Dean Jacobsen

Publications (4)11.43 Total impact

• Source

  Available from: Rodrigo Espinosa

  Article: Spatial variability in macroinvertebrate assemblages along and among neighbouring equatorial glacier‐fed streams

  JESPER KUHN, PATRICIO ANDINO, ROGER CALVEZ, RODRIGO ESPINOSA, LADISLAV HAMERLIK, SIGNE VIE, OLIVIER DANGLES, DEAN JACOBSEN
  [show abstract] [hide abstract]
  ABSTRACT: Summary1. During the past two decades, understanding of the structure and function of glacier-fed stream ecosystems at temperate latitudes has increased substantially. In contrast, information on their tropical counterparts is very limited. We studied three neighbouring glacier-fed streams in the tropical Andes of Ecuador. Our main goals were (i) to determine overall longitudinal patterns in density, taxon richness and the composition of macroinvertebrate assemblages and driving factors in equatorial glacial streams and (ii) to examine variability among replicate streams in faunal metrics and assemblages, and stream-specific effects of supposed environmental key factors.2. We measured four geographical and 17 environmental factors and collected five Surber samples (500 cm2) of macroinvertebrates at each of nine sites, three sites along three streams. The streams were located 1–5 km apart. In each stream, the three sites were placed at comparable distances from the glacier and were grouped as ‘upper’ (50–200 m), ‘middle’ (1.5 km) and ‘lower’ sites (3.5–5.6 km).3. In total, 2200 individuals (64% chironomids) were collected and 47 taxa (30 dipterans, 18 of these Chironomidae) identified. Density ranged from 176 to 372 ind. m−2, and the number of taxa ranged from 2 to 6 at the upper sites and 868–3044 ind. m−2 and 21–27 taxa at the lower sites. Density, number of taxa, rarefied richness and axis-1 coordinates from a MDS ordination increased logarithmically with distance from the glacier. These faunal metrics were equally related to altitude and glacier per cent of catchment and correlated with maximum conductivity, mean temperature, mean daily maximum temperature and a channel stability index. As expected, the mean difference in distance decay in similarity was higher at the upper (47% km−1) than at the lower reaches (20% km−1) of the streams.4. The number of taxa varied among sites within the upper and middle groups, but not among the lower sites. In contrast, but in accordance with our expectation, assemblage composition did not differ among upper sites but did so at middle and lower sites, following a supposed decrease in environmental harshness along the streams. Relationships between faunal metrics and the four environmental variables mean temperature, the stability index, chlorophyll a and coarse particulate organic matter also varied among the three streams. Generalised linear model analyses revealed that temperature interacted with stream on macroinvertebrate density, while chlorophyll a had a significant effect on the number of taxa in interaction with stream and stability.5. The basic predictions of the Milner et al. (2001a), model regarding longitudinal faunal patterns and temperature and stability as main driving factors were met by our three replicate equatorial glacial streams. Qualitative departures from the model were mainly because of zoogeographical differences. We demonstrated that variability in assemblages between comparable sites in closely situated streams was considerable, and the effect of key environmental factors varied among streams and interacted with other factors. Quantifying spatial variation in benthic assemblages may help us foresee possible consequences for biodiversity as a result of glacial retreat.
  Freshwater Biology 10/2011; 56(11):2226 - 2244. · 3.29 Impact Factor
• Article: Environmental harshness and global richness patterns in glacier‐fed streams

  Dean Jacobsen, Olivier Dangles
  [show abstract] [hide abstract]
  ABSTRACT: Aim  To test for a possible effect of environmental harshness on large-scale latitudinal and elevational patterns in taxon richness of macrofauna in arctic and alpine glacier-fed streams.Location  Svalbard (79° N), Iceland (65° N), Norway (62° N), Switzerland and Italy (46° N), France (43° N), New Zealand (43° S) and Ecuador (0°), covering an elevational gradient from sea level to 4800 m a.s.l.Methods  We gathered data from 63 sites along 13 streams and created an index of glacial influence (the glacial index, GI) as an integrative proxy for environmental harshness. The explicative power of the GI, environmental variables, latitude and elevation on taxon richness was tested in generalized linear models. Taxon richness along geographical gradients was analysed at standardized levels of GI in contour plots. Beta diversity and assemblage similarity was calculated at different GI intervals and compared with a null-model.Results  Overall, taxon richness decreased exponentially with increased GI (r2= 0.64), and of all included factors, GI had the highest explicative power. At low values of GI we found that local taxon richness varied along the coupled gradients of latitude and elevation in a hump-shaped manner. However, this pattern disappeared at high values of GI, i.e. when environmental harshness increased. Beta diversity increased, while similarity among assemblages decreased towards high GI values.Main conclusions  In our study system, the number of taxa able to cope with the harshest conditions was largely independent of the regional taxon pool, and environmental harshness constituted a ‘fixed’ constraint for local richness, irrespective of latitude and elevation. Contrary to expectations, we found that beta diversity was highest and similarity lowest among the harshest sites, suggesting that taxon richness was not solely driven by niche selection based on environmental tolerances, but also stochastic ecological drift, leading to dispersal-limited communities.
  Global Ecology and Biogeography. 07/2011; 21(6):647 - 656.
• Source

  Available from: Rodrigo Espinosa

  Article: Predicting richness effects on ecosystem function in natural communities: insights from high-elevation streams.

  Olivier Dangles, Verónica Crespo-Pérez, Patricio Andino, Rodrigo Espinosa, Roger Calvez, Dean Jacobsen
  [show abstract] [hide abstract]
  ABSTRACT: . Despite the increased complexity of experimental and theoretical studies on the biodiversity-ecosystem functioning (B-EF) relationship, a major challenge is to demonstrate whether the observed importance of biodiversity in controlled experimental systems also persists in nature. Due to their structural simplicity and their low levels of human impacts, extreme species-poor ecosystems may provide new insights into B-EF relationships in natural systems. We address this issue using shredder invertebrate communities and organic matter decomposition rates in 24 high-altitude (3200-3900 m) Neotropical streams as a study model. We first assessed the effects of stream characteristics and shredder diversity and abundance on organic matter decomposition rates in coarse- and fine-mesh bags. We found the interaction term shredder richness x shredder abundance had the most significant impact on decomposition rates in the field, although water discharge may also play a role locally. We also examined the relative contribution of the three most abundant shredders on decomposition rates by manipulating shredder richness and community composition in a field experiment. Transgressive overyielding was detected among the three shredder species, indicating complementary resource use and/or facilitation. By integrating survey and experimental data in surface response analyses we found that observed B-EF patterns fit those predicted by a linear model that described litter decomposition rates as a function of increasing shredder richness and the relative abundance of the most efficient shredders. Finally, the validity of our approach was tested in a broader context by using two independent but comparable data sets from 49 French and Swedish streams showing more complex shredder community structure. Results revealed that richness and identity effects on decomposition rates were lost with increasing shredder community complexity. Our approach of combining experimental and empirical data with modeling in species-poor ecosystems may serve as an impetus for new B-EF studies. If theory can explain B-EF in low-diversity ecosystems, it may also have credibility in more complex ones.
  Ecology 03/2011; 92(3):733-43. · 4.85 Impact Factor
• Source

  Available from: Rodrigo Espinosa

  Article: Longitudinal zonation of macroinvertebrates in an Ecuadorian glacier‐fed stream: do tropical glacial systems fit the temperate model?

  DEAN JACOBSEN, OLIVIER DANGLES, PATRICIO ANDINO, RODRIGO ESPINOSA, LADISLAV HAMERLÍK, ERIC CADIER
  [show abstract] [hide abstract]
  ABSTRACT: Summary1. The ecology of glacier-fed streams at temperate latitudes has been intensely studied in recent years, leading to the development of a well-validated conceptual model on the longitudinal distribution of macroinvertebrate communities downstream of the glacier margin (Freshwater Biology, 2001a; 46, 1833). However, to our knowledge, the ecology of tropical glacier-fed streams has not yet been studied.2. We sampled benthic macroinvertebrates and measured environmental variables at nine sites between 4730 and 4225 m altitude along a 4.3 km stretch of a glacier-fed stream 40 km south of the equator in the Ecuadorian Andes. Our goal was to study the longitudinal distribution of the fauna in relation to environmental factors and to compare this with the conceptual model based on temperate–arctic glacier-fed streams.3. Total density of invertebrates differed considerably at the two highest altitude sites; 4600 m−2 at a pro-glacial lake outlet and only 4 m−2 at a site originating directly from the glacier snout. Otherwise, there was a downstream decrease in density to about 825 m−2 at the three lowest sites. Taxon richness increased with distance from the glacier, very similar to the pattern predicted. A total of 28 taxa were collected; two at the glacier snout, seven at the nearby pro-glacial lake outlet, 13 at site 2 (<400 m from the glacier) and 20 at the lowest sites.4. The numerical percentage of Chironomidae (Diptera) decreased downstream from 100 to 44%. The subfamily Podonominae was numerous at the highest sites but became much less important further downstream. The Orthocladiinae were important both in numbers and species at all sites, while Diamesinae were numerous only in the middle of the reach studied and were completely absent from the upper three sites. The limited importance of the Diamesinae, and its replacement by Podonominae, is different from the pattern typically observed in north-temperate glacier-fed streams. This could be because of the fact that the genus Diamesa is missing from the Neotropics.5. Stream temperature and channel stability explained most of the variability in faunal composition and richness, supporting the model. Stability increased systematically downstream while temperature did not. Surprisingly, no classical kryal zone (Tmax < 4 °C) was found, as even the site closest to the glacier snout (50 m) had a Tmax of 15 °C and no site had Tmax < 8 °C. We propose that this might be a general feature of equatorial glacial streams.
  Freshwater Biology 05/2010; 55(6):1234 - 1248. · 3.29 Impact Factor