Disturbance and recovery of litter fauna: a contribution to environmental conservation

Disturbances play a great role in ecosystem functioning and, with the increasing anthropogenic activities, they have more and more influence on ecosystems. They have been studied for several decades but recovery, the ecological phenomenon following a disturbance, has seldom been the focus of research. In this thesis, I studied the impact of disturbances on the structure of soil and litter fauna communities and their ensuing recovery in varying environmental conditions, combined with the effect of productivity, life-history traits and community structure. I combined all the results to draw some conclusions on the main factors involved in recovery, how to improve recovery of ecosystems and how to make better predictions on recovery. In the second chapter of this thesis, I studied how the structure of soil and litter fauna communities from two climatically contrasting biomes was affected by a similar disturbance and how these communities recovered. I sampled litter macrofauna in a temperate and a boreal forest and, to be able to determine whether the communities had recovered, I created a “recovery index” that took into account the pre- and post-disturbance conditions of the disturbed and the control communities, taking into account natural variations. I hypothesised that the temperate communities would recover more rapidly due to the warmer temperatures and to higher species richness and abundance. Recovery was as fast in both biomes, which also had similar species richness. Contrary to my assumption, higher pre-disturbance species abundance did not favour the resistance of communities to the disturbance; on the contrary, high-abundance communities suffered a proportionally greater loss than other communities. Analyses based on life-history traits revealed that dispersal capabilities were the most relevant traits for species facing a disturbance and also for re-establishing. An unexpected factor that influenced the outcome of the disturbance was the litter layer, which, thick in the temperate forest and almost inexistent in the boreal one, protected the fauna of this former biome. The aim of my third chapter was to consider the main two theories of species assembly, the niche and neutral theory, in the context of a recovery. I considered these two theories not as mutually exclusive but as if they were at opposite ends of a stochasticity gradient. The neutral theory predicts recovered communities in a similar environment to be dissimilar from one another and the niche theory predicts the opposite, because, in this case, species assembly is driven by deterministic factors inherent to communities and to the environment. I used the same experiments as in the previous chapters and hypothesised that the more constraining environmental conditions of the boreal forest would lead to a species assembly rather driven by deterministic factors, with recovered communities more similar to each other than the temperate ones. These latter ones, from a less constraining environment, would be more dissimilar to each other. I found that the structure of each community before and one year after the disturbance was indeed more similar in the boreal forest. This would mean that, in low-productivity environments, the response of communities being less variable, it could be more easily predictable. In Chapter 4, I studied the structure of communities from a different perspective, using the density – body mass (DBM) relationship to detect changes in the structure of communities after a disturbance. I hypothesised that the slope of the relationship would be less steep if smaller organisms were mainly impacted or that it would be steeper if larger organisms were mainly impacted. By collecting the soil and litter fauna before, just after and again one and two months after a disturbance, I could establish that the DBM relationship reflected the changes of the structure of communities responding to modifications of the environment. In disturbed conditions, the slope of the DBM relationship of a community was less steep, because mainly the small organisms were impacted by the disturbance. I also showed that, at the very early stage of the recovery, the slope was even less steep, because of the large body mass of the first colonisers. This study confirmed the necessity to sample a broad spectrum of body masses and it was the first time that the DBM relationship was shown to be able to reflect changes of the structure of communities. I concluded by suggesting that it could be used for environmental biomonitoring. After the satisfying results of Chapter 4, I decided to test the ability of the DBM relationship to reflect different structures of communities living in environments varying by their productivity and subjected or not to a disturbance. I hypothesised that communities from low-productivity areas would have a less steep slope than high-productivity area communities and that disturbed communities would also have a less steep slope. To test this, soil and litter fauna were collected from a salt marsh at four elevation levels (hence subjected to varying sea inundation frequencies, from daily to annually), half of which were subjected to cattle grazing (i.e., the disturbance). I assumed that the least inundated sites were more productive and used the quantity of plant litter to confirm this. The only significant result was between the daily and annually inundated ungrazed areas, confirming that communities from high-productivity areas have a steeper DBM relationship slope. High productivity does not seem to equally affect all the trophic levels, certainly due to inefficient transfers of energy from one level to the other. In the synthesis, I suggested that recovery should first be properly defined to establish when a community has reached that stage and I advise to use pre- and post-disturbance states of control communities for that purpose. Besides, several environmental factors have to be taken into account instead of only focusing on one species or one ecosystem service, as I have showed that the species richness and abundance of communities, and the productivity and heterogeneity of the environment can influence the resistance and recovery of ecosystems. I also propose, in a first time, to study species assembly in constraining environments, where stochastic factors are limited, in order to obtain a better mechanistic understanding of the processes involved. As there is yet not such understanding, I suggest that managers in charge of environmental conservation rather use a phenomenological approach to quickly estimate outcomes of recovery.</p

Impact of anthropogenic disturbances on beetle communities of French Mediterranean coastal dunes

In coastal dunes, influenced by anthropogenic activities such as tourism, it is important to determine the relative influence of environmental factors at different spatial scales to evaluate the sensitivity of local communities to disturbances. We analyzed beetle communities of 14 dunes of the French Mediterranean coast: four in the relatively preserved Camargue area, and ten in the Var department, where tourism is intensive. Beetle communities were studied three times in early spring using sand sampling. Species-environment relationships were evaluated at the regional, landscape and local scale using redundancy analysis (RDA) and variability partitioning. About 28 species were identified, of which 15 were sand-specialist species, which accounted for more than 93% of total abundance. The beetle communities of Camargue were significantly different from those of the Var department owing to the pullulation of a Tenebrionid species (Trachyscelis aphodioides Latr.) in the Var, except for one restored dune where the community was very similar to those of Camargue. Our results showed no longitudinal gradient between the two regions. Local factors (dune height, preservation and disturbance index) significantly explained most of the variation in the dominance of T. aphodioides, while some other local factors were important for other psammophilous species. This study also suggests that dune beetle communities are strongly affected on beaches intensively managed for tourism, but beetles are still abundant in much disturbed sites

Disturbance and recovery of litter fauna: a contribution to environmental conservation

Disturbances play a great role in ecosystem functioning and, with the increasing anthropogenic activities, they have more and more influence on ecosystems. They have been studied for several decades but recovery, the ecological phenomenon following a disturbance, has seldom been the focus of research. In this thesis, I studied the impact of disturbances on the structure of soil and litter fauna communities and their ensuing recovery in varying environmental conditions, combined with the effect of productivity, life-history traits and community structure. I combined all the results to draw some conclusions on the main factors involved in recovery, how to improve recovery of ecosystems and how to make better predictions on recovery. In the second chapter of this thesis, I studied how the structure of soil and litter fauna communities from two climatically contrasting biomes was affected by a similar disturbance and how these communities recovered. I sampled litter macrofauna in a temperate and a boreal forest and, to be able to determine whether the communities had recovered, I created a “recovery index” that took into account the pre- and post-disturbance conditions of the disturbed and the control communities, taking into account natural variations. I hypothesised that the temperate communities would recover more rapidly due to the warmer temperatures and to higher species richness and abundance. Recovery was as fast in both biomes, which also had similar species richness. Contrary to my assumption, higher pre-disturbance species abundance did not favour the resistance of communities to the disturbance; on the contrary, high-abundance communities suffered a proportionally greater loss than other communities. Analyses based on life-history traits revealed that dispersal capabilities were the most relevant traits for species facing a disturbance and also for re-establishing. An unexpected factor that influenced the outcome of the disturbance was the litter layer, which, thick in the temperate forest and almost inexistent in the boreal one, protected the fauna of this former biome. The aim of my third chapter was to consider the main two theories of species assembly, the niche and neutral theory, in the context of a recovery. I considered these two theories not as mutually exclusive but as if they were at opposite ends of a stochasticity gradient. The neutral theory predicts recovered communities in a similar environment to be dissimilar from one another and the niche theory predicts the opposite, because, in this case, species assembly is driven by deterministic factors inherent to communities and to the environment. I used the same experiments as in the previous chapters and hypothesised that the more constraining environmental conditions of the boreal forest would lead to a species assembly rather driven by deterministic factors, with recovered communities more similar to each other than the temperate ones. These latter ones, from a less constraining environment, would be more dissimilar to each other. I found that the structure of each community before and one year after the disturbance was indeed more similar in the boreal forest. This would mean that, in low-productivity environments, the response of communities being less variable, it could be more easily predictable. In Chapter 4, I studied the structure of communities from a different perspective, using the density – body mass (DBM) relationship to detect changes in the structure of communities after a disturbance. I hypothesised that the slope of the relationship would be less steep if smaller organisms were mainly impacted or that it would be steeper if larger organisms were mainly impacted. By collecting the soil and litter fauna before, just after and again one and two months after a disturbance, I could establish that the DBM relationship reflected the changes of the structure of communities responding to modifications of the environment. In disturbed conditions, the slope of the DBM relationship of a community was less steep, because mainly the small organisms were impacted by the disturbance. I also showed that, at the very early stage of the recovery, the slope was even less steep, because of the large body mass of the first colonisers. This study confirmed the necessity to sample a broad spectrum of body masses and it was the first time that the DBM relationship was shown to be able to reflect changes of the structure of communities. I concluded by suggesting that it could be used for environmental biomonitoring. After the satisfying results of Chapter 4, I decided to test the ability of the DBM relationship to reflect different structures of communities living in environments varying by their productivity and subjected or not to a disturbance. I hypothesised that communities from low-productivity areas would have a less steep slope than high-productivity area communities and that disturbed communities would also have a less steep slope. To test this, soil and litter fauna were collected from a salt marsh at four elevation levels (hence subjected to varying sea inundation frequencies, from daily to annually), half of which were subjected to cattle grazing (i.e., the disturbance). I assumed that the least inundated sites were more productive and used the quantity of plant litter to confirm this. The only significant result was between the daily and annually inundated ungrazed areas, confirming that communities from high-productivity areas have a steeper DBM relationship slope. High productivity does not seem to equally affect all the trophic levels, certainly due to inefficient transfers of energy from one level to the other. In the synthesis, I suggested that recovery should first be properly defined to establish when a community has reached that stage and I advise to use pre- and post-disturbance states of control communities for that purpose. Besides, several environmental factors have to be taken into account instead of only focusing on one species or one ecosystem service, as I have showed that the species richness and abundance of communities, and the productivity and heterogeneity of the environment can influence the resistance and recovery of ecosystems. I also propose, in a first time, to study species assembly in constraining environments, where stochastic factors are limited, in order to obtain a better mechanistic understanding of the processes involved. As there is yet not such understanding, I suggest that managers in charge of environmental conservation rather use a phenomenological approach to quickly estimate outcomes of recovery

Productivity affects the density-body mass relationship of soil fauna communities

The productivity of ecosystems and their disturbance regime affect the structure of animal communities. However, it is not clear which trophic levels benefit the most from higher productivity or are the most impacted by disturbance. The density-body mass (DBM) relationship has been shown to reflect changes in the structure of communities subjected to environmental modifications, so far, mainly in aquatic systems. We tested how different seawater inundation frequencies and cattle grazing, which both disturbed and impacted the productivity of a terrestrial system, a salt marsh, affected the size structure of soil fauna communities, expressed by their DBM relationship. We hypothesized that either: (1) all the trophic levels of soil fauna would benefit from higher productivity (i.e., amount of litter mass), reflected by a higher Y-intercept of the DBM relationship; (2) only smaller animals would benefit, reflected by a lower slope of the relationship; (3) or only larger animals would benefit, reflected by a higher slope of the relationship. We collected a large range of soil fauna from different elevation levels in grazed and ungrazed areas, thence subjected to different levels of productivity, represented by litter mass, with the most inundated and grazed area as the least productive one. Considering that pore size must be smaller in inundated and grazed areas, productivity seemed to be a greater factor influencing species distribution than soil structure. We found slopes lower than 0.75, showing that large animals dominated the community. However, a difference between the DBM relationships of the most and least frequently inundated ungrazed sites indicated that higher productivity benefited the smaller animals. Our findings show that high productivity does not equally affect the different trophic levels of this soil fauna community, suggesting inefficient transfers of energy from one trophic level to another, as smaller species benefitted more from higher productivity

Responses of common buzzard (Buteo buteo) and Eurasian kestrel (Falco tinnunculus) to land use changes in agricultural landscapes of Western France

In front of land use changes, there has been a wide decline in biodiversity. In this study, we analysed the numerical response of two diurnal raptor species, the common buzzard and the Eurasian kestrel to different agricultural landscape contexts. We carried out a 3-year survey of the abundance of these two species in three agricultural landscape contexts that are known to form a gradient of land use intensity and hedgerow network fragmentation. We also considered the variation in prey abundance related to these land use changes. The two raptor species showed negative but differentiated response patterns to land use intensification and small mammal abundance. The abundance of the common buzzard decreased significantly with the reduction of hedgerows, woodlots and grasslands areas, as well as with the decrease in prey abundance at the landscape scale. The abundance of the Eurasian kestrel showed the same trend, but fall of abundance was less marked and not significant. This resulted in a reverse ratio of species density according to agricultural intensification and landscape openness. Specific feeding habits, nesting habitats and spatial partitioning in foraging areas could explain the difference in the numerical response observed for these two raptor species. Buzzards forage mainly on highly profitable prey areas and avoid highly fragmented woodland habitats for nesting. Kestrels seem less dependent on semi-natural habitats, are able to nest in more fragmented landscapes and are better adapted to exploit less abundant but more widespread small mammal prey species within the cultivated matrix. The study may help to evaluate carrying capacities of agricultural landscapes in accordance with ecological life traits of these two raptor specie