Latitudinal gradients in species richness

Transcription

1 Latitudinal gradients in species richness Multiple hypotheses proposed for why there are more species in the tropics (we will review these in discussion reading Wednesday) 1. Climate stability & refugia 2. Area effect 3. Narrower range distributions (climate niches) at low latitude 4. Differences in local species interactions 5. Energy and productivity 6. Energy and Evolutionary speed 7. Climatic niche diversification

2 6. Energy and evolutionary speed Rohde (1992) suggests that rather than trying to explain how productivity affects ecological interactions should view this correlation as one between energy supply and evolutionary speed. Mean age of 13 living bivalve faunas from east coast N. America. Similar data for foraminifera, mammals, brachiopods suggest higher evolutionary rates in the tropics

3 Rohde suggests that greater evolutionary speed might be due to shorter generation times of tropical organisms, and higher mutation rates. However: Generation times are not necessarily correlated with evolutionary rate (slow evolution in short generation opossums; fast evolution in elephants ) Some examples of temperature-dependence in mutation rates (e.g. Drosophila and E. coli - but indirect through shorter generation times).

4 7. Climatic niche diversification Janzen (1967) Why mountain passes are higher in the tropics (revisited by Ghalambor et al. 2006) Low latitude little seasonal variation in temperature organisms may have narrower physiological tolerances than temperate spp Kozak and Wiens (2007, 2010, 2012) explored whether groups of related species (clades) of salamanders show differences in their climatic distribution for tropical and temperate species Found less overlap for 14 tropical species pairs than 16 temperate pairs

5 Well resolved time-calibrated phylogeny exists for salamanders allowing inferences about the rate of species diversification. - Show more rapid diversification for tropical than temperate lineages More rapid diversification is associated with faster climatic niche evolution Climatic niche evolution in turn depends on geographic isolation of a clade (lack of competing groups of species that already occupy those niches)

6 Distribution of salamander spp in climatic niche space (PC axes) 3 clades: Eastern N. Am Western N. Am Tropical Am PC axes determined by combining data on temperature and rainfall

7 More recently (Kozak and Wiens 2012) looked at larger group of N. American salamanders. Slightly different conclusion Richness is strongly related to temp and precipitation in N. Am (not a latitudinal gradient) Richness really related to how long an area has been colonized rather than diversification rate

8 Diversity - productivity relationships Productivity: Rate of energy flow to a system (averaged over an appropriate time scale). Typically used by ecologists to denote net rate at which CO 2 is fixed by the ecosystem (Gross productivity) Diversity: Usually means S (species richness) in this context Productivity is difficult to measure directly. So, generally some indirect measure that is correlated to productivity is used.

9 For animals, diversity increases with PET up to a point

10 In plants and aquatic systems relationship is often humped Many examples of plant diversity showing a humped relationship with some index of productivity (see Morin book p. 333 for more examples) Mittelbach et al. (review in Ecology 2001) Unimodal relationship is common where biomass is used as the correlate for productivity.

11 Other correlates of productivity (e.g. moisture availability in prairie land:

12 Hump-shape relationships also common in lakes (Dodson 2000) Log species richness Species richness per lake after adjusting for lake size Log primary productivity

13 Just how common is the hump-relationship? Mittelbach et al (2001) looked at 171 published papers For plants found that predominant relationship between correlates of productivity and species richness was a humped (or unimodal) (~40 % of studies at all spatial scales - from local <20 km up to continental scale >4000 km) For aquatic systems, hump relationship especially common Dodson et al (2000) found evidence that phytoplankton, macrophytes, and zooplantkon all showed unimodal relationship. Not so for fish - diversity peaks at high productivity - no observed decline. Why would lakes commonly show hump relationships??

14 Hoffman and Dodson (2005). Unimodal productivity relationship in lakes may be an artefact Previous analyses did not account for human alteration of the watershed Pristine lakes Developed lakes All lakes Species richness

15 For animals, mostly no relationship of S with productivity surrogates at local-landscape scale (<200 km) but increasing proportion of studies found a humped relationship at larger scales (~40 % of studies at continental scales) Generally good support for a common occurrence of the humpshaped curve discussed in literature. However, lots of variation (positive, negative and U-shaped curves occur at all scales of observation ) Overall probability of finding a hump increases as the scale of variation in productivity increase (Mittelbach 2001).

16 What mechanisms could generate this pattern? 1. Resource supply rates: Tilman and Pacala (1993) suggest that Tilman s R* model could generate this pattern think two limiting resources Sedgwick Experimental Sedgwick Observation Rothamstead Experimental

17 N input rate (including background deposition) Just N alone can significantly reduce richness, even at relatively low levels of application (background N deposition rate in midwest is ~7kg/ha/year

18 Phoenix G.K. et al. (2006) Atmospheric nitrogen deposition in world biodiversity hotspots: the need for a greater global perspective in assessing N deposition impacts, Global Change Biology 12:

19 Can S recover if N addition is stopped?

20 2. Varying intensities of keystone predation Leibold (1996) Low productivity: predators rare; best competitors dominate Intermediate productivity: Higher predator densities - reduces the abundance of competitively superior (but predator sensitive) prey and allow coexistence with inferior (predator resistant) prey High productivity: Predators abundant. Predator sensitive prey eliminated. Only predator resistant prey survive. Little evidence that there is a consistent trade-off between competitive ability and sensitivity to predation (Viola et al. 2010, PNAS)

21 3. Assemblage level thinning (for plants) Stevens and Carson 1999, Oksanen Hump shaped is an artifact of area-based sampling? As productivity increases so does the size of individuals. Increased size = increased mortality = reduced density of individuals (selfthinning) Compare equal area samples then productive habitats contain fewer individuals and thus fewer species. Simulation of random thinning shows diversity decline.

22 How about reciprocal effects? To what extent is productivity itself dependent upon species composition? Consider coral reefs Rohde (1998): there is no reason to assume that the waters in which coral reefs are found were originally more productive than the surrounding sea. Evolution of corals and calcareous algae facilitated evolution of other primary producers leading to an increase in overall productivity If productivity is a consequence of diversity then what are the consequences of loss of diversity?

23 Classic studies of ecosystem effects of diversity came from the ECOTRON experiments (Silwood Park, UK) - a community microcosm including producers, consumers (2 levels), decomposers 16 replicate chambers

24 Naeem et al (1994) ECOTRON model communities Primary producers (2, 5, or 16 spp) all self-pollinating, herbaceous annual plants (=weeds) Primary consumers (3, 4, or 5 spp) - herbivorous insects and snails Secondary consumers (1 or 2) insect parasitoids Decomposers (3, 5 or 8 spp) - earthworms and collembola Can t run all possible spp combinations because of limited number of chambers (this has been a major criticism ) why? Is it diversity per se that generates these effects - Or, is it species composition?

25 ECOTRON experiment Predators Herbivores Plants Decomposers High Medium Low Low and Medium diversity treatments are nested subsets of the high diversity community Biodiversity Key result: - Lower diversity communities are less productive and consume significantly less CO 2, despite having equivalent population sizes. - Variation in vegetation structure, or plant community architecture, suggests a mechanism for this effect (niche complementarity).

26 - Higher diversity communities had greater space-filling canopies and intercepted more light, allowing higher photosynthetic rates and productivity. Low Medium High

27 Ecosystem consequences of biodiversity in natural settings Are more diverse sites more resistant to invasion? Kennedy et al. (2002) study with Tilman at Cedar Creek - Show that for small grassland plots, density and size of invasive spp negatively correlated with S.

28 Effect size of richness on invader success is quite small

29 Schnitzer et al. (2011) Maybe productivity relationships are driven by soil feedbacks? Increase in productivity with diversity has been attributed to niche complementarity (different resource use among competing species) Alternative: Low diversity systems are prone to disease mostly soil microbes. High diversity systems lower rates of disease transmission and/or greater likelihood of encountering a species that is not strongly impacted by soil feedback. How would you test the feedback hypothesis for the productivity relationship?

30 Strong effect of soil pathogens on the richness-productivity relationship

31 Conclusions Long standing relationships have been observed between diversity and ecosystem productivity. At large scales, relationships are often unimodal, indicating that highly productive communities support only modest diversity Multiple explanations have been given for this relationship; some may be an artefact of plant size, or disturbance Competition may play an important role, however niche complementarity is not the only explanation for positive effects of diversity on productivity. Soil feedbacks may also be important.