# Source code for networkx.algorithms.centrality.trophic

```
"""Trophic levels"""
import networkx as nx
from networkx.utils import not_implemented_for
__all__ = ["trophic_levels", "trophic_differences", "trophic_incoherence_parameter"]
[docs]
@not_implemented_for("undirected")
@nx._dispatchable(edge_attrs="weight")
def trophic_levels(G, weight="weight"):
r"""Compute the trophic levels of nodes.
The trophic level of a node $i$ is
.. math::
s_i = 1 + \frac{1}{k^{in}_i} \sum_{j} a_{ij} s_j
where $k^{in}_i$ is the in-degree of i
.. math::
k^{in}_i = \sum_{j} a_{ij}
and nodes with $k^{in}_i = 0$ have $s_i = 1$ by convention.
These are calculated using the method outlined in Levine [1]_.
Parameters
----------
G : DiGraph
A directed networkx graph
Returns
-------
nodes : dict
Dictionary of nodes with trophic level as the value.
References
----------
.. [1] Stephen Levine (1980) J. theor. Biol. 83, 195-207
"""
import numpy as np
# find adjacency matrix
a = nx.adjacency_matrix(G, weight=weight).T.toarray()
# drop rows/columns where in-degree is zero
rowsum = np.sum(a, axis=1)
p = a[rowsum != 0][:, rowsum != 0]
# normalise so sum of in-degree weights is 1 along each row
p = p / rowsum[rowsum != 0][:, np.newaxis]
# calculate trophic levels
nn = p.shape[0]
i = np.eye(nn)
try:
n = np.linalg.inv(i - p)
except np.linalg.LinAlgError as err:
# LinAlgError is raised when there is a non-basal node
msg = (
"Trophic levels are only defined for graphs where every "
+ "node has a path from a basal node (basal nodes are nodes "
+ "with no incoming edges)."
)
raise nx.NetworkXError(msg) from err
y = n.sum(axis=1) + 1
levels = {}
# all nodes with in-degree zero have trophic level == 1
zero_node_ids = (node_id for node_id, degree in G.in_degree if degree == 0)
for node_id in zero_node_ids:
levels[node_id] = 1
# all other nodes have levels as calculated
nonzero_node_ids = (node_id for node_id, degree in G.in_degree if degree != 0)
for i, node_id in enumerate(nonzero_node_ids):
levels[node_id] = y.item(i)
return levels
[docs]
@not_implemented_for("undirected")
@nx._dispatchable(edge_attrs="weight")
def trophic_differences(G, weight="weight"):
r"""Compute the trophic differences of the edges of a directed graph.
The trophic difference $x_ij$ for each edge is defined in Johnson et al.
[1]_ as:
.. math::
x_ij = s_j - s_i
Where $s_i$ is the trophic level of node $i$.
Parameters
----------
G : DiGraph
A directed networkx graph
Returns
-------
diffs : dict
Dictionary of edges with trophic differences as the value.
References
----------
.. [1] Samuel Johnson, Virginia Dominguez-Garcia, Luca Donetti, Miguel A.
Munoz (2014) PNAS "Trophic coherence determines food-web stability"
"""
levels = trophic_levels(G, weight=weight)
diffs = {}
for u, v in G.edges:
diffs[(u, v)] = levels[v] - levels[u]
return diffs
[docs]
@not_implemented_for("undirected")
@nx._dispatchable(edge_attrs="weight")
def trophic_incoherence_parameter(G, weight="weight", cannibalism=False):
r"""Compute the trophic incoherence parameter of a graph.
Trophic coherence is defined as the homogeneity of the distribution of
trophic distances: the more similar, the more coherent. This is measured by
the standard deviation of the trophic differences and referred to as the
trophic incoherence parameter $q$ by [1].
Parameters
----------
G : DiGraph
A directed networkx graph
cannibalism: Boolean
If set to False, self edges are not considered in the calculation
Returns
-------
trophic_incoherence_parameter : float
The trophic coherence of a graph
References
----------
.. [1] Samuel Johnson, Virginia Dominguez-Garcia, Luca Donetti, Miguel A.
Munoz (2014) PNAS "Trophic coherence determines food-web stability"
"""
import numpy as np
if cannibalism:
diffs = trophic_differences(G, weight=weight)
else:
# If no cannibalism, remove self-edges
self_loops = list(nx.selfloop_edges(G))
if self_loops:
# Make a copy so we do not change G's edges in memory
G_2 = G.copy()
G_2.remove_edges_from(self_loops)
else:
# Avoid copy otherwise
G_2 = G
diffs = trophic_differences(G_2, weight=weight)
return float(np.std(list(diffs.values())))
```