Predicting the Viability of Fish Populations in a Modified Riverine Environment

Riverine fishes evolved to life in a highly variable, flow-driven environment. During the two past centuries, large rivers have been substantially altered by human activities. This has resulted in declines of fish populations that depend on the large river environment. The research described here uses models to evaluate the effects of human activities on the viability of fish populations in rivers. I focused on five modifications of the river environment associated with impoundment: (1) seasonal allocation of river flow; (2) diversion of river flow; (3) fragmentation of the river habitat by dams; (4) conversion of free-flowing river to reservoir habitat; and (5) alteration of migration patterns. To understand the role of flow regulation on chinook salmon (Oncorhynchus tshawytscha) recruitment, I developed an individual-based model to predict recruitment as a function of seasonal flow patterns in the Tuolumne River, California. I used simulated annealing to find flow patterns that maximize chinook recruitment under wet and dry hydrologic conditions. As water availability increased, I found that the optimal flow pattern shifted from allocating low flows uniformly across seasons to a pattern with high spring flows. When I considered a new objective: maximizing the variance of spawning times among recruits, the optimal flow regime called for a winter pulse in flow just before the peak spawning date for the minority (late-fall) run. To evaluate the recovery options for chinook salmon in the Tuolumne River, Ideveloped an age-based model to conduct a population viability analysis (PVA). I developed a flow-dependent spawner-recruitment relationship from the recruitment model. Its shape depended on the flow regime, suggesting that such relationships are not fixed properties of species, but depend on environmental conditions. The PVA model suggested that recovery, in the absence of straying, would be enhanced most by significantly reducing ocean harvest, followed by reduced diversion of water from the river. For white sturgeon (Acipenser transmontanus) populations in the Snake River, Idaho a main concern is habitat fragmentation by dams resulting in smaller, isolated populations. Simulation experiments to evaluate the effects of fragmentation suggested that population viability was higher when dams were spaced widely enough apart to retain free-flowing habitat. A simulation experiment to evaluate the effects of altered migration patterns associated with impoundment showed that both the likelihood of persistence and the genetic diversity among white sturgeon populations were enhanced by balanced upstream and downstream migration rates. Models that simulate the responses of fish populations to modified river habitat do not consider the potential for an evolutionary response. I designed a PVA model simulating the genetic basis of age at maturity for individual fish. Simulated individual variation in this trait lead to increased population viability only when the variation was heritable and subjected to an altered selective regime. The results support the idea that predicting population viability depends on estimating the potential for evolution in fitness-related traits for populations exposed to anthropogenic changes in the environment that impose strong, directional selective forces

Mathematical modelling for red squirrel conservation

In this thesis we develop mathematical models to understand the impact of forest management and pine marten predation on red and grey squirrel disease-mediated competitive dynamics. Our work extends a well-tested, spatially-explicit, stochastic model framework to assess the impact of forest management plans undertaken in designated red squirrel stronghold forests in Scotland. We determine the impact of the forest management on the population viability of red squirrels, when threatened by grey squirrel invasion. The model framework is further extended to identify natural strongholds - forest sites that act as red squirrel strongholds without the need for specific forest composition plans or grey squirrel control. The results have direct implications for forest management practice and will inform an upcoming review of red squirrel conservation policy on the future of designated strongholds. The stochastic model framework was modified to include pine marten dynamics in order to assess the impact of pine marten predation on red and grey squirrel interactions. Over the last few decades pine marten have seen a recovery in their density and range in Scotland and Ireland, and this has coincided with a reduction in grey squirrel density and the recovery of red squirrels. We use the model to assess the impact of pine marten trans-locations into North Wales, with particular attention paid to the red squirrels resident on Anglesey and in Clocaenog (a forest site in North Wales). The thesis also develops a deterministic model framework to understand how a temporal refuge from predation can mediate the interactions of competing prey species. This deterministic framework is extended to consider the red and grey squirrel and pine marten system, where a temporal refuge can occur when pine marten predation is focussed on the peak phase of vole density cycles. The refuge from predation has the potential to reverse the beneficial advantage of pine marten for red squirrels, and provides a necessary note of caution for the role of pine marten as grey squirrel control agents. The work in this thesis highlights how important mathematical models are in understanding the potential impact of conservation policy on focal species in Scotland and Wales.UK Engineering and Physical Sciences Research Council (grant EP/L016508/01

Catastrophic vegetation dynamics and soil degradation in semi-arid grazing systems

When vegetation is drastically reduced as a result of drought or an increase in herbivore numbers, it does not simply recover if periods with normal rainfall follow or if herbivores are removed. These are commonly recognized catastrophic phenomena of semi-arid grazing systems in general and of the African Sahel in particular. The main aims of this thesis are to provide an effective explanation of the catastrophic properties of vegetation dynamics in these systems and to predict under which conditions they might be expected.We start with a description of Sahelian rangeland vegetation dynamics, to reveal its catastrophic properties. This exercise appeared a very useful first step in the growth of our ideas about catastrophic vegetation dynamics because: 1) it translated rather vague concepts into a verifiable format by deducing hypotheses about the conditions under which catastrophic vegetation dynamics might be expected, and 2) it generated the notion that soil degradation could somehow be an important factor attributing to catastrophic vegetation dynamics in semi-arid grazing systems. This is in contrast with models that emphasize herbivore feeding characteristics or plant competition as possible mechanisms underlying catastrophic vegetation dynamics. We tested the hypothesis that soil degradation, i.e. soil erosion by run-off and wind and the consequent loss of water and nutrients, is sufficient to explain catastrophic vegetation dynamics by mathematical modelling.Our model studies indeed show that soil degradation can effectively explain the catastrophic properties of semi-arid grazing systems. Soil degradation can cause a positive feedback between reduced resource (soil water and nutrients) availability and reduced vegetation biomass which may lead to collapse of the system. This positive feedback loop can be triggered by grazing. We argue on the basis of a large body of literature that this is an important mechanism causing catastrophic vegetation dynamics in semi-arid grazing systems. Furthermore, our model studies predict for which site-specific properties catastrophic vegetation dynamics may be expected, that is on loamy or clayey soils in case of water-limited vegetation biomass production, and on sandy soils in case of nutrient-limited biomass production. This is because sandy soils have higher water infiltration rates but are more vulnerable to nutrient loss through erosion than loamy or clayey soils.Based on our models, we hypothesized that the removal of aboveground herbaceous biomass would lead to a reduced soil water content and biomass production because of reduced water infiltration and increased run-off. We tested this hypothesis in a semi-arid savanna in Tanzania (East Africa). Indeed, as a consequence of biomass removal, a reduction in soil water content and biomass production occurred. But it appeared that increased loss of soil water through increased soil evaporation as a consequence of litter removal ultimately outbalanced all other effects on soil water content. Several factors might have contributed to the importance of increased soil evaporation, overriding that of reduced water infiltration and increased run-off. The soil in the research area was a sandy loam, with higher water infiltration rates than soils with a lower percentage sand and higher perentage clay, while rainfall primarily occurred in light showers. Thus, under these conditions, when the positive feedback between reduced water infiltration and reduced biomass does not operate, another positive feedback that is between increased soil evaporation and reduced biomass may become prominent.We further hypothesized that at a certain range of herbivore impact small initial differences in plant cover and amount of soil resources can magnify to alternative states which persist in time due to positive plant-soil feedbacks. We tested this hypothesis in a semi-arid grazing system in Burkina Faso (West Africa), where we studied vegetation patchiness along a gradient of herbivore impact. Indeed, the occurrence and likely persistence of a spatial pattern of vegetated patches alternating with bare soil at a certain range of herbivore impact could be explained by the positive plant-soil feedback between vegetation biomass and water infiltration.We stress the general applicability of our models by comparing catastrophic vegetation dynamics of the semi-arid grasslands of the African Sahel with the arctic salt marshes along the Hudson Bay in Canada. We argue that in both systems, an increase of herbivory triggered a catastrophic vegetation shift, which was ultimately caused by a positive plant-soil feedback, leading to desertification.One of our model assumptions was that herbivore density is not regulated by vegetation biomass. In the general discussion, I investigated the influence of a positive feedback between vegetation biomass and water infiltration on the dynamics of a plant-herbivore system, where herbivore density depends on vegetation biomass. As a consequence of the positive feedback and if herbivore reproduction is efficient, I predict that the plant-herbivore system could destabilize and collapse. In this chapter I also stress the practical relevance of our studies as our approach may finally lead to objective ecological criteria on which pastoral managers can base their decision how to evade the hazard of degradation of their rangelands.I highlight three topics which deserve more priority on the reseach agenda concerning semi-arid grazing systems in the near future. Hereby, I want to stress that it is important to put experimental and empirical studies into a clear theoretical framework, whereby mathematical modelling should play an important role. The three topics are:spatial heterogeneity and vegetation pattern formation,facilitation and competition between functional plant groups within the herbaceous layer andthe effects of positive plant-soil feedbacks on herbivore dynamics.</OL

Analysing effects of spatiotemporally distributed species interactions in Maculinea systems

We analyse effects of species-interaction and spatiotemporal host distribution on persistence of Maculinea populations at isolated habitat sites. Maculinea butterflies are parasites of Myrmica ants. The butterfly caterpillars infest host ant nest in the vicinity of their initial oviposition plant. Thus, spatial distribution of initial host plants (oviposition plants), implies a spatial distribution of parasitism. We develop a generic spatially-explicit rule-based simulation model for a Maculinea systems, to analyse the influence of different spatially relevant parameters on the performance of a Maculinea population. Parameter variation considers spatial host plant distribution, initial spatial host ant distribution and budding-range, which defines the probability distribution of colonising distances of ants. In simulation runs, it can be seen that the spatial distribution of host ants adapts to the spatially distributed parasitism from Maculinea caterpillars. Areas without host plants (unexploited areas) are completely inhabited by host ants. In areas with host plants (exploited areas) mean density of host ants is lower. Ant nests show dynamics. They are abandoned or nest sites are recolonised. Because of spatial segregation of exploitable and non-exploitable areas and by small scale dispersal of host ants (budding), areas of different effectiveness for Maculinea population performance can be distinguished on a Maculinea habitat site: These are reproductive area, buffer, sink and non-contributing area. Consequently, number of host plants is not the only decisive factor for performance of a Maculinea population. As well it is important how well empty nest sites in the vicinity of host plants can be colonised by host ants. We find two different mechanisms, which limit re-growth of the Maculinea butterfly population and lead to different types of dynamics. Population viability analysis (PVA) is a standard method for single species systems. However, the Maculinea system consists of interacting species with more complex dynamics. There is still no standardised methodology for PVA of multi-species systems. We derive a phenomenological description of distributions of extinction times for populations in a large class of systems of interacting species. We find in analytical calculations that the long term behaviour of distributions of times to extinction in multi-species systems can be analysed with the ln(1-P_{0})-method suggested for single species systems. Thus, this method can be transferred. Comparing distributions of times to extinction of single and interacting species, we find that long term extinction of established populations follows a similar process. However, on short time-scales, population cycles modulate extinction risk. In systems of interacting species, an initial transient phase can have strong influence on persistence of the population. In simulations of the Macu model, we observe that it takes up to 50 generations until the system reaches its established state. This can lead to a bottleneck of the population. We develop a landscape analysis method for multi-species systems. For that purpose, suitability of different landscapes is assessed for their ability to sustain particular populations. Suitability is measured by population persistence. Therefore, dynamics of the system and requirements of the populations are taken into account when assessing suitability. This method is applied to assess effects of the influence of different spatially relevant parameters. No major effects of initial host ant distribution on persistence of Maculinea populations can be found. In contrast, spatial distribution of host plants has a main effect. Thus, the host plant distribution can be considered as landscape structure of the Maculinea system on the site. Rules of thumb for suitable structures of host plant distributions and a spatial index are given. In general heterogeneous host plant distributions of intermediate density are a good option

Rangeland Systems: Processes, Management and Challenges

environmental management; environmental law; ecojustice; ecolog

South Carolina Wildlife, March-April 1998

The South Carolina Wildlife Magazines are published by the South Carolina Department of Natural Resources who are dedicated to educating citizens on the value, conservation, protection, and restoration of South Carolina's wildlife and natural resources. These magazines showcase the state’s natural resources and outdoor recreation opportunities by including articles and images of conservation, reflections and tales, field notes, recipes, and more. In this issue: Directions ; Events ; Forum ; Aliens Among Us ; Woodies By The Boxful ; Bed, Breakfast And The Great Outdoors ; A Forest For More Than Trees ; Nature To Go ; Rainbows And Rockfish ; For Wildlife Watchers: River Otter ; Field Trip: Forty-Acre Rock ; Roundtable