Donald R. Strong

Are Trophic Cascades All Wet? Differentiation and Donor‐Control in Speciose Ecosystems

Trophic cascades mean runaway consumption, downward dominance through the food chain. Especially vulnerable are the autotrophs. Standing crop and coverage of the plant community are reduced wholesale when one or a few species of potent herbivores are not suppressed. In archetypical trophic cascades, overwhelming effects propagate down through three trophic levels. Primary carnivores or diseases, by suppressing herbivores, switch the substrate from open and virtually bare to well occupied by plants. The discovery that these potent forces extend through four levels, in some instances (Carpenter and Kitchell 1988, Power 1990b), is surely one of the most important in all of ecology of the last decade; secondary carnivores, by suppressing primary carnivores, unleash herbivores that clear the substrate and greatly decrease standing crop of plants. In true trophic cascades, pervasive topdown influence combines with the always strong bottom-up influence through the food chain to produce acute intertwining between population, community, and ecosystem processes (Carpenter and Kitchell 1988, Oksanen 1990, Power 1992; M. J. Wiley, personal communication). True trophic cascades imply keystone species (Paine 1980), taxa with such top-down dominance that their removal causes precipitous change in the system. But all trophic interactions do not cascade, and simple top-down dominance is not the norm of communities or ecosystems. This point is central to the issues of this Special Feature, to Hunter and Prices (1992) proposal that the null hypothesis of food webs should be bottom-up forces, and Powers (1992) search for appropriate models of population dynamics of consumer and consumed. My premise is that true trophic cascades in the community sense are a relatively unusual sort of food web mechanics. I argue that, over the full range of ecological communities, evidence is that these cascades are restricted to fairly low-diversity places where great influence can issue from one or a few species; the majority of examples of true trophic cascades have algae at the

When is a trophic cascade a trophic cascade

Trophic cascades are the time-honored focal point of food-web dynamics. They are the best loved example of indirect effects in undergraduate ecology textbooks and they represent a potentially useful application of theory. Researchers have found them from the Arctic to the tropics. But, can we agree on what they are? Here, we seek to clarify the terminology of trophic cascades and call for a consensus on how to quantify cascading effects in the future.

Status, prediction and prevention of introduced cordgrass Spartina spp. invasions in Pacific estuaries, USA

Abstract Along the Pacific coast of North America, four introduced cordgrass species ( Spartina alterniflora, S. anglica, S. patens and S. densiflora ) have thus far invaded five isolated estuaries. Dense growth of introduced Spartina spp. reduces open mud feeding habitats of shorebirds, while in the upper intertidal, introduced Spartina spp. compete with native salt marsh vegetation. Prediction of Spartina invasions is facilitated by the remarkable restriction of these species to distinct estuarine habitats which generally lack interspecific competitors and herbivores. We used physical characteristics to identify 31 specific sites along the US Pacific coast that are vulnerable to future Spartina invasions and then used species characteristics, like native latitudinal range and past invasion success, to predict which Spartina species will be likely to invade these sites in the future. All 31 sites were predicted to be vulnerable to S. alterniflora , while the other invasive Spartina spp. may be restricted to a subset of the vulnerable sites. At a finer scale, within a vulnerable site, the mean tidal range can be used to predict the extent of spatial spread of a Spartina sp. after colonization. These prediction techniques might be used to identify and prioritize sites for protection against future invasions. We suggest that a cost-effective way to prevent the transformation of unique North American Pacific mudflat and saltmarsh communities into introduced Spartina -dominated marshes is to survey the vulnerable sites frequently and eliminate introduced Spartina spp. propagules before they spread.

Natural variability and the manifold mechanisms of ecological communities

In response to Roughgardens provocative essay, the editor of The American Naturalist has solicited articles that present the personal perspectives of some community ecologists and their sets of operating assumptions. Ecology is growing and changing rapidly, as is shown by this dispute over fundamentals, and my concern will be the importance of alternative and complementary forces in communities, in contrast to a singular emphasis on competition. I will emphasize density-vague population dynamics; independent and noncompetitive coexistence of species; weak and inconsequential competition; variable and diffuse species interactions; maintenance (by the weather, disturbance, and natural enemies) of populations well below densities that deplete resources to any important extent; and mutualistic and commensalistic relations between species. My purpose is to illustrate the commonness of communities that are quite different from those assumed by orthodox competition theory. I certainly do not deny that competition occurs in nature or that it is in some cases a dominant process. Competitively dominated communities have been amply reviewed elsewhere. Strong and persistent competition, however, does not necessarily square a set of species with orthodox competition and niche theory. Effects of competition are varied, and mathematical deductions about them need critical testing, both empirical and theoretical. Though interspecific competition, when and where it occurs, may affect aspects of population dynamics, shape niches, cause coevolution, or even result in character displacement of some species in some circumstances, it need not have ary of these effects. What are the possibilities for diffuse competition (Connell 1975)? What might nonequilibrium communities be like (Caswell 1982; Wiens and Rotenberry 1980)? What are the influences of habitat patchiness and high stochasticity (Levin 1976; Chesson 1978; Murdoch 1979)? My paper is in two parts. Part 1 deals with several logical and philosophical points. Part 2 is a description of some case studies that find factors other than interspecific competition to be of great influence in ecological communities. The dispute illustrated by this symposium was actually begun long ago (British Ecolog-

Spread of Exotic Cordgrasses and Hybrids (Spartina sp.) in the Tidal Marshes of San Francisco Bay, California, USA

Four species of exotic cordgrass (Spartina sp.) occur in the San Francisco estuary in addition to the California native Spartina foliosa. Our goal was to map the location and extent of all non-native Spartina in the estuary. Hybrids of S. alterniflora and S. foliosa are by far the most numerous exotic and are spreading rapidly. Radiating from sites of deliberate introduction, S. alterniflora and hybrids now cover ca. 190 ha, mainly in the South and Central Bay. Estimates of rate of aerial increase range from a constant value to an accelerating rate of increase. This could be due to the proliferation of hybrid clones capable of rapid expansion and having superior seed set and siring abilities. The total coverage of 195 ha by hybrids and other exotic cordgrass species is slightly less than 1% of the Bays tidal mudflats and marshes. Spartina anglica has not spread beyond its original 1970s introduction site. Spartina densiflora has spread to cover over 5 ha at 3 sites in the Central Bay. Spartina patens has expanded from 2 plants in 1970 to 42 plants at one site in Suisun Bay. Spartina seed floats on the tide, giving it the potential to export this invasion throughout the San Francisco estuary, and to estuaries outside of the Golden Gate. We found isolated plants of S. alterniflora and S. densiflora in outer coast estuaries north of the Bay suggesting the likelihood for the San Francisco Bay populations to found others on the Pacific coast.

Hybridization between introduced smooth cordgrass (Spartina alterniflora; Poaceae) and native California cordgrass (S. foliosa) in San Francisco Bay, California, USA.

Introduced Spartina alterniflora (smooth cordgrass) is rapidly invading intertidal mudflats in San Francisco Bay, California. At several sites, S. alterniflora co-occurs with native S. foliosa (California cordgrass), a species endemic to California salt marshes. In this study, random amplified polymorphic DNA markers (RAPDs) specific to each Spartina species were identified and used to test for hybridization between the native and introduced Spartina species in the greenhouse and in the field. Greenhouse crosses were made using S. alterniflora as the pollen donor and S. foliosa as the maternal plant, and these crosses produced viable seeds. The hybrid status of the crossed offspring was confirmed with the RAPD markers. Hybrids had low self-fertility but high fertility when back-crossed with S. foliosa pollen. Hybrids were also found established at two field sites in San Francisco Bay; these hybrids appeared vigorous and morphologically intermediate between the parental species. Field observations suggested that hybrids were recruiting more rapidly than the native S. foliosa. Previous work identified competition from introduced S. alterniflora as a threat to native S. foliosa. In this study, we identify introgression and the spread of hybrids as an additional, perhaps even more serious threat to conservation of S. foliosa in San Francisco Bay.

Life History Variation Among Populations of an Amphipod (Hyalella Azteca)

Inheritable differences in life history features are shown among three groups of populations of Hyallela azteca from Oregon. Hyallela individuals of coastal populations are relatively small as eggs, as young, and as adults. They grow slowly as young and adults, and the regression of clutch size on female size is relatively steep. Hyallela individuals from the Cascade Mountains are large as eggs, as young, and as adults. They grow rapidly as young and adults, and the regression of clutch size on female size is relatively less steep. A population of Hyalella from a hot springs is made up of animals that are intermediate in these characteristics. The maturation period and the rate of production of egg volume does not differ among the populations investigated. Selection for inconspicuousness, generated by visually orienting predaceous fish, is probably responsible for the small, slow—growing end of the character spectrum (coastal populations). Hyallela is a grazer and deposit feeder, it does not filter. Evidence indicates that larger Hyalella do not have a competitive advantage in predator—free environments, as do larger planktonic filter feeders.

Reduced herbivore resistance in introduced smooth cordgrass (Spartina alterniflora) after a century of herbivore-free growth.

Abstract We compared resistance to insect herbivory in two introduced populations of smooth cordgrass (Spartina alterniflora) differing in their history of herbivory. One population in Willapa Bay, Washington, has spread in the absence of herbivory for more than a century, while another population in San Francisco, California, was introduced 20 years ago and is fed upon by the Spartina-specialist planthopper, Prokelisia marginata. The planthopper is a sap-feeder common on the Atlantic and Gulf coasts of North America, where smooth cordgrass is native. Smooth cordgrass plants from Willapa Bay (WB), San Francisco Bay (SFB), and Maryland (the source of the SFB introduction) were exposed to P. marginata herbivory over two consecutive summers in a common greenhouse environment, and their growth was compared with that of control plants that were grown herbivore-free. The planthoppers had relatively little effect on the growth of SFB plants, with plants exposed to herbivores averaging 77% and 83% of the aboveground biomass of herbivore-free controls after the first and second season of herbivory, respectively. The growth of plants from Maryland was similarly little-affected by the planthoppers, with the plants exposed to herbivores averaging near 100% of the biomass of herbivore-free controls after two seasons. In contrast, the growth of the WB plants was greatly reduced by the planthopper, with the plants exposed to planthopper herbivory averaging only 30% and 12% of the aboveground biomass of herbivore-free controls after the first and second seasons of herbivory, respectively. By the end of the second season of herbivory, 37% of the WB plants exposed to herbivory had died, while none of the SFB plants exposed to herbivores had died. Among WB clones, there was variation in resistance; one WB clone suffered 0% mortality while another suffered 100% mortality when exposed to herbivores. Short-term herbivory experiments with the putative founder clone for the WB population suggested that the WB founder was similar to the more resistant WB clones in its susceptibility to planthopper herbivory. Nitrogen analyses of green leaf tissue indicated that WB plants, including the WB founder clone, averaged 70% more total leaf nitrogen than SFB and Maryland plants. In a planthopper choice experiment, more planthoppers were observed on WB plants than SFB plants after 95 days of exposure to herbivory. Planthopper preference for WB plants may have contributed to the lower resistance of WB plants to herbivory; however, even before planthoppers had become more abundant on the WB plants, the proportion of leaves with 50% or more dead tissue averaged significantly greater on the WB plants, suggesting a difference between populations in tolerance to herbivory as well. Multiple factors, including a founder effect, further loss of herbivore tolerance, and herbivore preference for WB plants, appear to account for the reduced planthopper resistance in the WB population.

Extent and degree of hybridization between exotic (Spartina alterniflora) and native (S. foliosa) cordgrass (Poaceae) in California, USA determined by random amplified polymorphic DNA (RAPDs)

Spartina alterniflora, smooth cordgrass, native to the eastern USA, was introduced into south San Francisco Bay ≈ 25 years ago. It has spread by purposeful introduction of rooted plants and dispersal of seeds on the tides. Previous work suggested that S. alterniflora was competitively superior to the native California cordgrass, S. foliosa, and that the two species hybridized. The present study determined the spread of S. alterniflora and S. foliosa × alterniflora hybrids in California and examined the degree of hybridization. We used nuclear DNA markers diagnostic for each species to detect the parental species and nine categories of hybrids. The California coast outside San Francisco Bay contained only the native species. All hybrid categories exist in the Bay, implying that several generations of crossing have occurred and that hybridization is bidirectional. Hybrids were found principally near sites of deliberate introduction of the exotic species. Where S. alterniflora was deliberately planted, we found approximately equal numbers of S. alterniflora and hybrid individuals; S. foliosa was virtually absent. Marshes colonized by water‐dispersed seed contained the full gamut of phenotypes with intermediate‐type hybrids predominating. The proliferation of hybrids could result in local extinction of S. foliosa. What is more, S. alterniflora has the ability to greatly modify the estuary ecosystem to the detriment of other native species and human uses of the Bay. To the extent that they share these engineering abilities, the proliferation of cordgrass hybrids could grossly alter the character of the San Francisco Bay.

Rapid Asymptotic Species Accumulation in Phytophagous Insect Communities: The Pests of Cacao

The number of cacao insect pests is described by a species-area curve. Either annual cacao productivity or area in cultivation of the crop predicts the number of associated insect pest species, when the worlds cacao-producing regions are compared. Analysis of covariance does not discriminate different species-area regressions for native as opposed to nonnative cacao-producing regions; the numbers of insect pest species per unit area of cacao in regions of long-standing cultivation do not exceed the numbers in regions of recent introduction. This demonstrates that the number of cacao insect pest species rises rapidly to an asymptote set by the area in cultivation in each region.