Manuel C. Molles

Frontiers of Ecology

integration and collaboration as we meet the challenge of understanding the great complexity of biological systems. Ecological subdisciplines are rapidly combining and incorporating other biological, physical, mathematical, and sociological disciplines. The burgeoning base of theoretical and empirical work, made possible by new methods, technologies, and funding opportunities, is providing the opportunity to reach robust answers to major ecological questions. In December 1999 the National Science Foundation convened a white paper committee to evaluate what we know and do not know about important ecological processes, what hurdles currently hamper our progress, and what intellectual and conceptual interfaces need to be encouraged. The committee distilled the discussion into four frontiers in research on the ecological structure of the earth’s biological diversity and the ways in which ecological processes continuously shape that structure (i.e., ecological dynamics). This article summarizes the discussions of those frontiers and explains why they are crucial to our understanding of how ecological processes shape patterns and dynamics of global biocomplexity. The frontiers are 1. Dynamics of coalescence in complex communities 2. Evolutionary and historical determinants of ecological processes: The role of ecological memory 3. Emergent properties of complex systems: Biophysical constraints and evolutionary attractors 4. Ecological topology: Defining the spatiotemporal domains of causality for ecological structure and processes Each of the four research frontiers takes a different approach to the overall ecological dynamics of biocomplexity, and all require integration and collaboration among those approaches. These overlapping frontiers themselves are not necessarily new. Within each frontier, however, are emerging questions and approaches that will help us understand how ecological processes are interconnected over multiple spatial and temporal scales, from local community structure to global patterns.

Trichopteran Communities of Streams Associated with Aspen and Conifer Forests: Long-Term Structural Change

A comparison of the trichopteran communities of streams associated with aspen, spruce-fir, and nixed-conifer forests demonstrated significant differences in structure. Though tri- chopteran species composition in aspen-associated and spruce-fir associated streams was virtually identical, relative abundances of shredder and grazer Trichoptera differed significantly in the two types of streams. The ratio of shredder to grazer trichopteran biomass in aspen streams was 1:17 while in conifer streams, the dominance was reversed with a shredder to grazer biomass ratio of 3:1. The difference in dominance of functional groups in the two types of streams was attributed to the order-of-magnitude greater standing crop of detritus found in the conifer streams. It is hypothesized that the greater accumulation of detritus in conifer streams is the result of habitat modification by logs. Five times the number of logs were found in conifer streams as compared to aspen streams. The greater accumulation of logs in the conifer streams is attributed to lower rates of decomposition of conifer wood. The data suggest a general model for long-term structural change in trichopteran communities of the southern Rocky Mountains and I suggest that these long-term changes result from changes in quantity and quality of wood inputs during forest succession.

Colonization of artificial substrates by stream insects: Influence of substrate size and diversity

A field experiment was performed in a New Mexico (U.S.A.) stream to investigate the relation between size and diversity of substrate and numbers and diversity of colonizing insects. Baskets with either small gravel, large gravel, or a 1 : 1 mixture of the two sizes were placed in a riffle area and colonization was monitored for 19 days.Colonization was rapid, and both total number of individuals and number of species colonizing the substrates had stopped increasing by the end of the experiment. Fewer individuals colonized baskets downstream, suggesting that the drift is a major source of insect colonists. Small substrate supported more individuals and more species than the larger stones. Total number of individuals and the number of species on the mixed substrate were between numbers on small and large substrate. These results do not support previous generalizations on the relation between substrate size and complexity and the structure of invertebrate communities in streams. Results of the study are compared with the findings of recent experimental studies and it is suggested that earlier generalizations on the role of substrate size and complexity need to be reexamined.

Influence of annual flooding on terrestrial arthropod assemblages of a Rio Grande riparian forest

Terrestrial arthropod communities remain poorly described for riparian ecosystems of the arid southwestern United States, and the effects of extensive river regulation and habitat alteration on these potentially important invertebrates are largely unknown. Beginning in 1991, surface-active arthropods were trapped at two riparian sites along the Rio Grande, in central New Mexico, for 2 years. One site was then experimentally flooded from mid-May to mid-June for each of the next 3 years to simulate historic, low intensity flooding, after which arthropod collections were continued. These primary sites, located outside the riverside levee, and isolated from flooding for about 50 years prior to the experiment, were compared with a naturally flooded site and a second non-flooded reference. Experimental flooding and observations of the naturally flooded site indicated that flooding did not affect total taxonomic richness, nor richness of spiders, beetles or ants. However, flooding may have slightly increased the number of carabid beetle taxa present. Flooding altered the overall composition for all taxa, insects, beetles and carabid beetles. Spider taxa composition may be insensitive to flooding, while ant responses were not clear. Abundance of terrestrial isopods and spiders decreased after flooding, while overall beetle abundance did not change. Abundance of crickets and carabid beetles increased, but the response was delayed until after the second flood. Changes in taxa composition and abundance after experimental flooding were generally consistent with arthropod community structure observed at a nearby naturally flooded site. This similarity suggests that reorganization of the terrestrial arthropod community may follow restoration of flooding to this riparian ecosystem. Copyright

Spatial and temporal variation of fish communities in secondary channels of the San Juan River, New Mexico and Utah

Spatial and temporal variation of fish communities in four secondary channels of the San Juan River between Shiprock, NM and Bluff, UT were investigated from July 1993 through November 1994. Fish abundance and habitat availability data were collected to determine if physical attributes of sites influenced spatial and temporal variation in their fish communities. Stability of habitat was shown to positively influence the stability of the fish community. Analysis of variance revealed greater spatial than temporal variation in the abundance of red shiner, Cyprinella lutrensis, fathead minnow, Pimephales promelas, and flannelmouth sucker Catostomus latipinnis, while speckled dace, Rhinichthys osculus showed greater temporal variation. Ordination, using detrended correspondence analysis, revealed variation in fish communities by site, date, and sample year. Spatial variation was greatest during low-flow periods when the greatest differences in habitat among the four sites occurred. Spring runoff had the greatest temporal effect on the fish communities in secondary channels and appeared to ‘reset’ the communities by displacing those species that were less resistant to increased current velocities. This annual event may help maintain native fish species adapted to these conditions in the San Juan River while moderating the abundance of nonnative fish species.

Ground Arthropods as Potential Indicators of Flooding Regime in the Riparian Forest of the Middle Rio Grande, New Mexico

Abstract In 2001 and 2002, we pit trapped arthropods at eight riparian forest sites along the middle Rio Grande, four characterized by flooding in some years (flood sites) and four others where periodic flooding no longer occurs (nonflood sites). All flood sites flooded in 2001 but not in 2002, while nonflood sites never flooded. Arthropod counts and hierarchical cluster analyses of the sites indicated (1) significantly greater abundance of carabid beetles and the isopod Porcellio laevis in 2001 than in 2002; (2) significantly greater carabid beetle abundance at flood sites during each year of the study, with the relative abundance of one carabid species, Calathus opaculus, significantly higher at nonflood sites in 2002; (3) marginally significantly higher (2001) or significantly higher (2002) carabid species richness at flood sites; (4) no obvious response of the other taxa examined (isopods, tenebrionid beetles, and the cricket Gryllus alogus) to flooding regime, although differences in isopod abundance between flood and nonflood sites approached significance in 2001; (5) successful classification of all nonflood sites and three flood sites using the Bray–Curtis Similarity Index and carabid abundance. Overall, our results suggest that carabid beetles are fairly sensitive indicators of hydrologic connectivity between the Rio Grande and its riparian forest, while the other taxa examined are not. With the number of ongoing or planned restoration efforts increasing along the middle Rio Grande, carabids may represent an important tool for monitoring the response of riparian areas to managed flooding.

Short-Term Effects of Annual Flooding on a Population of Peromyscus Leucopus in a Rio Grande Riparian Forest of Central New Mexico

-We studied the short-term effects of experimental flooding on a population of Peromyscus leucopus in a Rio Grande riparian forest that had not flooded for over 50 y. We monitored populations at two sites for 2 yr before flooding and then for 3 yr during which we flooded one site between mid-May and mid-June each year. Considerable interannual variation in density was detected at both sites, with no clear effect of flooding on density. Some mice stayed within the forest during flooding and used trees as refugia. Survivorship decreased slightly at the flood site after the first flood but did not differ from the reference site 2 mo after flooding that year, nor in the final year. There was no difference between sites in recruitment of new individuals.

Streams in Semiarid Regions as Sensitive Indicators of Global Climate Change

Revelle and Suess (1957) pointed out that human enterprise throughout our planet could yield “far-reaching insight into the processes determining weather and climate.” Today, scientists worldwide continue to search intensively for this insight into global weather and climate as the rate of emission of radiatively significant trace gases continues to increase. Much has been learned about the processes that determine weather and climate, but substantial uncertainty remains as predictions are made relative to future global warming and climate change.

Lightning, precipitation and vegetation at landscape scale

We investigated the question “Is there a relationship between seasonality in precipitation and vegetative cover in Pole Canyon, NM?” GIS and statistical methods were used to determine the degree of association between either summer or winter precipitation and percent canopy cover for trees, graminoids and total vegetation. Monsoon (summer) precipitation was predicted for the years 1986–1994 from lightning strike and relative humidity data by multiple regression. Winter precipitation, the percent of annual precipitation that occurs during winter, and vegetative cover were derived from the Forest Service Terrestrial Ecosystem Survey. Vegetation and precipitation data were ranked and classified (e.g., high, medium, low) and cross-tabulations were generated to compare the spatial distribution of vegetation classes within each precipitation class.Results indicate that seasonality in precipitation affects the distribution and spatial pattern of vegetation at landscape scales. Winter precipitation is a key factor that influences the distribution and spatial pattern of tree cover. Monsoon precipitation may affect the spatial pattern of graminoid cover where Bouteloua gracilis dominates. Winter precipitation may affect the distribution and spatial pattern of graminoid cover where Festuca arizonica dominates. Some of the unexplained relationships may be due to competition between trees and graminoids for moisture and other limiting factors. The importance of temperature was implicit in the division between summer (monsoon) and winter seasons. Annual precipitation, elevation, topography and edaphic factors probably contributed to the observed relationships.