Biotic Forces Research Articles

Empirical evaluation of observation scale effects in community time series

Natural communities are highly complex and dynamic over time, with populations structured by numerous abiotic and biotic forces acting through direct and indirect pathways. Multispecies Autoregressive (MAR) modeling can be used to partition effects of variables that are interrelated and temporally autocorrelated in time series from natural systems. Here we address two main questions in applying MAR models to community time series. First, what is the effect of observation scale on interpretation of community dynamics? We used a 10‐year weekly planktonic time series from Lake Washington to construct multiple “biweekly” and “monthly” data sets, and compared resulting community interaction models. Direct abiotic effects and intraspecific autocorrelation were apparent using all data sets. Biotic interactions were more apparent using biweekly and monthly data, indicating that time lags longer than one week were necessary to detect numerical response to interspecific interactions. Second, we examined effects of dropping the winter months from our analyses to simulate the common practice of sampling only during the “growing season” in long‐term ecological studies. We found that biotic interactions remained similarly characterized in models using only non‐winter months, but that the importance of seasonal physical factors nearly disappeared in non‐winter models. Exclusion of winter data in sampling designs may therefore allow us to characterize biotic interactions, although it may not help us understand populations’ relationships to seasonal abiotic variables. The models supported many previous findings from experimental and qualitative investigations of Lake Washington community interactions, implying that MARs provided plausible characterizations of community dynamics, but some previously unconsidered relationships did emerge, such as the importance of cryptomonads and picoplankton for zooplankton growth. We conclude that explicit consideration of time lags in biotic response is necessary to understand relative importance of abiotic and biotic factors, and that sampling regime can therefore strongly influence our interpretations of community dynamics.

Seasonal and diel patchiness of a Daphnia population: An acoustic analysis

Detailed information about the location and extent of zooplankton patches is fundamental to understand how abiotic and biotic forces interact to structure the spatial distribution of zooplankton. We mapped zooplankton patchiness in a Minnesota lake during spring, summer, and autumn with high‐frequency (192‐kHz) single‐beam sonar. Conventional plankton samples of aggregations detected acoustically revealed that Daphnia pulicaria (mean body length 1.6 mm, mean target strength —120 dB) scattered most (~63%) of the sound. Other taxa were smaller (<½ the length of D. pulicaria) and were usually less abundant and therefore scattered much less sound than D. pulicaria. Our acoustic estimates of Daphnia concentrations illustrate extreme patchiness, with concentrations varying by as much as four orders of magnitude over vertical distances of less than 1 m. Seasonal patterns of patchiness were related to predation by rainbow trout and to abiotic factors associated with stratification. Daphnia concentrations were highest from June to October in a deep‐water “refuge zone” where oxygen concentrations were between 3 and 5 mg L−1. These oxygen levels are suitable for Daphnia but are lower than those required by rainbow trout. Heterogeneity in Daphnia concentration along the lake's long axis was highest in May and June, when the population resided primarily in the oxic hypolimnion during the daytime. From July to October, as oxygen concentrations declined in the hypolimnion, the population became more metalimnetic and more uniformly distributed in the horizontal dimension. A diel study of the population in October indicated that the patchiness of population also changed dramatically between day and night. During the day the population aggregated densely in a thin layer (~2 m thick) in the thermocline. After sunset the population dispersed into the epilimnion, where concentrations were ~100,000 m23 less than they were during the day in the thermocline.

The influence of hydrological and biotic processes on brown trout (Salmo trutta) population dynamics

Hydrological and biotic forces constrain brown trout (Salmo trutta) population dynamics, but tests of their role across numerous streams are uncommon. In 30 French stream reaches, using 5–8 samples (1 per year) each, we investigated whether the year-to-year seasonal hydrology influenced annual trout densities within reaches, and whether the relationships were shared by all reaches. We also searched for intraspecific interactions between and within cohorts. Trout data were age class (0+, 1+, and adults) densities. For each year, hydrology was described using 13 variables, each computed for a reproduction, emergence, and growth period related to the biological cycle of trout. We used analyses of covariance (ANCOVA) to test how trout densities at year n – 1 and hydrology at year n influenced trout densities at year n. High flows during emergence significantly reduced the 0+ densities, consistently across the 30 reaches. Then, 1+ and adult densities were linked, respectively, to 0+ and 1+ densities from the previous year. Analyses also revealed density-dependent survival mechanisms for the 0+ cohort, suggesting intracohort competition. Therefore, hydrology constrains trout dynamics only during the critical emergence period, after which intracohort interactions regulate the 0+ density. Such mechanisms, validated across 30 environmentally different reaches, seem to be fundamental to trout population dynamics.

TESTING THE RELATIVE CONTRIBUTION OF POSITIVE AND NEGATIVE INTERACTIONS IN ROCKY INTERTIDAL COMMUNITIES

In contrast to many other biotic forces, such as competition and predation, the role played by habitat modification by plants and sessile animals in natural communities has not been given the experimental attention it deserves. To test the hypothesis that habitat modification by seaweed canopies can have direct positive effects on rocky intertidal communities, we quantified habitat amelioration by Ascophyllum nodosum canopies and its consequences on understory organisms in the Gulf of Maine, USA. At the upper and lower elevational borders of the algal canopy, we examined the recruitment, growth, and survivorship of common benthic organisms in canopy removal, canopy control, and shaded canopy removal plots intended to mimic canopy habitat modification. The algal canopy greatly reduced potential physical stresses, particularly at high tidal heights. Maximum daily rock temperatures were 5°–10°C lower and evaporative water loss was an order of magnitude less under the canopy than in canopy removal plots. The response of understory organisms to canopy removal, however, was species specific and somewhat idiosyncratic. Nonetheless, in general, at the high intertidal border of the canopy the recruitment, growth, and survival of understory organisms were enhanced by the canopy, whereas at the low intertidal border canopy effects were negative or neutral. Nearly half of the interactions we studied were positive in the high zone. In contrast to positive canopy effects on understory organism recruitment and growth at high tidal heights, consumer pressure was severe under the canopy, particularly at low tidal heights. Green crab predation is likely responsible for limiting understory mussel densities, while grazing by the snail, Littorina littorea, keeps understory substrate clear of algal recruits. The amelioration of harsh physical conditions by algal canopies can have strong direct positive effects in high rocky intertidal communities by enhancing organism recruitment, growth, and survival. These canopy effects, however, may often be offset by increased consumer pressure at low tidal heights. These types of habitat modification effects are likely to be pervasive in many other terrestrial and marine communities exposed to harsh physical conditions.

Testing the Relative Contribution of Positive and Negative Interactions in Rocky Intertidal Communities

In contrast to many other biotic forces, such as competition and predation, the role played by habitat modification by plants and sessile animals in natural communities has not been given the experimental attention it deserves. To test the hypothesis that habitat modification by seaweed canopies can have direct positive effects on rocky intertidal communities, we quantified habitat amelioration by Ascophyllum nodosum canopies and its consequences on understory organisms in the Gulf of Maine, USA. At the upper and lower elevational borders of the algal canopy, we examined the recruitment, growth, and survivorship of common benthic organisms in canopy removal, canopy control, and shaded canopy removal plots intended to mimic canopy habitat modification. The algal canopy greatly reduced potential physical stresses, particularly at high tidal heights. Maximum daily rock temperatures were 5°–10°C lower and evaporative water loss was an order of magnitude less under the canopy than in canopy removal plots. The response of understory organisms to canopy removal, however, was species specific and somewhat idiosyncratic. Nonetheless, in general, at the high intertidal border of the canopy the recruitment, growth, and survival of understory organisms were enhanced by the canopy, whereas at the low intertidal border canopy effects were negative or neutral. Nearly half of the interactions we studied were positive in the high zone. In contrast to positive canopy effects on understory organism recruitment and growth at high tidal heights, consumer pressure was severe under the canopy, particularly at low tidal heights. Green crab predation is likely responsible for limiting understory mussel densities, while grazing by the snail, Littorina littorea, keeps understory substrate clear of algal recruits. The amelioration of harsh physical conditions by algal canopies can have strong direct positive effects in high rocky intertidal communities by enhancing organism recruitment, growth, and survival. These canopy effects, however, may often be offset by increased consumer pressure at low tidal heights. These types of habitat modification effects are likely to be pervasive in many other terrestrial and marine communities exposed to harsh physical conditions.

Process-specific cues for recruitment in sedimentary environments: Geochemical signals?

The most biologically and geochemically active marine sediments are characterized by steep chemical gradients within the top centimeters of sediment (Berner, 1980).Acommon feature of these environments is disruptions of surface sediments by both physical and biotic forces. Growth and mortality rates for new recruits are affected by many of these surface perturbations.At the same time, these disturbances also impose a discontinuity in concentration across the sediment-water interface, and accordingly, a change in surface chemistry. In this paper we present evidence that the cue used by juveniles to distinguish between recently disturbed and undisturbed surfaces may be disruption of geochemical gradients that are typical of nearshore benthic systems. New juveniles exposed to ammonium concentrations typical of disturbed surface sediments exhibit behaviors consistent with rejection of the habitat. Conversely, new juveniles placed onto sediments containing ammonium levels typical of undisturbed sure cial sediments rapidly initiate burrowing activity, a sign of ‘ ‘ acceptability.’ ’ We also present a numerical model, which assesses the dynamics of small-scale chemical shifts that accompany sediment disruption, to determine (a) what is the magnitude of surface chemistry changes associated with disturbance (i.e. what is the signal strength)? and (b) what are the spatial and temporal scales associated with the return to the undisturbed condition (‘ ‘ recovery’ ’ )? Model results show that the signal strength, and the return to ‘ ‘ acceptable’ ’ conditions, are strongly ine uenced by the initial gradient. Model predictions of the time required to ‘ ‘ recover’ ’ indicate that times to recovery are longer than the interval between disturbance events, but are of the same temporal scale (minutes to hours). Thus, our results suggest that the dynamics of sure cial gradients provide a strong signal over appropriate time scales that may reveal the intensity of disturbance and the likelihood of mortality for juveniles.As such, transport-reaction processes which govern porewater concentrations in sure cial sediments may also play a role in recruitment processes.

Density‐Dependent Settler‐Recruit‐Juvenile Relationships in Blue Crabs

Current theory on the population dynamics of marine species with complex life history patterns posits that a suite of physical and biotic forces (e.g., habitat structure and density—dependent predation or emigration) control survival and abundance in early life history, particularly after settlement. We have conducted a long—term sampling effort accompanied by a series of field and laboratory experiments examining the joint effects of habitat type, body size, and population density upon abundance and survival of early juveniles of the blue crab, Callinectes sapidus. In addition, the chance occurrence of a tropical storm during one set of experiments provided an opportunity to assess the impact of a physical disturbance upon newly settled blue crab survival and abundance. In the 10—yr sampling effort, we quantified relationships between sequential life history stages (juvenile crab instars) in seagrass beds, the initial nursery habitat for blue crabs in the lower Chesapeake Bay. Inter—instar relationships were defined as the densities of larger instars as dependent on the densities of smaller instars. Inter—instar relationships for the youngest instars are described by hyperbolic functions until crabs begin to emigrate to unvegetated habitats at approximately the fifth instar. Inter—instar relationships between crabs larger than the fifth instar and smaller crabs become either parabolic or linear functions and decay as the number of instars between sequential life history stages increases. While both the hyperbolic and parabolic functions are indicative of populations regulated by density—dependent processes, either predation or emigration, the decay in the functions describing the inter—instar relationships for crabs larger than the fifth instar indicates that the suite of processes regulating this segment of the population changes qualitatively. In laboratory and field experiments, the effects of vegetated and unvegetated habitats and size—specific predation on newly settled juveniles were tested. Tethering was used to quantify relative rates of predation, and a laboratory study was conducted to determine if tethering induced treatment—specific bias. We found no statistically significant interactions between the tethering treatment and the factor treatments of crab size and habitat during the laboratory study, indicating that tethering did not produce treatment—specific bias. Thus, tethering provided a relative measure of predation that allowed comparisons between treatments of habitat and crab size on crab survival. In both laboratory and field experiments, survival was significantly higher in vegetated habitats and with increasing size until the ninth instar, when survival did not differ by habitat. This difference explains the dispersal from vegetated to unvegetated habitats that occurred between the fifth and seventh instars. In addition, survival of all crabs was significantly increased both during and after Tropical Storm Danielle compared to pre—storm conditions. A model is developed that describes juvenile survival as a function of crab size and habitat type. Survival curves in both habitats are represented by similar sigmoid functions with survival higher in vegetated habitats. Subsequently, the survival of newly settled blue crabs is likely dependent on the availability of complex habitat. Thus, a suite of biotic and physical processes, both density—dependent and density—independent, control the early life history after settlement for the blue crab.

Forest Occurrence of Trichoderma Species: Emphasis on Potential Organochlorine (Xenobiotic) Degradation

Fungi of the genus Trichoderma are recognized as "keystone or controller" organisms in forest soils due to their involvement in decomposition and nutrient cycling and their regulation of associated mycoflora. Recent evidence has identified additional potential importance in the decomposition of xenobiotics. Both T. harzianum and T. viride have been shown capable of organochlorine degradation in vitro. During the 1990 growing season soil immersion tubes were employed to sample Trichoderma species growing in the forest floors of the Hubbard Brook Experimental Forest and of near-by Mount Moosilauke in New Hampshire. At Hubbard Brook, sample sites included a mature forest (100 years old) and a regenerating forest (5 years old). On Mount Moosilauke, sample sites included haplorthod and cryofolist soil types and low, medium, and high elevation. T. harzianum was isolated from both sites in relatively high amount. T. viride was also present at both sites but in relatively low amount. T. polysporum was consistently isolated at all elevations but in relatively low amount. The abundance of T. harzianum decreased with increasing elevation, while the abundance of T. viride increased with increasing elevation. The evidence presented is consistent with other studies stressing the high spatial variability of members of this genus. Causes of this variability are presumed to be numerous and involve abiotic and biotic forces. Persistence of organochlorine contaminants in northern forest soils may be influenced by the abundance of fungal degraders such as T. harzianum and viride.

Patterns in the Evolution of Physiological Specialization in Salt-Marsh Animals

Patterns of tolerance to stressful abiotic variables among animals that have colonized salt marshes provide models for the study of the evolution of physiological specialization. We provide an overview of some closely related pairs or groups of species that show hierarchical gradations in tolerance, and thereby physiological mechanisms of compensation, along a line perpendicular to the shore. However, a full understanding of the evolution of physiological specialization requires that we evaluate the role of abiotic factors in light of the biotic interactions in which salt-marsh species are embedded. The common occurrence of salt-marsh endemics in a species-poor environment must be related to the unusual character of this habitat, in which the reduced importance of some biotic forces is paired with highly stressful abiotic factors. The salt marsh is probably not a refuge for competitively inferior forms so much as it is a highly selective environment that admits only those specialized species that can muster the necessary physiological adaptations. Thus the apparently lower competitive ability of salt-marsh species under conditions that are not abiotically stressful is likely to be a secondary consequence of the costs of mechanisms conferring tolerance, and of life in a species-poor habitat. Salt marshes provide an arena in which it is possible to experimentally evaluate the evolutionary and ecological importance of abiotic stress, especially in relation to parapatric or allotopic distributions. Yet very few studies have provided such a rigorous approach, despite a long-standing recognition that abiotic factors interact strongly with biotic forces in the salt marsh.

Sand dune stabilisation: an investigation in the Thar Desert of India

The morphometry of bushes plays an important role in the obstruction of sand, and thereby influences the size of the dunes formed around the bushes. Capparis decidua was found to be the best species for shelter belts to check the movement of sand. Dune stabilisation is initiated by physical forces and the process is promoted by biotic forces. Clay particles and organic carbon, coupled with iron and other trace elements, bring stability to the sand dunes in the Thar Desert. Vegetation cover is essential for further stabilisation.

Tropical Grasslands of Sri Lanka and India

A general survey is made of the comparative able for the development of woody vegetation. The man- ecology, structure, dynamics and productivity of grasslands made grasslands are the result of clearing of forest for of Asia with emphasis on those of Sri Lanka and India. human settlement and shifting cultivation, but also of Nearly 20% of the land-surface of Asia and more than deliberate fires. 10%. of that of Sri Lanka and India are under grasslands, The Sri Lankan grasslands are in a constant state of flux which include both lowland and montane communities. owing to the seasonality of climate, mainly the rainfall, They are the product of successional progression on aban- human interference, and invasion by graminaceous weeds, doned cultivated land or forest clearings due to human particularly Pennisetum polystachyon. The water-stress activities and/or natural forces such as fire, landslide, accentuated by prolonged droughts, nutrient deficiencies, alternate flooding and wild animals; most of them occupy a mainly that of nitrogen and phosphorus, aggravated by tension-zone of biotic and anthropogenic forces working erosion and biotic interference are the major forces which on potential forest lands. prevent the development of woody vegetation so that the In Sri Lanka, the majority of grasslands occur in the dry succession culminates and is maintained at the grassland- zone with an annual rainfall of less than 1675 mm and a dry stage. period of 2-5 months during May-September. They are

Recent lake acidification and cladoceran dynamics: surface sediment and core analyses from lakes in Norway, Scotland and Sweden

To interpret the remains of Cladocera in lake sediments in relation to pH history, fish abundance, vegetation change, trophic level change and other historic events, it is necessary to understand the balance of abiotic and biotic forces responsible for their present distribution, population dynamics and morphological types. Once these factors are understood, we can reverse the arguments to infer past lake conditions. Many cladoceran species are influenced by fish and invertebrate predation. Some species, especially in the plankton, also show a clear physiological relation to pH and aluminium levels in lakes. Moreover, several littoral-benthic species have a habitat distribution restricted to rock, sand, mud, vegetation, or a combination of some of these. Remains of littoral and planktonic cladocerans were analysed in surface sediments of 18 Norwegian lakes with pH ranging from 4.5-7.5. In addition, sediment cores from four sites in Norway, four in Scotland and one in Sweden were analysed. The majority of sites showed evidence of recent acidification. In lakes with non- planktivorous fish, analyses of cladoceran remains gave no information on past fish populations, but indicated the pH history of the lakes. In lakes with present or past populations of planktivorous fish, the cladoceran record could be used to assess past fish status as well as past pH. In some lakes changes in the cladoceran communities could be related to changes in macrophyte distribution.

Guidelines and limitations to cladoceran paleoecological interpretations

In order to interpret the remains of Cladocera in lake sediments we have to understand the balance of abiotic and biotic forces responsible for their distribution and abundance in present-day lakes. Once these factors are understood we can reverse the arguments to infer past lake environs from cladoceran remains in the sediments. Cladocera have physiological limits, and the physiochemical boundaries set by these limits establish the abiotic habitat in which biotic factors (predation, competition) operate. It is difficult to establish which factors are responsible for community structure in present-day communities. Consequently, a thoughtful and careful analysis of all available evidence is necessary to interpret paleoassemblages. Paleoecologists are faced with conceptual problems we have termed the “cosmic quadrat.“ The cosmic quadrat blends ecological levels of integration, temporal, and spatial dimensions that put logical constraints on our interpretations.

Geographic Size and Non-Size Variation in Rocky Mountain Tamiasciurus hudsonicus: Significance in Relation to Allen's Rule and Vicariant Biogeography

Multivariate analysis of cranial characters of Rocky Mountain populations of the red squirrel, Tamiasciurus hudsonicus , was employed in an evaluation of geographic variation. Principal components analysis yielded a relatively weak size component which was not significantly correlated with environmental variables. Homogeneous morphology of conifer cones in this area may not provide the strong pressure for size variation suggested in a similar analysis of T. douglasii . A relatively strong non-size component was not significantly correlated with indices of conifer cone morphology, was moderately correlated with temperature, and was highly correlated with latitude and humidity. Correlation of latitude with weather variables indicated the lack of an environment favorable for size variation according to Bergmann's ecogeographic rule but highly favorable to proportional variation according to Allen's rule. The vicariant biogeography of the Southwest is proposed as a means of enhancing the selective capabilities expected under Allen's rule. From this analysis it appears that, with the lack of strong biotic selective forces in the area, squirrels are being more heavily affected by abiotic pressures.

Flucutations in Biotic Communities I. Prairie‐Forest Ecotone of Central Illinois

The ecotone between deciduous forest and prairie is a tension zone which, by virtue of biotic forces, growth forms, and mores characters, results in a very unstable condition with dynamic annual, seasonal, and diurnal fluctuations in the biota. Although of composite character, the emergent result is, in some degree, a totally new community characterized by a different biota, or a somewhat changed or modified mores on the part of the constituent biota usually considered characteristic of either or both of the adjoining biomes (Bird, '30, p. 362). The entire tension area may be termed ecotone of the first order to distinguish it from the ecotone of the second order which is composed of forest edge thickets of individual forested plots in the larger tension zone. Plant studies in areas of this type have been made in various areas by Weaver and Himmel ('17), Pool, Weaver, and Jean ('18), Vestal ('18), Hanson ('22), and Clements and Weaver ('24). Biotic study of such areas has remained untouched. As has been pointed out by Shelford ('13) and Weese ('24) this area has its own fauna which ranges into the adjacent grassland and forest comnmunities; certain plants likewise have an overlapping zone of distribution in the second order ecotone. There is an environmental gradient within which, for the animals at least, exist a variety of micro-climates. Hence the entire biota is essentially a composite of forms of adjoining communities, but also includes indigeneous forms which often form an important part of the perennial nucleus. The area studied was in the ecotone parkland and savannah area of Illinois. Patches of woods and outlying thickets in the area originally occupied by the river forest of the Salt Fork of the Vermillion river in the vicinity of Urbana, Champaign County, Illinois, were selected as being representative relics of former wide spread communities. The woodlands selected are known locally as the Brownfield Woods (sec. 1, T 19 N, R 9 E) and the University Woods (sec. 34, T 20 N, R 9 E) and previously have been studied by McDougall ('22, '25, and '28), Weese ('24), Telford ('26), Blake ('27 and '31), and Smith-Davidson ('28, '30, and '32). Prairie communities nearby have been studied by Shackleford ('29) and Rice ('31). The forest border of thicket and brush in its original undisturbed condition occupied a width of approximately .004 miles. In northern and central Illinois this probably totaled one per cent of the total area.