Changes In Community Properties Research Articles

Species–area relationships as indicators of human impacts on demersal fish communities

AbstractAimThe relationship between number of species and area is a well‐known macro‐ecological property that can be used as an indicator of human impact on natural communities. This relationship can be described by species–area relationships. We explored the application of species–area relationships to data sets of bottom trawl surveys to determine their ability to capture fishing‐induced changes in demersal fish communities.LocationAustralia.MethodsWe performed numerical simulations to investigate whether species–area relationship was sensitive to changes in community properties, such as richness, species abundances and spatial distribution. Then, we analysed a data set of bottom trawl surveys carried out in south‐east Australia spanning 20 years. These data provided information on demersal fish communities at different stages of fishing exploitation. We built species–area relationships along spatial and temporal gradients of community exploitation and used the rate of species accumulation to characterize community structure. Linear mixed effects (LME) models were used to quantify the effect of fishing exploitation on species accumulation rates, while controlling for environmental and sampling factors.ResultsNumerical simulations showed that species–area relationships can capture changes in community properties. Observed species accumulation rate decreased with increasing exploitation and was sensitive to environmental variation along a latitudinal and depth gradient and to changes in sampling.Main conclusionsSpecies–area relationships applied to trawl survey data can capture fishing‐induced changes in the structure of demersal fish communities. We propose that species–area relationships can be used to inform on the status of demersal fish communities and aid community monitoring. If applied to data sets of bottom trawl surveys available world‐wide, species–area relationships could be used to rank demersal fish communities for their level of exploitation.

Consequences of changes in thermal regime for plankton diversity and trait composition in a polymictic lake: a matter of temporal scale

Summary 1. Changes in plankton species diversity and community structure as a result of global warming are of growing concern in ecological studies, as these properties contribute substantially to key ecosystem processes. 2. We analysed the effect of short-term temperature rise and changes in the thermal regime during summer on plankton diversity of the eutrophic and polymictic Muggelsee in Germany, from 26 years of summer records (1982–2007). We tested for changes in community properties, such as species richness, evenness and population size of phyto- and zooplankton, during alternating periods of thermal stratification and mixing, which were between 2 and 8 weeks long. Moreover, we tested for overall long-term temporal trends in annual averages of the community properties during stratified and mixed events. 3. We found that the overall number of stratification events increased significantly across the study period. When the lake was stratified, consistently higher surface water temperatures and lower epilimnetic nutrient concentrations were found. As the length of thermal stratification increased, the phytoplankton shifted towards a higher proportion of buoyant cyanobacteria capable of N-fixation (Aphanizomenon, Anabaena). Diatoms were at a disadvantage because of high temperature, exceeding their upper lethal limit and sedimentation losses. Zooplankton species with high thermal tolerances (i.e. Thermocyclops oithonoides, Thermocyclops crassus) and/or those that grow quickly at high temperatures (i.e. rotifers) became more common. 4. During periods of continuous mixing, the community remained largely unchanged, except for some minor increase in the biomass of diatoms. 5. While a noticeable shift towards N-fixing cyanobacteria was observed with increasing length of stratified events, and rotifers and copepods became the main predators, there were minimal changes in diversity, except for an increase in cyclopoid copepods and a decrease in diatom diversity. As for cyanobacteria, the net short-term effect on their diversity was neutral as a result of species replacements. In the long term, however, the diversity of cyanobacteria and cladocerans declined while that of rotifers increased. 6. Overall, our study presents a cautionary example of how we might fail to foresee the impact of climate-induced changes on ecosystem processes if we restrict our studies to seasonal or yearly temporal scales, thus neglecting the impact of substantial changes operating at smaller temporal scales.

Interactions between environmental variability and immigration rate control patterns of species diversity

Theories focussing on local competition processes predict that species diversity is maximised with high-frequency environmental variability or alternatively with intermediate frequencies. The models coupling regional processes to local explanations of diversity patterns predict that the immigration rate from a regional species pool can strongly increase diversity. However, the interaction between local and regional processes in temporally fluctuating environment has received little attention. We explored in a simulation study how the patterns of species diversity are affected by the frequency spectrum of stochastic environmental variations and density independent immigration rate in a competition model. Environmental variations were assumed to affect the growth rate of competing species. The growth rate of a species had a unimodal response to a continuous environmental variable, such as salinity and temperature. The location of optimum and breadth of the growth response in the environmental niche axis is treated as a species-dependent trait. Consequently, environmental variations can affect competitive ranking of the species. Decreasing the frequency of environmental variations produced monotonically decreasing or unimodal “intermediate disturbance type” diversity patterns in the competition model. The level of diversity and type of the pattern was determined by immigration rate, and the number and traits of the species initially placed along the environmental niche axis. Immigration generally increased diversity, but the amount of increase in intermediate immigration rates was strongly modified by the frequency spectrum of the environmental variation. Decreasing the frequency of the environmental variation always produced a monotonically decreasing diversity pattern in a two-species system where the growth optima are positioned symmetrically around the mode of the environmental noise. However, increasing the species number and asymmetry of optima makes the diversity pattern sensitive to immigration rate: diversity can be very low and unaffected by the frequency of environmental variation in the absence of immigration, peak in low-frequency environment with small immigration rate (i.e. produce the IDH type pattern), or decrease monotonically with high immigration rate. Diversity is most sensitive to small changes in the initial species number and position of the species growth optima when environmental fluctuations have high frequency. That immigration can change the impact of the frequency of the environmental variation on diversity from monotonically decreasing to unimodal (IDH type) pattern emphasizes that the regional process can be important for explanations of the diversity patterns, especially when the immigration rates are uncorrelated to the local conditions. The strong interactions between community properties, immigration rate, and the spectrum of environmental variation suggest that small changes in community properties or in dispersal rates can have a large impact on species diversity.

Changes in community properties during microbial succession

Succession is one of the oldest concepts in community ecology and various theoretical and empirical studies have addressed succession and it's underlying mechanisms, particularly in plant communities (Odum 1969, Connell and Slatyer 1977, Finegan 1984). The development of algal periphyton on submerged rocks in streams has also been viewed as an example of succession in autotrophic communities (Stevenson 1983, Johnson et al. 1997). More recently, the formation of bacterial communities on submerged surfaces (biofilms) has been viewed in the context of primary ecological succession (Jackson et al. 2001), and studies have used molecular techniques to examine the changes in bacterial community structure that occur during biofilm development (Santegoeds et al. 1998, Jackson et al. 2001). Briefly, when a fresh surface (glass slides, rocks etc.) is submerged in an aquatic habitat it is rapidly colonized by bacterial populations, some of which will grow and persist as part of the attached biofilm community. These initial heterotrophic populations are dependent upon the availability of dissolved organic carbon (DOC) from the overlying water, i.e. it is an open system with little internal cycling of nutrients or carbon. Over time, new populations of bacteria arrive through immigration from the water column, whereas other populations are lost from the community, either through cell death and losses due to sloughing, or because conditions in the biofilm change (largely because of microbial activity and growth) and those bacteria are no longer adapted to biofilm life. The latter case recalls the facilitation model of Connell and Slatyer (1977) in that early colonizing bacteria change the environmental conditions in the biofilm, facilitating the later dominance of different populations (indeed, those authors suggested that the facilitation mechanism may be particularly applicable when organisms are colonizing a new substrate, i.e. during primary succession, which is likely to be the case during biofilm formation). In the later stages of biofilm development, populations of heterotrophic bacteria are more dependent upon autotrophic production occurring within the biofilm (such as from algae or cyanobacteria), i.e. the community becomes more of a closed system. Based on the results of a recent study of epilithic biofilm development, Jackson et al. (2001) suggested certain conceptual changes that might occur in community properties during bacterial biofilm succession (Fig. 1). Initial colonization of a fresh surface leads to a rapid increase in species richness (S), which subsequently declines as some of the colonizing populations are less competitive and do not survive in the biofilm community. However, competition becomes less important in structuring the bacterial assemblage as the biofilm ages and new resources and habitats become available (such as through the accumulation of metabolic waste products that can serve as growth substrates for other organisms, the emergence of anoxic pockets permitting the growth of anaerobic bacteria, and the development of a three dimensional architecture allowing greater area for attachment and growth). Thus, S might increase again as the biofilm community matures, and community structure is driven by resource diversity rather than by competition. This model can however be extended, and a number of changes made to more accurately explain community changes. Firstly, the term resource diversity is vague. Originally the increase in resource diversity throughout community development was intended to convey both the appearance of new substrates for microbial growth, and a switch from a two dimensional community to a three dimensional one (so an increase in both habitat space and variety). An alternative term would be the number of ecological niches, and what was originally described as an increase in resource diversity can be more succinctly thought of as an increase in the amount of niche space (R, following MacArthurs' (1972) idea of a resource spectrum, and assuming that resource spectrum length is equivalent to niche space). Thus, as the biofilm develops, the increased heterogeneity of the community facilitates a continuous increase in the potential number