Ecology

Hudson, J. M. G.; Henry, G. H. R.

doi: 10.1890/09-0102.1pmid: 19886474

The Canadian High Arctic has been warming for several decades. Over this period, tundra plant communities have been influenced by regional climate change, as well as other disturbances. At a site on Ellesmere Island, Nunavut, Canada, we measured biomass and composition changes in a heath community over 13 years using a point‐intercept method in permanent plots (1995–2007) and over 27 years using a biomass harvest comparison (1981–2008). Results from both methods indicate that the community became more productive over time, suggesting that this ecosystem is currently in transition. Bryophyte and evergreen shrub abundances increased, while deciduous shrub, forb, graminoid, and lichen cover did not change. Species diversity also remained unchanged. Because of the greater evergreen shrub cover, canopy height increased. From 1995 to 2007, mean annual temperature and growing season length increased at the site. Maximum thaw depth increased, while soil water content did not change. We attribute the increased productivity of this community to regional warming over the past 30–50 years. This study provides the first plot‐based evidence for the recent pan‐Arctic increase in tundra productivity detected by satellite‐based remote‐sensing and repeat‐photography studies. These types of ground‐level observations are critical tools for detecting and projecting long‐term community‐level responses to warming.

Hudson, J. M. G.; Henry, G. H. R.

doi: 10.1890/09-0102.1pmid: 19886474

The Canadian High Arctic has been warming for several decades. Over this period, tundra plant communities have been influenced by regional climate change, as well as other disturbances. At a site on Ellesmere Island, Nunavut, Canada, we measured biomass and composition changes in a heath community over 13 years using a point-intercept method in permanent plots (1995––2007) and over 27 years using a biomass harvest comparison (1981––2008). Results from both methods indicate that the community became more productive over time, suggesting that this ecosystem is currently in transition. Bryophyte and evergreen shrub abundances increased, while deciduous shrub, forb, graminoid, and lichen cover did not change. Species diversity also remained unchanged. Because of the greater evergreen shrub cover, canopy height increased. From 1995 to 2007, mean annual temperature and growing season length increased at the site. Maximum thaw depth increased, while soil water content did not change. We attribute the increased productivity of this community to regional warming over the past 30––50 years. This study provides the first plot-based evidence for the recent pan-Arctic increase in tundra productivity detected by satellite-based remote-sensing and repeat-photography studies. These types of ground-level observations are critical tools for detecting and projecting long-term community-level responses to warming.

Lessard, Jean-Philippe; Fordyce, James A.; Gotelli, Nicholas J.; Sanders, Nathan J.

doi: 10.1890/09-0503.1pmid: 19886475

Invasive species displace native species and potentially alter the structure and function of ecological communities. In this study, we compared the generic composition of intact and invaded ant communities from 12 published studies and found that invasive ant species alter the phylogenetic structure of native ant communities. Intact ant communities were phylogenetically evenly dispersed, suggesting that competition structures communities. However, in the presence of an invasive ant species, these same communities were phylogenetically clustered. Phylogenetic clustering in invaded communities suggests that invasive species may act as strong environmental filters and prune the phylogenetic tree of native species in a nonrandom manner, such that only a few closely related taxa can persist in the face of a biological invasion. Taxa that were displaced by invasive ant species were evenly dispersed in the phylogeny, suggesting that diversity losses from invasive ant species are not clustered in particular lineages. Collectively, these results suggest that there is strong phylogenetic structuring in intact native ant communities, but the spread of invasive species disassembles those communities above and beyond the effect of simple reductions in diversity.

Lessard, Jean-Philippe; Fordyce, James A.; Gotelli, Nicholas J.; Sanders, Nathan J.

doi: 10.1890/09-0503.1pmid: 19886475

Invasive species displace native species and potentially alter the structure and function of ecological communities. In this study, we compared the generic composition of intact and invaded ant communities from 12 published studies and found that invasive ant species alter the phylogenetic structure of native ant communities. Intact ant communities were phylogenetically evenly dispersed, suggesting that competition structures communities. However, in the presence of an invasive ant species, these same communities were phylogenetically clustered. Phylogenetic clustering in invaded communities suggests that invasive species may act as strong environmental filters and prune the phylogenetic tree of native species in a nonrandom manner, such that only a few closely related taxa can persist in the face of a biological invasion. Taxa that were displaced by invasive ant species were evenly dispersed in the phylogeny, suggesting that diversity losses from invasive ant species are not clustered in particular lineages. Collectively, these results suggest that there is strong phylogenetic structuring in intact native ant communities, but the spread of invasive species disassembles those communities above and beyond the effect of simple reductions in diversity.

Berendonk, T. U.; Spitze, K.; Kerfoot, W. C.

doi: 10.1890/08-0667.1pmid: 19886476

One of the primary questions concerning the long-term preservation of nature and its diversity is the maintenance of genetic diversity. However, despite numerous theoretical investigations, comparative empirical information on how local extinctions influence regional genetic variation does not exist. To our knowledge, this is the first report of an empirical study comparing the genetic variation of permanent vs. ephemeral species at two scales (local variation, regional variation). This approach, utilizing a microsatellite analysis of six midge species of the genus Chaoborus generated intriguing scale-dependent results. Species that experienced repeated local extinctions had reduced genetic variation at the local level, yet the regional genetic variation was greater than in species with permanent populations. Our findings call into question the assumption that species with repeated local extinctions generally contain lower genetic diversity, especially if they experience a ““nomadic”” pattern of dispersal. We encourage comparative analyses of empirical genetic data at dual scales as molecular tools become more available in ecological studies.

Berendonk, T. U.; Spitze, K.; Kerfoot, W. C.

doi: 10.1890/08-0667.1pmid: 19886476

One of the primary questions concerning the long‐term preservation of nature and its diversity is the maintenance of genetic diversity. However, despite numerous theoretical investigations, comparative empirical information on how local extinctions influence regional genetic variation does not exist. To our knowledge, this is the first report of an empirical study comparing the genetic variation of permanent vs. ephemeral species at two scales (local variation, regional variation). This approach, utilizing a microsatellite analysis of six midge species of the genus Chaoborus generated intriguing scale‐dependent results. Species that experienced repeated local extinctions had reduced genetic variation at the local level, yet the regional genetic variation was greater than in species with permanent populations. Our findings call into question the assumption that species with repeated local extinctions generally contain lower genetic diversity, especially if they experience a “nomadic” pattern of dispersal. We encourage comparative analyses of empirical genetic data at dual scales as molecular tools become more available in ecological studies.

Efford, Murray G.; Dawson, Deanna K.; Borchers, David L.

doi: 10.1890/08-1735.1pmid: 19886477

The density of a closed population of animals occupying stable home ranges may be estimated from detections of individuals on an array of detectors, using newly developed methods for spatially explicit capture––recapture. Likelihood-based methods provide estimates for data from multi-catch traps or from devices that record presence without restricting animal movement (““proximity”” detectors such as camera traps and hair snags). As originally proposed, these methods require multiple sampling intervals. We show that equally precise and unbiased estimates may be obtained from a single sampling interval, using only the spatial pattern of detections. This considerably extends the range of possible applications, and we illustrate the potential by estimating density from simulated detections of bird vocalizations on a microphone array. Acoustic detection can be defined as occurring when received signal strength exceeds a threshold. We suggest detection models for binary acoustic data, and for continuous data comprising measurements of all signals above the threshold. While binary data are often sufficient for density estimation, modeling signal strength improves precision when the microphone array is small.

Efford, Murray G.; Dawson, Deanna K.; Borchers, David L.

doi: 10.1890/08-1735.1pmid: 19886477

The density of a closed population of animals occupying stable home ranges may be estimated from detections of individuals on an array of detectors, using newly developed methods for spatially explicit capture–recapture. Likelihood‐based methods provide estimates for data from multi‐catch traps or from devices that record presence without restricting animal movement (“proximity” detectors such as camera traps and hair snags). As originally proposed, these methods require multiple sampling intervals. We show that equally precise and unbiased estimates may be obtained from a single sampling interval, using only the spatial pattern of detections. This considerably extends the range of possible applications, and we illustrate the potential by estimating density from simulated detections of bird vocalizations on a microphone array. Acoustic detection can be defined as occurring when received signal strength exceeds a threshold. We suggest detection models for binary acoustic data, and for continuous data comprising measurements of all signals above the threshold. While binary data are often sufficient for density estimation, modeling signal strength improves precision when the microphone array is small.

Neubert, Michael G.; Caswell, Hal; Solow, Andrew R.

doi: 10.1890/08-2014.1pmid: 19886478

By definition, ecological systems at a stable equilibrium eventually return to the equilibrium point following a small perturbation. In the short term, however, perturbations can grow. Equilibria that exhibit transient growth following perturbation are said to be reactive. In this report, we present a statistical method for detecting reactivity from multivariate time series. The test is simple and computationally tractable, and it can be applied to short time series. Its main limitation is that it is based on a model of population dynamics that is linear on a logarithmic scale. Our results suggest that the test is robust when the dynamics are nonlinear on the log scale but that it may incorrectly classify an equilibrium as reactive when the reactivity is close to zero.

Neubert, Michael G.; Caswell, Hal; Solow, Andrew R.

doi: 10.1890/08-2014.1pmid: 19886478

By definition, ecological systems at a stable equilibrium eventually return to the equilibrium point following a small perturbation. In the short term, however, perturbations can grow. Equilibria that exhibit transient growth following perturbation are said to be reactive. In this report, we present a statistical method for detecting reactivity from multivariate time series. The test is simple and computationally tractable, and it can be applied to short time series. Its main limitation is that it is based on a model of population dynamics that is linear on a logarithmic scale. Our results suggest that the test is robust when the dynamics are nonlinear on the log scale but that it may incorrectly classify an equilibrium as reactive when the reactivity is close to zero.