The Nature of Cumulative Impacts on Biotic Diversity of Wetland Vertebrates

The Nature of Cumulative Impacts on Biotic Diversity of Wetland Vertebrates I.ARRu D. HARRIS about--makes using food chain support as a variable for Department of Wildlife and Range Sciences predicting environmental impacts very questionable. School of Forest Resources and Conservation Historical instances illustrate the effects of the accumula- University of Florida tion of impacts on vertebrates. At present it is nearly impos- Gainesville, Florida 32611, USA sible to predict the result of three or more different kinds of perturbations, although long-range effects can be observed. One case in point is waterfowl; while their ingestion of lead ABSTRACT/There is no longer any doubt that cumulative shot, harvesting by hunters during migration, and loss of impacts have important effects on wetland vertebrates. Inter- habitat have caused waterfowl populations to decline, the actions of species diversity and community structure produce proportional responsibility of these factors has not been de- a complex pattern in which environmental impacts can play termined. a highly significant role. Various examples show how wet- Further examples show muttiplicative effects of similar ac- lands maintain the biotic diversity within and among verte- tions, effects with long time lags, diffuse processes in the brate populations, and some of the ways that environmental landscape that may have concentrated effects on a compo- perturbations can interact to reduce this diversity. nent subsystem, and a variety of other interactions of in- The trophic and habitat pyramids are useful organizing creasing complexity. Not only is more information needed at concepts. Habitat fragmentation can have severe effects at all levels; impacts must be assessed on a landscape or re- all levels, reducing the usable range of the larger habitat gional scale to produce informed management decisions. I generalists while threatening the genetic integrity of small, conclude that a system of replicate wetland reserves that are isolated populations. The complexity of trophic interactions, allowed to interact naturally with the surrounding landscape and the propensity, or necessity, of vertebrates to switch will be more effective in preserving biotic diversity than iso- from one food source to another--something we know little lated sanctuaries. The National Environmental Policy Act and similar landscape or regional scale of analysis. This means that state legislation dictate the performance of cumulative the wetland manager and program administrator must impact analyses prior to specific environmental manip- be increasingly sensitive to the presence and meaning ulations. Still other legislation requires the mainte- of within-species biotic diversity, not only to among- nance of biological diversity on certain Federal lands species diversity. What follows are descriptions of how (e.g., the National Forest Management Act), and "the wetlands maintain vertebrate biotic diversity and of protection and propagation of a balanced indigenous some ways that various impacts have accumulated to population of shellfish, fish, and wildlife in the body of reduce biodiversity. The majority of examples cited water to which the discharge is to be made ..." [1972 are from southern wetlands and related systems only amendments to the Federal Water Pollution Control because of my familiarity with them. Act, Section 315(a)]. Although logic and observations dictate that the impacts of repeated, similar actions, or a sequence of different actions, can accumulate to Hierarchy of Biotic Diversity cause severe or irreparable damage to biotic diversity, A great deal has been learned about genetics over some suggest that the concept of cumulative impacts is the last 80 years. It is now recognized that the lowest only "an interesting hypothesis." While it may not be levels of genetically encoded biotic diversity occur at possible to predict cumulative impacts a priori, they the level of the gene allele. Other important aspects of certainly exist, can be described, and are critically im- genetic diversity within a species or subspecies include portant. To be effective, assessment of cumulative im- the proportion of loci that are polymorphic, the pacts on vertebrate communities must be done at a number and types of alleles at these loci, and the average level of heterozygosity. Moreover, we now KEY WORDS: Cumulative impacts; Landscape ecology; Vertebrate know that the expression of a dominant, heterozygous, biodiversity;Wetland habitat; Wetland impacts or recessive allele at a single locus may determine EnvironmentalManagement Vol. 12, No. 5, pp. 675-693 1988 Springer-VerlagNew York Inc. 676 L.D. Harris adaptive characteristics such as the presence or ab- Presence, relative abundance, and activity levels of sence of a critical enzyme, resistance to malaria, or oc- some wetland vertebrates--beaver, alligator (Alligator currence of sickle-cell anemia. mississippens/s), and to a lesser extent muskrat, and Our former understanding of genetics led ornithol- large fish and turtles--may transform the physical ogists and wetland wildlife scientists to recognize the structure of a site, altering hydroperiod and water lesser snow goose (formerly Chen hyperborea) and blue flow characteristics to such an extent that they influ- goose (Chen caerulescens) as distinct species. The same ence the very presence/absence of wetlands through- situation was true for the great blue heron (Ardea he- out the landscape. rodias) and the great white heron (formerly Arclea occi- Unless laws and permitting and regulating policies dental~s). In both cases further investigation revealed begin to consider biotic diversity across the full hierar- that the groups were, in fact, only expressions of allele chical spectrum, they will not ensure conservation of variation within the same genome. Because of rapid the wetland biodiversity resource (Noss and Harris progress in the classification of vertebrates during this 1986). Only regulation and planning strategies that century, bird species have been reclassified such that maintain the full gamut of diversity (including pro- the more than 20,000 recognized species in 1920 have cesses) to the landscape level and above stand to be been reduced to about 9000 at present (Mayr 1982). effective at maintaining biodiversity at all lower levels. Thus, what formerly constituted diversity among However, even more may need to be done to maintain species is now considered to be genetic diversity within long-term diversity at the community, ecosystem, or a species. landscape levels. Both genetic (genotypic) and environmentally in- duced (phenotypic) variance may be manifested either as a gradual or clinal change, or as distinct ecotypes Wetland Wildlife Habitat and Food because of the separation of populations, with little or Chain Support no opportunity for interbreeding. Both situations rep- resent legitimate, and ecologically important, aspects Wetland impact analysis procedures frequently re- of biotic diversity that must be considered. volve around two specific biological functions: the The number of species (richness) and some mea- provision of habitat and of food chain support. These sure of their relative abundance (equitability) combine are neither simple nor independent concepts, and thus to form the concept of species diversity. Because this some explanation is warranted. concept was so heavily reviewed and debated by ecologists in the 1960s and 1970s, it became the standard by' Wildlife Habitat which biotic diversity was monitored and evaluated. At least four different issues are integral to the as- Not until this decade have ecologists and regulators re- sessment of cumulative impacts on wetland habitat. alized that it is neither sufficiently broad nor sensitive These are (1) specialist vs generafist species; (2) alpha, to assess the complete spectrum of biotic diversity. beta, and gamma diversity; (3) component vs com- Interactive community processes, such as herbivory, pound ecosystems; and (4) animals' need to move. Nu- pollination, propagule dispersal, predation, parasitism, merous amphibian and reptile species are habitat spe- and competition, all function to influence the distribu- cialists: mammals such as bog lemmings (Synaptomys tion, abundance, form, and behavior of organisms. It spp) and round-tailed water rats (Neofiber allen,), and is the occurrence and magnitude of coevolved pro- breeding birds such as Swainson's warbler (Limnothlyp/s cesses such as these that distinguish functional biolog- swainsonii) and prothonotary warbler (Protonotaria ci- ical communities from random collections of species trea). Resident species that have specific habitat re- and determine biotic diversity at the community, level. quirements also usually have small home-range sizes, Thus, both the occurrence and relative abundance of and thus all their resource requirements may be pro- species, and the relative magnitude of key biological vided within a single wetland. In the classification processes, must be evaluated when assessing impacts scheme of Whittaker (1960), these within-habitat on wetland biotic diversity. Keystone wetland species species would constitute the alpha (et) component of such as muskrat (Ondatra zibethicus), nutria (Myocastor diversity. At the other extreme, large, frequently wide- coypus), beaver (Castor canadensis), manatee (Trichechus ranging species, such as bald eagles (Haliaeetus leucoce- manatus), and moose (Alces alces) can dramatically alter pha/us), mink (Mustela

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