template” upon which natural ecological and evolutionary processes act. As a result, conserving or reconstructing natural habitats defined by vegetation within urban areas is no guarantee that other components of the biological community will follow suit. Understanding the human‐controlled and natural processes that alter biodiversity is essential for conserving urban biodiversity. This urban biodiversity will comprise a growing fraction of the worlds repository of biodiversity in the future.

Urbanization and Spider Diversity: Influences of Human Modification of Habitat Structure and Productivity

As a part of the Central Arizona–Phoenix Long-Term Ecological Research project, we determined how land-use alteration influenced spider and harvestman diversity. We sampled spiders in six habitat types (desert parks, urban desert remnants, industrial, agricultural, xeric- and mesic-residential yards) and tested how habitat type and productivity affected spider diversity and abundance. As expected, agricultural fields and mesic yards were more productive than the other, xeric habitats. These more productive habitats were characterized by higher abundances but lower spider diversity and were dominated by Lycosidae (wolf spiders), followed by Linyphiidae (sheet-web weavers). The increase in wolf spider abundance was positively correlated with habitat productivity and negatively correlated with the abundance of other predatory arthropods that might compete with, or prey upon, wolf spiders.

Early Leaf Abscission: A Neglected Source of Mortality for Folivores

We present evidence for a simple, yet heretofore overlooked, cause of mortality for folivorous insects, early leaf abscission. We discuss characteristics of both herbivores and host plants that are likely to influence folivore mortality via leaf abscission. Many plants have been shown to abscind leaves that are diseased or damaged (Jacobs 1962). The complete mechanism by which plants do so, however, is less clear. Apparently, leaf abscission involves interactions of many plant compounds including ethylene, auxins, and abscisic acid (Milborrow 1974). Whatever the physiological mechanism of leaf abscission, the consequence for a leaf-feeding insect is disjunction from the host plant. Separation from the plant can increase herbivore mortality in a number of ways. The excised leaf itself is probably a poorer nutritional source than an intact leaf (Haukioja and Niemeld 1977) especially in plants that can melanize, such as oaks. Since many herbivorous insects require more than one leaf for development, starvation may ensue unless the insect can relocate on the same or another suitable host plant. Herbivores detached from the host plant could be more susceptible to predation and fungal attack while on the ground. The size and density of the host plant could influence the ability of a detached folivore to relocate the host. Obviously, an insect displaced by leaf abscission is less likely to return to the foliage of a large, solitary oak than to any individual plant in a dense patch of herbs. From the plants perspective, leaf abscission can be hypothesized as a trade-off resulting in the conservation of resources. If a leaf is damaged sufficiently, water loss might become prohibitive or photosynthate loss might be greater than the photosynthetic capability of the undamaged portion; consequently, abscission should occur when losses exceed gains. Long-lived plants that produce many leaves over their lifespan could abscise damaged leaves with relative impunity, but the cost would be high for annual herbs with only a few leaves. For example, Orians and Solbrig (1977) have demonstrated that plants with ephemeral leaves must compensate for constructing short-lived leaves with higher ates of photosynthesis. Furthermore, if there were some damage threshold for abscising leaves, large leaves should be less likely to abscind than small leaves with the same absolute area damaged by herbivores. The effect of leaf abscission on mortality of folivores hould depend not only on plant characteristics, but also on the relative mobility of the folivore. Species whose feeding habits restrict hem to a single leaf or a few leaves, such as leaf miners or gall formers, should experience greater mortality as a result of leaf abscission than should species that can readily move to other leaves. Moreover, mortality as a result of early leaf abscission should be greater in the more sedentary developmental stages (egg, larva, pupa). Folivorous insects that can