Many species are adapted to disturbance and occur within dynamic, mosaic landscapes that contain ... more Many species are adapted to disturbance and occur within dynamic, mosaic landscapes that contain early and late successional microhabitats. Human modification of disturbance regimes alters the availability of microhabitats and may affect the viability of species in these ecosystems. Because restoring historical disturbance regimes is typically expensive and requires action at large spatial scales, such restoration projects must be justified by linking the persistence of species with successional microhabitats. Coastal sand dune ecosystems worldwide are characterized by their endemic biodiversity and frequent disturbance. Dune-stabilizing invasive plants alter successional dynamics and may threaten species in these ecosystems. We examined the distribution and population dynamics of two federally endangered plant species, the annual Layia carnosa and the perennial Lupinus tidestromii, within a dune ecosystem in northern California, USA. We parameterized a matrix population model for L. tidestromii and examined the magnitude by which the successional stage of the habitat (early or late) influenced population dynamics. Both species had higher frequencies and L. tidestromii had higher frequency of seedlings in early successional habitats. Lupinus tidestromii plants in early successional microhabitats had higher projected rates of population growth than those associated with stabilized, late successional habitats, due primarily to higher rates of recruitment in early successional microhabitats. These results support the idea that restoration of disturbance is critical in historically dynamic landscapes. Our results suggest that large-scale restorations are necessary to allow persistence of the endemic plant species that characterize these ecosystems.
Using historic data sets, we quantified the degree to which global change over 120 years disrupte... more Using historic data sets, we quantified the degree to which global change over 120 years disrupted plant-pollinator interactions in a temperate forest understory community in Illinois, USA. We found degradation of interaction network structure and function and extirpation of 50% of bee species. Network changes can be attributed to shifts in forb and bee phenologies resulting in temporal mismatches, nonrandom species extinctions, and loss of spatial co-occurrences between extant species in modified landscapes. Quantity and quality of pollination services have declined through time. The historic network showed flexibility in response to disturbance; however, our data suggest that networks will be less resilient to future changes.
A new study of plants recolonising the land devasted when Mount St. Helens erupted in 1980 is pro... more A new study of plants recolonising the land devasted when Mount St. Helens erupted in 1980 is providing important new insights into the interactions with herbivores that determine the pattern and outcome of ecological succession.
Journal of vector ecology : journal of the Society for Vector Ecology, 2004
We examined the effect of predation by the backswimmer (Notonecta undulata; Hemiptera: Notonectid... more We examined the effect of predation by the backswimmer (Notonecta undulata; Hemiptera: Notonectidae), competition by zooplankton and snails, and both predation and competition on the survival and development time of larval Anopheles quadrimaculatus mosquitoes in experimental mesocosms. We found that both predation and interspecific competition greatly reduced the survivorship of larvae and the number of larvae emerging into adulthood. Treatments with both predators and competitors had fewer larvae transitioning among instars and into adulthood but not in an additive way. In addition, mosquito larvae in the presence of predators and competitors took two days longer to emerge than where predators and competitions were absent. Our work provides evidence that biotic interactions, such as predation and competition, can strongly regulate the number of mosquito larvae by reducing the number of larvae that survive through instars and to emergence and by increasing the generation time.
Mosquitoes are not only a nuisance, but also vector many important human and animal diseases. Her... more Mosquitoes are not only a nuisance, but also vector many important human and animal diseases. Here, in opposition with the dogma that increased precipitation predicts mosquito abundance, we hypothesize that mosquitoes should show population outbreaks after drought years. Specifically, we suggest that in wetlands that never dry (permanent), predators limit mosquito abundance, whereas in wetlands that dry yearly (temporary), competitors that are well adapted to predictable drying, limit mosquito abundance. However, in wetlands that dry only during drought years (semi-permanent), mosquito predators and competitors are eliminated and must recolonize following a drought, and the abundance of wetland mosquitoes can skyrocket. We present supportive evidence for this hypothesis from surveys of natural wetlands and from a controlled mesocosm experiment. We conclude that this framework may provide a reliable way to predict and prepare for year-to-year variation in mosquito abundances at large spatial scales.
Mosquitoes are not only a nuisance, but also vector many important human and animal diseases. Her... more Mosquitoes are not only a nuisance, but also vector many important human and animal diseases. Here, in opposition with the dogma that increased precipitation predicts mosquito abundance, we hypothesize that mosquitoes should show population outbreaks after drought years. Specifically, we suggest that in wetlands that never dry (permanent), predators limit mosquito abundance, whereas in wetlands that dry yearly (temporary), competitors
The shape of the species-area relationship (SAR) often varies with the amount of available energy... more The shape of the species-area relationship (SAR) often varies with the amount of available energy; SARs from high-energy habitats typically have higher intercepts and steeper slopes than SARs from low-energy habitats. Such patterns are often assumed to result from a shift in the mechanisms of coexistence between high and low energy habitats. However, a plausible but unexplored alternative mechanism emerges from proportional sampling, if there are simply more individuals in larger or more productive habitats, without the need to invoke differing coexistence mechanisms. Here, we examined proportional versus disproportional responses of a diverse assemblage of freshwater zooplankton to manipulations of experimental pond size and energy inputs. We found that higher energy treatments had higher species richness in large, but not small, ponds, leading to a steeper SAR with higher energy input. The total abundances of individuals also increased with energy in large, but not small ponds. By using a sample-independent rarefaction technique (probability of interspecific encounter), we found that SAR patterns resulted from changes in the total, but not relative, abundance of individuals, and thus proportional, rather than disproportional, responses of species. Overall, our results emphasize the need to consider how both the total and relative abundances of species respond to ecological drivers such as energy and area before inferring the underlying mechanisms that lead to biodiversity patterns. Further, our results may implicate a proportionally smaller influence of energy on patterns of biodiversity when habitats are destroyed.
ABSTRACT Background/Question/Methods Management by harvest (the often selective removal of indivi... more ABSTRACT Background/Question/Methods Management by harvest (the often selective removal of individuals from a population) is a common control tactic for invasive species. Harvest strategies, however, may backfire if density-dependent factors increase the survival of the individuals that escape harvest (e.g. if removing part of an overcrowded population affords the remainder more resources). These undesirable compensatory effects make it difficult to reduce, and may even amplify, long-term population densities, rendering attempts at management counterproductive. Although key population processes, including compensatory effects, are determined by seasonality and the timing of harvest, modeling the associated nonlinear density-dependent dynamics and seasonal effects raises challenging mathematical issues. To address these issues, we have developed a periodic, nonlinear, stage-structured matrix population model that is seasonally-explicit and incorporates selective harvest with respect to stage and timing. Results/Conclusions We have applied our model to the management of garlic mustard (Alliaria petiolata), a biennial European weed that is aggressively displacing North American woodland species. Garlic mustard life history was partitioned into relevant stages and seasons based on major life cycle transitions, and several popular garlic mustard management strategies (pulling flowering adults, clipping flowering or fruiting adults, applying herbicide in the spring or fall, and using weevils for biocontrol) were assessed for their projected effects on long-term population dynamics. We find that reducing seed germination or allowing adults to resprout after clipping has minimal effects on population size in all seasons, while reducing rosette, adult, or seed bank survival is more effective in decreasing average annual densities. We additionally find that population dynamics, including potential compensatory effects, depend on the season in which harvest is applied, and that each harvest strategy differentially increases or decreases densities in each season. These results affirm the importance of stage and seasonal considerations in directing population management efforts.
Abstract Due to a synergism of many human-mediated factors, white-tailed deer (Odocoileus virgini... more Abstract Due to a synergism of many human-mediated factors, white-tailed deer (Odocoileus virginianus) populations have increased dramatically throughout many forests throughout the northeastern North America over the last 50 years. The increase in deer populations ...
Many species are adapted to disturbance and occur within dynamic, mosaic landscapes that contain ... more Many species are adapted to disturbance and occur within dynamic, mosaic landscapes that contain early and late successional microhabitats. Human modification of disturbance regimes alters the availability of microhabitats and may affect the viability of species in these ecosystems. Because restoring historical disturbance regimes is typically expensive and requires action at large spatial scales, such restoration projects must be justified by linking the persistence of species with successional microhabitats. Coastal sand dune ecosystems worldwide are characterized by their endemic biodiversity and frequent disturbance. Dune-stabilizing invasive plants alter successional dynamics and may threaten species in these ecosystems. We examined the distribution and population dynamics of two federally endangered plant species, the annual Layia carnosa and the perennial Lupinus tidestromii, within a dune ecosystem in northern California, USA. We parameterized a matrix population model for L. tidestromii and examined the magnitude by which the successional stage of the habitat (early or late) influenced population dynamics. Both species had higher frequencies and L. tidestromii had higher frequency of seedlings in early successional habitats. Lupinus tidestromii plants in early successional microhabitats had higher projected rates of population growth than those associated with stabilized, late successional habitats, due primarily to higher rates of recruitment in early successional microhabitats. These results support the idea that restoration of disturbance is critical in historically dynamic landscapes. Our results suggest that large-scale restorations are necessary to allow persistence of the endemic plant species that characterize these ecosystems.
Using historic data sets, we quantified the degree to which global change over 120 years disrupte... more Using historic data sets, we quantified the degree to which global change over 120 years disrupted plant-pollinator interactions in a temperate forest understory community in Illinois, USA. We found degradation of interaction network structure and function and extirpation of 50% of bee species. Network changes can be attributed to shifts in forb and bee phenologies resulting in temporal mismatches, nonrandom species extinctions, and loss of spatial co-occurrences between extant species in modified landscapes. Quantity and quality of pollination services have declined through time. The historic network showed flexibility in response to disturbance; however, our data suggest that networks will be less resilient to future changes.
A new study of plants recolonising the land devasted when Mount St. Helens erupted in 1980 is pro... more A new study of plants recolonising the land devasted when Mount St. Helens erupted in 1980 is providing important new insights into the interactions with herbivores that determine the pattern and outcome of ecological succession.
Journal of vector ecology : journal of the Society for Vector Ecology, 2004
We examined the effect of predation by the backswimmer (Notonecta undulata; Hemiptera: Notonectid... more We examined the effect of predation by the backswimmer (Notonecta undulata; Hemiptera: Notonectidae), competition by zooplankton and snails, and both predation and competition on the survival and development time of larval Anopheles quadrimaculatus mosquitoes in experimental mesocosms. We found that both predation and interspecific competition greatly reduced the survivorship of larvae and the number of larvae emerging into adulthood. Treatments with both predators and competitors had fewer larvae transitioning among instars and into adulthood but not in an additive way. In addition, mosquito larvae in the presence of predators and competitors took two days longer to emerge than where predators and competitions were absent. Our work provides evidence that biotic interactions, such as predation and competition, can strongly regulate the number of mosquito larvae by reducing the number of larvae that survive through instars and to emergence and by increasing the generation time.
Mosquitoes are not only a nuisance, but also vector many important human and animal diseases. Her... more Mosquitoes are not only a nuisance, but also vector many important human and animal diseases. Here, in opposition with the dogma that increased precipitation predicts mosquito abundance, we hypothesize that mosquitoes should show population outbreaks after drought years. Specifically, we suggest that in wetlands that never dry (permanent), predators limit mosquito abundance, whereas in wetlands that dry yearly (temporary), competitors that are well adapted to predictable drying, limit mosquito abundance. However, in wetlands that dry only during drought years (semi-permanent), mosquito predators and competitors are eliminated and must recolonize following a drought, and the abundance of wetland mosquitoes can skyrocket. We present supportive evidence for this hypothesis from surveys of natural wetlands and from a controlled mesocosm experiment. We conclude that this framework may provide a reliable way to predict and prepare for year-to-year variation in mosquito abundances at large spatial scales.
Mosquitoes are not only a nuisance, but also vector many important human and animal diseases. Her... more Mosquitoes are not only a nuisance, but also vector many important human and animal diseases. Here, in opposition with the dogma that increased precipitation predicts mosquito abundance, we hypothesize that mosquitoes should show population outbreaks after drought years. Specifically, we suggest that in wetlands that never dry (permanent), predators limit mosquito abundance, whereas in wetlands that dry yearly (temporary), competitors
The shape of the species-area relationship (SAR) often varies with the amount of available energy... more The shape of the species-area relationship (SAR) often varies with the amount of available energy; SARs from high-energy habitats typically have higher intercepts and steeper slopes than SARs from low-energy habitats. Such patterns are often assumed to result from a shift in the mechanisms of coexistence between high and low energy habitats. However, a plausible but unexplored alternative mechanism emerges from proportional sampling, if there are simply more individuals in larger or more productive habitats, without the need to invoke differing coexistence mechanisms. Here, we examined proportional versus disproportional responses of a diverse assemblage of freshwater zooplankton to manipulations of experimental pond size and energy inputs. We found that higher energy treatments had higher species richness in large, but not small, ponds, leading to a steeper SAR with higher energy input. The total abundances of individuals also increased with energy in large, but not small ponds. By using a sample-independent rarefaction technique (probability of interspecific encounter), we found that SAR patterns resulted from changes in the total, but not relative, abundance of individuals, and thus proportional, rather than disproportional, responses of species. Overall, our results emphasize the need to consider how both the total and relative abundances of species respond to ecological drivers such as energy and area before inferring the underlying mechanisms that lead to biodiversity patterns. Further, our results may implicate a proportionally smaller influence of energy on patterns of biodiversity when habitats are destroyed.
ABSTRACT Background/Question/Methods Management by harvest (the often selective removal of indivi... more ABSTRACT Background/Question/Methods Management by harvest (the often selective removal of individuals from a population) is a common control tactic for invasive species. Harvest strategies, however, may backfire if density-dependent factors increase the survival of the individuals that escape harvest (e.g. if removing part of an overcrowded population affords the remainder more resources). These undesirable compensatory effects make it difficult to reduce, and may even amplify, long-term population densities, rendering attempts at management counterproductive. Although key population processes, including compensatory effects, are determined by seasonality and the timing of harvest, modeling the associated nonlinear density-dependent dynamics and seasonal effects raises challenging mathematical issues. To address these issues, we have developed a periodic, nonlinear, stage-structured matrix population model that is seasonally-explicit and incorporates selective harvest with respect to stage and timing. Results/Conclusions We have applied our model to the management of garlic mustard (Alliaria petiolata), a biennial European weed that is aggressively displacing North American woodland species. Garlic mustard life history was partitioned into relevant stages and seasons based on major life cycle transitions, and several popular garlic mustard management strategies (pulling flowering adults, clipping flowering or fruiting adults, applying herbicide in the spring or fall, and using weevils for biocontrol) were assessed for their projected effects on long-term population dynamics. We find that reducing seed germination or allowing adults to resprout after clipping has minimal effects on population size in all seasons, while reducing rosette, adult, or seed bank survival is more effective in decreasing average annual densities. We additionally find that population dynamics, including potential compensatory effects, depend on the season in which harvest is applied, and that each harvest strategy differentially increases or decreases densities in each season. These results affirm the importance of stage and seasonal considerations in directing population management efforts.
Abstract Due to a synergism of many human-mediated factors, white-tailed deer (Odocoileus virgini... more Abstract Due to a synergism of many human-mediated factors, white-tailed deer (Odocoileus virginianus) populations have increased dramatically throughout many forests throughout the northeastern North America over the last 50 years. The increase in deer populations ...
FULL TEXT FREELY AVAILABLE AT www.escholarship.org/uc/item/6837k4gc . . . Both means and year-to-... more FULL TEXT FREELY AVAILABLE AT www.escholarship.org/uc/item/6837k4gc . . . Both means and year-to-year variances of climate variables such as temperature and precipitation are predicted to change. However, the potential impact of changing climatic variability on the fate of populations has been largely unexamined. We analyzed multiyear demographic data for 36 plant and animal species with a broad range of life histories and types of environment to ask how sensitive their long-term stochastic population growth rates are likely to be to changes in the means and standard deviations of vital rates (survival, reproduction, growth) in response to changing climate. We quantified responsiveness using elasticities of the long-term population growth rate predicted by stochastic projection matrix models. Short-lived species (insects and annual plants and algae) are predicted to be more strongly (and negatively) affected by increasing vital rate variability relative to longer-lived species (perennial plants, birds, ungulates). Taxonomic affiliation has little power to explain sensitivity to increasing variability once longevity has been taken into account. Our results highlight the potential vulnerability of short-lived species to an increasingly variable climate, but also suggest that problems associated with short-lived undesirable species (agricultural pests, disease vectors, invasive weedy plants) may be exacerbated in regions where climate variability decreases.
Uploads
Papers by Tiffany Knight